KEYPROV Working Group A. Doherty Internet-Draft RSA, The Security Division of EMC Intended status: Standards Track M. Pei Expires: May 7, 2009 Verisign, Inc. S. Machani Diversinet Corp. M. Nystrom RSA, The Security Division of EMC November 3, 2008 Dynamic Symmetric Key Provisioning Protocol (DSKPP) draft-ietf-keyprov-dskpp-06.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 7, 2009. Abstract DSKPP is a client-server protocol for initialization (and configuration) of symmetric keys to locally and remotely accessible cryptographic modules. The protocol can be run with or without private-key capabilities in the cryptographic modules, and with or without an established public-key infrastructure. Two variations of the protocol support multiple usage scenarios. Doherty, et al. Expires May 7, 2009 [Page 1] Internet-Draft DSKPP November 2008 With the four-pass variant, keys are mutually generated by the provisioning server and cryptographic module; provisioned keys are not transferred over-the-wire or over-the-air. The two-pass variant enables secure and efficient download and installation of pre- generated symmetric keys to a cryptographic module. This document builds on information contained in [RFC4758], adding specific enhancements in response to implementation experience and liaison requests. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . 6 1.1.1. Single Key Request . . . . . . . . . . . . . . . . . 7 1.1.2. Multiple Key Requests . . . . . . . . . . . . . . . . 7 1.1.3. User Authentication . . . . . . . . . . . . . . . . . 7 1.1.4. Provisioning Time-Out Policy . . . . . . . . . . . . 7 1.1.5. Key Renewal . . . . . . . . . . . . . . . . . . . . . 7 1.1.6. Pre-Loaded Key Replacement . . . . . . . . . . . . . 8 1.1.7. Pre-Shared Manufacturing Key . . . . . . . . . . . . 8 1.1.8. End-to-End Protection of Key Material . . . . . . . . 8 1.2. Protocol Entities . . . . . . . . . . . . . . . . . . . . 9 1.3. Initiating DSKPP . . . . . . . . . . . . . . . . . . . . 10 1.4. Determining Which Protocol Variant to Use . . . . . . . . 11 1.4.1. Criteria for Using the Four-Pass Protocol . . . . . . 11 1.4.2. Criteria for Using the Two-Pass Protocol . . . . . . 12 1.5. Presentation Syntax . . . . . . . . . . . . . . . . . . . 12 1.5.1. Versions . . . . . . . . . . . . . . . . . . . . . . 12 1.5.2. Namespaces . . . . . . . . . . . . . . . . . . . . . 12 1.5.3. Identifiers . . . . . . . . . . . . . . . . . . . . . 13 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.1. Key Words . . . . . . . . . . . . . . . . . . . . . . . . 13 2.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 13 2.3. Notation . . . . . . . . . . . . . . . . . . . . . . . . 15 2.4. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 16 3. DSKPP Protocol Details . . . . . . . . . . . . . . . . . . . 17 3.1. Protocol Initiation . . . . . . . . . . . . . . . . . . . 17 3.1.1. Server Initiation . . . . . . . . . . . . . . . . . . 17 3.1.2. Client Initiation . . . . . . . . . . . . . . . . . . 18 3.2. Protocol Variations . . . . . . . . . . . . . . . . . . . 18 3.2.1. Four-Pass Protocol Interaction . . . . . . . . . . . 18 3.2.2. Two-Pass Protocol Interaction . . . . . . . . . . . . 20 3.3. Cryptographic Construction . . . . . . . . . . . . . . . 21 3.3.1. The DSKPP One-Way Pseudorandom Function, DSKPP-PRF . 21 3.4. Four-Pass Protocol Usage . . . . . . . . . . . . . . . . 22 3.4.1. Message Flow . . . . . . . . . . . . . . . . . . . . 22 Doherty, et al. Expires May 7, 2009 [Page 2] Internet-Draft DSKPP November 2008 3.4.2. Generation of Symmetric Keys for Cryptographic Modules . . . . . . . . . . . . . . . . . . . . . . . 25 3.4.3. Encryption of Pseudorandom Nonces Sent from the DSKPP Client . . . . . . . . . . . . . . . . . . . . 28 3.4.4. MAC Calculations . . . . . . . . . . . . . . . . . . 28 3.5. Two-Pass Protocol Usage . . . . . . . . . . . . . . . . . 30 3.5.1. Message Flow . . . . . . . . . . . . . . . . . . . . 30 3.5.2. Key Protection Profiles . . . . . . . . . . . . . . . 33 3.5.3. MAC Calculations . . . . . . . . . . . . . . . . . . 37 3.6. Device Identification . . . . . . . . . . . . . . . . . . 38 3.7. User Authentication . . . . . . . . . . . . . . . . . . . 39 3.7.1. Authentication Data . . . . . . . . . . . . . . . . . 39 3.7.2. Authentication Code Format . . . . . . . . . . . . . 40 3.7.3. Authentication Data Calculation . . . . . . . . . . . 42 4. DSKPP Message Formats . . . . . . . . . . . . . . . . . . . . 43 4.1. General XML Schema Requirements . . . . . . . . . . . . . 43 4.2. Components of the Message . . . . . . . 44 4.3. Components of the Request . . . . . 45 4.3.1. The DeviceIdentifierDataType Type . . . . . . . . . . 48 4.3.2. The ProtocolVariantsType Type . . . . . . . . . . . . 48 4.3.3. The KeyPackagesFormatType Type . . . . . . . . . . . 49 4.3.4. The AuthenticationDataType Type . . . . . . . . . . . 50 4.4. Components of the Response (Used Only in Four-Pass DSKPP) . . . . . . . . . . . . . . . . 50 4.5. Components of a Request (Used Only in Four-Pass DSKPP) . . . . . . . . . . . . . . . . 52 4.6. Components of a Response . . . . 53 4.7. The StatusCode Type . . . . . . . . . . . . . . . . . . . 55 5. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 57 5.1. The ClientInfoType Type . . . . . . . . . . . . . . . . . 57 5.2. The ServerInfoType Type . . . . . . . . . . . . . . . . . 57 6. Protocol Bindings . . . . . . . . . . . . . . . . . . . . . . 57 6.1. General Requirements . . . . . . . . . . . . . . . . . . 57 6.2. HTTP/1.1 Binding for DSKPP . . . . . . . . . . . . . . . 57 6.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 57 6.2.2. Identification of DSKPP Messages . . . . . . . . . . 58 6.2.3. HTTP Headers . . . . . . . . . . . . . . . . . . . . 58 6.2.4. HTTP Operations . . . . . . . . . . . . . . . . . . . 58 6.2.5. HTTP Status Codes . . . . . . . . . . . . . . . . . . 59 6.2.6. HTTP Authentication . . . . . . . . . . . . . . . . . 59 6.2.7. Initialization of DSKPP . . . . . . . . . . . . . . . 59 6.2.8. Example Messages . . . . . . . . . . . . . . . . . . 60 7. DSKPP Schema . . . . . . . . . . . . . . . . . . . . . . . . 60 8. Conformance Requirements . . . . . . . . . . . . . . . . . . 69 9. Security Considerations . . . . . . . . . . . . . . . . . . . 70 9.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 70 9.2. Active Attacks . . . . . . . . . . . . . . . . . . . . . 70 9.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 70 Doherty, et al. Expires May 7, 2009 [Page 3] Internet-Draft DSKPP November 2008 9.2.2. Message Modifications . . . . . . . . . . . . . . . . 70 9.2.3. Message Deletion . . . . . . . . . . . . . . . . . . 72 9.2.4. Message Insertion . . . . . . . . . . . . . . . . . . 72 9.2.5. Message Replay . . . . . . . . . . . . . . . . . . . 72 9.2.6. Message Reordering . . . . . . . . . . . . . . . . . 73 9.2.7. Man-in-the-Middle . . . . . . . . . . . . . . . . . . 73 9.3. Passive Attacks . . . . . . . . . . . . . . . . . . . . . 73 9.4. Cryptographic Attacks . . . . . . . . . . . . . . . . . . 73 9.5. Attacks on the Interaction between DSKPP and User Authentication . . . . . . . . . . . . . . . . . . . . . 74 9.6. Miscellaneous Considerations . . . . . . . . . . . . . . 75 9.6.1. Client Contributions to K_TOKEN Entropy . . . . . . . 75 9.6.2. Key Confirmation . . . . . . . . . . . . . . . . . . 75 9.6.3. Server Authentication . . . . . . . . . . . . . . . . 75 9.6.4. User Authentication . . . . . . . . . . . . . . . . . 75 9.6.5. Key Protection in Two-Pass DSKPP . . . . . . . . . . 76 10. Internationalization Considerations . . . . . . . . . . . . . 77 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 77 11.1. URN Sub-Namespace Registration . . . . . . . . . . . . . 77 11.2. XML Schema Registration . . . . . . . . . . . . . . . . . 78 11.3. MIME Media Type Registration . . . . . . . . . . . . . . 78 11.4. Status Code Registry . . . . . . . . . . . . . . . . . . 79 12. Intellectual Property Considerations . . . . . . . . . . . . 80 13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 80 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 80 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 81 15.1. Normative references . . . . . . . . . . . . . . . . . . 81 15.2. Informative references . . . . . . . . . . . . . . . . . 82 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 84 A.1. Trigger Message . . . . . . . . . . . . . . . . . . . . . 85 A.2. Four-Pass Protocol . . . . . . . . . . . . . . . . . . . 85 A.2.1. Without a Preceding Trigger . . 86 A.2.2. Assuming a Preceding Trigger . . 87 A.2.3. Without a Preceding Trigger . . 88 A.2.4. Assuming a Preceding Trigger . . 89 A.2.5. Using Default Encryption . . . . 89 A.2.6. Using Default Encryption . . 91 A.3. Two-Pass Protocol . . . . . . . . . . . . . . . . . . . . 91 A.3.1. Example Using the Key Transport Profile . . . . . . . 91 A.3.2. Example Using the Key Wrap Profile . . . . . . . . . 94 A.3.3. Example Using the Passphrase-Based Key Wrap Profile . 97 Appendix B. Integration with PKCS #11 . . . . . . . . . . . . . 100 B.1. The 4-pass Variant . . . . . . . . . . . . . . . . . . . 100 B.2. The 2-pass Variant . . . . . . . . . . . . . . . . . . . 100 Appendix C. Example of DSKPP-PRF Realizations . . . . . . . . . 103 C.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 103 C.2. DSKPP-PRF-AES . . . . . . . . . . . . . . . . . . . . . . 103 C.2.1. Identification . . . . . . . . . . . . . . . . . . . 103 Doherty, et al. Expires May 7, 2009 [Page 4] Internet-Draft DSKPP November 2008 C.2.2. Definition . . . . . . . . . . . . . . . . . . . . . 103 C.2.3. Example . . . . . . . . . . . . . . . . . . . . . . . 104 C.3. DSKPP-PRF-SHA256 . . . . . . . . . . . . . . . . . . . . 104 C.3.1. Identification . . . . . . . . . . . . . . . . . . . 105 C.3.2. Definition . . . . . . . . . . . . . . . . . . . . . 105 C.3.3. Example . . . . . . . . . . . . . . . . . . . . . . . 106 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 106 Intellectual Property and Copyright Statements . . . . . . . . . 108 Doherty, et al. Expires May 7, 2009 [Page 5] Internet-Draft DSKPP November 2008 1. Introduction While the range of problems for which symmetric key cryptography is the solution of choice is somewhat smaller than for public key cryptography, the problems it does solve, it solves exceedingly well. In particular, symmetric key algorithms are considerably faster than public key equivalents and allow for smaller key and signature sizes. Despite the clear advantages of employing symmetric keys as long term credentials or access keys in certain circumstances, it has generally been assumed that any protocol in which ease of key management is required will employ public key cryptography. In particular it is assumed that only private components of public keypairs will be employed as long term secrets and that symmetric cryptography will only play a supporting role. This document describes the Dynamic Symmetric Key Provisioning Protocol (DSKPP), which provides a mechanism for provisioning symmetric keys that provides the same degree of flexibility and convenience in use as equivalent infrastructures for public keys. DSKPP enables provisioning of symmetric keys to a symmetric key cryptographic module that provides data authentication and encryption services to software (or firmware) applications hosted on a wide range of hardware devices, such as personal computers, handheld mobile phones, one-time password tokens, USB flash drives, tape drives, etc. DSKPP provides an open and interoperable mechanism for initializing and configuring symmetric keys to cryptographic modules that are accessible over the Internet. The description is based on the information contained in [RFC4758], and contains specific enhancements, such as User Authentication and support for the [PSKC] format for transmission of keying material. DSKPP has two principal protocol variations. The four pass protocol variation permits a symmetric key to be established that includes randomness contributed by both the client and the server. The two pass protocol requires only one round trip instead of two and permits a server specified key to be established. 1.1. Usage Scenarios DSKPP is expected to be used to provision symmetric keys to cryptographic modules in a number of different scenarios, each with its own special requirements. Doherty, et al. Expires May 7, 2009 [Page 6] Internet-Draft DSKPP November 2008 1.1.1. Single Key Request The usual scenario is that a cryptographic module makes a request for a symmetric key from a provisioning server that is located on the local network or somewhere on the Internet. Depending upon the deployment scenario, the provisioning server may generate a new key on-the-fly or use a pre-generated key, e.g., one provided by a legacy back-end issuance server. The provisioning server assigns a unique key ID to the symmetric key and provisions it to the cryptographic module. 1.1.2. Multiple Key Requests A cryptographic module makes multiple requests for symmetric keys from the same provisioning server. The symmetric keys need not be of the same type, i.e., the keys may be used with different symmetric key cryptographic algorithms, including one-time password authentication algorithms, and the AES encryption algorithm. 1.1.3. User Authentication In some deployment scenarios, a key issuer may rely on a third party provisioning service. In this case, the issuer directs provisioning requests from the cryptographic module to the provisioning service. As such, it is the responsibility of the issuer to authenticate the user through some out-of-band means before granting him rights to acquire keys. Once the issuer has granted those rights, the issuer provides an authentication code to the user and makes it available to the provisioning service, so that the user can prove that he is authorized to acquire keys. 1.1.4. Provisioning Time-Out Policy An issuer may provide a time-limited authentication code to a user during registration, which the user will input into the cryptographic module to authenticate themselves with the provisioning server. The server will allow a key to be provisioned to the cryptographic module hosted by the user's device when user authentication is required only if the user inputs a valid authentication code within the fixed time period established by the issuer. 1.1.5. Key Renewal A cryptographic module requests renewal of the symmetric key material attached to a key ID, as opposed to keeping the key value constant and refreshing the metadata. Such a need may occur in the case when a user wants to upgrade her device that houses the cryptographic module or when a key has expired. When a user uses the same Doherty, et al. Expires May 7, 2009 [Page 7] Internet-Draft DSKPP November 2008 cryptographic module to, for example, perform strong authentication at multiple Web login sites, keeping the same key ID removes the need for the user to register a new key ID at each site. 1.1.6. Pre-Loaded Key Replacement This scenario represents a special case of symmetric key renewal in which a local administrator can authenticate the user procedurally before initiating the provisioning process. It also allows for a device issuer to pre-load a key onto a cryptographic module with a restriction that the key is replaced with a new key prior to use of the cryptographic module. Another variation of this scenario is the organization who recycles devices. In this case, a key issuer would provision a new symmetric key to a cryptographic module hosted on a device that was previously owned by another user. Note that this usage scenario is essentially the same as the previous scenario wherein the same key ID is used for renewal. 1.1.7. Pre-Shared Manufacturing Key A cryptographic module is loaded onto a smart card after the card is issued to a user. The symmetric key for the cryptographic module will then be provisioned using a secure channel mechanism present in many smart card platforms. This allows a direct secure channel to be established between the smart card chip and the provisioning server. For example, the card commands (i.e., Application Protocol Data Units, or APDUs) are encrypted with a pre-issued card manufacturer's key and sent directly to the smart card chip, allowing secure post- issuance in-the-field provisioning. This secure flow can pass Transport Layer Security (TLS) and other transport security boundaries. Note that two pre-conditions for this usage scenario are for the protocol to be tunneled and the provisioning server to know the correct pre-established manufacturer's key. 1.1.8. End-to-End Protection of Key Material In this scenario, transport layer security does not provide end-to- end protection of keying material transported from the provisioning server to the cryptographic module. For example, TLS may terminate at an application hosted on a PC rather than at the cryptographic module (i.e., the endpoint) located on a data storage device. Mutually authenticated key agreement provides end-to-end protection, which TLS cannot provide. Doherty, et al. Expires May 7, 2009 [Page 8] Internet-Draft DSKPP November 2008 1.2. Protocol Entities A DSKPP provisioning transaction has three entities: Server: The DSKPP provisioning server. Cryptographic Module: The cryptographic module to which the symmetric keys are to be provisioned. Client: The DSKPP client which manages communication between the cryptographic module and the provisioning server. While it is highly desirable for the entire communication between the DSKPP client and server to be protected by means of a transport providing confidentiality and integrity protection such as HTTP over Transport Layer Security (TLS), such protection is not sufficient to protect the exchange of the symmetric key data between the server and the cryptographic module and the DSKPP protocol is designed to permit implementations that satisfy this requirement. The server only communicates to the client. As far as the server is concerned, the client and cryptographic module may be considered to be a single entity. From a client-side security perspective, however, the client and the cryptographic module are separate logical entities and may in some implementations be separate physical entities as well. A high-level object model that describes the client-side entities and how they relate to each other is shown in Figure 1. Conceptually, each entity is represented by the definitions found in Section 2.2. Doherty, et al. Expires May 7, 2009 [Page 9] Internet-Draft DSKPP November 2008 ----------- ------------- | User | | Device | |---------|* owns *|-----------| | UserID |--------->| DeviceID | | ... | | ... | ----------- ------------- | 1 | | contains | | * V -------------------------- |Cryptographic Module | |------------------------| |Crypto Module ID | |Security Attribute List | |... | -------------------------- | 1 | | contains | | * V ----------------------- |Key Package | |---------------------| |Key ID | |Key Type | |... | ----------------------- Figure 1: Object Model It is assumed that a device will host an application layered above the cryptographic module, and this application will manage communication between the DSKPP client and cryptographic module. The manner in which the communicating application will transfer DSKPP protocol elements to and from the cryptographic module is transparent to the DSKPP server. One method for this transfer is described in [CT-KIP-P11]. 1.3. Initiating DSKPP To initiate DSKPP: Doherty, et al. Expires May 7, 2009 [Page 10] Internet-Draft DSKPP November 2008 1. A server may first send a DSKPP trigger message to a client application (e.g., in response to a user browsing to a Web site that requires a symmetric key for authentication), although this step is optional. 2. A client application calls on the DSKPP client to send a symmetric key request to a DSKPP server, thus beginning a DSKPP protocol run. One of the following actions may be used to contact a DSKPP server: 1. A user may indicate how the DSKPP client is to contact a certain DSKPP server during a browsing session. 2. A DSKPP client may be pre-configured to contact a certain DSKPP server. 3. A user may be informed out-of-band about the location of the DSKPP server. Once the location of the DSKPP server is known, the DSKPP client and the DSKPP server engage in a 4-pass or 2-pass protocol. 1.4. Determining Which Protocol Variant to Use The four-pass and two-pass protocols are appropriate in different deployment scenarios, as described in the sub-sections below. The biggest differentiator between the two is that the two-pass protocol supports transport of an existing key to a cryptographic module, while the four-pass involves key generation on-the-fly via key agreement. In either case, both protocol variants support algorithm agility through negotiation of encryption mechanisms and key types at the beginning of each protocol run. 1.4.1. Criteria for Using the Four-Pass Protocol The four-pass protocol is needed under one or more of the following conditions: o Policy requires that both parties engaged in the protocol jointly contribute entropy to the key. Enforcing this policy mitigates the risk of exposing a key during the provisioning process as the key is generated through mutual agreement without being transferred over-the-air or over-the-wire. It also mitigates risk of exposure after the key is provisioned, as the key will be not be vulnerable to a single point of attack in the system. o A cryptographic module does not have private-key capabilities. o The cryptographic module is hosted by a device that was neither pre-issued with a manufacturer's key or other form of pre-shared key (as might be the case with a smart card or SIM card) nor has a keypad that can be used for entering a passphrase (such as present Doherty, et al. Expires May 7, 2009 [Page 11] Internet-Draft DSKPP November 2008 on a mobile phone). 1.4.2. Criteria for Using the Two-Pass Protocol The two-pass protocol is needed under one or more of the following conditions: o Pre-existing (i.e., legacy) keys must be provisioned via transport to the cryptographic module. o The cryptographic module is hosted on a device that was pre-issued with a manufacturer's key (such as may exist on a smart card), or other form of pre-shared key (such as may exist on a SIM-card), and is capable of performing private-key operations. o The cryptographic module is hosted by a device that has a built-in keypad with which a user may enter a passphrase, useful for deriving a key wrapping key for distribution of keying material. 1.5. Presentation Syntax This documents presents DSKPP message formats and data elements using XML syntax. The main goal in using this syntax is to document DSKPP. Application of the syntax beyond this goal is OPTIONAL (i.e., an implementation that does not use XML and instead uses ASN.1 could claim compliance with this specification). 1.5.1. Versions There is a provision made in the syntax for an explicit version number. Only version "1.0" is currently specified. 1.5.2. Namespaces The XML namespace [XMLNS] URN that MUST be used by implementations of this syntax is: xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:dskpp:1.0" References to qualified elements in the DSKPP schema defined herein use the prefix "dskpp". This document relies on qualified elements already defined in the Portable Symmetric Key Container [PSKC] namespace, which is represented by the prefix "pskc" and declared as: xmlns:pskc="urn:ietf:params:xml:ns:keyprov:pskc:1.0" Finally, the DSKPP syntax presented in this document relies on algorithm identifiers defined in the XML Signature [XMLDSIG] Doherty, et al. Expires May 7, 2009 [Page 12] Internet-Draft DSKPP November 2008 namespace: xmlns:ds="http://www.w3.org/2000/09/xmldsig#" References to algorithm identifiers in the XML Signature namespace are represented by the prefix "ds". 1.5.3. Identifiers This document uses Uniform Resource Identifiers [RFC2396] to identify resources, algorithms, and semantics. 2. Terminology 2.1. Key Words The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2.2. Definitions The definitions provided below are defined as used in this document. The same terms may be defined differently in other documents. Authentication Code (AC): Client Authentication Code comprised of a string of numeric characters known to the device and the server and containing an identifier and a password Authentication Data (AD): Client Authentication Data that may be derived from the Authentication Code (AC) Cryptographic Module: A component of an application, which enables symmetric key cryptographic functionality CryptoModule ID: A unique identifier for an instance of the cryptographic module Device: A physical piece of hardware, or a software framework, that hosts symmetric key cryptographic modules Device ID (DeviceID): A unique identifier for the device Doherty, et al. Expires May 7, 2009 [Page 13] Internet-Draft DSKPP November 2008 DSKPP Client: Manages communication between the symmetric key cryptographic module and the DSKPP server DSKPP Server: The symmetric key provisioning server that participates in the DSKPP protocol run DSKPP Server ID (ServerID): The unique identifier of a DSKPP server Issuer: See "Key Issuer" Key Issuer: An organization that issues symmetric keys to end-users Key Package (KP): An object that encapsulates a symmetric key and its configuration data Key Package Header (KPH): Information about the Key Package, useful for two-pass DSKPP, e.g., the passing the ServerID and the Key Protection Method Key ID (KeyID): A unique identifier for the symmetric key Key Protection Method (KPM): The key transport method used during two-pass DSKPP Key Protection Method List (KPML): The list of key protection methods supported by a cryptographic module Key Provisioning Server: A lifecycle management system that provides a key issuer with the ability to provision keys to cryptographic modules hosted on end-users' devices Key Transport: A key establishment procedure whereby the DSKPP server selects and encrypts the keying material and then sends the material to the DSKPP client [NIST-SP800-57] Key Transport Key: The private key that resides on the cryptographic module. This key is paired with the DSKPP client's public key, which the DSKPP server uses to encrypt keying material during key transport [NIST-SP800-57] Key Type: The type of symmetric key cryptographic methods for which the key will be used (e.g., OATH HOTP or RSA SecurID authentication, AES encryption, etc.) Doherty, et al. Expires May 7, 2009 [Page 14] Internet-Draft DSKPP November 2008 Key Wrapping: A method of encrypting keys for key transport [NIST-SP800-57] Key Wrapping Key: A symmetric key encrypting key used for key wrapping [NIST-SP800-57] Keying Material: The data necessary (e.g., keys and key configuration data) necessary to establish and maintain cryptographic keying relationships [NIST-SP800-57] Manufacturer's Key A unique master key pre-issued to a hardware device, e.g., a smart card, during the manufacturing process. If present, this key may be used by a cryptographic module to derive secret keys Provisioning Service: See "Key Provisioning Server" Security Attribute List (SAL): A payload that contains the DSKPP version, DSKPP variation (four- or two-pass), key package formats, key types, and cryptographic algorithms that the cryptographic module is capable of supporting Security Context (SC): A payload that contains the DSKPP version, DSKPP variation (four- or two-pass), key package format, key type, and cryptographic algorithms relevant to the current protocol run User: The person or client to whom devices are issued User ID: A unique identifier for the user or client 2.3. Notation || String concatenation [x] Optional element x A ^ B Exclusive-OR operation on strings A and B (where A and B are of equal length) A typographical convention used in the body of the text Doherty, et al. Expires May 7, 2009 [Page 15] Internet-Draft DSKPP November 2008 DSKPP-PRF(k,s,dsLen) A keyed pseudo-random function (see Section 3.3.1) E(k,m) Encryption of m with the key k K Key used to encrypt R_C (either K_SERVER or K_SHARED), or in MAC or DSKPP_PRF computations K_AC Secret key that is derived from the Authentication Code and used for user authentication purposes K_MAC Secret key derived during a DSKPP exchange for use with key confirmation K_MAC' A second secret key used for server authentication K_PROV A provisioning master key from which two keys are derived: K_TOKEN and K_MAC K_SERVER Public key of the DSKPP server; used for encrypting R_C in the four-pass protocol variant K_SHARED Secret key that is pre-shared between the DSKPP client and the DSKPP server; used for encrypting R_C in the four-pass protocol variant K_TOKEN Secret key that is established in a cryptographic module using DSKPP R Pseudorandom value chosen by the DSKPP client and used for MAC computations R_C Pseudorandom value chosen by the DSKPP client and used as input to the generation of K_TOKEN R_S Pseudorandom value chosen by the DSKPP server and used as input to the generation of K_TOKEN R_TRIGGER Pseudorandom value chosen by the DSKPP server and used as input in a trigger message. URL_S DSKPP server address, as a URL 2.4. Abbreviations Doherty, et al. Expires May 7, 2009 [Page 16] Internet-Draft DSKPP November 2008 AC Authentication Code AD Authentication Data DSKPP Dynamic Symmetric Key Provisioning Protocol HTTP Hypertext Transfer Protocol KP Key Package KPH Key Package Header KPM Key Protection Method KPML Key Protection Method List MAC Message Authentication Code PC Personal Computer PDU Protocol Data Unit PKCS Public-Key Cryptography Standards PRF Pseudo-Random Function PSKC Portable Symmetric Key Container SAL Security Attribute List (see Section 2.2) SC Security Context (see Section 2.2) TLS Transport Layer Security URL Uniform Resource Locator USB Universal Serial Bus XML eXtensible Markup Language 3. DSKPP Protocol Details DSKPP enables symmetric key provisioning between a DSKPP server and DSKPP client. 3.1. Protocol Initiation The DSKPP protocol has two- and four-pass variations, either of which may be initiated by either the client or the server making four possible successful protocol interactions. In every case the first message sent from the client to the server is and the last message is and is sent from the server to the client. 3.1.1. Server Initiation The DSKPP protocol may be initiated by the server by means of a message to which the client responds with a message as shown in Figure 2. The trigger message always contains a nonce to allow the server to couple the trigger with a later request. Doherty, et al. Expires May 7, 2009 [Page 17] Internet-Draft DSKPP November 2008 +---------------+ +---------------+ | | | | | DSKPP Client | | DSKPP Server | | | | | +---------------+ +---------------+ | | | <--------- --------- | | | | ------- -------> | ... ... Figure 2: Server Initiated DSKPP (start) 3.1.2. Client Initiation The DSKPP protocol may be initiated by the client by means of the message Figure 3 message. +---------------+ +---------------+ | | | | | DSKPP Client | | DSKPP Server | | | | | +---------------+ +---------------+ | | | ------- -------> | ... ... Figure 3: Client Initiated DSKPP (start) 3.2. Protocol Variations Once contact has been initiated, the client and server MAY engage in either a two-pass or four-pass protocol depending on the protocol options specified in the message and the server configuration. 3.2.1. Four-Pass Protocol Interaction In the four-pass version of the protocol the server responds to the message with . The client then responds with and the server with as shown in Figure 4. Doherty, et al. Expires May 7, 2009 [Page 18] Internet-Draft DSKPP November 2008 +---------------+ +---------------+ | | | | | DSKPP Client | | DSKPP Server | | | | | +---------------+ +---------------+ | | | [ <--------- --------- ] | | | | ------- -------> | | | | <------ -------- | | | | ------- -------> | | | | <---- ------- | | | Figure 4: Four Pass DSKPP protocol (with OPTIONAL preceding trigger) [ Message]: The message is used to initiate a request from the server. The trigger message always contains a nonce to allow the server to couple the trigger with a later request. : The request is sent by a DSKPP client to initiate contact with the DSKPP server, indicating which protocol versions and variations (four-pass or two-pass), key types, encryption and MAC algorithms that it supports. In addition, the request may include client authentication data that the DSKPP server uses to verify proof- of-possession of the device. Server Processing of : Upon receiving a request, the DSKPP server uses the response to specify which protocol version and variation, key type, encryption algorithm, and MAC algorithm that will be used by the DSKPP server and DSKPP client during the protocol run. The decision of which variation, key type, and cryptographic algorithms to pick is policy- and implementation- dependent and therefore outside the scope of this document. : The response is only used in the four pass protocol, it includes the DSKPP server's random nonce, R_S. The response also consists of information about either a shared secret key, or its own public key, that the DSKPP client uses when sending its protected random nonce, R_C, in the request (see below). Doherty, et al. Expires May 7, 2009 [Page 19] Internet-Draft DSKPP November 2008 Optionally, the DSKPP server may provide a MAC that the DSKPP client may use for server authentication. Client Processing of : On receipt of , the client encrypts the random client nonce R_c under the (provided) server key K. : The request is only used in the four pass protocol, it is used to exchange protected data, i.e., the protected random nonce R_C. In addition, the request may include user authentication data that the DSKPP server uses to verify proof-of-possession of the device. : The response is a confirmation message that includes a key package that holds configuration data, but no keying material. Optionally, the DSKPP server may provide a MAC that the DSKPP client may use for server authentication. 3.2.2. Two-Pass Protocol Interaction In the two-pass version of the protocol the server responds to the message with a message Figure 5 +---------------+ +---------------+ | | | | | DSKPP Client | | DSKPP Server | | | | | +---------------+ +---------------+ | | | [ <--------- --------- ] | | | | ------- -------> | | | | <---- ------- | | | Figure 5: Two Pass DSKPP protocol (with OPTIONAL preceding trigger) [ Message]: The message is used to initiate a request from the server. The trigger message always contains a nonce to allow the server to couple the trigger with a later request. Doherty, et al. Expires May 7, 2009 [Page 20] Internet-Draft DSKPP November 2008 : The request is sent by a DSKPP client to initiate contact with the DSKPP server, indicating which protocol versions and variations (four-pass or two-pass), key types, encryption and MAC algorithms that it supports. In addition, the request may include client authentication data that the DSKPP server uses to verify proof- of-possession of the device. : The response is a confirmation message that includes a key package that holds configuration data and contain protected keying material. Optionally, the DSKPP server may provide a MAC that the DSKPP client may use for server authentication. 3.3. Cryptographic Construction 3.3.1. The DSKPP One-Way Pseudorandom Function, DSKPP-PRF 3.3.1.1. Introduction Regardless of the protocol variation employed, there is a requirement for a cryptographic primitive that provides a deterministic transformation of a secret key k and a varying length octet string s to a bitstring of specified length dsLen. This primitive must meet the same requirements as for a keyed hash function: It MUST take an arbitrary length input, and generate an output that is one-way and collision-free (for a definition of these terms, see, e.g., [FAQ]). Further, its output MUST be unpredictable even if other outputs for the same key are known. From the point of view of this specification, DSKPP-PRF is a "black- box" function that, given the inputs, generates a pseudorandom value and MAY be realized by any appropriate and competent cryptographic technique. Appendix C contains two example realizations of DSKPP- PRF. 3.3.1.2. Declaration DSKPP-PRF (k, s, dsLen) Input: Doherty, et al. Expires May 7, 2009 [Page 21] Internet-Draft DSKPP November 2008 k secret key in octet string format s octet string of varying length consisting of variable data distinguishing the particular string being derived dsLen desired length of the output Output: DS pseudorandom string, dsLen-octets long For the purposes of this document, the secret key k MUST be at least 16 octets long. 3.4. Four-Pass Protocol Usage This section describes the message flow and methods that comprise the four-pass protocol variant. 3.4.1. Message Flow The four-pass protocol flow consists of two message exchanges: 1: Pass 1 = , Pass 2 = 2: Pass 3 = , Pass 4 = The first pair of messages negotiate cryptographic algorithms and exchange nonces. The second pair of messages establishes a symmetric key using mutually authenticated key agreement. The DSKPP server MUST ensure that a generated key is associated with the correct cryptographic module, and if applicable, the correct user. To do this, the DSKPP server MAY couple an initial user authentication to the DSKPP execution using one of the mechanisms described in Section 3.7. The purpose and content of each message are described below, including the optional . DSKPP Client DSKPP Server ------------ ------------ [<---] R_TRIGGER, [DeviceID], [KeyID], [URL_S] The DSKPP server optionally sends a message to the DSKPP client. The trigger message MUST contain a nonce, R_TRIGGER, to allow the server to couple the trigger with a later request. MAY include a DeviceID to allow the client to select the device with which it will communicate (for more information about device identification, refer Doherty, et al. Expires May 7, 2009 [Page 22] Internet-Draft DSKPP November 2008 to Section 3.6). In the case of key renewal, MAY include the identifier for the key, KeyID, that is being replaced. Finally, the trigger MAY contain a URL for the DSKPP client to use when contacting the DSKPP server. DSKPP Client DSKPP Server ------------ ------------ SAL, [R_TRIGGER], [DeviceID], [KeyID] ---> The DSKPP client sends a message to the DSKPP server. This message MUST contain a Security Attribute List (SAL), identifying which DSKPP versions, protocol variations (in this case "four-pass"), key package formats, key types, encryption and MAC algorithms that the client supports. In addition, if a trigger message preceded , then it passes the parameters received in back to the DSKPP Server. In particular, it MUST include R_TRIGGER so that the DSKPP server can associate the client with the trigger message, and SHOULD include DeviceID and KeyID. DSKPP Client DSKPP Server ------------ ------------ <--- SC, R_S, [K], [MAC] The DSKPP server responds to the DSKPP client with a message, whose Status attribute is set to a return code for . If Status is not "Continue", only the Status and Version attributes will be present, and the DSKPP client MUST abort the protocol. If Status is set to "Continue", then the message MUST include a Security Context (SC). The DSKPP client will use the SC to select the DSKPP version and variation (e.g., four-pass), type of key to generate, and cryptographic algorithms that it will use for the remainder of the protocol run. MUST also include the server's random nonce, R_S, whose length may depend on the selected key type. In addition, the message MAY provide K, which represents its own public key (K_SERVER) or information about a shared secret key (K_SHARED) to use for encrypting the cryptographic module's random nonce (see description of below). Optionally, MAY include a MAC that the DSKPP client can use for server authentication in the case of key renewal (Section 3.4.4.1 describes how to calculate the MAC). Doherty, et al. Expires May 7, 2009 [Page 23] Internet-Draft DSKPP November 2008 DSKPP Client DSKPP Server ------------ ------------ E(K,R_C), [AD] ---> Based on the Security Context (SC) provided in the message, the cryptographic module generates a random nonce, R_C. The length of the nonce R_C will depend on the selected key type. The cryptographic module encrypts R_C using the selected encryption algorithm and with a key, K, that is either the DSKPP server's public key, K_SERVER, or a shared secret key, K_SHARED, as indicated by the DSKPP server. Note: If K is equivalent to K_SERVER, then the cryptographic module SHOULD verify the server's certificate before using it to encrypt R_C in accordance with [RFC5280]. Note: If successful execution of the protocol will result in the replacement of an existing key with a newly generated one, the DSKPP client MUST verify the MAC provided in the message. The DSKPP client MUST terminate the DSKPP session if the MAC does not verify, and MUST delete any nonces, keys, and/or secrets associated with the failed run. The DSKPP client MUST send the encrypted random nonce to the DSKPP server in a message, and MAY include client Authentication Data (AD), such as a MAC derived from an authentication code and R_C (refer to Section 3.7.1). Finally, the cryptographic module calculates and stores a symmetric key, K_TOKEN, of the key type specified in the SC received in (refer to Section 3.4.2.2. for a description of how K_TOKEN is generated). DSKPP Client DSKPP Server ------------ ------------ <--- KP, MAC If Authentication Data (AD) was received in the message, then the DSKPP server MUST authenticate the user in accordance with Section 3.7.1. If authentication fails, then DSKPP server MUST abort. Otherwise, the DSKPP server decrypts R_C, calculates K_TOKEN from the combination of the two random nonces R_S and R_C, the encryption key K, and possibly some other data (refer to Section 3.4.2.2 for a description of how K_TOKEN is generated). The server then associates K_TOKEN with the cryptographic module in a server-side data store. The intent is that the data store later on will be used by some service that needs to verify or decrypt data produced by the cryptographic module and the key. Doherty, et al. Expires May 7, 2009 [Page 24] Internet-Draft DSKPP November 2008 Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called . The confirmation message MUST include a Key Package (KP) that holds an identifier for the generated key (but not the key itself) and additional configuration information, e.g., the identity of the DSKPP server. The default symmetric key package format is based on the Portable Symmetric Key Container (PSKC) defined in [PSKC]. Alternative formats MAY include [SKPC-ASN.1], PKCS#12 [PKCS-12], or PKCS#5 XML [PKCS-5-XML] format. In addition to a Key Package, MUST also include a MAC that the DSKPP client will use to authenticate the message before committing K_TOKEN After receiving a message with Status = "Success", the DSKPP client MUST verify the MAC. The DSKPP client MUST terminate the DSKPP session if the MAC does not verify, and MUST, in this case, also delete any nonces, keys, and/or secrets associated with the failed run of the protocol. If has Status = "Success" and the MAC was verified, then the DSKPP client MUST associate the provided key package with the generated key K_TOKEN, and store this data permanently. After this operation, it MUST NOT be possible to overwrite the key unless knowledge of an authorizing key is proven through a MAC on a later (and ) message. 3.4.2. Generation of Symmetric Keys for Cryptographic Modules With 4-pass DSKPP, the symmetric key that is the target of provisioning, is generated on-the-fly without being transferred between the DSKPP client and DSKPP server. A sample data flow depicting how this works followed by computational information are provided in the subsections below. 3.4.2.1. Data Flow A sample data flow showing key generation during the 4-pass protocol is shown in Figure 6. Doherty, et al. Expires May 7, 2009 [Page 25] Internet-Draft DSKPP November 2008 +----------------------+ +-------+ +----------------------+ | +------------+ | | | | | | | Server key | | | | | | | +<-| Public |------>------------->-------------+---------+ | | | | Private | | | | | | | | | | +------------+ | | | | | | | | | | | | | | | | | | V V | | | | V V | | | +---------+ | | | | +---------+ | | | | | Decrypt |<-------<-------------<-----------| Encrypt | | | | | +---------+ | | | | +---------+ | | | | | +--------+ | | | | ^ | | | | | | Server | | | | | | | | | | | | Random |--->------------->------+ +----------+ | | | | | +--------+ | | | | | | Client | | | | | | | | | | | | | Random | | | | | | | | | | | | +----------+ | | | | | | | | | | | | | | | | V V | | | | V V | | | | +------------+ | | | | +------------+ | | | +-->| DSKPP PRF | | | | | | DSKPP PRF |<----+ | | +------------+ | | | | +------------+ | | | | | | | | | | V | | | | V | | +-------+ | | | | +-------+ | | | Key | | | | | | Key | | | +-------+ | | | | +-------+ | | +-------+ | | | | +-------+ | | |Key Id |-------->------------->------|Key Id | | | +-------+ | | | | +-------+ | +----------------------+ +-------+ +----------------------+ DSKPP Server DSKPP Client DSKPP Client (PC Host) (cryptographic module) Figure 6: Principal data flow for DSKPP key generation - using public server key Note: Conceptually, although R_C is one pseudorandom string, it may be viewed as consisting of two components, R_C1 and R_C2, where R_C1 is generated during the protocol run, and R_C2 can be pre-generated and loaded on the cryptographic module before the device is issued to the user. In that case, the latter string, R_C2, SHOULD be unique for each cryptographic module. The inclusion of the two random nonces R_S and R_C in the key generation provides assurance to both sides (the cryptographic module and the DSKPP server) that they have contributed to the key's randomness and that the key is unique. The inclusion of the Doherty, et al. Expires May 7, 2009 [Page 26] Internet-Draft DSKPP November 2008 encryption key K ensures that no man-in-the-middle may be present, or else the cryptographic module will end up with a key different from the one stored by the legitimate DSKPP server. Note: A man-in-the-middle (in the form of corrupt client software or a mistakenly contacted server) may present his own public key to the cryptographic module. This will enable the attacker to learn the client's version of K_TOKEN. However, the attacker is not able to persuade the legitimate server to derive the same value for K_TOKEN, since K_TOKEN is a function of the public key involved, and the attacker's public key must be different than the correct server's (or else the attacker would not be able to decrypt the information received from the client). Therefore, once the attacker is no longer "in the middle," the client and server will detect that they are "out of sync" when they try to use their keys. In the case of encrypting R_C with K_SERVER, it is therefore important to verify that K_SERVER really is the legitimate server's key. One way to do this is to independently validate a newly generated K_TOKEN against some validation service at the server (e.g. using a connection independent from the one used for the key generation). 3.4.2.2. Computing the Symmetric Key In DSKPP, K_TOKEN and K_MAC are derived from provisioning key, K_PROV, which is generated using the DSKPP-PRF function as follows (refer to Section 3.3.1): K_PROV = DSKPP-PRF(k,s,dsLen), where k = R_C (i.e., the secret random value chosen by the DSKPP client) s = "Key generation" || K || R_S (where K is the key used to encrypt R_C and R_S is the random value chosen by the DSKPP server) dsLen = (desired length of K_PROV whose first half constitutes K_MAC and second half constitutes K_TOKEN) Then K_TOKEN and K_MAC derived from K_PROV, where K_PROV = K_MAC || K_TOKEN When computing K_PROV, the derived keys, K_MAC and K_TOKEN, MAY be subject to an algorithm-dependent transform before being adopted as a key of the selected type. One example of this is the need for parity in DES keys. Doherty, et al. Expires May 7, 2009 [Page 27] Internet-Draft DSKPP November 2008 3.4.3. Encryption of Pseudorandom Nonces Sent from the DSKPP Client DSKPP client random nonce(s) are either encrypted with the public key provided by the DSKPP server or by a shared secret key. For example, in the case of a public RSA key, an RSA encryption scheme from PKCS #1 [PKCS-1] MAY be used. In the case of a shared secret key, to avoid dependence on other algorithms, the DSKPP client MAY use the DSKPP-PRF function described herein with the shared secret key K_SHARED as input parameter k (in this case, K_SHARED SHOULD be used solely for this purpose), the concatenation of the (ASCII) string "Encryption" and the server's nonce R_S as input parameter s, and dsLen set to the length of R_C: dsLen = len(R_C) DS = DSKPP-PRF(K_SHARED, "Encryption" || R_S, dsLen) This will produce a pseudorandom string DS of length equal to R_C. Encryption of R_C MAY then be achieved by XOR-ing DS with R_C: E(DS, R_C) = DS ^ R_C The DSKPP server will then perform the reverse operation to extract R_C from E(DS, R_C). 3.4.4. MAC Calculations 3.4.4.1. Server Authentication in the Case of Key Renewal A MAC MUST be present in the message if the DSKPP run will result in the replacement of an existing key with a new one, as proof that the DSKPP server is authenticated to perform the action. When the MAC value is used for server authentication, the value MAY be computed by using the DSKPP-PRF function of Section 3.3.1, in which case the input parameter k MUST be set to the existing MAC key K_MAC' (i.e., the value of the MAC key that existed before this protocol run); and input parameter s MUST be set to the concatenation of the (ASCII) string "MAC 1 computation", R (if sent by the client), and R_S. Note that the implementation MAY specify K_MAC' to be the value of the K_TOKEN that is being replaced, or a version of K_MAC from the previous protocol run. The input parameter dsLen MUST be set to the length of R_S: dsLen = len(R_S) MAC = DSKPP-PRF (K_MAC', "MAC 1 computation" || [R ||] R_S, dsLen) Doherty, et al. Expires May 7, 2009 [Page 28] Internet-Draft DSKPP November 2008 The MAC algorithm MUST be the same as the algorithm used for key confirmation purposes. 3.4.4.2. Key Confirmation To avoid a false "Commit" message causing the cryptographic module to end up in an initialized state in which the server does not recognize the stored key, messages MUST be authenticated with a MAC, calculated as follows: msg_hash = SHA-256(msg_1, ..., msg_n) dsLen = len(msg_hash) MAC = DSKPP-PRF (K_MAC, "MAC 2 computation" || msg_hash, dsLen) where MAC The MAC MUST be calculated using the already established MAC algorithm and MUST be computed on the (ASCII) string "MAC 2 computation" and msg_hash using the existing the MAC key K_MAC. K_MAC The key derived from K_PROV, as described in Section 3.4.2.2. msg_hash The message hash, defined below, of messages msg_1, ..., msg_n. If DSKPP-PRF (defined in Section 3.3.1) is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 2 computation" and msg_hash, and the parameter dsLen MUST be set to the length of msg_hash. 3.4.4.3. Message Hash Algorithm To compute a message hash for a MAC, given a sequence of DSKPP messages msg_1, ..., msg_n, the following operations MUST be carried out: a. The sequence of messages contains all DSKPP Request and Response messages up to but not including this message. b. Re-transmitted messages are removed from the sequence of messages. Note: The resulting sequence of messages MUST be an alternating sequence of DSKPP Request and DSKPP Response messages Doherty, et al. Expires May 7, 2009 [Page 29] Internet-Draft DSKPP November 2008 c. The contents of each message is concatenated together. d. The resulting string is hashed using SHA-256 in accordance with [FIPS180-SHA]. 3.5. Two-Pass Protocol Usage This section describes the message flow and methods that comprise the two-pass protocol variant. Two-pass DSKPP is essentially a transport of keying material from the DSKPP server to the DSKPP client. The keying material is contained in a package that is formatted in such a way that ensures that the symmetric key that is being established, K_TOKEN, is not exposed to any other entity than the DSKPP server and the cryptographic module itself. To ensure the keying material is adequately protected for all two-pass usage scenarios, the key package format MUST support the following key protection methods, as defined in Section 3.5.2: Key Transport This profile is intended for PKI-capable devices. Key transport is carried out using the public key of the DSKPP client, whose private key part resides in the cryptographic module as the key transport key. Key Wrap This profile is ideal for pre-keyed devices, e.g., SIM cards. Key wrap is carried out using a key wrapping key, which is known in advance by both the cryptographic module and the DSKPP server. Passphrase-Based Key Wrap This profile is a variation of the Key Wrap Profile. It is applicable to constrained devices with keypads, e.g., mobile phones. Key wrap is carried out using a passphrase-derived key wrapping key, known in advance by both the cryptographic module and DSKPP server. Key package formats that satisfy this criteria are [PSKC], [SKPC-ASN.1], PKCS#12 [PKCS-12], and PKCS#5 XML [PKCS-5-XML]. 3.5.1. Message Flow The two-pass protocol flow consists of one exchange: Doherty, et al. Expires May 7, 2009 [Page 30] Internet-Draft DSKPP November 2008 1: Pass 1 = , Pass 2 = The client's initial message is directly followed by a message (unlike the four-pass variant, there is no exchange of the and messages). However, as the two-pass variation of DSKPP consists of one round trip to the server, the client is still able to include its random nonce, R_C, algorithm preferences and supported key types in the message. Note that by including R_C in , the DSKPP client is able to ensure the server is alive before "committing" the key. The DSKPP server MUST ensure that a generated key is associated with the correct cryptographic module, and if applicable, the correct user. To ensure that the key K_TOKEN ends up associated with the correct cryptographic module and user, the DSKPP server MAY couple an initial user authentication to the DSKPP execution as described in Section 3.7. The purpose and content of each message are described below, including the optional . DSKPP Client DSKPP Server ------------ ------------ [<---] R_TRIGGER, [DeviceID], [KeyID], [URL_S] The DSKPP server optionally sends a message to the DSKPP client. The trigger message MUST contain a nonce, R_TRIGGER, to allow the server to couple the trigger with a later request. MAY include a DeviceID to allow the client to select the device with which it will communicate (for more information about device identification, refer to Section 3.6). In the case of key renewal, SHOULD include the identifier for the key, KeyID, that is being replaced. Finally, the trigger MAY contain a URL for the DSKPP client to use when contacting the DSKPP server. DSKPP Client DSKPP Server ------------ ------------ R_C, SAL, KPML, [AD], [R_TRIGGER], [DeviceID], [KeyID] ---> The DSKPP client sends a message to the DSKPP server. MUST include client nonce, R_C, and a Security Attribute List (SAL), identifying which DSKPP versions, protocol variations (in this case "two-pass"), key package formats, Doherty, et al. Expires May 7, 2009 [Page 31] Internet-Draft DSKPP November 2008 key types, encryption and MAC algorithms that the client supports. Unlike 4-pass DSKPP, the 2-pass DSKPP client uses the message to declare the list of Key Protection Method List (KPML) it supports, providing required payload information in accordance with Section 3.5.2. Optionally, the message MAY include client Authentication Data (AD), such as a MAC derived from an authentication code and R_C (refer to Section 3.7.1). In addition, if a trigger message preceded , then it passes the parameters received in back to the DSKPP Server. In particular, it MUST include R_TRIGGER so that the DSKPP server can associate the client with the trigger message, and SHOULD include DeviceID and KeyID. DSKPP Client DSKPP Server ------------ ------------ <--- KPH, KP, E(K,K_PROV), MAC, AD If Authentication Data (AD) was received, then the DSKPP server MUST authenticate the user in accordance with Section 3.7.1. If authentication fails, then DSKPP server MUST abort. Otherwise, the DSKPP server generates a key K_PROV from which two keys, K_TOKEN and K_MAC, are derived. (Alternatively, the key K_PROV may have been pre-generated as described in Section 1.1.1.) The DSKPP server selects a Key Protection Method (KPM) and applies it to K_PROV in accordance with Section 3.5.2. The server then associates K_TOKEN with the cryptographic module in a server-side data store. The intent is that the data store later will be used by some service that needs to verify or decrypt data produced by the cryptographic module and the key. Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called . For two-pass DSKPP, the confirmation message MUST include a Key Package Header (KPH) that contains the DSKPP Server's ID and KPM. The ServerID is used for authentication purposes, and the KPM informs the DSKPP client of the security context in which it will operate. In addition to the KPH, the confirmation message MUST include the Key Package (KP) that holds the KeyID, K_PROV from which K_TOKEN and K_MAC are derived, and additional configuration information. The default symmetric key package format is based on the Portable Symmetric Key Container (PSKC) defined in [PSKC]. Alternative formats MAY include [SKPC-ASN.1], PKCS#12 [PKCS-12], or PKCS#5 XML [PKCS-5-XML]. Finally, MUST include two MACs (MAC and AD) whose values are calculated with contribution from the client nonce, R_C, provided in the message. The MAC values will allow the cryptographic module to perform key confirmation and server Doherty, et al. Expires May 7, 2009 [Page 32] Internet-Draft DSKPP November 2008 authentication before "committing" the key (see Section 3.5.3 for more information). After receiving a message with Status = "Success", the DSKPP client MUST verify both MAC values (MAC and AD). The DSKPP client MUST terminate the DSKPP protocol run if either MAC does not verify, and MUST, in this case, also delete any nonces, keys, and/or secrets associated with the failed run of the protocol. If has Status = "Success" and the MACs were verified, then the DSKPP client MUST extract the key data from the provided key package, and store data locally. After this operation, it MUST NOT be possible to overwrite the key unless knowledge of an authorizing key is proven through a MAC on a later message. 3.5.2. Key Protection Profiles This section introduces three profiles of two-pass DSKPP for key protection. Further profiles MAY be defined by external entities or through the IETF process. 3.5.2.1. Key Transport Profile This profile establishes a symmetric key, K_TOKEN, in the cryptographic module using key transport and key derivation. Key transport is carried out using a public key whose private key part resides in the cryptographic module as the key transport key. A provisioning master key, K_PROV, MUST be transported from the DSKPP server to the client. From K_PROV, two keys are derived: the symmetric key to be established, K_TOKEN, and a key used to compute MACs, K_MAC. This profile MUST be identified with the following URN: urn:ietf:params:xml:schema:keyprov:dskpp#transport In the two-pass version of DSKPP, the client MUST send a payload with the Key Transport Profile. This payload MUST be of type ([XMLDSIG]), and only those choices of that identify a public key are allowed (i.e., , , , or ). The option of the alternative is RECOMMENDED when the public key corresponding to the private key on the cryptographic module has been certified. The server payload associated with this key protection method MUST be of type ([XMLENC]), and only those encryption methods utilizing a public key that are supported by the DSKPP client (as indicated in the element of the Doherty, et al. Expires May 7, 2009 [Page 33] Internet-Draft DSKPP November 2008 message in the case of 2-pass DSKPP) are allowed as values for the . Further, in the case of 2-pass DSKPP, MUST contain the same value (i.e. identify the same public key) as the of the corresponding supported key protection method in the message that triggered the response. MAY be present, but MUST, when present, contain the same value as the element of the message. The Type attribute of the MUST be present and MUST identify the type of the wrapped key. The type MUST be one of the types supported by the DSKPP client (as reported in the of the preceding message in the case of 2-pass DSKPP). The transported key, K_PROV, MUST consist of two parts of equal length. The first half constitutes K_MAC and the second half constitutes K_TOKEN. The length of K_TOKEN (and hence also the length of K_MAC) is determined by the type of K_TOKEN. DSKPP servers and cryptographic modules supporting this profile MUST support the http://www.w3.org/2001/04/xmlenc#rsa-1_5 key wrapping mechanism defined in [XMLENC]. When this profile is used, the MacAlgorithm attribute of the element of the message MUST be present and MUST identify the selected MAC algorithm. The selected MAC algorithm MUST be one of the MAC algorithms supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP). The MAC MUST be calculated as described in Section 3.5.3 for two-pass DSKPP. In addition, DSKPP servers MUST include the AuthenticationDataType element in their messages whenever a successful protocol run will result in an existing K_TOKEN being replaced. 3.5.2.2. Key Wrap Profile This profile establishes a symmetric key, K_TOKEN, in the cryptographic module through key wrap and key derivation. Key wrap is carried out using a symmetric key wrapping key, known in advance by both the cryptographic module and the DSKPP server. A provisioning master key, K_PROV, MUST be transported from the DSKPP server to the client. From K_PROV, two keys are derived: the symmetric key to be established, K_TOKEN, and a key used to compute MACs, K_MAC. This profile MUST be identified with the following URI: urn:ietf:params:xml:schema:keyprov:dskpp#wrap Doherty, et al. Expires May 7, 2009 [Page 34] Internet-Draft DSKPP November 2008 In the 2-pass version of DSKPP, the client MUST send a payload with the Key Wrap Profile. This payload MUST be of type ([XMLDSIG]), and only those choices of that identify a symmetric key are allowed (i.e., and ). The alternative is RECOMMENDED. The server payload associated with this key protection method MUST be of type ([XMLENC]), and only those encryption methods utilizing a symmetric key that are supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP) are allowed as values for the . Further, in the case of 2-pass DSKPP, MUST contain the same value (i.e. identify the same symmetric key) as the of the corresponding supported key protection method in the message that triggered the response. MAY be present, and MUST, when present, contain the same value as the element of the message. The Type attribute of the MUST be present and MUST identify the type of the wrapped key. The type MUST be one of the types supported by the DSKPP client (as reported in the of the preceding message in the case of 2-pass DSKPP). The wrapped key, K_PROV, MUST consist of two parts of equal length. The first half constitutes K_MAC and the second half constitutes K_TOKEN. The length of K_TOKEN (and hence also the length of K_MAC) is determined by the type of K_TOKEN. DSKPP servers and cryptographic modules supporting this profile MUST support the http://www.w3.org/2001/04/xmlenc#kw-aes128 key wrapping mechanism defined in [XMLENC]. When this profile is used, the MacAlgorithm attribute of the element of the message MUST be present and MUST identify the selected MAC algorithm. The selected MAC algorithm MUST be one of the MAC algorithms supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP). The MAC MUST be calculated as described in Section 3.5.3. In addition, DSKPP servers MUST include the AuthenticationDataType element in their messages whenever a successful protocol run will result in an existing K_TOKEN being replaced. 3.5.2.3. Passphrase-Based Key Wrap Profile This profile is a variation of the key wrap profile. It establishes a symmetric key, K_TOKEN, in the cryptographic module through key Doherty, et al. Expires May 7, 2009 [Page 35] Internet-Draft DSKPP November 2008 wrap and key derivation. Key wrap is carried out using a passphrase- derived key wrapping key. The passphrase is known in advance by both the user of the device and the DSKPP server. To preserve the property of not exposing K_TOKEN to any other entity than the DSKPP server and the cryptographic module itself, the method SHOULD be employed only when the device contains facilities (e.g. a keypad) for direct entry of the passphrase. A provisioning master key, K_PROV, MUST be transported from the DSKPP server to the client. From K_PROV, two keys are derived: the symmetric key to be established, K_TOKEN, and a key used to compute MACs, K_MAC. This profile MUST be identified with the following URI: urn:ietf:params:xml:schema:keyprov:dskpp#passphrase-wrap In the 2-pass version of DSKPP, the client MUST send a payload with the Passphrase-Based Key Wrap Profile. This payload MUST be of type ([XMLDSIG]). The option MUST be used and the key name MUST identify the passphrase that will be used by the server to generate the key wrapping key. As an example, the identifier could be a user identifier or a registration identifier issued by the server to the user during a session preceding the DSKPP protocol run. The server payload associated with this key protection method MUST be of type ([XMLENC]), and only those encryption methods utilizing a passphrase to derive the key wrapping key that are supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP) are allowed as values for the . Further, in the case of 2-pass DSKPP, MUST contain the same value (i.e. identify the same passphrase) as the of the corresponding supported key protection method in the message that triggered the response. MAY be present, and MUST, when present, contain the same value as the element of the message. The Type attribute of the MUST be present and MUST identify the type of the wrapped key. The type MUST be one of the types supported by the DSKPP client (as reported in the of the preceding message in the case of 2-pass DSKPP). The wrapped key, K_PROV, MUST consist of two parts of equal length. The first half constitutes K_MAC and the second half constitutes K_TOKEN. The length of K_TOKEN (and hence also the length of K_MAC) is determined by the type of K_TOKEN. DSKPP servers and cryptographic modules supporting this profile MUST support the PBES2 password based encryption scheme defined in [PKCS-5] (and identified as Doherty, et al. Expires May 7, 2009 [Page 36] Internet-Draft DSKPP November 2008 http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5#pbes2 in [PKCS-5-XML]), the PBKDF2 passphrase-based key derivation function also defined in [PKCS-5] (and identified as http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5#pbkdf2 in [PKCS-5-XML]), and the http://www.w3.org/2001/04/xmlenc#kw-aes128 key wrapping mechanism defined in [XMLENC]. When this profile is used, the MacAlgorithm attribute of the element of the message MUST be present and MUST identify the selected MAC algorithm. The selected MAC algorithm MUST be one of the MAC algorithms supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP). The MAC MUST be calculated as described in Section 3.5.3. In addition, DSKPP servers MUST include the AuthenticationDataType element in their messages whenever a successful protocol run will result in an existing K_TOKEN being replaced. 3.5.3. MAC Calculations 3.5.3.1. Key Confirmation The MAC value in the message MUST be calculated as follows: msg_hash = SHA-256(msg_1, ..., msg_n) dsLen = len(msg_hash) MAC = DSKPP-PRF (K_MAC, "MAC 1 computation" || msg_hash || ServerID, dsLen) where MAC The MAC MUST be calculated using the already established MAC algorithm and MUST be computed on the (ASCII) string "MAC 1 computation", msg_hash, and ServerID using the existing the MAC key K_MAC. K_MAC The key, along with K_TOKEN, that is derived from K_PROV which the DSKPP server MUST provide to the cryptographic module. Doherty, et al. Expires May 7, 2009 [Page 37] Internet-Draft DSKPP November 2008 msg_hash The message hash, defined in Section 3.4.4.3, of messages msg_1, ..., msg_n. ServerID The identifier that the DSKPP server MUST include in the element of . If DSKPP-PRF (defined in Section 3.3.1) is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 computation", msg_hash, and ServerID, and the parameter dsLen MUST be set to the length of msg_hash. 3.5.3.2. Server Authentication in the Case of Key Renewal A second MAC MUST be present in the message as proof that the DSKPP server is authorized to replace a key on the cryptographic module. In 2-pass DSKPP, servers provide the second MAC in the AuthenticationDataType element of . The MAC value in the AuthenticationDataType element MUST be computed on the (ASCII) string "MAC 2 computation", the server identifier ServerID, and R, using a pre-existing MAC key K_MAC' (the MAC key that existed before this protocol run). Note that the implementation may specify K_MAC' to be the value of the K_TOKEN that is being replaced, or a version of K_MAC from the previous protocol run. If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 2 computation" ServerID, and R. The parameter dsLen MUST be set to at least 16 (i.e. the length of the MAC MUST be at least 16 octets): dsLen >= 16 MAC = DSKPP-PRF (K_MAC', "MAC 2 computation" || ServerID || R, dsLen) The MAC algorithm MUST be the same as the algorithm used for key confirmation purposes. 3.6. Device Identification The DSKPP server MAY be pre-configured with a unique device identifier corresponding to a particular cryptographic module. The DSKPP server MAY then include this identifier in the DSKPP initialization trigger, in which case the DSKPP client MUST include it in its message(s) to the DSKPP server for authentication. Note that it is also legitimate for a DSKPP client to initiate the DSKPP protocol run without having received an initialization message from a server, but in this case any provided device identifier MUST NOT be accepted by the DSKPP server unless the server has access to a unique key for the identified device and that key will be used in the Doherty, et al. Expires May 7, 2009 [Page 38] Internet-Draft DSKPP November 2008 protocol. 3.7. User Authentication The DSKPP server MUST ensure that a generated key is associated with the correct cryptographic module, and if applicable, the correct user. If the user has not been authenticated by some out-of-band means, then the user SHOULD be authenticated within the DSKPP. When relying on DSKPP for user authentication, the DSKPP server SHOULD explicitly rely on client-provided Authentication Data (AD) to verify that a legitimate user is behind the wheel. For a further discussion of this, and threats related to man-in-the-middle attacks in this context, see Section 9.6.4. 3.7.1. Authentication Data As described in the message flows above (see Section 3.4.1 and Section 3.5.1), the DSKPP client MAY include Authentication Data (AD) in its request(s). Note that AD MAY be omitted if client certificate authentication has been provided by the transport channel such as TLS. Nonetheless, when AD is provided, the DSKPP server MUST verify the data before continuing with the protocol run. The data element that holds AD MUST include a Client ID and a value derived from an Authentication Code (AC). The Client ID represents a key request made by the user to the Provisioning Server. AC is a one-time use value that is a (potentially low entropy) shared secret between a user and the Provisioning Server. This secret is made available to the client before the DSKPP message exchange. Below are examples of how the DSKPP client may obtain the AC: a. A key issuer may deliver an AC to the user or device in response to a key request, which the user enters into an application hosted on their device. For example, a user runs an application that is resident on their device, e.g., a mobile phone. The application cannot proceed without a new symmetric key. The user is redirected to an issuer's Web site from where the user requests a key. The issuer's Web application processes the request, and returns an AC, which then appears on the user's display. The user then invokes a symmetric key-based application hosted on the device, which asks the user to input the AC using a keypad. The application invokes the DSKPP client, providing it with the AC. b. The provisioning server may send a trigger message, , to the DSKPP client, which sets the value of the trigger nonce, R_TRIGGER, to AC. When this method is used, a transport providing confidentiality and integrity MUST be used to deliver the DSKPP initialization trigger from the DSKPP server to Doherty, et al. Expires May 7, 2009 [Page 39] Internet-Draft DSKPP November 2008 the DSKPP client, e.g., HTTPS. A description of the AC and how it is used to derive AD is contained in the sub-sections below. 3.7.2. Authentication Code Format AC is encoded in Type-Length-Value (TLV) format. The format consists of a minimum of two TLVs and a variable number of additional TLVs, depending on implementation. See Figure 7 for TLV field layout. A 1 byte type field identifies the specific TLV, and a 1 byte length, in hexadecimal, indicates the length of the value field contained in the TLV. A TLV MUST start on a 4 byte boundary. Pad bytes MUST be placed at the end of the previous TLV in order to align the next TLV. These pad bytes are not counted in the length field of the TLV. 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Value[0] | ...Value[Length-1] +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: TLV Format The TLV fields are defined as follows: Type (1 byte) The integer value identifying the type of information contained in the value field. Length (1 byte) The length, in hexadecimal, of the value field to follow. Value (variable length) A variable-length hexadecimal value containing the instance-specific information for this TLV. Figure 8 summarizes the TLVs defined in this document. Optional TLVs are allowed for vendor-specific extensions with the constraint that the high bit MUST be set to indicate a vendor-specific type. Other TLVs are left for later revisions of this protocol. Doherty, et al. Expires May 7, 2009 [Page 40] Internet-Draft DSKPP November 2008 +------+------------+-------------------------------------------+ | Type | TLV Name | Conformance | Example Usage | +------+------------+-------------------------------------------+ | 1 | Client ID | Mandatory | { "AC00000A" } | +------+------------+-------------+-----------------------------+ | 2 | Password | Mandatory | { "3582" } | +------+------------+-------------+-----------------------------+ | 3 | Checksum | Optional | { 0x5F8D } | +------+------------+-------------+-----------------------------+ Figure 8: TLV Summary 3.7.2.1. Client ID (MANDATORY) The Client ID is a mandatory TLV that represents the user's key request. A summary of the Client ID TLV format is given in Figure 9. The fields are transmitted from left to right. 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0x1 | Length | clientID ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: ClientID TLV Format clientID is an ASCII string that identifies the key request. The clientID MUST be HEX encoded. For example, suppose clientID is set to "AC00000A", the hexadecimal equivalent is 0x4143303030303041, resulting in a TLV of {0x1, 0x8, 0x4143303030303041}. 3.7.2.2. Password (MANDATORY) The Password is a mandatory TLV the contains a one-time use shared secret known by the user and the Provisioning Server. A summary of the Password TLV format is given in Figure 10. The fields are transmitted from left to right. Doherty, et al. Expires May 7, 2009 [Page 41] Internet-Draft DSKPP November 2008 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0x2 | Length | password ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: Password TLV Format Password is a unique value that SHOULD be a random string to make AC more difficult to guess. The string MUST be UTF-8 encoded in accordance with [RFC3629]. For example, suppose password is set to "3582", then the TLV would be {0x2, 0x4, UTF-8("3582")}. 3.7.2.3. Checksum (OPTIONAL) The Checksum is an OPTIONAL TLV, which is generated by the issuing server and sent to the user as part of the AC. A summary of the Checksum TLV format is given in Figure 11. The fields are transmitted from left to right. 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0x3 | Length | checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11: Checksum TLV Format If included, the checksum MUST be computed using the CRC16 algorithm [ISO3309]. When the user enters the AC, the typed password is verified with the checksum to ensure it is correctly entered by the user. For example, suppose the Password is set to "3582", then the CRC16 calculation would generate a checksum of 0x5F8D, resulting in TLV {0x3, 0x2, 0x5F8D}. 3.7.3. Authentication Data Calculation The Authentication Data consists of a Client ID (extracted from the AC) and a value, which is derived from AC as follows (refer to Section 3.3.1 for a description of DSKPP-PRF in general and Appendix C for a description of DSKPP-PRF-AES): MAC = DSKPP-PRF(K_AC, AC->clientID||URL_S||R_C||[R_S], 16) Doherty, et al. Expires May 7, 2009 [Page 42] Internet-Draft DSKPP November 2008 In four-pass DSKPP, the cryptographic module uses R_C, R_S, and URL_S to calculate the MAC, where URL_S is the URL the DSKPP client uses when contacting the DSKPP server. In two-pass DSKPP, the cryptographic module does not have access to R_S, therefore only R_C is used in combination with URL_S to produce the MAC. In either case, K_AC MUST be derived from AC>password as follows [PKCS-5]: K_AC = PBKDF2(AC->password, R_C || K, iter_count, 16) One of the following values for K MUST be used: a. In four-pass: * The public key of the DSKPP server (K_SERVER), or (in the pre- shared key variant) the pre-shared key between the client and the server (K_SHARED) b. In two-pass: * The public key of the DSKPP client, or the public key of the device when a device certificate is available * The pre-shared key between the client and the server (K_SHARED) * A passphrase-derived key The iteration count, iter_count, MUST be set to at least 100,000 except for case (b) and (c), above, in which case it MUST be set to 1. 4. DSKPP Message Formats The message formats from the DSKPP XML schema, found in Section 7, are explained in this section. Examples can be found in Appendix A. The XML format for DSKPP messages has been designed to be extensible. However, it is possible that the use of extensions will harm interoperability; therefore, any use of extensions SHOULD be carefully considered. For example, if a particular implementation relies on the presence of a proprietary extension, then it may not be able to interoperate with independent implementations that have no knowledge of this extension. 4.1. General XML Schema Requirements Some DSKPP elements rely on the parties being able to compare received values with stored values. Unless otherwise noted, all elements in this document that have the XML Schema "xs:string" type, or a type derived from it, MUST be compared using an exact binary comparison. In particular, DSKPP implementations MUST NOT depend on case-insensitive string comparisons, normalization or trimming of white space, or conversion of locale-specific formats such as Doherty, et al. Expires May 7, 2009 [Page 43] Internet-Draft DSKPP November 2008 numbers. Implementations that compare values that are represented using different character encodings MUST use a comparison method that returns the same result as converting both values to the Unicode character encoding, Normalization Form C [UNICODE], and then performing an exact binary comparison. No collation or sorting order for attributes or element values is defined. Therefore, DSKPP implementations MUST NOT depend on specific sorting orders for values. 4.2. Components of the Message The DSKPP server MAY initialize the DSKPP protocol by sending a message. This message MAY, e.g., be sent in response to a user requesting key initialization in a browsing session. Doherty, et al. Expires May 7, 2009 [Page 44] Internet-Draft DSKPP November 2008 The element is intended for the DSKPP client and MAY inform the DSKPP client about the identifier for the device that houses the cryptographic module to be initialized, and optionally of the identifier for the key on that module. The latter would apply to key renewal. The trigger always contains a nonce to allow the DSKPP server to couple the trigger with a later DSKPP request. Finally, the trigger MAY contain a URL to use when contacting the DSKPP server. The elements are for future extensibility. Any provided or values MUST be used by the DSKPP client in the subsequent request. The OPTIONAL element informs the DSKPP client about the characteristics of the intended cryptographic module platform, and applies in the public-key variant of DSKPP in situations when the client potentially needs to decide which one of several modules to initialize. 4.3. Components of the Request This message is the initial message sent from the DSKPP client to the DSKPP server in both variations of the DSKPP. Doherty, et al. Expires May 7, 2009 [Page 45] Internet-Draft DSKPP November 2008 The components of this message have the following meaning: o Version: (attribute inherited from the AbstractRequestType type) The highest version of this protocol the client supports. Only version one ("1.0") is currently specified. o : An identifier for the cryptographic module as defined in Section 3.7 above. The identifier MUST only be present if such shared secrets exist or if the identifier was provided by the server in a element (see Section 6.2.7). In the latter case, it MUST have the same value as the identifier provided in that element. Doherty, et al. Expires May 7, 2009 [Page 46] Internet-Draft DSKPP November 2008 o : An identifier for the key that will be overwritten if the protocol run is successful. The identifier MUST only be present if the key exists or if the identifier was provided by the server in a element, in which case, it MUST have the same value as the identifier provided in that element (see a (Section 4.2) and Section 6.2.7). o : This is the nonce R, which, when present, MUST be used by the server when calculating MAC values (see below). It is RECOMMENDED that clients include this element whenever the element is present. o : This OPTIONAL element MUST be present if and only if the DSKPP run was initialized with a message (see Section 6.2.7), and MUST, in that case, have the same value as the child of that message. A server using nonces in this way MUST verify that the nonce is valid and that any device or key identifier values provided in the message match the corresponding identifier values in the message. o : A sequence of container elements that in turn contain URLs indicating the key types for which the cryptographic module is willing to generate keys through DSKPP. o : A sequence of container elements that in turn contain URLs indicating the encryption algorithms supported by the cryptographic module for the purposes of DSKPP. The DSKPP client MAY indicate the same algorithm both as a supported key type and as an encryption algorithm. o : A sequence of container elements that in turn contain URLs indicating the MAC algorithms supported by the cryptographic module for the purposes of DSKPP. The DSKPP client MAY indicate the same algorithm both as an encryption algorithm and as a MAC algorithm (e.g., http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128, which is defined in Appendix C). o : This OPTIONAL element is used by the DSKPP client to indicate support for four-pass or two-pass DSKPP. If two-pass support is specified, then MUST be set to nonce R in the message unless is already present. o : This OPTIONAL element is a sequence of container elements that in turn contain URLs indicating the key package formats supported by the DSKPP client. If this element is not provided, then the DSKPP server MUST proceed with "http://www.ietf.org/keyprov/pskc#KeyContainer" (see [PSKC]). o : This OPTIONAL element contains data that the DSKPP client uses to authenticate the user or device to the DSKPP server. The element is set as specified in Section 3.7. Doherty, et al. Expires May 7, 2009 [Page 47] Internet-Draft DSKPP November 2008 o : A sequence of OPTIONAL extensions. One extension is defined for this message in this version of DSKPP: the ClientInfoType (see Section 5). Some of the core elements of the message are described below. 4.3.1. The DeviceIdentifierDataType Type The DeviceIdentifierDataType type is used to uniquely identify the device that houses the cryptographic module, e.g., a mobile phone. The device identifier allows the DSKPP server to find, e.g., a pre- shared key transport key for 2-pass DSKPP and/or the correct shared secret for MAC'ing purposes. The default DeviceIdentifierDataType is defined in [PSKC]. 4.3.2. The ProtocolVariantsType Type The ProtocolVariantsType is a complex type that is a sequence of elements, each describing a DSKPP protocol variant. The DSKPP client MAY use the ProtocolVariantsType to identify which protocol variants it supports, i.e., by providing within a message. Selecting the element signals client support for 4-pass DSKPP as described in Section 3.4.1. Selecting the element signals client support for the 2-pass version of DSKPP as described in Section 3.5.1. The element is of type KeyProtectionDataType, which carries information that informs the server of supported two-pass key protection methods as described in Section 3.5.2, and provides OPTIONAL payload data to the DSKPP server. The payload is sent in an opportunistic fashion, and MAY be discarded by the DSKPP server if the server does not support the key protection method with which the payload is associated. If the DSKPP client does not include in the message, then the DSKPP server MUST proceed by using the 4-pass DSKPP variant. If the DSKPP server does not support 4-pass DSKPP, then the server MUST use the two-pass protocol variant. If it cannot support the two-pass protocol variant, then Doherty, et al. Expires May 7, 2009 [Page 48] Internet-Draft DSKPP November 2008 the protocol run MUST fail. The elements of this type have the following meaning: o : A two-pass key protection method supported by the DSKPP client. Multiple supported methods MAY be present, in which case they MUST be listed in order of precedence. o : An OPTIONAL payload associated with each supported key protection method. A DSKPP client that indicates support for two-pass DSKPP MUST also include the nonce R in its message (this will enable the client to verify that the DSKPP server it is communicating with is alive). 4.3.3. The KeyPackagesFormatType Type The OPTIONAL KeyPackagesFormatType type is a list of type-value pairs that a DSKPP client or server MAY use to define key package formats it supports. Key package formats are identified through URLs, e.g., the PSKC KeyContainer URL "http://www.ietf.org/keyprov/pskc#KeyContainer" (see [PSKC]). Doherty, et al. Expires May 7, 2009 [Page 49] Internet-Draft DSKPP November 2008 4.3.4. The AuthenticationDataType Type The OPTIONAL AuthenticationDataType type is used by DSKPP clients to carry authentication values in DSKPP messages as described in Section 3.7. The elements of the AuthenticationDataType type have the following meaning: o : A requester's identifier of maximum length 128. The value MAY be a user ID, a device ID, or a keyID associated with the requester's authentication value. o : An authentication MAC and additional information (e.g., MAC algorithm), derived as described in Section 3.7.3. 4.4. Components of the Response (Used Only in Four-Pass DSKPP) In a four-pass exchange, this message is the first message sent from Doherty, et al. Expires May 7, 2009 [Page 50] Internet-Draft DSKPP November 2008 the DSKPP server to the DSKPP client (assuming a trigger message has not been sent to initiate the protocol, in which case, this message is the second message sent from the DSKPP server to the DSKPP client). It is sent upon reception of a message. The components of this message have the following meaning: o Version: (attribute inherited from the AbstractResponseType type) The version selected by the DSKPP server. MAY be lower than the version indicated by the DSKPP client, in which case, local policy at the client MUST determine whether or not to continue the session. o SessionID: (attribute inherited from the AbstractResponseType type) An identifier for this session. The SessionID has a maximum length of 128. o Status: (attribute inherited from the AbstractResponseType type) Return code for the . If Status is not "Continue", only the Status and Version attributes will be present; otherwise, all the other element MUST be present as well. o : The type of the key to be generated. o : The encryption algorithm to use when protecting R_C. Doherty, et al. Expires May 7, 2009 [Page 51] Internet-Draft DSKPP November 2008 o : The MAC algorithm to be used by the DSKPP server. o : Information about the key to use when encrypting R_C. It will either be the server's public key (the alternative of ds:KeyInfoType) or an identifier for a shared secret key (the alternative of ds:KeyInfoType). o : The key package format type to be used by the DSKPP server. The default setting relies on the KeyPackageType element defined in "urn:ietf:params:xml:schema:keyprov:pskc" [PSKC]. o : The actual payload. For this version of the protocol, only one payload is defined: the pseudorandom string R_S. o : A list of server extensions. Two extensions are defined for this message in this version of DSKPP: the ClientInfoType and the ServerInfoType (see Section 5). o : The MAC MUST be present if the DSKPP run will result in the replacement of an existing symmetric key with a new one (i.e., if the element was present in the Request (Used Only in Four- Pass DSKPP) In a four-pass DSKPP exchange, this message contains the nonce R_C that was chosen by the cryptographic module, and encrypted by the negotiated encryption key and encryption algorithm. Doherty, et al. Expires May 7, 2009 [Page 52] Internet-Draft DSKPP November 2008 The components of this message have the following meaning: o Version: (inherited from the AbstractRequestType type) MUST be the same version as in the message. o : (attribute inherited from the AbstractResponseType type) MUST have the same value as the SessionID attribute in the received message. SessionID has maximum length of 128. o : The nonce generated and encrypted by the cryptographic module. The encryption MUST be made using the selected encryption algorithm and identified key, and as specified in Section 3.3.1. o : The authentication data value MUST be set as specified in Section 3.7 and Section 4.3.4. o : A list of OPTIONAL extensions. Two extensions are defined for this message in this version of DSKPP: the ClientInfoType and the ServerInfoType (see Section 5). 4.6. Components of a Response This message is the last message of the DSKPP protocol run. In a 4-pass exchange, the DSKPP server sends this message in response to a message, whereas in a 2-pass exchange, the DSKPP server sends this message in response to a message. Doherty, et al. Expires May 7, 2009 [Page 53] Internet-Draft DSKPP November 2008 The components of this message have the following meaning: o Version: (inherited from the AbstractResponseType type) The DSKPP version used in this session. o SessionID: (inherited from the AbstractResponseType type) The previously established identifier for this session. The SessionID is of maximum length 128. o Status: (inherited from the AbstractResponseType type) Return code for the message. If Status is not "Success", only the Status, SessionID, and Version attributes will be present (the presence of the SessionID attribute is dependent on the type of reported error); otherwise, all the other elements MUST be present as well. In this latter case, the message can be seen as a "Commit" message, instructing the cryptographic module to store the generated key and associate the given key identifier with this key. o : The key package containing keying material in accordance with four- and two-pass DSKPP usage (see Section 3.4 and Section 3.5). The default package format is based on the KeyContainerType type from PSKC, as defined in [PSKC]. o : A list of extensions chosen by the DSKPP server. For this message, this version of DSKPP defines one extension, the ClientInfoType (see Section 5). o : To avoid a false "Commit" message causing the cryptographic module to end up in an initialized state for which the server does not know the stored key, messages MUST always be authenticated with a MAC. The MAC MUST be made using the already established MAC algorithm. Doherty, et al. Expires May 7, 2009 [Page 54] Internet-Draft DSKPP November 2008 o : This OPTIONAL element contains a MAC value that the DSKPP server provides in a two-pass message exchange as proof that the server is authorized to replace a key on the cryptographic module. The MAC MUST be calculated as specified in Section 3.5.3.2. 4.7. The StatusCode Type The StatusCode type enumerates all possible return codes: Upon transmission or receipt of a message for which the Status attribute's value is not "Success" or "Continue", the default behavior, unless explicitly stated otherwise below, is that both the DSKPP server and the DSKPP client MUST immediately terminate the DSKPP protocol run. DSKPP servers and DSKPP clients MUST delete any secret values generated as a result of failed runs of the DSKPP protocol. Session identifiers MAY be retained from successful or failed protocol runs for replay detection purposes, but such retained identifiers MUST NOT be reused for subsequent runs of the protocol. When possible, the DSKPP client SHOULD present an appropriate error message to the user. These status codes are valid in all DSKPP Response messages unless explicitly stated otherwise: Doherty, et al. Expires May 7, 2009 [Page 55] Internet-Draft DSKPP November 2008 o "Continue" indicates that the DSKPP server is ready for a subsequent request from the DSKPP client. It cannot be sent in the server's final message. o "Success" indicates successful completion of the DSKPP session. It can only be sent in the server's final message. o "Abort" indicates that the DSKPP server rejected the DSKPP client's request for unspecified reasons. o "AccessDenied" indicates that the DSKPP client is not authorized to contact this DSKPP server. o "MalformedRequest" indicates that the DSKPP server failed to parse the DSKPP client's request. o "UnknownRequest" indicates that the DSKPP client made a request that is unknown to the DSKPP server. o "UnknownCriticalExtension" indicates that a critical DSKPP extension (see below) used by the DSKPP client was not supported or recognized by the DSKPP server. o "UnsupportedVersion" indicates that the DSKPP client used a DSKPP protocol version not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "NoSupportedKeyTypes" indicates that the DSKPP client only suggested key types that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "NoSupportedEncryptionAlgorithms" indicates that the DSKPP client only suggested encryption algorithms that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "NoSupportedMacAlgorithms" indicates that the DSKPP client only suggested MAC algorithms that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "NoProtocolVariants" indicates that the DSKPP client only suggested a protocol variation (either 2-pass or 4-pass) that is not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "NoSupportedKeyPackages" indicates that the DSKPP client only suggested key package formats that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "AuthenticationDataMissing" indicates that the DSKPP client didn't provide authentication data that the DSKPP server required. o "AuthenticationDataInvalid" indicates that the DSKPP client supplied user authentication data that the DSKPP server failed to validate. o "InitializationFailed" indicates that the DSKPP server could not generate a valid key given the provided data. When this status code is received, the DSKPP client SHOULD try to restart DSKPP, as it is possible that a new run will succeed. Doherty, et al. Expires May 7, 2009 [Page 56] Internet-Draft DSKPP November 2008 o "ProvisioningPeriodExpired" indicates that the provisioning period set by the DSKPP server has expired. When the status code is received, the DSKPP client SHOULD report the reason for key initialization failure to the user and the user MUST register with the DSKPP server to initialize a new key. 5. Protocol Extensions 5.1. The ClientInfoType Type Present in a or a message, the OPTIONAL ClientInfoType extension contains DSKPP client-specific information that is custom to an implementation. DSKPP servers MUST support this extension. DSKPP servers MUST NOT attempt to interpret the data it carries and, if received, MUST include it unmodified in the current protocol run's next server response. Servers need not retain the ClientInfoType's data after that response has been generated. 5.2. The ServerInfoType Type When present, the OPTIONAL ServerInfoType extension contains DSKPP server-specific information that is custom to an implementation. This extension is only valid in messages for which Status = "Continue". DSKPP clients MUST support this extension. DSKPP clients MUST NOT attempt to interpret the data it carries and, if received, MUST include it unmodified in the current protocol run's next client request (i.e., the message). DSKPP clients need not retain the ServerInfoType's data after that request has been generated. This extension MAY be used, e.g., for state management in the DSKPP server. 6. Protocol Bindings 6.1. General Requirements DSKPP assumes a reliable transport. 6.2. HTTP/1.1 Binding for DSKPP 6.2.1. Introduction This section presents a binding of the previous messages to HTTP/1.1 [RFC2616]. Note that the HTTP client normally will be different from the DSKPP client, i.e., the HTTP client will only exist to "proxy" DSKPP messages from the DSKPP client to the DSKPP server. Likewise, Doherty, et al. Expires May 7, 2009 [Page 57] Internet-Draft DSKPP November 2008 on the HTTP server side, the DSKPP server MAY receive DSKPP PDUs from a "front-end" HTTP server. The DSKPP server will be identified by a specific URL, which may be pre-configured, or provided to the client during initialization. 6.2.2. Identification of DSKPP Messages The MIME-type for all DSKPP messages MUST be application/vnd.ietf.keyprov.dskpp+xml 6.2.3. HTTP Headers In order to avoid caching of responses carrying DSKPP messages by proxies, the following holds: o When using HTTP/1.1, requesters SHOULD: * Include a Cache-Control header field set to "no-cache, no- store". * Include a Pragma header field set to "no-cache". o When using HTTP/1.1, responders SHOULD: * Include a Cache-Control header field set to "no-cache, no-must- revalidate, private". * Include a Pragma header field set to "no-cache". * NOT include a Validator, such as a Last-Modified or ETag header. To handle content negotiation, HTTP requests MAY include an HTTP Accept header field. This header field SHOULD have the value application/vnd.ietf.keyprov.dskpp+xml as defined in Section 6.2.2. The Accept header MAY include additional content types defined by future versions of this protocol. There are no other restrictions on HTTP headers, besides the requirement to set the Content-Type header value according to Section 6.2.2. 6.2.4. HTTP Operations Persistent connections as defined in HTTP/1.1 are OPTIONAL. DSKPP requests are mapped to HTTP requests with the POST method. DSKPP responses are mapped to HTTP responses. For the 4-pass DSKPP, messages within the protocol run are bound together. In particular, is bound to the preceding by being transmitted in the corresponding HTTP response. MUST have a Doherty, et al. Expires May 7, 2009 [Page 58] Internet-Draft DSKPP November 2008 SessionID attribute, and the SessionID attribute of the subsequent message MUST be identical. is then once again bound to the rest through HTTP (and possibly through a SessionID). 6.2.5. HTTP Status Codes A DSKPP HTTP responder that refuses to perform a message exchange with a DSKPP HTTP requester SHOULD return a 403 (Forbidden) response. In this case, the content of the HTTP body is not significant. In the case of an HTTP error while processing a DSKPP request, the HTTP server MUST return a 500 (Internal Server Error) response. This type of error SHOULD be returned for HTTP-related errors detected before control is passed to the DSKPP processor, or when the DSKPP processor reports an internal error (for example, the DSKPP XML namespace is incorrect, or the DSKPP schema cannot be located). If a request is received that is not a DSKPP client message, the DSKPP responder MUST return a 400 (Bad request) response. In these cases (i.e., when the HTTP response code is 4xx or 5xx), the content of the HTTP body is not significant. Redirection status codes (3xx) apply as usual. Whenever the HTTP POST is successfully invoked, the DSKPP HTTP responder MUST use the 200 status code and provide a suitable DSKPP message (possibly with DSKPP error information included) in the HTTP body. 6.2.6. HTTP Authentication No support for HTTP/1.1 authentication is assumed. 6.2.7. Initialization of DSKPP If a user requests key initialization in a browsing session, and if that request has an appropriate Accept header (e.g., to a specific DSKPP server URL), the DSKPP server MAY respond by sending a DSKPP initialization message in an HTTP response with Content-Type set according to Section 6.2.2 and response code set to 200 (OK). The initialization message MAY carry data in its body, such as the URL for the DSKPP client to use when contacting the DSKPP server. If the message does carry data, the data MUST be a valid instance of a element. Note that if the user's request was directed to some other resource, the DSKPP server MUST NOT respond by combining the DSKPP content type with response code 200. In that case, the DSKPP server SHOULD Doherty, et al. Expires May 7, 2009 [Page 59] Internet-Draft DSKPP November 2008 respond by sending a DSKPP initialization message in an HTTP response with Content-Type set according to Section 6.2.2 and response code set to 406 (Not Acceptable). 6.2.8. Example Messages a. Initialization from DSKPP server: HTTP/1.1 200 OK Cache-Control: no-store Content-Type: application/vnd.ietf.keyprov.dskpp+xml Content-Length: DSKPP initialization data in XML form... b. Initial request from DSKPP client: POST http://example.com/cgi-bin/DSKPP-server HTTP/1.1 Cache-Control: no-cache, no-store Pragma: no-cache Host: www.example.com Content-Type: application/vnd.ietf.keyprov.dskpp+xml Content-Length: DSKPP data in XML form (supported version, supported algorithms...) c. Initial response from DSKPP server: HTTP/1.1 200 OK Cache-Control: no-cache, no-must-revalidate, private Pragma: no-cache Content-Type: application/vnd.ietf.keyprov.dskpp+xml Content-Length: DSKPP data in XML form (server random nonce, server public key, ...) 7. DSKPP Schema Basic types Basic types Doherty, et al. Expires May 7, 2009 [Page 61] Internet-Draft DSKPP November 2008 Doherty, et al. Expires May 7, 2009 [Page 62] Internet-Draft DSKPP November 2008 This element is only valid for two-pass DSKPP. Authentication data contains a MAC. Doherty, et al. Expires May 7, 2009 [Page 64] Internet-Draft DSKPP November 2008 Extension types DSKPP PDUs Message used to trigger the device to initiate a DSKPP protocol run. KeyProvClientHello PDU Message sent from DSKPP client to DSKPP server to initiate a DSKPP session. Doherty, et al. Expires May 7, 2009 [Page 66] Internet-Draft DSKPP November 2008 KeyProvServerHello PDU Response message sent from DSKPP server to DSKPP client in four-pass DSKPP. KeyProvClientNonce PDU Response message sent from DSKPP client to DSKPP server in a four-pass DSKPP session. Doherty, et al. Expires May 7, 2009 [Page 67] Internet-Draft DSKPP November 2008 KeyProvServerFinished PDU Final message sent from DSKPP server to DSKPP client in a DSKPP session. A MAC value serves for key confirmation, and optional AuthenticationData serves for server authentication. Doherty, et al. Expires May 7, 2009 [Page 68] Internet-Draft DSKPP November 2008 8. Conformance Requirements In order to assure that all implementations of DSKPP can interoperate, the DSKPP server: a. MUST implement the four-pass variation of the protocol (Section 3.4) b. MUST implement the two-pass variation of the protocol (Section 3.5) c. MUST support user authentication (Section 3.7) d. MUST support the following Key Derivation Functions: * DSKPP-PRF-AES DSKPP-PRF realization (Appendix C) * DSKPP-PRF-SHA256 DSKPP-PRF realization (Appendix C) e. MUST support the following Encryption mechanisms for protection of the client nonce in the four-pass protocol: * Mechanism described in Section 3.4.3 f. MUST support the following Encryption algorithms for symmetric key operations, e.g., key wrap: * AES-CBC-128 [FIPS197-AES] g. MUST support the following Encryption algorithms for asymmetric key operations, e.g., key transport: * RSA Encryption Scheme [PKCS-1] h. MUST support the following Integrity/KDF MAC functions: * HMAC-SHA256 [FIPS180-SHA] * AES-CMAC-128 [FIPS197-AES] i. MUST support the PSKC key package [PSKC]; all three PSKC key protection profiles (Key Transport, Key Wrap, and Passphrase- Based Key Wrap) MUST be implemented j. MAY support the ASN.1 key package as defined in [SKPC-ASN.1] DSKPP clients MUST support either the two-pass or the four-pass variant of the protocol. DSKPP clients MUST fulfill all requirements listed in item (c) - (j). Of course, DSKPP is a security protocol, and one of its major functions is to allow only authorized parties to successfully initialize a cryptographic module with a new symmetric key. Therefore, a particular implementation may be configured with any of a number of restrictions concerning algorithms and trusted Doherty, et al. Expires May 7, 2009 [Page 69] Internet-Draft DSKPP November 2008 authorities that will prevent universal interoperability. 9. Security Considerations 9.1. General DSKPP is designed to protect generated keying material from exposure. No other entities than the DSKPP server and the cryptographic module will have access to a generated K_TOKEN if the cryptographic algorithms used are of sufficient strength and, on the DSKPP client side, generation and encryption of R_C and generation of K_TOKEN take place as specified in the cryptographic module. This applies even if malicious software is present in the DSKPP client. However, as discussed in the following sub-sections, DSKPP does not protect against certain other threats resulting from man-in-the-middle attacks and other forms of attacks. DSKPP SHOULD, therefore, be run over a transport providing confidentiality and integrity, such as HTTP over Transport Layer Security (TLS) with a suitable ciphersuite, when such threats are a concern. Note that TLS ciphersuites with anonymous key exchanges are not suitable in those situations. 9.2. Active Attacks 9.2.1. Introduction An active attacker MAY attempt to modify, delete, insert, replay, or reorder messages for a variety of purposes including service denial and compromise of generated keying material. Section 9.2.2 through Section 9.2.7. 9.2.2. Message Modifications Modifications to a message will either cause denial- of-service (modifications of any of the identifiers or the nonce) or will cause the DSKPP client to contact the wrong DSKPP server. The latter is in effect a man-in-the-middle attack and is discussed further in Section 9.2.7. An attacker may modify a message. This means that the attacker could indicate a different key or device than the one intended by the DSKPP client, and could also suggest other cryptographic algorithms than the ones preferred by the DSKPP client, e.g., cryptographically weaker ones. The attacker could also suggest earlier versions of the DSKPP protocol, in case these versions have been shown to have vulnerabilities. These modifications could lead to an attacker succeeding in initializing or modifying another cryptographic module than the one intended (i.e., the server Doherty, et al. Expires May 7, 2009 [Page 70] Internet-Draft DSKPP November 2008 assigning the generated key to the wrong module), or gaining access to a generated key through the use of weak cryptographic algorithms or protocol versions. DSKPP implementations MAY protect against the latter by having strict policies about what versions and algorithms they support and accept. The former threat (assignment of a generated key to the wrong module) is not possible when the shared- key variant of DSKPP is employed (assuming existing shared keys are unique per cryptographic module), but is possible in the public-key variation. Therefore, DSKPP servers MUST NOT accept unilaterally provided device identifiers in the public-key variation. This is also indicated in the protocol description. In the shared-key variation, however, an attacker may be able to provide the wrong identifier (possibly also leading to the incorrect user being associated with the generated key) if the attacker has real-time access to the cryptographic module with the identified key. In other words, the generated key is associated with the correct cryptographic module but the module is associated with the incorrect user. See further Section 9.5 for a discussion of this threat and possible countermeasures. An attacker may also modify a message. This means that the attacker could indicate different key types, algorithms, or protocol versions than the legitimate server would, e.g., cryptographically weaker ones. The attacker may also provide a different nonce than the one sent by the legitimate server. Clients MAY protect against the former through strict adherence to policies regarding permissible algorithms and protocol versions. The latter (wrong nonce) will not constitute a security problem, as a generated key will not match the key generated on the legitimate server. Also, whenever the DSKPP run would result in the replacement of an existing key, the element protects against modifications of R_S. Modifications of messages are also possible. If an attacker modifies the SessionID attribute, then, in effect, a switch to another session will occur at the server, assuming the new SessionID is valid at that time on the server. It still will not allow the attacker to learn a generated K_TOKEN since R_C has been wrapped for the legitimate server. Modifications of the element, e.g., replacing it with a value for which the attacker knows an underlying R'C, will not result in the client changing its pre-DSKPP state, since the server will be unable to provide a valid MAC in its final message to the client. The server MAY, however, end up storing K'TOKEN rather than K_TOKEN. If the cryptographic module has been associated with a particular user, then this could constitute a security problem. For a further discussion about this threat, and a possible countermeasure, see Section 9.5 below. Note that use of TLS does not protect against this attack if the attacker has access to the DSKPP client (e.g., through malicious Doherty, et al. Expires May 7, 2009 [Page 71] Internet-Draft DSKPP November 2008 software, "Trojans"). Finally, attackers may also modify the message. Replacing the element will only result in denial-of- service. Replacement of any other element may cause the DSKPP client to associate, e.g., the wrong service with the generated key. DSKPP SHOULD be run over a transport providing confidentiality and integrity when this is a concern. 9.2.3. Message Deletion Message deletion will not cause any other harm than denial-of- service, since a cryptographic module MUST NOT change its state (i.e., "commit" to a generated key) until it receives the final message from the DSKPP server and successfully has processed that message, including validation of its MAC. A deleted message will not cause the server to end up in an inconsistent state vis-a-vis the cryptographic module if the server implements the suggestions in Section 9.5. 9.2.4. Message Insertion An active attacker may initiate a DSKPP run at any time, and suggest any device identifier. DSKPP server implementations MAY receive some protection against inadvertently initializing a key or inadvertently replacing an existing key or assigning a key to a cryptographic module by initializing the DSKPP run by use of the . The element allows the server to associate a DSKPP protocol run with, e.g., an earlier user-authenticated session. The security of this method, therefore, depends on the ability to protect the element in the DSKPP initialization message. If an eavesdropper is able to capture this message, he may race the legitimate user for a key initialization. DSKPP over a transport providing confidentiality and integrity, coupled with the recommendations in Section 9.5, is RECOMMENDED when this is a concern. Insertion of other messages into an existing protocol run is seen as equivalent to modification of legitimately sent messages. 9.2.5. Message Replay During 4-pass DSKPP, attempts to replay a previously recorded DSKPP message will be detected, as the use of nonces ensures that both parties are live. For example, a DSKPP client knows that a server it is communicating with is "live" since the server MUST create a MAC on information sent by the client. Doherty, et al. Expires May 7, 2009 [Page 72] Internet-Draft DSKPP November 2008 The same is true for 2-pass DSKPP thanks to the requirement that the client sends R in the message and that the server includes R in the MAC computation. 9.2.6. Message Reordering An attacker may attempt to re-order 4-pass DSKPP messages but this will be detected, as each message is of a unique type. Note: Message re-ordering attacks cannot occur in 2-pass DSKPP since each party sends at most one message each. 9.2.7. Man-in-the-Middle In addition to other active attacks, an attacker posing as a man in the middle may be able to provide his own public key to the DSKPP client. This threat and countermeasures to it are discussed in Section 3.4.2.1. An attacker posing as a man-in-the-middle may also be acting as a proxy and, hence, may not interfere with DSKPP runs but still learn valuable information; see Section 9.3. 9.3. Passive Attacks Passive attackers may eavesdrop on DSKPP runs to learn information that later on may be used to impersonate users, mount active attacks, etc. If DSKPP is not run over a transport providing confidentiality, a passive attacker may learn: o What cryptographic modules a particular user is in possession of; o The identifiers of keys on those cryptographic modules and other attributes pertaining to those keys, e.g., the lifetime of the keys; o DSKPP versions and cryptographic algorithms supported by a particular DSKPP client or server; and o Any value present in an that is part of Whenever the above is a concern, DSKPP SHOULD be run over a transport providing confidentiality. If man-in-the-middle attacks for the purposes described above are a concern, the transport SHOULD also offer server-side authentication. 9.4. Cryptographic Attacks An attacker with unlimited access to an initialized cryptographic module may use the module as an "oracle" to pre-compute values that later on may be used to impersonate the DSKPP server. Section 3.4.3 and Section 3 contain discussions of this threat and steps Doherty, et al. Expires May 7, 2009 [Page 73] Internet-Draft DSKPP November 2008 RECOMMENDED to protect against it. Implementers SHOULD also be aware that cryptographic algorithms become weaker with time. As new cryptographic techniques are developed and computing performance improves, the work factor to break a particular cryptographic algorithm will reduce. Therefore, cryptographic algorithm implementations SHOULD be modular allowing new algorithms to be readily inserted. That is, implementers SHOULD be prepared to regularly update the algorithms in their implementations. 9.5. Attacks on the Interaction between DSKPP and User Authentication If keys generated in DSKPP will be associated with a particular user at the DSKPP server (or a server trusted by, and communicating with the DSKPP server), then in order to protect against threats where an attacker replaces a client-provided encrypted R_C with his own R'C (regardless of whether the public-key variation or the shared-secret variation of DSKPP is employed to encrypt the client nonce), the server SHOULD not commit to associate a generated K_TOKEN with the given cryptographic module until the user simultaneously has proven both possession of the device that hosts the cryptographic module containing K_TOKEN and some out-of-band provided authenticating information (e.g., a temporary password). For example, if the cryptographic module is a one-time password token, the user could be required to authenticate with both a one-time password generated by the cryptographic module and an out-of-band provided temporary PIN in order to have the server "commit" to the generated OTP value for the given user. Preferably, the user SHOULD perform this operation from another host than the one used to initialize keys on the cryptographic module, in order to minimize the risk of malicious software on the client interfering with the process. Note: This scenario, wherein the attacker replaces a client-provided R_C with his own R'C, does not apply to 2-pass DSKPP as the client does not provide any entropy to K_TOKEN. The attack as such (and its countermeasures) still applies to 2-pass DSKPP, however, as it essentially is a man-in-the-middle attack. Another threat arises when an attacker is able to trick a user to authenticate to the attacker rather than to the legitimate service before the DSKPP protocol run. If successful, the attacker will then be able to impersonate the user towards the legitimate service, and subsequently receive a valid DSKPP trigger. If the public-key variant of DSKPP is used, this may result in the attacker being able to (after a successful DSKPP protocol run) impersonate the user. Ordinary precautions MUST, therefore, be in place to ensure that users authenticate only to legitimate services. Doherty, et al. Expires May 7, 2009 [Page 74] Internet-Draft DSKPP November 2008 9.6. Miscellaneous Considerations 9.6.1. Client Contributions to K_TOKEN Entropy In 4-pass DSKPP, both the client and the server provide randomizing material to K_TOKEN, in a manner that allows both parties to verify that they did contribute to the resulting key. In the 2-pass DSKPP version defined herein, only the server contributes to the entropy of K_TOKEN. This means that a broken or compromised (pseudo-)random number generator in the server may cause more damage than it would in the 4-pass variation. Server implementations SHOULD therefore take extreme care to ensure that this situation does not occur. 9.6.2. Key Confirmation 4-pass DSKPP servers provide key confirmation through the MAC on R_C in the message. In the 2-pass DSKPP variation described herein, key confirmation is provided by the MAC including R, using K_MAC. 9.6.3. Server Authentication DSKPP servers MUST authenticate themselves whenever a successful DSKPP 2-pass protocol run would result in an existing K_TOKEN being replaced by a K_TOKEN', or else a denial-of-service attack where an unauthorized DSKPP server replaces a K_TOKEN with another key would be possible. In 2-pass DSKPP, servers authenticate by including the AuthenticationDataType extension containing a MAC as described in Section 3.5 for two-pass DSKPP. 9.6.4. User Authentication A DSKPP server MUST authenticate a client to ensure that K_TOKEN is delivered to the intended device. The following measures SHOULD be considered: o When an Authentication Code is used for client authentication, a password dictionary attack on the authentication data is possible. o The length of the Authentication Code when used over a non-secure channel SHOULD be longer than what is used over a secure channel. When a device, e.g., some mobile phones with small screens, cannot handle a long Authentication Code in a user-friendly manner, DSKPP SHOULD rely on a secure channel for communication. o In the case that a non-secure channel has to be used, the Authentication Code SHOULD be sent to the server MAC'd as specified in Section 3.7. The Authentication Code and nonce value MUST be strong enough to prevent offline brute-force recovery of the Authentication Code from the HMAC data. Given that the nonce Doherty, et al. Expires May 7, 2009 [Page 75] Internet-Draft DSKPP November 2008 value is sent in plaintext format over a non-secure transport, the cryptographic strength of the Authentication Data depends more on the quality of the Authentication Code. o When the Authentication Code is sent from the DSKPP server to the device in a DSKPP initialization trigger message, an eavesdropper may be able to capture this message and race the legitimate user for a key initialization. To prevent this, the transport layer used to send the DSKPP trigger MUST provide confidentiality and integrity e.g. secure browser session. 9.6.5. Key Protection in Two-Pass DSKPP Three key protection profiles are defined for the different usages of 2-pass DSKPP, which MUST be supported by a key package format, such as [PSKC] and [SKPC-ASN.1]. Therefore, key protection in the two- pass DSKPP is dependent upon the security of the key package format selected for a protocol run. Some considerations for the Passphrase profile follow. The passphrase-based key wrap profile SHOULD depend upon the PBKDF2 function from [PKCS-5] to generate an encryption key from a passphrase and salt string. It is important to note that passphrase- based encryption is generally limited in the security that it provides despite the use of salt and iteration count in PBKDF2 to increase the complexity of attack. Implementations SHOULD therefore take additional measures to strengthen the security of the passphrase-based key wrap profile. The following measures SHOULD be considered where applicable: o The passphrase SHOULD be selected well, and usage guidelines such as the ones in [NIST-PWD] SHOULD be taken into account. o A different passphrase SHOULD be used for every key initialization wherever possible (the use of a global passphrase for a batch of cryptographic modules SHOULD be avoided, for example). One way to achieve this is to use randomly-generated passphrases. o The passphrase SHOULD be protected well if stored on the server and/or on the cryptographic module and SHOULD be delivered to the device's user using secure methods. o User per-authentication SHOULD be implemented to ensure that K_TOKEN is not delivered to a rogue recipient. o The iteration count in PBKDF2 SHOULD be high to impose more work for an attacker using brute-force methods (see [PKCS-5] for recommendations). However, it MUST be noted that the higher the count, the more work is required on the legitimate cryptographic module to decrypt the newly delivered K_TOKEN. Servers MAY use relatively low iteration counts to accommodate devices with limited processing power such as some PDA and cell phones when other security measures are implemented and the security of the Doherty, et al. Expires May 7, 2009 [Page 76] Internet-Draft DSKPP November 2008 passphrase-based key wrap method is not weakened. o Transport level security (e.g. TLS) SHOULD be used where possible to protect a two-pass protocol run. Transport level security provides a second layer of protection for the newly generated K_TOKEN. 10. Internationalization Considerations The DSKPP protocol is mostly meant for machine-to-machine communications; as such, most of its elements are tokens not meant for direct human consumption. If these tokens are presented to the end user, some localization may need to occur. DSKPP exchanges information using XML. All XML processors are required to understand UTF-8 and UTF-16 encoding, and therefore all DSKPP clients and servers MUST understand UTF-8 and UTF-16 encoded XML. Additionally, DSKPP servers and clients MUST NOT encode XML with encodings other than UTF-8 or UTF-16. 11. IANA Considerations This document requires several IANA registrations, detailed below. 11.1. URN Sub-Namespace Registration This section registers a new XML namespace, "urn:ietf:params:xml:ns:keyprov:dskpp:1.0" per the guidelines in [RFC3688]: URI: urn:ietf:params:xml:ns:keyprov:dskpp:1.0 Registrant Contact: IETF, KEYPROV Working Group (keyprov@ietf.org), Andrea Doherty (andrea.doherty@rsa.com) XML: Doherty, et al. Expires May 7, 2009 [Page 77] Internet-Draft DSKPP November 2008 BEGIN DSKPP Messsages

Namespace for DSKPP Messages

urn:ietf:params:xml:ns:keyprov:dskpp:1.0

[NOTE TO IANA/RFC-EDITOR: Please replace XXXX below with the RFC number for this specification.]

See RFCXXXX

END 11.2. XML Schema Registration This section registers an XML schema as per the guidelines in [RFC3688]. URI: urn:ietf:params:xml:ns:keyprov:dskpp:1.0 Registrant Contact: IETF, KEYPROV Working Group (keyprov@ietf.org), Andrea Doherty (andrea.doherty@rsa.com) Schema The XML for this schema can be found as the entirety of Section 7 of this document. 11.3. MIME Media Type Registration This section registers the "application/dskpp+xml" MIME type: To: ietf-types@iana.org Subject: Registration of MIME media type application/dskpp+xml MIME media type name: application MIME subtype name: dskpp+xml Required parameters: (none) Optional parameters: charset Indicates the character encoding of enclosed XML. Default is UTF-8. Encoding considerations: Uses XML, which can employ 8-bit characters, depending on the character encoding used. See [RFC3203], Section 3.2. Doherty, et al. Expires May 7, 2009 [Page 78] Internet-Draft DSKPP November 2008 Security considerations: This content type is designed to carry protocol data related to key management. Security mechanisms are built into the protocol to ensure that various threats are dealt with. Interoperability considerations: This content type provides a basis for a protocol. Published specification: RFC XXXX [NOTE TO IANA/RFC-EDITOR: Please replace XXXX with the RFC number for this specification.] Applications which use this media type: Protocol for key exchange. Additional information: Magic Number(s): (none) File extension(s): .xmls Macintosh File Type Code(s): (none) Person & email address to contact for further information: Andrea Doherty (andrea.doherty@rsa.com) Intended usage: LIMITED USE Author/Change controller: The IETF Other information: This media type is a specialization of application/xml [RFC3203], and many of the considerations described there also apply to application/dskpp+xml. 11.4. Status Code Registry This section registers status codes included in each DSKPP response message. The status codes are defined in the schema in the type definition as described in the XML schema in Section 7. The following summarizes the registry: Related Registry: KEYPROV DSKPP Registries, Status codes for DSKPP Defining RFC: RFC XXXX [NOTE TO IANA/RFC-EDITOR: Please replace XXXX with the RFC number for this specification.] Registration/Assignment Procedures: Following the policies outlined in [RFC3575], the IANA policy for assigning new values for the status codes for DSKPP MUST be "Specification Required" and their meanings MUST be documented in an RFC or in some other permanent and readily available reference, in sufficient detail that interoperability between independent implementations is possible. No mechanism to mark entries as "deprecated" is envisioned. It is possible to delete or update entries from the registry. Doherty, et al. Expires May 7, 2009 [Page 79] Internet-Draft DSKPP November 2008 Registrant Contact: IETF, KEYPROV working group (keyprov@ietf.org), Andrea Doherty (andrea.doherty@rsa.com) This section pre-registers the initial status codes, as described above in Section 4.7, including their textual description. The list in Section 4.7, starts with the StatusCode "Continue" and ends with the StatusCode "InitializationFailed". 12. Intellectual Property Considerations RSA and RSA Security are registered trademarks or trademarks of RSA Security Inc. in the United States and/or other countries. The names of other products and services mentioned may be the trademarks of their respective owners. 13. Contributors This work is based on information contained in [RFC4758], authored by Magnus Nystrom, with enhancements (esp. Client Authentication, and support for multiple key package formats) from an individual Internet-Draft co-authored by Mingliang Pei and Salah Machani. We would like to thank Shuh Chang for contributing the DSKPP object model, and Philip Hoyer for his work in aligning DSKPP and PSKC schemas. We would also like to thank Hannes Tschofenig for his draft reviews, feedback, and text contributions. 14. Acknowledgements We would like to thank the following for review of previous DSKPP document versions: o Dr. Ulrike Meyer (Review June 2007) o Niklas Neumann (Review June 2007) o Shuh Chang (Review June 2007) o Hannes Tschofenig (Review June 2007 and again in August 2007) o Sean Turner (Reviews August 2007 and again in July 2008) o John Linn (Review August 2007) o Philip Hoyer (Review September 2007) o Thomas Roessler (Review November 2007) Doherty, et al. Expires May 7, 2009 [Page 80] Internet-Draft DSKPP November 2008 o Lakshminath Dondeti (Comments December 2007) o Pasi Eronen (Comments December 2007) o Phillip Hallam-Baker (Review November 2008) We would also like to thank the following for their input to selected design aspects of the DSKPP protocol: o Anders Rundgren (Key Package Format and Client Authentication Data) o Thomas Roessler (HTTP Binding) o Hannes Tschofenig (HTTP Binding) o Phillip Hallam-Baker (Registry for Algorithms) Finally, we would like to thank Robert Griffin for opening communication channels for us with the IEEE P1619.3 Key Management Group, and facilitating our groups in staying informed of potential areas (esp. key provisioning and global key identifiers of collaboration) of collaboration. 15. References 15.1. Normative references [FIPS180-SHA] National Institute of Standards and Technology, "Secure Hash Standard", FIPS 180-2, February 2004, . [FIPS197-AES] National Institute of Standards and Technology, "Specification for the Advanced Encryption Standard (AES)", FIPS 197, November 2001, . [PKCS-1] RSA Laboratories, "RSA Cryptography Standard", PKCS #1 Version 2.1, June 2002, . [PKCS-5] RSA Laboratories, "Password-Based Cryptography Standard", PKCS #5 Version 2.0, March 1999, . [PKCS-5-XML] RSA Laboratories, "XML Schema for PKCS #5 Version 2.0", PKCS #5 Version 2.0 Amd.1 (FINAL DRAFT), October 2006, . Doherty, et al. Expires May 7, 2009 [Page 81] Internet-Draft DSKPP November 2008 [PSKC] "Portable Symmetric Key Container", 2008, . [RFC2104] Krawzcyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997, . [RFC2119] "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997, . [RFC3629] "UTF-8, a transformation format of ISO10646", STD 63, RFC 3629, November 2003, . [UNICODE] Davis, M. and M. Duerst, "Unicode Normalization Forms", March 2001, . [XMLDSIG] W3C, "XML Signature Syntax and Processing", W3C Recommendation, February 2002, . [XMLENC] W3C, "XML Encryption Syntax and Processing", W3C Recommendation, December 2002, . 15.2. Informative references [CT-KIP-P11] RSA Laboratories, "PKCS #11 Mechanisms for the Cryptographic Token Key Initialization Protocol", PKCS #11 Version 2.20 Amd.2, December 2005, . [FAQ] RSA Laboratories, "Frequently Asked Questions About Today's Cryptography", Version 4.1, 2000. [ISO3309] "ISO Information Processing Systems - Data Communication - High-Level Data Link Control Procedure - Frame Structure", IS 3309, 3rd Edition, October 1984. [NIST-PWD] National Institute of Standards and Technology, "Password Usage", FIPS 112, May 1985, Doherty, et al. Expires May 7, 2009 [Page 82] Internet-Draft DSKPP November 2008 . [NIST-SP800-38B] International Organization for Standardization, "Recommendations for Block Cipher Modes of Operation: The CMAC Mode for Authentication", NIST SP800-38B, May 2005, < http://csrc.nist.gov/publications/nistpubs/800-38B/ SP_800-38B.pdf>. [NIST-SP800-57] National Institute of Standards and Technology, "Recommendation for Key Management - Part I: General (Revised)", NIST 800-57, March 2007, . [PKCS-11] RSA Laboratories, "Cryptographic Token Interface Standard", PKCS #11 Version 2.20, June 2004, . [PKCS-12] "Personal Information Exchange Syntax Standard", PKCS #12 Version 1.0, 2005, . [RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998, . [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999, . [RFC3203] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types", RFC 3203, January 2001, . [RFC3575] Aboba, B., "IANA Considerations for RADIUS", RFC 3575, July 2003, . [RFC3688] Mealling, M., "The IETF XML Registry", RFC 3688, BCP 81, January 2004, . [RFC4758] RSA, The Security Division of EMC, "Cryptographic Token Key Initialization Protocol (CT-KIP)", November 2006, . Doherty, et al. Expires May 7, 2009 [Page 83] Internet-Draft DSKPP November 2008 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008, . [SKPC-ASN.1] "Symmetric Key Package Content Type", 2007, . [XMLNS] W3C, "Namespaces in XML", W3C Recommendation, January 1999, . Appendix A. Examples This appendix contains example messages that illustrate parameters, encoding, and semantics in four-and two- pass DSKPP exchanges. The examples are written using XML, and are syntactically correct. MAC and cipher values are fictitious however. Doherty, et al. Expires May 7, 2009 [Page 84] Internet-Draft DSKPP November 2008 A.1. Trigger Message ManufacturerABC XL0000000001234 U2 SE9UUDAwMDAwMDAx 112dsdfwf312asder394jw== https://www.somekeyprovservice.com/ A.2. Four-Pass Protocol Doherty, et al. Expires May 7, 2009 [Page 85] Internet-Draft DSKPP November 2008 A.2.1. Without a Preceding Trigger ManufacturerABC XL0000000001234 U2 http://www.ietf.org/keyprov/pskc#hotp http://www.rsa.com/rsalabs/otps/schemas/2005/09/ otps-wst#SecurID-AES http://www.w3.org/2001/05/xmlenc#rsa_1_5 http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 http://www.ietf.org/keyprov/pskc#KeyContainer Doherty, et al. Expires May 7, 2009 [Page 86] Internet-Draft DSKPP November 2008 A.2.2. Assuming a Preceding Trigger ManufacturerABC XL0000000001234 U2 SE9UUDAwMDAwMDAx 112dsdfwf312asder394jw== http://www.ietf.org/keyprov/pskc#hotp http://www.rsa.com/rsalabs/otps/schemas/2005/09/ otps-wst#SecurID-AES http://www.w3.org/2001/05/xmlenc#rsa_1_5 http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 http://www.ietf.org/keyprov/pskc#KeyContainer Doherty, et al. Expires May 7, 2009 [Page 87] Internet-Draft DSKPP November 2008 A.2.3. Without a Preceding Trigger http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst#SecurID-AES http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 KEY-1 http://www.ietf.org/keyprov/pskc#KeyContainer qw2ewasde312asder394jw== Doherty, et al. Expires May 7, 2009 [Page 88] Internet-Draft DSKPP November 2008 A.2.4. Assuming a Preceding Trigger urn:ietf:params:xml:schema:keyprov:otpalg#SecurID-AES http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 KEY-1 http://www.ietf.org/keyprov/pskc#KeyContainer qw2ewasde312asder394jw== cXcycmFuZG9tMzEyYXNkZXIzOTRqdw== A.2.5. Using Default Encryption This message contains the nonce chosen by the cryptographic module, R_C, encrypted by the specified encryption key and encryption algorithm. Doherty, et al. Expires May 7, 2009 [Page 89] Internet-Draft DSKPP November 2008 VXENc+Um/9/NvmYKiHDLaErK0gk= 31300257 512 4bRJf9xXd3KchKoTenHJiw== Doherty, et al. Expires May 7, 2009 [Page 90] Internet-Draft DSKPP November 2008 A.2.6. Using Default Encryption http://www.w3.org/2000/09/xmldsig#hmac-sha1 CredentialIssuer MyFirstToken 0 2012-12-31T00:00:00 miidfasde312asder394jw== A.3. Two-Pass Protocol A.3.1. Example Using the Key Transport Profile The client indicates support all the Key Transport, Key Wrap, and Passphrase-Based Key Wrap profiles: Doherty, et al. Expires May 7, 2009 [Page 91] Internet-Draft DSKPP November 2008 ManufacturerABC XL0000000001234 U2 xwQzwEl0CjPAiQeDxwRJdQ== http://www.ietf.org/keyprov/pskc#hotp http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst#SecurID-AES http://www.w3.org/2001/05/xmlenc#rsa_1_5 http://www.w3.org/2001/04/xmlenc#kw-aes128 http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 urn:ietf:params:xml:schema:keyprov:dskpp#wrap Key_001 urn:ietf:params:xml:schema:keyprov:dskpp#transport urn:ietf:params:xml:schema:keyprov:dskpp#passphrase-wrap miib Doherty, et al. Expires May 7, 2009 [Page 92] Internet-Draft DSKPP November 2008 http://www.ietf.org/keyprov/pskc#KeyContainer 31300257 512 4bRJf9xXd3KchKoTenHJiw== In this example, the server responds to the previous request using the key transport profile. https://www.somedskppservice.com/ urn:ietf:params:xml:schema:keyprov:dskpp#transport miib ACME 0755225266 AnIssuer Doherty, et al. Expires May 7, 2009 [Page 93] Internet-Draft DSKPP November 2008 rf4dx3rvEPO0vKtKL14NbeVu8nk= 0 miidfasde312asder394jw== 4bRJf9xXd3KchKoTenHJiw== A.3.2. Example Using the Key Wrap Profile The client sends a request that specifies a shared key to protect the K_TOKEN, and the server responds using the Key Wrap Profile. Authentication data in this example is based on an authentication code rather than a device certificate. ManufacturerABC XL0000000001234 Doherty, et al. Expires May 7, 2009 [Page 94] Internet-Draft DSKPP November 2008 U2 xwQzwEl0CjPAiQeDxwRJdQ== http://www.ietf.org/keyprov/pskc#hotp http://www.rsa.com/rsalabs/otps/schemas/2005/09/ otps-wst#SecurID-AES http://www.w3.org/2001/05/xmlenc#rsa_1_5 http://www.w3.org/2001/04/xmlenc#kw-aes128 http://www.rsasecurity.com/rsalabs/pkcs/schemas/ pkcs-5#pbes2 http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 urn:ietf:params:xml:schema:keyprov:dskpp#wrap Key_001 http://www.ietf.org/keyprov/pskc#KeyContainer 31300257 512 4bRJf9xXd3KchKoTenHJiw== Doherty, et al. Expires May 7, 2009 [Page 95] Internet-Draft DSKPP November 2008 In this example, the server responds to the previous request using the key wrap profile. https://www.somedskppservice.com/ urn:ietf:params:xml:schema:keyprov:dskpp#wrap PRE_SHARED_KEY http://www.w3.org/2000/09/xmldsig#hmac-sha1 CredentialIssuer MyFirstToken kyzrWTJuhJKQHhZtf2CWbKC5H3LdfAPvKzHHQ8SdxyE= cwJI898rRpGBytTqCAsegaQqPZA= 1/pskc:PlainValue> Doherty, et al. Expires May 7, 2009 [Page 96] Internet-Draft DSKPP November 2008 2012-12-31T00:00:00 miidfasde312asder394jw== 4bRJf9xXd3KchKoTenHJiw== A.3.3. Example Using the Passphrase-Based Key Wrap Profile The client sends a request similar to that in Appendix A.3.1 with authentication data based on an authentication code, and the server responds using the Passphrase-Based Key Wrap Profile. The authentication data is set in clear text when it is sent over a secure transport channel such as TLS. ManufacturerABC XL0000000001234 U2 xwQzwEl0CjPAiQeDxwRJdQ== http://www.ietf.org/keyprov/pskc#hotp http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst#SecurID-AES http://www.w3.org/2001/05/xmlenc#rsa_1_5 Doherty, et al. Expires May 7, 2009 [Page 97] Internet-Draft DSKPP November 2008 http://www.w3.org/2001/04/xmlenc#kw-aes128 http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5#pbes2 http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 urn:ietf:params:xml:schema:keyprov:dskpp#wrap Key_001 urn:ietf:params:xml:schema:keyprov:dskpp#passphrase-wrap http://www.ietf.org/keyprov/pskc#KeyContainer 31300257 512 4bRJf9xXd3KchKoTenHJiw== In this example, the server responds to the previous request using the Passphrase-Based Key Wrap Profile. (preamble) Doherty, et al. Expires May 7, 2009 [Page 98] Internet-Draft DSKPP November 2008 https://www.somedskppservice.com/ urn:ietf:params:xml:schema:keyprov:protocol#passphrase-wrap Passphrase1 P1ciQdGbrI0= 2000 16 Manufacturer 0755225266 AnIssuer rf4dx3rvEPO0vKtKL14NbeVu8nk= Doherty, et al. Expires May 7, 2009 [Page 99] Internet-Draft DSKPP November 2008 0 miidfasde312asder394jw== 4bRJf9xXd3KchKoTenHJiw== (postamble) Appendix B. Integration with PKCS #11 A DSKPP client that needs to communicate with a connected cryptographic module to perform a DSKPP exchange MAY use PKCS #11 [PKCS-11] as a programming interface. B.1. The 4-pass Variant When performing 4-pass DSKPP with a cryptographic module using the PKCS #11 programming interface, the procedure described in [CT-KIP-P11], Appendix B, is RECOMMENDED. B.2. The 2-pass Variant A suggested procedure to perform 2-pass DSKPP with a cryptographic module through the PKCS #11 interface using the mechanisms defined in [CT-KIP-P11] is as follows: a. On the client side, 1. The client selects a suitable slot and token (e.g. through use of the or the element of the DSKPP trigger message). 2. A nonce R is generated, e.g. by calling C_SeedRandom and C_GenerateRandom. 3. The client sends its first message to the server, including the nonce R. b. On the server side, Doherty, et al. Expires May 7, 2009 [Page 100] Internet-Draft DSKPP November 2008 1. A generic key K_PROV = K_TOKEN | K_MAC (where '|' denotes concatenation) is generated, e.g. by calling C_GenerateKey (using key type CKK_GENERIC_SECRET). The template for K_PROV MUST allow it to be exported (but only in wrapped form, i.e. CKA_SENSITIVE MUST be set to CK_TRUE and CKA_EXTRACTABLE MUST also be set to CK_TRUE), and also to be used for further key derivation. From K, a token key K_TOKEN of suitable type is derived by calling C_DeriveKey using the PKCS #11 mechanism CKM_EXTRACT_KEY_FROM_KEY and setting the CK_EXTRACT_PARAMS to the first bit of the generic secret key (i.e. set to 0). Likewise, a MAC key K_MAC is derived from K_PROV by calling C_DeriveKey using the CKM_EXTRACT_KEY_FROM_KEY mechanism, this time setting CK_EXTRACT_PARAMS to the length of K_PROV (in bits) divided by two. 2. The server wraps K_PROV with either the public key of the DSKPP client or device, the pre-shared secret key, or the derived shared secret key by using C_WrapKey. If use of the DSKPP key wrap algorithm has been negotiated then the CKM_KIP_WRAP mechanism MUST be used to wrap K. When calling C_WrapKey, the hKey handle in the CK_KIP_PARAMS structure MUST be set to NULL_PTR. The pSeed parameter in the CK_KIP_PARAMS structure MUST point to the nonce R provided by the DSKPP client, and the ulSeedLen parameter MUST indicate the length of R. The hWrappingKey parameter in the call to C_WrapKey MUST be set to refer to the key wrapping key. 3. Next, the server needs to calculate a MAC using K_MAC. If use of the DSKPP MAC algorithm has been negotiated, then the MAC is calculated by calling C_SignInit with the CKM_KIP_MAC mechanism followed by a call to C_Sign. In the call to C_SignInit, K_MAC MUST be the signature key, the hKey parameter in the CK_KIP_PARAMS structure MUST be set to NULL_PTR, the pSeed parameter of the CT_KIP_PARAMS structure MUST be set to NULL_PTR, and the ulSeedLen parameter MUST be set to zero. In the call to C_Sign, the pData parameter MUST be set to the concatenation of the string ServerID and the nonce R, and the ulDataLen parameter MUST be set to the length of the concatenated string. The desired length of the MAC MUST be specified through the pulSignatureLen parameter and MUST be set to the length of R. 4. If the server also needs to authenticate its message (due to an existing K_TOKEN being replaced), the server MUST calculate a second MAC. Again, if use of the DSKPP MAC algorithm has been negotiated, then the MAC is calculated by calling C_SignInit with the CKM_KIP_MAC mechanism followed by a call to C_Sign. In this call to C_SignInit, the K_MAC' existing before this DSKPP protocol run MUST be the signature key (the implementation may specify K_MAC' to be the value of the K_TOKEN that is being replaced, or a version of K_MAC Doherty, et al. Expires May 7, 2009 [Page 101] Internet-Draft DSKPP November 2008 from the previous protocol run), the hKey parameter in the CK_KIP_PARAMS structure MUST be set to NULL, the pSeed parameter of the CT_KIP_PARAMS structure MUST be set to NULL_PTR, and the ulSeedLen parameter MUST be set to zero. In the call to C_Sign, the pData parameter MUST be set to the concatenation of the string ServerID and the nonce R, and the ulDataLen parameter MUST be set to the length of concatenated string. The desired length of the MAC MUST be specified through the pulSignatureLen parameter and MUST be set to the length of R. 5. The server sends its message to the client, including the wrapped key K_TOKEN, the MAC and possibly also the authenticating MAC. c. On the client side, 1. The client calls C_UnwrapKey to receive a handle to K. After this, the client calls C_DeriveKey twice: Once to derive K_TOKEN and once to derive K_MAC. The client MUST use the same mechanism (CKM_EXTRACT_KEY_FROM_KEY) and the same mechanism parameters as used by the server above. When calling C_UnwrapKey and C_DeriveKey, the pTemplate parameter MUST be used to set additional key attributes in accordance with local policy and as negotiated and expressed in the protocol. In particular, the value of the element in the server's response message MAY be used as CKA_ID for K_TOKEN. The key K_PROV MUST be destroyed after deriving K_TOKEN and K_MAC. 2. The MAC is verified in a reciprocal fashion as it was generated by the server. If use of the CKM_KIP_MAC mechanism has been negotiated, then in the call to C_VerifyInit, the hKey parameter in the CK_KIP_PARAMS structure MUST be set to NULL_PTR, the pSeed parameter MUST be set to NULL_PTR, and ulSeedLen MUST be set to 0. The hKey parameter of C_VerifyInit MUST refer to K_MAC. In the call to C_Verify, pData MUST be set to the concatenation of the string ServerID and the nonce R, and the ulDataLen parameter MUST be set to the length of the concatenated string, pSignature to the MAC value received from the server, and ulSignatureLen to the length of the MAC. If the MAC does not verify the protocol session ends with a failure. The token MUST be constructed to not "commit" to the new K_TOKEN or the new K_MAC unless the MAC verifies. 3. If an authenticating MAC was received (REQUIRED if the new K_TOKEN will replace an existing key on the token), then it is verified in a similar vein but using the K_MAC' associated with this server and existing before the protocol run (the implementation may specify K_MAC' to be the value of the K_TOKEN that is being replaced, or a version of K_MAC from the previous protocol run). Again, if the MAC does not Doherty, et al. Expires May 7, 2009 [Page 102] Internet-Draft DSKPP November 2008 verify the protocol session ends with a failure, and the token MUST be constructed no to "commit" to the new K_TOKEN or the new K_MAC unless the MAC verifies. Appendix C. Example of DSKPP-PRF Realizations C.1. Introduction This example appendix defines DSKPP-PRF in terms of AES [FIPS197-AES] and HMAC [RFC2104]. C.2. DSKPP-PRF-AES C.2.1. Identification For cryptographic modules supporting this realization of DSKPP-PRF, the following URL MAY be used to identify this algorithm in DSKPP: http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes-128 When this URL is used to identify the encryption algorithm, the method for encryption of R_C values described in Section 3.4.3 MUST be used. C.2.2. Definition DSKPP-PRF-AES (k, s, dsLen) Input: k Encryption key to use s Octet string consisting of randomizing material. The length of the string s is sLen. dsLen Desired length of the output Output: DS A pseudorandom string, dsLen-octets long Steps: 1. Let bLen be the output block size of AES in octets: bLen = (AES output block length in octets) (normally, bLen = 16) Doherty, et al. Expires May 7, 2009 [Page 103] Internet-Draft DSKPP November 2008 2. If dsLen > (2**32 - 1) * bLen, output "derived data too long" and stop 3. Let n be the number of bLen-octet blocks in the output data, rounding up, and let j be the number of octets in the last block: n = CEILING( dsLen / bLen) j = dsLen - (n - 1) * bLen 4. For each block of the pseudorandom string DS, apply the function F defined below to the key k, the string s and the block index to compute the block: B1 = F (k, s, 1) , B2 = F (k, s, 2) , ... Bn = F (k, s, n) The function F is defined in terms of the CMAC construction from [NIST-SP800-38B], using AES as the block cipher: F (k, s, i) = CMAC-AES (k, INT (i) || s) where INT (i) is a four-octet encoding of the integer i, most significant octet first, and the output length of CMAC is set to bLen. Concatenate the blocks and extract the first dsLen octets to product the desired data string DS: DS = B1 || B2 || ... || Bn<0..j-1> Output the derived data DS. C.2.3. Example If we assume that dsLen = 16, then: n = 16 / 16 = 1 j = 16 - (1 - 1) * 16 = 16 DS = B1 = F (k, s, 1) = CMAC-AES (k, INT (1) || s) C.3. DSKPP-PRF-SHA256 Doherty, et al. Expires May 7, 2009 [Page 104] Internet-Draft DSKPP November 2008 C.3.1. Identification For cryptographic modules supporting this realization of DSKPP-PRF, the following URL MAY be used to identify this algorithm in DSKPP: http://www.ietf.org/keyprov/dskpp#dskpp-prf-sha256 When this URL is used to identify the encryption algorithm to use, the method for encryption of R_C values described in Section 3.4.3 MUST be used. C.3.2. Definition DSKPP-PRF-SHA256 (k, s, dsLen) Input: k Encryption key to use s Octet string consisting of randomizing material. The length of the string s is sLen. dsLen Desired length of the output Output: DS A pseudorandom string, dsLen-octets long Steps: 1. Let bLen be the output size of SHA-256 in octets of [FIPS180-SHA] (no truncation is done on the HMAC output): bLen = 32 (normally, bLen = 16) 2. If dsLen > (2**32 - 1) * bLen, output "derived data too long" and stop 3. Let n be the number of bLen-octet blocks in the output data, rounding up, and let j be the number of octets in the last block: n = CEILING( dsLen / bLen) j = dsLen - (n - 1) * bLen 4. For each block of the pseudorandom string DS, apply the function F defined below to the key k, the string s and the block index to compute the block: B1 = F (k, s, 1), Doherty, et al. Expires May 7, 2009 [Page 105] Internet-Draft DSKPP November 2008 B2 = F (k, s, 2), ... Bn = F (k, s, n) The function F is defined in terms of the HMAC construction from [RFC2104], using SHA-256 as the digest algorithm: F (k, s, i) = HMAC-SHA256 (k, INT (i) || s) where INT (i) is a four-octet encoding of the integer i, most significant octet first, and the output length of HMAC is set to bLen. Concatenate the blocks and extract the first dsLen octets to product the desired data string DS: DS = B1 || B2 || ... || Bn<0..j-1> Output the derived data DS. C.3.3. Example If we assume that sLen = 256 (two 128-octet long values) and dsLen = 16, then: n = CEILING( 16 / 32 ) = 1 j = 16 - (1 - 1) * 32 = 16 B1 = F (k, s, 1) = HMAC-SHA256 (k, INT (1) || s) DS = B1<0 ... 15> That is, the result will be the first 16 octets of the HMAC output. Authors' Addresses Andrea Doherty RSA, The Security Division of EMC 174 Middlesex Tpk. Bedford, MA 01730 USA Email: andrea.doherty@rsa.com Doherty, et al. Expires May 7, 2009 [Page 106] Internet-Draft DSKPP November 2008 Mingliang Pei Verisign, Inc. 487 E. Middlefield Road Mountain View, CA 94043 USA Email: mpei@verisign.com Salah Machani Diversinet Corp. 2225 Sheppard Avenue East, Suite 1801 Toronto, Ontario M2J 5C2 Canada Email: smachani@diversinet.com Magnus Nystrom RSA, The Security Division of EMC Arenavagen 29 Stockholm, Stockholm Ln 121 29 SE Email: magnus.nystrom@rsa.com Doherty, et al. Expires May 7, 2009 [Page 107] Internet-Draft DSKPP November 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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