KEYPROV Working Group A. Doherty Internet-Draft RSA, The Security Division of EMC Intended status: Standards Track M. Pei Expires: January 27, 2008 VeriSign, Inc. M. Nystroem RSA, The Security Division of EMC S. Machani Diversinet Corp. July 26, 2007 Dynamic Symmetric Key Provisioning Protocol (DSKPP) draft-ietf-keyprov-dskpp-00.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 January 27, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). 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 Doherty, et al. Expires January 27, 2008 [Page 1] Internet-Draft DSKPP July 2007 private-key capabilities in the cryptographic modules, and with or without an established public-key infrastructure. Three variations of the protocol support multiple usage scenarios. The four-pass (i.e., two round-trip) variant enables key generation in near real-time. 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. Two- and one-pass variants enable secure and efficient download and installation of symmetric keys to a cryptographic module in environments where near real-time communication may not be possible. This document builds on information contained in [RFC4758], adding specific enhancements in response to implementation experience and liaison requests. It is intended, therefore, that this document or a successor version thereto will become the basis for subsequent progression of a symmetric key provisioning protocol specification on the standards track. Doherty, et al. Expires January 27, 2008 [Page 2] Internet-Draft DSKPP July 2007 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7 1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 7 2. Notation and Terminology . . . . . . . . . . . . . . . . . . 8 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. A cryptographic module obtains a symmetric key . . . . . 9 3.2. A cryptographic module acquires multiple symmetric keys of different types . . . . . . . . . . . . . . . . . 9 3.3. A provisioning server imposes a validity period policy for provisioning sessions . . . . . . . . . . . . . . . . 10 3.4. A symmetric key issuer uses a third party provisioning service provider . . . . . . . . . . . . . . . . . . . . 10 3.5. A cryptographic module renews its symmetric key with the same key ID . . . . . . . . . . . . . . . . . . . . . 10 3.6. An administrator initiates a symmetric key replacement before it can be used . . . . . . . . . . . . . . . . . . 10 3.7. A cryptographic module hosted by a smart card uses a pre-shared transport key to communicate with the provisioning server . . . . . . . . . . . . . . . . . . . 11 3.8. A cryptographic module hosted by a mobile device downloads a symmetric key through SMS . . . . . . . . . . 11 3.9. A cryptographic module acquires a symmetric key over a transport protocol that does not ensure data confidentiality . . . . . . . . . . . . . . . . . . . . . 12 3.10. A cryptographic module acquires a symmetric key over a transport protocol that does not provide authentication . 12 4. DSKPP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.1. Entities . . . . . . . . . . . . . . . . . . . . . . . . 12 4.2. Principles of Operation . . . . . . . . . . . . . . . . . 14 4.2.1. Four-pass DSKPP . . . . . . . . . . . . . . . . . . . 15 4.2.2. Two-pass DSKPP . . . . . . . . . . . . . . . . . . . 19 4.2.3. One-pass DSKPP . . . . . . . . . . . . . . . . . . . 21 4.3. Authentication . . . . . . . . . . . . . . . . . . . . . 22 4.3.1. Client Authentication (Applicable to Four- and Two-Pass DSKPP) . . . . . . . . . . . . . . . . . . . 22 4.3.2. Server Authentication . . . . . . . . . . . . . . . . 25 4.4. Symmetric Key Container Format . . . . . . . . . . . . . 25 4.5. The DSKPP One-Way Pseudorandom Function, DSKPP-PRF . . . 25 4.5.1. Introduction . . . . . . . . . . . . . . . . . . . . 25 4.5.2. Declaration . . . . . . . . . . . . . . . . . . . . . 26 4.6. Generation of Symmetric Keys for Cryptographic Modules . 26 4.7. Encryption of Pseudorandom Nonces Sent from the DSKPP Client . . . . . . . . . . . . . . . . . . . . . . . . . 27 4.8. MAC calculations . . . . . . . . . . . . . . . . . . . . 27 4.8.1. Four-pass DSKPP . . . . . . . . . . . . . . . . . . . 27 4.8.2. Two-pass DSKPP . . . . . . . . . . . . . . . . . . . 28 Doherty, et al. Expires January 27, 2008 [Page 3] Internet-Draft DSKPP July 2007 4.8.3. One-pass DSKPP . . . . . . . . . . . . . . . . . . . 29 4.9. DSKPP Schema Basics . . . . . . . . . . . . . . . . . . . 30 4.9.1. The AbstractRequestType Type . . . . . . . . . . . . 31 4.9.2. The AbstractResponseType Type . . . . . . . . . . . . 31 4.9.3. The VersionType Type . . . . . . . . . . . . . . . . 32 4.9.4. The IdentifierType Type . . . . . . . . . . . . . . . 32 4.9.5. The StatusCode Type . . . . . . . . . . . . . . . . . 32 4.9.6. The DeviceIdentifierDataType Type . . . . . . . . . . 34 4.9.7. The TokenPlatformInfoType and PlatformType Types . . 35 4.9.8. The NonceType Type . . . . . . . . . . . . . . . . . 35 4.9.9. The AlgorithmsType Type . . . . . . . . . . . . . . . 36 4.9.10. The ProtocolVariantsType and the TwoPassSupportType Types . . . . . . . . . . . . . . 36 4.9.11. The KeyContainersFormatTypeType . . . . . . . . . . . 37 4.9.12. The AuthenticationDataType Type . . . . . . . . . . . 38 4.9.13. The PayloadType Type . . . . . . . . . . . . . . . . 40 4.9.14. The MacType Type . . . . . . . . . . . . . . . . . . 40 4.9.15. The KeyContainerType Type . . . . . . . . . . . . . . 40 4.9.16. The ExtensionsType and the AbstractExtensionType Types . . . . . . . . . . . . . . . . . . . . . . . . 41 4.10. DSKPP Messages . . . . . . . . . . . . . . . . . . . . . 41 4.10.1. Introduction . . . . . . . . . . . . . . . . . . . . 41 4.10.2. DSKPP Initialization (OPTIONAL) . . . . . . . . . . . 41 4.10.3. The DSKPP Client's Initial PDU (2- and 4-Pass) . . . 43 4.10.4. The DSKPP Server's Initial PDU (4-Pass Only) . . . . 46 4.10.5. The DSKPP Client's Second PDU (4-Pass Only) . . . . . 47 4.10.6. The DSKPP Server's Final PDU (1-, 2-, and 4-Pass) . . 48 4.11. Protocol Extensions . . . . . . . . . . . . . . . . . . . 50 4.11.1. The ClientInfoType Type . . . . . . . . . . . . . . . 50 4.11.2. The ServerInfoType Type . . . . . . . . . . . . . . . 50 4.11.3. The KeyInitializationDataType Type . . . . . . . . . 51 5. Protocol Bindings . . . . . . . . . . . . . . . . . . . . . . 52 5.1. General Requirements . . . . . . . . . . . . . . . . . . 52 5.2. HTTP/1.1 Binding for DSKPP . . . . . . . . . . . . . . . 52 5.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 52 5.2.2. Identification of DSKPP Messages . . . . . . . . . . 53 5.2.3. HTTP Headers . . . . . . . . . . . . . . . . . . . . 53 5.2.4. HTTP Operations . . . . . . . . . . . . . . . . . . . 53 5.2.5. HTTP Status Codes . . . . . . . . . . . . . . . . . . 53 5.2.6. HTTP Authentication . . . . . . . . . . . . . . . . . 54 5.2.7. Initialization of DSKPP . . . . . . . . . . . . . . . 54 5.2.8. Example Messages . . . . . . . . . . . . . . . . . . 54 6. DSKPP Schema . . . . . . . . . . . . . . . . . . . . . . . . 55 7. Security Considerations . . . . . . . . . . . . . . . . . . . 63 7.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 63 7.2. Active Attacks . . . . . . . . . . . . . . . . . . . . . 63 7.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 63 7.2.2. Message Modifications . . . . . . . . . . . . . . . . 64 Doherty, et al. Expires January 27, 2008 [Page 4] Internet-Draft DSKPP July 2007 7.2.3. Message Deletion . . . . . . . . . . . . . . . . . . 65 7.2.4. Message Insertion . . . . . . . . . . . . . . . . . . 65 7.2.5. Message Replay . . . . . . . . . . . . . . . . . . . 66 7.2.6. Message Reordering . . . . . . . . . . . . . . . . . 66 7.2.7. Man-in-the-Middle . . . . . . . . . . . . . . . . . . 66 7.3. Passive Attacks . . . . . . . . . . . . . . . . . . . . . 66 7.4. Cryptographic Attacks . . . . . . . . . . . . . . . . . . 67 7.5. Attacks on the Interaction between DSKPP and User Authentication . . . . . . . . . . . . . . . . . . . . . 67 7.6. Additional Considerations Specific to 2- and 1-pass DSKPP . . . . . . . . . . . . . . . . . . . . . . . . . . 68 7.6.1. Client Contributions to K_TOKEN Entropy . . . . . . . 68 7.6.2. Key Confirmation . . . . . . . . . . . . . . . . . . 68 7.6.3. Server Authentication . . . . . . . . . . . . . . . . 68 7.6.4. Client Authentication . . . . . . . . . . . . . . . . 68 7.6.5. Key Protection in the Passphrase Profile . . . . . . 69 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 70 9. Intellectual Property Considerations . . . . . . . . . . . . 70 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 70 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 70 11.1. Normative references . . . . . . . . . . . . . . . . . . 70 11.2. Informative references . . . . . . . . . . . . . . . . . 71 Appendix A. Key Initialization Profiles of DSKPP . . . . . . . . 72 A.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 73 A.2. Key Transport Profile . . . . . . . . . . . . . . . . . . 73 A.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 73 A.2.2. Identification . . . . . . . . . . . . . . . . . . . 73 A.2.3. Payloads . . . . . . . . . . . . . . . . . . . . . . 73 A.3. Key wrap profile . . . . . . . . . . . . . . . . . . . . 74 A.3.1. Introduction . . . . . . . . . . . . . . . . . . . . 74 A.3.2. Identification . . . . . . . . . . . . . . . . . . . 74 A.3.3. Payloads . . . . . . . . . . . . . . . . . . . . . . 74 A.4. Passphrase-based key wrap profile . . . . . . . . . . . . 76 A.4.1. Introduction . . . . . . . . . . . . . . . . . . . . 76 A.4.2. Identification . . . . . . . . . . . . . . . . . . . 76 A.4.3. Payloads . . . . . . . . . . . . . . . . . . . . . . 76 Appendix B. Example Messages . . . . . . . . . . . . . . . . . . 77 B.1. Example Messages in a Four-pass Exchange . . . . . . . . 77 B.1.1. Example of a DSKPP Initialization (Trigger) Message . 78 B.1.2. Example of a Message . . . . . . . . . 79 B.1.3. Example of a Message . . . . . . . . . 80 B.1.4. Example of a Message . . . . . . . . . 80 B.1.5. Example of a Message . . . . . . . . 80 B.2. Example Messages in a Two- or One-pass Exchange . . . . . 81 B.2.1. Example of a Message Indicating Support for Two-pass DSKPP . . . . . . . . . . . . . 81 B.2.2. Example of a Message Using the Key Transport Profile . . . . . . . . . . . . . . . . 83 Doherty, et al. Expires January 27, 2008 [Page 5] Internet-Draft DSKPP July 2007 B.2.3. Example of a Message Using the Key Wrap Profile . . . . . . . . . . . . . . . . . . 85 B.2.4. Example of a Message using the Passphrase-based Key Wrap Profile . . . . . . . . . . 86 Appendix C. Requirements . . . . . . . . . . . . . . . . . . . . 88 Appendix D. Integration with PKCS #11 . . . . . . . . . . . . . 90 D.1. The 4-pass Variant . . . . . . . . . . . . . . . . . . . 91 D.2. The 2-pass Variant . . . . . . . . . . . . . . . . . . . 91 D.3. The 1-pass Variant . . . . . . . . . . . . . . . . . . . 93 Appendix E. Example of DSKPP-PRF Realizations . . . . . . . . . 95 E.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 96 E.2. DSKPP-PRF-AES . . . . . . . . . . . . . . . . . . . . . . 96 E.2.1. Identification . . . . . . . . . . . . . . . . . . . 96 E.2.2. Definition . . . . . . . . . . . . . . . . . . . . . 96 E.2.3. Example . . . . . . . . . . . . . . . . . . . . . . . 97 E.3. DSKPP-PRF-SHA256 . . . . . . . . . . . . . . . . . . . . 97 E.3.1. Identification . . . . . . . . . . . . . . . . . . . 97 E.3.2. Definition . . . . . . . . . . . . . . . . . . . . . 98 E.3.3. Example . . . . . . . . . . . . . . . . . . . . . . . 99 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 99 Intellectual Property and Copyright Statements . . . . . . . . . 100 Doherty, et al. Expires January 27, 2008 [Page 6] Internet-Draft DSKPP July 2007 1. Introduction 1.1. Scope This document describes 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. The objectives of this protocol are to: o Provide a secure method of initializing cryptographic modules with symmetric keys without exposing generated, secret material to any other entities than the server and the cryptographic module itself. o Provide a secure method of generating and transporting symmetric keys to a cryptographic module in environments where near real-time communication is not possible. o Provide a secure method of transporting pre-generated (i.e., legacy) keys to a cryptographic module. o Provide a solution that is easy to administer and scales well. The mechanism is intended for general use within computer and communications systems employing symmetric cryptographic modules that are locally (i.e., over-the-wire) or remotely (i.e., over-the-air) accessible. 1.2. Background A symmetric cryptographic module may be hosted by a hand-held hardware device (e.g., a mobile phone), a hardware device connected to a personal computer through an electronic interface, such as USB, or a software application resident on a personal computer. The cryptographic module offers symmetric cryptographic functionality that may be used to authenticate a user towards some service, perform data encryption, etc. Increasingly, these modules enable their programmatic initialization as well as programmatic retrieval of their output values. This document intends to meet the need for an open and inter-operable mechanism to programmatically initialize and configure symmetric keys to locally and remotely accessible cryptographic modules. The target mechanism addressed herein is a symmetric key provisioning server. In an ideal deployment scenario, near real-time communication is possible between the provisioning server and the cryptographic module. In such an environment, it is possible for the cryptographic module and provisioning server to mutually generate a symmetric key, and to ensure that keys are not transported between Doherty, et al. Expires January 27, 2008 [Page 7] Internet-Draft DSKPP July 2007 them. There are, however, several deployment scenarios that make mutual key generation less suitable. Specifically, scenarios where near real- time communication between the symmetric key provisioning server and the cryptographic module is not possible, and scenarios with significant design constraints. Examples include work-flow constraints (e.g., policies that require incremental administrative approval), network design constraints that create network latency, and budget constraints that sustain reliance upon legacy systems that already have supplies of pre-generated keys. In these situations, the cryptographic module is required to download and install a symmetric key from the provisioning server in a secure and efficient manner. This document tries to meet the needs of these scenarios by describing three variations to DSKPP for the provisioning of symmetric keys in two round trips or less. The four-pass (i.e., two round-trip) variant enables key generation in near real-time. With this variant, keys are mutually generated by the provisioning server and cryptographic module; provisioned keys are not transferred over- the-wire or over-the-air. In contrast, two- and one-pass variants enable secure and efficient download and installation of symmetric keys to a cryptographic module in environments where near real-time communication is not possible. 2. Notation and Terminology 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]. The following notations are used in this document: || String concatenation [x] Optional element x A ^ B Exclusive-OR operation on strings A and B (where A and B are of equal length) DSKPP client Manages communication between the symmetric cryptographic module and the DSKPP server DSKPP server The symmetric key provisioning server that participates in the DSKPP protocol run ID_C Identifier for DSKPP client Doherty, et al. Expires January 27, 2008 [Page 8] Internet-Draft DSKPP July 2007 ID_S Identifier for DSKPP server K Key used to encrypt R_C (either K_SERVER or K_SHARED) K_AUTH Secret key used for server authentication purposes in 4-pass DSKPP K_CLIENT Public key of the DSKPP client K_DERIVED Secret key derived from a passphrase that is known to both the DSKPP client or user and the DSKPP server K_MAC Secret key used for key confirmation and server authentication purposes, and generated in DSKPP K_MAC' A second secret key used for server authentication purposes in 2- and 1-pass DSKPP K_SERVER Public key of the DSKPP server K_SHARED Secret key shared between the DSKPP client and the DSKPP server K_TOKEN Secret key used for cryptographic module computations, and generated in DSKPP R Pseudorandom value chosen by the DSKPP client and used for MAC computations, which is mandatory for 2-pass DSKPP and optional for 4-pass 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 The following typographical convention is used in the body of the text: . 3. Use Cases This section describes typical use cases. 3.1. A cryptographic module obtains a symmetric key A cryptographic module hosted by a device, such as a mobile phone, makes a request for a symmetric key from a provisioning server. Depending upon how the system is deployed, 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. 3.2. A cryptographic module acquires multiple symmetric keys of different types A cryptographic module makes multiple requests for symmetric keys from the same provisioning server. The symmetric keys may or may not be of the same type, i.e., the keys may be used with different Doherty, et al. Expires January 27, 2008 [Page 9] Internet-Draft DSKPP July 2007 symmetric cryptographic algorithms, including the HMAC-Based One-Time Password (HOTP), RSA SecurID, challenge-response, etc. 3.3. A provisioning server imposes a validity period policy for provisioning sessions Once a cryptographic module initiates a symmetric key request, the provisioning server may require that any subsequent actions to complete the provisioning cycle occur within a certain time window. For example, 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. As long as the user inputs a valid authentication code within the fixed time period established by the issuer, the server will provision a key to the cryptographic module hosted by the user's device. 3.4. A symmetric key issuer uses a third party provisioning service provider A symmetric key issuer outsources its key provisioning to a third party key provisioning server provider. The issuer is responsible for authenticating and granting rights to users to acquire keys while acting as a proxy to the cryptographic module to acquire symmetric keys from the provisioning server; the cryptographic module communicates with the issuer proxy server, which forwards provisioning requests to the provisioning server. 3.5. A cryptographic module renews its symmetric key with the same key ID A cryptographic module requests renewal of a symmetric key using the same key ID already associated with the key. 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 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. 3.6. An administrator initiates a symmetric key replacement before it can be used This use case 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 an 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. Doherty, et al. Expires January 27, 2008 [Page 10] Internet-Draft DSKPP July 2007 Bulk initialization under controlled conditions, e.g., during manufacture, is likely to meet the security needs of most applications. However, reliance on a pre-disclosed secret is unacceptable in some circumstances. One such circumstance is when cryptographic modules are issued for classified government use or high security applications. In such cases, the issuer requires the ability to remove all secret information already installed on the cryptographic module and replace it with symmetric keys established under conditions controlled by the issuer. Another variation of this use case is the issuer who recycles devices. In this case, an 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 use case is essentially the same as the last use case wherein the same key ID is used for renewal. 3.7. A cryptographic module hosted by a smart card uses a pre-shared transport key to communicate with the provisioning server 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-shared transport 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 use case are for the protocol to be tunneled and the provisioning server to know the correct pre- established transport key. 3.8. A cryptographic module hosted by a mobile device downloads a symmetric key through SMS A mobile device supports Short Message Service (SMS) but is not able to support a data service allowing for HTTP or HTTPS transports. In addition, the cryptographic module can ensure that SMS will provide an acceptable level of protection for download of the symmetric key. In such a case, the cryptographic module hosted by the mobile device may initiate a symmetric key request from a desktop computer and ask the server to send the key to the mobile device through SMS. User authentication is carried out via the online communication established between the desktop computer and the provisioning server. Doherty, et al. Expires January 27, 2008 [Page 11] Internet-Draft DSKPP July 2007 3.9. A cryptographic module acquires a symmetric key over a transport protocol that does not ensure data confidentiality Some devices are not able to support a secure transport channel such as SSL or TLS to provide data confidentiality. A cryptographic module hosted by such a device requests a symmetric key from the provisioning server. It is up to DSKPP to ensure data confidentiality over non-secure networks. 3.10. A cryptographic module acquires a symmetric key over a transport protocol that does not provide authentication Some devices are not able to use a transport protocol that provides server authentication such as SSL or TLS. A cryptographic module hosted by such a device wants to be sure that it sends a request for a symmetric key to a legitimate provisioning server. It is up to DSKPP to provide proper client and server authentication. 4. DSKPP 4.1. Entities In principle, the protocol involves a DSKPP client and a DSKPP server. The DSKPP client manages communication between the cryptographic module and the provisioning server. The DSKPP server herein represents the provisioning server. 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 represents the following: User The person or client to whom devices are issued UserID A unique identifier for the user or client Device A physical piece of hardware that hosts symmetric cryptographic modules DeviceID A unique identifier for the device Cryptographic Module A low-level component of an application, which enables symmetric cryptographic functionality CryptoModuleID A unique identifier for an instance of the cryptographic module Encryption Algorithms Encryption algorithms supported by the cryptographic module Doherty, et al. Expires January 27, 2008 [Page 12] Internet-Draft DSKPP July 2007 MAC Algorithms MAC algorithms supported by the cryptographic module Key Container An object that encapsulates a symmetric key and its configuration data KeyID A unique identifier for the symmetric key Key Type The type of symmetric cryptographic methods for which the key will be used (e.g., OATH HOTP or RSA SecurID authentication, AES encryption, etc.) ----------- ------------- | User | | Device | |---------|* owns *|-----------| | UserID |--------->| DeviceID | | ... | | ... | ----------- ------------- | 1 | | contains | | * V ----------------------- |Cryptographic Module | |---------------------| |CryptoModuleID |Encryption Algorithms| |MAC Algorithms | |... | ----------------------- | 1 | | contains | | * V ----------------------- |Key Container | |---------------------| |KeyID | |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 Doherty, et al. Expires January 27, 2008 [Page 13] Internet-Draft DSKPP July 2007 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]. 4.2. Principles of Operation To initiate a DSKPP session, a user may use a browser to connect to a web server. The user may then identify and optionally authenticate herself and possibly indicate how the DSKPP client has to contact the DSKPP server. There are also other alternatives for DSKPP session initiation, such as the DSKPP client being pre-configured to contact a certain DSKPP server, or the user being informed out-of-band about the address 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, 2-pass, or 1-pass protocol. 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. Two- and one-pass variants enable secure and efficient download and installation of symmetric keys to a cryptographic module in environments where near real-time communication may not be possible. DSKPP protocol variants may be applied to the use cases described in Section 3, as shown below: Doherty, et al. Expires January 27, 2008 [Page 14] Internet-Draft DSKPP July 2007 ========================================================== Protocol Applicable Applicable Variant Use Cases Deployment Scenarios ========================================================== 4-pass All but 3.6 and Near real-time 3.8 if mutual key communication is generation is desired; possible none if transport of a pre-generated key is required ----------------------------------------------------------- 2-pass All Either near real-time or non real-time communication may be possible ----------------------------------------------------------- 1-pass All but 3.8 Either near real-time or non real-time communication may be possible ========================================================== Figure 2: Mapping of use cases to protocol variants 4.2.1. Four-pass DSKPP The 4-pass protocol flow is suitable for environments wherein there is near real-time communication possible between the DSKPP client and DSKPP server. It is not suitable for environments wherein administrative approval is a required step in the flow, nor for provisioning of pre-generated keys. The 4-pass protocol flow, shown in Figure 3 and expanded in Figure 4, consists of two round trips between the DSKPP client and server. Doherty, et al. Expires January 27, 2008 [Page 15] Internet-Draft DSKPP July 2007 +---------------+ +---------------+ | | | | | DSKPP client | | DSKPP server | | | | | +---------------+ +---------------+ | | | [ <---- DSKPP trigger ----- ] | | | | ------- Client Hello -------> | | | | <------ Server Hello -------- | | | | ------- Client Nonce -------> | | | | <----- Server Finished ------ | | | Figure 3: The 4-pass DSKPP protocol (with OPTIONAL preceeding trigger) a. The DSKPP client sends a message to the DSKPP server. The message provides information to the DSKPP server about the DSKPP versions, protocol variants, key types, encryption and MAC algorithms supported by the cryptographic module for the purposes of this protocol. The message may also include client authentication data, such as a certificate or authentication code. b. The DSKPP server responds to the DSKPP client with a message, whose content includes a random nonce, R_S, along with information about the type of key to generate, and the encryption algorithm chosen to protect sensitive data sent in the protocol. The length of the nonce R_S may depend on the selected key type. The message also provides information about either a shared secret key to use for encrypting the cryptographic module's random nonce (see description of below), or its own public key. Optionally, may include a MAC that the DSKPP client may use for server authentication. c. Based on information contained in the message, the cryptographic module generates a random nonce, R_C. The length of the nonce R_C may 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. If K is equivalent to K_SERVER, then the cryptographic module SHOULD verify the server's certificate before using it to encrypt R_C. The DSKPP client then sends the encrypted random nonce to the DSKPP Doherty, et al. Expires January 27, 2008 [Page 16] Internet-Draft DSKPP July 2007 server in a message, and may include client authentication data, such as a certificate or authentication code. Finally, the cryptographic module calculates a symmetric key, K_TOKEN, of the selected type from the combination of the two random nonces R_S and R_C, the encryption key K, and possibly some other data, using the DSKPP-PRF function defined in Section 4.5. d. 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, using the DSKPP-PRF function defined in Section 4.5. 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. e. Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called . The confirmation message includes a key container 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. Optionally, may include a MAC that the DSKPP client may use for server authentication. f. Upon receipt of the DSKPP server's confirmation message, the cryptographic module associates the provided key container with the generated key K_TOKEN, and stores any provided configuration data. 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 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 Doherty, et al. Expires January 27, 2008 [Page 17] Internet-Draft DSKPP July 2007 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 synch" 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. by using a connection independent from the one used for the key generation). +----------------------+ +-------+ +----------------------+ | +------------+ | | | | | | | 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 4: Principal data flow for DSKPP key generation - using public server key Doherty, et al. Expires January 27, 2008 [Page 18] Internet-Draft DSKPP July 2007 4.2.2. Two-pass DSKPP The 2-pass protocol flow is suitable for environments wherein near real-time communication between the DSKPP client and server may not be possible. It is also suitable for environments wherein administrative approval is a required step in the flow, and for provisioning of pre-generated keys. In the 2-pass protocol flow, shown in Figure 5, the client's initial message is directly followed by a message. There is no exchange of the message or the message. However, as the two-pass variant 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 than by including R_C in , the DSKPP client is able to ensure the server is alive before "commiting" the key. Also note that the DSKPP "trigger" message MAY be used to trigger the client's sending of the message. Essentially, two-pass DSKPP is a transport of key material from the DSKPP server to the DSKPP client. Two-pass DSKPP supports multiple key initialization methods that ensure K_TOKEN is not exposed to any other entity than the DSKPP server and the cryptographic module itself. Currently, three such key initialization methods are defined (refer to Appendix A), each supporting a different usage of 2-pass DSKPP: Key Transport This profile is intended for PKI-capable devices. Key transport is carried out using a public key, K_CLIENT, whose private key part resides in the cryptographic module as the transport key. Key Wrap This profile is ideal for pre-keyed devices, e.g., SIM cards. Key wrap is carried out using a symmetric key- wrapping key, K_SHARED, 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, K_DERIVED, which is known in advance by both the cryptographic module and DSKPP server. Doherty, et al. Expires January 27, 2008 [Page 19] Internet-Draft DSKPP July 2007 +---------------+ +---------------+ | | | | | DSKPP client | | DSKPP server | | | | | +---------------+ +---------------+ | | | [ <---- DSKPP trigger ----- ] | | | | ------- Client Hello -------> | | | | <----- Server Finished ------ | | | Figure 5: The 2-pass DSKPP protocol (with OPTIONAL preceding trigger) a. The DSKPP client sends a message to the DSKPP server. The message provides the client nonce, R_C, and information about the DSKPP versions, protocol variants, key types, encryption and MAC algorithms supported by the cryptographic module for the purposes of this protocol. The message may also include client authentication data, such as a certificate or authentication code. Unlike 4-pass DSKPP, 2-pass DSKPP client uses the message to declare which key initialization method it supports, providing required payload information, e.g., K_CLIENT for the Key Transport Profile. b. The DSKPP server generates a key K from which two keys, K_TOKEN and K_MAC are derived. K is either transported or wrapped in accordance with the key initialization method specified by the DSKPP client in the message. 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. c. Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called . The confirmation message includes a key container that holds an identifier for the key, the key K from which K_TOKEN and K_MAC are derived, and additional configuration information (note that the latter MUST include the identity of the DSKPP server for authentication purposes). In addition, MUST include two MACs 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 authentication before "commiting" the key. Doherty, et al. Expires January 27, 2008 [Page 20] Internet-Draft DSKPP July 2007 d. Upon receipt of the DSKPP server's confirmation message, the cryptographic module extracts the key data from the provided key container, uses the two MAC values to perform key confirmation and server authentication, and stores the key material locally. 4.2.3. One-pass DSKPP The one-pass protocol flow is suitable for environments wherein near real-time communication between the DSKPP client and server may not be possible. It is also suitable for environments wherein administrative approval is a required step in the flow, and for provisioning of pre-generated keys. In one-pass DSKPP, shown in Figure 6, the server simply sends a message to the DSKPP client. In this case, there is no exchange of the , , and DSKPP messages, and hence there is no way for the client to express supported algorithms or key types. Before attempting one-pass DSKPP, the server MUST therefore have prior knowledge not only that the client is able and willing to accept this variant of DSKPP, but also of algorithms and key types supported by the client. Essentially, one-pass DSKPP is a transport of key material from the DSKPP server to the DSKPP client. As with two-pass DSKPP, the one- pass variant relies on key initialization methods that ensure K_TOKEN is not exposed to any other entity than the DSKPP server and the cryptographic module itself. The same key initialization profiles are defined as described in Section 4.2.2 and Appendix A. Outside the specific cases where one-pass DSKPP is desired, clients SHOULD be constructed and configured to only accept DSKPP server messages in response to client-initiated transactions. +---------------+ +---------------+ | | | | | DSKPP client | | DSKPP server | | | | | +---------------+ +---------------+ | | | <----- Server Finished ------ | | | Figure 6: The 1-pass DSKPP protocol Doherty, et al. Expires January 27, 2008 [Page 21] Internet-Draft DSKPP July 2007 a. The DSKPP server generates a key K from which two keys, K_TOKEN and K_MAC are derived. K is either transported or wrapped in accordance with the key initialization method known in advance by the DSKPP server. 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. b. Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called . The confirmation message includes a key container that holds an identifier for the key, the key K from which K_TOKEN and K_MAC are derived, and additional configuration information (note that the latter MUST include the identity of the DSKPP server for authentication purposes). In addition, MUST include two MACs, which will allow the cryptographic module to perform key confirmation and server authentication before "commiting" the key. Note that unlike two-pass DSKPP, in the one-pass variant, the server does not have the client nonce, R_C, and therefore the MACs values are calculated with contribution from an unsigned integer, I, generated by the server during the protocol run. c. Upon receipt of the DSKPP server's confirmation message, the cryptographic module extracts the key data from the provided key container, uses the two MAC values to perform key confirmation and server authentication, and stores the key material locally. 4.3. Authentication 4.3.1. Client Authentication (Applicable to Four- and Two-Pass DSKPP) To ensure that a generated 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 in several ways, as discussed in the following sub-sections. Whatever the method, the DSKPP server MUST ensure that a generated key is associated with the correct cryptographic module, and if applicable, the correct user. For a further discussion of this, and threats related to man-in-the- middle attacks in this context, see Section 7. 4.3.1.1. Device Certificate Instead of requiring an Authentication Code for in-band authentication, a device certificate could be used, which was supplied with the cryptographic module by its issuer. Doherty, et al. Expires January 27, 2008 [Page 22] Internet-Draft DSKPP July 2007 4.3.1.2. Device Identifier The provisioning server could be pre-configured with a device identifier. The DSKPP server MAY then include this identifier in the DSKPP initialization trigger, and the DSKPP client would 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 protocol. 4.3.1.3. One-time Use Authentication Code A key issuer may provide a one-time value, called an Authentication Code, to the user or device out-of-band and require this value to be used by the DSKPP client when contacting the DSKPP server. The DSKPP client MAY include the authentication data in its (and for four-pass) message, and the DSKPP server MUST verify the data before continuing with the protocol run. Note: An alternate method for getting the Authentication Code to the client, is for the DSKPP server to place the value in the element of the DSKPP initialization trigger (if triggers are used; see Section 5.2.7) . +------------+ Get Authentication Code +------------+ | User |<------------------------->| Issuer | +------------+ +------------+ | | | | | | V V +--------------+ +--------------+ | Provisioning | Authentication Data | Provisioning | | Client |----------------------->| Server | +--------------+ +--------------+ Figure 7: User Authentication with One-Time Code Considering an Authentication Code as a special form of shared secret between a user and a provisioning server, Authentication Data can have one of the following forms: o AuthenticationData = Hash (Authentication Code) Doherty, et al. Expires January 27, 2008 [Page 23] Internet-Draft DSKPP July 2007 When an Authentication Code is used to initiate the protocol run, the Authentication Code MUST be sent to the DSKPP server in a secure manner. If the underlying transport channel is secure, the authentication data MAY contain the plaintext format or the hashed format of the Authentication Code using a hash function. o AuthenticationData = HMAC(Authentication Code, K_AUTH) If the underlying transport is not secure, the client MUST use a key K_AUTH and the Authentication Code to derive authentication data. For example, if the Authentication Code has a fixed format, e.g., AuthenticationCode = passwordLength || ID || password || checksum then AuthenticationData MAY be calculated as follows: AuthenticationData = AuthenticationCode->ID || B64(Digest) where for four-pass DSKPP, the cryptographic module uses the server nonce R_S in combination with the server URL to calculate the Digest: Digest = DSKPP-PRF-AES(K_AUTH, AuthCode->ID || serverURL || R_S, 16) Refer to Section 4.5 for a description of DSKPP-PRF in general and Appendix E for a description of DSKPP-PRF-AES. For two-pass DSKPP, the cryptographic module does not have access to the server nonce R_S in combination and so: Digest = DSKPP-PRF-AES(K_AUTH, AuthenticationCode->ID || serverURL, 16) In either case, K_AUTH MAY be derived AES key from AuthenticationCode->password as in: K_AUTH = truncate( Hash( Hash(...n times...( AuthCode->password ) ) ) ) where truncate() returns the first 16 bytes from the result of the last hash iteration, and n is the number of hash iterations (set to fixed values, e.g., between 10 and 100). Doherty, et al. Expires January 27, 2008 [Page 24] Internet-Draft DSKPP July 2007 o AuthenticationData = When a certificate is used for authentication, the authentication data MAY be client-signed. Authentication data MAY be omitted if client certificate authentication has been provided by the transport channel such as TLS. When an issuer delegates symmetric key provisioning to a third party provisioning service provider, both client authentication and issuer authentication are required by the provisioning server. Client authentication to the Issuer MAY be in-band or out-of-band as described above. The issuer acts as a proxy for the provisioning server. The issuer authenticates to the provisioning service provider either using a certificate or a pre-established secret key. 4.3.2. Server Authentication A DSKPP server MUST authenticate itself to avoid a false "Commit" of a symmetric key that which could cause the cryptographic module to end up in an initialized state for which the server does not know the stored key. To do this, the DSKPP server authenticates itself by including a MAC in each of its responses to the client. In 2-pass and 1-pass DSKPP, servers authenticate themselves by including a second MAC value in the response message. In addition, a DSKPP server can leverage transport layer authentication if it is available. 4.4. Symmetric Key Container Format The default symmetric key container format that is used in the message is based on the Portable Symmetric Key Container (PSKC) defined in [PSKC]. Alternative formats MAY include PKCS#12 [PKCS-12] or PKCS#5 XML [PKCS-5-XML] format. 4.5. The DSKPP One-Way Pseudorandom Function, DSKPP-PRF 4.5.1. Introduction The general requirements on DSKPP-PRF are the same as on keyed hash functions: It MUST take an arbitrary length input, and be one-way and collision-free (for a definition of these terms, see, e.g., [FAQ]). Further, the DSKPP-PRF function MUST be capable of generating a variable-length output, and its output MUST be unpredictable even if other outputs for the same key are known. It is assumed that any realization of DSKPP-PRF takes three input parameters: A secret key k, some combination of variable data, and the desired length of the output. The combination of variable data Doherty, et al. Expires January 27, 2008 [Page 25] Internet-Draft DSKPP July 2007 can, without loss of generalization, be considered as a salt value (see PKCS#5 Version 2.0 [PKCS-5], Section 4), and this characterization of DSKPP-PRF SHOULD fit all actual PRF algorithms implemented by cryptographic modules. From the point of view of this specification, DSKPP-PRF is a "black-box" function that, given the inputs, generates a pseudorandom value. Separate specifications MAY define the implementation of DSKPP-PRF for various types of cryptographic modules. Appendix E contains two example realizations of DSKPP-PRF. 4.5.2. Declaration DSKPP-PRF (k, s, dsLen) Input: 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 16 octets long. 4.6. Generation of Symmetric Keys for Cryptographic Modules In DSKPP, keys are generated using the DSKPP-PRF function, a secret random value R_C chosen by the DSKPP client, a random value R_S chosen by the DSKPP server, and the key k used to encrypt R_C. The input parameter s of DSKPP-PRF is set to the concatenation of the (ASCII) string "Key generation", k, and R_S, and the input parameter dsLen is set to the desired length of the key, K_TOKEN (the length of K_TOKEN is given by the key's type): dsLen = (desired length of K_TOKEN) K_TOKEN = DSKPP-PRF (R_C, "Key generation" || k || R_S, dsLen) When computing K_TOKEN above, the output of DSKPP-PRF 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 January 27, 2008 [Page 26] Internet-Draft DSKPP July 2007 4.7. 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: Enc-R_C = DS ^ R_C The DSKPP server will then perform the reverse operation to extract R_C from Enc-R_C. Note: It may appear that an attacker, who learns a previous value of R_C, may be able to replay the corresponding R_S and, hence, learn a new R_C as well. However, this attack is mitigated by the requirement for a server to show knowledge of K_AUTH (see below) in order to successfully complete a key re-generation. 4.8. MAC calculations 4.8.1. Four-pass DSKPP 4.8.1.1. Server Authentication: The MAC value MUST be computed on the (ASCII) string "MAC 1 computation", the client's nonce R (if sent), and the server's nonce R_S using an authentication key K_AUTH that SHOULD be a special authentication key used only for this purpose but MAY be the current K_TOKEN. The MAC value MAY be computed by using the DSKPP-PRF function of Section 4.5, in which case the 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, and k MUST be set to K_AUTH. The input Doherty, et al. Expires January 27, 2008 [Page 27] Internet-Draft DSKPP July 2007 parameter dsLen MUST be set to the length of R_S: dsLen = len(R_S) MAC = DSKPP-PRF (K_AUTH, "MAC 1 computation" || [R ||] R_S, dsLen) 4.8.1.2. Server Authentication: The MAC value MUST be computed on the (ASCII) string "MAC 2 computation" and R_C using an authentication key K_AUTH. Again, this SHOULD be a special authentication key used only for this purpose, but MAY also be an existing K_TOKEN. (In this case, implementations MUST protect against attacks where K_TOKEN is used to pre-compute MAC values.) If no authentication key is present in the cryptographic module, and no K_TOKEN existed before the DSKPP run, K_AUTH MUST be the newly generated K_TOKEN. 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", R_C, the parameter dsLen MUST be set to the length of R_C: dsLen = len(R_C) MAC = DSKPP-PRF (K_AUTH, "MAC 2 computation" || R_C, dsLen) 4.8.2. Two-pass DSKPP 4.8.2.1. Key Confirmation In two-pass DSKPP, the client is REQUIRED to include a nonce R in the message. Further, the server is REQUIRED to include an identifier, ID_S, for itself (via the key container) in the message. The MAC value in the message MUST be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and R using a MAC key K_MAC. Again, in contrast with the MAC calculation in the four-pass DSKPP, this key MUST be provided together with K_TOKEN to the cryptographic module, and hence there is no need for a K_AUTH for key confirmation purposes. If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 computation" and R, and the parameter dsLen MUST be set to the length of R: dsLen = len(R) Doherty, et al. Expires January 27, 2008 [Page 28] Internet-Draft DSKPP July 2007 MAC = DSKPP-PRF (K_MAC, "MAC 1 computation" || ID_S || R, dsLen) 4.8.2.2. Server Authentication As discussed in Section 4.3.2, servers need to authenticate themselves when attempting to replace an existing K_TOKEN. In 2-pass DSKPP, servers authenticate themselves by including a second MAC value in the AuthenticationDataType element. The MAC value in the AuthenticationDataType element MUST be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and R, using the existing MAC key K_MAC' (the MAC key that existed before this protocol run). The MAC algorithm MUST be the same as the algorithm used for key confirmation purposes. If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 computation" ID_S, 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 1 computation" || ID_S || R, dsLen) 4.8.3. One-pass DSKPP 4.8.3.1. Key Confirmation In one-pass DSKPP, the server MUST include an identifier, ID_S, for itself (via the key container) in the message. The MAC value in the message MUST be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and an unsigned integer value I, using a MAC key K_MAC. The value I MUST be monotonically increasing and guaranteed not to be used again by this server towards this cryptographic module. It could for example be the number of seconds since some point in time with sufficient granularity, a counter value, or a combination of the two where the counter value is reset for each new time value. In contrast to the MAC calculation in four-pass DSKPP, the MAC key K_MAC MUST be provided together with K_TOKEN to the cryptographic module, and hence there is no need for a K_AUTH for key confirmation purposes. Note: The integer I does not necessarily need to be maintained per cryptographic module by the DSKPP server (it is enough if the server can guarantee that the same value is never being sent twice to the same cryptographic module). If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 Doherty, et al. Expires January 27, 2008 [Page 29] Internet-Draft DSKPP July 2007 computation", ID_S, and I. 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 1 computation" || ID_S || I, dsLen) The server MUST provide I to the client in the Nonce attribute of the element of the message using the AuthenticationCodeMacType defined in Section 4.9.12. 4.8.3.2. Server Authentication As discussed in Section 4.3.2, servers need to authenticate themselves when attempting to replace an existing K_TOKEN. In 1-pass DSKPP, servers authenticate themselves by including a second MAC value in the AuthenticationDataType element. The MAC value in the AuthenticationDataType element MUST be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and a new value I', I' > I, using the existing MAC key K_MAC' (the MAC key that existed before this protocol run). The MAC algorithm MUST be the same as the algorithm used for key confirmation purposes. If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 computation" ID_S, and I'. 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 1 computation" || ID_S || I', dsLen) The server MUST provide I' to the client in the Nonce attribute of the element of the AuthenticationDataType extension. If the protocol run is successful, the client stores I' as the new value of I for this server. 4.9. DSKPP Schema Basics This section describes the schema used by DSKPP. The DSKPP XML schema itself can be found in Section 6. Specific protocol message elements are defined in Section 4.10. Examples can be found in Appendix B. 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 Doherty, et al. Expires January 27, 2008 [Page 30] Internet-Draft DSKPP July 2007 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 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. 4.9.1. The AbstractRequestType Type All DSKPP requests are defined as extensions to the abstract AbstractRequestType type. The elements of the AbstractRequestType, therefore, apply to all DSKPP requests. All DSKPP requests MUST contain a Version attribute. For this version of this specification, Version MUST be set to "1.0". 4.9.2. The AbstractResponseType Type All DSKPP responses are defined as extensions to the abstract AbstractResponseType type. The elements of the AbstractResponseType, therefore, apply to all DSKPP responses. All DSKPP responses contain a Version attribute indicating the version that was used. A Status attribute, which indicates whether the preceding request was successful or not MUST also be present. Finally, all responses MAY contain a SessionID attribute identifying the particular DSKPP session. The SessionID attribute needs only be present if more than one roundtrip is REQUIRED for a successful protocol run (this is the case with the protocol version described herein). Doherty, et al. Expires January 27, 2008 [Page 31] Internet-Draft DSKPP July 2007 4.9.3. The VersionType Type The VersionType type is used within DSKPP messages to identify the highest version of this protocol supported by the DSKPP client and server. 4.9.4. The IdentifierType Type The IdentifierType type is used to identify various DSKPP elements, such as sessions, users, and services. Identifiers MUST NOT be longer than 128 octets. 4.9.5. The StatusCode Type The StatusCode type enumerates all possible return codes: Doherty, et al. Expires January 27, 2008 [Page 32] Internet-Draft DSKPP July 2007 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 session. 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: 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. Doherty, et al. Expires January 27, 2008 [Page 33] Internet-Draft DSKPP July 2007 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. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested encryption algorithms. 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. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested MAC algorithms. o "NoProtocolVariants" indicates that the DSKPP client only suggested a protocol variant (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. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested protocol variants. o "NoSupportedKeyContainers" indicates that the DSKPP client only suggested key container formats that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested key container formats. o "AuthenticationDataInvalid" indicates that the DSKPP client supplied user or device 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. 4.9.6. The DeviceIdentifierDataType Type The DeviceIdentifierDataType type is used to uniquely identify the device that houses the cryptographic module, e.g., a mobile phone. Doherty, et al. Expires January 27, 2008 [Page 34] Internet-Draft DSKPP July 2007 The device identifier allows the DSKPP server to find, e.g., a pre- shared transport key for 2-pass DSKPP and/or the correct shared secret for MAC'ing purposes. The default DeviceIdentifierDataType is defined in [PSKC]. 4.9.7. The TokenPlatformInfoType and PlatformType Types The TokenPlatformInfoType type is used to carry characteristics of the intended cryptographic module platform, and applies in the public-key variant of DSKPP in situations when the client potentially needs to select a cryptographic module to initialize. 4.9.8. The NonceType Type The NonceType type is used to carry pseudorandom values in DSKPP messages. A nonce, as the name implies, MUST be used only once. For each DSKPP message that requires a nonce element to be sent, a fresh nonce MUST be generated each time. Nonce values MUST be at least 16 octets long. Doherty, et al. Expires January 27, 2008 [Page 35] Internet-Draft DSKPP July 2007 4.9.9. The AlgorithmsType Type The AlgorithmsType type is a list of type-value pairs that define algorithms supported by a DSKPP client or server. Algorithms are identified through URIs. 4.9.10. The ProtocolVariantsType and the TwoPassSupportType Types The ProtocolVariantsType type is OPTIONALLY used by the DSKPP client to indicate the number of passes of the DSKPP protocol that it supports (see Section 4.2). The ProtocolVariantsType MAY be used to indicate support for 4-pass or 2-pass DSKPP. Because 1-pass DSKPP does not include a client request to the server, the ProtocolVariantsType type MAY NOT be used to indicate support for 1-pass DSKPP. If the ProtocolVariantsType is not used, then the DSKPP server will proceed with ordinary 4-pass DSKPP. However, it does not support 4-pass DSKPP, then the server MUST find a suitable two-pass variant or else the protocol run will fail. Doherty, et al. Expires January 27, 2008 [Page 36] Internet-Draft DSKPP July 2007 The TwoPassSupportType type signals client support for the 2-pass version of DSKPP, informs the server of supported two-pass variants, 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 two-pass variant the payload is associated with. The elements of this type have the following meaning: o : A two-pass key initialization 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 initialization 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). 4.9.11. The KeyContainersFormatTypeType The KeyContainersFormatType type is a list of type-value pairs that are OPTIONALLY used to define key container formats supported by a DSKPP client or server. Key container formats are identified through URIs, e.g., the PSKC URI "http://www.openauthentication.org/OATH/2006/10/PSKC#KeyContainer" (see [PSKC]. Doherty, et al. Expires January 27, 2008 [Page 37] Internet-Draft DSKPP July 2007 4.9.12. The AuthenticationDataType Type The AuthenticationDataType type is OPTIONALLY used to carry client or server authentication values in DSKPP messages (see Section 4.3). The element MAY be used as follows: a. A DSKPP client MAY include a one-time use AuthenticationCode that was given by the issuer to the user for acquiring a symmetric key. An AuthenticationCode MAY or MAY NOT contain alphanumeric characters in addition to numeric digits depending on the device type and policy of the issuer. For example, if the device is a mobile phone, a code that the user enters on the keypad would typically be restricted to numeric digits for ease of use. An activation code can be sent to the DSKPP server in plaintext form, hashed data form, or keyed hash data form depending on the underlying transport protocol. b. A DSKPP client MAY include an AuthenticationCertificate that contains a certificate issued with the device by the issuer. c. A DSKPP server MAY use the AuthenticationDataType element AuthenticationCodeMac to carry a MAC for authenticating itself to the client. For example, when a successful 1- or 2-pass DSKPP protocol run will result in an existing key being replaced, then the DSKPP server MUST include a MAC proving to the DSKPP client that the server knows the value of the key it is about to replace. Doherty, et al. Expires January 27, 2008 [Page 38] Internet-Draft DSKPP July 2007 The element of the AuthenticationDataType type have the following meaning: o : A requestor's identifier. The value MAY be a user ID, a device ID, or a keyID associated with the requestor's authentication value. When the authentication data is based on a certificate, can be omitted, as the certificate itself Doherty, et al. Expires January 27, 2008 [Page 39] Internet-Draft DSKPP July 2007 is typically sufficient to identify the requestor. Also, if a message was provided by the server to initiate the DSKPP protocol run, can be omitted, as the DeviceID, KeyID, and/or nonce provided in the element ought to be sufficient to identify the requestor. o : A one-time use value sent in the clear to the DSKPP server. o : A one-time use value sent in digest form to the DSKPP server. o : An authentication MAC and OPTIONAL additional information (e.g., MAC algorithm). The value could be a one-time use value sent as a MAC value to the DSKPP server; or, it could be a MAC value sent to the DSKPP client, where the MAC is calculated as described in Section 4.8. o : A device certificate sent to the DSKPP server. 4.9.13. The PayloadType Type The PayloadType type is used to carry data in a DSKPP client or server message. For this version of the protocol, only one payload is defined, the pseudorandom string R_S, for one message, the DSKPP . 4.9.14. The MacType Type The MacType type is used by the DSKPP server to carry a MAC value that the DSKPP server uses to authenticate itself to the client. 4.9.15. The KeyContainerType Type The KeyContainerType type is used by the DSKPP server in its final message to carry symmetric key(s) (in the 2- and 1-pass exchanges) Doherty, et al. Expires January 27, 2008 [Page 40] Internet-Draft DSKPP July 2007 and configuration data. The default element defined for the KeyContainerType is contained in the namespace defined in the PSKC namespace as KeyContainerType (see [PSKC]. 4.9.16. The ExtensionsType and the AbstractExtensionType Types The ExtensionsType type is a list of type-value pairs that define OPTIONAL DSKPP features supported by a DSKPP client or server. Extensions MAY be sent with any DSKPP message. Please see the description of individual DSKPP messages in Section 4.11 of this document for applicable extensions. All DSKPP extensions are defined as extensions to the AbstractExtensionType type. The elements of the AbstractExtensionType, therefore, apply to all DSKPP extensions. Unless an extension is marked as Critical, a receiving party need not be able to interpret it. A receiving party is always free to disregard any (non-critical) extensions. 4.10. DSKPP Messages 4.10.1. Introduction In this section, DSKPP messages, including their parameters, encoding and semantics are defined. 4.10.2. DSKPP Initialization (OPTIONAL) 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 January 27, 2008 [Page 41] Internet-Draft DSKPP July 2007 Message used to trigger the device to initiate a DSKPP protocol run. 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. Doherty, et al. Expires January 27, 2008 [Page 42] Internet-Draft DSKPP July 2007 The Version attribute MUST be set to "1.0" for this version of DSKPP. 4.10.3. The DSKPP Client's Initial PDU (2- and 4-Pass) This message is the initial message sent from the DSKPP client to the DSKPP server. Doherty, et al. Expires January 27, 2008 [Page 43] Internet-Draft DSKPP July 2007 Message sent from DSKPP client to DSKPP server to initiate a DSKPP session. 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 4.3.1 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 5.2.7 below). In the latter case, it MUST have the same Doherty, et al. Expires January 27, 2008 [Page 44] Internet-Draft DSKPP July 2007 value as the identifier provided in that element. 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 was provided by the server in a element (see Section 5.2.7 below). In the latter case, it MUST have the same value as the identifier provided in that element. 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 5.2.7 below), 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 URIs indicating the key types for which the cryptographic module is willing to generate keys through DSKPP. o : A sequence of URIs 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 URIs 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., urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes defined in Appendix E). 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 URIs indicating the key container formats supported by the DSKPP client. If this element is not provided, then the DSKPP server MUST proceed with "http://www.openauthentication.org/OATH/2006/10/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 4.3.1. Doherty, et al. Expires January 27, 2008 [Page 45] Internet-Draft DSKPP July 2007 o : A sequence of extensions. One extension is defined for this message in this version of DSKPP: the ClientInfoType (see Section 4.11). 4.10.4. The DSKPP Server's Initial PDU (4-Pass Only) This message is the first message sent from 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. Message sent from DSKPP server to DSKPP client in response to a received ClientHello PDU. The components of this message have the following meaning: Doherty, et al. Expires January 27, 2008 [Page 46] Internet-Draft DSKPP July 2007 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. 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. 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 container format type to be used by the DSKPP server. The default setting relies on the KeyContainerType element defined in "urn:ietf:params:xml:schema:keyprov:container" [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 4.11). 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 Second message sent from DSKPP client to DSKPP server in a DSKPP session. 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 : MUST have the same value as the SessionID attribute in the received message. 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 4.5. o : The authentication data value, which MAY OPTIONALLY be the same as provided in the , MUST be set as specified in Section 4.3.1. o : A list of extensions. Two extensions are defined for this message in this version of DSKPP: the ClientInfoType and the ServerInfoType (see Section 4.11). 4.10.6. The DSKPP Server's Final PDU (1-, 2-, and 4-Pass) 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. In a Doherty, et al. Expires January 27, 2008 [Page 48] Internet-Draft DSKPP July 2007 1-pass exchange, the DSKPP server sends only this message to the client. Final message sent from DSKPP server to DSKPP client in a DSKPP session. 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. 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 container containing symmetric key values (in the case of a 2- or 1-pass exchange) and configuration data. The default container 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 4.11). Doherty, et al. Expires January 27, 2008 [Page 49] Internet-Draft DSKPP July 2007 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. The MAC value MUST be computed as specified in Section 4.8.1.2. When receiving a message with Status="Success" for which the MAC verifies, the DSKPP client MUST associate the generated key K_TOKEN with the provided key identifier 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. 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 DSKPP protocol. The MacType's MacAlgorithm attribute MUST, when present, identify the negotiated MAC algorithm. 4.11. Protocol Extensions 4.11.1. The ClientInfoType Type When present in a or a message, the OPTIONAL ClientInfoType extension contains DSKPP client-specific information. 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. 4.11.2. The ServerInfoType Type When present, the OPTIONAL ServerInfoType extension contains DSKPP server-specific information. This extension is only valid in messages for which Status = "Continue". DSKPP clients Doherty, et al. Expires January 27, 2008 [Page 50] Internet-Draft DSKPP July 2007 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. 4.11.3. The KeyInitializationDataType Type This extension is used for 2- and 1-pass DSKPP exchange; it carries an identifier for the selected key initialization method as well as key initialization method-dependent payload data. Servers MAY include this extension in a message that is being sent in response to a received message if and only if that message selected TwoPassSupport as the ProtocolVariantType and the client indicated support for the selected key initialization method. Servers MUST include this extension in a message that is sent as part of a 1-pass DSKPP. Doherty, et al. Expires January 27, 2008 [Page 51] Internet-Draft DSKPP July 2007 This extension is only valid in ServerFinished PDUs. It contains key initialization data and its presence results in a two-pass (or one-pass, if no ClientHello was sent) DSKPP exchange. The elements of this type have the following meaning: o : A two-pass key initialization method supported by the DSKPP client. o : A payload associated with the key initialization method. Since the syntax is a shorthand for , any well-formed payloads can be carried in this element. 5. Protocol Bindings 5.1. General Requirements DSKPP assumes a reliable transport. 5.2. HTTP/1.1 Binding for DSKPP 5.2.1. 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, on the HTTP server side, the DSKPP server MAY receive DSKPP PDUs from a "front-end" HTTP server. Doherty, et al. Expires January 27, 2008 [Page 52] Internet-Draft DSKPP July 2007 5.2.2. Identification of DSKPP Messages The MIME-type for all DSKPP messages MUST be application/vnd.ietf.keyprov.dskpp+xml 5.2.3. HTTP Headers HTTP proxies MUST NOT cache responses carrying DSKPP messages. For this reason, 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. There are no other restrictions on HTTP headers, besides the requirement to set the Content-Type header value according to Section 5.2.2. 5.2.4. HTTP Operations Persistent connections as defined in HTTP/1.1 are assumed but not required. DSKPP requests are mapped to HTTP POST operations. DSKPP responses are mapped to HTTP responses. 5.2.5. 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 the type of a DSKPP request cannot be determined, 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. Doherty, et al. Expires January 27, 2008 [Page 53] Internet-Draft DSKPP July 2007 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. 5.2.6. HTTP Authentication No support for HTTP/1.1 authentication is assumed. 5.2.7. Initialization of DSKPP The DSKPP server MAY initialize the DSKPP protocol by sending an HTTP response with Content-Type set according to Section 5.2.2 and response code set to 200 (OK). This message MAY, e.g., be sent in response to a user requesting key initialization in a browsing session. The initialization message MAY carry data in its body. If this is the case, the data MUST be a valid instance of a element. 5.2.8. 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-store Pragma: no-cache Host: 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-store Content-Type: application/vnd.ietf.keyprov.dskpp+xml Content-Length: Doherty, et al. Expires January 27, 2008 [Page 54] Internet-Draft DSKPP July 2007 DSKPP data in XML form (server random nonce, server public key, ...) 6. DSKPP Schema Doherty, et al. Expires January 27, 2008 [Page 55] Internet-Draft DSKPP July 2007 Doherty, et al. Expires January 27, 2008 [Page 56] Internet-Draft DSKPP July 2007 Doherty, et al. Expires January 27, 2008 [Page 57] Internet-Draft DSKPP July 2007 Doherty, et al. Expires January 27, 2008 [Page 58] Internet-Draft DSKPP July 2007 Doherty, et al. Expires January 27, 2008 [Page 59] Internet-Draft DSKPP July 2007 This extension is only valid in ServerFinished PDUs. It contains key initialization data and its presence results in a two-pass (or one-pass, if no ClientHello was sent) DSKPP exchange. Message used to trigger the device to initiate a DSKPP protocol run. Message sent from DSKPP client to DSKPP server to initiate a DSKPP session. Doherty, et al. Expires January 27, 2008 [Page 60] Internet-Draft DSKPP July 2007 Message sent from DSKPP server to DSKPP client in response to a received ClientHello PDU. Second message sent from DSKPP client to DSKPP server in a DSKPP session. Doherty, et al. Expires January 27, 2008 [Page 62] Internet-Draft DSKPP July 2007 Final message sent from DSKPP server to DSKPP client in a DSKPP session. 7. Security Considerations 7.1. General DSKPP is designed to protect generated key 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 and in the cryptographic module. This applies even if malicious software is present in the DSKPP client. However, as discussed in the following, 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 privacy 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. 7.2. Active Attacks 7.2.1. Introduction An active attacker MAY attempt to modify, delete, insert, replay, or reorder messages for a variety of purposes including service denial Doherty, et al. Expires January 27, 2008 [Page 63] Internet-Draft DSKPP July 2007 and compromise of generated key material. Section 7.2.2 through Section 7.2.7. 7.2.2. Message Modifications Modifications to a message will either cause denial- of-service (modifications of any of the identifiers or the nonce) or 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 7.2.7. 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 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 variant. Therefore, DSKPP servers MUST NOT accept unilaterally provided device identifiers in the public-key variant. This is also indicated in the protocol description. In the shared-key variant, 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 7.5 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 could also provide a different nonce than the one sent by the legitimate server. Clients will 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 Doherty, et al. Expires January 27, 2008 [Page 64] Internet-Draft DSKPP July 2007 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 7.5 below. Note that use of Secure Socket Layer (SSL) or TLS does not protect against this attack if the attacker has access to the DSKPP client (e.g., through malicious 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 privacy and integrity when this is a concern. 7.2.3. 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 7.5. 7.2.4. 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 Doherty, et al. Expires January 27, 2008 [Page 65] Internet-Draft DSKPP July 2007 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 privacy and integrity, coupled with the recommendations in Section 7.5, 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. 7.2.5. 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. 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. In 1-pass DSKPP clients that record the latest I used by a particular server (as identified by ID_S) will be able to detect replays. 7.2.6. 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- and 1-pass DSKPP since each party sends at most one message each. 7.2.7. 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 4.2. 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 7.3. 7.3. 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. Doherty, et al. Expires January 27, 2008 [Page 66] Internet-Draft DSKPP July 2007 If DSKPP is not run over a transport providing privacy, a passive attacker may learn: o What cryptographic modules a particular user is in possession of; o The identifiers of keys on those crypotgraphic modules and other attributes pertaining to those keys, e.g., the lifetime of the keys; and o DSKPP versions and cryptographic algorithms supported by a particular DSKPP client or server. Whenever the above is a concer, DSKPP SHOULD be run over a transport providing privacy. If man-in-the-middle attacks for the purposes described above are a concern, the transport SHOULD also offer server-side authentication. 7.4. 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 4.7 and Section 4.10 contain discussions of this threat and steps RECOMMENDED to protect against it. 7.5. 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 variant or the shared-secret variant 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- and 1-pass DSKPP as the client does not provide any entropy to K_TOKEN. The attack as such (and its countermeasures) still applies to 2- and 1-pass DSKPP, however, as it essentially is a man-in-the-middle attack. Doherty, et al. Expires January 27, 2008 [Page 67] Internet-Draft DSKPP July 2007 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. 7.6. Additional Considerations Specific to 2- and 1-pass DSKPP 7.6.1. 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 1- and 2-pass DSKPP versions 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 variant. Server implementations SHOULD therefore take extreme care to ensure that this situation does not occur. 7.6.2. Key Confirmation 4-pass DSKPP servers provide key confirmation through the MAC on R_C in the message. In the 1- and 2-pass DSKPP variants described herein, key confirmation is provided by the MAC including I (in the 1-pass case) or R (2-pass case), using K_MAC. 7.6.3. Server Authentication DSKPP servers MUST authenticate themselves whenever a successful DSKPP 1- or 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 1- and 2-pass DSKPP, servers authenticate by including the AuthenticationDataType extension containing a MAC as described in Section 4.8 above. 7.6.4. Client 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: Doherty, et al. Expires January 27, 2008 [Page 68] Internet-Draft DSKPP July 2007 o When a device certificate is used for client authentication, the DSKPP server SHOULD follow standard certificate verification processes to ensure that it is a trusted device. o When an Authentication Code is used for client authentication, a password dictionary attack on the authentication data is possible. When a secure channel, e.g., SSL or TLS, is established between a DSKPP client and server, an attacker could successfully brute- force guess an Authentication Code, allowing him to illegitimately receive K_TOKEN. o The length the 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's with a DSKPP server's nonce value. The Authentication Code and nonce value MUST be strong enough to prevent offline brute-force recovery of the Authentication Code from the HMAC data. Because the nonce value is almost public across a non-secure channel, the key strength is dependent on the Authentication Code. 7.6.5. Key Protection in the Passphrase Profile The passphrase-based key wrap profile uses the PBKDF2 function from [PKCS-5] to generate an encryption key from a passphrase and salt string. The derived key, K_DERIVED is used by the server to encrypt K_TOKEN and by the cryptographic module to decrypt the newly delivered K_TOKEN. 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. Doherty, et al. Expires January 27, 2008 [Page 69] Internet-Draft DSKPP July 2007 o User pre-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 passphrase-based key wrap method is not weakened. o Transport level security (e.g. TLS) SHOULD be used where possible to protect a 2-pass or 1-pass protocol run. Transport level security provides a second layer of protection for the newly generated K_TOKEN. 8. IANA Considerations This document calls for registration of new URNs within the IETF sub- namespace per RFC3553 [RFC3553]. The following URNs are RECOMMENDED: o DSKPP XML schema: "urn:ietf:params:xml:schema:keyprov:protocol" o DSKPP XML namespace: "urn:ietf:params:xml:ns:keyprov:protocol" 9. 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. 10. Acknowledgements The authors would like to thank all the members of OATH [OATH] and participants of OTPS workshops for their review and comments related to this document. 11. References 11.1. Normative references [UNICODE] Davis, M. and M. Duerst, "Unicode Normalization Forms", March 2001, . Doherty, et al. Expires January 27, 2008 [Page 70] Internet-Draft DSKPP July 2007 [XMLDSIG] W3C, "XML Signature Syntax and Processing", W3C Recommendation, February 2002, . [XMLENC] W3C, "XML Encryption Syntax and Processing", W3C Recommendation, December 2002, . 11.2. 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. [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, . [FSE2003] Iwata, T. and K. Kurosawa, "OMAC: One-Key CBC MAC. In Fast Software Encryption", FSE 2003, Springer-Verlag , 2003, . [NIST-PWD] National Institute of Standards and Technology, "Password Usage", FIPS 112, May 1985, . [OATH] "Initiative for Open AuTHentication", 2005, . [PKCS-1] RSA Laboratories, "RSA Cryptography Standard", PKCS #1 Version 2.1, June 2002, . [PKCS-11] RSA Laboratories, "Cryptographic Token Interface Doherty, et al. Expires January 27, 2008 [Page 71] Internet-Draft DSKPP July 2007 Standard", PKCS #11 Version 2.20, June 2004, . [PKCS-12] "Personal Information Exchange Syntax Standard", PKCS #12 Version 1.0, 2005, . [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, . [PSKC] "Portable Symmetric Key Container", 2005, . [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, . [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, . [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An IETF URN Sub-namespace for Registered Protocol Parameters", RFC 3553, BCP 73, June 2003. [RFC4758] RSA, The Security Division of EMC, "Cryptographic Token Key Initialization Protocol (CT-KIP)", November 2006, . Appendix A. Key Initialization Profiles of DSKPP Doherty, et al. Expires January 27, 2008 [Page 72] Internet-Draft DSKPP July 2007 A.1. Introduction This appendix introduces three profiles of DSKPP for key initialization. They MAY all be used for two- as well as one-pass initialization of cryptographic modules. Further profiles MAY be defined by external entities or through the IETF process. A.2. Key Transport Profile A.2.1. Introduction This profile initializes the cryptographic module with a symmetric key, K_TOKEN, through key transport and key derivation. The key transport is carried out using a public key, K_CLIENT, whose private key part resides in the cryptographic module as the transport key. A key K from which two keys, K_TOKEN and K_MAC are derived MUST be transported. A.2.2. Identification This profile MUST be identified with the following URN: urn:ietf:params:xml:schema:keyprov:protocol#transport A.2.3. Payloads In the two-pass version of DSKPP, the client MUST send a payload associated with this key initialization method. The payload MUST be of type ds:KeyInfoType ([XMLDSIG]), and only those choices of the ds: KeyInfoType that identify a public key are allowed. The ds: X509Certificate option of the ds:X509Data 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 initialization method MUST be of type xenc:EncryptedKeyType ([XMLENC]), and only those encryption methods utilizing a public key that are supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP) are allowed as values for the element. Further, in the case of 2-pass DSKPP, the element MUST contain the same value (i.e. identify the same public key) as the of the corresponding supported key initialization method in the message that triggered the response. The element MAY be present, but MUST, when present, contain the same value as the element of the message. The Type attribute of the xenc:EncryptedKeyType MUST be Doherty, et al. Expires January 27, 2008 [Page 73] Internet-Draft DSKPP July 2007 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, or as otherwise known in the case of 1-pass DSKPP). The transported key 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, or as otherwise known in the case of 1-pass DSKPP). The MAC MUST be calculated as described in Section 4.8 In addition, DSKPP servers MUST include the AuthenticationDataType element (see further Section 4.8) in their messages whenever a successful protocol run will result in an existing K_TOKEN being replaced. A.3. Key wrap profile A.3.1. Introduction This profile initializes the cryptographic module with a symmetric key, K_TOKEN, through key wrap and key derivation. The key wrap MUST be carried out using a (symmetric) key-wrapping key, K_SHARED, known in advance by both the cryptographic module and the DSKPP server. A key K from which two keys, K_TOKEN and K_MAC are derived MUST be wrapped. A.3.2. Identification This profile MUST be identified with the following URI: urn:ietf:params:xml:schema:keyprov:protocol#wrap A.3.3. Payloads In the 2-pass version of DSKPP, the client MUST send a payload associated with this key initialization method. The payload MUST be Doherty, et al. Expires January 27, 2008 [Page 74] Internet-Draft DSKPP July 2007 of type ds:KeyInfoType ([XMLDSIG]), and only those choices of the ds: KeyInfoType that identify a symmetric key are allowed. The ds: KeyName alternative is RECOMMENDED. The server payload associated with this key initialization method MUST be of type xenc:EncryptedKeyType ([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, or as otherwise known in the case of 1-pass DSKPP) are allowed as values for the element. Further, in the case of 2-pass DSKPP, the element MUST contain the same value (i.e. identify the same symmetric key) as the of the corresponding supported key initialization method in the message that triggered the response. The element MAY be present, and MUST, when present, contain the same value as the element of the message. The Type attribute of the xenc:EncryptedKeyType 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, or as otherwise known in the case of 1-pass DSKPP). The wrapped key 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. DSKP 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, or as otherwise known in the case of 1-pass DSKPP). The MAC MUST be calculated as described in Section 4.8 In addition, DSKPP servers MUST include the AuthenticationDataType element (see further Section 4.8) in their messages whenever a successful protocol run will result in an existing K_TOKEN being replaced. Doherty, et al. Expires January 27, 2008 [Page 75] Internet-Draft DSKPP July 2007 A.4. Passphrase-based key wrap profile A.4.1. Introduction This profile is a variation of the key wrap profile. It initializes the cryptographic module with a symmetric key, K_TOKEN, through key wrap and key derivation, using a passphrase-derived key-wrapping key, K_DERIVED. The passphrase is known in advance by both the device user 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 key K from which two keys, K_TOKEN and K_MAC are derived MUST be wrapped. A.4.2. Identification This profile MUST be identified with the following URI: urn:ietf:params:xml:schema:keyprov:protocol#passphrase-wrap A.4.3. Payloads In the 2-pass version of DSKPP, the client MUST send a payload associated with this key initialization method. The payload MUST be of type ds:KeyInfoType ([XMLDSIG]). The ds:KeyName 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 initialization method MUST be of type xenc:EncryptedKeyType ([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, or as otherwise known in the case of 1-pass DSKPP) are allowed as values for the element. Further, in the case of 2-pass DSKPP, the element MUST contain the same value (i.e. identify the same passphrase) as the of the corresponding supported key initialization method in the message that triggered the response. The element MAY be present, and MUST, when present, contain the same value as the element of the message. The Type attribute of the xenc: EncryptedKeyType MUST be present and MUST identify the type of the wrapped key. The type MUST be one of the types supported by the Doherty, et al. Expires January 27, 2008 [Page 76] Internet-Draft DSKPP July 2007 DSKPP client (as reported in the of the preceding message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The wrapped key 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 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, or as otherwise known in the case of 1-pass DSKPP). The MAC MUST be calculated as described in Section 4.8 In addition, DSKPP servers MUST include the AuthenticationDataType element (see further Section 4.8) in their messages whenever a successful protocol run will result in an existing K_TOKEN being replaced. Appendix B. Example Messages All examples are syntactically correct. MAC and cipher values are fictitious however. B.1. Example Messages in a Four-pass Exchange The examples below illustrate a complete four-pass DSKPP exchange. Doherty, et al. Expires January 27, 2008 [Page 77] Internet-Draft DSKPP July 2007 B.1.1. Example of a DSKPP Initialization (Trigger) Message ManufacturerABC XL0000000001234 U2 112dsdfwf312asder394jw== Doherty, et al. Expires January 27, 2008 [Page 78] Internet-Draft DSKPP July 2007 B.1.2. Example of a Message ManufacturerABC XL0000000001234 U2 112dsdfwf312asder394jw== http://www.rsa.com/rsalabs/otps/schemas/2005/09/ otps-wst#SecurID-AES http://www.openauthentication.org/OATH/2006/10/PSKC# HOTP http://www.w3.org/2001/05/xmlenc#rsa_1_5 urn:ietf:params:xml:schema:keyprov:protocol# dskpp-prf-aes urn:ietf:params:xml:schema:keyprov:protocol# dskpp-prf-aes FourPass urn:ietf:params:xml:schema:keyprov:container 1erd354657689102abcd Doherty, et al. Expires January 27, 2008 [Page 79] Internet-Draft DSKPP July 2007 B.1.3. Example of a Message http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst# SecurID-AES urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes KEY-1 urn:ietf:params:xml:schema:keyprov:container qw2ewasde312asder394jw== B.1.4. Example of a Message VXENc+Um/9/NvmYKiHDLaErK0gk= 1erd354657689102abcd B.1.5. Example of a Message Doherty, et al. Expires January 27, 2008 [Page 80] Internet-Draft DSKPP July 2007 CredentialIssuer MyFirstToken Time 10/30/2009 miidfasde312asder394jw== B.2. Example Messages in a Two- or One-pass Exchange The examples illustrate a complete two-pass DSKPP exchange. The server messages MAY also constitute the only messages in a one-pass DSKPP exchange. B.2.1. Example of a Message Indicating Support for Two- pass DSKPP The client indicates support both for the two-pass key transport variant as well as the two-pass key wrap variant. Doherty, et al. Expires January 27, 2008 [Page 81] Internet-Draft DSKPP July 2007 ManufacturerABC XL0000000001234 U2 1523sdfxe798jowie913ol== http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst# SecurID-AES http://www.openauthentication.org/OATH/2006/10/PSKC#HOTP 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 urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes urn:ietf:params:xml:schema:keyprov:protocol# dskpp-prf-aes urn:ietf:params:xml:schema:keyprov:protocol#wrap Key_001 urn:ietf:params:xml:schema:keyprov:protocol#transport miib Doherty, et al. Expires January 27, 2008 [Page 82] Internet-Draft DSKPP July 2007 urn:ietf:params:xml:schema:keyprov:container 1erd354657689102abcd B.2.2. Example of a Message Using the Key Transport Profile In this example, the server responds to the previous request using the key transport profile. Doherty, et al. Expires January 27, 2008 [Page 83] Internet-Draft DSKPP July 2007 43212093< miib CredentialIssuer MyFirstToken 7JHUyp3azOkqJENSsh6b2vxXzwGBYypzJxEr+ikQAa229KV/BgZhGA== 9AEDpd4td44mRSASab625oPqlvHHIplzADer+pPOlL118JW/AhIoHB== 1 10/30/2009 miidfasde312asder394jw== Doherty, et al. Expires January 27, 2008 [Page 84] Internet-Draft DSKPP July 2007 B.2.3. Example of a Message Using the Key Wrap Profile In this example, the server responds to the previous request using the key wrap profile. Doherty, et al. Expires January 27, 2008 [Page 85] Internet-Draft DSKPP July 2007 43212093 Key-001 CredentialIssuer MyFirstToken 7JHUyp3azOkqJENSsh6b2vxXzwGBYypzJxEr+ikQAa229KV/BgZhGA== 9AEDpd4td44mRSASab625oPqlvHHIplzADer+pPOlL118JW/AhIoHB== 1 10/30/2009 miidfasde312asder394jw== B.2.4. Example of a Message using the Passphrase-based Key Wrap Profile In this example, the server responds to the previous request using Doherty, et al. Expires January 27, 2008 [Page 86] Internet-Draft DSKPP July 2007 the passphrase-based key wrap profile. 32113435 1024 128 43212093 Passphrase1 CredentialIssuer MyFirstToken 7JHUyp3azOkqJENSsh6b2vxXzwGBYypzJxEr+ikQAa229KV/BgZhGA== Doherty, et al. Expires January 27, 2008 [Page 87] Internet-Draft DSKPP July 2007 9AEDpd4td44mRSASab625oPqlvHHIplzADer+pPOlL118JW/AhIoHB== 1 10/30/2009 miidfasde312asder394jw== Appendix C. Requirements This section specifies mandatory and desirable protocol requirements. Req-1: The protocol MUST support provisioning of keys for use with multiple types of symmetric cryptographic algorithms. Req-2: The protocol MUST support pre-generated symmetric keys (by separate key issuance service) or locally generated keys in real- time (by provisioning server). Req-3: The protocol MUST support mutually generated symmetric keys by both client and server (i.e., joint key control). Req-4: The protocol MUST allow cryptographic modules to acquire multiple symmetric keys; each key MAY be acquired in a separate provisioning session. Doherty, et al. Expires January 27, 2008 [Page 88] Internet-Draft DSKPP July 2007 Req-5: The protocol MUST support renewal of a symmetric key with the original key ID. Req-6: The protocol MUST allow clients to specify their cryptographic capabilities to the server and the server to indicate the cryptography and algorithm types that it will be using. Req-7: The protocol MUST support mutual authentication and confidentiality of sensitive data during provisioning. Req-8: The protocol MAY use a public-key infrastructure and the use of client certificates for device authentication or symmetric key data protection. The protocol MUST allow for other mechanisms, such as symmetric key-based techniques, to be used. Req-9: The protocol SHOULD NOT only rely on transport layer security. It SHOULD be compatible with transport layer security when available. Req-10: The protocol SHOULD allow the server to use pre-loaded symmetric transport keys if available on the device that hosts the cryptographic module (i.e., smart card update keys, such as used by Global Platform for establishing a secure channel). Req-11: The protocol MUST protect against replay attacks. Req-12: The protocol MUST protect against MITM attacks. Req-13: The protocol MAY support a cryptographic module request to acquire multiple symmetric keys in the same session. Doherty, et al. Expires January 27, 2008 [Page 89] Internet-Draft DSKPP July 2007 Req-14: The protocol MAY allow the provisioning server to verify that the key has been correctly provisioned to the cryptographic module (i.e., key confirmation). Req-15: The protocol MAY allow a cryptographic module to notify the provisioning server upon symmetric key deletion. Req-16: The protocol MAY limit a protocol run to complete within a certain time window. Req-17: The protocol MAY support download of a key to a cryptographic module via SMS depending upon whether the application can provide an acceptable level of protection for transport of the symmetric key. The following is a list of features that are not required by the protocol: Non-Req-1: Support for cryptographic module generated symmetric key upload to a provisioning server. Non-Req-2: Support for other key lifecycle management functions, such as key suspension, lock, and activation. These functions are supported in a symmetric key-based application, such as an authentication system. Non-Req-3: Support for asymmetric key pair provisioning. Appendix D. Integration with PKCS #11 A DSKPP client that needs to communicate with a conncected cryptographic module to perform a DSKPP exchange MAY use PKCS #11 Doherty, et al. Expires January 27, 2008 [Page 90] Internet-Draft DSKPP July 2007 [PKCS-11]as a programming interface. D.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. D.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, 1. A generic key K = 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 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 by calling C_DeriveKey using the CKM_EXTRACT_KEY_FROM_KEY mechanism, this time setting CK_EXTRACT_PARAMS to the length of K (in bits) divided by two. 2. The server wraps K with either the token's public key K_CLIENT, the shared secret key K_SHARED, or the derived shared secret key K_DERIVED 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 wrapping key. Doherty, et al. Expires January 27, 2008 [Page 91] Internet-Draft DSKPP July 2007 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 ID_S 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 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 ulSeeidLen parameter MUST be set to zero. In the call to C_Sign, the pData parameter MUST be set to the concatenation of the string ID_S 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, 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 MUST be destroyed after deriving K_TOKEN and K_MAC. Doherty, et al. Expires January 27, 2008 [Page 92] Internet-Draft DSKPP July 2007 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 ID_S 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. Again, if the MAC does not 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. D.3. The 1-pass Variant A suggested procedure to perform 1-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 server side, 1. A generic key K = 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 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 by calling C_DeriveKey using the CKM_EXTRACT_KEY_FROM_KEY mechanism, this time setting CK_EXTRACT_PARAMS to the length of K (in bits) divided by two. Doherty, et al. Expires January 27, 2008 [Page 93] Internet-Draft DSKPP July 2007 2. The server wraps K with either the token's public key, K_CLIENT, the shared secret key, K_SHARED, or the derived shared secret key, K_DERIVED 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 octet-string representation of an integer I whose value MUST be incremented before each protocol run, and the ulSeedLen parameter MUST indicate the length of the octet-string representation of I. The hWrappingKey parameter in the call to C_WrapKey MUST be set to refer to the wrapping key. Note: The integer-to-octet string conversion MUST be made using the I2OSP primitive from [PKCS-1]. There MUST be no leading zeros. 3. For the server's message to the client, 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 ID_S and the octet-string representation of the integer I, 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 as usual, and MUST be equal to, or greater than, sixteen (16). 4. If the server also needs to authenticate its message (due to an existing K_TOKEN being replaced), the server calculates a second MAC. If the DSKPP MAC mechanism is used, the server does this by calling C_SignInit with the CKM_KIP_MAC mechanism followed by a call to C_Sign. In the call to C_SignInit, the K_MAC existing on the token before this protocol run 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 ID_S and the octet-string representation of the integer I+1 (i.e. I MUST be incremented before each use), 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 as usual, and MUST be equal to, or Doherty, et al. Expires January 27, 2008 [Page 94] Internet-Draft DSKPP July 2007 greater than, sixteen (16). 5. The server sends its message to the client, including the MAC and possibly also the authenticating MAC. b. 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 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 ID_S and the octet-string representation of the provided value for I, 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 or if the provided value of I is not larger than any stored value I' for the identified server ID_S 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. If the verification succeeds, the token MUST store the provided value of I as a new I' for ID_S. 3. If an authenticating MAC was received (REQUIRED if K_TOKEN will replace an existing key on the token), it is verified in a similar vein but using the K_MAC existing before the protocol run. Again, if the MAC does not 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 E. Example of DSKPP-PRF Realizations Doherty, et al. Expires January 27, 2008 [Page 95] Internet-Draft DSKPP July 2007 E.1. Introduction This example appendix defines DSKPP-PRF in terms of AES [FIPS197-AES] and HMAC [RFC2104]. E.2. DSKPP-PRF-AES E.2.1. Identification For cryptographic modules supporting this realization of DSKPP-PRF, the following URI MAY be used to identify this algorithm in DSKPP: urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes When this URI is used to identify the encryption algorithm to use, the method for encryption of R_C values described in Section 4.7 MUST be used. E.2.2. Definition DSKPP-PRF-AES (k, s, dsLen) Input: k Encryption keyto 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) 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 = ROUND( dsLen / bLen) j = dsLen - (n - 1) * bLen Doherty, et al. Expires January 27, 2008 [Page 96] Internet-Draft DSKPP July 2007 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 OMAC1 construction from [FSE2003], using AES as the block cipher: F (k, s, i) = OMAC1-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 OMAC1 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. E.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) = OMAC1-AES (k, INT (1) || s) E.3. DSKPP-PRF-SHA256 E.3.1. Identification For cryptographic modules supporting this realization of DSKPP-PRF, the following URI MAY be used to identify this algorithm in DSKPP: urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-sha256 When this URI is used to identify the encryption algorithm to use, the method for encryption of R_C values described in Section 4.7 MUST be used. Doherty, et al. Expires January 27, 2008 [Page 97] Internet-Draft DSKPP July 2007 E.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 = ROUND( 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 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: Doherty, et al. Expires January 27, 2008 [Page 98] Internet-Draft DSKPP July 2007 DS = B1 || B2 || ... || Bn<0..j-1> Output the derived data DS. E.3.3. Example If we assume that sLen = 256 (two 128-octet long values) and dsLen = 16, then: n = ROUND ( 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 Email: adoherty@rsa.com Mingliang Pei VeriSign, Inc. Email: mpei@verisign.com Magnus Nystroem RSA, The Security Division of EMC Email: magnus@rsa.com Salah Machani Diversinet Corp. Email: smachani@diversinet.com Doherty, et al. Expires January 27, 2008 [Page 99] Internet-Draft DSKPP July 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Doherty, et al. Expires January 27, 2008 [Page 100]