INTERNET-DRAFT R. Housley Internet Engineering Task Force (IETF) Vigil Security Intended Status: Standards Track T. Polk NIST S. Hartman Painless Security D. Zhang Huawei Expires: 22 April 2014 22 October 2013 Database of Long-Lived Symmetric Cryptographic Keys Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and 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." 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Housley, et al [Page 1] INTERNET-DRAFT October 21, 2013 Abstract This document specifies the information contained in a conceptual database of long-lived cryptographic keys used by many different routing protocols for message security. The database is designed to support both manual and automated key management. In addition to describing the schema for the database, this document describes the operations that can be performed on the database as well as the requirements for the routing protocols that wish to use the database. In many typical scenarios, the protocols do not directly use the long-lived key, but rather a key derivation function is used to derive a short-lived key from a long-lived key. 1. Introduction This document specifies the information that needs to be included in a database of long-lived cryptographic keys in order to key the authentication of cryptographic authentication for routing protocols. This conceptual database is designed to separate protocol-specific aspects from both manual and automated key management. The intent is to allow many different implementation approaches to the specified cryptographic key database, while simplifying specification and heterogeneous deployments. This conceptual database avoids the need to build knowledge of any security protocol into key management protocols. It minimizes protocol-specific knowledge in operational/management interfaces, but it constrains where that knowledge can appear. Textual conventions are provided for the representation of keys and other identifiers. These conventions should be used when presenting keys and identifiers to operational/management interfaces or reading keys/identifiers from these interfaces. It is an operational requirement that all implementations represent the keys and key identifiers in the same way so that cross-vendor configuration instructions can be provided. Routing protocols can employ the services of more generic security protocols such as TCP-AO [RFC5925]. Implementations of routing protocols may need to supply keys to databases specific to these security protocols as the associated entries in this document's conceptual database are manipulated. In many instances, the long-lived keys are not used directly in security protocols, but rather a key derivation function is used to derive short-lived key from the long-lived keys in the database. In other instances, security protocols will directly use the long-lived key from the database. The database design supports both use cases. Housley, et al [Page 2] INTERNET-DRAFT October 21, 2013 1.1. Requirements Notation 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 RFC 2119 [RFC2119]. 2. Conceptual Database Structure The database is characterized as a table, where each row represents a single long-lived symmetric cryptographic key. Normally, each key should only have one row. Only in the (hopefully) very rare cases where a key is used for more than one purpose, or where the same key is used with multiple key derivation functions (KDFs) will multiple rows contain the same key value. The columns in the table represent the key value and attributes of the key. To accommodate manual key management, the format of the fields has been purposefully chosen to allow updates with a plain text editor and to provide equivalent display on multiple systems. The columns that the table consists of are listed as follows: AdminKeyName The AdminKeyName field contains a string identifying the key by humans. The same string can be used on the local system and peer systems, but this is not required. Protocols do not make use of this string; protocols use the LocalKeyName and the PeerKeyName. Implementations can use this field to uniquely identify rows in the key table. LocalKeyName The LocalKeyName field contains a string identifying the key. It can be used to retrieve the key in the local database when received in a message. As discussed in Section 4, the protocol defines the form of this field. For example, many routing protocols restrict the format of their key names to integers that can be represented in 16 or 32 bits. Typically this field does not contain data in human character sets requiring internationalization. If there ever are any Protocols with key names requiring internationalization, those specifications need to address issues of canonicalization and normalization so that key names can be compared using binary comparison. Housley, et al [Page 3] INTERNET-DRAFT October 21, 2013 PeerKeyName For unicast communication, the PeerKeyName of a key on a system matches the LocalKeyName of the identical key that is maintained on one or multiple peer systems. Similar to LocalKeyName, a protocol defines the form of this identifier and will often restrict it to be an integer. For group keys, the protocol will typically require this field be an empty string as the sending and the receiving key names need to be the same. Peers Typically for unicast keys, this field lists the peer systems that have this key in their database. For group keys this field names the groups for which the key is appropriate. For example, this might name a routing area for a multicast routing protocol. Formally, this field provides a protocol-specific set of restrictions on the scope in which the key is appropriate. The format of the identifiers in the Peers field is specified by the protocol. Interfaces The Interfaces field identifies the set of physical and/or virtual interfaces for which it is appropriate to use this key. When the long-lived value in the Key field is intended for use on any interface, this field is set to "all". The interfaces field consists of a set of strings; the form of these strings is specified by the implementation and is independent of the protocol in question. Protocols may require support for the interfaces field or may indicate that support for constraining keys based on interface is not required. As an example, TCP-AO implementations are unlikely to make the decision of what interface to use prior to key selection. In this case, the implementations are expected to use the same keying material across all of the interfaces and then require the "all" setting. Protocol The Protocol field identifies a single security protocol where this key may be used to provide cryptographic protection. This specification establishes a registry for this field; the registry also specifies the format of the following field, ProtocolSpecificInfo, for each registered protocol. ProtocolSpecificInfo This field contains the protocol-specified information which may be useful for a protocol to apply the key correctly. Note that such information must not be required for a protocol to Housley, et al [Page 4] INTERNET-DRAFT October 21, 2013 locate an appropriate key. When a protocol does not need the information in ProtocolSpecificInfo, it will require this field be empty. Key table rows MAY specify a direction of "both". As a result, the encoding of this field needs to support encoding protocol specific information for sending and receiving in the same row. KDF The KDF field indicates the key derivation function which is used to generate short-lived keys from the long-lived value in the Key field. When the long-lived value in the Key field is intended for direct use, the KDF field is set to "none". A key derivation function is a one-way function that provides cryptographic separation of key material. The KDF MAY use inputs from the row in the key table and the message being sent or received but MUST NOT depend on other configuration state. This document establishes an IANA registry for the values in the KDF field to simplify references in future specifications. The protocol indicates what (if any) KDFs are valid. AlgID The AlgID field indicates which cryptographic algorithm to be used with the security protocol for the specified peer or peers. Such an algorithm can be an encryption algorithm and mode (e.g., AES-128-CBC), an authentication algorithm (e.g., HMAC-SHA1-96 or AES-128-CMAC), or any other symmetric cryptographic algorithm needed by a security protocol. If the KDF field contains "none", then the long-lived key is used directly with this algorithm, otherwise the derived short-lived key is used with this algorithm. When the long-lived key is used to generate a set of short-lived keys for use with the security protocol, the AlgID field identifies a ciphersuite rather than a single cryptographic algorithm. This document establishes an IANA registry for the values in the AlgID field to simplify references in future specifications. Protocols indicate which algorithms are appropriate. Key The Key field contains a long-lived symmetric cryptographic key in the format of a lower-case hexadecimal string. The size of the Key depends on the KDF and the AlgID. For instance, a KDF=none and AlgID=AES128 requires a 128-bit key, which is represented by 32 hexadecimal digits. Direction The Direction field indicates whether this key may be used for inbound traffic, outbound traffic, both, or whether the key has been disabled and may not currently be used at all. The Housley, et al [Page 5] INTERNET-DRAFT October 21, 2013 supported values are "in", "out", "both", and "disabled", respectively. The Protocol field will determine which of these values are valid. SendLifetimeStart The SendLifetimeStart field specifies the earliest date and time in Coordinated Universal Time (UTC) at which this key should be considered for use when sending traffic. The format is YYYYMMDDHHSSZ, where four digits specify the year, two digits specify the month, two digits specify the day, two digits specify the hour, two digits specify the minute, and two digits specify the second. The "Z" is included as a clear indication that the time is in UTC. SendLifeTimeEnd The SendLifeTimeEnd field specifies the latest date and time at which this key should be considered for use when sending traffic. The format is the same as the SendLifetimeStart field. AcceptLifeTimeStart The AcceptLifeTimeStart field specifies the earliest date and time in Coordinated Universal Time (UTC) at which this key should be considered for use when processing received traffic. The format is YYYYMMDDHHSSZ, where four digits specify the year, two digits specify the month, two digits specify the day, two digits specify the hour, two digits specify the minute, and two digits specify the second. The "Z" is included as a clear indication that the time is in UTC. AcceptLifeTimeEnd The AcceptLifeTimeEnd field specifies the latest date and time at which this key should be considered for use when processing the received traffic. The format of this field is identical to the format of AcceptLifeTimeStart. 3. Key Selection and Rollover A protocol may directly consult the key table to find the key to use on an outgoing message. The protocol provides a protocol (P) and a peer identifier (H) into the key selection function. Optionally, an interface identifier (I) may also need to be provided. Any key that satisfies the following conditions may be selected: (1) the Peers field includes H; (2) the Protocol field matches P; Housley, et al [Page 6] INTERNET-DRAFT October 21, 2013 (3) If an interface is specified by the protocol, the Interfaces field in the key table row includes I or "all"; (4) the Direction field is either "out" or "both"; and (5) SendLifetimeStart <= current time <= SendLifeTimeEnd. During key selection, multiple entries may simultaneously exist associated with different cryptographic algorithms or ciphersuites. Systems should support selection of keys based on algorithm preference to facilitate algorithm transition. In addition, multiple entries with overlapping valid periods are expected to be available for orderly key rollover. In these cases, the expectation is that systems will transition to the newest key available. To meet this requirement, this specification recommends supplementing the key selection algorithm with the following differentiation: select the long-lived key specifying the most recent time in the SendLifetimeStart field. Housley, et al [Page 7] INTERNET-DRAFT October 21, 2013 In order to look up a key for verifying an incoming message, the protocol provides its protocol (P), the peer identifier (H), the key identifier (L), and optionally the interface (I). If one key matches the following conditions it is selected: (1) the Peer field includes H; (2) the Protocol field matches P; (3) if the Interface field is provided, it includes I or is "all"; (4) the Direction field is either "in" or "both"; (5) the LocalKeyName is L; and (6) AcceptLifeTimeStart <= current time <= AcceptLifeTimeEnd. Note that the key usage is loosely bound by the times specified in the AcceptLifeTimeStart and AcceptLifeTimeEnd fields. New security associations should not be established except within the period of use specified by these fields, while allowing some grace time for clock skew. However, if a security association has already been established based on a particular long-lived key, exceeding the lifetime does not have any direct impact. The implementations of security protocols that involve long-lived security association should be designed to periodically interrogate the database and rollover to new keys without tearing down the security association. Rather than consulting the conceptual database, a security protocol such as TCP-AO may update its own tables as keys are added and removed. In this case, the protocol needs to maintain its own key information. Some routing protocols use IP Security (IPsec) to provide integrity. If a specification describes how to use the conceptual database described in this document to configure keys for these routing protocols, similar concerns apply. The specification mapping those routing protocols onto this conceptual database needs to describe how the Security Policy Database is manipulated as rows are added and removed from the conceptual database. 4. Application of the Database in a Security Protocol In order to use the key table database in a protocol specification, a protocol needs to specify certain information. This section enumerates items that a protocol must specify. (1) The ways of mapping the information in a key table row to the information needed to produce an outgoing message; specified Housley, et al [Page 8] INTERNET-DRAFT October 21, 2013 either as an explanation of how to fill in authentication-related fields in a message based on key table information, or for protocols such as TCP-AO how to construct Master Key Tuples (MKTs) or other protocol-specific structures from a key table row (2) The ways of locating the peer identifier (a member of the Peers set) and the LocalKeyName inside an incoming message (3) The methods of verifying a message given a key table row; this may be stated directly or in terms of protocol-specific structures such as MKTs (4) The form and validation rules for LocalKeyName and PeerKeyName; if either of these is an integer, the conventions in Section 5.1 are used as a vendor-independent format (5) The form and validation rules for members of the Peers set (6) The algorithms and KDFs supported (7) The form of the ProtocolSpecifics field (8) The rules for canonicalizing LocalKeyName, PeerKeyName, entries in the Peers set, or ProtocolSpecifics; this may include normalizations such as lower-casing hexadecimal strings (9) The Indication whether the support for Interfaces is required by this protocol The form of the interfaces field is not protocol-specific but instead is shared among all protocols on an implementation. If a protocol needs to distinguish instances running over the same interface, this is included in the specification of peers. Generally it is desirable to define the specification of peers so that an operator can use the interfaces field to refer to all instances of a protocol on a link without having to specify both generic interfaces information and protocol-specific peer information. 5. Textual Conventions 5.1 Key Names When a key for a given protocol is identified by an integer key identifier, the associated key name will be represented as lower case hexadecimal integers with the most significant octet first. This integer is padded with leading 0's until the width of the key identifier field in the protocol is reached. Housley, et al [Page 9] INTERNET-DRAFT October 21, 2013 5.2 Keys A key is represented as a lower-case hexadecimal string with the most significant octet of the key first. As discussed in Section 2, the length of this string depends on the associated algorithm and KDF. 6. Operational Considerations If the valid periods for long-lived keys do not overlap or the system clocks are inconsistent, it is possible to construct scenarios where systems cannot agree upon a long-lived key. When installing a series of keys to be used one after another, operators should configure the SendLifetimeStart field of the key to be several hours after the AcceptLifeTimeStart field of the key to guarantee there is some overlap. This overlap is intended to address the clock skew issue and allow for basic operational considerations. Operators may choose to specify a longer overlap (e.g., several days) to allow for exceptional circumstances. 7. Security Considerations Management of encryption and authentication keys has been a significant operational problem, both in terms of key synchronization and key selection. For instance, the current guidance [RFC3562] warns against sharing TCP MD5 keying material between systems, and recommends changing keys according to a schedule. The same general operational issues are relevant for the management of other cryptographic keys. It has been recognized in [RFC4107] that automated key management is not viable in multiple scenarios. The conceptual database specified in this document is designed to accommodate both manual key management and automated key management. A future specification to automatically populate rows in the database is envisioned. Designers should recognize the warning provided in [RFC4107]: Automated key management and manual key management provide very different features. In particular, the protocol associated with an automated key management technique will confirm the liveness of the peer, protect against replay, authenticate the source of the short-term session key, associate protocol state information with the short-term session key, and ensure that a fresh short-term session key is generated. Moreover, an automated key management protocol can improve the interoperability by including negotiation mechanisms for cryptographic algorithms. These valuable features are impossible or extremely cumbersome to accomplish with manual key management. Housley, et al [Page 10] INTERNET-DRAFT October 21, 2013 8. IANA Considerations This specification defines three registries. 8.1. KeyTable Protocols This document requests establishment of a registry called "KeyTable Protocols". All assignments to the KeyTable Protocols registry are made on a specification required basis per Section 4.1 of [RFC5226]. Each registration entry must contain the three fields: - Protocol Name (unique within the registry); - Protocol Specific Info; and - Specification. The specification needs to describe parameters required for using the conceptual database as outlined in Section 4. This typically means that the specification focuses more on the application of security protocols with the key tables rather than being a new security protocol specification for general purposes. New protocols may of course combine information on how to use the key tables database with the protocol specification. The registry has three columns. The first column is a string of UTF-8 characters representing the name protocol. The second column is a string of UTF-8 characters providing a brief description of Protocol Specific Info. The third column is a reference to a specification defining how the protocol is used with the key table. There are no initial registrations. 8.2. KeyTable KDFs This document requests the establishment of a registry called "KeyTable KDFs". The remainder of this section describes the registry. All assignments to the KeyTable KDFs registry are made on a First Come First Served basis per Section 4.1 of RFC 5226. The registry has three columns. The first column is a string of UTF-8 characters representing the name of a KDF. The second column is a string of UTF-8 characters providing a brief description of the KDF. The third column is a reference to a specification defining the KDF, Housley, et al [Page 11] INTERNET-DRAFT October 21, 2013 if available. KDF Description Reference --- ----------- --------- none No KDF is used with this key 802.1X-01 IEEE 802.1X Table 9.1 [IEEE802.1X-2010] 8.3. KeyTable AlgIDs This document requests establishment of a registry called "KeyTable AlgIDs". The remainder of this section describes the registry. All assignments to the KeyTable AlgIDs registry are made on a First Come First Served basis per Section 4.1 of RFC 5226. The registry has three columns. The first column is a string of UTF-8 characters representing the name of an AlgID. The second column is a string of UTF-8 characters providing a brief description of the AlgID. The third column is a reference to a specification defining the AlgID, if available. AlgID Description Reference ----- ----------- --------- AES-128-CMAC AES-CMAC using 128-bit keys [RFC4493] 9. Acknowledgments This document reflects many discussions with many different people over many years. In particular, the authors thank Jari Arkko, Ran Atkinson, Ron Bonica, Ross Callon, Lars Eggert, Pasi Eronen, Adrian Farrel, Gregory Lebovitz, Acee Lindem, Sandy Murphy, Eric Rescorla, Mike Shand, Dave Ward, and Brian Weis for their insights. The authors additionally thank Brian Weis for supplying text to address IANA concerns and for help with formatting. Sam Hartman's work on this draft is funded by Huawei. 10. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Housley, et al [Page 12] INTERNET-DRAFT October 21, 2013 11. Informational References [IEEE802.1X-2010] IEEE Standard for Local and Metropolitan Area Networks -- Port-Based Network Access Control", February 2010. [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5 Signature Option", RFC 3562, July 2003. [RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key Management", RFC 4107, BCP 107, June 2005. [RFC4493] Song, J., Lee, J., Poovendran, R., and T. Iwata, "The AES-CMAC Algorithm", RFC 4493, June 2006. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP Authentication Option", RFC 5925, June 2010. Authors' Addresses Russell Housley Vigil Security, LLC 918 Spring Knoll Drive Herndon, VA 20170 USA EMail: housley@vigilsec.com Tim Polk National Institute of Standards and Technology 100 Bureau Drive, Mail Stop 8930 Gaithersburg, MD 20899-8930 USA EMail: tim.polk@nist.gov Sam Hartman Painless Security, LLC USA Email: hartmans@painless-security.com Dacheng Zhang Huawei China Email: zhangdacheng@huawei.com Housley, et al [Page 13]