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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 INTERNET-DRAFT R. Housley 3 Internet Engineering Task Force (IETF) Vigil Security 4 Intended Status: Standards Track T. Polk 5 NIST 6 Expires: 16 November 2011 16 May 2011 8 Database of Long-Lived Symmetric Cryptographic Keys 9 11 Status of this Memo 13 This Internet-Draft is submitted to IETF in full conformance with the 14 provisions of BCP 78 and BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that other 18 groups may also distribute working documents as Internet-Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as "work in progress." 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/1id-abstracts.html 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html 31 Copyright Notice 33 Copyright (c) 2011 IETF Trust and the persons identified as the 34 document authors. All rights reserved. 36 This document is subject to BCP 78 and the IETF Trust's Legal 37 Provisions Relating to IETF Documents 38 (http://trustee.ietf.org/license-info) in effect on the date of 39 publication of this document. Please review these documents 40 carefully, as they describe your rights and restrictions with respect 41 to this document. Code Components extracted from this document must 42 include Simplified BSD License text as described in Section 4.e of 43 the Trust Legal Provisions and are provided without warranty as 44 described in the Simplified BSD License. 46 Abstract 48 This document specifies the information contained in a database of 49 long-lived cryptographic keys used by many different security 50 protocols. The database design supports both manual and automated 51 key management. In many instances, the security protocols do not 52 directly use the long-lived key, but rather a key derivation function 53 is used to derive a short-lived key from a long-lived key. 55 1. Introduction 57 This document specifies the information that needs to be included in 58 a database of long-lived cryptographic keys. This conceptual 59 database is designed to support both manual key management and 60 automated key management. The intent is to allow many different 61 implementation approaches to the specified cryptographic key 62 database. 64 Security protocols such as TCP-AO [RFC5925] are expected to use an 65 application program interface (API) to select a long-lived key from 66 the database. In many instances, the long-lived keys are not used 67 directly in security protocols, but rather a key derivation function 68 is used to derive short-lived key from the long-lived keys in the 69 database. In other instances, security protocols will directly use 70 the long-lived key from the database. The database design supports 71 both use cases. 73 2. Conceptual Database Structure 75 The database is characterized as a table, where each row represents a 76 single long-lived symmetric cryptographic key. Each key should only 77 have one row; however, in the (hopefully) very rare cases where the 78 same key is used for more than one purpose, multiple rows will 79 contain the same key value. The columns in the table represent the 80 key value and attributes of the key. 82 To accommodate manual key management, then formatting of the fields 83 has been purposefully chosen to allow updates with a plain text 84 editor. 86 The table has the following columns: 88 LocalKeyID 89 LocalKeyID is a 16-bit integer in hexadecimal. The LocalKeyID 90 can be used by a peer to identify this entry in the database. 91 For pairwise keys, the most significant bit in LocalKeyID is 92 set to zero, and the integer value must be unique among all the 93 pairwise keys in the database. For group keys, the most 94 significant bit in LocalKeyID is set to one, but collisions 95 among group key identifiers must be accommodated. 97 PeerKeyID 98 For pairwise keys, the PeerKeyID field is a 16 bit integer in 99 hexadecimal provided by the peer. If the peer has not yet 100 provided this value, the PeerKeyID is set to "unknown". For 101 group keying, the PeerKeyID field is set to "group", which 102 easily accommodates group keys generated by a third party. 104 Peers 105 The Peers field identifies the peer system or set of systems 106 that have this key configured in their own database of long- 107 lived keys. For pairwise keys, the database on the peer system 108 LocalKeyID field will contain the value specified in the 109 PeerKeyID field in the local database. For group keying, the 110 Peers field names the group, not the individual systems that 111 comprise the group. 113 Interfaces 114 The Interfaces field identifies the set of interfaces for which 115 it is appropriate to use this key. When the long-lived value 116 in the Key field is intended for use on any interface, the 117 Interfaces field is set to "all". 119 Protocol 120 The Protocol field identifies a single security protocol where 121 this key may be used to provide cryptographic protection. 123 KDF 124 The KDF field indicates which key derivation function is used 125 to generate short-lived keys from the long-lived value in the 126 Key field. When the long-lived value in the Key field is 127 intended for direct use, the KDF field is set to "none". 129 KDFInputs 130 The KDFInputs field is used when supplementary public or 131 private data is supplied to the KDF. For protocols that do not 132 require additional information for the KDF, the KDFInputs field 133 is set to "none". The Protocol field will determine the format 134 of this field if it is not "none". 136 AlgID 137 The AlgID field indicates which cryptographic algorithm to be 138 used with the security protocol for the specified peer. The 139 algorithm may be an encryption algorithm and mode (such as 140 AES-128-CBC), an authentication algorithm (such as HMAC-SHA1-96 141 or AES-128-CMAC), or any other symmetric cryptographic 142 algorithm needed by a security protocol. If the KDF field 143 contains "none", then the long-lived key is used directly with 144 this algorithm, otherwise the derived short-lived key is used 145 with this algorithm. When the long-lived key is used to 146 generate a set of short-lived keys for use with the security 147 protocol, the AlgID field identifies a ciphersuite rather than 148 a single cryptographic algorithm. 150 Key 151 The Key is a hexadecimal string representing a long-lived 152 symmetric cryptographic key. The size of the Key depends on 153 the KDF and the AlgID. For example, a KDF=none and 154 AlgID=AES128 requires a 128-bit key, which is represented by 32 155 hexadecimal digits. 157 Direction 158 The Direction field indicates whether this key may be used for 159 inbound traffic, outbound traffic, or both. The supported 160 values are "in", "out", and "both", respectively. The Protocol 161 field will determine which of these values are valid. 163 NotBefore 164 The NotBefore field specifies the earliest date and time in 165 Universal Coordinated Time (UTC) at which this key should be 166 considered for use. The format is YYYYMMDDHHSSZ, where four 167 digits specify the year, two digits specify the month, two 168 digits specify the day, two digits specify the hour, and two 169 digits specify the minute. The "Z" is included as a clear 170 indication that the time is in UTC. 172 NotAfter 173 The NotAfter field specifies the latest date and time at which 174 this key should be considered for use. The format is the same 175 as the NotBefore field. 177 Note that some security protocols use a KeyID value of zero for 178 special purposes, so care is needed if this KeyID value is included 179 in the table. 181 3. Key Selection and Rollover 183 When a system desires to protect a unicast protocol data unit for a 184 remote system H using security protocol P via interface I, the local 185 system selects a long-lived key at time T from the database, any key 186 that satisfies the following conditions may be used: 188 (1) the Peer field includes H; 190 (2) the PeerKeyID field is not "unknown"; 192 (3) the Protocol field matches P; 193 (4) the Interfaces field includes I; 195 (5) the Direction field is either "out" or "both"; and 197 (6) NotBefore <= T <= NotAfter. 199 The value in the PeerKeyID field is used to identify the selected key 200 to the remote system H. 202 Group key selection is different than pairwise key selection. When a 203 system desires to protect a multicast protocol data unit for a group 204 of systems G using security protocol P via interface I, the local 205 system selects a long-lived key at time T from the database, any key 206 that satisfies the following conditions may be used: 208 (1) the Peer field includes the multicast group G; 210 (2) the PeerKeyID field is "group"; 212 (3) the Protocol field matches P; 214 (4) the Interfaces field includes I; 216 (5) the Direction field is either "out" or "both"; and 218 (6) NotBefore <= T <= NotAfter. 220 The value in the LocalKeyID field is used to identify the selected 221 key since all of the systems in the group G use the same identifier. 223 During algorithm transition, multiple entries may exist associated 224 with different cryptographic algorithms or ciphersuites. Systems 225 should support selection of keys based on algorithm preference. 227 In addition, multiple entries with overlapping use periods are 228 expected to be employed to provide orderly key rollover. In these 229 cases, the expectation is that systems will transition to the newest 230 key available. To meet this requirement, this specification 231 recommends supplementing the key selection algorithm with the 232 following differentiation: select the long-lived key specifying the 233 most recent time in the NotBefore field. 235 When a system participates in a security protocol, a sending peer 236 system H has selected a long-lived key and the LocalKeyID is included 237 in the protocol control information. When retrieving the long-lived 238 key (for direct use or for key derivation), the local system should 239 confirm the following conditions are satisfied before use: 241 (1) the Peer field includes H; 243 (2) the Protocol field matches P; 245 (3) the Interface field includes I; 247 (4) the Direction field is either "in" or "both"; and 249 (5) NotBefore <= T <= NotAfter. 251 Note that the key usage is loosely bound by the times specified in 252 the NotBefore and NotAfter fields. New security associations should 253 not be established except within the period of use specified by these 254 fields, while allowing some grace time for clock skew. However, if a 255 security association has already been established based on a 256 particular long-lived key, exceeding the lifetime does not have any 257 direct impact. Implementations of protocols that involve long-lived 258 security association should be designed to periodically interrogate 259 the database and rollover to new keys without tearing down the 260 security association. 262 For group keying, the local system should confirm the following 263 conditions are satisfied before use: 265 (1) the Peer field includes the multicast group G; 267 (2) the PeerKeyID field is "group"; 269 (3) the Protocol field matches P; 271 (4) the Interface field includes I; 273 (5) the Direction field is either "in" or "both"; and 275 (6) NotBefore <= T <= NotAfter. 277 As long as a key remains in the database, the key may be used for 278 received traffic. Any key that is unacceptable for received traffic 279 needs to be removed from the database. 281 4. Operational Considerations 283 If usage periods for long-lived keys do not overlap and system clocks 284 are inconsistent, it is possible to construct scenarios where systems 285 cannot agree upon a long-lived key. When installing a series of keys 286 to be used one after the other (sometimes called a key chain), 287 operators should configure the NotAfter field of the preceding key to 288 be several days after the NotBefore field of the subsequent key to 289 ensure that clock skew is not a concern. 291 For group keys, the most significant bit in LocalKeyID must be set to 292 one. Collisions among group key identifiers can be avoided by 293 subdividing the remaining 15 bits of the LocalKeyID field into an 294 identifier of the group key generator and an identifier assigned by 295 that generator. 297 5. Security Considerations 299 Management of encryption and authentication keys has been a 300 significant operational problem, both in terms of key synchronization 301 and key selection. For example, current guidance [RFC3562] warns 302 against sharing TCP MD5 keying material between systems, and 303 recommends changing keys according to a schedule. The same general 304 operational issues are relevant for the management of other 305 cryptographic keys. 307 It is recognized in [RFC4107] that automated key management is not 308 viable in some situations. The conceptual database specified in this 309 document is intended to accommodate both manual key management and 310 automated key management. A future specification to automatically 311 populate rows in the database is envisioned. 313 Designers should recognize the warning provided in [RFC4107]: 315 Automated key management and manual key management provide very 316 different features. In particular, the protocol associated with 317 an automated key management technique will confirm the liveness of 318 the peer, protect against replay, authenticate the source of the 319 short-term session key, associate protocol state information with 320 the short-term session key, and ensure that a fresh short-term 321 session key is generated. Further, an automated key management 322 protocol can improve interoperability by including negotiation 323 mechanisms for cryptographic algorithms. These valuable features 324 are impossible or extremely cumbersome to accomplish with manual 325 key management. 327 6. IANA Considerations 329 No IANA actions are required. 331 {{{ RFC Editor: Please remove this section prior to publication. }}} 333 7. Acknowledgments 335 This document reflects many discussions with many different people 336 over many years. In particular, the authors thank Jari Arkko, Ran 337 Atkinson, Ron Bonica, Ross Callon, Lars Eggert, Pasi Eronen, Adrian 338 Farrel, Sam Hartman, Gregory Lebovitz, Sandy Murphy, Eric Rescorla, 339 Mike Shand, Dave Ward, and Brian Weis for their insights. 341 8. Informational References 343 [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5 344 Signature Option", RFC 3562, July 2003. 346 [RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic 347 Key Management", RFC 4107, BCP 107, June 2005. 349 [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP 350 Authentication Option", RFC 5925, June 2010. 352 Authors' Addresses 354 Russell Housley 355 Vigil Security, LLC 356 918 Spring Knoll Drive 357 Herndon, VA 20170 358 USA 359 EMail: housley@vigilsec.com 361 Tim Polk 362 National Institute of Standards and Technology 363 100 Bureau Drive, Mail Stop 8930 364 Gaithersburg, MD 20899-8930 365 USA 366 EMail: tim.polk@nist.gov