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'6') Summary: 17 errors (**), 0 flaws (~~), 4 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DRAFT RIP-II Cryptographic Authentication March 1995 4 RIP-II Cryptographic Authentication | 5 draft-ietf-ripv2-md5-01.txt | 7 Fri Mar 17 11:41:29 PST 1995 9 Fred Baker 10 Cisco Systems 11 fred@cisco.com 13 Randall Atkinson 14 Information Technology Division 15 Naval Research Laboratory 16 atkinson@itd.nrl.navy.mil 18 Status of this Memo 20 This document is an Internet Draft. Internet Drafts are working 21 documents of the Internet Engineering Task Force (IETF), its Areas, and 22 its Working Groups. Note that other groups may also distribute working 23 documents as Internet Drafts. 25 Internet Drafts are valid for a maximum of six months and may be 26 updated, replaced, or obsoleted by other documents at any time. It is 27 inappropriate to use Internet Drafts as reference material or to cite 28 them other than as a "work in progress". 30 1. Introduction 32 Growth in the Internet has made us aware of the need for improved 33 authentication of routing information. RIP-II provides for 34 unauthenticated service (as in classical RIP), or password 35 authentication. Both are vulnerable to passive attacks currently 36 widespread in the Internet. Well-understood security issues exist in 37 routing protocols [4]. Clear text passwords, currently specified for 38 use with RIP-II, are no longer considered sufficient [5]. 40 If authentication is disabled, then only simple misconfigurations are 41 detected. Simple passwords transmitted in the clear will further 42 protect against the honest neighbor, but are useless in the general 43 case. By simply capturing information on the wire - straightforward 44 even in a remote environment - a hostile process can learn the password 45 and overcome the network. 47 We propose that RIP-II use an authentication algorithm, as in SNMP 48 Version 2, augmented by a sequence number. MD5 is proposed as the 49 standard authentication algorithm for RIP-II, but the mechanism is 50 intended to be algorithm-independent. While this mechanism is not 51 unbreakable (no known mechanism is), it provides a greatly enhanced 52 probability that a system being attacked will detect and ignore hostile 53 messages. This is because we transmit the output of an authentication 54 algorithm (e.g., MD5) rather than the secret RIP-II Authentication Key. 55 This output is a one-way function of a message and a secret RIP-II 56 Authentication Key. This RIP-II Authentication Key is never sent over 57 the network in the clear, thus providing protection against the passive 58 attacks now commonplace in the Internet. 60 In this way, protection is afforded against forgery or message 61 modification. It is possible to replay a message until the sequence 62 number changes, but the sequence number makes replay in the long term 63 less of an issue. The mechanism does not afford confidentiality, since 64 messages stay in the clear; however, the mechanism is also exportable 65 from most countries, which test a confidentiality algorithm would fail. 67 Other relevant rationales for the approach are that MD5 is used in SNMP 68 Version 2, and is therefore present in routers already, as is some form 69 of password management. A similar approach has been proposed for 70 authentication in IP version 6 (IPv6). 72 2. Implementation Approach 74 Implementation requires three issues to be addressed: 76 (1) A changed packet format, 78 (2) Authentication procedures, and 80 (3) Management controls. 82 2.1. RIP-II PDU Format 84 The basic RIP-II message format provides for an 8 byte header with an 85 array of 20 byte records as its data content. When MD5 is used, the 86 same header and content are used, except that the 16 byte 87 "authentication key" field is reused to describe a "Keyed Message 88 Digest" trailer. This consists in five fields: 90 (1) The "Authentication Type" is Keyed Message Digest Algorithm, 91 indicated by the value 3 (1 and 2 indicate "IP Route" and 92 "Password", respectively). 94 (2) A 16 bit offset from the RIP-II header to the record containing the 95 cryptogtaphic digest. This value effectively points to the end of 96 routing data in the packet.. 98 (3) An unsigned 8-bit field that contains the Key Identifier or Key-ID. 99 This identifies the key used to create the Authentication Data for 100 this RIP-II message. A key is associated with an interface. 102 (4) An unsigned 8-bit field that contains the length in octets of the 103 trailing Authentication Data field. The presence of this field 104 permits other algorithms (e.g., SHA) to be substituted for MD5 if 105 desired. 107 (5) An unsigned 32 bit non-decreasing sequence number. 109 The trailer consists of the Authentication Data, which is the output of 110 the Keyed Message Digest Algorithm. When the Authentication Algorithm 111 is MD5, the output data is 16 bytes; during digest calculation, this is 112 effectively followed by a pad field and a length field as defined by RFC 113 1321. The field is contained in a record reminiscient of other 114 entiries, to be kind to ancient RIP implementations, but the actual 115 length of the digest varies by algorithm. 117 2.2. Processing Algorithm 119 When the authentication type is "Keyed Message Digest", message 120 processing is changed in message creation and reception. 121 0 1 2 3 3 122 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 123 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 124 | Command (1) | Version (1) | Routing Domain (2) | 125 +---------------+---------------+-------------------------------+ 126 | 0xFFFF | AuType=Keyed Message Digest | 127 +-------------------------------+-------------------------------+ 128 | RIP-II Packet Length | Key ID | Auth Data Len | 129 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 130 | Sequence Number (non-decreasing) | 131 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 132 | reserved must be zero | 133 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 134 | reserved must be zero | 135 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 136 | | 137 / (RIP-II Packet length-24) bytes Data / 138 | | 139 +---------------+---------------+-------------------------------+ 140 | 0xFFFF | 0x01 | 141 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 142 / Authentication Data (var. length; 16 bytes when MD5 is used) / 143 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 145 The MD5 algorithm logically appends the following information to the 146 packet during the MD5 calculation. 148 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 149 | zero or more pad bytes (defined by RFC 1321 when MD5 is used) | 150 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 151 | 64 bit message length MSW | 152 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 153 | 64 bit message length LSW | 154 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 155 2.2.1. Message Generation 157 The RIP-II Packet is created as usual, with these exceptions: 159 (1) The UDP checksum need not be calculated. If it is not, it MUST be 160 set to zero. 162 (2) The authentication type field indicates the Keyed Message Digest 163 Algorithm (2). 165 (3) The authentication "password" field is reused to store a packet 166 offset to the Authentication Data, a Key Identifier, the 167 Authentication Data Length, and a non-decreasing sequence number. 169 The value used in the sequence number is arbitrary, but two suggestions 170 are the time of the message's creation or a simple message counter. 172 The RIP-II Authentication Key is selected by the sender based on the 173 outgoing interface. Each key has a lifetime associated with it. No key 174 is ever used outside its lifetime. If more than one key is currently 175 alive, then the youngest key (the key whose lifetime most recently 176 started) SHOULD be used. Since the key's algorithm is an attribute of 177 the key, stored in the sender and receiver along with it, the Key ID 178 effectively indicates which authentication algorithm is in use if the 179 implementation supports more than one authentication algorithm. 181 (1) The RIP-II header's packet length field indicates the standard 182 RIP-II portion of the packet. 184 (2) The Authentication Data Offset, Key Identifier, and Authentication 185 Data size fields are filled in appropriately. 187 (3) The RIP-II Authentication Key, which is 16 bytes long when the MD5 188 algorithm is used, is now appended to the data. For all 189 algorithms, the RIP-II Authentication Key is never longer than the 190 output of the algorithm in use. 192 (4) Trailing pad and length fields are added and the digest calculated 193 using the indicated algorithm. When MD5 is the algorithm in use, 194 these are calculated per RFC 1321. 196 (5) The digest is written over the RIP-II Authentication Key. When MD5 197 is used, this digest will be 16 bytes long. 199 The trailing pad is not actually transmitted, as it is entirely 200 predictable from the message length and algorithm in use. 202 2.2.2. Message Reception 204 When the message is received, the process is reversed: 206 (1) The digest is set aside, 208 (2) The appropriate algorithm and key are determined from the value of 209 the Key Identifier field, 211 (3) The RIP-II Authentication Key is written into the appropriate 212 number (16 when MD5 is used) of bytes starting at the offset 213 indicated, 215 (4) Appropriate padding is added as needed, and 217 (5) A new digest calculated using the indicated algorithm. 219 If the calculated digest does not match the received digest, the message 220 is discarded unprocessed. If the neighbor has been heard from recently 221 enough to have viable routes in the route table and the received 222 sequence number is less than the last one received, the message likewise 223 is discarded unprocessed. The received sequence number must, of course, 224 be stored by neighbor and zeroed whenever it determines that 225 connectivity to the neighbor has been lost. Acceptable messages are now 226 truncated to RIP-II message itself and treated normally. 228 3. Management Procedures 230 3.1. Key Management Requirements 232 It is strongly desirable that a hypothetical security breach in one 233 Internet protocol not automatically compromise other Internet protocols. 234 The Authentication Key of this specification SHOULD NOT be stored using 235 protocols or algorithms that have known flaws or fail to afford perfect 236 forward secrecy. 238 Implementations MUST support the storage of more than one key at the 239 same time, although it is recognized that only one key will normally be 240 active on an interface. They MUST associate a specific lifetime (i.e., 241 data/time first valid and data/time no longer valid) and a key 242 identifier with each key, and MUST support manual key distribution 243 (e.g., the privileged user manually typing in the key, key lifetime, and 244 key identifier on the router console). The lifetime may be infinite. 245 If more than one algorithm is supported, then the implementation MUST 246 require that the algorithm be specified for each key at the time the 247 other key information is entered. Keys that are out of date MAY be 248 deleted at will by the implementation without requiring human 249 intervention. Manual deletion of active keys SHOULD also be supported. 251 Note that there are four "times" that are important with respect to a 252 key: 254 + The time the system starts accepting received packets signed with | 255 the key (KeyStartReceive). 256 + The time the system starts signing packets with the key | 257 (KeyStartSign). 258 + The time the system stops signing packets with the key, which is to | 259 say, the time it starts signing with the next key (KeyStopSign). 260 + The time the system stops accepting received packets signed with the | 261 key (KeyStopReceive). 263 The times SHOULD be in the order listed, which is to say that none of 264 these times occurs before the one mentioned before it. There needs to 265 be some distance between starts and between stops in order to get a 266 seamless transition. Each system sends with whichever key has the most 267 recent "start" time, and makes its first attempt at validation of 268 incoming traffic with the same key. If this validation fails and | 269 another (older) key is also active, the system should attempt to | 270 validate with any other active keys it may possess. 272 Note that the concept of a "key lifetime" does not require a hardware 273 time of day clock or the use of NTP, although one or the other is 274 advised; it merely requires that the earliest and latest times that the 275 key is valid must be programmable in a way the router understands. 277 It is likely that the IETF will define a standard key management 278 protocol. It is strongly desirable to use that key management protocol 279 to distribute RIP-II Authentication Keys among communicating RIP-II 280 implementations. Such a protocol would provide scalability and 281 significantly reduce the human administrative burden. The Key ID can be 282 used as a hook between RIP-II and such a future protocol. Key 283 management protocols have a long history of subtle flaws that are often 284 discovered long after the protocol was first described in public. To 285 avoid having to change all RIP-II implementations should such a flaw be 286 discovered, integrated key management protocol techniques were 287 deliberately omitted from this specification. 289 3.2. Key Management Procedures 291 As with all security methods using keys, it is necessary to change the 292 RIP-II Authentication Key on a regular basis. To maintain routing 293 stability during such changes, implementations are required to store and 294 support the use of more than one RIP-II Authentication Key on a given 295 interface at the same time. 297 Each key will have its own Key Identifier, which is stored locally. The 298 combination of the Key Identifier and the interface associated with the 299 message uniquely identifies the Authentication Algorithm and RIP-II 300 Authentication Key in use. 302 As noted above in Section 2.2.1, the party creating the RIP-II message 303 will select a valid key from the set of valid keys for that interface. 304 The receiver will use the Key Identifier and interface to determine 305 which key to use for authentication of the received message. More than 306 one key may be associated with an interface at the same time. 308 Hence it is possible to have fairly smooth RIP-II Authentication Key 309 rollovers without losing legitimate RIP-II messages because the stored 310 key is incorrect and without requiring people to change all the keys at 311 once. To ensure a smooth rollover, each communicating RIP-II system 312 must be updated with the new key several minutes before they current key 313 will expire and several minutes before the new key lifetime begins. The 314 new key should have a lifetime that starts several minutes before the 315 old key expires. This gives time for each system to learn of the new 316 RIP-II Authentication Key before that key will be used. It also ensures 317 that the new key will begin being used and the current key will go out 318 of use before the current key's lifetime expires. For the duration of 319 the overlap in key lifetimes, a system may receive messages using either 320 key and authenticate the message. 322 Key storage SHOULD persist across a system restart, warm or cold, to 323 avoid operational issues. Key lifetime is an obvious issue, to be 324 solved by the implementation. Obvious solutions include the use of the 325 Network Time Protocol, hardware time of day clocks, or waiting some 326 period of time before emitting the initial RIP REQUEST to determine what 327 key other systems are signing with. The matter is left for the 328 implementor. 330 3.3. Pathological Cases 332 Two pathological cases exist which must be handled, which are failures 333 of the network manager. Both of these should be exceedingly rare. 335 During key switchover, devices may exist which have not yet been 336 successfully configured with the new key. Therefore, routers MAY 337 implement (and would be well advised to implement) an algorithm that 338 detects the set of keys being used by its neighbors, and transmits its 339 messages using both the new and old keys until all of the neighbors are 340 using the new key or the lifetime of the old key expires. Under normal 341 circumstances, this elevated transmission rate will exist for a single 342 update interval. 344 In the event that the last key associated with an interface expires, it 345 is unacceptable to revert to an unauthenticated condition, and not 346 advisable to disrupt routing. Therefore, the router should send a "last 347 authentication key expiration" notification to the network manager and 348 treat the key as having an infinite lifetime until the lifetime is 349 extended, the key is deleted by network management, or a new key is 350 configured. 352 4. Conformance Requirements 354 To conform to this specification, an implementation MUST support all of 355 its aspects. The MD5 authentication algorithm defined in RFC-1321 MUST 356 be implemented by all conforming implementations. A conforming 357 implementation MAY also support other authentication algorithms such as 358 NIST's Secure Hash Algorithm (SHA). Manual key distribution as 359 described above MUST be supported by all conforming implementations. 360 All implementations MUST support the smooth key rollover described under 361 "Key Change Procedures." 363 The user documentation provided with the implementation MUST contain 364 clear instructions on how to ensure that smooth key rollover occurs. 366 Implementations SHOULD support a standard key management protocol for 367 secure distribution of RIP-II Authentication Keys once such a key 368 management protocol is standardized by the IETF. 370 5. Acknowledgments 372 This work was done by the RIP-II Working Group, of which Gary Malkin is 373 the Chair. This suggestion was originally made by Christian Huitema on 374 behalf of the IAB. Jeff Honig (Cornell) and Dennis Ferguson (ANS) built 375 the first operational prototype, proving out the algorithms. The 376 authors gladly acknowledge significant inputs from each of these 377 sources. 379 6. References 381 [1] Malkin, Gary, "RIP Version 2 Carrying Additional Information", RFC 382 1388, January 1993. 384 [2] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 385 1992. 387 [3] Malkin, G., and F. Baker, "RIP Version 2 MIB Extension", RFC 1389, 388 Xylogics, Inc., Advanced Computer Communications, January 1993. 390 [4] S. Bellovin, "Security Problems in the TCP/IP Protocol Suite", ACM 391 Computer Communications Review, Volume 19, Number 2, pp.32-48, 392 April 1989. 394 [5] N. Haller, R. Atkinson, "On Internet Authentication", Request for 395 Comments 1704, DDN Network Information Center, October 1994. 397 [6] R. Braden, D. Clark, S. Crocker, & C. Huitema, "Report of IAB 398 Workshop on Security in the Internet Architecture", Request for 399 Comments 1636, DDN Network Information Center, June 1994. 401 7. Security Considerations 403 This entire memo describes and specifies an authentication mechanism for 404 the RIP-II routing protocol that is believed to be secure against active 405 and passive attacks. Passive attacks are clearly widespread in the 406 Internet at present. Protection against active attacks is also needed 407 even though such attacks are not currently widespread. 409 Users need to understand that the quality of the security provided by 410 this mechanism depends completely on the strength of the implemented 411 authentication algorithms, the strength of the key being used, and the 412 correct implementation of the security mechanism in all communicating 413 RIP-II implementations. This mechanism also depends on the RIP-II 414 Authentication Key being kept confidential by all parties. If any of 415 these incorrect or insufficiently secure, then no real security will be 416 provided to the users of this mechanism. 418 Specifically with respect to the use of SNMP, compromise of SNMP 419 security has the necessary result that the various RIP-II configuration 420 parameters (e.g. routing table, RIP-II Authentication Key) managable via 421 SNMP could be compromised as well. Changing Authentication Keys using 422 non-encrypted SNMP is no more secure than sending passwords in the 423 clear. 425 Confidentiality is not provided by this mechanism. Work is underway 426 within the IETF to specify a standard mechanism for IP-layer encryption. 427 That mechanism might be used to provide confidentiality for RIP-II in 428 the future. Protection against traffic analysis is also not provided. 429 Mechanisms such as bulk link encryption might be used when protection 430 against traffic analysis is required. 432 The memo is written to address a security consideration in RIP-II 433 Version 2 that was raised during the IAB's recent security review [6]. 435 8. Chairman's Address 437 Gary Scott Malkin 438 Xylogics, Inc. 439 53 Third Avenue 440 Burlington, MA 01803 441 Phone: (617) 272-8140 442 EMail: gmalkin@Xylogics.COM 444 9. Author's Address 446 Fred Baker 447 Cisco Systems 448 519 Lado Drive 449 Santa Barbara, California 93111 450 Phone: (805) 681 0115 451 Email: fred@cisco.com 453 Randall Atkinson 454 Information Technology Division 455 Naval Research Laboratory 456 Washington, DC 20375-5320 457 Voice: (DSN) 354-8590 458 Fax: (DSN) 354-7942 459 Email: atkinson@itd.nrl.navy.mil 460 Table of Contents 462 1 Introduction .................................................... 2 463 2 Implementation Approach ......................................... 3 464 2.1 RIP-II PDU Format ............................................. 3 465 2.2 Processing Algorithm .......................................... 4 466 2.2.1 Message Generation .......................................... 5 467 2.2.2 Message Reception ........................................... 6 468 3 Management Procedures ........................................... 6 469 3.1 Key Management Requirements ................................... 6 470 3.2 Key Management Procedures ..................................... 8 471 3.3 Pathological Cases ............................................ 8 472 4 Conformance Requirements ........................................ 9 473 5 Acknowledgments ................................................. 9 474 6 References ...................................................... 10 475 7 Security Considerations ......................................... 10 476 8 Chairman's Address .............................................. 11 477 9 Author's Address ................................................ 11