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'6') ** Obsolete normative reference: RFC 1826 (ref. '7') (Obsoleted by RFC 2402) ** Obsolete normative reference: RFC 1827 (ref. '8') (Obsoleted by RFC 2406) Summary: 19 errors (**), 0 flaws (~~), 5 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DRAFT RIP-II MD5 Authentication February 1995 4 RIP-II MD5 Authentication 5 draft-ietf-ripv2-md5-03.txt 7 Fri Feb 23 16:23:57 PST 1996 9 Fred Baker 10 Cisco Systems 11 fred@cisco.com 13 Randall Atkinson 14 cisco Systems 15 rja@cisco.com 17 Status of this Memo 19 This document is an Internet Draft. Internet Drafts are working 20 documents of the Internet Engineering Task Force (IETF), its Areas, and 21 its Working Groups. Note that other groups may also distribute working 22 documents as Internet Drafts. 24 Internet Drafts are valid for a maximum of six months and may be 25 updated, replaced, or obsoleted by other documents at any time. It is 26 inappropriate to use Internet Drafts as reference material or to cite 27 them other than as a "work in progress". 29 DRAFT RIP-II MD5 Authentication February 1995 31 1. Introduction 33 Growth in the Internet has made us aware of the need for improved 34 authentication of routing information. RIP-II provides for 35 unauthenticated service (as in classical RIP), or password 36 authentication. Both are vulnerable to passive attacks currently 37 widespread in the Internet. Well-understood security issues exist in 38 routing protocols [4]. Clear text passwords, currently specified for 39 use with RIP-II, are no longer considered sufficient [5]. 41 If authentication is disabled, then only simple misconfigurations are 42 detected. Simple passwords transmitted in the clear will further 43 protect against the honest neighbor, but are useless in the general 44 case. By simply capturing information on the wire - straightforward 45 even in a remote environment - a hostile process can learn the password 46 and overcome the network. 48 We propose that RIP-II use an authentication algorithm, as was 49 originally proposed for SNMP Version 2, augmented by a sequence number. 50 Keyed MD5 is proposed as the standard authentication algorithm for RIP- 51 II, but the mechanism is intended to be algorithm-independent. While 52 this mechanism is not unbreakable (no known mechanism is), it provides a 53 greatly enhanced probability that a system being attacked will detect 54 and ignore hostile messages. This is because we transmit the output of 55 an authentication algorithm (e.g., Keyed MD5) rather than the secret 56 RIP-II Authentication Key. This output is a one-way function of a 57 message and a secret RIP-II Authentication Key. This RIP-II 58 Authentication Key is never sent over the network in the clear, thus 59 providing protection against the passive attacks now commonplace in the 60 Internet. 62 In this way, protection is afforded against forgery or message 63 modification. It is possible to replay a message until the sequence 64 number changes, but the sequence number makes replay in the long term 65 less of an issue. The mechanism does not afford confidentiality, since 66 messages stay in the clear; however, the mechanism is also exportable 67 from most countries, which test a privacy algorithm would fail. 69 Other relevant rationales for the approach are that Keyed MD5 is being 70 used for OSPF cryptographic authentication, and is therefore present in 71 routers already, as is some form of password management. A similar 72 approach has been standardised for use in IP-layer authentication. [7] 74 DRAFT RIP-II MD5 Authentication February 1995 76 2. Implementation Approach 78 Implementation requires three issues to be addressed: 80 (1) A changed packet format, 82 (2) Authentication procedures, and 84 (3) Management controls. 86 2.1. RIP-II PDU Format 88 The basic RIP-II message format provides for an 8 byte header with an 89 array of 20 byte records as its data content. When Keyed MD5 is used, 90 the same header and content are used, except that the 16 byte 91 "authentication key" field is reused to describe a "Keyed Message 92 Digest" trailer. This consists in five fields: 94 (1) The "Authentication Type" is Keyed Message Digest Algorithm, 95 indicated by the value 3 (1 and 2 indicate "IP Route" and 96 "Password", respectively). 98 (2) A 16 bit offset from the RIP-II header to the MD5 digest (if no 99 other trailer fields are ever defined, this value equals the RIP-II 100 Data Length). 102 (3) An unsigned 8-bit field that contains the Key Identifier or Key-ID. 103 This identifies the key used to create the Authentication Data for 104 this RIP-II message. In implementations supporting more than one 105 authentication algorithm, the Key-ID also indicates the 106 authentication algorithm in use for this message. A key is 107 associated with an interface. 109 (4) An unsigned 8-bit field that contains the length in octets of the 110 trailing Authentication Data field. The presence of this field 111 permits other algorithms (e.g., Keyed SHA) to be substituted for 112 Keyed MD5 if desired. 114 (5) An unsigned 32 bit sequence number. The sequence number MUST be 115 non-decreasing for all messages sent with the same Key ID. 117 The trailer consists of the Authentication Data, which is the output of 118 the Keyed Message Digest Algorithm. When the Authentication Algorithm 119 is Keyed MD5, the output data is 16 bytes; during digest calculation, 120 this is effectively followed by a pad field and a length field as 122 DRAFT RIP-II MD5 Authentication February 1995 124 defined by RFC 1321. 126 DRAFT RIP-II MD5 Authentication February 1995 128 2.2. Processing Algorithm 130 When the authentication type is "Keyed Message Digest", message 131 processing is changed in message creation and reception. 132 0 1 2 3 3 133 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 1 134 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 135 | Command (1) | Version (1) | Routing Domain (2) | 136 +---------------+---------------+-------------------------------+ 137 | 0xFFFF | AuType=Keyed Message Digest | 138 +-------------------------------+-------------------------------+ 139 | RIP-II Packet Length | Key ID | Auth Data Len | 140 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 141 | Sequence Number (non-decreasing) | 142 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 143 | reserved must be zero | 144 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 145 | reserved must be zero | 146 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 147 | | 148 / (RIP-II Packet Length - 24) bytes of Data / 149 | | 150 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 151 | 0xFFFF | 0x01 | 152 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 153 / Authentication Data (var. length; 16 bytes with Keyed MD5) / 154 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 156 In memory, the following trailer is appended by the MD5 algorithm and 157 treated as though it were part of the message. 159 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 160 | sixteen octets of MD5 "secret" | 161 / / 162 | | 163 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 164 | zero or more pad bytes (defined by RFC 1321 when MD5 is used) | 165 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 166 | 64 bit message length MSW | 167 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 168 | 64 bit message length LSW | 169 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 171 DRAFT RIP-II MD5 Authentication February 1995 173 2.2.1. Message Generation 175 The RIP-II Packet is created as usual, with these exceptions: 177 (1) The UDP checksum need not be calculated, but MAY be set to zero. 179 (2) The authentication type field indicates the Keyed Message Digest 180 Algorithm (3). 182 (3) The authentication "password" field is reused to store a packet 183 offset to the Authentication Data, a Key Identifier, the 184 Authentication Data Length, and a non-decreasing sequence number. 186 The value used in the sequence number is arbitrary, but two suggestions 187 are the time of the message's creation or a simple message counter. 189 The RIP-II Authentication Key is selected by the sender based on the 190 outgoing interface. Each key has a lifetime associated with it. No key 191 is ever used outside its lifetime. Since the key's algorithm is related 192 to the key itself, stored in the sender and receiver along with it, the 193 Key ID effectively indicates which authentication algorithm is in use if 194 the implementation supports more than one authentication algorithm. 196 (1) The RIP-II header's packet length field indicates the standard 197 RIP-II portion of the packet. 199 (2) The Authentication Data Offset, Key Identifier, and Authentication 200 Data size fields are filled in appropriately. 202 (3) The RIP-II Authentication Key, which is 16 bytes long when the 203 Keyed MD5 algorithm is used, is now appended to the data. For all 204 algorithms, the RIP-II Authentication Key is never longer than the 205 output of the algorithm in use. 207 (4) Trailing pad and length fields are added and the digest calculated 208 using the indicated algorithm. When Keyed MD5 is the algorithm in 209 use, these are calculated per RFC 1321. 211 (5) The digest is written over the RIP-II Authentication Key. When MD5 212 is used, this digest will be 16 bytes long. 214 The trailing pad is not actually transmitted, as it is entirely 215 predictable from the message length and algorithm in use. 217 DRAFT RIP-II MD5 Authentication February 1995 219 2.2.2. Message Reception 221 When the message is received, the process is reversed: 223 (1) The digest is set aside, 225 (2) The appropriate algorithm and key are determined from the value of 226 the Key Identifier field, 228 (3) The RIP-II Authentication Key is written into the appropriate 229 number (16 when Keyed MD5 is used) of bytes starting at the offset 230 indicated, 232 (4) Appropriate padding is added as needed, and 234 (5) A new digest calculated using the indicated algorithm. 236 If the calculated digest does not match the received digest, the message 237 is discarded unprocessed. If the neighbor has been heard from recently 238 enough to have viable routes in the route table and the received 239 sequence number is less than the last one received, the message likewise 240 is discarded unprocessed. When connectivity to the neighbor has been 241 lost, the receiver SHOULD be ready to accept either: 242 - a message with a sequence number of zero 243 - a message with a higher sequence number than the last received sequence 244 number. 246 A router that has forgotten its current sequence number but remembers 247 its key and Key-ID MUST send its first packet with a sequence number of 248 zero. This leaves a small opening for a replay attack. Router vendors 249 are encouraged to provide stable storage for keys, key lifetimes, Key- 250 IDs, and the related sequence numbers. 252 Acceptable messages are now truncated to RIP-II message itself and 253 treated normally. 255 3. Management Procedures 257 3.1. Key Management Requirements 259 It is strongly desirable that a hypothetical security breach in one 260 Internet protocol not automatically compromise other Internet protocols. 261 The Authentication Key of this specification SHOULD NOT be stored using 262 protocols or algorithms that have known flaws. 264 DRAFT RIP-II MD5 Authentication February 1995 266 Implementations MUST support the storage of more than one key at the 267 same time, although it is recognized that only one key will normally be 268 active on an interface. They MUST associate a specific lifetime (i.e., 269 date/time first valid and date/time no longer valid) and a key 270 identifier with each key, and MUST support manual key distribution 271 (e.g., the privileged user manually typing in the key, key lifetime, and 272 key identifier on the router console). The lifetime may be infinite. 273 If more than one algorithm is supported, then the implementation MUST 274 require that the algorithm be specified for each key at the time the 275 other key information is entered. Keys that are out of date MAY be 276 deleted at will by the implementation without requiring human 277 intervention. Manual deletion of active keys SHOULD also be supported. 279 It is likely that the IETF will define a standard key management 280 protocol. It is strongly desirable to use that key management protocol 281 to distribute RIP-II Authentication Keys among communicating RIP-II 282 implementations. Such a protocol would provide scalability and 283 significantly reduce the human administrative burden. The Key ID can be 284 used as a hook between RIP-II and such a future protocol. Key 285 management protocols have a long history of subtle flaws that are often 286 discovered long after the protocol was first described in public. To 287 avoid having to change all RIP-II implementations should such a flaw be 288 discovered, integrated key management protocol techniques were 289 deliberately omitted from this specification. 291 3.2. Key Management Procedures 293 As with all security methods using keys, it is necessary to change the 294 RIP-II Authentication Key on a regular basis. To maintain routing 295 stability during such changes, implementations MUST be able to store and 296 use more than one RIP-II Authentication Key on a given interface at the 297 same time. 299 Each key will have its own Key Identifier, which is stored locally. The 300 combination of the Key Identifier and the interface associated with the 301 message uniquely identifies the Authentication Algorithm and RIP-II 302 Authentication Key in use. 304 As noted above in Section 2.2.1, the party creating the RIP-II message 305 will select a valid key from the set of valid keys for that interface. 306 The receiver will use the Key Identifier and interface to determine 307 which key to use for authentication of the received message. More than 308 one key may be associated with an interface at the same time. 310 DRAFT RIP-II MD5 Authentication February 1995 312 Hence it is possible to have fairly smooth RIP-II Authentication Key 313 rollovers without losing legitimate RIP-II messages because the stored 314 key is incorrect and without requiring people to change all the keys at 315 once. To ensure a smooth rollover, each communicating RIP-II system 316 must be updated with the new key several minutes before the current key 317 will expire and several minutes before the new key lifetime begins. The 318 new key should have a lifetime that starts several minutes before the 319 old key expires. This gives time for each system to learn of the new 320 RIP-II Authentication Key before that key will be used. It also ensures 321 that the new key will begin being used and the current key will go out 322 of use before the current key's lifetime expires. For the duration of 323 the overlap in key lifetimes, a system may receive messages using either 324 key and authenticate the message. The Key-ID in the received message is 325 used to select the appropriate key for authentication. 327 3.3. Pathological Cases 329 Two pathological cases exist which must be handled, which are failures 330 of the network manager. Both of these should be exceedingly rare. 332 During key switchover, devices may exist which have not yet been 333 successfully configured with the new key. Therefore, routers SHOULD 334 implement (and would be well advised to implement) an algorithm that 335 detects the set of keys being used by its neighbors, and transmits its 336 messages using both the new and old keys until all of the neighbors are 337 using the new key or the lifetime of the old key expires. Under normal 338 circumstances, this elevated transmission rate will exist for a single 339 update interval. 341 In the event that the last key associated with an interface expires, it 342 is unacceptable to revert to an unauthenticated condition, and not 343 advisable to disrupt routing. Therefore, the router should send a "last 344 authentication key expiration" notification to the network manager and 345 treat the key as having an infinite lifetime until the lifetime is 346 extended, the key is deleted by network management, or a new key is 347 configured. 349 4. Conformance Requirements 351 To conform to this specification, an implementation MUST support all of 352 its aspects. The Keyed MD5 authentication algorithm MUST be implemented 353 by all conforming implementations. MD5 is defined in RFC-1321. A 354 conforming implementation MAY also support other authentication 355 algorithms such as Keyed Secure Hash Algorithm (SHA). Manual key 356 distribution as described above MUST be supported by all conforming 358 DRAFT RIP-II MD5 Authentication February 1995 360 implementations. All implementations MUST support the smooth key 361 rollover described under "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, "Internet Authentication Guidelines", 395 RFC-1704, October 1994. 397 [6] R. Braden, D. Clark, S. Crocker, & C. Huitema, "Report of IAB 398 Workshop on Security in the Internet Architecture", RFC-1636, June 399 1994. 401 [7] R. Atkinson, "IP Authentication Header", RFC-1826, August 1995. 403 DRAFT RIP-II MD5 Authentication February 1995 405 [8] R. Atkinson, "IP Encapsulating Security Payload", RFC-1827, August 406 1995. 408 7. Security Considerations 410 This entire memo describes and specifies an authentication mechanism for 411 the RIP-II routing protocol that is believed to be secure against active 412 and passive attacks. Passive attacks are clearly widespread in the 413 Internet at present. Protection against active attacks is also needed 414 because active attacks are becoming more common. 416 Users need to understand that the quality of the security provided by 417 this mechanism depends completely on the strength of the implemented 418 authentication algorithms, the strength of the key being used, and the 419 correct implementation of the security mechanism in all communicating 420 RIP-II implementations. This mechanism also depends on the RIP-II 421 Authentication Key being kept confidential by all parties. If any of 422 these incorrect or insufficiently secure, then no real security will be 423 provided to the users of this mechanism. 425 Specifically with respect to the use of SNMP, compromise of SNMP 426 security has the necessary result that the various RIP-II configuration 427 parameters (e.g. routing table, RIP-II Authentication Key) managable via 428 SNMP could be compromised as well. Changing Authentication Keys using 429 non-encrypted SNMP is no more secure than sending passwords in the 430 clear. 432 Confidentiality is not provided by this mechanism. Recent work in the 433 IETf provides a standard mechanism for IP-layer encryption. [8] That 434 mechanism might be used to provide confidentiality for RIP-II in the 435 future. Protection against traffic analysis is also not provided. 436 Mechanisms such as bulk link encryption might be used when protection 437 against traffic analysis is required. 439 The memo is written to address a security consideration in RIP-II 440 Version 2 that was raised during the IAB's recent security review [6]. 442 8. Chairman's Address 444 Gary Scott Malkin 445 Xylogics, Inc. 446 53 Third Avenue 447 Burlington, MA 01803 448 Phone: (617) 272-8140 450 DRAFT RIP-II MD5 Authentication February 1995 452 EMail: gmalkin@Xylogics.COM 454 9. Authors' Addresses 456 Fred Baker 457 Cisco Systems 458 519 Lado Drive 459 Santa Barbara, California 93111 460 Phone: (805) 681 0115 461 Email: fred@cisco.com 463 Randall Atkinson 464 cisco Systems 465 170 West Tasman Drive 466 San Jose, CA 95134-1706 467 Voice: (408) 526-6566 468 Email: rja@cisco.com 470 DRAFT RIP-II MD5 Authentication February 1995 472 Table of Contents 474 1 Introduction .................................................... 2 475 2 Implementation Approach ......................................... 3 476 2.1 RIP-II PDU Format ............................................. 3 477 2.2 Processing Algorithm .......................................... 5 478 2.2.1 Message Generation .......................................... 6 479 2.2.2 Message Reception ........................................... 7 480 3 Management Procedures ........................................... 7 481 3.1 Key Management Requirements ................................... 7 482 3.2 Key Management Procedures ..................................... 8 483 3.3 Pathological Cases ............................................ 9 484 4 Conformance Requirements ........................................ 9 485 5 Acknowledgments ................................................. 10 486 6 References ...................................................... 10 487 7 Security Considerations ......................................... 11 488 8 Chairman's Address .............................................. 11 489 9 Authors' Addresses .............................................. 12