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'1' -- Possible downref: Non-RFC (?) normative reference: ref. '2' -- Possible downref: Non-RFC (?) normative reference: ref. '3' -- Possible downref: Non-RFC (?) normative reference: ref. '4' ** Obsolete normative reference: RFC 1750 (ref. '8') (Obsoleted by RFC 4086) ** Downref: Normative reference to an Informational RFC: RFC 1983 (ref. '9') ** Downref: Normative reference to an Informational RFC: RFC 2104 (ref. '12') ** Obsolete normative reference: RFC 2279 (ref. '14') (Obsoleted by RFC 3629) ** Obsolete normative reference: RFC 2234 (ref. '15') (Obsoleted by RFC 4234) ** Obsolete normative reference: RFC 2373 (ref. '16') (Obsoleted by RFC 3513) ** Obsolete normative reference: RFC 2434 (ref. '17') (Obsoleted by RFC 5226) ** Obsolete normative reference: RFC 2440 (ref. '18') (Obsoleted by RFC 4880) ** Obsolete normative reference: RFC 2574 (ref. '19') (Obsoleted by RFC 3414) ** Obsolete normative reference: RFC 3164 (ref. '20') (Obsoleted by RFC 5424) -- Possible downref: Non-RFC (?) normative reference: ref. '23' Summary: 17 errors (**), 0 flaws (~~), 18 warnings (==), 12 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 syslog Working Group J. Kelsey 2 Internet-Draft 3 Expires: November 23, 2005 J. Callas 4 PGP Corporation 5 May 23, 2005 7 Signed syslog Messages 8 draft-ietf-syslog-sign-16.txt 10 Status of this Memo 12 This document is an Internet-Draft and is subject to all provisions 13 of Section 3 of RFC 3667. By submitting this Internet-Draft, each 14 author represents that any applicable patent or other IPR claims of 15 which he or she is aware have been or will be disclosed, and any of 16 which he or she becomes aware will be disclosed, in accordance with 17 Section 6 of BCP 79. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt. 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 This Internet-Draft will expire on November 23, 2005. 37 Copyright Notice 39 Copyright (C) The Internet Society (2005). 41 Abstract 43 This document describes a mechanism to add origin authentication, 44 message integrity, replay-resistance, message sequencing, and 45 detection of missing messages to the transmitted syslog messages. 47 Table of Contents 49 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 50 2. syslog Message Format . . . . . . . . . . . . . . . . . . . . 6 51 3. Signature Block Format and Fields . . . . . . . . . . . . . . 7 52 3.1 syslog Packets Containing a Signature Block . . . . . . . 7 53 3.2 Cookie . . . . . . . . . . . . . . . . . . . . . . . . . . 7 54 3.3 Version . . . . . . . . . . . . . . . . . . . . . . . . . 8 55 3.4 Reboot Session ID . . . . . . . . . . . . . . . . . . . . 8 56 3.5 Signature Group and Signature Priority . . . . . . . . . . 8 57 3.6 Global Block Counter . . . . . . . . . . . . . . . . . . . 10 58 3.7 First Message Number . . . . . . . . . . . . . . . . . . . 11 59 3.8 Count . . . . . . . . . . . . . . . . . . . . . . . . . . 11 60 3.9 Hash Block . . . . . . . . . . . . . . . . . . . . . . . . 11 61 3.10 Signature . . . . . . . . . . . . . . . . . . . . . . . . 11 62 4. Payload and Certificate Blocks . . . . . . . . . . . . . . . . 12 63 4.1 Preliminaries: Key Management and Distribution Issues . . 12 64 4.2 Building the Payload Block . . . . . . . . . . . . . . . . 12 65 4.3 Building the Certificate Block . . . . . . . . . . . . . . 13 66 4.3.1 Cookie . . . . . . . . . . . . . . . . . . . . . . . . 14 67 4.3.2 Version . . . . . . . . . . . . . . . . . . . . . . . 14 68 4.3.3 Reboot Session ID . . . . . . . . . . . . . . . . . . 14 69 4.3.4 Signature Group and Signature Priority . . . . . . . . 15 70 4.3.5 Total Payload Block Length . . . . . . . . . . . . . . 15 71 4.3.6 Index into Payload Block . . . . . . . . . . . . . . . 15 72 4.3.7 Fragment Length . . . . . . . . . . . . . . . . . . . 15 73 4.3.8 Signature . . . . . . . . . . . . . . . . . . . . . . 15 74 5. Redundancy and Flexibility . . . . . . . . . . . . . . . . . . 16 75 5.1 Redundancy . . . . . . . . . . . . . . . . . . . . . . . . 16 76 5.1.1 Certificate Blocks . . . . . . . . . . . . . . . . . . 16 77 5.1.2 Signature Blocks . . . . . . . . . . . . . . . . . . . 16 78 5.2 Flexibility . . . . . . . . . . . . . . . . . . . . . . . 17 79 6. Efficient Verification of Logs . . . . . . . . . . . . . . . . 18 80 6.1 Offline Review of Logs . . . . . . . . . . . . . . . . . . 18 81 6.2 Online Review of Logs . . . . . . . . . . . . . . . . . . 19 82 7. Security Considerations . . . . . . . . . . . . . . . . . . . 21 83 7.1 Cryptography Constraints . . . . . . . . . . . . . . . . . 21 84 7.2 Packet Parameters . . . . . . . . . . . . . . . . . . . . 21 85 7.3 Message Authenticity . . . . . . . . . . . . . . . . . . . 22 86 7.4 Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 22 87 7.5 Replaying . . . . . . . . . . . . . . . . . . . . . . . . 22 88 7.6 Reliable Delivery . . . . . . . . . . . . . . . . . . . . 22 89 7.7 Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 22 90 7.8 Message Integrity . . . . . . . . . . . . . . . . . . . . 23 91 7.9 Message Observation . . . . . . . . . . . . . . . . . . . 23 92 7.10 Man In The Middle . . . . . . . . . . . . . . . . . . . . 23 93 7.11 Denial of Service . . . . . . . . . . . . . . . . . . . . 23 94 7.12 Covert Channels . . . . . . . . . . . . . . . . . . . . . 23 96 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 97 8.1 Version Field . . . . . . . . . . . . . . . . . . . . . . 25 98 8.2 SIG Field . . . . . . . . . . . . . . . . . . . . . . . . 27 99 8.3 Key Blob Type . . . . . . . . . . . . . . . . . . . . . . 27 100 9. Authors and Working Group Chair . . . . . . . . . . . . . . . 28 101 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29 102 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 103 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30 104 Intellectual Property and Copyright Statements . . . . . . . . 31 106 1. Introduction 108 This document describes a mechanism that adds origin authentication, 109 message integrity, replay resistance, message sequencing, and 110 detection of missing messages to syslog. Essentially, this is 111 accomplished by sending a cryptographically signed syslog message 112 containing the signatures of previously sent syslog messages. The 113 contents of this message is called a Signature Block. Each Signature 114 Block contains, in effect, a detached signature on some number of 115 previously sent messages. While most implementations of syslog 116 involve only a single device as the generator of each message and a 117 single receiver as the collector of each message, provisions need to 118 be made to cover messages being sent to multiple receivers. This is 119 generally performed based upon the Priority value of the individual 120 messages. For example, messages from any Facility with a Severity 121 value of 3, 2, 1 or 0 may be sent to one collector while all messages 122 of Facilities 4, 10, 13, and 14 may be sent to another collector. 123 Appropriate syslog-sign messages must be kept with their proper 124 syslog messages. To address this, syslog-sign uses a signature- 125 group. A signature group identifies a group of messages that are all 126 kept together for signing purposes by the device. A Signature Block 127 always belongs to exactly one signature group and it always signs 128 messages belonging only to that signature group. 130 Additionally, a device will send a Certificate Block to provide key 131 management information between the sender and the receiver. This 132 Certificate Block has a field to denote the type of key material 133 which may be such things as a PKIX certificate, an OpenPGP 134 certificate, or even an indication that a key had been 135 predistributed. In all cases, these messages still use the syslog 136 packet format described in this document. In the cases of 137 certificates being sent, the certificates may have to be split across 138 multiple packets. 140 The receiver of the previous messages may verify that the digital 141 signature of each received message matches the signature contained in 142 the Signature Block. A collector may process these Signature Blocks 143 as they arrive, building an authenticated log file. Alternatively, 144 it may store all the log messages in the order they were received. 145 This allows a network operator to authenticate the log file at the 146 time the logs are reviewed. 148 This specification is independent of the actual transport protocol 149 selected. It may be used with syslog packets over traditional UDP 150 [5] as described in RFC 3164 [20]. It may be used with other event 151 notification protocols, and it may be used with the Reliable Delivery 152 of syslog as described in RFC 3195 [21]. Other efforts to define 153 event notification messages should consider this specification in 154 their design. 156 2. syslog Message Format 158 This specification does not rely upon any specific syslog message 159 format. It MAY be transported over a traditional syslog message 160 format such as that defined in the informational RFC 3164 [20], or it 161 MAY be used over the Reliable Delivery of syslog Messages as defined 162 in RFC 3195 [21]. Care must be taken when choosing a transport for 163 this mechanism, however. Since the device generating the Signature 164 Block message signs each message in its entirety, it is imperative 165 that the messages MUST NOT be changed in transit. It is equally 166 imperative that the syslog-sign messages MUST NOT be changed in 167 transit. Specifically, a relay, as described in RFC 3164 MAY make 168 changes to a syslog packet if specific fields are not found. If this 169 occurs, the entire mechanism is rendered useless. 171 For convenience, this document will use the syslog message format in 172 the terms described in RFC 3164. That document describes the 3 parts 173 of a syslog message; the PRI, HEADER, and MSG parts. The MSG part is 174 composed of TAG and CONTENT parts. Space characters separate each of 175 the fields. 177 3. Signature Block Format and Fields 179 This section describes the Signature Block format and the fields used 180 within the Signature Block. 182 3.1 syslog Packets Containing a Signature Block 184 Signature Block messages MUST be encompassed within completely formed 185 syslog messages. It SHOULD also contain a valid TAG field. It is 186 RECOMMENDED that the TAG field have the value of "syslog " (without 187 the double quotes) to signify that this message was generated by the 188 syslog process. The CONTENT field of the syslog Signature Block 189 messages MUST have the following fields. Each of these fields are 190 separated by a single space character. 192 The Signature Block is composed of the following fields. Each field 193 must be printable ASCII, and any binary values are base-64 encoded. 195 Field Designation Size in bytes 196 ----- ----------- ---- -- ----- 198 Cookie Cookie 8 200 Version Ver 4 202 Reboot Session ID RSID 1-10 204 Signature Group SIG 1 206 Signature Priority SPRI 1-3 208 Global Block Counter GBC 1-10 210 First Message Number FMN 1-10 212 Count Count 1-2 214 Hash Block Hash Block variable, size of hash 215 (base-64 encoded binary) 217 Signature Signature variable 218 (base-64 encoded binary) 220 These fields are described below. 222 3.2 Cookie 224 The cookie is a eight-byte sequence to signal that this is a 225 Signature Block. This sequence is "@#sigSIG" (without the double 226 quotes). As noted, a space character follows this, and all other 227 fields. 229 3.3 Version 231 The signature group version field is 4 characters in length and is 232 terminated with a space character. The value in this field specifies 233 the version of the syslog-sign protocol. This is extensible to allow 234 for different hash algorithms and signature schemes to be used in the 235 future. The value of this field is the grouping of the protocol 236 version (2 bytes), the hash algorithm (1 byte) and the signature 237 scheme (1 byte). 239 Protocol Version - 2 bytes with the first version as described in 240 this document being value of 01 to denote Version 1. 242 Hash Algorithm - 1 byte with the definition that 1 denotes SHA1 as 243 defined in FIPS-180-1.1995 [2]. 245 Signature Scheme - 1 byte with the definition that 1 denotes 246 OpenPGP DSA - RFC 2440 [18], FIPS.186-1.1998 [1]. 248 As such, the version, hash algorithm and signature scheme defined in 249 this document may be represented as "0111" (without the quote marks). 251 3.4 Reboot Session ID 253 The reboot session ID is a value between 1 and 10 bytes, which is 254 required to never repeat or decrease. The acceptable values for this 255 are between 0 and 9999999999. If the value latches at 9999999999, 256 then manual intervention may be required to reset it to 0. 257 Implementors MAY wish to consider using the snmpEngineBoots value as 258 a source for this counter as defined in RFC 2574 [19]. 260 3.5 Signature Group and Signature Priority 262 The SIG identifier as described above may take on any value from 0-3 263 inclusive. The SPRI may take any value from 0-191. Each of these 264 fields are followed by a space character. These fields taken 265 together allows network administrators to associate groupings of 266 syslog messages with appropriate Signature Blocks and Certificate 267 Blocks. For example, in some cases, network administrators may send 268 syslog messages of Facilities 0 through 15 to one destination while 269 sending messages with Facilities 16 through 23 to another. 270 Associated Signature Blocks should be sent to these different syslog 271 servers as well. 273 In some cases, an administrator may wish the Signature Blocks to go 274 to the same destination as the syslog messages themselves. This may 275 be to different syslog servers if the destinations of syslog messages 276 is being controlled by the Facilities or the Severities of the 277 messages. In other cases, administrators may wish to send the 278 Signature Blocks to an altogether different destination. 280 Syslog-sign provides four options for handling signature groups, 281 linking them with PRI values so they may be routed to the destination 282 commensurate with the appropriate syslog messages. In all cases, no 283 more than 192 signature groups (0-191) are permitted. 285 a. '0' -- There is only one signature group. All Signature Block 286 messages use a single PRI value which is the same SPRI value. In 287 this case, the administrators want all Signature Blocks to be 288 sent to a single destination. In all likelihood, all of the 289 syslog messages will also be going to that same destination. As 290 one example, if SIG=0, then PRI and SPRI may be 46 to indicate 291 that they are informational messages from the syslog daemon. If 292 the device is configured to send all messages with the local5 293 Facility (21), then the PRI and SPRI may be 174 to indicate that 294 they are also from the local5 Facility with a Severity of 6. 296 b. '1' -- Each PRI value has its own signature group. Signature 297 Blocks for a given signature group have SPRI = PRI for that 298 signature group. In this case, the administrator of a device may 299 not know where any of the syslog messages will ultimately go. 300 This use ensures that a Signature Block follows each of the 301 syslog messages to each destination. This may be seen to be 302 inefficient if groups of syslog messages are actually going to 303 the same syslog server. Examine an example of a device being 304 configured to have a SIG value of 1, which generates 16 syslog 305 messages with 307 4 from PRI=132 (Facility 16, Severity 4), 308 4 from PRI=148 (Facility 18, Severity 4), 309 4 from PRI=164, (Facility 20, Severity 4), and 310 4 from PRI=180 (Facility 22, Severity 4). 312 In actuality, the messages from Facilities local0 and local2 go 313 to one syslog server and messages from Facilities local4 and 314 local6 go to a different one. Then, the first syslog server 315 receives 2 Signature Blocks, the first with PRI=134 and the 316 second from PRI=150 - the PRI values matching the SPRI values. 317 The second syslog server would also receive two Signature Block 318 messages, the first from PRI=164 and the second from PRI=180. In 319 each of those Signature Blocks, the SPRI values matches their 320 respective PRI values. In each of these cases, the Signature 321 Blocks going to each respective syslog server could have been 322 combined. One way to do this more efficiently is explained using 323 SIG=2. 325 c. '2' -- Each signature group contains a range of PRI values. 326 Signature groups are assigned sequentially. A Signature Block 327 for a given signature group has its own SPRI value denoting the 328 highest PRI value in that signature group. For flexibility, the 329 PRI does not have to be that upper-boundary SPRI value. 330 Continuing the above example, the administrator of the device may 331 configure SIG=2 with upper-bound SPRIs of 151 and 191. The lower 332 group contains all PRIs between 0 and 151, and the second group 333 contains all PRIs between 152 and 191. The administrator may 334 then wish to configure the lower group to send all of the lower 335 group Signature Blocks using PRI=150 (Facility 18, Severity 6), 336 and the upper group using PRI=182 (Facility 22, Severity 6). The 337 receiving syslog servers then each receive a single Signature 338 Block describing the 8 syslog messages sent to it. 340 d. '3' -- Signature groups are not assigned with any simple 341 relationship to PRI values. This has to be some predefined 342 arrangement between the sender and the intended receivers. In 343 this case, the administrators of the devices and syslog servers 344 may, as an example, use SIG=3 with a SPRI of 1 to denote that all 345 Warning and above syslog messages from all Facilities are sent 346 using a PRI of 46 (Facility 5, Severity 6). 348 One reasonable way to configure some installations is to have only 349 one signature group with SIG=0. The devices send messages to many 350 collectors and also send a copy of each Signature Block to each 351 collector. This won't allow any collector to detect gaps in the 352 messages, but it allows all messages that arrive at each collector to 353 be put into the right order, and to be verified. It also allows each 354 collector to detect duplicates and any messages that are not 355 associated with a Signature Block. 357 3.6 Global Block Counter 359 The global block counter is a value representing the number of 360 Signature Blocks sent out by syslog-sign before this one, in this 361 reboot session. This takes at least 1 byte and at most 10 bytes 362 displayed as a decimal counter and the acceptable values for this are 363 between 0 and 9999999999. If the value latches at 9999999999, then 364 the reboot session counter must be incremented by 1 and the global 365 block counter resumes at 0. Note that this counter crosses signature 366 groups; it allows us to roughly synchronize when two messages were 367 sent, even though they went to different collectors. 369 3.7 First Message Number 371 This is a value between 1 and 10 bytes. It contains the unique 372 message number within this signature group of the first message whose 373 hash appears in this block. The very first message of the reboot 374 session will be numbered "1". 376 For example, if this signature group has processed 1000 messages so 377 far and message number 1001 is the first message whose hash appears 378 in this Signature Block, then this field contains 1001. 380 3.8 Count 382 The count is a 1 or 2 byte field displaying the number of message 383 hashes to follow. The valid values for this field are between 1 and 384 99. 386 3.9 Hash Block 388 The hash block is a block of hashes, each separately encoded in 389 base-64. Each hash in the hash block is the hash of the entire 390 syslog message represented by the hash. The hashing algorithm used 391 effectively specified by the Version field determines the size of 392 each hash, but the size MUST NOT be shorter than 160 bits. It is 393 base-64 encoded as per RFC 2045. 395 3.10 Signature 397 This is a digital signature, encoded in base-64, as per RFC 2045. 398 The signature is calculated over all fields but excludes the space 399 characters between them. The Version field effectively specifies the 400 original encoding of the signature. The signature is a signature 401 over the entire data, including all of the PRI, HEADER, and hashes in 402 the hash block. To reiterate, the signature is calculated over the 403 completely formatted syslog-message, excluding spaces between fields, 404 and also excluding this signature field. 406 4. Payload and Certificate Blocks 408 Certificate Blocks and Payload Blocks provide key management in 409 syslog-sign. 411 4.1 Preliminaries: Key Management and Distribution Issues 413 The purpose of Certificate Blocks is to support key management using 414 public key cryptosystems. All devices send at least one Certificate 415 Block at the beginning of a new reboot session, carrying useful 416 information about the reboot session. 418 There are three key points to understand about Certificate Blocks: 420 a. They handle a variable-sized payload, fragmenting it if necessary 421 and transmitting the fragments as legal syslog messages. This 422 payload is built (as described below) at the beginning of a 423 reboot session and is transmitted in pieces with each Certificate 424 Block carrying a piece. Note that there is exactly one Payload 425 Block per reboot session. 427 b. The Certificate Blocks are digitally signed. The device does not 428 sign the Payload Block, but the signatures on the Certificate 429 Blocks ensure its authenticity. Note that it may not even be 430 possible to verify the signature on the Certificate Blocks 431 without the information in the Payload Block; in this case the 432 Payload Block is reconstructed, the key is extracted, and then 433 the Certificate Blocks are verified. (This is necessary even 434 when the Payload Block carries a certificate, since some other 435 fields of the Payload Block aren't otherwise verified.) In 436 practice, most installations keep the same public key over long 437 periods of time, so that most of the time, it's easy to verify 438 the signatures on the Certificate Blocks, and use the Payload 439 Block to provide other useful per-session information. 441 c. The kind of Payload Block that is expected is determined by what 442 kind of key material is on the collector that receives it. The 443 device and collector (or offline log viewer) has both some key 444 material (such as a root public key, or predistributed public 445 key), and an acceptable value for the Key Blob Type in the 446 Payload Block, below. The collector or offline log viewer MUST 447 NOT accept a Payload Block of the wrong type. 449 4.2 Building the Payload Block 451 The Payload Block is built when a new reboot session is started. 452 There is a one-to-one correspondence of reboot sessions to Payload 453 Blocks. That is, each reboot session has only one Payload Block, 454 regardless of how many signature groups it may support. Like syslog 455 packets containing the Signature Block, Payload Block messages MUST 456 be completely formed syslog messages. It is RECOMMENDED that the TAG 457 field have the value of "syslog " (without the double quotes) to 458 signify that this message was generated by the syslog process. The 459 CONTENT field of the syslog Payload Block messages MUST have the 460 following fields. Each of these fields are separated by a single 461 space character. 463 a. Unique identifier of sender; by default, the sender's IP address 464 in dotted-decimal (IPv4) or colon-separated (IPv6) notation. 466 b. Full local time stamp for the device at the time the reboot 467 session started. This must be in TIMESTAMP-3339 format. 469 c. Key Blob Type, a one-byte field which holds one of five values: 471 1. 'C' -- a PKIX certificate. 473 2. 'P' -- an OpenPGP certificate. 475 3. 'K' -- the public key whose corresponding private key is 476 being used to sign these messages. 478 4. 'N' -- no key information sent; key is predistributed. 480 5. 'U' -- installation-specific key exchange information 482 d. The key blob, consisting of the raw key data, if any, base-64 483 encoded. 485 4.3 Building the Certificate Block 487 The Certificate Block must get the Payload Block to the collector. 488 Since certificates can legitimately be much longer than 1024 bytes, 489 each Certificate Block carries a piece of the Payload Block. Note 490 that the device MAY make the Certificate Blocks of any legal length 491 (that is, any length less than 1024 bytes) which holds all the 492 required fields. Software that processes Certificate Blocks MUST 493 deal correctly with blocks of any legal length. 495 The Certificate Block is composed of the following fields. Each 496 field must be printable ASCII, and any binary values are base-64 497 encoded. 499 Field Designation Size in bytes 500 ----- ----------- ---- -- ----- 502 Cookie Cookie 8 504 Version Ver 4 506 Reboot Session ID RSID 1-10 508 Signature Group SIG 1 510 Signature Priority SPRI 1-3 512 Total Payload Block Length TPBL 1-8 514 Index into Payload Block Index 1-8 516 Fragment Length FragLen 1-3 518 Payload Block Fragment Fragment variable 519 (base-64 encoded binary) 521 Signature Signature variable 522 (base-64 encoded binary) 524 4.3.1 Cookie 526 The cookie is a eight-byte sequence to signal that this is a 527 Signature Block. This sequence is "@#sigCER" (without the double 528 quotes). As noted, a space character follows this, and all other 529 fields. 531 4.3.2 Version 533 The signature group version field is 4 characters in length and is 534 terminated with a space character. This field is identical in nature 535 to the Version field described in Section 3.3. As such, the version, 536 hash algorithm and signature scheme defined in this document may be 537 represented as "0111" (without the quote marks). 539 4.3.3 Reboot Session ID 541 The Reboot Session ID is identical in characteristics to the RSID 542 field described in Section 3.4. 544 4.3.4 Signature Group and Signature Priority 546 The SIG field is identical in characteristics to the SIG field 547 described in Section 3.10. Also, the SPRI field is identical to the 548 SPRI field described there. 550 4.3.5 Total Payload Block Length 552 The Total Payload Block Length is a value representing the total 553 length of the Payload Block in bytes in decimal. This will be one to 554 eight bytes. 556 4.3.6 Index into Payload Block 558 This is a value between 1 and 8 bytes. It contains the number of 559 bytes into the Payload Block where this fragment starts. The first 560 byte of the first fragment is numbered "1". 562 4.3.7 Fragment Length 564 The total length of this fragment expressed as a decimal integer. 565 This will be one to three bytes. 567 4.3.8 Signature 569 This is a digital signature, encoded in base-64, as per RFC 2045. 570 The signature is calculated over all fields but excludes the space 571 characters between them. The Version field effectively specifies the 572 original encoding of the signature. The signature is a signature 573 over the entire data, including all of the PRI, HEADER, and hashes in 574 the hash block. This is consistent with the method of calculating 575 the signature as specified in Section 3.10. To reiterate, the 576 signature is calculated over the completely formatted syslog-message, 577 excluding spaces between fields, and also excluding this signature 578 field. 580 5. Redundancy and Flexibility 582 There is a general rule that determines how redundancy works and what 583 level of flexibility the device and collector have in message 584 formats: in general, the device is allowed to send Signature and 585 Certificate Blocks multiple times, to send Signature and Certificate 586 Blocks of any legal length, to include fewer hashes in hash blocks, 587 etc. 589 5.1 Redundancy 591 Syslog messages are sent over unreliable transport, which means that 592 they can be lost in transit. However, the collector must receive 593 Signature and Certificate Blocks or many messages may not be able to 594 be verified. Sending Signature and Certificate Blocks multiple times 595 provides redundancy; since the collector MUST ignore Signature/ 596 Certificate Blocks it has already received and authenticated, the 597 device can in principle change its redundancy level for any reason, 598 without communicating this fact to the collector. 600 Although the device isn't constrained in how it decides to send 601 redundant Signature and Certificate Blocks, or even in whether it 602 decides to send along multiple copies of normal syslog messages, here 603 we define some redundancy parameters below which may be useful in 604 controlling redundant transmission from the device to the collector. 606 5.1.1 Certificate Blocks 608 certInitialRepeat = number of times each Certificate Block should be 609 sent before the first message is sent. 611 certResendDelay = maximum time delay in seconds to delay before next 612 redundant sending. 614 certResendCount = maximum number of sent messages to delay before 615 next redundant sending. 617 5.1.2 Signature Blocks 619 sigNumberResends = number of times a Signature Block is resent. 621 sigResendDelay = maximum time delay in seconds from original 622 sending to next redundant sending. 624 sigResendCount = maximum number of sent messages to delay before 625 next redundant sending. 627 5.2 Flexibility 629 The device may change many things about the makeup of Signature and 630 Certificate Blocks in a given reboot session. The things it cannot 631 change are: 633 * The version 635 * The number or arrangements of signature groups 637 It is legitimate for a device to send out short Signature Blocks, in 638 order to keep the collector able to verify messages quickly. In 639 general, unless something verified by the Payload Block or 640 Certificate Blocks is changed within the reboot session ID, any 641 change is allowed to the Signature or Certificate Blocks during the 642 session. 644 6. Efficient Verification of Logs 646 The logs secured with syslog-sign may either be reviewed online or 647 offline. Online review is somewhat more complicated and 648 computationally expensive, but not prohibitively so. 650 6.1 Offline Review of Logs 652 When the collector stores logs and reviewed later, they can be 653 authenticated offline just before they are reviewed. Reviewing these 654 logs offline is simple and relatively cheap in terms of resources 655 used, so long as there is enough space available on the reviewing 656 machine. Here, we consider that the stored log files have already 657 been separated by sender, reboot session ID, and signature group. 658 This can be done very easily with a script file. We then do the 659 following: 661 a. First, we go through the raw log file, and split its contents 662 into three files. Each message in the raw log file is classified 663 as a normal message, a Signature Block, or a Certificate Block. 664 Certificate Blocks and Signature Blocks are stored in their own 665 files. Normal messages are stored in a keyed file, indexed on 666 their hash values. 668 b. We sort the Certificate Block file by index value, and check to 669 see if we have a set of Certificate Blocks that can reconstruct 670 the Payload Block. If so, we reconstruct the Payload Block, 671 verify any key-identifying information, and then use this to 672 verify the signatures on the Certificate Blocks we've received. 673 When this is done, we have verified the reboot session and key 674 used for the rest of the process. 676 c. We sort the Signature Block file by firstMessageNumber. We now 677 create an authenticated log file, which consists of some header 678 information, and then a sequence of message number, message text 679 pairs. We next go through the Signature Block file. For each 680 Signature Block in the file, we do the following: 682 1. Verify the signature on the Block. 684 2. For each hashed message in the Block: 686 a. Look up the hash value in the keyed message file. 688 b. If the message is found, write (message number, message 689 text) to the authenticated log file. 691 3. Skip all other Signature Blocks with the same 692 firstMessageNumber. 694 d. The resulting authenticated log file contains all messages that 695 have been authenticated, and implicitly indicates (by missing 696 message numbers) all gaps in the authenticated messages. 698 It's pretty easy to see that, assuming sufficient space for building 699 the keyed file, this whole process is linear in the number of 700 messages (generally two seeks, one to write and the other to read, 701 per normal message received), and O(N lg N) in the number of 702 Signature Blocks. This estimate comes with two caveats: first, the 703 Signature Blocks arrive very nearly in sorted order, and so can 704 probably be sorted more cheaply on average than O(N lg N) steps. 705 Second, the signature verification on each Signature Block almost 706 certainly is more expensive than the sorting step in practice. We 707 haven't discussed error-recovery, which may be necessary for the 708 Certificate Blocks. In practice, a very simple error-recovery 709 strategy is probably good enough -- if the Payload Block doesn't come 710 out as valid, then we can just try an alternate instance of each 711 Certificate Block, if such are available, until we get the Payload 712 Block right. 714 It's easy for an attacker to flood us with plausible-looking 715 messages, Signature Blocks, and Certificate Blocks. 717 6.2 Online Review of Logs 719 Some processes on the collector machine may need to monitor log 720 messages in something very close to real-time. This can be done with 721 syslog-sign, though it is somewhat more complex than the offline 722 analysis. This is done as follows: 724 a. We have an output queue, into which we write (message number, 725 message text) pairs which have been authenticated. Again, we'll 726 assume we're handling only one signature group, and only one 727 reboot session ID, at any given time. 729 b. We have three data structures: A queue into which (message 730 number, hash of message) pairs is kept in sorted order, a queue 731 into which (arrival sequence, hash of message) is kept in sorted 732 order, and a hash table which stores (message text, count) 733 indexed by hash value. In this file, count may be any number 734 greater than zero; when count is zero, the entry in the hash 735 table is cleared. 737 c. We must receive all the Certificate Blocks before any other 738 processing can really be done. (This is why they're sent first.) 739 Once that's done, any Certificate Block that arrives is 740 discarded. 742 d. Whenever a normal message arrives, we add (arrival sequence, hash 743 of message) to our message queue. If our hash table has an entry 744 for the message's hash value, we increment its count by one; 745 otherwise, we create a new entry with count = 1. When the 746 message queue is full, we roll the oldest messages off the queue 747 by taking the last entry in the queue, and using it to index the 748 hash table. If that entry has count is 1, we delete the entry in 749 the hash table; otherwise, we decrement its count. We then 750 delete the last entry in the queue. 752 e. Whenever a Signature Block arrives, we first check to see if the 753 firstMessageNumber value is too old, or if another Signature 754 Block with that firstMessageNumber has already been received. If 755 so, we discard the Signature Block unread. Otherwise, we check 756 its signature, and discard it if the signature isn't valid. A 757 Signature Block contains a sequence of (message number, message 758 hash) pairs. For each pair, we first check to see if the message 759 hash is in the hash table. If so, we write out the (message 760 number, message text) in the authenticated message queue. 761 Otherwise, we write the (message number, message hash) to the 762 message number queue. This generally involves rolling the oldest 763 entry out of this queue: before this is done, that entry's hash 764 value is again searched for in the hash table. If a matching 765 entry is found, the (message number, message text) pair is 766 written out to the authenticated message queue. In either case, 767 the oldest entry is then discarded. 769 f. The result of this is a sequence of messages in the authenticated 770 message queue, each of which has been authenticated, and which 771 are combined with numbers showing their order of original 772 transmission. 774 It's not too hard to see that this whole process is roughly linear in 775 the number of messages, and also in the number of Signature Blocks 776 received. The process is susceptible to flooding attacks; an 777 attacker can send enough normal messages that the messages roll off 778 their queue before their Signature Blocks can be processed. 780 7. Security Considerations 782 Normal syslog event messages are unsigned and have most of the 783 security attributes described in Section 6 of RFC 3164. This 784 document also describes Certificate Blocks and Signature Blocks which 785 are signed syslog messages. The Signature Blocks contains signature 786 information of previously sent syslog event messages. All of this 787 information may be used to authenticate syslog messages and to 788 minimize or obviate many of the security concerns described in RFC 789 3164. 791 7.1 Cryptography Constraints 793 As with any technology involving cryptography, you should check the 794 current literature to determine if any algorithms used here have been 795 found to be vulnerable to attack. 797 This specification uses Public Key Cryptography technologies. The 798 proper party or parties must control the private key portion of a 799 public-private key pair. Any party that controls a private key may 800 sign anything they please. 802 Certain operations in this specification involve the use of random 803 numbers. An appropriate entropy source should be used to generate 804 these numbers. See RFC 1750 [8]. 806 7.2 Packet Parameters 808 The message length must not exceed 1024 bytes. Various problems may 809 result if a device sends out messages with a length greater than 1024 810 bytes. As seen in RFC 3164, relays MAY truncate messages with 811 lengths greater than 1024 bytes which would result in a problem for 812 receivers trying to validate a hash of the packet. In this case, as 813 with all others, it is best to be conservative with what you send but 814 liberal in what you receive, and accept more than 1024 bytes. 816 Similarly, senders must rigidly enforce the correctness of the 817 message body. This document specifies an enhancement to the syslog 818 protocol but does not stipulate any specific syslog message format. 819 Nonetheless, problems may arise if the receiver does not fully accept 820 the syslog packets sent from a device, or if it has problems with the 821 format of the Certificate Block or Signature Block messages. 823 Finally, receivers must not malfunction if they receive syslog 824 messages containing characters other than those specified in this 825 document. 827 7.3 Message Authenticity 829 Event messages being sent through syslog do not strongly associate 830 the message with the message sender. That fact is established by the 831 receiver upon verification of the Signature Block as described above. 832 Before a Signature Block is used to ascertain the authenticity of an 833 event message, it may be received, stored and reviewed by a person or 834 automated parser. Both of these should maintain doubt about the 835 authenticity of the message until after it has been validated by 836 checking the contents of the Signature Block. 838 With the Signature Block checking, an attacker may only forge 839 messages if they can compromise the private key of the true sender. 841 7.4 Sequenced Delivery 843 Event messages may be recorded and replayed by an attacker. However 844 the information contained in the Signature Blocks allows a reviewer 845 to determine if the received messages are the ones originally sent by 846 a device. This process also alerts the reviewer to replayed 847 messages. 849 7.5 Replaying 851 Event messages may be recorded and replayed by an attacker. However 852 the information contained in the Signature Blocks will allow a 853 reviewer to determine if the received messages are the ones 854 originally sent by a device. This process will also alert the 855 reviewer to replayed messages. 857 7.6 Reliable Delivery 859 RFC 3195 may be used for the reliable delivery of all syslog 860 messages. This document acknowledges that event messages sent over 861 UDP may be lost in transit. A proper review of the Signature Block 862 information may pinpoint any messages sent by the sender but not 863 received by the receiver. The overlap of information in subsequent 864 Signature Block information allows a reviewer to determine if any 865 Signature Block messages were also lost in transit. 867 7.7 Sequenced Delivery 869 Related to the above, syslog messages delivered over UDP not only may 870 be lost, but they may arrive out of sequence. The information 871 contained in the Signature Block allows a receiver to correctly order 872 the event messages. Beyond that, the timestamp information contained 873 in the packet may help the reviewer to visually order received 874 messages even if they are received out of order. 876 7.8 Message Integrity 878 syslog messages may be damaged in transit. A review of the 879 information in the Signature Block determines if the received message 880 was the intended message sent by the sender. A damaged Signature 881 Block or Certificate Block will be evident since the receiver will 882 not be able to validate that it was signed by the sender. 884 7.9 Message Observation 886 Event messages, Certificate Blocks and Signature Blocks are all sent 887 in plaintext. Generally this has had the benefit of allowing network 888 administrators to read the message when sniffing the wire. However, 889 this also allows an attacker to see the contents of event messages 890 and perhaps to use that information for malicious purposes. 892 7.10 Man In The Middle 894 It is conceivable that an attacker may intercept Certificate Blocks 895 and insert their own Certificate information. In that case, the 896 attacker would be able to receive event messages from the actual 897 sender and then relay modified messages, insert new messages, or 898 deleted messages. They would then be able to construct a Signature 899 Block and sign it with their own private key. The network 900 administrators should verify that the key contained in the 901 Certificate Block is indeed the key being used on the actual device. 902 If that is indeed the case, then this MITM attack will not succeed. 904 7.11 Denial of Service 906 An attacker may be able to overwhelm a receiver by sending it invalid 907 Signature Block messages. If the receiver is attempting to process 908 these messages online, it may consume all available resources. For 909 this reason, it may be appropriate to just receive the Signature 910 Block messages and process them as time permits. 912 As with any system, an attacker may also just overwhelm a receiver by 913 sending more messages to it than can be handled by the infrastructure 914 or the device itself. Implementors should attempt to provide 915 features that minimize this threat. Such as only receiving syslog 916 messages from known IP addresses. 918 7.12 Covert Channels 920 Nothing in this protocol attempts to eliminate covert channels. 921 Indeed, the unformatted message syntax in the packets could be very 922 amenable to sending embedded secret messages. In fact, just about 923 every aspect of syslog messages lends itself to the conveyance of 924 covert signals. For example, a collusionist could send odd and even 925 PRI values to indicate Morse Code dashes and dots. 927 8. IANA Considerations 929 Two syslog packet types are specified in this document; the Signature 930 Block and the Certificate Block. Each of these has several fields 931 specified that should be controlled by the IANA. Essentially these 932 packet types may be differentiated based upon the value in the Cookie 933 field. The Signature Block packet may be identified by a value of 934 "@#sigSIG" in the Cookie field. The Certificate Block packet may be 935 identified by a value of "@#sigCER" in the Cookie field. Each of 936 these packet types share fields that should be consistent; 937 specifically, the Certificate Block packet types may be considered to 938 be an announcement of capabilities and the Signature Block packets 939 SHOULD have the same values in the fields described in this section. 940 This document allows that there may be some really fine reason for 941 the values to be different between the two packet types but the 942 authors and contributors can't see any valid reason for that at this 943 time. 945 This document also upholds the Facilities and Severities listed in 946 RFC 3164 [20]. Those values range from 0 to 191. This document also 947 instructs the IANA to reserve all other possible values of the 948 Severities and Facilities above the value of 191 and to distribute 949 them via the consensus process as defined in RFC 2434 [17]. 951 The following fields are to be controlled by the IANA in both the 952 Signature Block packets and the Certificate Block packets. 954 8.1 Version Field 956 The Version field (Ver) is a 4 byte field. The first two bytes of 957 this field define the version of the Signature Block packets and the 958 Certificate Block Packets. This allows for future efforts to 959 redefine the subsequent fields in the Signature Block packets and 960 Certificate Block packets. A value of "00" is reserved and not used. 961 This document describes the fields for the version value of "01". It 962 is expected that this value be incremented monotonically with decimal 963 values up through "50" for IANA assigned values. Values "02" through 964 "50" will be assigned by the IANA using the "IETF Consensus" policy 965 defined in RFC 2434 [17]. It is not anticipated that these values 966 will be reused. Values of "51" through "99" will be vendor-specific, 967 and values in this range are not to be assigned by the IANA. 969 In the case of vendor-specific assigned Version numbers, all 970 subsequent values defined in the packet will then have vendor- 971 specific meaning. They may, or may not, align with the values 972 assigned by the IANA for these fields. For example, a vendor may 973 choose to define their own Version of "51" still containing values of 974 "1" for the Hash Algorithm and Signature Scheme which aligns with the 975 IANA assigned values as defined in this document. However, they may 976 then choose to define a value of "5" for the Signature Group for 977 their own reasons. 979 The third byte of the Ver field defines the Hash Algorithm. It is 980 envisioned that this will also be a monotonically increasing value 981 with a maximum value of "9". The value of "1" is defined in this 982 document as the first assigned value and is SHA1 FIPS-180-1.1995 [2]. 983 Subsequent values will be assigned by the IANA using the "IETF 984 Consensus" policy defined in RFC 2434 [17]. 986 The forth and final byte of the Ver field defines the Signature 987 Scheme. It is envisioned that this too will be a monotonically 988 increasing value with a maximum value of "9". The value of "1" is 989 defined in this document as OpenPGP DSA - RFC 2440 [18], FIPS.186- 990 1.1998 [1]. Subsequent values will be assigned by the IANA using the 991 "IETF Consensus" policy defined in RFC 2434 [17]. The fields, values 992 assigned in this document and ranges are illustrated in the following 993 table. 995 Field Value Defined IANA Assigned Vendor Specific 996 in this Document Range Range 997 ----- ---------------- ------------- --------------- 998 Ver 999 ver 01 01-50 50-99 1000 hash 1 0-9 -none- 1001 sig 1 0-9 -none- 1003 If either the Hash Algorithm field or the Signature Scheme field is 1004 needed to go beyond "9" within the current version (first two bytes), 1005 the IANA should increment the first two bytes of this 4 byte field to 1006 be the next value with the definition that all of the subsequent 1007 values of fields described in this section are reset to "0" while 1008 retaining the latest definitions given by the IANA. For example, 1009 consider the case that the first two characters are "23" and the 1010 latest Signature Algorithm is 4. Let's say that the latest Hash 1011 Algorithm value is "9" but a better Hash Algorithm is defined. In 1012 that case, the IANA will increment the first two bytes to become 1013 "24", retain the current Hash Algorithm to be "0", define the new 1014 Hash Algorithm to be "1" in this scheme, and define the current 1015 Signature Scheme to also be "0". This example is illustrated in the 1016 following table. 1018 Current New - Equivalent New with Later 1019 to "Current" Algorithms 1020 ------- -------------- --------------- 1021 ver = 23 ver = 24 ver = 24 1022 hash = 9 hash = 0 hash = 1 1023 sig = 4 sig = 0 sig = 0 1025 8.2 SIG Field 1027 The SIG field values are numbers as defined in Section 3.5. Values 1028 "0" through "3" are assigned in this document. The IANA shall assign 1029 values "4" through "7" using the "IETF Consensus" policy defined in 1030 RFC 2434 [17]. Values "8" and "9" shall be left as vendor specific 1031 and shall not be assigned by the IANA. 1033 8.3 Key Blob Type 1035 Section Section 4.2 defines five, one character identifiers for the 1036 key blob type. These are the uppercase letters, "C", "P", "K", "N", 1037 and "U". All other uppercase letters shall be assigned by the IANA 1038 using the "IETF Consensus" policy defined in RFC 2434 [17]. 1039 Lowercase letters are left as vendor specific and shall not be 1040 assigned by the IANA. 1042 9. Authors and Working Group Chair 1044 The working group can be contacted via the mailing list: 1046 syslog-sec@employees.org 1048 The current Chair of the Working Group may be contacted at: 1050 Chris Lonvick 1051 Cisco Systems 1052 Email: clonvick@cisco.com 1054 The authors of this draft are: 1056 John Kelsey 1057 Email: kelsey.j@ix.netcom.com 1059 Jon Callas 1060 Email: jon@callas.org 1062 10. Acknowledgements 1064 The authors wish to thank Alex Brown, Chris Calabrese, Carson Gaspar, 1065 Drew Gross, Chris Lonvick, Darrin New, Marshall Rose, Holt Sorenson, 1066 Rodney Thayer, Andrew Ross, Rainer Gerhards, Albert Mietus, and the 1067 many Counterpane Internet Security engineering and operations people 1068 who commented on various versions of this proposal. 1070 11. References 1072 [1] National Institute of Standards and Technology, "Digital 1073 Signature Standard", FIPS PUB 186-1, December 1998, 1074 . 1076 [2] National Institute of Standards and Technology, "Secure Hash 1077 Standard", FIPS PUB 180-1, April 1995, 1078 . 1080 [3] American National Standards Institute, "USA Code for 1081 Information Interchange", ANSI X3.4, 1968. 1083 [4] Menezes, A., van Oorschot, P., and S. Vanstone, ""Handbook of 1084 Applied Cryptography", CRC Press", 1996. 1086 [5] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 1087 August 1980. 1089 [6] Mockapetris, P., "Domain names - concepts and facilities", 1090 STD 13, RFC 1034, November 1987. 1092 [7] Mockapetris, P., "Domain names - implementation and 1093 specification", STD 13, RFC 1035, November 1987. 1095 [8] Eastlake, D., Crocker, S., and J. Schiller, "Randomness 1096 Recommendations for Security", RFC 1750, December 1994. 1098 [9] Malkin, G., "Internet Users' Glossary", RFC 1983, August 1996. 1100 [10] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 1101 Extensions (MIME) Part One: Format of Internet Message Bodies", 1102 RFC 2045, November 1996. 1104 [11] Oehler, M. and R. Glenn, "HMAC-MD5 IP Authentication with 1105 Replay Prevention", RFC 2085, February 1997. 1107 [12] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing 1108 for Message Authentication", RFC 2104, February 1997. 1110 [13] Bradner, S., "Key words for use in RFCs to Indicate Requirement 1111 Levels", BCP 14, RFC 2119, March 1997. 1113 [14] Yergeau, F., "UTF-8, a transformation format of ISO 10646", 1114 RFC 2279, January 1998. 1116 [15] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1117 Specifications: ABNF", RFC 2234, November 1997. 1119 [16] Hinden, R. and S. Deering, "IP Version 6 Addressing 1120 Architecture", RFC 2373, July 1998. 1122 [17] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA 1123 Considerations Section in RFCs", BCP 26, RFC 2434, 1124 October 1998. 1126 [18] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, 1127 "OpenPGP Message Format", RFC 2440, November 1998. 1129 [19] Blumenthal, U. and B. Wijnen, "User-based Security Model (USM) 1130 for version 3 of the Simple Network Management Protocol 1131 (SNMPv3)", RFC 2574, April 1999. 1133 [20] Lonvick, C., "The BSD Syslog Protocol", RFC 3164, August 2001. 1135 [21] New, D. and M. Rose, "Reliable Delivery for syslog", RFC 3195, 1136 November 2001. 1138 [22] Klyne, G. and C. Newman, "Date and Time on the Internet: 1139 Timestamps", RFC 3339, July 2002. 1141 [23] Schneier, B., "Applied Cryptography Second Edition: protocols, 1142 algorithms, and source code in C", 1996. 1144 Authors' Addresses 1146 John Kelsey 1148 Email: kelsey.j@ix.netcom.com 1150 Jon Callas 1151 PGP Corporation 1153 Email: jon@callas.org 1155 Intellectual Property Statement 1157 The IETF takes no position regarding the validity or scope of any 1158 Intellectual Property Rights or other rights that might be claimed to 1159 pertain to the implementation or use of the technology described in 1160 this document or the extent to which any license under such rights 1161 might or might not be available; nor does it represent that it has 1162 made any independent effort to identify any such rights. 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