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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 TRANS (Public Notary Transparency) B. Laurie 3 Internet-Draft A. Langley 4 Obsoletes: 6962 (if approved) E. Kasper 5 Intended status: Experimental E. Messeri 6 Expires: 14 November 2021 Google 7 R. Stradling 8 Sectigo 9 13 May 2021 11 Certificate Transparency Version 2.0 12 draft-ietf-trans-rfc6962-bis-36 14 Abstract 16 This document describes version 2.0 of the Certificate Transparency 17 (CT) protocol for publicly logging the existence of Transport Layer 18 Security (TLS) server certificates as they are issued or observed, in 19 a manner that allows anyone to audit certification authority (CA) 20 activity and notice the issuance of suspect certificates as well as 21 to audit the certificate logs themselves. The intent is that 22 eventually clients would refuse to honor certificates that do not 23 appear in a log, effectively forcing CAs to add all issued 24 certificates to the logs. 26 This document obsoletes RFC 6962. It also specifies a new TLS 27 extension that is used to send various CT log artifacts. 29 Logs are network services that implement the protocol operations for 30 submissions and queries that are defined in this document. 32 [RFC Editor: please update 'RFCXXXX' to refer to this document, once 33 its RFC number is known, through the document.] 35 Status of This Memo 37 This Internet-Draft is submitted in full conformance with the 38 provisions of BCP 78 and BCP 79. 40 Internet-Drafts are working documents of the Internet Engineering 41 Task Force (IETF). Note that other groups may also distribute 42 working documents as Internet-Drafts. The list of current Internet- 43 Drafts is at https://datatracker.ietf.org/drafts/current/. 45 Internet-Drafts are draft documents valid for a maximum of six months 46 and may be updated, replaced, or obsoleted by other documents at any 47 time. It is inappropriate to use Internet-Drafts as reference 48 material or to cite them other than as "work in progress." 49 This Internet-Draft will expire on 14 November 2021. 51 Copyright Notice 53 Copyright (c) 2021 IETF Trust and the persons identified as the 54 document authors. All rights reserved. 56 This document is subject to BCP 78 and the IETF Trust's Legal 57 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 58 license-info) in effect on the date of publication of this document. 59 Please review these documents carefully, as they describe your rights 60 and restrictions with respect to this document. Code Components 61 extracted from this document must include Simplified BSD License text 62 as described in Section 4.e of the Trust Legal Provisions and are 63 provided without warranty as described in the Simplified BSD License. 65 Table of Contents 67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 68 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5 69 1.2. Data Structures . . . . . . . . . . . . . . . . . . . . . 5 70 1.3. Major Differences from CT 1.0 . . . . . . . . . . . . . . 5 71 2. Cryptographic Components . . . . . . . . . . . . . . . . . . 7 72 2.1. Merkle Hash Trees . . . . . . . . . . . . . . . . . . . . 7 73 2.1.1. Definition of the Merkle Tree . . . . . . . . . . . . 7 74 2.1.2. Verifying a Tree Head Given Entries . . . . . . . . . 8 75 2.1.3. Merkle Inclusion Proofs . . . . . . . . . . . . . . . 9 76 2.1.4. Merkle Consistency Proofs . . . . . . . . . . . . . . 10 77 2.1.5. Example . . . . . . . . . . . . . . . . . . . . . . . 13 78 2.2. Signatures . . . . . . . . . . . . . . . . . . . . . . . 14 79 3. Submitters . . . . . . . . . . . . . . . . . . . . . . . . . 15 80 3.1. Certificates . . . . . . . . . . . . . . . . . . . . . . 15 81 3.2. Precertificates . . . . . . . . . . . . . . . . . . . . . 15 82 3.2.1. Binding Intent to Issue . . . . . . . . . . . . . . . 17 83 4. Log Format and Operation . . . . . . . . . . . . . . . . . . 17 84 4.1. Log Parameters . . . . . . . . . . . . . . . . . . . . . 18 85 4.2. Evaluating Submissions . . . . . . . . . . . . . . . . . 19 86 4.2.1. Minimum Acceptance Criteria . . . . . . . . . . . . . 19 87 4.2.2. Discretionary Acceptance Criteria . . . . . . . . . . 20 88 4.3. Log Entries . . . . . . . . . . . . . . . . . . . . . . . 20 89 4.4. Log ID . . . . . . . . . . . . . . . . . . . . . . . . . 21 90 4.5. TransItem Structure . . . . . . . . . . . . . . . . . . . 21 91 4.6. Log Artifact Extensions . . . . . . . . . . . . . . . . . 22 92 4.7. Merkle Tree Leaves . . . . . . . . . . . . . . . . . . . 23 93 4.8. Signed Certificate Timestamp (SCT) . . . . . . . . . . . 24 94 4.9. Merkle Tree Head . . . . . . . . . . . . . . . . . . . . 25 95 4.10. Signed Tree Head (STH) . . . . . . . . . . . . . . . . . 26 96 4.11. Merkle Consistency Proofs . . . . . . . . . . . . . . . . 26 97 4.12. Merkle Inclusion Proofs . . . . . . . . . . . . . . . . . 27 98 4.13. Shutting down a log . . . . . . . . . . . . . . . . . . . 28 99 5. Log Client Messages . . . . . . . . . . . . . . . . . . . . . 28 100 5.1. Submit Entry to Log . . . . . . . . . . . . . . . . . . . 30 101 5.2. Retrieve Latest Signed Tree Head . . . . . . . . . . . . 32 102 5.3. Retrieve Merkle Consistency Proof between Two Signed Tree 103 Heads . . . . . . . . . . . . . . . . . . . . . . . . . . 32 104 5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash . . 33 105 5.5. Retrieve Merkle Inclusion Proof, Signed Tree Head and 106 Consistency Proof by Leaf Hash . . . . . . . . . . . . . 34 107 5.6. Retrieve Entries and STH from Log . . . . . . . . . . . . 35 108 5.7. Retrieve Accepted Trust Anchors . . . . . . . . . . . . . 37 109 6. TLS Servers . . . . . . . . . . . . . . . . . . . . . . . . . 38 110 6.1. TLS Client Authentication . . . . . . . . . . . . . . . . 38 111 6.2. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . . 39 112 6.3. TransItemList Structure . . . . . . . . . . . . . . . . . 39 113 6.4. Presenting SCTs, inclusions proofs and STHs . . . . . . . 40 114 6.5. transparency_info TLS Extension . . . . . . . . . . . . . 40 115 7. Certification Authorities . . . . . . . . . . . . . . . . . . 41 116 7.1. Transparency Information X.509v3 Extension . . . . . . . 41 117 7.1.1. OCSP Response Extension . . . . . . . . . . . . . . . 41 118 7.1.2. Certificate Extension . . . . . . . . . . . . . . . . 41 119 7.2. TLS Feature X.509v3 Extension . . . . . . . . . . . . . . 41 120 8. Clients . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 121 8.1. TLS Client . . . . . . . . . . . . . . . . . . . . . . . 42 122 8.1.1. Receiving SCTs and inclusion proofs . . . . . . . . . 42 123 8.1.2. Reconstructing the TBSCertificate . . . . . . . . . . 42 124 8.1.3. Validating SCTs . . . . . . . . . . . . . . . . . . . 42 125 8.1.4. Fetching inclusion proofs . . . . . . . . . . . . . . 43 126 8.1.5. Validating inclusion proofs . . . . . . . . . . . . . 43 127 8.1.6. Evaluating compliance . . . . . . . . . . . . . . . . 44 128 8.2. Monitor . . . . . . . . . . . . . . . . . . . . . . . . . 44 129 8.3. Auditing . . . . . . . . . . . . . . . . . . . . . . . . 45 130 9. Algorithm Agility . . . . . . . . . . . . . . . . . . . . . . 46 131 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47 132 10.1. New Entry to the TLS ExtensionType Registry . . . . . . 47 133 10.2. Hash Algorithms . . . . . . . . . . . . . . . . . . . . 47 134 10.2.1. Specification Required guidance . . . . . . . . . . 47 135 10.3. Signature Algorithms . . . . . . . . . . . . . . . . . . 48 136 10.3.1. Expert Review guidelines . . . . . . . . . . . . . . 49 137 10.4. VersionedTransTypes . . . . . . . . . . . . . . . . . . 49 138 10.5. Log Artifact Extension Registry . . . . . . . . . . . . 50 139 10.5.1. Specification Required guidance . . . . . . . . . . 51 140 10.6. Object Identifiers . . . . . . . . . . . . . . . . . . . 51 141 10.6.1. Log ID Registry . . . . . . . . . . . . . . . . . . 51 142 10.7. URN Sub-namespace for TRANS errors 143 (urn:ietf:params:trans:error) . . . . . . . . . . . . . 52 144 10.7.1. TRANS Error Types . . . . . . . . . . . . . . . . . 53 146 11. Security Considerations . . . . . . . . . . . . . . . . . . . 54 147 11.1. Misissued Certificates . . . . . . . . . . . . . . . . . 55 148 11.2. Detection of Misissue . . . . . . . . . . . . . . . . . 55 149 11.3. Misbehaving Logs . . . . . . . . . . . . . . . . . . . . 55 150 11.4. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . 56 151 11.5. Leakage of DNS Information . . . . . . . . . . . . . . . 56 152 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 56 153 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 57 154 13.1. Normative References . . . . . . . . . . . . . . . . . . 57 155 13.2. Informative References . . . . . . . . . . . . . . . . . 59 156 Appendix A. Supporting v1 and v2 simultaneously (Informative) . 60 157 Appendix B. An ASN.1 Module (Informative) . . . . . . . . . . . 61 158 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 62 160 1. Introduction 162 Certificate Transparency aims to mitigate the problem of misissued 163 certificates by providing append-only logs of issued certificates. 164 The logs do not themselves prevent misissuance, but they ensure that 165 interested parties (particularly those named in certificates) can 166 detect such misissuance. Note that this is a general mechanism that 167 could be used for transparently logging any form of binary data, 168 subject to some kind of inclusion criteria. In this document, we 169 only describe its use for public TLS server certificates (i.e., where 170 the inclusion criteria is a valid certificate issued by a public 171 certification authority (CA)). A typical definition of "public" can 172 be found in [CABBR]. 174 Each log contains certificate chains, which can be submitted by 175 anyone. It is expected that public CAs will contribute all their 176 newly issued certificates to one or more logs; however certificate 177 holders can also contribute their own certificate chains, as can 178 third parties. In order to avoid logs being rendered useless by the 179 submission of large numbers of spurious certificates, it is required 180 that each chain ends with a trust anchor that is accepted by the log. 181 A log may also limit the length of the chain it is willing to accept; 182 such chains must also end with an acceptable trust anchor. When a 183 chain is accepted by a log, a signed timestamp is returned, which can 184 later be used to provide evidence to TLS clients that the chain has 185 been submitted. TLS clients can thus require that all certificates 186 they accept as valid are accompanied by signed timestamps. 188 Those who are concerned about misissuance can monitor the logs, 189 asking them regularly for all new entries, and can thus check whether 190 domains for which they are responsible have had certificates issued 191 that they did not expect. What they do with this information, 192 particularly when they find that a misissuance has happened, is 193 beyond the scope of this document. However, broadly speaking, they 194 can invoke existing business mechanisms for dealing with misissued 195 certificates, such as working with the CA to get the certificate 196 revoked, or with maintainers of trust anchor lists to get the CA 197 removed. Of course, anyone who wants can monitor the logs and, if 198 they believe a certificate is incorrectly issued, take action as they 199 see fit. 201 Similarly, those who have seen signed timestamps from a particular 202 log can later demand a proof of inclusion from that log. If the log 203 is unable to provide this (or, indeed, if the corresponding 204 certificate is absent from monitors' copies of that log), that is 205 evidence of the incorrect operation of the log. The checking 206 operation is asynchronous to allow clients to proceed without delay, 207 despite possible issues such as network connectivity and the vagaries 208 of firewalls. 210 The append-only property of each log is achieved using Merkle Trees, 211 which can be used to efficiently prove that any particular instance 212 of the log is a superset of any particular previous instance and to 213 efficiently detect various misbehaviors of the log (e.g., issuing a 214 signed timestamp for a certificate that is not subsequently logged). 216 It is necessary to treat each log as a trusted third party, because 217 the log auditing mechanisms described in this document can be 218 circumvented by a misbehaving log that shows different, inconsistent 219 views of itself to different clients. While mechanisms are being 220 developed to address these shortcomings and thereby avoid the need to 221 blindly trust logs, such mechanisms are outside the scope of this 222 document. 224 1.1. Requirements Language 226 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 227 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 228 "OPTIONAL" in this document are to be interpreted as described in BCP 229 14 [RFC2119] [RFC8174] when, and only when, they appear in all 230 capitals, as shown here. 232 1.2. Data Structures 234 Data structures are defined and encoded according to the conventions 235 laid out in Section 3 of [RFC8446]. 237 1.3. Major Differences from CT 1.0 239 This document revises and obsoletes the CT 1.0 [RFC6962] protocol, 240 drawing on insights gained from CT 1.0 deployments and on feedback 241 from the community. The major changes are: 243 * Hash and signature algorithm agility: permitted algorithms are now 244 specified in IANA registries. 246 * Precertificate format: precertificates are now CMS objects rather 247 than X.509 certificates, which avoids violating the certificate 248 serial number uniqueness requirement in Section 4.1.2.2 of 249 [RFC5280]. 251 * Removed precertificate signing certificates and the precertificate 252 poison extension: the change of precertificate format means that 253 these are no longer needed. 255 * Logs IDs: each log is now identified by an OID rather than by the 256 hash of its public key. OID allocations are managed by an IANA 257 registry. 259 * "TransItem" structure: this new data structure is used to 260 encapsulate most types of CT data. A "TransItemList", consisting 261 of one or more "TransItem" structures, can be used anywhere that 262 "SignedCertificateTimestampList" was used in [RFC6962]. 264 * Merkle tree leaves: the "MerkleTreeLeaf" structure has been 265 replaced by the "TransItem" structure, which eases extensibility 266 and simplifies the leaf structure by removing one layer of 267 abstraction. 269 * Unified leaf format: the structure for both certificate and 270 precertificate entries now includes only the TBSCertificate 271 (whereas certificate entries in [RFC6962] included the entire 272 certificate). 274 * Log Artifact Extensions: these are now typed and managed by an 275 IANA registry, and they can now appear not only in SCTs but also 276 in STHs. 278 * API outputs: complete "TransItem" structures are returned, rather 279 than the constituent parts of each structure. 281 * get-all-by-hash: new client API for obtaining an inclusion proof 282 and the corresponding consistency proof at the same time. 284 * submit-entry: new client API, replacing add-chain and add-pre- 285 chain. 287 * Presenting SCTs with proofs: TLS servers may present SCTs together 288 with the corresponding inclusion proofs using any of the 289 mechanisms that [RFC6962] defined for presenting SCTs only. 290 (Presenting SCTs only is still supported). 292 * CT TLS extension: the "signed_certificate_timestamp" TLS extension 293 has been replaced by the "transparency_info" TLS extension. 295 * Verification algorithms: added detailed algorithms for verifying 296 inclusion proofs, for verifying consistency between two STHs, and 297 for verifying a root hash given a complete list of the relevant 298 leaf input entries. 300 * Extensive clarifications and editorial work. 302 2. Cryptographic Components 304 2.1. Merkle Hash Trees 306 A full description of Merkle Hash Tree is beyond the scope of this 307 document. Briefly, it is a binary tree where each non-leaf node is a 308 hash of its children. For CT, the number of children is at most two. 309 Additional information can be found in the Introduction and Reference 310 section of [RFC8391]. 312 2.1.1. Definition of the Merkle Tree 314 The log uses a binary Merkle Hash Tree for efficient auditing. The 315 hash algorithm used is one of the log's parameters (see Section 4.1). 316 This document establishes a registry of acceptable hash algorithms 317 (see Section 10.2). Throughout this document, the hash algorithm in 318 use is referred to as HASH and the size of its output in bytes as 319 HASH_SIZE. The input to the Merkle Tree Hash is a list of data 320 entries; these entries will be hashed to form the leaves of the 321 Merkle Hash Tree. The output is a single HASH_SIZE Merkle Tree Hash. 322 Given an ordered list of n inputs, D_n = {d[0], d[1], ..., d[n-1]}, 323 the Merkle Tree Hash (MTH) is thus defined as follows: 325 The hash of an empty list is the hash of an empty string: 327 MTH({}) = HASH(). 329 The hash of a list with one entry (also known as a leaf hash) is: 331 MTH({d[0]}) = HASH(0x00 || d[0]). 333 For n > 1, let k be the largest power of two smaller than n (i.e., k 334 < n <= 2k). The Merkle Tree Hash of an n-element list D_n is then 335 defined recursively as 337 MTH(D_n) = HASH(0x01 || MTH(D[0:k]) || MTH(D[k:n])), 339 where: 341 * || denotes concatenation 343 * : denotes concatenation of lists 345 * D[k1:k2] = D'_(k2-k1) denotes the list {d'[0] = d[k1], d'[1] = 346 d[k1+1], ..., d'[k2-k1-1] = d[k2-1]} of length (k2 - k1). 348 Note that the hash calculations for leaves and nodes differ; this 349 domain separation is required to give second preimage resistance. 351 Note that we do not require the length of the input list to be a 352 power of two. The resulting Merkle Tree may thus not be balanced; 353 however, its shape is uniquely determined by the number of leaves. 354 (Note: This Merkle Tree is essentially the same as the history tree 355 [CrosbyWallach] proposal, except our definition handles non-full 356 trees differently). 358 2.1.2. Verifying a Tree Head Given Entries 360 When a client has a complete list of "entries" from "0" up to 361 "tree_size - 1" and wishes to verify this list against a tree head 362 "root_hash" returned by the log for the same "tree_size", the 363 following algorithm may be used: 365 1. Set "stack" to an empty stack. 367 2. For each "i" from "0" up to "tree_size - 1": 369 1. Push "HASH(0x00 || entries[i])" to "stack". 371 2. Set "merge_count" to the lowest value ("0" included) such 372 that "LSB(i >> merge_count)" is not set, where "LSB" means 373 the least significant bit. In other words, set "merge_count" 374 to the number of consecutive "1"s found starting at the least 375 significant bit of "i". 377 3. Repeat "merge_count" times: 379 1. Pop "right" from "stack". 381 2. Pop "left" from "stack". 383 3. Push "HASH(0x01 || left || right)" to "stack". 385 3. If there is more than one element in the "stack", repeat the same 386 merge procedure (the sub-items of Step 2.3 above) until only a 387 single element remains. 389 4. The remaining element in "stack" is the Merkle Tree hash for the 390 given "tree_size" and should be compared by equality against the 391 supplied "root_hash". 393 2.1.3. Merkle Inclusion Proofs 395 A Merkle inclusion proof for a leaf in a Merkle Hash Tree is the 396 shortest list of additional nodes in the Merkle Tree required to 397 compute the Merkle Tree Hash for that tree. Each node in the tree is 398 either a leaf node or is computed from the two nodes immediately 399 below it (i.e., towards the leaves). At each step up the tree 400 (towards the root), a node from the inclusion proof is combined with 401 the node computed so far. In other words, the inclusion proof 402 consists of the list of missing nodes required to compute the nodes 403 leading from a leaf to the root of the tree. If the root computed 404 from the inclusion proof matches the true root, then the inclusion 405 proof proves that the leaf exists in the tree. 407 2.1.3.1. Generating an Inclusion Proof 409 Given an ordered list of n inputs to the tree, D_n = {d[0], d[1], 410 ..., d[n-1]}, the Merkle inclusion proof PATH(m, D_n) for the (m+1)th 411 input d[m], 0 <= m < n, is defined as follows: 413 The proof for the single leaf in a tree with a one-element input list 414 D[1] = {d[0]} is empty: 416 PATH(0, {d[0]}) = {} 418 For n > 1, let k be the largest power of two smaller than n. The 419 proof for the (m+1)th element d[m] in a list of n > m elements is 420 then defined recursively as 422 PATH(m, D_n) = PATH(m, D[0:k]) : MTH(D[k:n]) for m < k; and 424 PATH(m, D_n) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for m >= k, 426 The : operator and D[k1:k2] are defined the same as in Section 2.1.1. 428 2.1.3.2. Verifying an Inclusion Proof 430 When a client has received an inclusion proof (e.g., in a "TransItem" 431 of type "inclusion_proof_v2") and wishes to verify inclusion of an 432 input "hash" for a given "tree_size" and "root_hash", the following 433 algorithm may be used to prove the "hash" was included in the 434 "root_hash": 436 1. Compare "leaf_index" from the "inclusion_proof_v2" structure 437 against "tree_size". If "leaf_index" is greater than or equal to 438 "tree_size" then fail the proof verification. 440 2. Set "fn" to "leaf_index" and "sn" to "tree_size - 1". 442 3. Set "r" to "hash". 444 4. For each value "p" in the "inclusion_path" array: 446 If "sn" is 0, stop the iteration and fail the proof verification. 448 If "LSB(fn)" is set, or if "fn" is equal to "sn", then: 450 1. Set "r" to "HASH(0x01 || p || r)" 452 2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn" 453 equally until either "LSB(fn)" is set or "fn" is "0". 455 Otherwise: 457 1. Set "r" to "HASH(0x01 || r || p)" 459 Finally, right-shift both "fn" and "sn" one time. 461 5. Compare "sn" to 0. Compare "r" against the "root_hash". If "sn" 462 is equal to 0, and "r" and the "root_hash" are equal, then the 463 log has proven the inclusion of "hash". Otherwise, fail the 464 proof verification. 466 2.1.4. Merkle Consistency Proofs 468 Merkle consistency proofs prove the append-only property of the tree. 469 A Merkle consistency proof for a Merkle Tree Hash MTH(D_n) and a 470 previously advertised hash MTH(D[0:m]) of the first m leaves, m <= n, 471 is the list of nodes in the Merkle Tree required to verify that the 472 first m inputs D[0:m] are equal in both trees. Thus, a consistency 473 proof must contain a set of intermediate nodes (i.e., commitments to 474 inputs) sufficient to verify MTH(D_n), such that (a subset of) the 475 same nodes can be used to verify MTH(D[0:m]). We define an algorithm 476 that outputs the (unique) minimal consistency proof. 478 2.1.4.1. Generating a Consistency Proof 480 Given an ordered list of n inputs to the tree, D_n = {d[0], d[1], 481 ..., d[n-1]}, the Merkle consistency proof PROOF(m, D_n) for a 482 previous Merkle Tree Hash MTH(D[0:m]), 0 < m < n, is defined as: 484 PROOF(m, D_n) = SUBPROOF(m, D_n, true) 486 In SUBPROOF, the boolean value represents whether the subtree created 487 from D[0:m] is a complete subtree of the Merkle Tree created from 488 D_n, and, consequently, whether the subtree Merkle Tree Hash 489 MTH(D[0:m]) is known. The initial call to SUBPROOF sets this to be 490 true, and SUBPROOF is then defined as follows: 492 The subproof for m = n is empty if m is the value for which PROOF was 493 originally requested (meaning that the subtree created from D[0:m] is 494 a complete subtree of the Merkle Tree created from the original D_n 495 for which PROOF was requested, and the subtree Merkle Tree Hash 496 MTH(D[0:m]) is known): 498 SUBPROOF(m, D_m, true) = {} 500 Otherwise, the subproof for m = n is the Merkle Tree Hash committing 501 inputs D[0:m]: 503 SUBPROOF(m, D_m, false) = {MTH(D_m)} 505 For m < n, let k be the largest power of two smaller than n. The 506 subproof is then defined recursively, using the appropriate step 507 below: 509 If m <= k, the right subtree entries D[k:n] only exist in the current 510 tree. We prove that the left subtree entries D[0:k] are consistent 511 and add a commitment to D[k:n]: 513 SUBPROOF(m, D_n, b) = SUBPROOF(m, D[0:k], b) : MTH(D[k:n]) 515 If m > k, the left subtree entries D[0:k] are identical in both 516 trees. We prove that the right subtree entries D[k:n] are consistent 517 and add a commitment to D[0:k]. 519 SUBPROOF(m, D_n, b) = SUBPROOF(m - k, D[k:n], false) : MTH(D[0:k]) 521 The number of nodes in the resulting proof is bounded above by 522 ceil(log2(n)) + 1. 524 The : operator and D[k1:k2] are defined the same as in Section 2.1.1. 526 2.1.4.2. Verifying Consistency between Two Tree Heads 528 When a client has a tree head "first_hash" for tree size "first", a 529 tree head "second_hash" for tree size "second" where "0 < first < 530 second", and has received a consistency proof between the two (e.g., 531 in a "TransItem" of type "consistency_proof_v2"), the following 532 algorithm may be used to verify the consistency proof: 534 1. If "consistency_path" is an empty array, stop and fail the proof 535 verification. 537 2. If "first" is an exact power of 2, then prepend "first_hash" to 538 the "consistency_path" array. 540 3. Set "fn" to "first - 1" and "sn" to "second - 1". 542 4. If "LSB(fn)" is set, then right-shift both "fn" and "sn" equally 543 until "LSB(fn)" is not set. 545 5. Set both "fr" and "sr" to the first value in the 546 "consistency_path" array. 548 6. For each subsequent value "c" in the "consistency_path" array: 550 If "sn" is 0, stop the iteration and fail the proof verification. 552 If "LSB(fn)" is set, or if "fn" is equal to "sn", then: 554 1. Set "fr" to "HASH(0x01 || c || fr)" 556 Set "sr" to "HASH(0x01 || c || sr)" 558 2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn" 559 equally until either "LSB(fn)" is set or "fn" is "0". 561 Otherwise: 563 1. Set "sr" to "HASH(0x01 || sr || c)" 565 Finally, right-shift both "fn" and "sn" one time. 567 7. After completing iterating through the "consistency_path" array 568 as described above, verify that the "fr" calculated is equal to 569 the "first_hash" supplied, that the "sr" calculated is equal to 570 the "second_hash" supplied and that "sn" is 0. 572 2.1.5. Example 574 The binary Merkle Tree with 7 leaves: 576 hash 577 / \ 578 / \ 579 / \ 580 / \ 581 / \ 582 k l 583 / \ / \ 584 / \ / \ 585 / \ / \ 586 g h i j 587 / \ / \ / \ | 588 a b c d e f d6 589 | | | | | | 590 d0 d1 d2 d3 d4 d5 592 The inclusion proof for d0 is [b, h, l]. 594 The inclusion proof for d3 is [c, g, l]. 596 The inclusion proof for d4 is [f, j, k]. 598 The inclusion proof for d6 is [i, k]. 600 The same tree, built incrementally in four steps: 602 hash0 hash1=k 603 / \ / \ 604 / \ / \ 605 / \ / \ 606 g c g h 607 / \ | / \ / \ 608 a b d2 a b c d 609 | | | | | | 610 d0 d1 d0 d1 d2 d3 612 hash2 hash 613 / \ / \ 614 / \ / \ 615 / \ / \ 616 / \ / \ 617 / \ / \ 618 k i k l 619 / \ / \ / \ / \ 620 / \ e f / \ / \ 621 / \ | | / \ / \ 622 g h d4 d5 g h i j 623 / \ / \ / \ / \ / \ | 624 a b c d a b c d e f d6 625 | | | | | | | | | | 626 d0 d1 d2 d3 d0 d1 d2 d3 d4 d5 628 The consistency proof between hash0 and hash is PROOF(3, D[7]) = [c, 629 d, g, l]. c, g are used to verify hash0, and d, l are additionally 630 used to show hash is consistent with hash0. 632 The consistency proof between hash1 and hash is PROOF(4, D[7]) = [l]. 633 hash can be verified using hash1=k and l. 635 The consistency proof between hash2 and hash is PROOF(6, D[7]) = [i, 636 j, k]. k, i are used to verify hash2, and j is additionally used to 637 show hash is consistent with hash2. 639 2.2. Signatures 641 When signing data structures, a log MUST use one of the signature 642 algorithms from the IANA CT Signature Algorithms registry, described 643 in Section 10.3. 645 3. Submitters 647 Submitters submit certificates or preannouncements of certificates 648 prior to issuance (precertificates) to logs for public auditing, as 649 described below. In order to enable attribution of each logged 650 certificate or precertificate to its issuer, each submission MUST be 651 accompanied by all additional certificates required to verify the 652 chain up to an accepted trust anchor (Section 5.7). The trust anchor 653 (a root or intermediate CA certificate) MAY be omitted from the 654 submission. 656 If a log accepts a submission, it will return a Signed Certificate 657 Timestamp (SCT) (see Section 4.8). The submitter SHOULD validate the 658 returned SCT as described in Section 8.1 if they understand its 659 format and they intend to use it directly in a TLS handshake or to 660 construct a certificate. If the submitter does not need the SCT (for 661 example, the certificate is being submitted simply to make it 662 available in the log), it MAY validate the SCT. 664 3.1. Certificates 666 Any entity can submit a certificate (Section 5.1) to a log. Since it 667 is anticipated that TLS clients will reject certificates that are not 668 logged, it is expected that certificate issuers and subjects will be 669 strongly motivated to submit them. 671 3.2. Precertificates 673 CAs may preannounce a certificate prior to issuance by submitting a 674 precertificate (Section 5.1) that the log can use to create an entry 675 that will be valid against the issued certificate. The CA MAY 676 incorporate the returned SCT in the issued certificate. One example 677 of where the returned SCT is not incorporated in the issued 678 certificate is when a CA sends the precertificate to multiple logs, 679 but only incorporates the SCTs that are returned first. 681 A precertificate is a CMS [RFC5652] "signed-data" object that 682 conforms to the following profile: 684 * It MUST be DER encoded as described in [X690]. 686 * "SignedData.version" MUST be v3(3). 688 * "SignedData.digestAlgorithms" MUST be the same as the 689 "SignerInfo.digestAlgorithm" OID value (see below). 691 * "SignedData.encapContentInfo": 693 - "eContentType" MUST be the OID 1.3.101.78. 695 - "eContent" MUST contain a TBSCertificate [RFC5280] that will be 696 identical to the TBSCertificate in the issued certificate, 697 except that the Transparency Information (Section 7.1) 698 extension MUST be omitted. 700 * "SignedData.certificates" MUST be omitted. 702 * "SignedData.crls" MUST be omitted. 704 * "SignedData.signerInfos" MUST contain one "SignerInfo": 706 - "version" MUST be v3(3). 708 - "sid" MUST use the "subjectKeyIdentifier" option. 710 - "digestAlgorithm" MUST be one of the hash algorithm OIDs listed 711 in the IANA CT Hash Algorithms Registry, described in 712 Section 10.2. 714 - "signedAttrs" MUST be present and MUST contain two attributes: 716 o A content-type attribute whose value is the same as 717 "SignedData.encapContentInfo.eContentType". 719 o A message-digest attribute whose value is the message digest 720 of "SignedData.encapContentInfo.eContent". 722 - "signatureAlgorithm" MUST be the same OID as 723 "TBSCertificate.signature". 725 - "signature" MUST be from the same (root or intermediate) CA 726 that intends to issue the corresponding certificate (see 727 Section 3.2.1). 729 - "unsignedAttrs" MUST be omitted. 731 "SignerInfo.signedAttrs" is included in the message digest 732 calculation process (see Section 5.4 of [RFC5652]), which ensures 733 that the "SignerInfo.signature" value will not be a valid X.509v3 734 signature that could be used in conjunction with the TBSCertificate 735 (from "SignedData.encapContentInfo.eContent") to construct a valid 736 certificate. 738 3.2.1. Binding Intent to Issue 740 Under normal circumstances, there will be a short delay between 741 precertificate submission and issuance of the corresponding 742 certificate. Longer delays are to be expected occasionally (e.g., 743 due to log server downtime), and in some cases the CA might not 744 actually issue the corresponding certificate. Nevertheless, a 745 precertificate's "signature" indicates the CA's binding intent to 746 issue the corresponding certificate, which means that: 748 * Misissuance of a precertificate is considered equivalent to 749 misissuance of the corresponding certificate. The CA should 750 expect to be held to account, even if the corresponding 751 certificate has not actually been issued. 753 * Upon observing a precertificate, a client can reasonably presume 754 that the corresponding certificate has been issued. A client may 755 wish to obtain status information (e.g., by using the Online 756 Certificate Status Protocol [RFC6960] or by checking a Certificate 757 Revocation List [RFC5280]) about a certificate that is presumed to 758 exist, especially if there is evidence or suspicion that the 759 corresponding precertificate was misissued. 761 * TLS clients may have policies that require CAs to be able to 762 revoke, and to provide certificate status services for, each 763 certificate that is presumed to exist based on the existence of a 764 corresponding precertificate. 766 4. Log Format and Operation 768 A log is a single, append-only Merkle Tree of submitted certificate 769 and precertificate entries. 771 When it receives and accepts a valid submission, the log MUST return 772 an SCT that corresponds to the submitted certificate or 773 precertificate. If the log has previously seen this valid 774 submission, it SHOULD return the same SCT as it returned before, as 775 discussed in Section 11.3. If different SCTs are produced for the 776 same submission, multiple log entries will have to be created, one 777 for each SCT (as the timestamp is a part of the leaf structure). 778 Note that if a certificate was previously logged as a precertificate, 779 then the precertificate's SCT of type "precert_sct_v2" would not be 780 appropriate; instead, a fresh SCT of type "x509_sct_v2" should be 781 generated. 783 An SCT is the log's promise to append to its Merkle Tree an entry for 784 the accepted submission. Upon producing an SCT, the log MUST fulfil 785 this promise by performing the following actions within a fixed 786 amount of time known as the Maximum Merge Delay (MMD), which is one 787 of the log's parameters (see Section 4.1): 789 * Allocate a tree index to the entry representing the accepted 790 submission. 792 * Calculate the root of the tree. 794 * Sign the root of the tree (see Section 4.10). 796 The log may append multiple entries before signing the root of the 797 tree. 799 Log operators SHOULD NOT impose any conditions on retrieving or 800 sharing data from the log. 802 4.1. Log Parameters 804 A log is defined by a collection of immutable parameters, which are 805 used by clients to communicate with the log and to verify log 806 artifacts. Except for the Final Signed Tree Head (STH), each of 807 these parameters MUST be established before the log operator begins 808 to operate the log. 810 Base URL: The prefix used to construct URLs ([RFC3986]) for client 811 messages (see Section 5). The base URL MUST be an "https" URL, 812 MAY contain a port, MAY contain a path with any number of path 813 segments, but MUST NOT contain a query string, fragment, or 814 trailing "/". Example: https://ct.example.org/blue 816 Hash Algorithm: The hash algorithm used for the Merkle Tree (see 817 Section 10.2). 819 Signature Algorithm: The signature algorithm used (see Section 2.2). 821 Public Key: The public key used to verify signatures generated by 822 the log. A log MUST NOT use the same keypair as any other log. 824 Log ID: The OID that uniquely identifies the log. 826 Maximum Merge Delay: The MMD the log has committed to. This 827 document deliberately does not specify any limits on the value, to 828 allow for experimentation. 830 Version: The version of the protocol supported by the log (currently 831 1 or 2). 833 Maximum Chain Length: The longest certificate chain submission the 834 log is willing to accept, if the log imposes any limit. 836 STH Frequency Count: The maximum number of STHs the log may produce 837 in any period equal to the "Maximum Merge Delay" (see 838 Section 4.10). 840 Final STH: If a log has been closed down (i.e., no longer accepts 841 new entries), existing entries may still be valid. In this case, 842 the client should know the final valid STH in the log to ensure no 843 new entries can be added without detection. This value MUST be 844 provided in the form of a TransItem of type "signed_tree_head_v2". 845 If a log is still accepting entries, this value should not be 846 provided. 848 [JSON.Metadata] is an example of a metadata format which includes the 849 above elements. 851 4.2. Evaluating Submissions 853 A log determines whether to accept or reject a submission by 854 evaluating it against the minimum acceptance criteria (see 855 Section 4.2.1) and against the log's discretionary acceptance 856 criteria (see Section 4.2.2). 858 If the acceptance criteria are met, the log SHOULD accept the 859 submission. (A log may decide, for example, to temporarily reject 860 acceptable submissions to protect itself against denial-of-service 861 attacks). 863 The log SHALL allow retrieval of its list of accepted trust anchors 864 (see Section 5.7), each of which is a root or intermediate CA 865 certificate. This list might usefully be the union of root 866 certificates trusted by major browser vendors. 868 4.2.1. Minimum Acceptance Criteria 870 To ensure that logged certificates and precertificates are 871 attributable to an accepted trust anchor, and to set clear 872 expectations for what monitors would find in the log, and to avoid 873 being overloaded by invalid submissions, the log MUST reject a 874 submission if any of the following conditions are not met: 876 * The "submission", "type" and "chain" inputs MUST be set as 877 described in Section 5.1. The log MUST NOT accommodate misordered 878 CA certificates or use any other source of intermediate CA 879 certificates to attempt certification path construction. 881 * Each of the zero or more intermediate CA certificates in the chain 882 MUST have one or both of the following features: 884 - The Basic Constraints extension with the cA boolean asserted. 886 - The Key Usage extension with the keyCertSign bit asserted. 888 * Each certificate in the chain MUST fall within the limits imposed 889 by the zero or more Basic Constraints pathLenConstraint values 890 found higher up the chain. 892 * Precertificate submissions MUST conform to all of the requirements 893 in Section 3.2. 895 4.2.2. Discretionary Acceptance Criteria 897 If the minimum acceptance criteria are met but the submission is not 898 fully valid according to [RFC5280] verification rules (e.g., the 899 certificate or precertificate has expired, is not yet valid, has been 900 revoked, exhibits ASN.1 DER encoding errors but the log can still 901 parse it, etc), then the acceptability of the submission is left to 902 the log's discretion. It is useful for logs to accept such 903 submissions in order to accommodate quirks of CA certificate-issuing 904 software and to facilitate monitoring of CA compliance with 905 applicable policies and technical standards. However, it is 906 impractical for this document to enumerate, and for logs to consider, 907 all of the ways that a submission might fail to comply with 908 [RFC5280]. 910 Logs SHOULD limit the length of chain they will accept. The maximum 911 chain length is one of the log's parameters (see Section 4.1). 913 4.3. Log Entries 915 If a submission is accepted and an SCT issued, the accepting log MUST 916 store the entire chain used for verification. This chain MUST 917 include the certificate or precertificate itself, the zero or more 918 intermediate CA certificates provided by the submitter, and the trust 919 anchor used to verify the chain (even if it was omitted from the 920 submission). The log MUST provide this chain for auditing upon 921 request (see Section 5.6) so that the CA cannot avoid blame by 922 logging a partial or empty chain. Each log entry is a "TransItem" 923 structure of type "x509_entry_v2" or "precert_entry_v2". However, a 924 log may store its entries in any format. If a log does not store 925 this "TransItem" in full, it must store the "timestamp" and 926 "sct_extensions" of the corresponding 927 "TimestampedCertificateEntryDataV2" structure. The "TransItem" can 928 be reconstructed from these fields and the entire chain that the log 929 used to verify the submission. 931 4.4. Log ID 933 Each log is identified by an OID, which is one of the log's 934 parameters (see Section 4.1) and which MUST NOT be used to identify 935 any other log. A log's operator MUST either allocate the OID 936 themselves or request an OID from the Log ID Registry (see 937 Section 10.6.1). The only advantage of the registry is that the DER 938 encoding can be small. (Recall that OID allocations do not require a 939 central registration, although logs will most likely want to make 940 themselves known to potential clients through out of band means.) 941 Various data structures include the DER encoding of this OID, 942 excluding the ASN.1 tag and length bytes, in an opaque vector: 944 opaque LogID<2..127>; 946 Note that the ASN.1 length and the opaque vector length are identical 947 in size (1 byte) and value, so the full DER encoding (including the 948 tag and length) of the OID can be reproduced simply by prepending an 949 OBJECT IDENTIFIER tag (0x06) to the opaque vector length and 950 contents. 952 The OID used to identify a log is limited such that the DER encoding 953 of its value, excluding the tag and length, MUST be no longer than 954 127 octets. 956 4.5. TransItem Structure 958 Various data structures are encapsulated in the "TransItem" structure 959 to ensure that the type and version of each one is identified in a 960 common fashion: 962 enum { 963 reserved(0), 964 x509_entry_v2(1), precert_entry_v2(2), 965 x509_sct_v2(3), precert_sct_v2(4), 966 signed_tree_head_v2(5), consistency_proof_v2(6), 967 inclusion_proof_v2(7), 968 (65535) 969 } VersionedTransType; 971 struct { 972 VersionedTransType versioned_type; 973 select (versioned_type) { 974 case x509_entry_v2: TimestampedCertificateEntryDataV2; 975 case precert_entry_v2: TimestampedCertificateEntryDataV2; 976 case x509_sct_v2: SignedCertificateTimestampDataV2; 977 case precert_sct_v2: SignedCertificateTimestampDataV2; 978 case signed_tree_head_v2: SignedTreeHeadDataV2; 979 case consistency_proof_v2: ConsistencyProofDataV2; 980 case inclusion_proof_v2: InclusionProofDataV2; 981 } data; 982 } TransItem; 984 "versioned_type" is a value from the IANA registry in Section 10.4 985 that identifies the type of the encapsulated data structure and the 986 earliest version of this protocol to which it conforms. This 987 document is v2. 989 "data" is the encapsulated data structure. The various structures 990 named with the "DataV2" suffix are defined in later sections of this 991 document. 993 Note that "VersionedTransType" combines the v1 [RFC6962] type 994 enumerations "Version", "LogEntryType", "SignatureType" and 995 "MerkleLeafType". Note also that v1 did not define "TransItem", but 996 this document provides guidelines (see Appendix A) on how v2 997 implementations can co-exist with v1 implementations. 999 Future versions of this protocol may reuse "VersionedTransType" 1000 values defined in this document as long as the corresponding data 1001 structures are not modified, and may add new "VersionedTransType" 1002 values for new or modified data structures. 1004 4.6. Log Artifact Extensions 1005 enum { 1006 reserved(65535) 1007 } ExtensionType; 1009 struct { 1010 ExtensionType extension_type; 1011 opaque extension_data<0..2^16-1>; 1012 } Extension; 1014 The "Extension" structure provides a generic extensibility for log 1015 artifacts, including Signed Certificate Timestamps (Section 4.8) and 1016 Signed Tree Heads (Section 4.10). The interpretation of the 1017 "extension_data" field is determined solely by the value of the 1018 "extension_type" field. 1020 This document does not define any extensions, but it does establish a 1021 registry for future "ExtensionType" values (see Section 10.5). Each 1022 document that registers a new "ExtensionType" must specify the 1023 context in which it may be used (e.g., SCT, STH, or both) and 1024 describe how to interpret the corresponding "extension_data". 1026 4.7. Merkle Tree Leaves 1028 The leaves of a log's Merkle Tree correspond to the log's entries 1029 (see Section 4.3). Each leaf is the leaf hash (Section 2.1) of a 1030 "TransItem" structure of type "x509_entry_v2" or "precert_entry_v2", 1031 which encapsulates a "TimestampedCertificateEntryDataV2" structure. 1032 Note that leaf hashes are calculated as HASH(0x00 || TransItem), 1033 where the hash algorithm is one of the log's parameters. 1035 opaque TBSCertificate<1..2^24-1>; 1037 struct { 1038 uint64 timestamp; 1039 opaque issuer_key_hash<32..2^8-1>; 1040 TBSCertificate tbs_certificate; 1041 Extension sct_extensions<0..2^16-1>; 1042 } TimestampedCertificateEntryDataV2; 1044 "timestamp" is the date and time at which the certificate or 1045 precertificate was accepted by the log, in the form of a 64-bit 1046 unsigned number of milliseconds elapsed since the Unix Epoch (1 1047 January 1970 00:00:00 UTC - see [UNIXTIME]), ignoring leap seconds, 1048 in network byte order. Note that the leaves of a log's Merkle Tree 1049 are not required to be in strict chronological order. 1051 "issuer_key_hash" is the HASH of the public key of the CA that issued 1052 the certificate or precertificate, calculated over the DER encoding 1053 of the key represented as SubjectPublicKeyInfo [RFC5280]. This is 1054 needed to bind the CA to the certificate or precertificate, making it 1055 impossible for the corresponding SCT to be valid for any other 1056 certificate or precertificate whose TBSCertificate matches 1057 "tbs_certificate". The length of the "issuer_key_hash" MUST match 1058 HASH_SIZE. 1060 "tbs_certificate" is the DER encoded TBSCertificate from the 1061 submission. (Note that a precertificate's TBSCertificate can be 1062 reconstructed from the corresponding certificate as described in 1063 Section 8.1.2). 1065 "sct_extensions" is byte-for-byte identical to the SCT extensions of 1066 the corresponding SCT. 1068 The type of the "TransItem" corresponds to the value of the "type" 1069 parameter supplied in the Section 5.1 call. 1071 4.8. Signed Certificate Timestamp (SCT) 1073 An SCT is a "TransItem" structure of type "x509_sct_v2" or 1074 "precert_sct_v2", which encapsulates a 1075 "SignedCertificateTimestampDataV2" structure: 1077 struct { 1078 LogID log_id; 1079 uint64 timestamp; 1080 Extension sct_extensions<0..2^16-1>; 1081 opaque signature<1..2^16-1>; 1082 } SignedCertificateTimestampDataV2; 1084 "log_id" is this log's unique ID, encoded in an opaque vector as 1085 described in Section 4.4. 1087 "timestamp" is equal to the timestamp from the corresponding 1088 "TimestampedCertificateEntryDataV2" structure. 1090 "sct_extensions" is a vector of 0 or more SCT extensions. This 1091 vector MUST NOT include more than one extension with the same 1092 "extension_type". The extensions in the vector MUST be ordered by 1093 the value of the "extension_type" field, smallest value first. All 1094 SCT extensions are similar to non-critical X.509v3 extensions (i.e., 1095 the "mustUnderstand" field is not set), and a recipient SHOULD ignore 1096 any extension it does not understand. Furthermore, an implementation 1097 MAY choose to ignore any extension(s) that it does understand. 1099 "signature" is computed over a "TransItem" structure of type 1100 "x509_entry_v2" or "precert_entry_v2" (see Section 4.7) using the 1101 signature algorithm declared in the log's parameters (see 1102 Section 4.1). 1104 4.9. Merkle Tree Head 1106 The log stores information about its Merkle Tree in a 1107 "TreeHeadDataV2": 1109 opaque NodeHash<32..2^8-1>; 1111 struct { 1112 uint64 timestamp; 1113 uint64 tree_size; 1114 NodeHash root_hash; 1115 Extension sth_extensions<0..2^16-1>; 1116 } TreeHeadDataV2; 1118 The length of NodeHash MUST match HASH_SIZE of the log. 1120 "timestamp" is the current date and time, using the format defined in 1121 {tree_leaves}. 1123 "tree_size" is the number of entries currently in the log's Merkle 1124 Tree. 1126 "root_hash" is the root of the Merkle Hash Tree. 1128 "sth_extensions" is a vector of 0 or more STH extensions. This 1129 vector MUST NOT include more than one extension with the same 1130 "extension_type". The extensions in the vector MUST be ordered by 1131 the value of the "extension_type" field, smallest value first. If an 1132 implementation sees an extension that it does not understand, it 1133 SHOULD ignore that extension. Furthermore, an implementation MAY 1134 choose to ignore any extension(s) that it does understand. 1136 4.10. Signed Tree Head (STH) 1138 Periodically each log SHOULD sign its current tree head information 1139 (see Section 4.9) to produce an STH. When a client requests a log's 1140 latest STH (see Section 5.2), the log MUST return an STH that is no 1141 older than the log's MMD. However, since STHs could be used to mark 1142 individual clients (by producing a new STH for each query), a log 1143 MUST NOT produce STHs more frequently than its parameters declare 1144 (see Section 4.1). In general, there is no need to produce a new STH 1145 unless there are new entries in the log; however, in the event that a 1146 log does not accept any submissions during an MMD period, the log 1147 MUST sign the same Merkle Tree Hash with a fresh timestamp. 1149 An STH is a "TransItem" structure of type "signed_tree_head_v2", 1150 which encapsulates a "SignedTreeHeadDataV2" structure: 1152 struct { 1153 LogID log_id; 1154 TreeHeadDataV2 tree_head; 1155 opaque signature<0..2^16-1>; 1156 } SignedTreeHeadDataV2; 1158 "log_id" is this log's unique ID, encoded in an opaque vector as 1159 described in Section 4.4. 1161 The "timestamp" in "tree_head" MUST be at least as recent as the most 1162 recent SCT timestamp in the tree. Each subsequent timestamp MUST be 1163 more recent than the timestamp of the previous update. 1165 "tree_head" contains the latest tree head information (see 1166 Section 4.9). 1168 "signature" is computed over the "tree_head" field using the 1169 signature algorithm declared in the log's parameters (see 1170 Section 4.1). 1172 4.11. Merkle Consistency Proofs 1174 To prepare a Merkle Consistency Proof for distribution to clients, 1175 the log produces a "TransItem" structure of type 1176 "consistency_proof_v2", which encapsulates a "ConsistencyProofDataV2" 1177 structure: 1179 struct { 1180 LogID log_id; 1181 uint64 tree_size_1; 1182 uint64 tree_size_2; 1183 NodeHash consistency_path<0..2^16-1>; 1184 } ConsistencyProofDataV2; 1186 "log_id" is this log's unique ID, encoded in an opaque vector as 1187 described in Section 4.4. 1189 "tree_size_1" is the size of the older tree. 1191 "tree_size_2" is the size of the newer tree. 1193 "consistency_path" is a vector of Merkle Tree nodes proving the 1194 consistency of two STHs as described in {consistency}. 1196 4.12. Merkle Inclusion Proofs 1198 To prepare a Merkle Inclusion Proof for distribution to clients, the 1199 log produces a "TransItem" structure of type "inclusion_proof_v2", 1200 which encapsulates an "InclusionProofDataV2" structure: 1202 struct { 1203 LogID log_id; 1204 uint64 tree_size; 1205 uint64 leaf_index; 1206 NodeHash inclusion_path<0..2^16-1>; 1207 } InclusionProofDataV2; 1209 "log_id" is this log's unique ID, encoded in an opaque vector as 1210 described in Section 4.4. 1212 "tree_size" is the size of the tree on which this inclusion proof is 1213 based. 1215 "leaf_index" is the 0-based index of the log entry corresponding to 1216 this inclusion proof. 1218 "inclusion_path" is a vector of Merkle Tree nodes proving the 1219 inclusion of the chosen certificate or precertificate as described in 1220 {merkle_inclusion_proof}. 1222 4.13. Shutting down a log 1224 Log operators may decide to shut down a log for various reasons, such 1225 as deprecation of the signature algorithm. If there are entries in 1226 the log for certificates that have not yet expired, simply making TLS 1227 clients stop recognizing that log will have the effect of 1228 invalidating SCTs from that log. In order to avoid that, the 1229 following actions SHOULD be taken: 1231 * Make it known to clients and monitors that the log will be frozen. 1232 This is not part of the API, so it will have to be done via a 1233 relevant out-of-band mechanism. 1235 * Stop accepting new submissions (the error code "shutdown" should 1236 be returned for such requests). 1238 * Once MMD from the last accepted submission has passed and all 1239 pending submissions are incorporated, issue a final STH and 1240 publish it as one of the log's parameters. Having an STH with a 1241 timestamp that is after the MMD has passed from the last SCT 1242 issuance allows clients to audit this log regularly without 1243 special handling for the final STH. At this point the log's 1244 private key is no longer needed and can be destroyed. 1246 * Keep the log running until the certificates in all of its entries 1247 have expired or exist in other logs (this can be determined by 1248 scanning other logs or connecting to domains mentioned in the 1249 certificates and inspecting the SCTs served). 1251 5. Log Client Messages 1253 Messages are sent as HTTPS GET or POST requests. Parameters for 1254 POSTs and all responses are encoded as JavaScript Object Notation 1255 (JSON) objects [RFC8259]. Parameters for GETs are encoded as order- 1256 independent key/value URL parameters, using the "application/x-www- 1257 form-urlencoded" format described in the "HTML 4.01 Specification" 1258 [HTML401]. Binary data is base64 encoded according to section 4 of 1259 [RFC4648] as specified in the individual messages. 1261 Clients are configured with a log's base URL, which is one of the 1262 log's parameters. Clients construct URLs for requests by appending 1263 suffixes to this base URL. This structure places some degree of 1264 restriction on how log operators can deploy these services, as noted 1265 in [RFC7320]. However, operational experience with version 1 of this 1266 protocol has not indicated that these restrictions are a problem in 1267 practice. 1269 Note that JSON objects and URL parameters may contain fields not 1270 specified here, to allow for experimentation. Any fields that are 1271 not understood SHOULD be ignored. 1273 In practice, log servers may include multiple front-end machines. 1274 Since it is impractical to keep these machines in perfect sync, 1275 errors may occur that are caused by skew between the machines. Where 1276 such errors are possible, the front-end will return additional 1277 information (as specified below) making it possible for clients to 1278 make progress, if progress is possible. Front-ends MUST only serve 1279 data that is free of gaps (that is, for example, no front-end will 1280 respond with an STH unless it is also able to prove consistency from 1281 all log entries logged within that STH). 1283 For example, when a consistency proof between two STHs is requested, 1284 the front-end reached may not yet be aware of one or both STHs. In 1285 the case where it is unaware of both, it will return the latest STH 1286 it is aware of. Where it is aware of the first but not the second, 1287 it will return the latest STH it is aware of and a consistency proof 1288 from the first STH to the returned STH. The case where it knows the 1289 second but not the first should not arise (see the "no gaps" 1290 requirement above). 1292 If the log is unable to process a client's request, it MUST return an 1293 HTTP response code of 4xx/5xx (see [RFC7231]), and, in place of the 1294 responses outlined in the subsections below, the body SHOULD be a 1295 JSON Problem Details Object (see [RFC7807] Section 3), containing: 1297 type: A URN reference identifying the problem. To facilitate 1298 automated response to errors, this document defines a set of 1299 standard tokens for use in the "type" field, within the URN 1300 namespace of: "urn:ietf:params:trans:error:". 1302 detail: A human-readable string describing the error that prevented 1303 the log from processing the request, ideally with sufficient 1304 detail to enable the error to be rectified. 1306 e.g., In response to a request of "/ct/v2/get- 1307 entries?start=100&end=99", the log would return a "400 Bad Request" 1308 response code with a body similar to the following: 1310 { 1311 "type": "urn:ietf:params:trans:error:endBeforeStart", 1312 "detail": "'start' cannot be greater than 'end'" 1313 } 1315 Most error types are specific to the type of request and are defined 1316 in the respective subsections below. The one exception is the 1317 "malformed" error type, which indicates that the log server could not 1318 parse the client's request because it did not comply with this 1319 document: 1321 +===========+==================================+ 1322 | type | detail | 1323 +===========+==================================+ 1324 | malformed | The request could not be parsed. | 1325 +-----------+----------------------------------+ 1327 Table 1 1329 Clients SHOULD treat "500 Internal Server Error" and "503 Service 1330 Unavailable" responses as transient failures and MAY retry the same 1331 request without modification at a later date. Note that as per 1332 [RFC7231], in the case of a 503 response the log MAY include a 1333 "Retry-After:" header in order to request a minimum time for the 1334 client to wait before retrying the request. In the absence of this 1335 header, this document does not specify a minimum. 1337 Clients SHOULD treat any 4xx error as a problem with the request and 1338 not attempt to resubmit without some modification to the request. 1339 The full status code MAY provide additional details. 1341 This document deliberately does not provide more specific guidance on 1342 the use of HTTP status codes. 1344 5.1. Submit Entry to Log 1346 POST /ct/v2/submit-entry 1348 Inputs: submission: The base64 encoded certificate or 1349 precertificate. 1351 type: The "VersionedTransType" integer value that indicates 1352 the type of the "submission": 1 for "x509_entry_v2", or 2 for 1353 "precert_entry_v2". 1355 chain: An array of zero or more JSON strings, each of which 1356 is a base64 encoded CA certificate. The first element is the 1357 certifier of the "submission"; the second certifies the first; 1358 etc. The last element of "chain" (or, if "chain" is an empty 1359 array, the "submission") is certified by an accepted trust 1360 anchor. 1362 Outputs: sct: A base64 encoded "TransItem" of type "x509_sct_v2" or 1363 "precert_sct_v2", signed by this log, that corresponds to the 1364 "submission". 1366 If the submitted entry is immediately appended to (or already 1367 exists in) this log's tree, then the log SHOULD also output: 1369 sth: A base64 encoded "TransItem" of type "signed_tree_head_v2", 1370 signed by this log. 1372 inclusion: A base64 encoded "TransItem" of type 1373 "inclusion_proof_v2" whose "inclusion_path" array of Merkle 1374 Tree nodes proves the inclusion of the "submission" in the 1375 returned "sth". 1377 Error codes: 1379 +================+==============================================+ 1380 | type | detail | 1381 +================+==============================================+ 1382 | badSubmission | "submission" is neither a valid certificate | 1383 | | nor a valid precertificate. | 1384 +----------------+----------------------------------------------+ 1385 | badType | "type" is neither 1 nor 2. | 1386 +----------------+----------------------------------------------+ 1387 | badChain | The first element of "chain" is not the | 1388 | | certifier of the "submission", or the second | 1389 | | element does not certify the first, etc. | 1390 +----------------+----------------------------------------------+ 1391 | badCertificate | One or more certificates in the "chain" are | 1392 | | not valid (e.g., not properly encoded). | 1393 +----------------+----------------------------------------------+ 1394 | unknownAnchor | The last element of "chain" (or, if "chain" | 1395 | | is an empty array, the "submission") both is | 1396 | | not, and is not certified by, an accepted | 1397 | | trust anchor. | 1398 +----------------+----------------------------------------------+ 1399 | shutdown | The log is no longer accepting submissions. | 1400 +----------------+----------------------------------------------+ 1402 Table 2 1404 If the version of "sct" is not v2, then a v2 client may be unable to 1405 verify the signature. It MUST NOT construe this as an error. This 1406 is to avoid forcing an upgrade of compliant v2 clients that do not 1407 use the returned SCTs. 1409 If a log detects bad encoding in a chain that otherwise verifies 1410 correctly then the log MUST either log the certificate or return the 1411 "bad certificate" error. If the certificate is logged, an SCT MUST 1412 be issued. Logging the certificate is useful, because monitors 1413 (Section 8.2) can then detect these encoding errors, which may be 1414 accepted by some TLS clients. 1416 If "submission" is an accepted trust anchor whose certifier is 1417 neither an accepted trust anchor nor the first element of "chain", 1418 then the log MUST return the "unknown anchor" error. A log is not 1419 able to generate an SCT for a submission if it does not have access 1420 to the issuer's public key. 1422 If the returned "sct" is intended to be provided to TLS clients, then 1423 "sth" and "inclusion" (if returned) SHOULD also be provided to TLS 1424 clients. For example, if "type" was 2 (indicating "precert_sct_v2") 1425 then all three "TransItem"s could be embedded in the certificate. 1427 5.2. Retrieve Latest Signed Tree Head 1429 GET /ct/v2/get-sth 1431 No inputs. 1433 Outputs: sth: A base64 encoded "TransItem" of type 1434 "signed_tree_head_v2", signed by this log, that is no older 1435 than the log's MMD. 1437 5.3. Retrieve Merkle Consistency Proof between Two Signed Tree Heads 1439 GET /ct/v2/get-sth-consistency 1441 Inputs: first: The tree_size of the older tree, in decimal. 1443 second: The tree_size of the newer tree, in decimal 1444 (optional). 1446 Both tree sizes must be from existing v2 STHs. However, because 1447 of skew, the receiving front-end may not know one or both of the 1448 existing STHs. If both are known, then only the "consistency" 1449 output is returned. If the first is known but the second is not 1450 (or has been omitted), then the latest known STH is returned, 1451 along with a consistency proof between the first STH and the 1452 latest. If neither are known, then the latest known STH is 1453 returned without a consistency proof. 1455 Outputs: consistency: A base64 encoded "TransItem" of type 1456 "consistency_proof_v2", whose "tree_size_1" MUST match the 1457 "first" input. If the "sth" output is omitted, then 1458 "tree_size_2" MUST match the "second" input. If "first" and 1459 "second" are equal and correspond to a known STH, the returned 1460 consistency proof MUST be empty (a "consistency_path" array 1461 with zero elements). 1463 sth: A base64 encoded "TransItem" of type 1464 "signed_tree_head_v2", signed by this log. 1466 Note that no signature is required for the "consistency" output as 1467 it is used to verify the consistency between two STHs, which are 1468 signed. 1470 Error codes: 1472 +===================+======================================+ 1473 | type | detail | 1474 +===================+======================================+ 1475 | firstUnknown | "first" is before the latest known | 1476 | | STH but is not from an existing STH. | 1477 +-------------------+--------------------------------------+ 1478 | secondUnknown | "second" is before the latest known | 1479 | | STH but is not from an existing STH. | 1480 +-------------------+--------------------------------------+ 1481 | secondBeforeFirst | "second" is smaller than "first". | 1482 +-------------------+--------------------------------------+ 1484 Table 3 1486 See Section 2.1.4.2 for an outline of how to use the "consistency" 1487 output. 1489 5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash 1491 GET /ct/v2/get-proof-by-hash 1493 Inputs: hash: A base64 encoded v2 leaf hash. 1495 tree_size: The tree_size of the tree on which to base the 1496 proof, in decimal. 1498 The "hash" must be calculated as defined in Section 4.7. A v2 STH 1499 must exist for the "tree_size". Because of skew, the front-end 1500 may not know the requested tree head. In that case, it will 1501 return the latest STH it knows, along with an inclusion proof to 1502 that STH. If the front-end knows the requested tree head then 1503 only "inclusion" is returned. 1505 Outputs: inclusion: A base64 encoded "TransItem" of type 1506 "inclusion_proof_v2" whose "inclusion_path" array of Merkle 1507 Tree nodes proves the inclusion of the certificate (as 1508 specified by the "hash" parameter) in the selected STH. 1510 sth: A base64 encoded "TransItem" of type 1511 "signed_tree_head_v2", signed by this log. 1513 Note that no signature is required for the "inclusion" output as 1514 it is used to verify inclusion in the selected STH, which is 1515 signed. 1517 Error codes: 1519 +=================+=====================================+ 1520 | type | detail | 1521 +=================+=====================================+ 1522 | hashUnknown | "hash" is not the hash of a known | 1523 | | leaf (may be caused by skew or by a | 1524 | | known certificate not yet merged). | 1525 +-----------------+-------------------------------------+ 1526 | treeSizeUnknown | "hash" is before the latest known | 1527 | | STH but is not from an existing | 1528 | | STH. | 1529 +-----------------+-------------------------------------+ 1531 Table 4 1533 See Section 2.1.3.2 for an outline of how to use the "inclusion" 1534 output. 1536 5.5. Retrieve Merkle Inclusion Proof, Signed Tree Head and Consistency 1537 Proof by Leaf Hash 1539 GET /ct/v2/get-all-by-hash 1541 Inputs: hash: A base64 encoded v2 leaf hash. 1543 tree_size: The tree_size of the tree on which to base the 1544 proofs, in decimal. 1546 The "hash" must be calculated as defined in Section 4.7. A v2 STH 1547 must exist for the "tree_size". 1549 Because of skew, the front-end may not know the requested tree head 1550 or the requested hash, which leads to a number of cases: 1552 +=====================+=====================================+ 1553 | Case | Response | 1554 +=====================+=====================================+ 1555 | latest STH < | Return latest STH | 1556 | requested tree head | | 1557 +---------------------+-------------------------------------+ 1558 | latest STH > | Return latest STH and a consistency | 1559 | requested tree head | proof between it and the requested | 1560 | | tree head (see Section 5.3) | 1561 +---------------------+-------------------------------------+ 1562 | index of requested | Return "inclusion" | 1563 | hash < latest STH | | 1564 +---------------------+-------------------------------------+ 1566 Table 5 1568 Note that more than one case can be true, in which case the returned 1569 data is their union. It is also possible for none to be true, in 1570 which case the front-end MUST return an empty response. 1572 Outputs: inclusion: A base64 encoded "TransItem" of type 1573 "inclusion_proof_v2" whose "inclusion_path" array of Merkle 1574 Tree nodes proves the inclusion of the certificate (as 1575 specified by the "hash" parameter) in the selected STH. 1577 sth: A base64 encoded "TransItem" of type 1578 "signed_tree_head_v2", signed by this log. 1580 consistency: A base64 encoded "TransItem" of type 1581 "consistency_proof_v2" that proves the consistency of the 1582 requested tree head and the returned STH. 1584 Note that no signature is required for the "inclusion" or 1585 "consistency" outputs as they are used to verify inclusion in and 1586 consistency of STHs, which are signed. 1588 Errors are the same as in Section 5.4. 1590 See Section 2.1.3.2 for an outline of how to use the "inclusion" 1591 output, and see Section 2.1.4.2 for an outline of how to use the 1592 "consistency" output. 1594 5.6. Retrieve Entries and STH from Log 1596 GET /ct/v2/get-entries 1598 Inputs: start: 0-based index of first entry to retrieve, in 1599 decimal. 1601 end: 0-based index of last entry to retrieve, in decimal. 1603 Outputs: entries: An array of objects, each consisting of 1605 log_entry: The base64 encoded "TransItem" structure of type 1606 "x509_entry_v2" or "precert_entry_v2" (see Section 4.3). 1608 submitted_entry: JSON object equivalent to inputs that were 1609 submitted to "submit-entry", with the addition of the trust 1610 anchor to the "chain" field if the submission did not 1611 include it. 1613 sct: The base64 encoded "TransItem" of type "x509_sct_v2" or 1614 "precert_sct_v2" corresponding to this log entry. 1616 sth: A base64 encoded "TransItem" of type 1617 "signed_tree_head_v2", signed by this log. 1619 Note that this message is not signed -- the "entries" data can be 1620 verified by constructing the Merkle Tree Hash corresponding to a 1621 retrieved STH. All leaves MUST be v2. However, a compliant v2 1622 client MUST NOT construe an unrecognized TransItem type as an error. 1623 This means it may be unable to parse some entries, but note that each 1624 client can inspect the entries it does recognize as well as verify 1625 the integrity of the data by treating unrecognized leaves as opaque 1626 input to the tree. 1628 The "start" and "end" parameters SHOULD be within the range 0 <= x < 1629 "tree_size" as returned by "get-sth" in Section 5.2. 1631 The "start" parameter MUST be less than or equal to the "end" 1632 parameter. 1634 Each "submitted_entry" output parameter MUST include the trust anchor 1635 that the log used to verify the "submission", even if that trust 1636 anchor was not provided to "submit-entry" (see Section 5.1). If the 1637 "submission" does not certify itself, then the first element of 1638 "chain" MUST be present and MUST certify the "submission". 1640 Log servers MUST honor requests where 0 <= "start" < "tree_size" and 1641 "end" >= "tree_size" by returning a partial response covering only 1642 the valid entries in the specified range. "end" >= "tree_size" could 1643 be caused by skew. Note that the following restriction may also 1644 apply: 1646 Logs MAY restrict the number of entries that can be retrieved per 1647 "get-entries" request. If a client requests more than the permitted 1648 number of entries, the log SHALL return the maximum number of entries 1649 permissible. These entries SHALL be sequential beginning with the 1650 entry specified by "start". Note that limit on the number of entries 1651 is not immutable and therefore the restriction may be changed or 1652 lifted at any time and is not listed with the other Log Parameters in 1653 Section 4.1. 1655 Because of skew, it is possible the log server will not have any 1656 entries between "start" and "end". In this case it MUST return an 1657 empty "entries" array. 1659 In any case, the log server MUST return the latest STH it knows 1660 about. 1662 See Section 2.1.2 for an outline of how to use a complete list of 1663 "log_entry" entries to verify the "root_hash". 1665 Error codes: 1667 +================+====================================+ 1668 | type | detail | 1669 +================+====================================+ 1670 | startUnknown | "start" is greater than the number | 1671 | | of entries in the Merkle tree. | 1672 +----------------+------------------------------------+ 1673 | endBeforeStart | "start" cannot be greater than | 1674 | | "end". | 1675 +----------------+------------------------------------+ 1677 Table 6 1679 5.7. Retrieve Accepted Trust Anchors 1681 GET /ct/v2/get-anchors 1683 No inputs. 1685 Outputs: certificates: An array of JSON strings, each of which is a 1686 base64 encoded CA certificate that is acceptable to the log. 1687 max_chain_length: 1689 If the server has chosen to limit the length of chains it 1690 accepts, this is the maximum number of certificates in the 1691 chain, in decimal. If there is no limit, this is omitted. 1693 This data is not signed and the protocol depends on the security 1694 guarantees of TLS to ensure correctness. 1696 6. TLS Servers 1698 CT-using TLS servers MUST use at least one of the mechanisms 1699 described below to present one or more SCTs from one or more logs to 1700 each TLS client during full TLS handshakes, where each SCT 1701 corresponds to the server certificate. (Of course, a server can only 1702 send a TLS extension if the client has specified it first.) Servers 1703 SHOULD also present corresponding inclusion proofs and STHs. 1705 A server can provide SCTs using a TLS 1.3 extension (Section 4.2 of 1706 [RFC8446]) with type "transparency_info" (see Section 6.5). This 1707 mechanism allows TLS servers to participate in CT without the 1708 cooperation of CAs, unlike the other two mechanisms. It also allows 1709 SCTs and inclusion proofs to be updated on the fly. 1711 The server may also use an Online Certificate Status Protocol (OCSP) 1712 [RFC6960] response extension (see Section 7.1.1), providing the OCSP 1713 response as part of the TLS handshake. Providing a response during a 1714 TLS handshake is popularly known as "OCSP stapling." For TLS 1.3, 1715 the information is encoded as an extension in the "status_request" 1716 extension data; see Section 4.4.2.1 of [RFC8446]. For TLS 1.2 1717 ([RFC5246]), the information is encoded as an extension in the 1718 "CertificateStatus" message; see Section 8 of [RFC6066]. Using 1719 stapling also allows SCTs and inclusion proofs to be updated on the 1720 fly. 1722 CT information can also be encoded as an extension in the X.509v3 1723 certificate (see Section 7.1.2). This mechanism allows the use of 1724 unmodified TLS servers, but the SCTs and inclusion proofs cannot be 1725 updated on the fly. Since the logs from which the SCTs and inclusion 1726 proofs originated won't necessarily be accepted by TLS clients for 1727 the full lifetime of the certificate, there is a risk that TLS 1728 clients may subsequently consider the certificate to be non-compliant 1729 and in need of re-issuance or the use of one of the other two methods 1730 for delivering CT information. 1732 6.1. TLS Client Authentication 1734 This specification includes no description of how a TLS server can 1735 use CT for TLS client certificates. While this may be useful, it is 1736 not documented here for the following reasons: 1738 * The greater security exposure is for clients to end up interacting 1739 with an illegitimate server. 1741 * In general, TLS client certificates are not expected to be 1742 submitted to CT logs, particularly those intended for general 1743 public use. 1745 A future version could include such information. 1747 6.2. Multiple SCTs 1749 CT-using TLS servers SHOULD send SCTs from multiple logs, because: 1751 * One or more logs may not have become acceptable to all CT-using 1752 TLS clients. Note that client discovery, trust, and distrust of 1753 logs is expected to be handled out-of-band and is out of scope of 1754 this document. 1756 * If a CA and a log collude, it is possible to temporarily hide 1757 misissuance from clients. When a TLS client requires SCTs from 1758 multiple logs to be provided, it is more difficult to mount this 1759 attack. 1761 * If a log misbehaves or suffers a key compromise, a consequence may 1762 be that clients cease to trust it. Since the time an SCT may be 1763 in use can be considerable (several years is common in current 1764 practice when embedded in a certificate), including SCTs from 1765 multiple logs reduces the probability of the certificate being 1766 rejected by TLS clients. 1768 * TLS clients may have policies related to the above risks requiring 1769 TLS servers to present multiple SCTs. For example, at the time of 1770 writing, Chromium [Chromium.Log.Policy] requires multiple SCTs to 1771 be presented with EV certificates in order for the EV indicator to 1772 be shown. 1774 To select the logs from which to obtain SCTs, a TLS server can, for 1775 example, examine the set of logs popular TLS clients accept and 1776 recognize. 1778 6.3. TransItemList Structure 1780 Multiple SCTs, inclusion proofs, and indeed "TransItem" structures of 1781 any type, are combined into a list as follows: 1783 opaque SerializedTransItem<1..2^16-1>; 1785 struct { 1786 SerializedTransItem trans_item_list<1..2^16-1>; 1787 } TransItemList; 1789 Here, "SerializedTransItem" is an opaque byte string that contains 1790 the serialized "TransItem" structure. This encoding ensures that TLS 1791 clients can decode each "TransItem" individually (so, for example, if 1792 there is a version upgrade, out-of-date clients can still parse old 1793 "TransItem" structures while skipping over new "TransItem" structures 1794 whose versions they don't understand). 1796 6.4. Presenting SCTs, inclusions proofs and STHs 1798 In each "TransItemList" that is sent during a TLS handshake, the TLS 1799 server MUST include a "TransItem" structure of type "x509_sct_v2" or 1800 "precert_sct_v2". 1802 Presenting inclusion proofs and STHs in the TLS handshake helps to 1803 protect the client's privacy (see Section 8.1.4) and reduces load on 1804 log servers. Therefore, if the TLS server can obtain them, it SHOULD 1805 also include "TransItem"s of type "inclusion_proof_v2" and 1806 "signed_tree_head_v2" in the "TransItemList". 1808 6.5. transparency_info TLS Extension 1810 Provided that a TLS client includes the "transparency_info" extension 1811 type in the ClientHello and the TLS server supports the 1812 "transparency_info" extension: 1814 * The TLS server MUST verify that the received "extension_data" is 1815 empty. 1817 * The TLS server MUST construct a "TransItemList" of relevant 1818 "TransItem"s (see Section 6.4), which SHOULD omit any "TransItem"s 1819 that are already embedded in the server certificate or the stapled 1820 OCSP response (see Section 7.1). If the constructed 1821 "TransItemList" is not empty, then the TLS server MUST include the 1822 "transparency_info" extension with the "extension_data" set to 1823 this "TransItemList". If the list is empty then the server SHOULD 1824 omit the "extension_data" element, but MAY send it with an empty 1825 array. 1827 TLS servers MUST only include this extension in the following 1828 messages: 1830 * the ServerHello message (for TLS 1.2 or earlier). 1832 * the Certificate or CertificateRequest message (for TLS 1.3). 1834 TLS servers MUST NOT process or include this extension when a TLS 1835 session is resumed, since session resumption uses the original 1836 session information. 1838 7. Certification Authorities 1840 7.1. Transparency Information X.509v3 Extension 1842 The Transparency Information X.509v3 extension, which has OID 1843 1.3.101.75 and SHOULD be non-critical, contains one or more 1844 "TransItem" structures in a "TransItemList". This extension MAY be 1845 included in OCSP responses (see Section 7.1.1) and certificates (see 1846 Section 7.1.2). Since RFC5280 requires the "extnValue" field (an 1847 OCTET STRING) of each X.509v3 extension to include the DER encoding 1848 of an ASN.1 value, a "TransItemList" MUST NOT be included directly. 1849 Instead, it MUST be wrapped inside an additional OCTET STRING, which 1850 is then put into the "extnValue" field: 1852 TransparencyInformationSyntax ::= OCTET STRING 1854 "TransparencyInformationSyntax" contains a "TransItemList". 1856 7.1.1. OCSP Response Extension 1858 A certification authority MAY include a Transparency Information 1859 X.509v3 extension in the "singleExtensions" of a "SingleResponse" in 1860 an OCSP response. All included SCTs and inclusion proofs MUST be for 1861 the certificate identified by the "certID" of that "SingleResponse", 1862 or for a precertificate that corresponds to that certificate. 1864 7.1.2. Certificate Extension 1866 A certification authority MAY include a Transparency Information 1867 X.509v3 extension in a certificate. All included SCTs and inclusion 1868 proofs MUST be for a precertificate that corresponds to this 1869 certificate. 1871 7.2. TLS Feature X.509v3 Extension 1873 A certification authority SHOULD NOT issue any certificate that 1874 identifies the "transparency_info" TLS extension in a TLS feature 1875 extension [RFC7633], because TLS servers are not required to support 1876 the "transparency_info" TLS extension in order to participate in CT 1877 (see Section 6). 1879 8. Clients 1881 There are various different functions clients of logs might perform. 1882 We describe here some typical clients and how they should function. 1883 Any inconsistency may be used as evidence that a log has not behaved 1884 correctly, and the signatures on the data structures prevent the log 1885 from denying that misbehavior. 1887 All clients need various parameters in order to communicate with logs 1888 and verify their responses. These parameters are described in 1889 Section 4.1, but note that this document does not describe how the 1890 parameters are obtained, which is implementation-dependent (see, for 1891 example, [Chromium.Policy]). 1893 8.1. TLS Client 1895 8.1.1. Receiving SCTs and inclusion proofs 1897 TLS clients receive SCTs and inclusion proofs alongside or in 1898 certificates. CT-using TLS clients MUST implement all of the three 1899 mechanisms by which TLS servers may present SCTs (see Section 6). 1901 TLS clients that support the "transparency_info" TLS extension (see 1902 Section 6.5) SHOULD include it in ClientHello messages, with empty 1903 "extension_data". If a TLS server includes the "transparency_info" 1904 TLS extension when resuming a TLS session, the TLS client MUST abort 1905 the handshake. 1907 8.1.2. Reconstructing the TBSCertificate 1909 Validation of an SCT for a certificate (where the "type" of the 1910 "TransItem" is "x509_sct_v2") uses the unmodified TBSCertificate 1911 component of the certificate. 1913 Before an SCT for a precertificate (where the "type" of the 1914 "TransItem" is "precert_sct_v2") can be validated, the TBSCertificate 1915 component of the precertificate needs to be reconstructed from the 1916 TBSCertificate component of the certificate as follows: 1918 * Remove the Transparency Information extension (see Section 7.1). 1920 * Remove embedded v1 SCTs, identified by OID 1.3.6.1.4.1.11129.2.4.2 1921 (see section 3.3 of [RFC6962]). This allows embedded v1 and v2 1922 SCTs to co-exist in a certificate (see Appendix A). 1924 8.1.3. Validating SCTs 1926 In order to make use of a received SCT, the TLS client MUST first 1927 validate it as follows: 1929 * Compute the signature input by constructing a "TransItem" of type 1930 "x509_entry_v2" or "precert_entry_v2", depending on the SCT's 1931 "TransItem" type. The "TimestampedCertificateEntryDataV2" 1932 structure is constructed in the following manner: 1934 - "timestamp" is copied from the SCT. 1936 - "tbs_certificate" is the reconstructed TBSCertificate portion 1937 of the server certificate, as described in Section 8.1.2. 1939 - "issuer_key_hash" is computed as described in Section 4.7. 1941 - "sct_extensions" is copied from the SCT. 1943 * Verify the SCT's "signature" against the computed signature input 1944 using the public key of the corresponding log, which is identified 1945 by the "log_id". The required signature algorithm is one of the 1946 log's parameters. 1948 If the TLS client does not have the corresponding log's parameters, 1949 it cannot attempt to validate the SCT. When evaluating compliance 1950 (see Section 8.1.6), the TLS client will consider only those SCTs 1951 that it was able to validate. 1953 Note that SCT validation is not a substitute for the normal 1954 validation of the server certificate and its chain. 1956 8.1.4. Fetching inclusion proofs 1958 When a TLS client has validated a received SCT but does not yet 1959 possess a corresponding inclusion proof, the TLS client MAY request 1960 the inclusion proof directly from a log using "get-proof-by-hash" 1961 (Section 5.4) or "get-all-by-hash" (Section 5.5). 1963 Note that fetching inclusion proofs directly from a log will disclose 1964 to the log which TLS server the client has been communicating with. 1965 This may be regarded as a significant privacy concern, and so it is 1966 preferable for the TLS server to send the inclusion proofs (see 1967 Section 6.4). 1969 8.1.5. Validating inclusion proofs 1971 When a TLS client has received, or fetched, an inclusion proof (and 1972 an STH), it SHOULD proceed to verifying the inclusion proof to the 1973 provided STH. The TLS client SHOULD also verify consistency between 1974 the provided STH and an STH it knows about. 1976 If the TLS client holds an STH that predates the SCT, it MAY, in the 1977 process of auditing, request a new STH from the log (Section 5.2), 1978 then verify it by requesting a consistency proof (Section 5.3). Note 1979 that if the TLS client uses "get-all-by-hash", then it will already 1980 have the new STH. 1982 8.1.6. Evaluating compliance 1984 It is up to a client's local policy to specify the quantity and form 1985 of evidence (SCTs, inclusion proofs or a combination) needed to 1986 achieve compliance and how to handle non-compliance. 1988 A TLS client can only evaluate compliance if it has given the TLS 1989 server the opportunity to send SCTs and inclusion proofs by any of 1990 the three mechanisms that are mandatory to implement for CT-using TLS 1991 clients (see Section 8.1.1). Therefore, a TLS client MUST NOT 1992 evaluate compliance if it did not include both the 1993 "transparency_info" and "status_request" TLS extensions in the 1994 ClientHello. 1996 8.2. Monitor 1998 Monitors watch logs to check that they behave correctly, for 1999 certificates of interest, or both. For example, a monitor may be 2000 configured to report on all certificates that apply to a specific 2001 domain name when fetching new entries for consistency validation. 2003 A monitor MUST at least inspect every new entry in every log it 2004 watches, and it MAY also choose to keep copies of entire logs. 2006 To inspect all of the existing entries, the monitor SHOULD follow 2007 these steps once for each log: 2009 1. Fetch the current STH (Section 5.2). 2011 2. Verify the STH signature. 2013 3. Fetch all the entries in the tree corresponding to the STH 2014 (Section 5.6). 2016 4. If applicable, check each entry to see if it's a certificate of 2017 interest. 2019 5. Confirm that the tree made from the fetched entries produces the 2020 same hash as that in the STH. 2022 To inspect new entries, the monitor SHOULD follow these steps 2023 repeatedly for each log: 2025 1. Fetch the current STH (Section 5.2). Repeat until the STH 2026 changes. This document does not specify the polling frequency, 2027 to allow for experimentation. 2029 2. Verify the STH signature. 2031 3. Fetch all the new entries in the tree corresponding to the STH 2032 (Section 5.6). If they remain unavailable for an extended 2033 period, then this should be viewed as misbehavior on the part of 2034 the log. 2036 4. If applicable, check each entry to see if it's a certificate of 2037 interest. 2039 5. Either: 2041 1. Verify that the updated list of all entries generates a tree 2042 with the same hash as the new STH. 2044 Or, if it is not keeping all log entries: 2046 1. Fetch a consistency proof for the new STH with the previous 2047 STH (Section 5.3). 2049 2. Verify the consistency proof. 2051 3. Verify that the new entries generate the corresponding 2052 elements in the consistency proof. 2054 6. Repeat from step 1. 2056 8.3. Auditing 2058 Auditing ensures that the current published state of a log is 2059 reachable from previously published states that are known to be good, 2060 and that the promises made by the log in the form of SCTs have been 2061 kept. Audits are performed by monitors or TLS clients. 2063 In particular, there are four log behavior properties that should be 2064 checked: 2066 * The Maximum Merge Delay (MMD). 2068 * The STH Frequency Count. 2070 * The append-only property. 2072 * The consistency of the log view presented to all query sources. 2074 A benign, conformant log publishes a series of STHs over time, each 2075 derived from the previous STH and the submitted entries incorporated 2076 into the log since publication of the previous STH. This can be 2077 proven through auditing of STHs. SCTs returned to TLS clients can be 2078 audited by verifying against the accompanying certificate, and using 2079 Merkle Inclusion Proofs, against the log's Merkle tree. 2081 The action taken by the auditor if an audit fails is not specified, 2082 but note that in general if audit fails, the auditor is in possession 2083 of signed proof of the log's misbehavior. 2085 A monitor (Section 8.2) can audit by verifying the consistency of 2086 STHs it receives, ensure that each entry can be fetched and that the 2087 STH is indeed the result of making a tree from all fetched entries. 2089 A TLS client (Section 8.1) can audit by verifying an SCT against any 2090 STH dated after the SCT timestamp + the Maximum Merge Delay by 2091 requesting a Merkle inclusion proof (Section 5.4). It can also 2092 verify that the SCT corresponds to the server certificate it arrived 2093 with (i.e., the log entry is that certificate, or is a precertificate 2094 corresponding to that certificate). 2096 Checking of the consistency of the log view presented to all entities 2097 is more difficult to perform because it requires a way to share log 2098 responses among a set of CT-using entities, and is discussed in 2099 Section 11.3. 2101 9. Algorithm Agility 2103 It is not possible for a log to change any of its algorithms part way 2104 through its lifetime: 2106 Signature algorithm: SCT signatures must remain valid so signature 2107 algorithms can only be added, not removed. 2109 Hash algorithm: A log would have to support the old and new hash 2110 algorithms to allow backwards-compatibility with clients that are 2111 not aware of a hash algorithm change. 2113 Allowing multiple signature or hash algorithms for a log would 2114 require that all data structures support it and would significantly 2115 complicate client implementation, which is why it is not supported by 2116 this document. 2118 If it should become necessary to deprecate an algorithm used by a 2119 live log, then the log MUST be frozen as specified in Section 4.13 2120 and a new log SHOULD be started. Certificates in the frozen log that 2121 have not yet expired and require new SCTs SHOULD be submitted to the 2122 new log and the SCTs from that log used instead. 2124 10. IANA Considerations 2126 The assignment policy criteria mentioned in this section refer to the 2127 policies outlined in [RFC8126]. 2129 10.1. New Entry to the TLS ExtensionType Registry 2131 IANA is asked to add an entry for "transparency_info(TBD)" to the 2132 "TLS ExtensionType Values" registry defined in [RFC8446], setting the 2133 "Recommended" value to "Y", setting the "TLS 1.3" value to "CH, CR, 2134 CT", and citing this document as the "Reference". 2136 10.2. Hash Algorithms 2138 IANA is asked to establish a registry of hash algorithm values, named 2139 "CT Hash Algorithms", that initially consists of: 2141 +========+============+========================+===================+ 2142 | Value | Hash | OID | Reference / | 2143 | | Algorithm | | Assignment Policy | 2144 +========+============+========================+===================+ 2145 | 0x00 | SHA-256 | 2.16.840.1.101.3.4.2.1 | [RFC6234] | 2146 +--------+------------+------------------------+-------------------+ 2147 | 0x01 - | Unassigned | | Specification | 2148 | 0xDF | | | Required | 2149 +--------+------------+------------------------+-------------------+ 2150 | 0xE0 - | Reserved | | Experimental Use | 2151 | 0xEF | | | | 2152 +--------+------------+------------------------+-------------------+ 2153 | 0xF0 - | Reserved | | Private Use | 2154 | 0xFF | | | | 2155 +--------+------------+------------------------+-------------------+ 2157 Table 7 2159 10.2.1. Specification Required guidance 2161 The appointed Expert(s) should ensure that the proposed algorithm has 2162 a public specification and is suitable for use as a cryptographic 2163 hash algorithm with no known preimage or collision attacks. These 2164 attacks can damage the integrity of the log. 2166 10.3. Signature Algorithms 2168 IANA is asked to establish a registry of signature algorithm values, 2169 named "CT Signature Algorithms" 2171 The following notes should be added: 2173 * This is a subset of the TLS SignatureScheme Registry, limited to 2174 those algorithms that are appropriate for CT. A major advantage 2175 of this is leveraging the expertise of the TLS working group and 2176 its designated experts. 2178 * The value "0x0403" appears twice. While this may be confusing, it 2179 is okay because the verification process is the same for both 2180 algorithms, and the choice of which to use when generating a 2181 signature is purely internal to the log server. 2183 The registry should initially consist of: 2185 +================================+==================+==============+ 2186 | SignatureScheme Value | Signature | Reference / | 2187 | | Algorithm | Assignment | 2188 | | | Policy | 2189 +================================+==================+==============+ 2190 | 0x0000 - 0x0402 | Unassigned | Expert | 2191 | | | Review | 2192 +--------------------------------+------------------+--------------+ 2193 | ecdsa_secp256r1_sha256(0x0403) | ECDSA (NIST | [FIPS186-4] | 2194 | | P-256) with | | 2195 | | SHA-256 | | 2196 +--------------------------------+------------------+--------------+ 2197 | ecdsa_secp256r1_sha256(0x0403) | Deterministic | [RFC6979] | 2198 | | ECDSA (NIST | | 2199 | | P-256) with | | 2200 | | HMAC-SHA256 | | 2201 +--------------------------------+------------------+--------------+ 2202 | 0x0404 - 0x0806 | Unassigned | Expert | 2203 | | | Review | 2204 +--------------------------------+------------------+--------------+ 2205 | ed25519(0x0807) | Ed25519 | [RFC8032] | 2206 | | (PureEdDSA with | | 2207 | | the edwards25519 | | 2208 | | curve) | | 2209 +--------------------------------+------------------+--------------+ 2210 | 0x0808 - 0xFDFF | Unassigned | Expert | 2211 | | | Review | 2212 +--------------------------------+------------------+--------------+ 2213 | 0xFE00 - 0xFEFF | Reserved | Experimental | 2214 | | | Use | 2215 +--------------------------------+------------------+--------------+ 2216 | 0xFF00 - 0xFFFF | Reserved | Private Use | 2217 +--------------------------------+------------------+--------------+ 2219 Table 8 2221 10.3.1. Expert Review guidelines 2223 The appointed Expert should ensure that the proposed algorithm has a 2224 public specification, has a value assigned to it in the TLS 2225 SignatureScheme Registry (that IANA is asked to establish in 2226 [RFC8446]) and is suitable for use as a cryptographic signature 2227 algorithm. 2229 10.4. VersionedTransTypes 2231 IANA is asked to establish a registry of "VersionedTransType" values, 2232 named "CT VersionedTransTypes", that initially consists of: 2234 +==========+======================+===============================+ 2235 | Value | Type and Version | Reference / Assignment Policy | 2236 +==========+======================+===============================+ 2237 | 0x0000 | Reserved | [RFC6962] * | 2238 +----------+----------------------+-------------------------------+ 2239 | 0x0001 | x509_entry_v2 | RFCXXXX | 2240 +----------+----------------------+-------------------------------+ 2241 | 0x0002 | precert_entry_v2 | RFCXXXX | 2242 +----------+----------------------+-------------------------------+ 2243 | 0x0003 | x509_sct_v2 | RFCXXXX | 2244 +----------+----------------------+-------------------------------+ 2245 | 0x0004 | precert_sct_v2 | RFCXXXX | 2246 +----------+----------------------+-------------------------------+ 2247 | 0x0005 | signed_tree_head_v2 | RFCXXXX | 2248 +----------+----------------------+-------------------------------+ 2249 | 0x0006 | consistency_proof_v2 | RFCXXXX | 2250 +----------+----------------------+-------------------------------+ 2251 | 0x0007 | inclusion_proof_v2 | RFCXXXX | 2252 +----------+----------------------+-------------------------------+ 2253 | 0x0008 - | Unassigned | Specification Required | 2254 | 0xDFFF | | | 2255 +----------+----------------------+-------------------------------+ 2256 | 0xE000 - | Reserved | Experimental Use | 2257 | 0xEFFF | | | 2258 +----------+----------------------+-------------------------------+ 2259 | 0xF000 - | Reserved | Private Use | 2260 | 0xFFFF | | | 2261 +----------+----------------------+-------------------------------+ 2263 Table 9 2265 * The 0x0000 value is reserved so that v1 SCTs are distinguishable 2266 from v2 SCTs and other "TransItem" structures. ### Specification 2267 Required guidance 2269 The appointed Expert should review the public specification to ensure 2270 that it is detailed enough to ensure implementation interoperability. 2272 10.5. Log Artifact Extension Registry 2274 IANA is asked to establish a registry of "ExtensionType" values, 2275 named "CT Log Artifact Extensions", that initially consists of: 2277 +===============+============+=====+===============================+ 2278 | ExtensionType | Status | Use | Reference / Assignment Policy | 2279 +===============+============+=====+===============================+ 2280 | 0x0000 - | Unassigned | n/a | Specification Required | 2281 | 0xDFFF | | | | 2282 +---------------+------------+-----+-------------------------------+ 2283 | 0xE000 - | Reserved | n/a | Experimental Use | 2284 | 0xEFFF | | | | 2285 +---------------+------------+-----+-------------------------------+ 2286 | 0xF000 - | Reserved | n/a | Private Use | 2287 | 0xFFFF | | | | 2288 +---------------+------------+-----+-------------------------------+ 2290 Table 10 2292 The "Use" column should contain one or both of the following values: 2294 * "SCT", for extensions specified for use in Signed Certificate 2295 Timestamps. 2297 * "STH", for extensions specified for use in Signed Tree Heads. 2299 10.5.1. Specification Required guidance 2301 The appointed Expert should review the public specification to ensure 2302 that it is detailed enough to ensure implementation interoperability. 2303 The Expert should also verify that the extension is appropriate to 2304 the contexts in which it is specified to be used (SCT, STH, or both). 2306 10.6. Object Identifiers 2308 This document uses object identifiers (OIDs) to identify Log IDs (see 2309 Section 4.4), the precertificate CMS "eContentType" (see 2310 Section 3.2), and X.509v3 extensions in certificates (see 2311 Section 7.1.2) and OCSP responses (see Section 7.1.1). The OIDs are 2312 defined in an arc that was selected due to its short encoding. 2314 10.6.1. Log ID Registry 2316 IANA is asked to establish a registry of Log IDs, named "CT Log ID 2317 Registry", that initially consists of: 2319 +================+==============+==============+===================+ 2320 | Log ID | Log Base URL | Log Operator | Reference / | 2321 | | | | Assignment Policy | 2322 +================+==============+==============+===================+ 2323 | 1.3.101.8192 - | Unassigned | Unassigned | First Come First | 2324 | 1.3.101.16383 | | | Served | 2325 +----------------+--------------+--------------+-------------------+ 2326 | 1.3.101.80.0 - | Unassigned | Unassigned | First Come First | 2327 | 1.3.101.80.* | | | Served | 2328 +----------------+--------------+--------------+-------------------+ 2330 Table 11 2332 All OIDs in the range from 1.3.101.8192 to 1.3.101.16383 have been 2333 set aside for Log IDs. This is a limited resource of 8,192 OIDs, 2334 each of which has an encoded length of 4 octets. 2336 The 1.3.101.80 arc has also been set assigned for LogIDs. This is an 2337 unlimited resource, but only the 128 OIDs from 1.3.101.80.0 to 2338 1.3.101.80.127 have an encoded length of only 4 octets. 2340 Each application for the allocation of a Log ID MUST be accompanied 2341 by: 2343 * the Log's Base URL (see Section 4.1). 2345 * the Log Operator's contact details. 2347 IANA is asked to reject any request to update a Log ID or Log Base 2348 URL in this registry, because these fields are immutable (see 2349 Section 4.1). 2351 IANA is asked to accept requests from log operators to update their 2352 contact details in this registry. 2354 Since log operators can choose to not use this registry (see 2355 Section 4.4), it is not expected to be a global directory of all 2356 logs. 2358 10.7. URN Sub-namespace for TRANS errors (urn:ietf:params:trans:error) 2360 IANA is requested to add a new entry in the "IETF URN Sub-namespace 2361 for Registered Protocol Parameter Identifiers" registry, following 2362 the template in [RFC3553]: 2364 Registry name: trans:error 2366 Specification: RFCXXXX 2367 Repository: https://www.iana.org/assignments/trans 2369 Index value: No transformation needed. 2371 10.7.1. TRANS Error Types 2373 IANA is requested to create a new registry for errors. Requirements 2374 for this registry are Specification Required. 2376 This registry should have the following three fields: 2378 +============+========+===========+ 2379 | Field Name | Type | Reference | 2380 +============+========+===========+ 2381 | identifier | string | RFCXXXX | 2382 +------------+--------+-----------+ 2383 | meaning | string | RFCXXXX | 2384 +------------+--------+-----------+ 2385 | reference | string | RFCXXXX | 2386 +------------+--------+-----------+ 2388 Table 12 2390 The initial values are as follows, taken from the text above: 2392 +===================+===============================+===========+ 2393 | Identifier | Meaning | Reference | 2394 +===================+===============================+===========+ 2395 | malformed | The request could not be | RFCXXXX | 2396 | | parsed. | | 2397 +-------------------+-------------------------------+-----------+ 2398 | badSubmission | "submission" is neither a | RFCXXXX | 2399 | | valid certificate nor a valid | | 2400 | | precertificate | | 2401 +-------------------+-------------------------------+-----------+ 2402 | badType | "type" is neither 1 nor 2 | RFCXXXX | 2403 +-------------------+-------------------------------+-----------+ 2404 | badChain | The first element of "chain" | RFCXXXX | 2405 | | is not the certifier of the | | 2406 | | "submission", or the second | | 2407 | | element does not certify the | | 2408 | | first, etc. | | 2409 +-------------------+-------------------------------+-----------+ 2410 | badCertificate | One or more certificates in | RFCXXXX | 2411 | | the "chain" are not valid | | 2412 | | (e.g., not properly encoded) | | 2413 +-------------------+-------------------------------+-----------+ 2414 | unknownAnchor | The last element of "chain" | RFCXXXX | 2415 | | (or, if "chain" is an empty | | 2416 | | array, the "submission") both | | 2417 | | is not, and is not certified | | 2418 | | by, an accepted trust anchor | | 2419 +-------------------+-------------------------------+-----------+ 2420 | shutdown | The log is no longer | RFCXXXX | 2421 | | accepting submissions | | 2422 +-------------------+-------------------------------+-----------+ 2423 | firstUnknown | "first" is before the latest | RFCXXXX | 2424 | | known STH but is not from an | | 2425 | | existing STH. | | 2426 +-------------------+-------------------------------+-----------+ 2427 | secondUnknown | "second" is before the latest | RFCXXXX | 2428 | | known STH but is not from an | | 2429 | | existing STH. | | 2430 +-------------------+-------------------------------+-----------+ 2431 | secondBeforeFirst | "second" is smaller than | RFCXXXX | 2432 | | "first". | | 2433 +-------------------+-------------------------------+-----------+ 2434 | hashUnknown | "hash" is not the hash of a | RFCXXXX | 2435 | | known leaf (may be caused by | | 2436 | | skew or by a known | | 2437 | | certificate not yet merged). | | 2438 +-------------------+-------------------------------+-----------+ 2439 | treeSizeUnknown | "hash" is before the latest | RFCXXXX | 2440 | | known STH but is not from an | | 2441 | | existing STH. | | 2442 +-------------------+-------------------------------+-----------+ 2443 | startUnknown | "start" is greater than the | RFCXXXX | 2444 | | number of entries in the | | 2445 | | Merkle tree. | | 2446 +-------------------+-------------------------------+-----------+ 2447 | endBeforeStart | "start" cannot be greater | RFCXXXX | 2448 | | than "end". | | 2449 +-------------------+-------------------------------+-----------+ 2451 Table 13 2453 11. Security Considerations 2455 With CAs, logs, and servers performing the actions described here, 2456 TLS clients can use logs and signed timestamps to reduce the 2457 likelihood that they will accept misissued certificates. If a server 2458 presents a valid signed timestamp for a certificate, then the client 2459 knows that a log has committed to publishing the certificate. From 2460 this, the client knows that monitors acting for the subject of the 2461 certificate have had some time to notice the misissuance and take 2462 some action, such as asking a CA to revoke a misissued certificate. 2464 A signed timestamp does not guarantee this though, since appropriate 2465 monitors might not have checked the logs or the CA might have refused 2466 to revoke the certificate. 2468 In addition, if TLS clients will not accept unlogged certificates, 2469 then site owners will have a greater incentive to submit certificates 2470 to logs, possibly with the assistance of their CA, increasing the 2471 overall transparency of the system. 2473 11.1. Misissued Certificates 2475 Misissued certificates that have not been publicly logged, and thus 2476 do not have a valid SCT, are not considered compliant. Misissued 2477 certificates that do have an SCT from a log will appear in that 2478 public log within the Maximum Merge Delay, assuming the log is 2479 operating correctly. Since a log is allowed to serve an STH of any 2480 age up to the MMD, the maximum period of time during which a 2481 misissued certificate can be used without being available for audit 2482 is twice the MMD. 2484 11.2. Detection of Misissue 2486 The logs do not themselves detect misissued certificates; they rely 2487 instead on interested parties, such as domain owners, to monitor them 2488 and take corrective action when a misissue is detected. 2490 11.3. Misbehaving Logs 2492 A log can misbehave in several ways. Examples include: failing to 2493 incorporate a certificate with an SCT in the Merkle Tree within the 2494 MMD; presenting different, conflicting views of the Merkle Tree at 2495 different times and/or to different parties; issuing STHs too 2496 frequently; mutating the signature of a logged certificate; and 2497 failing to present a chain containing the certifier of a logged 2498 certificate. 2500 Violation of the MMD contract is detected by log clients requesting a 2501 Merkle inclusion proof (Section 5.4) for each observed SCT. These 2502 checks can be asynchronous and need only be done once per 2503 certificate. However, note that there may be privacy concerns (see 2504 Section 8.1.4). 2506 Violation of the append-only property or the STH issuance rate limit 2507 can be detected by multiple clients comparing their instances of the 2508 Signed Tree Heads. This technique, known as "gossip," is an active 2509 area of research and not defined here. Proof of misbehavior in such 2510 cases would be: a series of STHs that were issued too closely 2511 together, proving violation of the STH issuance rate limit; or an STH 2512 with a root hash that does not match the one calculated from a copy 2513 of the log, proving violation of the append-only property. 2515 Clients that report back SCTs can be tracked or traced if a log 2516 produces multiple STHs or SCTs with the same timestamp and data but 2517 different signatures. Logs SHOULD mitigate this risk by either: 2519 * Using deterministic signature schemes, or 2521 * Producing no more than one SCT for each distinct submission and no 2522 more than one STH for each distinct tree_size. Each of these SCTs 2523 and STHs can be stored by the log and served to other clients that 2524 submit the same certificate or request the same STH. 2526 11.4. Multiple SCTs 2528 By requiring TLS servers to offer multiple SCTs, each from a 2529 different log, TLS clients reduce the effectiveness of an attack 2530 where a CA and a log collude (see Section 6.2). 2532 11.5. Leakage of DNS Information 2534 Malicious monitors can use logs to learn about the existence of 2535 domain names that might not otherwise be easy to discover. Some 2536 subdomain labels may reveal information about the service and 2537 software for which the subdomain is used, which in turn might 2538 facilitate targeted attacks. 2540 12. Acknowledgements 2542 The authors would like to thank Erwann Abelea, Robin Alden, Andrew 2543 Ayer, Richard Barnes, Al Cutter, David Drysdale, Francis Dupont, Adam 2544 Eijdenberg, Stephen Farrell, Daniel Kahn Gillmor, Paul Hadfield, Brad 2545 Hill, Jeff Hodges, Paul Hoffman, Jeffrey Hutzelman, Kat Joyce, 2546 Stephen Kent, SM, Alexey Melnikov, Linus Nordberg, Chris Palmer, 2547 Trevor Perrin, Pierre Phaneuf, Eric Rescorla, Rich Salz, Melinda 2548 Shore, Ryan Sleevi, Martin Smith, Carl Wallace and Paul Wouters for 2549 their valuable contributions. 2551 A big thank you to Symantec for kindly donating the OIDs from the 2552 1.3.101 arc that are used in this document. 2554 13. References 2556 13.1. Normative References 2558 [FIPS186-4] 2559 NIST, "FIPS PUB 186-4", 1 July 2013, 2560 . 2563 [HTML401] Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01 2564 Specification", World Wide Web Consortium Recommendation 2565 REC-html401-19991224, 24 December 1999, 2566 . 2568 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2569 Requirement Levels", BCP 14, RFC 2119, 2570 DOI 10.17487/RFC2119, March 1997, 2571 . 2573 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 2574 IETF URN Sub-namespace for Registered Protocol 2575 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 2576 2003, . 2578 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2579 Resource Identifier (URI): Generic Syntax", STD 66, 2580 RFC 3986, DOI 10.17487/RFC3986, January 2005, 2581 . 2583 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 2584 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 2585 . 2587 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 2588 (TLS) Protocol Version 1.2", RFC 5246, 2589 DOI 10.17487/RFC5246, August 2008, 2590 . 2592 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 2593 Housley, R., and W. Polk, "Internet X.509 Public Key 2594 Infrastructure Certificate and Certificate Revocation List 2595 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 2596 . 2598 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, 2599 RFC 5652, DOI 10.17487/RFC5652, September 2009, 2600 . 2602 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 2603 Extensions: Extension Definitions", RFC 6066, 2604 DOI 10.17487/RFC6066, January 2011, 2605 . 2607 [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms 2608 (SHA and SHA-based HMAC and HKDF)", RFC 6234, 2609 DOI 10.17487/RFC6234, May 2011, 2610 . 2612 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., 2613 Galperin, S., and C. Adams, "X.509 Internet Public Key 2614 Infrastructure Online Certificate Status Protocol - OCSP", 2615 RFC 6960, DOI 10.17487/RFC6960, June 2013, 2616 . 2618 [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature 2619 Algorithm (DSA) and Elliptic Curve Digital Signature 2620 Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2621 2013, . 2623 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 2624 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 2625 DOI 10.17487/RFC7231, June 2014, 2626 . 2628 [RFC7633] Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS) 2629 Feature Extension", RFC 7633, DOI 10.17487/RFC7633, 2630 October 2015, . 2632 [RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP 2633 APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016, 2634 . 2636 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 2637 Signature Algorithm (EdDSA)", RFC 8032, 2638 DOI 10.17487/RFC8032, January 2017, 2639 . 2641 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2642 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2643 May 2017, . 2645 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 2646 Interchange Format", STD 90, RFC 8259, 2647 DOI 10.17487/RFC8259, December 2017, 2648 . 2650 [RFC8391] Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A. 2651 Mohaisen, "XMSS: eXtended Merkle Signature Scheme", 2652 RFC 8391, DOI 10.17487/RFC8391, May 2018, 2653 . 2655 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 2656 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 2657 . 2659 [UNIXTIME] IEEE, "The Open Group Base Specifications Issue 7 IEEE Std 2660 1003.1-2008, 2016 Edition", n.d., 2661 . 2665 [X690] ITU-T, "Information technology - ASN.1 encoding Rules: 2666 Specification of Basic Encoding Rules (BER), Canonical 2667 Encoding Rules (CER) and Distinguished Encoding Rules 2668 (DER)", ISO/IEC 8825-1:2002, November 2015. 2670 13.2. Informative References 2672 [CABBR] CA/Browser Forum, "Baseline Requirements for the Issuance 2673 and Management of Publicly-Trusted Certificates", 2020, 2674 . 2677 [Chromium.Log.Policy] 2678 The Chromium Projects, "Chromium Certificate Transparency 2679 Log Policy", 2014, . 2682 [Chromium.Policy] 2683 The Chromium Projects, "Chromium Certificate 2684 Transparency", 2014, . 2687 [CrosbyWallach] 2688 Crosby, S. and D. Wallach, "Efficient Data Structures for 2689 Tamper-Evident Logging", Proceedings of the 18th USENIX 2690 Security Symposium, Montreal, August 2009, 2691 . 2694 [JSON.Metadata] 2695 The Chromium Projects, "Chromium Log Metadata JSON 2696 Schema", 2014, . 2699 [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate 2700 Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, 2701 . 2703 [RFC7320] Nottingham, M., "URI Design and Ownership", RFC 7320, 2704 DOI 10.17487/RFC7320, July 2014, 2705 . 2707 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2708 Writing an IANA Considerations Section in RFCs", BCP 26, 2709 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2710 . 2712 Appendix A. Supporting v1 and v2 simultaneously (Informative) 2714 Certificate Transparency logs have to be either v1 (conforming to 2715 [RFC6962]) or v2 (conforming to this document), as the data 2716 structures are incompatible and so a v2 log could not issue a valid 2717 v1 SCT. 2719 CT clients, however, can support v1 and v2 SCTs, for the same 2720 certificate, simultaneously, as v1 SCTs are delivered in different 2721 TLS, X.509 and OCSP extensions than v2 SCTs. 2723 v1 and v2 SCTs for X.509 certificates can be validated independently. 2724 For precertificates, v2 SCTs should be embedded in the TBSCertificate 2725 before submission of the TBSCertificate (inside a v1 precertificate, 2726 as described in Section 3.1. of [RFC6962]) to a v1 log so that TLS 2727 clients conforming to [RFC6962] but not this document are oblivious 2728 to the embedded v2 SCTs. An issuer can follow these steps to produce 2729 an X.509 certificate with embedded v1 and v2 SCTs: 2731 * Create a CMS precertificate as described in Section 3.2 and submit 2732 it to v2 logs. 2734 * Embed the obtained v2 SCTs in the TBSCertificate, as described in 2735 Section 7.1.2. 2737 * Use that TBSCertificate to create a v1 precertificate, as 2738 described in Section 3.1. of [RFC6962] and submit it to v1 logs. 2740 * Embed the v1 SCTs in the TBSCertificate, as described in 2741 Section 3.3 of [RFC6962]. 2743 * Sign that TBSCertificate (which now contains v1 and v2 SCTs) to 2744 issue the final X.509 certificate. 2746 Appendix B. An ASN.1 Module (Informative) 2748 The following ASN.1 module may be useful to implementors. 2750 CertificateTransparencyV2Module-2021 2751 -- { OID Needed, but no point in using a short one } 2752 DEFINITIONS IMPLICIT TAGS ::= BEGIN 2754 -- EXPORTS ALL -- 2756 IMPORTS 2757 EXTENSION 2758 FROM PKIX-CommonTypes-2009 -- RFC 5912 2759 { iso(1) identified-organization(3) dod(6) internet(1) 2760 security(5) mechanisms(5) pkix(7) id-mod(0) 2761 id-mod-pkixCommon-02(57) } 2763 CONTENT-TYPE 2764 FROM CryptographicMessageSyntax-2010 -- RFC 6268 2765 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) 2766 pkcs-9(9) smime(16) modules(0) id-mod-cms-2009(58) } 2768 TBSCertificate 2769 FROM PKIX1Explicit-2009 -- RFC 5912 2770 { iso(1) identified-organization(3) dod(6) internet(1) 2771 security(5) mechanisms(5) pkix(7) id-mod(0) 2772 id-mod-pkix1-explicit-02(51) } 2773 ; 2775 -- 2776 -- Section 3.2. Precertificates 2777 -- 2779 ct-tbsCertificate CONTENT-TYPE ::= { 2780 TYPE TBSCertificate 2781 IDENTIFIED BY id-ct-tbsCertificate } 2783 id-ct-tbsCertificate OBJECT IDENTIFIER ::= { 1 3 101 78 } 2785 -- 2786 -- Section 7.1. Transparency Information X.509v3 Extension 2787 -- 2789 ext-transparencyInfo EXTENSION ::= { 2790 SYNTAX TransparencyInformationSyntax 2791 IDENTIFIED BY id-ce-transparencyInfo 2792 CRITICALITY { FALSE } } 2794 id-ce-transparencyInfo OBJECT IDENTIFIER ::= { 1 3 101 75 } 2796 TransparencyInformationSyntax ::= OCTET STRING 2798 -- 2799 -- Section 7.1.1. OCSP Response Extension 2800 -- 2802 ext-ocsp-transparencyInfo EXTENSION ::= { 2803 SYNTAX TransparencyInformationSyntax 2804 IDENTIFIED BY id-pkix-ocsp-transparencyInfo 2805 CRITICALITY { FALSE } } 2807 id-pkix-ocsp-transparencyInfo OBJECT IDENTIFIER ::= 2808 id-ce-transparencyInfo 2810 -- 2811 -- Section 8.1.2. Reconstructing the TBSCertificate 2812 -- 2814 ext-embeddedSCT-CTv1 EXTENSION ::= { 2815 SYNTAX SignedCertificateTimestampList 2816 IDENTIFIED BY id-ce-embeddedSCT-CTv1 2817 CRITICALITY { FALSE } } 2819 id-ce-embeddedSCT-CTv1 OBJECT IDENTIFIER ::= { 2820 1 3 6 1 4 1 11129 2 4 2 } 2822 SignedCertificateTimestampList ::= OCTET STRING 2824 END 2826 Authors' Addresses 2828 Ben Laurie 2829 Google UK Ltd. 2831 Email: benl@google.com 2833 Adam Langley 2834 Google Inc. 2836 Email: agl@google.com 2837 Emilia Kasper 2838 Google Switzerland GmbH 2840 Email: ekasper@google.com 2842 Eran Messeri 2843 Google UK Ltd. 2845 Email: eranm@google.com 2847 Rob Stradling 2848 Sectigo Ltd. 2850 Email: rob@sectigo.com