idnits 2.17.1 draft-ietf-trans-rfc6962-bis-37.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- == There are 7 instances of lines with non-RFC6890-compliant IPv4 addresses in the document. If these are example addresses, they should be changed. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (13 May 2021) is 1078 days in the past. Is this intentional? Checking references for intended status: Experimental ---------------------------------------------------------------------------- -- Looks like a reference, but probably isn't: '0' on line 477 -- Looks like a reference, but probably isn't: '1' on line 477 -- Looks like a reference, but probably isn't: '7' on line 632 ** Obsolete normative reference: RFC 5246 (Obsoleted by RFC 8446) ** Obsolete normative reference: RFC 7231 (Obsoleted by RFC 9110) ** Obsolete normative reference: RFC 7807 (Obsoleted by RFC 9457) -- Obsolete informational reference (is this intentional?): RFC 6962 (Obsoleted by RFC 9162) Summary: 3 errors (**), 0 flaws (~~), 2 warnings (==), 5 comments (--). 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-37 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. Additions to existing registries . . . . . . . . . . . . 47 133 10.1.1. New Entry to the TLS ExtensionType Registry . . . . 47 134 10.1.2. URN Sub-namespace for TRANS errors 135 (urn:ietf:params:trans:error) . . . . . . . . . . . . 47 136 10.2. New CT-Related registries . . . . . . . . . . . . . . . 47 137 10.2.1. Hash Algorithms . . . . . . . . . . . . . . . . . . 47 138 10.2.2. Signature Algorithms . . . . . . . . . . . . . . . . 48 139 10.2.3. VersionedTransTypes . . . . . . . . . . . . . . . . 49 140 10.2.4. Log Artifact Extension Registry . . . . . . . . . . 50 141 10.2.5. Object Identifiers . . . . . . . . . . . . . . . . . 51 142 10.2.6. CT Error Types Registry . . . . . . . . . . . . . . 52 143 11. Security Considerations . . . . . . . . . . . . . . . . . . . 54 144 11.1. Misissued Certificates . . . . . . . . . . . . . . . . . 55 145 11.2. Detection of Misissue . . . . . . . . . . . . . . . . . 55 146 11.3. Misbehaving Logs . . . . . . . . . . . . . . . . . . . . 55 147 11.4. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . 56 148 11.5. Leakage of DNS Information . . . . . . . . . . . . . . . 56 149 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 56 150 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 56 151 13.1. Normative References . . . . . . . . . . . . . . . . . . 56 152 13.2. Informative References . . . . . . . . . . . . . . . . . 59 153 Appendix A. Supporting v1 and v2 simultaneously (Informative) . 60 154 Appendix B. An ASN.1 Module (Informative) . . . . . . . . . . . 60 155 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 62 157 1. Introduction 159 Certificate Transparency aims to mitigate the problem of misissued 160 certificates by providing append-only logs of issued certificates. 161 The logs do not themselves prevent misissuance, but they ensure that 162 interested parties (particularly those named in certificates) can 163 detect such misissuance. Note that this is a general mechanism that 164 could be used for transparently logging any form of binary data, 165 subject to some kind of inclusion criteria. In this document, we 166 only describe its use for public TLS server certificates (i.e., where 167 the inclusion criteria is a valid certificate issued by a public 168 certification authority (CA)). A typical definition of "public" can 169 be found in [CABBR]. 171 Each log contains certificate chains, which can be submitted by 172 anyone. It is expected that public CAs will contribute all their 173 newly issued certificates to one or more logs; however certificate 174 holders can also contribute their own certificate chains, as can 175 third parties. In order to avoid logs being rendered useless by the 176 submission of large numbers of spurious certificates, it is required 177 that each chain ends with a trust anchor that is accepted by the log. 178 A log may also limit the length of the chain it is willing to accept; 179 such chains must also end with an acceptable trust anchor. When a 180 chain is accepted by a log, a signed timestamp is returned, which can 181 later be used to provide evidence to TLS clients that the chain has 182 been submitted. TLS clients can thus require that all certificates 183 they accept as valid are accompanied by signed timestamps. 185 Those who are concerned about misissuance can monitor the logs, 186 asking them regularly for all new entries, and can thus check whether 187 domains for which they are responsible have had certificates issued 188 that they did not expect. What they do with this information, 189 particularly when they find that a misissuance has happened, is 190 beyond the scope of this document. However, broadly speaking, they 191 can invoke existing business mechanisms for dealing with misissued 192 certificates, such as working with the CA to get the certificate 193 revoked, or with maintainers of trust anchor lists to get the CA 194 removed. Of course, anyone who wants can monitor the logs and, if 195 they believe a certificate is incorrectly issued, take action as they 196 see fit. 198 Similarly, those who have seen signed timestamps from a particular 199 log can later demand a proof of inclusion from that log. If the log 200 is unable to provide this (or, indeed, if the corresponding 201 certificate is absent from monitors' copies of that log), that is 202 evidence of the incorrect operation of the log. The checking 203 operation is asynchronous to allow clients to proceed without delay, 204 despite possible issues such as network connectivity and the vagaries 205 of firewalls. 207 The append-only property of each log is achieved using Merkle Trees, 208 which can be used to efficiently prove that any particular instance 209 of the log is a superset of any particular previous instance and to 210 efficiently detect various misbehaviors of the log (e.g., issuing a 211 signed timestamp for a certificate that is not subsequently logged). 213 It is necessary to treat each log as a trusted third party, because 214 the log auditing mechanisms described in this document can be 215 circumvented by a misbehaving log that shows different, inconsistent 216 views of itself to different clients. While mechanisms are being 217 developed to address these shortcomings and thereby avoid the need to 218 blindly trust logs, such mechanisms are outside the scope of this 219 document. 221 1.1. Requirements Language 223 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 224 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 225 "OPTIONAL" in this document are to be interpreted as described in BCP 226 14 [RFC2119] [RFC8174] when, and only when, they appear in all 227 capitals, as shown here. 229 1.2. Data Structures 231 Data structures are defined and encoded according to the conventions 232 laid out in Section 3 of [RFC8446]. 234 1.3. Major Differences from CT 1.0 236 This document revises and obsoletes the CT 1.0 [RFC6962] protocol, 237 drawing on insights gained from CT 1.0 deployments and on feedback 238 from the community. The major changes are: 240 * Hash and signature algorithm agility: permitted algorithms are now 241 specified in IANA registries. 243 * Precertificate format: precertificates are now CMS objects rather 244 than X.509 certificates, which avoids violating the certificate 245 serial number uniqueness requirement in Section 4.1.2.2 of 246 [RFC5280]. 248 * Removed precertificate signing certificates and the precertificate 249 poison extension: the change of precertificate format means that 250 these are no longer needed. 252 * Logs IDs: each log is now identified by an OID rather than by the 253 hash of its public key. OID allocations are managed by an IANA 254 registry. 256 * "TransItem" structure: this new data structure is used to 257 encapsulate most types of CT data. A "TransItemList", consisting 258 of one or more "TransItem" structures, can be used anywhere that 259 "SignedCertificateTimestampList" was used in [RFC6962]. 261 * Merkle tree leaves: the "MerkleTreeLeaf" structure has been 262 replaced by the "TransItem" structure, which eases extensibility 263 and simplifies the leaf structure by removing one layer of 264 abstraction. 266 * Unified leaf format: the structure for both certificate and 267 precertificate entries now includes only the TBSCertificate 268 (whereas certificate entries in [RFC6962] included the entire 269 certificate). 271 * Log Artifact Extensions: these are now typed and managed by an 272 IANA registry, and they can now appear not only in SCTs but also 273 in STHs. 275 * API outputs: complete "TransItem" structures are returned, rather 276 than the constituent parts of each structure. 278 * get-all-by-hash: new client API for obtaining an inclusion proof 279 and the corresponding consistency proof at the same time. 281 * submit-entry: new client API, replacing add-chain and add-pre- 282 chain. 284 * Presenting SCTs with proofs: TLS servers may present SCTs together 285 with the corresponding inclusion proofs using any of the 286 mechanisms that [RFC6962] defined for presenting SCTs only. 287 (Presenting SCTs only is still supported). 289 * CT TLS extension: the "signed_certificate_timestamp" TLS extension 290 has been replaced by the "transparency_info" TLS extension. 292 * Verification algorithms: added detailed algorithms for verifying 293 inclusion proofs, for verifying consistency between two STHs, and 294 for verifying a root hash given a complete list of the relevant 295 leaf input entries. 297 * Extensive clarifications and editorial work. 299 2. Cryptographic Components 301 2.1. Merkle Hash Trees 303 A full description of Merkle Hash Tree is beyond the scope of this 304 document. Briefly, it is a binary tree where each non-leaf node is a 305 hash of its children. For CT, the number of children is at most two. 306 Additional information can be found in the Introduction and Reference 307 section of [RFC8391]. 309 2.1.1. Definition of the Merkle Tree 311 The log uses a binary Merkle Hash Tree for efficient auditing. The 312 hash algorithm used is one of the log's parameters (see Section 4.1). 313 This document establishes a registry of acceptable hash algorithms 314 (see Section 10.2.1). Throughout this document, the hash algorithm 315 in use is referred to as HASH and the size of its output in bytes as 316 HASH_SIZE. The input to the Merkle Tree Hash is a list of data 317 entries; these entries will be hashed to form the leaves of the 318 Merkle Hash Tree. The output is a single HASH_SIZE Merkle Tree Hash. 319 Given an ordered list of n inputs, D_n = {d[0], d[1], ..., d[n-1]}, 320 the Merkle Tree Hash (MTH) is thus defined as follows: 322 The hash of an empty list is the hash of an empty string: 324 MTH({}) = HASH(). 326 The hash of a list with one entry (also known as a leaf hash) is: 328 MTH({d[0]}) = HASH(0x00 || d[0]). 330 For n > 1, let k be the largest power of two smaller than n (i.e., k 331 < n <= 2k). The Merkle Tree Hash of an n-element list D_n is then 332 defined recursively as 334 MTH(D_n) = HASH(0x01 || MTH(D[0:k]) || MTH(D[k:n])), 336 where: 338 * || denotes concatenation 340 * : denotes concatenation of lists 342 * D[k1:k2] = D'_(k2-k1) denotes the list {d'[0] = d[k1], d'[1] = 343 d[k1+1], ..., d'[k2-k1-1] = d[k2-1]} of length (k2 - k1). 345 Note that the hash calculations for leaves and nodes differ; this 346 domain separation is required to give second preimage resistance. 348 Note that we do not require the length of the input list to be a 349 power of two. The resulting Merkle Tree may thus not be balanced; 350 however, its shape is uniquely determined by the number of leaves. 351 (Note: This Merkle Tree is essentially the same as the history tree 352 [CrosbyWallach] proposal, except our definition handles non-full 353 trees differently). 355 2.1.2. Verifying a Tree Head Given Entries 357 When a client has a complete list of "entries" from "0" up to 358 "tree_size - 1" and wishes to verify this list against a tree head 359 "root_hash" returned by the log for the same "tree_size", the 360 following algorithm may be used: 362 1. Set "stack" to an empty stack. 364 2. For each "i" from "0" up to "tree_size - 1": 366 1. Push "HASH(0x00 || entries[i])" to "stack". 368 2. Set "merge_count" to the lowest value ("0" included) such 369 that "LSB(i >> merge_count)" is not set, where "LSB" means 370 the least significant bit. In other words, set "merge_count" 371 to the number of consecutive "1"s found starting at the least 372 significant bit of "i". 374 3. Repeat "merge_count" times: 376 1. Pop "right" from "stack". 378 2. Pop "left" from "stack". 380 3. Push "HASH(0x01 || left || right)" to "stack". 382 3. If there is more than one element in the "stack", repeat the same 383 merge procedure (the sub-items of Step 2.3 above) until only a 384 single element remains. 386 4. The remaining element in "stack" is the Merkle Tree hash for the 387 given "tree_size" and should be compared by equality against the 388 supplied "root_hash". 390 2.1.3. Merkle Inclusion Proofs 392 A Merkle inclusion proof for a leaf in a Merkle Hash Tree is the 393 shortest list of additional nodes in the Merkle Tree required to 394 compute the Merkle Tree Hash for that tree. Each node in the tree is 395 either a leaf node or is computed from the two nodes immediately 396 below it (i.e., towards the leaves). At each step up the tree 397 (towards the root), a node from the inclusion proof is combined with 398 the node computed so far. In other words, the inclusion proof 399 consists of the list of missing nodes required to compute the nodes 400 leading from a leaf to the root of the tree. If the root computed 401 from the inclusion proof matches the true root, then the inclusion 402 proof proves that the leaf exists in the tree. 404 2.1.3.1. Generating an Inclusion Proof 406 Given an ordered list of n inputs to the tree, D_n = {d[0], d[1], 407 ..., d[n-1]}, the Merkle inclusion proof PATH(m, D_n) for the (m+1)th 408 input d[m], 0 <= m < n, is defined as follows: 410 The proof for the single leaf in a tree with a one-element input list 411 D[1] = {d[0]} is empty: 413 PATH(0, {d[0]}) = {} 415 For n > 1, let k be the largest power of two smaller than n. The 416 proof for the (m+1)th element d[m] in a list of n > m elements is 417 then defined recursively as 419 PATH(m, D_n) = PATH(m, D[0:k]) : MTH(D[k:n]) for m < k; and 421 PATH(m, D_n) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for m >= k, 423 The : operator and D[k1:k2] are defined the same as in Section 2.1.1. 425 2.1.3.2. Verifying an Inclusion Proof 427 When a client has received an inclusion proof (e.g., in a "TransItem" 428 of type "inclusion_proof_v2") and wishes to verify inclusion of an 429 input "hash" for a given "tree_size" and "root_hash", the following 430 algorithm may be used to prove the "hash" was included in the 431 "root_hash": 433 1. Compare "leaf_index" from the "inclusion_proof_v2" structure 434 against "tree_size". If "leaf_index" is greater than or equal to 435 "tree_size" then fail the proof verification. 437 2. Set "fn" to "leaf_index" and "sn" to "tree_size - 1". 439 3. Set "r" to "hash". 441 4. For each value "p" in the "inclusion_path" array: 443 If "sn" is 0, stop the iteration and fail the proof verification. 445 If "LSB(fn)" is set, or if "fn" is equal to "sn", then: 447 1. Set "r" to "HASH(0x01 || p || r)" 449 2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn" 450 equally until either "LSB(fn)" is set or "fn" is "0". 452 Otherwise: 454 1. Set "r" to "HASH(0x01 || r || p)" 456 Finally, right-shift both "fn" and "sn" one time. 458 5. Compare "sn" to 0. Compare "r" against the "root_hash". If "sn" 459 is equal to 0, and "r" and the "root_hash" are equal, then the 460 log has proven the inclusion of "hash". Otherwise, fail the 461 proof verification. 463 2.1.4. Merkle Consistency Proofs 465 Merkle consistency proofs prove the append-only property of the tree. 466 A Merkle consistency proof for a Merkle Tree Hash MTH(D_n) and a 467 previously advertised hash MTH(D[0:m]) of the first m leaves, m <= n, 468 is the list of nodes in the Merkle Tree required to verify that the 469 first m inputs D[0:m] are equal in both trees. Thus, a consistency 470 proof must contain a set of intermediate nodes (i.e., commitments to 471 inputs) sufficient to verify MTH(D_n), such that (a subset of) the 472 same nodes can be used to verify MTH(D[0:m]). We define an algorithm 473 that outputs the (unique) minimal consistency proof. 475 2.1.4.1. Generating a Consistency Proof 477 Given an ordered list of n inputs to the tree, D_n = {d[0], d[1], 478 ..., d[n-1]}, the Merkle consistency proof PROOF(m, D_n) for a 479 previous Merkle Tree Hash MTH(D[0:m]), 0 < m < n, is defined as: 481 PROOF(m, D_n) = SUBPROOF(m, D_n, true) 483 In SUBPROOF, the boolean value represents whether the subtree created 484 from D[0:m] is a complete subtree of the Merkle Tree created from 485 D_n, and, consequently, whether the subtree Merkle Tree Hash 486 MTH(D[0:m]) is known. The initial call to SUBPROOF sets this to be 487 true, and SUBPROOF is then defined as follows: 489 The subproof for m = n is empty if m is the value for which PROOF was 490 originally requested (meaning that the subtree created from D[0:m] is 491 a complete subtree of the Merkle Tree created from the original D_n 492 for which PROOF was requested, and the subtree Merkle Tree Hash 493 MTH(D[0:m]) is known): 495 SUBPROOF(m, D_m, true) = {} 497 Otherwise, the subproof for m = n is the Merkle Tree Hash committing 498 inputs D[0:m]: 500 SUBPROOF(m, D_m, false) = {MTH(D_m)} 502 For m < n, let k be the largest power of two smaller than n. The 503 subproof is then defined recursively, using the appropriate step 504 below: 506 If m <= k, the right subtree entries D[k:n] only exist in the current 507 tree. We prove that the left subtree entries D[0:k] are consistent 508 and add a commitment to D[k:n]: 510 SUBPROOF(m, D_n, b) = SUBPROOF(m, D[0:k], b) : MTH(D[k:n]) 512 If m > k, the left subtree entries D[0:k] are identical in both 513 trees. We prove that the right subtree entries D[k:n] are consistent 514 and add a commitment to D[0:k]. 516 SUBPROOF(m, D_n, b) = SUBPROOF(m - k, D[k:n], false) : MTH(D[0:k]) 518 The number of nodes in the resulting proof is bounded above by 519 ceil(log2(n)) + 1. 521 The : operator and D[k1:k2] are defined the same as in Section 2.1.1. 523 2.1.4.2. Verifying Consistency between Two Tree Heads 525 When a client has a tree head "first_hash" for tree size "first", a 526 tree head "second_hash" for tree size "second" where "0 < first < 527 second", and has received a consistency proof between the two (e.g., 528 in a "TransItem" of type "consistency_proof_v2"), the following 529 algorithm may be used to verify the consistency proof: 531 1. If "consistency_path" is an empty array, stop and fail the proof 532 verification. 534 2. If "first" is an exact power of 2, then prepend "first_hash" to 535 the "consistency_path" array. 537 3. Set "fn" to "first - 1" and "sn" to "second - 1". 539 4. If "LSB(fn)" is set, then right-shift both "fn" and "sn" equally 540 until "LSB(fn)" is not set. 542 5. Set both "fr" and "sr" to the first value in the 543 "consistency_path" array. 545 6. For each subsequent value "c" in the "consistency_path" array: 547 If "sn" is 0, stop the iteration and fail the proof verification. 549 If "LSB(fn)" is set, or if "fn" is equal to "sn", then: 551 1. Set "fr" to "HASH(0x01 || c || fr)" 553 Set "sr" to "HASH(0x01 || c || sr)" 555 2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn" 556 equally until either "LSB(fn)" is set or "fn" is "0". 558 Otherwise: 560 1. Set "sr" to "HASH(0x01 || sr || c)" 562 Finally, right-shift both "fn" and "sn" one time. 564 7. After completing iterating through the "consistency_path" array 565 as described above, verify that the "fr" calculated is equal to 566 the "first_hash" supplied, that the "sr" calculated is equal to 567 the "second_hash" supplied and that "sn" is 0. 569 2.1.5. Example 571 The binary Merkle Tree with 7 leaves: 573 hash 574 / \ 575 / \ 576 / \ 577 / \ 578 / \ 579 k l 580 / \ / \ 581 / \ / \ 582 / \ / \ 583 g h i j 584 / \ / \ / \ | 585 a b c d e f d6 586 | | | | | | 587 d0 d1 d2 d3 d4 d5 589 The inclusion proof for d0 is [b, h, l]. 591 The inclusion proof for d3 is [c, g, l]. 593 The inclusion proof for d4 is [f, j, k]. 595 The inclusion proof for d6 is [i, k]. 597 The same tree, built incrementally in four steps: 599 hash0 hash1=k 600 / \ / \ 601 / \ / \ 602 / \ / \ 603 g c g h 604 / \ | / \ / \ 605 a b d2 a b c d 606 | | | | | | 607 d0 d1 d0 d1 d2 d3 609 hash2 hash 610 / \ / \ 611 / \ / \ 612 / \ / \ 613 / \ / \ 614 / \ / \ 615 k i k l 616 / \ / \ / \ / \ 617 / \ e f / \ / \ 618 / \ | | / \ / \ 619 g h d4 d5 g h i j 620 / \ / \ / \ / \ / \ | 621 a b c d a b c d e f d6 622 | | | | | | | | | | 623 d0 d1 d2 d3 d0 d1 d2 d3 d4 d5 625 The consistency proof between hash0 and hash is PROOF(3, D[7]) = [c, 626 d, g, l]. c, g are used to verify hash0, and d, l are additionally 627 used to show hash is consistent with hash0. 629 The consistency proof between hash1 and hash is PROOF(4, D[7]) = [l]. 630 hash can be verified using hash1=k and l. 632 The consistency proof between hash2 and hash is PROOF(6, D[7]) = [i, 633 j, k]. k, i are used to verify hash2, and j is additionally used to 634 show hash is consistent with hash2. 636 2.2. Signatures 638 When signing data structures, a log MUST use one of the signature 639 algorithms from the IANA CT Signature Algorithms registry, described 640 in Section 10.2.2. 642 3. Submitters 644 Submitters submit certificates or preannouncements of certificates 645 prior to issuance (precertificates) to logs for public auditing, as 646 described below. In order to enable attribution of each logged 647 certificate or precertificate to its issuer, each submission MUST be 648 accompanied by all additional certificates required to verify the 649 chain up to an accepted trust anchor (Section 5.7). The trust anchor 650 (a root or intermediate CA certificate) MAY be omitted from the 651 submission. 653 If a log accepts a submission, it will return a Signed Certificate 654 Timestamp (SCT) (see Section 4.8). The submitter SHOULD validate the 655 returned SCT as described in Section 8.1 if they understand its 656 format and they intend to use it directly in a TLS handshake or to 657 construct a certificate. If the submitter does not need the SCT (for 658 example, the certificate is being submitted simply to make it 659 available in the log), it MAY validate the SCT. 661 3.1. Certificates 663 Any entity can submit a certificate (Section 5.1) to a log. Since it 664 is anticipated that TLS clients will reject certificates that are not 665 logged, it is expected that certificate issuers and subjects will be 666 strongly motivated to submit them. 668 3.2. Precertificates 670 CAs may preannounce a certificate prior to issuance by submitting a 671 precertificate (Section 5.1) that the log can use to create an entry 672 that will be valid against the issued certificate. The CA MAY 673 incorporate the returned SCT in the issued certificate. One example 674 of where the returned SCT is not incorporated in the issued 675 certificate is when a CA sends the precertificate to multiple logs, 676 but only incorporates the SCTs that are returned first. 678 A precertificate is a CMS [RFC5652] "signed-data" object that 679 conforms to the following profile: 681 * It MUST be DER encoded as described in [X690]. 683 * "SignedData.version" MUST be v3(3). 685 * "SignedData.digestAlgorithms" MUST be the same as the 686 "SignerInfo.digestAlgorithm" OID value (see below). 688 * "SignedData.encapContentInfo": 690 - "eContentType" MUST be the OID 1.3.101.78. 692 - "eContent" MUST contain a TBSCertificate [RFC5280] that will be 693 identical to the TBSCertificate in the issued certificate, 694 except that the Transparency Information (Section 7.1) 695 extension MUST be omitted. 697 * "SignedData.certificates" MUST be omitted. 699 * "SignedData.crls" MUST be omitted. 701 * "SignedData.signerInfos" MUST contain one "SignerInfo": 703 - "version" MUST be v3(3). 705 - "sid" MUST use the "subjectKeyIdentifier" option. 707 - "digestAlgorithm" MUST be one of the hash algorithm OIDs listed 708 in the IANA CT Hash Algorithms Registry, described in 709 Section 10.2.1. 711 - "signedAttrs" MUST be present and MUST contain two attributes: 713 o A content-type attribute whose value is the same as 714 "SignedData.encapContentInfo.eContentType". 716 o A message-digest attribute whose value is the message digest 717 of "SignedData.encapContentInfo.eContent". 719 - "signatureAlgorithm" MUST be the same OID as 720 "TBSCertificate.signature". 722 - "signature" MUST be from the same (root or intermediate) CA 723 that intends to issue the corresponding certificate (see 724 Section 3.2.1). 726 - "unsignedAttrs" MUST be omitted. 728 "SignerInfo.signedAttrs" is included in the message digest 729 calculation process (see Section 5.4 of [RFC5652]), which ensures 730 that the "SignerInfo.signature" value will not be a valid X.509v3 731 signature that could be used in conjunction with the TBSCertificate 732 (from "SignedData.encapContentInfo.eContent") to construct a valid 733 certificate. 735 3.2.1. Binding Intent to Issue 737 Under normal circumstances, there will be a short delay between 738 precertificate submission and issuance of the corresponding 739 certificate. Longer delays are to be expected occasionally (e.g., 740 due to log server downtime), and in some cases the CA might not 741 actually issue the corresponding certificate. Nevertheless, a 742 precertificate's "signature" indicates the CA's binding intent to 743 issue the corresponding certificate, which means that: 745 * Misissuance of a precertificate is considered equivalent to 746 misissuance of the corresponding certificate. The CA should 747 expect to be held to account, even if the corresponding 748 certificate has not actually been issued. 750 * Upon observing a precertificate, a client can reasonably presume 751 that the corresponding certificate has been issued. A client may 752 wish to obtain status information (e.g., by using the Online 753 Certificate Status Protocol [RFC6960] or by checking a Certificate 754 Revocation List [RFC5280]) about a certificate that is presumed to 755 exist, especially if there is evidence or suspicion that the 756 corresponding precertificate was misissued. 758 * TLS clients may have policies that require CAs to be able to 759 revoke, and to provide certificate status services for, each 760 certificate that is presumed to exist based on the existence of a 761 corresponding precertificate. 763 4. Log Format and Operation 765 A log is a single, append-only Merkle Tree of submitted certificate 766 and precertificate entries. 768 When it receives and accepts a valid submission, the log MUST return 769 an SCT that corresponds to the submitted certificate or 770 precertificate. If the log has previously seen this valid 771 submission, it SHOULD return the same SCT as it returned before, as 772 discussed in Section 11.3. If different SCTs are produced for the 773 same submission, multiple log entries will have to be created, one 774 for each SCT (as the timestamp is a part of the leaf structure). 775 Note that if a certificate was previously logged as a precertificate, 776 then the precertificate's SCT of type "precert_sct_v2" would not be 777 appropriate; instead, a fresh SCT of type "x509_sct_v2" should be 778 generated. 780 An SCT is the log's promise to append to its Merkle Tree an entry for 781 the accepted submission. Upon producing an SCT, the log MUST fulfil 782 this promise by performing the following actions within a fixed 783 amount of time known as the Maximum Merge Delay (MMD), which is one 784 of the log's parameters (see Section 4.1): 786 * Allocate a tree index to the entry representing the accepted 787 submission. 789 * Calculate the root of the tree. 791 * Sign the root of the tree (see Section 4.10). 793 The log may append multiple entries before signing the root of the 794 tree. 796 Log operators SHOULD NOT impose any conditions on retrieving or 797 sharing data from the log. 799 4.1. Log Parameters 801 A log is defined by a collection of immutable parameters, which are 802 used by clients to communicate with the log and to verify log 803 artifacts. Except for the Final Signed Tree Head (STH), each of 804 these parameters MUST be established before the log operator begins 805 to operate the log. 807 Base URL: The prefix used to construct URLs ([RFC3986]) for client 808 messages (see Section 5). The base URL MUST be an "https" URL, 809 MAY contain a port, MAY contain a path with any number of path 810 segments, but MUST NOT contain a query string, fragment, or 811 trailing "/". Example: https://ct.example.org/blue 813 Hash Algorithm: The hash algorithm used for the Merkle Tree (see 814 Section 10.2.1). 816 Signature Algorithm: The signature algorithm used (see Section 2.2). 818 Public Key: The public key used to verify signatures generated by 819 the log. A log MUST NOT use the same keypair as any other log. 821 Log ID: The OID that uniquely identifies the log. 823 Maximum Merge Delay: The MMD the log has committed to. This 824 document deliberately does not specify any limits on the value, to 825 allow for experimentation. 827 Version: The version of the protocol supported by the log (currently 828 1 or 2). 830 Maximum Chain Length: The longest certificate chain submission the 831 log is willing to accept, if the log imposes any limit. 833 STH Frequency Count: The maximum number of STHs the log may produce 834 in any period equal to the "Maximum Merge Delay" (see 835 Section 4.10). 837 Final STH: If a log has been closed down (i.e., no longer accepts 838 new entries), existing entries may still be valid. In this case, 839 the client should know the final valid STH in the log to ensure no 840 new entries can be added without detection. This value MUST be 841 provided in the form of a TransItem of type "signed_tree_head_v2". 842 If a log is still accepting entries, this value should not be 843 provided. 845 [JSON.Metadata] is an example of a metadata format which includes the 846 above elements. 848 4.2. Evaluating Submissions 850 A log determines whether to accept or reject a submission by 851 evaluating it against the minimum acceptance criteria (see 852 Section 4.2.1) and against the log's discretionary acceptance 853 criteria (see Section 4.2.2). 855 If the acceptance criteria are met, the log SHOULD accept the 856 submission. (A log may decide, for example, to temporarily reject 857 acceptable submissions to protect itself against denial-of-service 858 attacks). 860 The log SHALL allow retrieval of its list of accepted trust anchors 861 (see Section 5.7), each of which is a root or intermediate CA 862 certificate. This list might usefully be the union of root 863 certificates trusted by major browser vendors. 865 4.2.1. Minimum Acceptance Criteria 867 To ensure that logged certificates and precertificates are 868 attributable to an accepted trust anchor, and to set clear 869 expectations for what monitors would find in the log, and to avoid 870 being overloaded by invalid submissions, the log MUST reject a 871 submission if any of the following conditions are not met: 873 * The "submission", "type" and "chain" inputs MUST be set as 874 described in Section 5.1. The log MUST NOT accommodate misordered 875 CA certificates or use any other source of intermediate CA 876 certificates to attempt certification path construction. 878 * Each of the zero or more intermediate CA certificates in the chain 879 MUST have one or both of the following features: 881 - The Basic Constraints extension with the cA boolean asserted. 883 - The Key Usage extension with the keyCertSign bit asserted. 885 * Each certificate in the chain MUST fall within the limits imposed 886 by the zero or more Basic Constraints pathLenConstraint values 887 found higher up the chain. 889 * Precertificate submissions MUST conform to all of the requirements 890 in Section 3.2. 892 4.2.2. Discretionary Acceptance Criteria 894 If the minimum acceptance criteria are met but the submission is not 895 fully valid according to [RFC5280] verification rules (e.g., the 896 certificate or precertificate has expired, is not yet valid, has been 897 revoked, exhibits ASN.1 DER encoding errors but the log can still 898 parse it, etc), then the acceptability of the submission is left to 899 the log's discretion. It is useful for logs to accept such 900 submissions in order to accommodate quirks of CA certificate-issuing 901 software and to facilitate monitoring of CA compliance with 902 applicable policies and technical standards. However, it is 903 impractical for this document to enumerate, and for logs to consider, 904 all of the ways that a submission might fail to comply with 905 [RFC5280]. 907 Logs SHOULD limit the length of chain they will accept. The maximum 908 chain length is one of the log's parameters (see Section 4.1). 910 4.3. Log Entries 912 If a submission is accepted and an SCT issued, the accepting log MUST 913 store the entire chain used for verification. This chain MUST 914 include the certificate or precertificate itself, the zero or more 915 intermediate CA certificates provided by the submitter, and the trust 916 anchor used to verify the chain (even if it was omitted from the 917 submission). The log MUST provide this chain for auditing upon 918 request (see Section 5.6) so that the CA cannot avoid blame by 919 logging a partial or empty chain. Each log entry is a "TransItem" 920 structure of type "x509_entry_v2" or "precert_entry_v2". However, a 921 log may store its entries in any format. If a log does not store 922 this "TransItem" in full, it must store the "timestamp" and 923 "sct_extensions" of the corresponding 924 "TimestampedCertificateEntryDataV2" structure. The "TransItem" can 925 be reconstructed from these fields and the entire chain that the log 926 used to verify the submission. 928 4.4. Log ID 930 Each log is identified by an OID, which is one of the log's 931 parameters (see Section 4.1) and which MUST NOT be used to identify 932 any other log. A log's operator MUST either allocate the OID 933 themselves or request an OID from the Log ID Registry (see 934 Section 10.2.5.1). The only advantage of the registry is that the 935 DER encoding can be small. (Recall that OID allocations do not 936 require a central registration, although logs will most likely want 937 to make themselves known to potential clients through out of band 938 means.) Various data structures include the DER encoding of this 939 OID, excluding the ASN.1 tag and length bytes, in an opaque vector: 941 opaque LogID<2..127>; 943 Note that the ASN.1 length and the opaque vector length are identical 944 in size (1 byte) and value, so the full DER encoding (including the 945 tag and length) of the OID can be reproduced simply by prepending an 946 OBJECT IDENTIFIER tag (0x06) to the opaque vector length and 947 contents. 949 The OID used to identify a log is limited such that the DER encoding 950 of its value, excluding the tag and length, MUST be no longer than 951 127 octets. 953 4.5. TransItem Structure 955 Various data structures are encapsulated in the "TransItem" structure 956 to ensure that the type and version of each one is identified in a 957 common fashion: 959 enum { 960 reserved(0), 961 x509_entry_v2(1), precert_entry_v2(2), 962 x509_sct_v2(3), precert_sct_v2(4), 963 signed_tree_head_v2(5), consistency_proof_v2(6), 964 inclusion_proof_v2(7), 965 (65535) 966 } VersionedTransType; 968 struct { 969 VersionedTransType versioned_type; 970 select (versioned_type) { 971 case x509_entry_v2: TimestampedCertificateEntryDataV2; 972 case precert_entry_v2: TimestampedCertificateEntryDataV2; 973 case x509_sct_v2: SignedCertificateTimestampDataV2; 974 case precert_sct_v2: SignedCertificateTimestampDataV2; 975 case signed_tree_head_v2: SignedTreeHeadDataV2; 976 case consistency_proof_v2: ConsistencyProofDataV2; 977 case inclusion_proof_v2: InclusionProofDataV2; 978 } data; 979 } TransItem; 981 "versioned_type" is a value from the IANA registry in Section 10.2.3 982 that identifies the type of the encapsulated data structure and the 983 earliest version of this protocol to which it conforms. This 984 document is v2. 986 "data" is the encapsulated data structure. The various structures 987 named with the "DataV2" suffix are defined in later sections of this 988 document. 990 Note that "VersionedTransType" combines the v1 [RFC6962] type 991 enumerations "Version", "LogEntryType", "SignatureType" and 992 "MerkleLeafType". Note also that v1 did not define "TransItem", but 993 this document provides guidelines (see Appendix A) on how v2 994 implementations can co-exist with v1 implementations. 996 Future versions of this protocol may reuse "VersionedTransType" 997 values defined in this document as long as the corresponding data 998 structures are not modified, and may add new "VersionedTransType" 999 values for new or modified data structures. 1001 4.6. Log Artifact Extensions 1002 enum { 1003 reserved(65535) 1004 } ExtensionType; 1006 struct { 1007 ExtensionType extension_type; 1008 opaque extension_data<0..2^16-1>; 1009 } Extension; 1011 The "Extension" structure provides a generic extensibility for log 1012 artifacts, including Signed Certificate Timestamps (Section 4.8) and 1013 Signed Tree Heads (Section 4.10). The interpretation of the 1014 "extension_data" field is determined solely by the value of the 1015 "extension_type" field. 1017 This document does not define any extensions, but it does establish a 1018 registry for future "ExtensionType" values (see Section 10.2.4). 1019 Each document that registers a new "ExtensionType" must specify the 1020 context in which it may be used (e.g., SCT, STH, or both) and 1021 describe how to interpret the corresponding "extension_data". 1023 4.7. Merkle Tree Leaves 1025 The leaves of a log's Merkle Tree correspond to the log's entries 1026 (see Section 4.3). Each leaf is the leaf hash (Section 2.1) of a 1027 "TransItem" structure of type "x509_entry_v2" or "precert_entry_v2", 1028 which encapsulates a "TimestampedCertificateEntryDataV2" structure. 1029 Note that leaf hashes are calculated as HASH(0x00 || TransItem), 1030 where the hash algorithm is one of the log's parameters. 1032 opaque TBSCertificate<1..2^24-1>; 1034 struct { 1035 uint64 timestamp; 1036 opaque issuer_key_hash<32..2^8-1>; 1037 TBSCertificate tbs_certificate; 1038 Extension sct_extensions<0..2^16-1>; 1039 } TimestampedCertificateEntryDataV2; 1041 "timestamp" is the date and time at which the certificate or 1042 precertificate was accepted by the log, in the form of a 64-bit 1043 unsigned number of milliseconds elapsed since the Unix Epoch (1 1044 January 1970 00:00:00 UTC - see [UNIXTIME]), ignoring leap seconds, 1045 in network byte order. Note that the leaves of a log's Merkle Tree 1046 are not required to be in strict chronological order. 1048 "issuer_key_hash" is the HASH of the public key of the CA that issued 1049 the certificate or precertificate, calculated over the DER encoding 1050 of the key represented as SubjectPublicKeyInfo [RFC5280]. This is 1051 needed to bind the CA to the certificate or precertificate, making it 1052 impossible for the corresponding SCT to be valid for any other 1053 certificate or precertificate whose TBSCertificate matches 1054 "tbs_certificate". The length of the "issuer_key_hash" MUST match 1055 HASH_SIZE. 1057 "tbs_certificate" is the DER encoded TBSCertificate from the 1058 submission. (Note that a precertificate's TBSCertificate can be 1059 reconstructed from the corresponding certificate as described in 1060 Section 8.1.2). 1062 "sct_extensions" is byte-for-byte identical to the SCT extensions of 1063 the corresponding SCT. 1065 The type of the "TransItem" corresponds to the value of the "type" 1066 parameter supplied in the Section 5.1 call. 1068 4.8. Signed Certificate Timestamp (SCT) 1070 An SCT is a "TransItem" structure of type "x509_sct_v2" or 1071 "precert_sct_v2", which encapsulates a 1072 "SignedCertificateTimestampDataV2" structure: 1074 struct { 1075 LogID log_id; 1076 uint64 timestamp; 1077 Extension sct_extensions<0..2^16-1>; 1078 opaque signature<1..2^16-1>; 1079 } SignedCertificateTimestampDataV2; 1081 "log_id" is this log's unique ID, encoded in an opaque vector as 1082 described in Section 4.4. 1084 "timestamp" is equal to the timestamp from the corresponding 1085 "TimestampedCertificateEntryDataV2" structure. 1087 "sct_extensions" is a vector of 0 or more SCT extensions. This 1088 vector MUST NOT include more than one extension with the same 1089 "extension_type". The extensions in the vector MUST be ordered by 1090 the value of the "extension_type" field, smallest value first. All 1091 SCT extensions are similar to non-critical X.509v3 extensions (i.e., 1092 the "mustUnderstand" field is not set), and a recipient SHOULD ignore 1093 any extension it does not understand. Furthermore, an implementation 1094 MAY choose to ignore any extension(s) that it does understand. 1096 "signature" is computed over a "TransItem" structure of type 1097 "x509_entry_v2" or "precert_entry_v2" (see Section 4.7) using the 1098 signature algorithm declared in the log's parameters (see 1099 Section 4.1). 1101 4.9. Merkle Tree Head 1103 The log stores information about its Merkle Tree in a 1104 "TreeHeadDataV2": 1106 opaque NodeHash<32..2^8-1>; 1108 struct { 1109 uint64 timestamp; 1110 uint64 tree_size; 1111 NodeHash root_hash; 1112 Extension sth_extensions<0..2^16-1>; 1113 } TreeHeadDataV2; 1115 The length of NodeHash MUST match HASH_SIZE of the log. 1117 "timestamp" is the current date and time, using the format defined in 1118 {tree_leaves}. 1120 "tree_size" is the number of entries currently in the log's Merkle 1121 Tree. 1123 "root_hash" is the root of the Merkle Hash Tree. 1125 "sth_extensions" is a vector of 0 or more STH extensions. This 1126 vector MUST NOT include more than one extension with the same 1127 "extension_type". The extensions in the vector MUST be ordered by 1128 the value of the "extension_type" field, smallest value first. If an 1129 implementation sees an extension that it does not understand, it 1130 SHOULD ignore that extension. Furthermore, an implementation MAY 1131 choose to ignore any extension(s) that it does understand. 1133 4.10. Signed Tree Head (STH) 1135 Periodically each log SHOULD sign its current tree head information 1136 (see Section 4.9) to produce an STH. When a client requests a log's 1137 latest STH (see Section 5.2), the log MUST return an STH that is no 1138 older than the log's MMD. However, since STHs could be used to mark 1139 individual clients (by producing a new STH for each query), a log 1140 MUST NOT produce STHs more frequently than its parameters declare 1141 (see Section 4.1). In general, there is no need to produce a new STH 1142 unless there are new entries in the log; however, in the event that a 1143 log does not accept any submissions during an MMD period, the log 1144 MUST sign the same Merkle Tree Hash with a fresh timestamp. 1146 An STH is a "TransItem" structure of type "signed_tree_head_v2", 1147 which encapsulates a "SignedTreeHeadDataV2" structure: 1149 struct { 1150 LogID log_id; 1151 TreeHeadDataV2 tree_head; 1152 opaque signature<0..2^16-1>; 1153 } SignedTreeHeadDataV2; 1155 "log_id" is this log's unique ID, encoded in an opaque vector as 1156 described in Section 4.4. 1158 The "timestamp" in "tree_head" MUST be at least as recent as the most 1159 recent SCT timestamp in the tree. Each subsequent timestamp MUST be 1160 more recent than the timestamp of the previous update. 1162 "tree_head" contains the latest tree head information (see 1163 Section 4.9). 1165 "signature" is computed over the "tree_head" field using the 1166 signature algorithm declared in the log's parameters (see 1167 Section 4.1). 1169 4.11. Merkle Consistency Proofs 1171 To prepare a Merkle Consistency Proof for distribution to clients, 1172 the log produces a "TransItem" structure of type 1173 "consistency_proof_v2", which encapsulates a "ConsistencyProofDataV2" 1174 structure: 1176 struct { 1177 LogID log_id; 1178 uint64 tree_size_1; 1179 uint64 tree_size_2; 1180 NodeHash consistency_path<0..2^16-1>; 1181 } ConsistencyProofDataV2; 1183 "log_id" is this log's unique ID, encoded in an opaque vector as 1184 described in Section 4.4. 1186 "tree_size_1" is the size of the older tree. 1188 "tree_size_2" is the size of the newer tree. 1190 "consistency_path" is a vector of Merkle Tree nodes proving the 1191 consistency of two STHs as described in {consistency}. 1193 4.12. Merkle Inclusion Proofs 1195 To prepare a Merkle Inclusion Proof for distribution to clients, the 1196 log produces a "TransItem" structure of type "inclusion_proof_v2", 1197 which encapsulates an "InclusionProofDataV2" structure: 1199 struct { 1200 LogID log_id; 1201 uint64 tree_size; 1202 uint64 leaf_index; 1203 NodeHash inclusion_path<0..2^16-1>; 1204 } InclusionProofDataV2; 1206 "log_id" is this log's unique ID, encoded in an opaque vector as 1207 described in Section 4.4. 1209 "tree_size" is the size of the tree on which this inclusion proof is 1210 based. 1212 "leaf_index" is the 0-based index of the log entry corresponding to 1213 this inclusion proof. 1215 "inclusion_path" is a vector of Merkle Tree nodes proving the 1216 inclusion of the chosen certificate or precertificate as described in 1217 {merkle_inclusion_proof}. 1219 4.13. Shutting down a log 1221 Log operators may decide to shut down a log for various reasons, such 1222 as deprecation of the signature algorithm. If there are entries in 1223 the log for certificates that have not yet expired, simply making TLS 1224 clients stop recognizing that log will have the effect of 1225 invalidating SCTs from that log. In order to avoid that, the 1226 following actions SHOULD be taken: 1228 * Make it known to clients and monitors that the log will be frozen. 1229 This is not part of the API, so it will have to be done via a 1230 relevant out-of-band mechanism. 1232 * Stop accepting new submissions (the error code "shutdown" should 1233 be returned for such requests). 1235 * Once MMD from the last accepted submission has passed and all 1236 pending submissions are incorporated, issue a final STH and 1237 publish it as one of the log's parameters. Having an STH with a 1238 timestamp that is after the MMD has passed from the last SCT 1239 issuance allows clients to audit this log regularly without 1240 special handling for the final STH. At this point the log's 1241 private key is no longer needed and can be destroyed. 1243 * Keep the log running until the certificates in all of its entries 1244 have expired or exist in other logs (this can be determined by 1245 scanning other logs or connecting to domains mentioned in the 1246 certificates and inspecting the SCTs served). 1248 5. Log Client Messages 1250 Messages are sent as HTTPS GET or POST requests. Parameters for 1251 POSTs and all responses are encoded as JavaScript Object Notation 1252 (JSON) objects [RFC8259]. Parameters for GETs are encoded as order- 1253 independent key/value URL parameters, using the "application/x-www- 1254 form-urlencoded" format described in the "HTML 4.01 Specification" 1255 [HTML401]. Binary data is base64 encoded according to section 4 of 1256 [RFC4648] as specified in the individual messages. 1258 Clients are configured with a log's base URL, which is one of the 1259 log's parameters. Clients construct URLs for requests by appending 1260 suffixes to this base URL. This structure places some degree of 1261 restriction on how log operators can deploy these services, as noted 1262 in [RFC8820]. However, operational experience with version 1 of this 1263 protocol has not indicated that these restrictions are a problem in 1264 practice. 1266 Note that JSON objects and URL parameters may contain fields not 1267 specified here, to allow for experimentation. Any fields that are 1268 not understood SHOULD be ignored. 1270 In practice, log servers may include multiple front-end machines. 1271 Since it is impractical to keep these machines in perfect sync, 1272 errors may occur that are caused by skew between the machines. Where 1273 such errors are possible, the front-end will return additional 1274 information (as specified below) making it possible for clients to 1275 make progress, if progress is possible. Front-ends MUST only serve 1276 data that is free of gaps (that is, for example, no front-end will 1277 respond with an STH unless it is also able to prove consistency from 1278 all log entries logged within that STH). 1280 For example, when a consistency proof between two STHs is requested, 1281 the front-end reached may not yet be aware of one or both STHs. In 1282 the case where it is unaware of both, it will return the latest STH 1283 it is aware of. Where it is aware of the first but not the second, 1284 it will return the latest STH it is aware of and a consistency proof 1285 from the first STH to the returned STH. The case where it knows the 1286 second but not the first should not arise (see the "no gaps" 1287 requirement above). 1289 If the log is unable to process a client's request, it MUST return an 1290 HTTP response code of 4xx/5xx (see [RFC7231]), and, in place of the 1291 responses outlined in the subsections below, the body SHOULD be a 1292 JSON Problem Details Object (see [RFC7807] Section 3), containing: 1294 type: A URN reference identifying the problem. To facilitate 1295 automated response to errors, this document defines a set of 1296 standard tokens for use in the "type" field, within the URN 1297 namespace of: "urn:ietf:params:trans:error:". 1299 detail: A human-readable string describing the error that prevented 1300 the log from processing the request, ideally with sufficient 1301 detail to enable the error to be rectified. 1303 e.g., In response to a request of "/ct/v2/get- 1304 entries?start=100&end=99", the log would return a "400 Bad Request" 1305 response code with a body similar to the following: 1307 { 1308 "type": "urn:ietf:params:trans:error:endBeforeStart", 1309 "detail": "'start' cannot be greater than 'end'" 1310 } 1312 Most error types are specific to the type of request and are defined 1313 in the respective subsections below. The one exception is the 1314 "malformed" error type, which indicates that the log server could not 1315 parse the client's request because it did not comply with this 1316 document: 1318 +===========+==================================+ 1319 | type | detail | 1320 +===========+==================================+ 1321 | malformed | The request could not be parsed. | 1322 +-----------+----------------------------------+ 1324 Table 1 1326 Clients SHOULD treat "500 Internal Server Error" and "503 Service 1327 Unavailable" responses as transient failures and MAY retry the same 1328 request without modification at a later date. Note that as per 1329 [RFC7231], in the case of a 503 response the log MAY include a 1330 "Retry-After:" header in order to request a minimum time for the 1331 client to wait before retrying the request. In the absence of this 1332 header, this document does not specify a minimum. 1334 Clients SHOULD treat any 4xx error as a problem with the request and 1335 not attempt to resubmit without some modification to the request. 1336 The full status code MAY provide additional details. 1338 This document deliberately does not provide more specific guidance on 1339 the use of HTTP status codes. 1341 5.1. Submit Entry to Log 1343 POST /ct/v2/submit-entry 1345 Inputs: submission: The base64 encoded certificate or 1346 precertificate. 1348 type: The "VersionedTransType" integer value that indicates 1349 the type of the "submission": 1 for "x509_entry_v2", or 2 for 1350 "precert_entry_v2". 1352 chain: An array of zero or more JSON strings, each of which 1353 is a base64 encoded CA certificate. The first element is the 1354 certifier of the "submission"; the second certifies the first; 1355 etc. The last element of "chain" (or, if "chain" is an empty 1356 array, the "submission") is certified by an accepted trust 1357 anchor. 1359 Outputs: sct: A base64 encoded "TransItem" of type "x509_sct_v2" or 1360 "precert_sct_v2", signed by this log, that corresponds to the 1361 "submission". 1363 If the submitted entry is immediately appended to (or already 1364 exists in) this log's tree, then the log SHOULD also output: 1366 sth: A base64 encoded "TransItem" of type "signed_tree_head_v2", 1367 signed by this log. 1369 inclusion: A base64 encoded "TransItem" of type 1370 "inclusion_proof_v2" whose "inclusion_path" array of Merkle 1371 Tree nodes proves the inclusion of the "submission" in the 1372 returned "sth". 1374 Error codes: 1376 +================+==============================================+ 1377 | type | detail | 1378 +================+==============================================+ 1379 | badSubmission | "submission" is neither a valid certificate | 1380 | | nor a valid precertificate. | 1381 +----------------+----------------------------------------------+ 1382 | badType | "type" is neither 1 nor 2. | 1383 +----------------+----------------------------------------------+ 1384 | badChain | The first element of "chain" is not the | 1385 | | certifier of the "submission", or the second | 1386 | | element does not certify the first, etc. | 1387 +----------------+----------------------------------------------+ 1388 | badCertificate | One or more certificates in the "chain" are | 1389 | | not valid (e.g., not properly encoded). | 1390 +----------------+----------------------------------------------+ 1391 | unknownAnchor | The last element of "chain" (or, if "chain" | 1392 | | is an empty array, the "submission") both is | 1393 | | not, and is not certified by, an accepted | 1394 | | trust anchor. | 1395 +----------------+----------------------------------------------+ 1396 | shutdown | The log is no longer accepting submissions. | 1397 +----------------+----------------------------------------------+ 1399 Table 2 1401 If the version of "sct" is not v2, then a v2 client may be unable to 1402 verify the signature. It MUST NOT construe this as an error. This 1403 is to avoid forcing an upgrade of compliant v2 clients that do not 1404 use the returned SCTs. 1406 If a log detects bad encoding in a chain that otherwise verifies 1407 correctly then the log MUST either log the certificate or return the 1408 "bad certificate" error. If the certificate is logged, an SCT MUST 1409 be issued. Logging the certificate is useful, because monitors 1410 (Section 8.2) can then detect these encoding errors, which may be 1411 accepted by some TLS clients. 1413 If "submission" is an accepted trust anchor whose certifier is 1414 neither an accepted trust anchor nor the first element of "chain", 1415 then the log MUST return the "unknown anchor" error. A log is not 1416 able to generate an SCT for a submission if it does not have access 1417 to the issuer's public key. 1419 If the returned "sct" is intended to be provided to TLS clients, then 1420 "sth" and "inclusion" (if returned) SHOULD also be provided to TLS 1421 clients. For example, if "type" was 2 (indicating "precert_sct_v2") 1422 then all three "TransItem"s could be embedded in the certificate. 1424 5.2. Retrieve Latest Signed Tree Head 1426 GET /ct/v2/get-sth 1428 No inputs. 1430 Outputs: sth: A base64 encoded "TransItem" of type 1431 "signed_tree_head_v2", signed by this log, that is no older 1432 than the log's MMD. 1434 5.3. Retrieve Merkle Consistency Proof between Two Signed Tree Heads 1436 GET /ct/v2/get-sth-consistency 1438 Inputs: first: The tree_size of the older tree, in decimal. 1440 second: The tree_size of the newer tree, in decimal 1441 (optional). 1443 Both tree sizes must be from existing v2 STHs. However, because 1444 of skew, the receiving front-end may not know one or both of the 1445 existing STHs. If both are known, then only the "consistency" 1446 output is returned. If the first is known but the second is not 1447 (or has been omitted), then the latest known STH is returned, 1448 along with a consistency proof between the first STH and the 1449 latest. If neither are known, then the latest known STH is 1450 returned without a consistency proof. 1452 Outputs: consistency: A base64 encoded "TransItem" of type 1453 "consistency_proof_v2", whose "tree_size_1" MUST match the 1454 "first" input. If the "sth" output is omitted, then 1455 "tree_size_2" MUST match the "second" input. If "first" and 1456 "second" are equal and correspond to a known STH, the returned 1457 consistency proof MUST be empty (a "consistency_path" array 1458 with zero elements). 1460 sth: A base64 encoded "TransItem" of type 1461 "signed_tree_head_v2", signed by this log. 1463 Note that no signature is required for the "consistency" output as 1464 it is used to verify the consistency between two STHs, which are 1465 signed. 1467 Error codes: 1469 +===================+======================================+ 1470 | type | detail | 1471 +===================+======================================+ 1472 | firstUnknown | "first" is before the latest known | 1473 | | STH but is not from an existing STH. | 1474 +-------------------+--------------------------------------+ 1475 | secondUnknown | "second" is before the latest known | 1476 | | STH but is not from an existing STH. | 1477 +-------------------+--------------------------------------+ 1478 | secondBeforeFirst | "second" is smaller than "first". | 1479 +-------------------+--------------------------------------+ 1481 Table 3 1483 See Section 2.1.4.2 for an outline of how to use the "consistency" 1484 output. 1486 5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash 1488 GET /ct/v2/get-proof-by-hash 1490 Inputs: hash: A base64 encoded v2 leaf hash. 1492 tree_size: The tree_size of the tree on which to base the 1493 proof, in decimal. 1495 The "hash" must be calculated as defined in Section 4.7. A v2 STH 1496 must exist for the "tree_size". Because of skew, the front-end 1497 may not know the requested tree head. In that case, it will 1498 return the latest STH it knows, along with an inclusion proof to 1499 that STH. If the front-end knows the requested tree head then 1500 only "inclusion" is returned. 1502 Outputs: inclusion: A base64 encoded "TransItem" of type 1503 "inclusion_proof_v2" whose "inclusion_path" array of Merkle 1504 Tree nodes proves the inclusion of the certificate (as 1505 specified by the "hash" parameter) in the selected STH. 1507 sth: A base64 encoded "TransItem" of type 1508 "signed_tree_head_v2", signed by this log. 1510 Note that no signature is required for the "inclusion" output as 1511 it is used to verify inclusion in the selected STH, which is 1512 signed. 1514 Error codes: 1516 +=================+=====================================+ 1517 | type | detail | 1518 +=================+=====================================+ 1519 | hashUnknown | "hash" is not the hash of a known | 1520 | | leaf (may be caused by skew or by a | 1521 | | known certificate not yet merged). | 1522 +-----------------+-------------------------------------+ 1523 | treeSizeUnknown | "hash" is before the latest known | 1524 | | STH but is not from an existing | 1525 | | STH. | 1526 +-----------------+-------------------------------------+ 1528 Table 4 1530 See Section 2.1.3.2 for an outline of how to use the "inclusion" 1531 output. 1533 5.5. Retrieve Merkle Inclusion Proof, Signed Tree Head and Consistency 1534 Proof by Leaf Hash 1536 GET /ct/v2/get-all-by-hash 1538 Inputs: hash: A base64 encoded v2 leaf hash. 1540 tree_size: The tree_size of the tree on which to base the 1541 proofs, in decimal. 1543 The "hash" must be calculated as defined in Section 4.7. A v2 STH 1544 must exist for the "tree_size". 1546 Because of skew, the front-end may not know the requested tree head 1547 or the requested hash, which leads to a number of cases: 1549 +=====================+=====================================+ 1550 | Case | Response | 1551 +=====================+=====================================+ 1552 | latest STH < | Return latest STH | 1553 | requested tree head | | 1554 +---------------------+-------------------------------------+ 1555 | latest STH > | Return latest STH and a consistency | 1556 | requested tree head | proof between it and the requested | 1557 | | tree head (see Section 5.3) | 1558 +---------------------+-------------------------------------+ 1559 | index of requested | Return "inclusion" | 1560 | hash < latest STH | | 1561 +---------------------+-------------------------------------+ 1563 Table 5 1565 Note that more than one case can be true, in which case the returned 1566 data is their union. It is also possible for none to be true, in 1567 which case the front-end MUST return an empty response. 1569 Outputs: inclusion: A base64 encoded "TransItem" of type 1570 "inclusion_proof_v2" whose "inclusion_path" array of Merkle 1571 Tree nodes proves the inclusion of the certificate (as 1572 specified by the "hash" parameter) in the selected STH. 1574 sth: A base64 encoded "TransItem" of type 1575 "signed_tree_head_v2", signed by this log. 1577 consistency: A base64 encoded "TransItem" of type 1578 "consistency_proof_v2" that proves the consistency of the 1579 requested tree head and the returned STH. 1581 Note that no signature is required for the "inclusion" or 1582 "consistency" outputs as they are used to verify inclusion in and 1583 consistency of STHs, which are signed. 1585 Errors are the same as in Section 5.4. 1587 See Section 2.1.3.2 for an outline of how to use the "inclusion" 1588 output, and see Section 2.1.4.2 for an outline of how to use the 1589 "consistency" output. 1591 5.6. Retrieve Entries and STH from Log 1593 GET /ct/v2/get-entries 1595 Inputs: start: 0-based index of first entry to retrieve, in 1596 decimal. 1598 end: 0-based index of last entry to retrieve, in decimal. 1600 Outputs: entries: An array of objects, each consisting of 1602 log_entry: The base64 encoded "TransItem" structure of type 1603 "x509_entry_v2" or "precert_entry_v2" (see Section 4.3). 1605 submitted_entry: JSON object equivalent to inputs that were 1606 submitted to "submit-entry", with the addition of the trust 1607 anchor to the "chain" field if the submission did not 1608 include it. 1610 sct: The base64 encoded "TransItem" of type "x509_sct_v2" or 1611 "precert_sct_v2" corresponding to this log entry. 1613 sth: A base64 encoded "TransItem" of type 1614 "signed_tree_head_v2", signed by this log. 1616 Note that this message is not signed -- the "entries" data can be 1617 verified by constructing the Merkle Tree Hash corresponding to a 1618 retrieved STH. All leaves MUST be v2. However, a compliant v2 1619 client MUST NOT construe an unrecognized TransItem type as an error. 1620 This means it may be unable to parse some entries, but note that each 1621 client can inspect the entries it does recognize as well as verify 1622 the integrity of the data by treating unrecognized leaves as opaque 1623 input to the tree. 1625 The "start" and "end" parameters SHOULD be within the range 0 <= x < 1626 "tree_size" as returned by "get-sth" in Section 5.2. 1628 The "start" parameter MUST be less than or equal to the "end" 1629 parameter. 1631 Each "submitted_entry" output parameter MUST include the trust anchor 1632 that the log used to verify the "submission", even if that trust 1633 anchor was not provided to "submit-entry" (see Section 5.1). If the 1634 "submission" does not certify itself, then the first element of 1635 "chain" MUST be present and MUST certify the "submission". 1637 Log servers MUST honor requests where 0 <= "start" < "tree_size" and 1638 "end" >= "tree_size" by returning a partial response covering only 1639 the valid entries in the specified range. "end" >= "tree_size" could 1640 be caused by skew. Note that the following restriction may also 1641 apply: 1643 Logs MAY restrict the number of entries that can be retrieved per 1644 "get-entries" request. If a client requests more than the permitted 1645 number of entries, the log SHALL return the maximum number of entries 1646 permissible. These entries SHALL be sequential beginning with the 1647 entry specified by "start". Note that limit on the number of entries 1648 is not immutable and therefore the restriction may be changed or 1649 lifted at any time and is not listed with the other Log Parameters in 1650 Section 4.1. 1652 Because of skew, it is possible the log server will not have any 1653 entries between "start" and "end". In this case it MUST return an 1654 empty "entries" array. 1656 In any case, the log server MUST return the latest STH it knows 1657 about. 1659 See Section 2.1.2 for an outline of how to use a complete list of 1660 "log_entry" entries to verify the "root_hash". 1662 Error codes: 1664 +================+====================================+ 1665 | type | detail | 1666 +================+====================================+ 1667 | startUnknown | "start" is greater than the number | 1668 | | of entries in the Merkle tree. | 1669 +----------------+------------------------------------+ 1670 | endBeforeStart | "start" cannot be greater than | 1671 | | "end". | 1672 +----------------+------------------------------------+ 1674 Table 6 1676 5.7. Retrieve Accepted Trust Anchors 1678 GET /ct/v2/get-anchors 1680 No inputs. 1682 Outputs: certificates: An array of JSON strings, each of which is a 1683 base64 encoded CA certificate that is acceptable to the log. 1684 max_chain_length: 1686 If the server has chosen to limit the length of chains it 1687 accepts, this is the maximum number of certificates in the 1688 chain, in decimal. If there is no limit, this is omitted. 1690 This data is not signed and the protocol depends on the security 1691 guarantees of TLS to ensure correctness. 1693 6. TLS Servers 1695 CT-using TLS servers MUST use at least one of the mechanisms 1696 described below to present one or more SCTs from one or more logs to 1697 each TLS client during full TLS handshakes, where each SCT 1698 corresponds to the server certificate. (Of course, a server can only 1699 send a TLS extension if the client has specified it first.) Servers 1700 SHOULD also present corresponding inclusion proofs and STHs. 1702 A server can provide SCTs using a TLS 1.3 extension (Section 4.2 of 1703 [RFC8446]) with type "transparency_info" (see Section 6.5). This 1704 mechanism allows TLS servers to participate in CT without the 1705 cooperation of CAs, unlike the other two mechanisms. It also allows 1706 SCTs and inclusion proofs to be updated on the fly. 1708 The server may also use an Online Certificate Status Protocol (OCSP) 1709 [RFC6960] response extension (see Section 7.1.1), providing the OCSP 1710 response as part of the TLS handshake. Providing a response during a 1711 TLS handshake is popularly known as "OCSP stapling." For TLS 1.3, 1712 the information is encoded as an extension in the "status_request" 1713 extension data; see Section 4.4.2.1 of [RFC8446]. For TLS 1.2 1714 ([RFC5246]), the information is encoded as an extension in the 1715 "CertificateStatus" message; see Section 8 of [RFC6066]. Using 1716 stapling also allows SCTs and inclusion proofs to be updated on the 1717 fly. 1719 CT information can also be encoded as an extension in the X.509v3 1720 certificate (see Section 7.1.2). This mechanism allows the use of 1721 unmodified TLS servers, but the SCTs and inclusion proofs cannot be 1722 updated on the fly. Since the logs from which the SCTs and inclusion 1723 proofs originated won't necessarily be accepted by TLS clients for 1724 the full lifetime of the certificate, there is a risk that TLS 1725 clients may subsequently consider the certificate to be non-compliant 1726 and in need of re-issuance or the use of one of the other two methods 1727 for delivering CT information. 1729 6.1. TLS Client Authentication 1731 This specification includes no description of how a TLS server can 1732 use CT for TLS client certificates. While this may be useful, it is 1733 not documented here for the following reasons: 1735 * The greater security exposure is for clients to end up interacting 1736 with an illegitimate server. 1738 * In general, TLS client certificates are not expected to be 1739 submitted to CT logs, particularly those intended for general 1740 public use. 1742 A future version could include such information. 1744 6.2. Multiple SCTs 1746 CT-using TLS servers SHOULD send SCTs from multiple logs, because: 1748 * One or more logs may not have become acceptable to all CT-using 1749 TLS clients. Note that client discovery, trust, and distrust of 1750 logs is expected to be handled out-of-band and is out of scope of 1751 this document. 1753 * If a CA and a log collude, it is possible to temporarily hide 1754 misissuance from clients. When a TLS client requires SCTs from 1755 multiple logs to be provided, it is more difficult to mount this 1756 attack. 1758 * If a log misbehaves or suffers a key compromise, a consequence may 1759 be that clients cease to trust it. Since the time an SCT may be 1760 in use can be considerable (several years is common in current 1761 practice when embedded in a certificate), including SCTs from 1762 multiple logs reduces the probability of the certificate being 1763 rejected by TLS clients. 1765 * TLS clients may have policies related to the above risks requiring 1766 TLS servers to present multiple SCTs. For example, at the time of 1767 writing, Chromium [Chromium.Log.Policy] requires multiple SCTs to 1768 be presented with EV certificates in order for the EV indicator to 1769 be shown. 1771 To select the logs from which to obtain SCTs, a TLS server can, for 1772 example, examine the set of logs popular TLS clients accept and 1773 recognize. 1775 6.3. TransItemList Structure 1777 Multiple SCTs, inclusion proofs, and indeed "TransItem" structures of 1778 any type, are combined into a list as follows: 1780 opaque SerializedTransItem<1..2^16-1>; 1782 struct { 1783 SerializedTransItem trans_item_list<1..2^16-1>; 1784 } TransItemList; 1786 Here, "SerializedTransItem" is an opaque byte string that contains 1787 the serialized "TransItem" structure. This encoding ensures that TLS 1788 clients can decode each "TransItem" individually (so, for example, if 1789 there is a version upgrade, out-of-date clients can still parse old 1790 "TransItem" structures while skipping over new "TransItem" structures 1791 whose versions they don't understand). 1793 6.4. Presenting SCTs, inclusions proofs and STHs 1795 In each "TransItemList" that is sent during a TLS handshake, the TLS 1796 server MUST include a "TransItem" structure of type "x509_sct_v2" or 1797 "precert_sct_v2". 1799 Presenting inclusion proofs and STHs in the TLS handshake helps to 1800 protect the client's privacy (see Section 8.1.4) and reduces load on 1801 log servers. Therefore, if the TLS server can obtain them, it SHOULD 1802 also include "TransItem"s of type "inclusion_proof_v2" and 1803 "signed_tree_head_v2" in the "TransItemList". 1805 6.5. transparency_info TLS Extension 1807 Provided that a TLS client includes the "transparency_info" extension 1808 type in the ClientHello and the TLS server supports the 1809 "transparency_info" extension: 1811 * The TLS server MUST verify that the received "extension_data" is 1812 empty. 1814 * The TLS server MUST construct a "TransItemList" of relevant 1815 "TransItem"s (see Section 6.4), which SHOULD omit any "TransItem"s 1816 that are already embedded in the server certificate or the stapled 1817 OCSP response (see Section 7.1). If the constructed 1818 "TransItemList" is not empty, then the TLS server MUST include the 1819 "transparency_info" extension with the "extension_data" set to 1820 this "TransItemList". If the list is empty then the server SHOULD 1821 omit the "extension_data" element, but MAY send it with an empty 1822 array. 1824 TLS servers MUST only include this extension in the following 1825 messages: 1827 * the ServerHello message (for TLS 1.2 or earlier). 1829 * the Certificate or CertificateRequest message (for TLS 1.3). 1831 TLS servers MUST NOT process or include this extension when a TLS 1832 session is resumed, since session resumption uses the original 1833 session information. 1835 7. Certification Authorities 1837 7.1. Transparency Information X.509v3 Extension 1839 The Transparency Information X.509v3 extension, which has OID 1840 1.3.101.75 and SHOULD be non-critical, contains one or more 1841 "TransItem" structures in a "TransItemList". This extension MAY be 1842 included in OCSP responses (see Section 7.1.1) and certificates (see 1843 Section 7.1.2). Since RFC5280 requires the "extnValue" field (an 1844 OCTET STRING) of each X.509v3 extension to include the DER encoding 1845 of an ASN.1 value, a "TransItemList" MUST NOT be included directly. 1846 Instead, it MUST be wrapped inside an additional OCTET STRING, which 1847 is then put into the "extnValue" field: 1849 TransparencyInformationSyntax ::= OCTET STRING 1851 "TransparencyInformationSyntax" contains a "TransItemList". 1853 7.1.1. OCSP Response Extension 1855 A certification authority MAY include a Transparency Information 1856 X.509v3 extension in the "singleExtensions" of a "SingleResponse" in 1857 an OCSP response. All included SCTs and inclusion proofs MUST be for 1858 the certificate identified by the "certID" of that "SingleResponse", 1859 or for a precertificate that corresponds to that certificate. 1861 7.1.2. Certificate Extension 1863 A certification authority MAY include a Transparency Information 1864 X.509v3 extension in a certificate. All included SCTs and inclusion 1865 proofs MUST be for a precertificate that corresponds to this 1866 certificate. 1868 7.2. TLS Feature X.509v3 Extension 1870 A certification authority SHOULD NOT issue any certificate that 1871 identifies the "transparency_info" TLS extension in a TLS feature 1872 extension [RFC7633], because TLS servers are not required to support 1873 the "transparency_info" TLS extension in order to participate in CT 1874 (see Section 6). 1876 8. Clients 1878 There are various different functions clients of logs might perform. 1879 We describe here some typical clients and how they should function. 1880 Any inconsistency may be used as evidence that a log has not behaved 1881 correctly, and the signatures on the data structures prevent the log 1882 from denying that misbehavior. 1884 All clients need various parameters in order to communicate with logs 1885 and verify their responses. These parameters are described in 1886 Section 4.1, but note that this document does not describe how the 1887 parameters are obtained, which is implementation-dependent (see, for 1888 example, [Chromium.Policy]). 1890 8.1. TLS Client 1892 8.1.1. Receiving SCTs and inclusion proofs 1894 TLS clients receive SCTs and inclusion proofs alongside or in 1895 certificates. CT-using TLS clients MUST implement all of the three 1896 mechanisms by which TLS servers may present SCTs (see Section 6). 1898 TLS clients that support the "transparency_info" TLS extension (see 1899 Section 6.5) SHOULD include it in ClientHello messages, with empty 1900 "extension_data". If a TLS server includes the "transparency_info" 1901 TLS extension when resuming a TLS session, the TLS client MUST abort 1902 the handshake. 1904 8.1.2. Reconstructing the TBSCertificate 1906 Validation of an SCT for a certificate (where the "type" of the 1907 "TransItem" is "x509_sct_v2") uses the unmodified TBSCertificate 1908 component of the certificate. 1910 Before an SCT for a precertificate (where the "type" of the 1911 "TransItem" is "precert_sct_v2") can be validated, the TBSCertificate 1912 component of the precertificate needs to be reconstructed from the 1913 TBSCertificate component of the certificate as follows: 1915 * Remove the Transparency Information extension (see Section 7.1). 1917 * Remove embedded v1 SCTs, identified by OID 1.3.6.1.4.1.11129.2.4.2 1918 (see section 3.3 of [RFC6962]). This allows embedded v1 and v2 1919 SCTs to co-exist in a certificate (see Appendix A). 1921 8.1.3. Validating SCTs 1923 In order to make use of a received SCT, the TLS client MUST first 1924 validate it as follows: 1926 * Compute the signature input by constructing a "TransItem" of type 1927 "x509_entry_v2" or "precert_entry_v2", depending on the SCT's 1928 "TransItem" type. The "TimestampedCertificateEntryDataV2" 1929 structure is constructed in the following manner: 1931 - "timestamp" is copied from the SCT. 1933 - "tbs_certificate" is the reconstructed TBSCertificate portion 1934 of the server certificate, as described in Section 8.1.2. 1936 - "issuer_key_hash" is computed as described in Section 4.7. 1938 - "sct_extensions" is copied from the SCT. 1940 * Verify the SCT's "signature" against the computed signature input 1941 using the public key of the corresponding log, which is identified 1942 by the "log_id". The required signature algorithm is one of the 1943 log's parameters. 1945 If the TLS client does not have the corresponding log's parameters, 1946 it cannot attempt to validate the SCT. When evaluating compliance 1947 (see Section 8.1.6), the TLS client will consider only those SCTs 1948 that it was able to validate. 1950 Note that SCT validation is not a substitute for the normal 1951 validation of the server certificate and its chain. 1953 8.1.4. Fetching inclusion proofs 1955 When a TLS client has validated a received SCT but does not yet 1956 possess a corresponding inclusion proof, the TLS client MAY request 1957 the inclusion proof directly from a log using "get-proof-by-hash" 1958 (Section 5.4) or "get-all-by-hash" (Section 5.5). 1960 Note that fetching inclusion proofs directly from a log will disclose 1961 to the log which TLS server the client has been communicating with. 1962 This may be regarded as a significant privacy concern, and so it is 1963 preferable for the TLS server to send the inclusion proofs (see 1964 Section 6.4). 1966 8.1.5. Validating inclusion proofs 1968 When a TLS client has received, or fetched, an inclusion proof (and 1969 an STH), it SHOULD proceed to verifying the inclusion proof to the 1970 provided STH. The TLS client SHOULD also verify consistency between 1971 the provided STH and an STH it knows about. 1973 If the TLS client holds an STH that predates the SCT, it MAY, in the 1974 process of auditing, request a new STH from the log (Section 5.2), 1975 then verify it by requesting a consistency proof (Section 5.3). Note 1976 that if the TLS client uses "get-all-by-hash", then it will already 1977 have the new STH. 1979 8.1.6. Evaluating compliance 1981 It is up to a client's local policy to specify the quantity and form 1982 of evidence (SCTs, inclusion proofs or a combination) needed to 1983 achieve compliance and how to handle non-compliance. 1985 A TLS client can only evaluate compliance if it has given the TLS 1986 server the opportunity to send SCTs and inclusion proofs by any of 1987 the three mechanisms that are mandatory to implement for CT-using TLS 1988 clients (see Section 8.1.1). Therefore, a TLS client MUST NOT 1989 evaluate compliance if it did not include both the 1990 "transparency_info" and "status_request" TLS extensions in the 1991 ClientHello. 1993 8.2. Monitor 1995 Monitors watch logs to check that they behave correctly, for 1996 certificates of interest, or both. For example, a monitor may be 1997 configured to report on all certificates that apply to a specific 1998 domain name when fetching new entries for consistency validation. 2000 A monitor MUST at least inspect every new entry in every log it 2001 watches, and it MAY also choose to keep copies of entire logs. 2003 To inspect all of the existing entries, the monitor SHOULD follow 2004 these steps once for each log: 2006 1. Fetch the current STH (Section 5.2). 2008 2. Verify the STH signature. 2010 3. Fetch all the entries in the tree corresponding to the STH 2011 (Section 5.6). 2013 4. If applicable, check each entry to see if it's a certificate of 2014 interest. 2016 5. Confirm that the tree made from the fetched entries produces the 2017 same hash as that in the STH. 2019 To inspect new entries, the monitor SHOULD follow these steps 2020 repeatedly for each log: 2022 1. Fetch the current STH (Section 5.2). Repeat until the STH 2023 changes. This document does not specify the polling frequency, 2024 to allow for experimentation. 2026 2. Verify the STH signature. 2028 3. Fetch all the new entries in the tree corresponding to the STH 2029 (Section 5.6). If they remain unavailable for an extended 2030 period, then this should be viewed as misbehavior on the part of 2031 the log. 2033 4. If applicable, check each entry to see if it's a certificate of 2034 interest. 2036 5. Either: 2038 1. Verify that the updated list of all entries generates a tree 2039 with the same hash as the new STH. 2041 Or, if it is not keeping all log entries: 2043 1. Fetch a consistency proof for the new STH with the previous 2044 STH (Section 5.3). 2046 2. Verify the consistency proof. 2048 3. Verify that the new entries generate the corresponding 2049 elements in the consistency proof. 2051 6. Repeat from step 1. 2053 8.3. Auditing 2055 Auditing ensures that the current published state of a log is 2056 reachable from previously published states that are known to be good, 2057 and that the promises made by the log in the form of SCTs have been 2058 kept. Audits are performed by monitors or TLS clients. 2060 In particular, there are four log behavior properties that should be 2061 checked: 2063 * The Maximum Merge Delay (MMD). 2065 * The STH Frequency Count. 2067 * The append-only property. 2069 * The consistency of the log view presented to all query sources. 2071 A benign, conformant log publishes a series of STHs over time, each 2072 derived from the previous STH and the submitted entries incorporated 2073 into the log since publication of the previous STH. This can be 2074 proven through auditing of STHs. SCTs returned to TLS clients can be 2075 audited by verifying against the accompanying certificate, and using 2076 Merkle Inclusion Proofs, against the log's Merkle tree. 2078 The action taken by the auditor if an audit fails is not specified, 2079 but note that in general if audit fails, the auditor is in possession 2080 of signed proof of the log's misbehavior. 2082 A monitor (Section 8.2) can audit by verifying the consistency of 2083 STHs it receives, ensure that each entry can be fetched and that the 2084 STH is indeed the result of making a tree from all fetched entries. 2086 A TLS client (Section 8.1) can audit by verifying an SCT against any 2087 STH dated after the SCT timestamp + the Maximum Merge Delay by 2088 requesting a Merkle inclusion proof (Section 5.4). It can also 2089 verify that the SCT corresponds to the server certificate it arrived 2090 with (i.e., the log entry is that certificate, or is a precertificate 2091 corresponding to that certificate). 2093 Checking of the consistency of the log view presented to all entities 2094 is more difficult to perform because it requires a way to share log 2095 responses among a set of CT-using entities, and is discussed in 2096 Section 11.3. 2098 9. Algorithm Agility 2100 It is not possible for a log to change any of its algorithms part way 2101 through its lifetime: 2103 Signature algorithm: SCT signatures must remain valid so signature 2104 algorithms can only be added, not removed. 2106 Hash algorithm: A log would have to support the old and new hash 2107 algorithms to allow backwards-compatibility with clients that are 2108 not aware of a hash algorithm change. 2110 Allowing multiple signature or hash algorithms for a log would 2111 require that all data structures support it and would significantly 2112 complicate client implementation, which is why it is not supported by 2113 this document. 2115 If it should become necessary to deprecate an algorithm used by a 2116 live log, then the log MUST be frozen as specified in Section 4.13 2117 and a new log SHOULD be started. Certificates in the frozen log that 2118 have not yet expired and require new SCTs SHOULD be submitted to the 2119 new log and the SCTs from that log used instead. 2121 10. IANA Considerations 2123 The assignment policy criteria mentioned in this section refer to the 2124 policies outlined in [RFC8126]. 2126 10.1. Additions to existing registries 2128 This sub-section defines additions to existing registries. 2130 10.1.1. New Entry to the TLS ExtensionType Registry 2132 IANA is asked to add an entry for "transparency_info(TBD)" to the 2133 "TLS ExtensionType Values" registry defined in [RFC8446], setting the 2134 "Recommended" value to "Y", setting the "TLS 1.3" value to "CH, CR, 2135 CT", and citing this document as the "Reference". 2137 10.1.2. URN Sub-namespace for TRANS errors 2138 (urn:ietf:params:trans:error) 2140 IANA is requested to add a new entry in the "IETF URN Sub-namespace 2141 for Registered Protocol Parameter Identifiers" registry, following 2142 the template in [RFC3553]: 2144 Registry name: trans:error 2146 Specification: RFCXXXX 2148 Repository: https://www.iana.org/assignments/trans 2150 Index value: No transformation needed. 2152 10.2. New CT-Related registries 2154 This sub-section defines new registries for CT. They should be made 2155 available at https://www.iana.org/assignments/ 2157 10.2.1. Hash Algorithms 2159 IANA is asked to establish a registry of hash algorithm values, named 2160 "CT Hash Algorithms", that initially consists of: 2162 +========+============+========================+===================+ 2163 | Value | Hash | OID | Reference / | 2164 | | Algorithm | | Assignment Policy | 2165 +========+============+========================+===================+ 2166 | 0x00 | SHA-256 | 2.16.840.1.101.3.4.2.1 | [RFC6234] | 2167 +--------+------------+------------------------+-------------------+ 2168 | 0x01 - | Unassigned | | Specification | 2169 | 0xDF | | | Required | 2170 +--------+------------+------------------------+-------------------+ 2171 | 0xE0 - | Reserved | | Experimental Use | 2172 | 0xEF | | | | 2173 +--------+------------+------------------------+-------------------+ 2174 | 0xF0 - | Reserved | | Private Use | 2175 | 0xFF | | | | 2176 +--------+------------+------------------------+-------------------+ 2178 Table 7 2180 The Designated Expert(s) should ensure that the proposed algorithm 2181 has a public specification and is suitable for use as a cryptographic 2182 hash algorithm with no known preimage or collision attacks. These 2183 attacks can damage the integrity of the log. 2185 10.2.2. Signature Algorithms 2187 IANA is asked to establish a registry of signature algorithm values, 2188 named "CT Signature Algorithms". 2190 The following notes should be added: 2192 * This is a subset of the TLS SignatureScheme Registry, limited to 2193 those algorithms that are appropriate for CT. A major advantage 2194 of this is leveraging the expertise of the TLS working group and 2195 its Designated Expert(s). 2197 * The value "0x0403" appears twice. While this may be confusing, it 2198 is okay because the verification process is the same for both 2199 algorithms, and the choice of which to use when generating a 2200 signature is purely internal to the log server. 2202 The registry should initially consist of: 2204 +================================+==================+==============+ 2205 | SignatureScheme Value | Signature | Reference / | 2206 | | Algorithm | Assignment | 2207 | | | Policy | 2208 +================================+==================+==============+ 2209 | 0x0000 - 0x0402 | Unassigned | Expert | 2210 | | | Review | 2211 +--------------------------------+------------------+--------------+ 2212 | ecdsa_secp256r1_sha256(0x0403) | ECDSA (NIST | [FIPS186-4] | 2213 | | P-256) with | | 2214 | | SHA-256 | | 2215 +--------------------------------+------------------+--------------+ 2216 | ecdsa_secp256r1_sha256(0x0403) | Deterministic | [RFC6979] | 2217 | | ECDSA (NIST | | 2218 | | P-256) with | | 2219 | | HMAC-SHA256 | | 2220 +--------------------------------+------------------+--------------+ 2221 | 0x0404 - 0x0806 | Unassigned | Expert | 2222 | | | Review | 2223 +--------------------------------+------------------+--------------+ 2224 | ed25519(0x0807) | Ed25519 | [RFC8032] | 2225 | | (PureEdDSA with | | 2226 | | the edwards25519 | | 2227 | | curve) | | 2228 +--------------------------------+------------------+--------------+ 2229 | 0x0808 - 0xFDFF | Unassigned | Expert | 2230 | | | Review | 2231 +--------------------------------+------------------+--------------+ 2232 | 0xFE00 - 0xFEFF | Reserved | Experimental | 2233 | | | Use | 2234 +--------------------------------+------------------+--------------+ 2235 | 0xFF00 - 0xFFFF | Reserved | Private Use | 2236 +--------------------------------+------------------+--------------+ 2238 Table 8 2240 The Designated Expert(s) should ensure that the proposed algorithm 2241 has a public specification, has a value assigned to it in the TLS 2242 SignatureScheme Registry (that IANA is asked to establish in 2243 [RFC8446]) and is suitable for use as a cryptographic signature 2244 algorithm. 2246 10.2.3. VersionedTransTypes 2248 IANA is asked to establish a registry of "VersionedTransType" values, 2249 named "CT VersionedTransTypes", that initially consists of: 2251 +==========+======================+===============================+ 2252 | Value | Type and Version | Reference / Assignment Policy | 2253 +==========+======================+===============================+ 2254 | 0x0000 | Reserved | [RFC6962] * | 2255 +----------+----------------------+-------------------------------+ 2256 | 0x0001 | x509_entry_v2 | RFCXXXX | 2257 +----------+----------------------+-------------------------------+ 2258 | 0x0002 | precert_entry_v2 | RFCXXXX | 2259 +----------+----------------------+-------------------------------+ 2260 | 0x0003 | x509_sct_v2 | RFCXXXX | 2261 +----------+----------------------+-------------------------------+ 2262 | 0x0004 | precert_sct_v2 | RFCXXXX | 2263 +----------+----------------------+-------------------------------+ 2264 | 0x0005 | signed_tree_head_v2 | RFCXXXX | 2265 +----------+----------------------+-------------------------------+ 2266 | 0x0006 | consistency_proof_v2 | RFCXXXX | 2267 +----------+----------------------+-------------------------------+ 2268 | 0x0007 | inclusion_proof_v2 | RFCXXXX | 2269 +----------+----------------------+-------------------------------+ 2270 | 0x0008 - | Unassigned | Specification Required | 2271 | 0xDFFF | | | 2272 +----------+----------------------+-------------------------------+ 2273 | 0xE000 - | Reserved | Experimental Use | 2274 | 0xEFFF | | | 2275 +----------+----------------------+-------------------------------+ 2276 | 0xF000 - | Reserved | Private Use | 2277 | 0xFFFF | | | 2278 +----------+----------------------+-------------------------------+ 2280 Table 9 2282 * The 0x0000 value is reserved so that v1 SCTs are distinguishable 2283 from v2 SCTs and other "TransItem" structures. 2285 The Designated Expert(s) should review the public specification to 2286 ensure that it is detailed enough to ensure implementation 2287 interoperability. 2289 10.2.4. Log Artifact Extension Registry 2291 IANA is asked to establish a registry of "ExtensionType" values, 2292 named "CT Log Artifact Extensions", that initially consists of: 2294 +===============+============+=====+===============================+ 2295 | ExtensionType | Status | Use | Reference / Assignment Policy | 2296 +===============+============+=====+===============================+ 2297 | 0x0000 - | Unassigned | n/a | Specification Required | 2298 | 0xDFFF | | | | 2299 +---------------+------------+-----+-------------------------------+ 2300 | 0xE000 - | Reserved | n/a | Experimental Use | 2301 | 0xEFFF | | | | 2302 +---------------+------------+-----+-------------------------------+ 2303 | 0xF000 - | Reserved | n/a | Private Use | 2304 | 0xFFFF | | | | 2305 +---------------+------------+-----+-------------------------------+ 2307 Table 10 2309 The "Use" column should contain one or both of the following values: 2311 * "SCT", for extensions specified for use in Signed Certificate 2312 Timestamps. 2314 * "STH", for extensions specified for use in Signed Tree Heads. 2316 The Designated Expert(s) should review the public specification to 2317 ensure that it is detailed enough to ensure implementation 2318 interoperability. They should also verify that the extension is 2319 appropriate to the contexts in which it is specified to be used (SCT, 2320 STH, or both). 2322 10.2.5. Object Identifiers 2324 This document uses object identifiers (OIDs) to identify Log IDs (see 2325 Section 4.4), the precertificate CMS "eContentType" (see 2326 Section 3.2), and X.509v3 extensions in certificates (see 2327 Section 7.1.2) and OCSP responses (see Section 7.1.1). The OIDs are 2328 defined in an arc that was selected due to its short encoding. 2330 10.2.5.1. Log ID Registry 2332 IANA is asked to establish a registry of Log IDs, named "CT Log ID 2333 Registry", that initially consists of: 2335 +================+==============+==============+===================+ 2336 | Log ID | Log Base URL | Log Operator | Reference / | 2337 | | | | Assignment Policy | 2338 +================+==============+==============+===================+ 2339 | 1.3.101.8192 - | Unassigned | Unassigned | First Come First | 2340 | 1.3.101.16383 | | | Served | 2341 +----------------+--------------+--------------+-------------------+ 2342 | 1.3.101.80.0 - | Unassigned | Unassigned | First Come First | 2343 | 1.3.101.80.* | | | Served | 2344 +----------------+--------------+--------------+-------------------+ 2346 Table 11 2348 All OIDs in the range from 1.3.101.8192 to 1.3.101.16383 have been 2349 set aside for Log IDs. This is a limited resource of 8,192 OIDs, 2350 each of which has an encoded length of 4 octets. 2352 The 1.3.101.80 arc has also been set assigned for LogIDs. This is an 2353 unlimited resource, but only the 128 OIDs from 1.3.101.80.0 to 2354 1.3.101.80.127 have an encoded length of only 4 octets. 2356 Each application for the allocation of a Log ID MUST be accompanied 2357 by: 2359 * the Log's Base URL (see Section 4.1). 2361 * the Log Operator's contact details. 2363 IANA is asked to reject any request to update a Log ID or Log Base 2364 URL in this registry, because these fields are immutable (see 2365 Section 4.1). 2367 IANA is asked to accept requests from log operators to update their 2368 contact details in this registry. 2370 Since log operators can choose to not use this registry (see 2371 Section 4.4), it is not expected to be a global directory of all 2372 logs. 2374 10.2.6. CT Error Types Registry 2376 IANA is requested to create a new registry for errors, the "CT Error 2377 Types" registry. 2379 Requirements for this registry are Specification Required. 2381 This registry should have the following three fields: 2383 +============+========+===========+ 2384 | Field Name | Type | Reference | 2385 +============+========+===========+ 2386 | identifier | string | RFCXXXX | 2387 +------------+--------+-----------+ 2388 | meaning | string | RFCXXXX | 2389 +------------+--------+-----------+ 2390 | reference | string | RFCXXXX | 2391 +------------+--------+-----------+ 2393 Table 12 2395 The initial values are as follows, taken from the text above: 2397 +===================+===============================+===========+ 2398 | Identifier | Meaning | Reference | 2399 +===================+===============================+===========+ 2400 | malformed | The request could not be | RFCXXXX | 2401 | | parsed. | | 2402 +-------------------+-------------------------------+-----------+ 2403 | badSubmission | "submission" is neither a | RFCXXXX | 2404 | | valid certificate nor a valid | | 2405 | | precertificate | | 2406 +-------------------+-------------------------------+-----------+ 2407 | badType | "type" is neither 1 nor 2 | RFCXXXX | 2408 +-------------------+-------------------------------+-----------+ 2409 | badChain | The first element of "chain" | RFCXXXX | 2410 | | is not the certifier of the | | 2411 | | "submission", or the second | | 2412 | | element does not certify the | | 2413 | | first, etc. | | 2414 +-------------------+-------------------------------+-----------+ 2415 | badCertificate | One or more certificates in | RFCXXXX | 2416 | | the "chain" are not valid | | 2417 | | (e.g., not properly encoded) | | 2418 +-------------------+-------------------------------+-----------+ 2419 | unknownAnchor | The last element of "chain" | RFCXXXX | 2420 | | (or, if "chain" is an empty | | 2421 | | array, the "submission") both | | 2422 | | is not, and is not certified | | 2423 | | by, an accepted trust anchor | | 2424 +-------------------+-------------------------------+-----------+ 2425 | shutdown | The log is no longer | RFCXXXX | 2426 | | accepting submissions | | 2427 +-------------------+-------------------------------+-----------+ 2428 | firstUnknown | "first" is before the latest | RFCXXXX | 2429 | | known STH but is not from an | | 2430 | | existing STH. | | 2431 +-------------------+-------------------------------+-----------+ 2432 | secondUnknown | "second" is before the latest | RFCXXXX | 2433 | | known STH but is not from an | | 2434 | | existing STH. | | 2435 +-------------------+-------------------------------+-----------+ 2436 | secondBeforeFirst | "second" is smaller than | RFCXXXX | 2437 | | "first". | | 2438 +-------------------+-------------------------------+-----------+ 2439 | hashUnknown | "hash" is not the hash of a | RFCXXXX | 2440 | | known leaf (may be caused by | | 2441 | | skew or by a known | | 2442 | | certificate not yet merged). | | 2443 +-------------------+-------------------------------+-----------+ 2444 | treeSizeUnknown | "hash" is before the latest | RFCXXXX | 2445 | | known STH but is not from an | | 2446 | | existing STH. | | 2447 +-------------------+-------------------------------+-----------+ 2448 | startUnknown | "start" is greater than the | RFCXXXX | 2449 | | number of entries in the | | 2450 | | Merkle tree. | | 2451 +-------------------+-------------------------------+-----------+ 2452 | endBeforeStart | "start" cannot be greater | RFCXXXX | 2453 | | than "end". | | 2454 +-------------------+-------------------------------+-----------+ 2456 Table 13 2458 11. Security Considerations 2460 With CAs, logs, and servers performing the actions described here, 2461 TLS clients can use logs and signed timestamps to reduce the 2462 likelihood that they will accept misissued certificates. If a server 2463 presents a valid signed timestamp for a certificate, then the client 2464 knows that a log has committed to publishing the certificate. From 2465 this, the client knows that monitors acting for the subject of the 2466 certificate have had some time to notice the misissuance and take 2467 some action, such as asking a CA to revoke a misissued certificate. 2468 A signed timestamp does not guarantee this though, since appropriate 2469 monitors might not have checked the logs or the CA might have refused 2470 to revoke the certificate. 2472 In addition, if TLS clients will not accept unlogged certificates, 2473 then site owners will have a greater incentive to submit certificates 2474 to logs, possibly with the assistance of their CA, increasing the 2475 overall transparency of the system. 2477 11.1. Misissued Certificates 2479 Misissued certificates that have not been publicly logged, and thus 2480 do not have a valid SCT, are not considered compliant. Misissued 2481 certificates that do have an SCT from a log will appear in that 2482 public log within the Maximum Merge Delay, assuming the log is 2483 operating correctly. Since a log is allowed to serve an STH of any 2484 age up to the MMD, the maximum period of time during which a 2485 misissued certificate can be used without being available for audit 2486 is twice the MMD. 2488 11.2. Detection of Misissue 2490 The logs do not themselves detect misissued certificates; they rely 2491 instead on interested parties, such as domain owners, to monitor them 2492 and take corrective action when a misissue is detected. 2494 11.3. Misbehaving Logs 2496 A log can misbehave in several ways. Examples include: failing to 2497 incorporate a certificate with an SCT in the Merkle Tree within the 2498 MMD; presenting different, conflicting views of the Merkle Tree at 2499 different times and/or to different parties; issuing STHs too 2500 frequently; mutating the signature of a logged certificate; and 2501 failing to present a chain containing the certifier of a logged 2502 certificate. 2504 Violation of the MMD contract is detected by log clients requesting a 2505 Merkle inclusion proof (Section 5.4) for each observed SCT. These 2506 checks can be asynchronous and need only be done once per 2507 certificate. However, note that there may be privacy concerns (see 2508 Section 8.1.4). 2510 Violation of the append-only property or the STH issuance rate limit 2511 can be detected by multiple clients comparing their instances of the 2512 Signed Tree Heads. This technique, known as "gossip," is an active 2513 area of research and not defined here. Proof of misbehavior in such 2514 cases would be: a series of STHs that were issued too closely 2515 together, proving violation of the STH issuance rate limit; or an STH 2516 with a root hash that does not match the one calculated from a copy 2517 of the log, proving violation of the append-only property. 2519 Clients that report back SCTs can be tracked or traced if a log 2520 produces multiple STHs or SCTs with the same timestamp and data but 2521 different signatures. Logs SHOULD mitigate this risk by either: 2523 * Using deterministic signature schemes, or 2524 * Producing no more than one SCT for each distinct submission and no 2525 more than one STH for each distinct tree_size. Each of these SCTs 2526 and STHs can be stored by the log and served to other clients that 2527 submit the same certificate or request the same STH. 2529 11.4. Multiple SCTs 2531 By requiring TLS servers to offer multiple SCTs, each from a 2532 different log, TLS clients reduce the effectiveness of an attack 2533 where a CA and a log collude (see Section 6.2). 2535 11.5. Leakage of DNS Information 2537 Malicious monitors can use logs to learn about the existence of 2538 domain names that might not otherwise be easy to discover. Some 2539 subdomain labels may reveal information about the service and 2540 software for which the subdomain is used, which in turn might 2541 facilitate targeted attacks. 2543 12. Acknowledgements 2545 The authors would like to thank Erwann Abelea, Robin Alden, Andrew 2546 Ayer, Richard Barnes, Al Cutter, David Drysdale, Francis Dupont, Adam 2547 Eijdenberg, Stephen Farrell, Daniel Kahn Gillmor, Paul Hadfield, Brad 2548 Hill, Jeff Hodges, Paul Hoffman, Jeffrey Hutzelman, Kat Joyce, 2549 Stephen Kent, SM, Alexey Melnikov, Linus Nordberg, Chris Palmer, 2550 Trevor Perrin, Pierre Phaneuf, Eric Rescorla, Rich Salz, Melinda 2551 Shore, Ryan Sleevi, Martin Smith, Carl Wallace and Paul Wouters for 2552 their valuable contributions. 2554 A big thank you to Symantec for kindly donating the OIDs from the 2555 1.3.101 arc that are used in this document. 2557 13. References 2559 13.1. Normative References 2561 [FIPS186-4] 2562 NIST, "FIPS PUB 186-4", 1 July 2013, 2563 . 2566 [HTML401] Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01 2567 Specification", World Wide Web Consortium Recommendation 2568 REC-html401-19991224, 24 December 1999, 2569 . 2571 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2572 Requirement Levels", BCP 14, RFC 2119, 2573 DOI 10.17487/RFC2119, March 1997, 2574 . 2576 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 2577 IETF URN Sub-namespace for Registered Protocol 2578 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 2579 2003, . 2581 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2582 Resource Identifier (URI): Generic Syntax", STD 66, 2583 RFC 3986, DOI 10.17487/RFC3986, January 2005, 2584 . 2586 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 2587 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 2588 . 2590 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 2591 (TLS) Protocol Version 1.2", RFC 5246, 2592 DOI 10.17487/RFC5246, August 2008, 2593 . 2595 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 2596 Housley, R., and W. Polk, "Internet X.509 Public Key 2597 Infrastructure Certificate and Certificate Revocation List 2598 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 2599 . 2601 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, 2602 RFC 5652, DOI 10.17487/RFC5652, September 2009, 2603 . 2605 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 2606 Extensions: Extension Definitions", RFC 6066, 2607 DOI 10.17487/RFC6066, January 2011, 2608 . 2610 [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms 2611 (SHA and SHA-based HMAC and HKDF)", RFC 6234, 2612 DOI 10.17487/RFC6234, May 2011, 2613 . 2615 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., 2616 Galperin, S., and C. Adams, "X.509 Internet Public Key 2617 Infrastructure Online Certificate Status Protocol - OCSP", 2618 RFC 6960, DOI 10.17487/RFC6960, June 2013, 2619 . 2621 [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature 2622 Algorithm (DSA) and Elliptic Curve Digital Signature 2623 Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2624 2013, . 2626 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 2627 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 2628 DOI 10.17487/RFC7231, June 2014, 2629 . 2631 [RFC7633] Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS) 2632 Feature Extension", RFC 7633, DOI 10.17487/RFC7633, 2633 October 2015, . 2635 [RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP 2636 APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016, 2637 . 2639 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 2640 Signature Algorithm (EdDSA)", RFC 8032, 2641 DOI 10.17487/RFC8032, January 2017, 2642 . 2644 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2645 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2646 May 2017, . 2648 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 2649 Interchange Format", STD 90, RFC 8259, 2650 DOI 10.17487/RFC8259, December 2017, 2651 . 2653 [RFC8391] Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A. 2654 Mohaisen, "XMSS: eXtended Merkle Signature Scheme", 2655 RFC 8391, DOI 10.17487/RFC8391, May 2018, 2656 . 2658 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 2659 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 2660 . 2662 [UNIXTIME] IEEE, "The Open Group Base Specifications Issue 7 IEEE Std 2663 1003.1-2008, 2016 Edition", n.d., 2664 . 2668 [X690] ITU-T, "Information technology - ASN.1 encoding Rules: 2669 Specification of Basic Encoding Rules (BER), Canonical 2670 Encoding Rules (CER) and Distinguished Encoding Rules 2671 (DER)", ISO/IEC 8825-1:2002, November 2015. 2673 13.2. Informative References 2675 [CABBR] CA/Browser Forum, "Baseline Requirements for the Issuance 2676 and Management of Publicly-Trusted Certificates", 2020, 2677 . 2680 [Chromium.Log.Policy] 2681 The Chromium Projects, "Chromium Certificate Transparency 2682 Log Policy", 2014, . 2685 [Chromium.Policy] 2686 The Chromium Projects, "Chromium Certificate 2687 Transparency", 2014, . 2690 [CrosbyWallach] 2691 Crosby, S. and D. Wallach, "Efficient Data Structures for 2692 Tamper-Evident Logging", Proceedings of the 18th USENIX 2693 Security Symposium, Montreal, August 2009, 2694 . 2697 [JSON.Metadata] 2698 The Chromium Projects, "Chromium Log Metadata JSON 2699 Schema", 2014, . 2702 [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate 2703 Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, 2704 . 2706 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2707 Writing an IANA Considerations Section in RFCs", BCP 26, 2708 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2709 . 2711 [RFC8820] Nottingham, M., "URI Design and Ownership", BCP 190, 2712 RFC 8820, DOI 10.17487/RFC8820, June 2020, 2713 . 2715 Appendix A. Supporting v1 and v2 simultaneously (Informative) 2717 Certificate Transparency logs have to be either v1 (conforming to 2718 [RFC6962]) or v2 (conforming to this document), as the data 2719 structures are incompatible and so a v2 log could not issue a valid 2720 v1 SCT. 2722 CT clients, however, can support v1 and v2 SCTs, for the same 2723 certificate, simultaneously, as v1 SCTs are delivered in different 2724 TLS, X.509 and OCSP extensions than v2 SCTs. 2726 v1 and v2 SCTs for X.509 certificates can be validated independently. 2727 For precertificates, v2 SCTs should be embedded in the TBSCertificate 2728 before submission of the TBSCertificate (inside a v1 precertificate, 2729 as described in Section 3.1. of [RFC6962]) to a v1 log so that TLS 2730 clients conforming to [RFC6962] but not this document are oblivious 2731 to the embedded v2 SCTs. An issuer can follow these steps to produce 2732 an X.509 certificate with embedded v1 and v2 SCTs: 2734 * Create a CMS precertificate as described in Section 3.2 and submit 2735 it to v2 logs. 2737 * Embed the obtained v2 SCTs in the TBSCertificate, as described in 2738 Section 7.1.2. 2740 * Use that TBSCertificate to create a v1 precertificate, as 2741 described in Section 3.1. of [RFC6962] and submit it to v1 logs. 2743 * Embed the v1 SCTs in the TBSCertificate, as described in 2744 Section 3.3 of [RFC6962]. 2746 * Sign that TBSCertificate (which now contains v1 and v2 SCTs) to 2747 issue the final X.509 certificate. 2749 Appendix B. An ASN.1 Module (Informative) 2751 The following ASN.1 module may be useful to implementors. 2753 CertificateTransparencyV2Module-2021 2754 -- { OID Needed, but no point in using a short one } 2755 DEFINITIONS IMPLICIT TAGS ::= BEGIN 2757 -- EXPORTS ALL -- 2758 IMPORTS 2759 EXTENSION 2760 FROM PKIX-CommonTypes-2009 -- RFC 5912 2761 { iso(1) identified-organization(3) dod(6) internet(1) 2762 security(5) mechanisms(5) pkix(7) id-mod(0) 2763 id-mod-pkixCommon-02(57) } 2765 CONTENT-TYPE 2766 FROM CryptographicMessageSyntax-2010 -- RFC 6268 2767 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) 2768 pkcs-9(9) smime(16) modules(0) id-mod-cms-2009(58) } 2770 TBSCertificate 2771 FROM PKIX1Explicit-2009 -- RFC 5912 2772 { iso(1) identified-organization(3) dod(6) internet(1) 2773 security(5) mechanisms(5) pkix(7) id-mod(0) 2774 id-mod-pkix1-explicit-02(51) } 2775 ; 2777 -- 2778 -- Section 3.2. Precertificates 2779 -- 2781 ct-tbsCertificate CONTENT-TYPE ::= { 2782 TYPE TBSCertificate 2783 IDENTIFIED BY id-ct-tbsCertificate } 2785 id-ct-tbsCertificate OBJECT IDENTIFIER ::= { 1 3 101 78 } 2787 -- 2788 -- Section 7.1. Transparency Information X.509v3 Extension 2789 -- 2791 ext-transparencyInfo EXTENSION ::= { 2792 SYNTAX TransparencyInformationSyntax 2793 IDENTIFIED BY id-ce-transparencyInfo 2794 CRITICALITY { FALSE } } 2796 id-ce-transparencyInfo OBJECT IDENTIFIER ::= { 1 3 101 75 } 2798 TransparencyInformationSyntax ::= OCTET STRING 2800 -- 2801 -- Section 7.1.1. OCSP Response Extension 2802 -- 2804 ext-ocsp-transparencyInfo EXTENSION ::= { 2805 SYNTAX TransparencyInformationSyntax 2806 IDENTIFIED BY id-pkix-ocsp-transparencyInfo 2807 CRITICALITY { FALSE } } 2809 id-pkix-ocsp-transparencyInfo OBJECT IDENTIFIER ::= 2810 id-ce-transparencyInfo 2812 -- 2813 -- Section 8.1.2. Reconstructing the TBSCertificate 2814 -- 2816 ext-embeddedSCT-CTv1 EXTENSION ::= { 2817 SYNTAX SignedCertificateTimestampList 2818 IDENTIFIED BY id-ce-embeddedSCT-CTv1 2819 CRITICALITY { FALSE } } 2821 id-ce-embeddedSCT-CTv1 OBJECT IDENTIFIER ::= { 2822 1 3 6 1 4 1 11129 2 4 2 } 2824 SignedCertificateTimestampList ::= OCTET STRING 2826 END 2828 Authors' Addresses 2830 Ben Laurie 2831 Google UK Ltd. 2833 Email: benl@google.com 2835 Adam Langley 2836 Google Inc. 2838 Email: agl@google.com 2840 Emilia Kasper 2841 Google Switzerland GmbH 2843 Email: ekasper@google.com 2845 Eran Messeri 2846 Google UK Ltd. 2848 Email: eranm@google.com 2849 Rob Stradling 2850 Sectigo Ltd. 2852 Email: rob@sectigo.com