idnits 2.17.1 draft-ietf-trans-rfc6962-bis-38.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 (14 May 2021) is 1071 days in the past. Is this intentional? Checking references for intended status: Experimental ---------------------------------------------------------------------------- -- Looks like a reference, but probably isn't: '0' on line 483 -- Looks like a reference, but probably isn't: '1' on line 483 -- Looks like a reference, but probably isn't: '7' on line 637 ** 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: 15 November 2021 Google 7 R. Stradling 8 Sectigo 9 14 May 2021 11 Certificate Transparency Version 2.0 12 draft-ietf-trans-rfc6962-bis-38 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 15 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 . . . . . . . . . . . . . . 6 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 . . . . . . . . . . . . . . 11 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 . . . . . . . . . . . . . . . . . . 48 138 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. Log ID Registry . . . . . . . . . . . . . . . . . . 51 142 10.2.6. Error Types Registry . . . . . . . . . . . . . . . . 52 143 11. Security Considerations . . . . . . . . . . . . . . . . . . . 54 144 11.1. Misissued Certificates . . . . . . . . . . . . . . . . . 54 145 11.2. Detection of Misissue . . . . . . . . . . . . . . . . . 54 146 11.3. Misbehaving Logs . . . . . . . . . . . . . . . . . . . . 55 147 11.4. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . 55 148 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 . . . . . . . . . . . . . . . . . 58 153 Appendix A. Supporting v1 and v2 simultaneously (Informative) . 59 154 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 This document uses object identifiers (OIDs) to identify Log IDs (see 235 Section 4.4), the precertificate CMS "eContentType" (see 236 Section 3.2), and X.509v3 extensions in certificates (see 237 Section 7.1.2) and OCSP responses (see Section 7.1.1). The OIDs are 238 defined in an arc that was selected due to its short encoding. 240 1.3. Major Differences from CT 1.0 242 This document revises and obsoletes the CT 1.0 [RFC6962] protocol, 243 drawing on insights gained from CT 1.0 deployments and on feedback 244 from the community. The major changes are: 246 * Hash and signature algorithm agility: permitted algorithms are now 247 specified in IANA registries. 249 * Precertificate format: precertificates are now CMS objects rather 250 than X.509 certificates, which avoids violating the certificate 251 serial number uniqueness requirement in Section 4.1.2.2 of 252 [RFC5280]. 254 * Removed precertificate signing certificates and the precertificate 255 poison extension: the change of precertificate format means that 256 these are no longer needed. 258 * Logs IDs: each log is now identified by an OID rather than by the 259 hash of its public key. OID allocations are managed by an IANA 260 registry. 262 * "TransItem" structure: this new data structure is used to 263 encapsulate most types of CT data. A "TransItemList", consisting 264 of one or more "TransItem" structures, can be used anywhere that 265 "SignedCertificateTimestampList" was used in [RFC6962]. 267 * Merkle tree leaves: the "MerkleTreeLeaf" structure has been 268 replaced by the "TransItem" structure, which eases extensibility 269 and simplifies the leaf structure by removing one layer of 270 abstraction. 272 * Unified leaf format: the structure for both certificate and 273 precertificate entries now includes only the TBSCertificate 274 (whereas certificate entries in [RFC6962] included the entire 275 certificate). 277 * Log Artifact Extensions: these are now typed and managed by an 278 IANA registry, and they can now appear not only in SCTs but also 279 in STHs. 281 * API outputs: complete "TransItem" structures are returned, rather 282 than the constituent parts of each structure. 284 * get-all-by-hash: new client API for obtaining an inclusion proof 285 and the corresponding consistency proof at the same time. 287 * submit-entry: new client API, replacing add-chain and add-pre- 288 chain. 290 * Presenting SCTs with proofs: TLS servers may present SCTs together 291 with the corresponding inclusion proofs using any of the 292 mechanisms that [RFC6962] defined for presenting SCTs only. 293 (Presenting SCTs only is still supported). 295 * CT TLS extension: the "signed_certificate_timestamp" TLS extension 296 has been replaced by the "transparency_info" TLS extension. 298 * Verification algorithms: added detailed algorithms for verifying 299 inclusion proofs, for verifying consistency between two STHs, and 300 for verifying a root hash given a complete list of the relevant 301 leaf input entries. 303 * Extensive clarifications and editorial work. 305 2. Cryptographic Components 307 2.1. Merkle Hash Trees 309 A full description of Merkle Hash Tree is beyond the scope of this 310 document. Briefly, it is a binary tree where each non-leaf node is a 311 hash of its children. For CT, the number of children is at most two. 312 Additional information can be found in the Introduction and Reference 313 section of [RFC8391]. 315 2.1.1. Definition of the Merkle Tree 317 The log uses a binary Merkle Hash Tree for efficient auditing. The 318 hash algorithm used is one of the log's parameters (see Section 4.1). 319 This document establishes a registry of acceptable hash algorithms 320 (see Section 10.2.1). Throughout this document, the hash algorithm 321 in use is referred to as HASH and the size of its output in bytes as 322 HASH_SIZE. The input to the Merkle Tree Hash is a list of data 323 entries; these entries will be hashed to form the leaves of the 324 Merkle Hash Tree. The output is a single HASH_SIZE Merkle Tree Hash. 325 Given an ordered list of n inputs, D_n = {d[0], d[1], ..., d[n-1]}, 326 the Merkle Tree Hash (MTH) is thus defined as follows: 328 The hash of an empty list is the hash of an empty string: 330 MTH({}) = HASH(). 332 The hash of a list with one entry (also known as a leaf hash) is: 334 MTH({d[0]}) = HASH(0x00 || d[0]). 336 For n > 1, let k be the largest power of two smaller than n (i.e., k 337 < n <= 2k). The Merkle Tree Hash of an n-element list D_n is then 338 defined recursively as 340 MTH(D_n) = HASH(0x01 || MTH(D[0:k]) || MTH(D[k:n])), 342 where: 344 * || denotes concatenation 346 * : denotes concatenation of lists 348 * D[k1:k2] = D'_(k2-k1) denotes the list {d'[0] = d[k1], d'[1] = 349 d[k1+1], ..., d'[k2-k1-1] = d[k2-1]} of length (k2 - k1). 351 Note that the hash calculations for leaves and nodes differ; this 352 domain separation is required to give second preimage resistance. 354 Note that we do not require the length of the input list to be a 355 power of two. The resulting Merkle Tree may thus not be balanced; 356 however, its shape is uniquely determined by the number of leaves. 357 (Note: This Merkle Tree is essentially the same as the history tree 358 [CrosbyWallach] proposal, except our definition handles non-full 359 trees differently). 361 2.1.2. Verifying a Tree Head Given Entries 363 When a client has a complete list of "entries" from "0" up to 364 "tree_size - 1" and wishes to verify this list against a tree head 365 "root_hash" returned by the log for the same "tree_size", the 366 following algorithm may be used: 368 1. Set "stack" to an empty stack. 370 2. For each "i" from "0" up to "tree_size - 1": 372 1. Push "HASH(0x00 || entries[i])" to "stack". 374 2. Set "merge_count" to the lowest value ("0" included) such 375 that "LSB(i >> merge_count)" is not set, where "LSB" means 376 the least significant bit. In other words, set "merge_count" 377 to the number of consecutive "1"s found starting at the least 378 significant bit of "i". 380 3. Repeat "merge_count" times: 382 1. Pop "right" from "stack". 384 2. Pop "left" from "stack". 386 3. Push "HASH(0x01 || left || right)" to "stack". 388 3. If there is more than one element in the "stack", repeat the same 389 merge procedure (the sub-items of Step 2.3 above) until only a 390 single element remains. 392 4. The remaining element in "stack" is the Merkle Tree hash for the 393 given "tree_size" and should be compared by equality against the 394 supplied "root_hash". 396 2.1.3. Merkle Inclusion Proofs 398 A Merkle inclusion proof for a leaf in a Merkle Hash Tree is the 399 shortest list of additional nodes in the Merkle Tree required to 400 compute the Merkle Tree Hash for that tree. Each node in the tree is 401 either a leaf node or is computed from the two nodes immediately 402 below it (i.e., towards the leaves). At each step up the tree 403 (towards the root), a node from the inclusion proof is combined with 404 the node computed so far. In other words, the inclusion proof 405 consists of the list of missing nodes required to compute the nodes 406 leading from a leaf to the root of the tree. If the root computed 407 from the inclusion proof matches the true root, then the inclusion 408 proof proves that the leaf exists in the tree. 410 2.1.3.1. Generating an Inclusion Proof 412 Given an ordered list of n inputs to the tree, D_n = {d[0], d[1], 413 ..., d[n-1]}, the Merkle inclusion proof PATH(m, D_n) for the (m+1)th 414 input d[m], 0 <= m < n, is defined as follows: 416 The proof for the single leaf in a tree with a one-element input list 417 D[1] = {d[0]} is empty: 419 PATH(0, {d[0]}) = {} 421 For n > 1, let k be the largest power of two smaller than n. The 422 proof for the (m+1)th element d[m] in a list of n > m elements is 423 then defined recursively as 425 PATH(m, D_n) = PATH(m, D[0:k]) : MTH(D[k:n]) for m < k; and 427 PATH(m, D_n) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for m >= k, 429 The : operator and D[k1:k2] are defined the same as in Section 2.1.1. 431 2.1.3.2. Verifying an Inclusion Proof 433 When a client has received an inclusion proof (e.g., in a "TransItem" 434 of type "inclusion_proof_v2") and wishes to verify inclusion of an 435 input "hash" for a given "tree_size" and "root_hash", the following 436 algorithm may be used to prove the "hash" was included in the 437 "root_hash": 439 1. Compare "leaf_index" from the "inclusion_proof_v2" structure 440 against "tree_size". If "leaf_index" is greater than or equal to 441 "tree_size" then fail the proof verification. 443 2. Set "fn" to "leaf_index" and "sn" to "tree_size - 1". 445 3. Set "r" to "hash". 447 4. For each value "p" in the "inclusion_path" array: 449 If "sn" is 0, stop the iteration and fail the proof verification. 451 If "LSB(fn)" is set, or if "fn" is equal to "sn", then: 453 1. Set "r" to "HASH(0x01 || p || r)" 455 2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn" 456 equally until either "LSB(fn)" is set or "fn" is "0". 458 Otherwise: 460 1. Set "r" to "HASH(0x01 || r || p)" 462 Finally, right-shift both "fn" and "sn" one time. 464 5. Compare "sn" to 0. Compare "r" against the "root_hash". If "sn" 465 is equal to 0, and "r" and the "root_hash" are equal, then the 466 log has proven the inclusion of "hash". Otherwise, fail the 467 proof verification. 469 2.1.4. Merkle Consistency Proofs 471 Merkle consistency proofs prove the append-only property of the tree. 472 A Merkle consistency proof for a Merkle Tree Hash MTH(D_n) and a 473 previously advertised hash MTH(D[0:m]) of the first m leaves, m <= n, 474 is the list of nodes in the Merkle Tree required to verify that the 475 first m inputs D[0:m] are equal in both trees. Thus, a consistency 476 proof must contain a set of intermediate nodes (i.e., commitments to 477 inputs) sufficient to verify MTH(D_n), such that (a subset of) the 478 same nodes can be used to verify MTH(D[0:m]). We define an algorithm 479 that outputs the (unique) minimal consistency proof. 481 2.1.4.1. Generating a Consistency Proof 483 Given an ordered list of n inputs to the tree, D_n = {d[0], d[1], 484 ..., d[n-1]}, the Merkle consistency proof PROOF(m, D_n) for a 485 previous Merkle Tree Hash MTH(D[0:m]), 0 < m < n, is defined as: 487 PROOF(m, D_n) = SUBPROOF(m, D_n, true) 489 In SUBPROOF, the boolean value represents whether the subtree created 490 from D[0:m] is a complete subtree of the Merkle Tree created from 491 D_n, and, consequently, whether the subtree Merkle Tree Hash 492 MTH(D[0:m]) is known. The initial call to SUBPROOF sets this to be 493 true, and SUBPROOF is then defined as follows: 495 The subproof for m = n is empty if m is the value for which PROOF was 496 originally requested (meaning that the subtree created from D[0:m] is 497 a complete subtree of the Merkle Tree created from the original D_n 498 for which PROOF was requested, and the subtree Merkle Tree Hash 499 MTH(D[0:m]) is known): 501 SUBPROOF(m, D_m, true) = {} 503 Otherwise, the subproof for m = n is the Merkle Tree Hash committing 504 inputs D[0:m]: 506 SUBPROOF(m, D_m, false) = {MTH(D_m)} 508 For m < n, let k be the largest power of two smaller than n. The 509 subproof is then defined recursively, using the appropriate step 510 below: 512 If m <= k, the right subtree entries D[k:n] only exist in the current 513 tree. We prove that the left subtree entries D[0:k] are consistent 514 and add a commitment to D[k:n]: 516 SUBPROOF(m, D_n, b) = SUBPROOF(m, D[0:k], b) : MTH(D[k:n]) 517 If m > k, the left subtree entries D[0:k] are identical in both 518 trees. We prove that the right subtree entries D[k:n] are consistent 519 and add a commitment to D[0:k]. 521 SUBPROOF(m, D_n, b) = SUBPROOF(m - k, D[k:n], false) : MTH(D[0:k]) 523 The number of nodes in the resulting proof is bounded above by 524 ceil(log2(n)) + 1. 526 The : operator and D[k1:k2] are defined the same as in Section 2.1.1. 528 2.1.4.2. Verifying Consistency between Two Tree Heads 530 When a client has a tree head "first_hash" for tree size "first", a 531 tree head "second_hash" for tree size "second" where "0 < first < 532 second", and has received a consistency proof between the two (e.g., 533 in a "TransItem" of type "consistency_proof_v2"), the following 534 algorithm may be used to verify the consistency proof: 536 1. If "consistency_path" is an empty array, stop and fail the proof 537 verification. 539 2. If "first" is an exact power of 2, then prepend "first_hash" to 540 the "consistency_path" array. 542 3. Set "fn" to "first - 1" and "sn" to "second - 1". 544 4. If "LSB(fn)" is set, then right-shift both "fn" and "sn" equally 545 until "LSB(fn)" is not set. 547 5. Set both "fr" and "sr" to the first value in the 548 "consistency_path" array. 550 6. For each subsequent value "c" in the "consistency_path" array: 552 If "sn" is 0, stop the iteration and fail the proof verification. 554 If "LSB(fn)" is set, or if "fn" is equal to "sn", then: 556 1. Set "fr" to "HASH(0x01 || c || fr)" 558 Set "sr" to "HASH(0x01 || c || sr)" 560 2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn" 561 equally until either "LSB(fn)" is set or "fn" is "0". 563 Otherwise: 565 1. Set "sr" to "HASH(0x01 || sr || c)" 567 Finally, right-shift both "fn" and "sn" one time. 569 7. After completing iterating through the "consistency_path" array 570 as described above, verify that the "fr" calculated is equal to 571 the "first_hash" supplied, that the "sr" calculated is equal to 572 the "second_hash" supplied and that "sn" is 0. 574 2.1.5. Example 576 The binary Merkle Tree with 7 leaves: 578 hash 579 / \ 580 / \ 581 / \ 582 / \ 583 / \ 584 k l 585 / \ / \ 586 / \ / \ 587 / \ / \ 588 g h i j 589 / \ / \ / \ | 590 a b c d e f d6 591 | | | | | | 592 d0 d1 d2 d3 d4 d5 594 The inclusion proof for d0 is [b, h, l]. 596 The inclusion proof for d3 is [c, g, l]. 598 The inclusion proof for d4 is [f, j, k]. 600 The inclusion proof for d6 is [i, k]. 602 The same tree, built incrementally in four steps: 604 hash0 hash1=k 605 / \ / \ 606 / \ / \ 607 / \ / \ 608 g c g h 609 / \ | / \ / \ 610 a b d2 a b c d 611 | | | | | | 612 d0 d1 d0 d1 d2 d3 614 hash2 hash 615 / \ / \ 616 / \ / \ 617 / \ / \ 618 / \ / \ 619 / \ / \ 620 k i k l 621 / \ / \ / \ / \ 622 / \ e f / \ / \ 623 / \ | | / \ / \ 624 g h d4 d5 g h i j 625 / \ / \ / \ / \ / \ | 626 a b c d a b c d e f d6 627 | | | | | | | | | | 628 d0 d1 d2 d3 d0 d1 d2 d3 d4 d5 630 The consistency proof between hash0 and hash is PROOF(3, D[7]) = [c, 631 d, g, l]. c, g are used to verify hash0, and d, l are additionally 632 used to show hash is consistent with hash0. 634 The consistency proof between hash1 and hash is PROOF(4, D[7]) = [l]. 635 hash can be verified using hash1=k and l. 637 The consistency proof between hash2 and hash is PROOF(6, D[7]) = [i, 638 j, k]. k, i are used to verify hash2, and j is additionally used to 639 show hash is consistent with hash2. 641 2.2. Signatures 643 When signing data structures, a log MUST use one of the signature 644 algorithms from the IANA CT Signature Algorithms registry, described 645 in Section 10.2.2. 647 3. Submitters 649 Submitters submit certificates or preannouncements of certificates 650 prior to issuance (precertificates) to logs for public auditing, as 651 described below. In order to enable attribution of each logged 652 certificate or precertificate to its issuer, each submission MUST be 653 accompanied by all additional certificates required to verify the 654 chain up to an accepted trust anchor (Section 5.7). The trust anchor 655 (a root or intermediate CA certificate) MAY be omitted from the 656 submission. 658 If a log accepts a submission, it will return a Signed Certificate 659 Timestamp (SCT) (see Section 4.8). The submitter SHOULD validate the 660 returned SCT as described in Section 8.1 if they understand its 661 format and they intend to use it directly in a TLS handshake or to 662 construct a certificate. If the submitter does not need the SCT (for 663 example, the certificate is being submitted simply to make it 664 available in the log), it MAY validate the SCT. 666 3.1. Certificates 668 Any entity can submit a certificate (Section 5.1) to a log. Since it 669 is anticipated that TLS clients will reject certificates that are not 670 logged, it is expected that certificate issuers and subjects will be 671 strongly motivated to submit them. 673 3.2. Precertificates 675 CAs may preannounce a certificate prior to issuance by submitting a 676 precertificate (Section 5.1) that the log can use to create an entry 677 that will be valid against the issued certificate. The CA MAY 678 incorporate the returned SCT in the issued certificate. One example 679 of where the returned SCT is not incorporated in the issued 680 certificate is when a CA sends the precertificate to multiple logs, 681 but only incorporates the SCTs that are returned first. 683 A precertificate is a CMS [RFC5652] "signed-data" object that 684 conforms to the following profile: 686 * It MUST be DER encoded as described in [X690]. 688 * "SignedData.version" MUST be v3(3). 690 * "SignedData.digestAlgorithms" MUST be the same as the 691 "SignerInfo.digestAlgorithm" OID value (see below). 693 * "SignedData.encapContentInfo": 695 - "eContentType" MUST be the OID 1.3.101.78. 697 - "eContent" MUST contain a TBSCertificate [RFC5280] that will be 698 identical to the TBSCertificate in the issued certificate, 699 except that the Transparency Information (Section 7.1) 700 extension MUST be omitted. 702 * "SignedData.certificates" MUST be omitted. 704 * "SignedData.crls" MUST be omitted. 706 * "SignedData.signerInfos" MUST contain one "SignerInfo": 708 - "version" MUST be v3(3). 710 - "sid" MUST use the "subjectKeyIdentifier" option. 712 - "digestAlgorithm" MUST be one of the hash algorithm OIDs listed 713 in the IANA CT Hash Algorithms Registry, described in 714 Section 10.2.1. 716 - "signedAttrs" MUST be present and MUST contain two attributes: 718 o A content-type attribute whose value is the same as 719 "SignedData.encapContentInfo.eContentType". 721 o A message-digest attribute whose value is the message digest 722 of "SignedData.encapContentInfo.eContent". 724 - "signatureAlgorithm" MUST be the same OID as 725 "TBSCertificate.signature". 727 - "signature" MUST be from the same (root or intermediate) CA 728 that intends to issue the corresponding certificate (see 729 Section 3.2.1). 731 - "unsignedAttrs" MUST be omitted. 733 "SignerInfo.signedAttrs" is included in the message digest 734 calculation process (see Section 5.4 of [RFC5652]), which ensures 735 that the "SignerInfo.signature" value will not be a valid X.509v3 736 signature that could be used in conjunction with the TBSCertificate 737 (from "SignedData.encapContentInfo.eContent") to construct a valid 738 certificate. 740 3.2.1. Binding Intent to Issue 742 Under normal circumstances, there will be a short delay between 743 precertificate submission and issuance of the corresponding 744 certificate. Longer delays are to be expected occasionally (e.g., 745 due to log server downtime), and in some cases the CA might not 746 actually issue the corresponding certificate. Nevertheless, a 747 precertificate's "signature" indicates the CA's binding intent to 748 issue the corresponding certificate, which means that: 750 * Misissuance of a precertificate is considered equivalent to 751 misissuance of the corresponding certificate. The CA should 752 expect to be held to account, even if the corresponding 753 certificate has not actually been issued. 755 * Upon observing a precertificate, a client can reasonably presume 756 that the corresponding certificate has been issued. A client may 757 wish to obtain status information (e.g., by using the Online 758 Certificate Status Protocol [RFC6960] or by checking a Certificate 759 Revocation List [RFC5280]) about a certificate that is presumed to 760 exist, especially if there is evidence or suspicion that the 761 corresponding precertificate was misissued. 763 * TLS clients may have policies that require CAs to be able to 764 revoke, and to provide certificate status services for, each 765 certificate that is presumed to exist based on the existence of a 766 corresponding precertificate. 768 4. Log Format and Operation 770 A log is a single, append-only Merkle Tree of submitted certificate 771 and precertificate entries. 773 When it receives and accepts a valid submission, the log MUST return 774 an SCT that corresponds to the submitted certificate or 775 precertificate. If the log has previously seen this valid 776 submission, it SHOULD return the same SCT as it returned before, as 777 discussed in Section 11.3. If different SCTs are produced for the 778 same submission, multiple log entries will have to be created, one 779 for each SCT (as the timestamp is a part of the leaf structure). 780 Note that if a certificate was previously logged as a precertificate, 781 then the precertificate's SCT of type "precert_sct_v2" would not be 782 appropriate; instead, a fresh SCT of type "x509_sct_v2" should be 783 generated. 785 An SCT is the log's promise to append to its Merkle Tree an entry for 786 the accepted submission. Upon producing an SCT, the log MUST fulfil 787 this promise by performing the following actions within a fixed 788 amount of time known as the Maximum Merge Delay (MMD), which is one 789 of the log's parameters (see Section 4.1): 791 * Allocate a tree index to the entry representing the accepted 792 submission. 794 * Calculate the root of the tree. 796 * Sign the root of the tree (see Section 4.10). 798 The log may append multiple entries before signing the root of the 799 tree. 801 Log operators SHOULD NOT impose any conditions on retrieving or 802 sharing data from the log. 804 4.1. Log Parameters 806 A log is defined by a collection of immutable parameters, which are 807 used by clients to communicate with the log and to verify log 808 artifacts. Except for the Final Signed Tree Head (STH), each of 809 these parameters MUST be established before the log operator begins 810 to operate the log. 812 Base URL: The prefix used to construct URLs ([RFC3986]) for client 813 messages (see Section 5). The base URL MUST be an "https" URL, 814 MAY contain a port, MAY contain a path with any number of path 815 segments, but MUST NOT contain a query string, fragment, or 816 trailing "/". Example: https://ct.example.org/blue 818 Hash Algorithm: The hash algorithm used for the Merkle Tree (see 819 Section 10.2.1). 821 Signature Algorithm: The signature algorithm used (see Section 2.2). 823 Public Key: The public key used to verify signatures generated by 824 the log. A log MUST NOT use the same keypair as any other log. 826 Log ID: The OID that uniquely identifies the log. 828 Maximum Merge Delay: The MMD the log has committed to. This 829 document deliberately does not specify any limits on the value, to 830 allow for experimentation. 832 Version: The version of the protocol supported by the log (currently 833 1 or 2). 835 Maximum Chain Length: The longest certificate chain submission the 836 log is willing to accept, if the log imposes any limit. 838 STH Frequency Count: The maximum number of STHs the log may produce 839 in any period equal to the "Maximum Merge Delay" (see 840 Section 4.10). 842 Final STH: If a log has been closed down (i.e., no longer accepts 843 new entries), existing entries may still be valid. In this case, 844 the client should know the final valid STH in the log to ensure no 845 new entries can be added without detection. This value MUST be 846 provided in the form of a TransItem of type "signed_tree_head_v2". 847 If a log is still accepting entries, this value should not be 848 provided. 850 [JSON.Metadata] is an example of a metadata format which includes the 851 above elements. 853 4.2. Evaluating Submissions 855 A log determines whether to accept or reject a submission by 856 evaluating it against the minimum acceptance criteria (see 857 Section 4.2.1) and against the log's discretionary acceptance 858 criteria (see Section 4.2.2). 860 If the acceptance criteria are met, the log SHOULD accept the 861 submission. (A log may decide, for example, to temporarily reject 862 acceptable submissions to protect itself against denial-of-service 863 attacks). 865 The log SHALL allow retrieval of its list of accepted trust anchors 866 (see Section 5.7), each of which is a root or intermediate CA 867 certificate. This list might usefully be the union of root 868 certificates trusted by major browser vendors. 870 4.2.1. Minimum Acceptance Criteria 872 To ensure that logged certificates and precertificates are 873 attributable to an accepted trust anchor, and to set clear 874 expectations for what monitors would find in the log, and to avoid 875 being overloaded by invalid submissions, the log MUST reject a 876 submission if any of the following conditions are not met: 878 * The "submission", "type" and "chain" inputs MUST be set as 879 described in Section 5.1. The log MUST NOT accommodate misordered 880 CA certificates or use any other source of intermediate CA 881 certificates to attempt certification path construction. 883 * Each of the zero or more intermediate CA certificates in the chain 884 MUST have one or both of the following features: 886 - The Basic Constraints extension with the cA boolean asserted. 888 - The Key Usage extension with the keyCertSign bit asserted. 890 * Each certificate in the chain MUST fall within the limits imposed 891 by the zero or more Basic Constraints pathLenConstraint values 892 found higher up the chain. 894 * Precertificate submissions MUST conform to all of the requirements 895 in Section 3.2. 897 4.2.2. Discretionary Acceptance Criteria 899 If the minimum acceptance criteria are met but the submission is not 900 fully valid according to [RFC5280] verification rules (e.g., the 901 certificate or precertificate has expired, is not yet valid, has been 902 revoked, exhibits ASN.1 DER encoding errors but the log can still 903 parse it, etc), then the acceptability of the submission is left to 904 the log's discretion. It is useful for logs to accept such 905 submissions in order to accommodate quirks of CA certificate-issuing 906 software and to facilitate monitoring of CA compliance with 907 applicable policies and technical standards. However, it is 908 impractical for this document to enumerate, and for logs to consider, 909 all of the ways that a submission might fail to comply with 910 [RFC5280]. 912 Logs SHOULD limit the length of chain they will accept. The maximum 913 chain length is one of the log's parameters (see Section 4.1). 915 4.3. Log Entries 917 If a submission is accepted and an SCT issued, the accepting log MUST 918 store the entire chain used for verification. This chain MUST 919 include the certificate or precertificate itself, the zero or more 920 intermediate CA certificates provided by the submitter, and the trust 921 anchor used to verify the chain (even if it was omitted from the 922 submission). The log MUST provide this chain for auditing upon 923 request (see Section 5.6) so that the CA cannot avoid blame by 924 logging a partial or empty chain. Each log entry is a "TransItem" 925 structure of type "x509_entry_v2" or "precert_entry_v2". However, a 926 log may store its entries in any format. If a log does not store 927 this "TransItem" in full, it must store the "timestamp" and 928 "sct_extensions" of the corresponding 929 "TimestampedCertificateEntryDataV2" structure. The "TransItem" can 930 be reconstructed from these fields and the entire chain that the log 931 used to verify the submission. 933 4.4. Log ID 935 Each log is identified by an OID, which is one of the log's 936 parameters (see Section 4.1) and which MUST NOT be used to identify 937 any other log. A log's operator MUST either allocate the OID 938 themselves or request an OID from the Log ID Registry (see 939 Section 10.2.5). The only advantage of the registry is that the DER 940 encoding can be small. (Recall that OID allocations do not require a 941 central registration, although logs will most likely want to make 942 themselves known to potential clients through out of band means.) 943 Various data structures include the DER encoding of this OID, 944 excluding the ASN.1 tag and length bytes, in an opaque vector: 946 opaque LogID<2..127>; 948 Note that the ASN.1 length and the opaque vector length are identical 949 in size (1 byte) and value, so the full DER encoding (including the 950 tag and length) of the OID can be reproduced simply by prepending an 951 OBJECT IDENTIFIER tag (0x06) to the opaque vector length and 952 contents. 954 The OID used to identify a log is limited such that the DER encoding 955 of its value, excluding the tag and length, MUST be no longer than 956 127 octets. 958 4.5. TransItem Structure 960 Various data structures are encapsulated in the "TransItem" structure 961 to ensure that the type and version of each one is identified in a 962 common fashion: 964 enum { 965 reserved(0), 966 x509_entry_v2(1), precert_entry_v2(2), 967 x509_sct_v2(3), precert_sct_v2(4), 968 signed_tree_head_v2(5), consistency_proof_v2(6), 969 inclusion_proof_v2(7), 970 (65535) 971 } VersionedTransType; 973 struct { 974 VersionedTransType versioned_type; 975 select (versioned_type) { 976 case x509_entry_v2: TimestampedCertificateEntryDataV2; 977 case precert_entry_v2: TimestampedCertificateEntryDataV2; 978 case x509_sct_v2: SignedCertificateTimestampDataV2; 979 case precert_sct_v2: SignedCertificateTimestampDataV2; 980 case signed_tree_head_v2: SignedTreeHeadDataV2; 981 case consistency_proof_v2: ConsistencyProofDataV2; 982 case inclusion_proof_v2: InclusionProofDataV2; 983 } data; 984 } TransItem; 986 "versioned_type" is a value from the IANA registry in Section 10.2.3 987 that identifies the type of the encapsulated data structure and the 988 earliest version of this protocol to which it conforms. This 989 document is v2. 991 "data" is the encapsulated data structure. The various structures 992 named with the "DataV2" suffix are defined in later sections of this 993 document. 995 Note that "VersionedTransType" combines the v1 [RFC6962] type 996 enumerations "Version", "LogEntryType", "SignatureType" and 997 "MerkleLeafType". Note also that v1 did not define "TransItem", but 998 this document provides guidelines (see Appendix A) on how v2 999 implementations can co-exist with v1 implementations. 1001 Future versions of this protocol may reuse "VersionedTransType" 1002 values defined in this document as long as the corresponding data 1003 structures are not modified, and may add new "VersionedTransType" 1004 values for new or modified data structures. 1006 4.6. Log Artifact Extensions 1007 enum { 1008 reserved(65535) 1009 } ExtensionType; 1011 struct { 1012 ExtensionType extension_type; 1013 opaque extension_data<0..2^16-1>; 1014 } Extension; 1016 The "Extension" structure provides a generic extensibility for log 1017 artifacts, including Signed Certificate Timestamps (Section 4.8) and 1018 Signed Tree Heads (Section 4.10). The interpretation of the 1019 "extension_data" field is determined solely by the value of the 1020 "extension_type" field. 1022 This document does not define any extensions, but it does establish a 1023 registry for future "ExtensionType" values (see Section 10.2.4). 1024 Each document that registers a new "ExtensionType" must specify the 1025 context in which it may be used (e.g., SCT, STH, or both) and 1026 describe how to interpret the corresponding "extension_data". 1028 4.7. Merkle Tree Leaves 1030 The leaves of a log's Merkle Tree correspond to the log's entries 1031 (see Section 4.3). Each leaf is the leaf hash (Section 2.1) of a 1032 "TransItem" structure of type "x509_entry_v2" or "precert_entry_v2", 1033 which encapsulates a "TimestampedCertificateEntryDataV2" structure. 1034 Note that leaf hashes are calculated as HASH(0x00 || TransItem), 1035 where the hash algorithm is one of the log's parameters. 1037 opaque TBSCertificate<1..2^24-1>; 1039 struct { 1040 uint64 timestamp; 1041 opaque issuer_key_hash<32..2^8-1>; 1042 TBSCertificate tbs_certificate; 1043 Extension sct_extensions<0..2^16-1>; 1044 } TimestampedCertificateEntryDataV2; 1046 "timestamp" is the date and time at which the certificate or 1047 precertificate was accepted by the log, in the form of a 64-bit 1048 unsigned number of milliseconds elapsed since the Unix Epoch (1 1049 January 1970 00:00:00 UTC - see [UNIXTIME]), ignoring leap seconds, 1050 in network byte order. Note that the leaves of a log's Merkle Tree 1051 are not required to be in strict chronological order. 1053 "issuer_key_hash" is the HASH of the public key of the CA that issued 1054 the certificate or precertificate, calculated over the DER encoding 1055 of the key represented as SubjectPublicKeyInfo [RFC5280]. This is 1056 needed to bind the CA to the certificate or precertificate, making it 1057 impossible for the corresponding SCT to be valid for any other 1058 certificate or precertificate whose TBSCertificate matches 1059 "tbs_certificate". The length of the "issuer_key_hash" MUST match 1060 HASH_SIZE. 1062 "tbs_certificate" is the DER encoded TBSCertificate from the 1063 submission. (Note that a precertificate's TBSCertificate can be 1064 reconstructed from the corresponding certificate as described in 1065 Section 8.1.2). 1067 "sct_extensions" is byte-for-byte identical to the SCT extensions of 1068 the corresponding SCT. 1070 The type of the "TransItem" corresponds to the value of the "type" 1071 parameter supplied in the Section 5.1 call. 1073 4.8. Signed Certificate Timestamp (SCT) 1075 An SCT is a "TransItem" structure of type "x509_sct_v2" or 1076 "precert_sct_v2", which encapsulates a 1077 "SignedCertificateTimestampDataV2" structure: 1079 struct { 1080 LogID log_id; 1081 uint64 timestamp; 1082 Extension sct_extensions<0..2^16-1>; 1083 opaque signature<1..2^16-1>; 1084 } SignedCertificateTimestampDataV2; 1086 "log_id" is this log's unique ID, encoded in an opaque vector as 1087 described in Section 4.4. 1089 "timestamp" is equal to the timestamp from the corresponding 1090 "TimestampedCertificateEntryDataV2" structure. 1092 "sct_extensions" is a vector of 0 or more SCT extensions. This 1093 vector MUST NOT include more than one extension with the same 1094 "extension_type". The extensions in the vector MUST be ordered by 1095 the value of the "extension_type" field, smallest value first. All 1096 SCT extensions are similar to non-critical X.509v3 extensions (i.e., 1097 the "mustUnderstand" field is not set), and a recipient SHOULD ignore 1098 any extension it does not understand. Furthermore, an implementation 1099 MAY choose to ignore any extension(s) that it does understand. 1101 "signature" is computed over a "TransItem" structure of type 1102 "x509_entry_v2" or "precert_entry_v2" (see Section 4.7) using the 1103 signature algorithm declared in the log's parameters (see 1104 Section 4.1). 1106 4.9. Merkle Tree Head 1108 The log stores information about its Merkle Tree in a 1109 "TreeHeadDataV2": 1111 opaque NodeHash<32..2^8-1>; 1113 struct { 1114 uint64 timestamp; 1115 uint64 tree_size; 1116 NodeHash root_hash; 1117 Extension sth_extensions<0..2^16-1>; 1118 } TreeHeadDataV2; 1120 The length of NodeHash MUST match HASH_SIZE of the log. 1122 "timestamp" is the current date and time, using the format defined in 1123 {tree_leaves}. 1125 "tree_size" is the number of entries currently in the log's Merkle 1126 Tree. 1128 "root_hash" is the root of the Merkle Hash Tree. 1130 "sth_extensions" is a vector of 0 or more STH extensions. This 1131 vector MUST NOT include more than one extension with the same 1132 "extension_type". The extensions in the vector MUST be ordered by 1133 the value of the "extension_type" field, smallest value first. If an 1134 implementation sees an extension that it does not understand, it 1135 SHOULD ignore that extension. Furthermore, an implementation MAY 1136 choose to ignore any extension(s) that it does understand. 1138 4.10. Signed Tree Head (STH) 1140 Periodically each log SHOULD sign its current tree head information 1141 (see Section 4.9) to produce an STH. When a client requests a log's 1142 latest STH (see Section 5.2), the log MUST return an STH that is no 1143 older than the log's MMD. However, since STHs could be used to mark 1144 individual clients (by producing a new STH for each query), a log 1145 MUST NOT produce STHs more frequently than its parameters declare 1146 (see Section 4.1). In general, there is no need to produce a new STH 1147 unless there are new entries in the log; however, in the event that a 1148 log does not accept any submissions during an MMD period, the log 1149 MUST sign the same Merkle Tree Hash with a fresh timestamp. 1151 An STH is a "TransItem" structure of type "signed_tree_head_v2", 1152 which encapsulates a "SignedTreeHeadDataV2" structure: 1154 struct { 1155 LogID log_id; 1156 TreeHeadDataV2 tree_head; 1157 opaque signature<0..2^16-1>; 1158 } SignedTreeHeadDataV2; 1160 "log_id" is this log's unique ID, encoded in an opaque vector as 1161 described in Section 4.4. 1163 The "timestamp" in "tree_head" MUST be at least as recent as the most 1164 recent SCT timestamp in the tree. Each subsequent timestamp MUST be 1165 more recent than the timestamp of the previous update. 1167 "tree_head" contains the latest tree head information (see 1168 Section 4.9). 1170 "signature" is computed over the "tree_head" field using the 1171 signature algorithm declared in the log's parameters (see 1172 Section 4.1). 1174 4.11. Merkle Consistency Proofs 1176 To prepare a Merkle Consistency Proof for distribution to clients, 1177 the log produces a "TransItem" structure of type 1178 "consistency_proof_v2", which encapsulates a "ConsistencyProofDataV2" 1179 structure: 1181 struct { 1182 LogID log_id; 1183 uint64 tree_size_1; 1184 uint64 tree_size_2; 1185 NodeHash consistency_path<0..2^16-1>; 1186 } ConsistencyProofDataV2; 1188 "log_id" is this log's unique ID, encoded in an opaque vector as 1189 described in Section 4.4. 1191 "tree_size_1" is the size of the older tree. 1193 "tree_size_2" is the size of the newer tree. 1195 "consistency_path" is a vector of Merkle Tree nodes proving the 1196 consistency of two STHs as described in {consistency}. 1198 4.12. Merkle Inclusion Proofs 1200 To prepare a Merkle Inclusion Proof for distribution to clients, the 1201 log produces a "TransItem" structure of type "inclusion_proof_v2", 1202 which encapsulates an "InclusionProofDataV2" structure: 1204 struct { 1205 LogID log_id; 1206 uint64 tree_size; 1207 uint64 leaf_index; 1208 NodeHash inclusion_path<0..2^16-1>; 1209 } InclusionProofDataV2; 1211 "log_id" is this log's unique ID, encoded in an opaque vector as 1212 described in Section 4.4. 1214 "tree_size" is the size of the tree on which this inclusion proof is 1215 based. 1217 "leaf_index" is the 0-based index of the log entry corresponding to 1218 this inclusion proof. 1220 "inclusion_path" is a vector of Merkle Tree nodes proving the 1221 inclusion of the chosen certificate or precertificate as described in 1222 {merkle_inclusion_proof}. 1224 4.13. Shutting down a log 1226 Log operators may decide to shut down a log for various reasons, such 1227 as deprecation of the signature algorithm. If there are entries in 1228 the log for certificates that have not yet expired, simply making TLS 1229 clients stop recognizing that log will have the effect of 1230 invalidating SCTs from that log. In order to avoid that, the 1231 following actions SHOULD be taken: 1233 * Make it known to clients and monitors that the log will be frozen. 1234 This is not part of the API, so it will have to be done via a 1235 relevant out-of-band mechanism. 1237 * Stop accepting new submissions (the error code "shutdown" should 1238 be returned for such requests). 1240 * Once MMD from the last accepted submission has passed and all 1241 pending submissions are incorporated, issue a final STH and 1242 publish it as one of the log's parameters. Having an STH with a 1243 timestamp that is after the MMD has passed from the last SCT 1244 issuance allows clients to audit this log regularly without 1245 special handling for the final STH. At this point the log's 1246 private key is no longer needed and can be destroyed. 1248 * Keep the log running until the certificates in all of its entries 1249 have expired or exist in other logs (this can be determined by 1250 scanning other logs or connecting to domains mentioned in the 1251 certificates and inspecting the SCTs served). 1253 5. Log Client Messages 1255 Messages are sent as HTTPS GET or POST requests. Parameters for 1256 POSTs and all responses are encoded as JavaScript Object Notation 1257 (JSON) objects [RFC8259]. Parameters for GETs are encoded as order- 1258 independent key/value URL parameters, using the "application/x-www- 1259 form-urlencoded" format described in the "HTML 4.01 Specification" 1260 [HTML401]. Binary data is base64 encoded according to section 4 of 1261 [RFC4648] as specified in the individual messages. 1263 Clients are configured with a log's base URL, which is one of the 1264 log's parameters. Clients construct URLs for requests by appending 1265 suffixes to this base URL. This structure places some degree of 1266 restriction on how log operators can deploy these services, as noted 1267 in [RFC8820]. However, operational experience with version 1 of this 1268 protocol has not indicated that these restrictions are a problem in 1269 practice. 1271 Note that JSON objects and URL parameters may contain fields not 1272 specified here, to allow for experimentation. Any fields that are 1273 not understood SHOULD be ignored. 1275 In practice, log servers may include multiple front-end machines. 1276 Since it is impractical to keep these machines in perfect sync, 1277 errors may occur that are caused by skew between the machines. Where 1278 such errors are possible, the front-end will return additional 1279 information (as specified below) making it possible for clients to 1280 make progress, if progress is possible. Front-ends MUST only serve 1281 data that is free of gaps (that is, for example, no front-end will 1282 respond with an STH unless it is also able to prove consistency from 1283 all log entries logged within that STH). 1285 For example, when a consistency proof between two STHs is requested, 1286 the front-end reached may not yet be aware of one or both STHs. In 1287 the case where it is unaware of both, it will return the latest STH 1288 it is aware of. Where it is aware of the first but not the second, 1289 it will return the latest STH it is aware of and a consistency proof 1290 from the first STH to the returned STH. The case where it knows the 1291 second but not the first should not arise (see the "no gaps" 1292 requirement above). 1294 If the log is unable to process a client's request, it MUST return an 1295 HTTP response code of 4xx/5xx (see [RFC7231]), and, in place of the 1296 responses outlined in the subsections below, the body SHOULD be a 1297 JSON Problem Details Object (see [RFC7807] Section 3), containing: 1299 type: A URN reference identifying the problem. To facilitate 1300 automated response to errors, this document defines a set of 1301 standard tokens for use in the "type" field, within the URN 1302 namespace of: "urn:ietf:params:trans:error:". 1304 detail: A human-readable string describing the error that prevented 1305 the log from processing the request, ideally with sufficient 1306 detail to enable the error to be rectified. 1308 e.g., In response to a request of "/ct/v2/get- 1309 entries?start=100&end=99", the log would return a "400 Bad Request" 1310 response code with a body similar to the following: 1312 { 1313 "type": "urn:ietf:params:trans:error:endBeforeStart", 1314 "detail": "'start' cannot be greater than 'end'" 1315 } 1317 Most error types are specific to the type of request and are defined 1318 in the respective subsections below. The one exception is the 1319 "malformed" error type, which indicates that the log server could not 1320 parse the client's request because it did not comply with this 1321 document: 1323 +===========+==================================+ 1324 | type | detail | 1325 +===========+==================================+ 1326 | malformed | The request could not be parsed. | 1327 +-----------+----------------------------------+ 1329 Table 1 1331 Clients SHOULD treat "500 Internal Server Error" and "503 Service 1332 Unavailable" responses as transient failures and MAY retry the same 1333 request without modification at a later date. Note that as per 1334 [RFC7231], in the case of a 503 response the log MAY include a 1335 "Retry-After:" header in order to request a minimum time for the 1336 client to wait before retrying the request. In the absence of this 1337 header, this document does not specify a minimum. 1339 Clients SHOULD treat any 4xx error as a problem with the request and 1340 not attempt to resubmit without some modification to the request. 1341 The full status code MAY provide additional details. 1343 This document deliberately does not provide more specific guidance on 1344 the use of HTTP status codes. 1346 5.1. Submit Entry to Log 1348 POST /ct/v2/submit-entry 1350 Inputs: submission: The base64 encoded certificate or 1351 precertificate. 1353 type: The "VersionedTransType" integer value that indicates 1354 the type of the "submission": 1 for "x509_entry_v2", or 2 for 1355 "precert_entry_v2". 1357 chain: An array of zero or more JSON strings, each of which 1358 is a base64 encoded CA certificate. The first element is the 1359 certifier of the "submission"; the second certifies the first; 1360 etc. The last element of "chain" (or, if "chain" is an empty 1361 array, the "submission") is certified by an accepted trust 1362 anchor. 1364 Outputs: sct: A base64 encoded "TransItem" of type "x509_sct_v2" or 1365 "precert_sct_v2", signed by this log, that corresponds to the 1366 "submission". 1368 If the submitted entry is immediately appended to (or already 1369 exists in) this log's tree, then the log SHOULD also output: 1371 sth: A base64 encoded "TransItem" of type "signed_tree_head_v2", 1372 signed by this log. 1374 inclusion: A base64 encoded "TransItem" of type 1375 "inclusion_proof_v2" whose "inclusion_path" array of Merkle 1376 Tree nodes proves the inclusion of the "submission" in the 1377 returned "sth". 1379 Error codes: 1381 +================+==============================================+ 1382 | type | detail | 1383 +================+==============================================+ 1384 | badSubmission | "submission" is neither a valid certificate | 1385 | | nor a valid precertificate. | 1386 +----------------+----------------------------------------------+ 1387 | badType | "type" is neither 1 nor 2. | 1388 +----------------+----------------------------------------------+ 1389 | badChain | The first element of "chain" is not the | 1390 | | certifier of the "submission", or the second | 1391 | | element does not certify the first, etc. | 1392 +----------------+----------------------------------------------+ 1393 | badCertificate | One or more certificates in the "chain" are | 1394 | | not valid (e.g., not properly encoded). | 1395 +----------------+----------------------------------------------+ 1396 | unknownAnchor | The last element of "chain" (or, if "chain" | 1397 | | is an empty array, the "submission") both is | 1398 | | not, and is not certified by, an accepted | 1399 | | trust anchor. | 1400 +----------------+----------------------------------------------+ 1401 | shutdown | The log is no longer accepting submissions. | 1402 +----------------+----------------------------------------------+ 1404 Table 2 1406 If the version of "sct" is not v2, then a v2 client may be unable to 1407 verify the signature. It MUST NOT construe this as an error. This 1408 is to avoid forcing an upgrade of compliant v2 clients that do not 1409 use the returned SCTs. 1411 If a log detects bad encoding in a chain that otherwise verifies 1412 correctly then the log MUST either log the certificate or return the 1413 "bad certificate" error. If the certificate is logged, an SCT MUST 1414 be issued. Logging the certificate is useful, because monitors 1415 (Section 8.2) can then detect these encoding errors, which may be 1416 accepted by some TLS clients. 1418 If "submission" is an accepted trust anchor whose certifier is 1419 neither an accepted trust anchor nor the first element of "chain", 1420 then the log MUST return the "unknown anchor" error. A log is not 1421 able to generate an SCT for a submission if it does not have access 1422 to the issuer's public key. 1424 If the returned "sct" is intended to be provided to TLS clients, then 1425 "sth" and "inclusion" (if returned) SHOULD also be provided to TLS 1426 clients. For example, if "type" was 2 (indicating "precert_sct_v2") 1427 then all three "TransItem"s could be embedded in the certificate. 1429 5.2. Retrieve Latest Signed Tree Head 1431 GET /ct/v2/get-sth 1433 No inputs. 1435 Outputs: sth: A base64 encoded "TransItem" of type 1436 "signed_tree_head_v2", signed by this log, that is no older 1437 than the log's MMD. 1439 5.3. Retrieve Merkle Consistency Proof between Two Signed Tree Heads 1441 GET /ct/v2/get-sth-consistency 1443 Inputs: first: The tree_size of the older tree, in decimal. 1445 second: The tree_size of the newer tree, in decimal 1446 (optional). 1448 Both tree sizes must be from existing v2 STHs. However, because 1449 of skew, the receiving front-end may not know one or both of the 1450 existing STHs. If both are known, then only the "consistency" 1451 output is returned. If the first is known but the second is not 1452 (or has been omitted), then the latest known STH is returned, 1453 along with a consistency proof between the first STH and the 1454 latest. If neither are known, then the latest known STH is 1455 returned without a consistency proof. 1457 Outputs: consistency: A base64 encoded "TransItem" of type 1458 "consistency_proof_v2", whose "tree_size_1" MUST match the 1459 "first" input. If the "sth" output is omitted, then 1460 "tree_size_2" MUST match the "second" input. If "first" and 1461 "second" are equal and correspond to a known STH, the returned 1462 consistency proof MUST be empty (a "consistency_path" array 1463 with zero elements). 1465 sth: A base64 encoded "TransItem" of type 1466 "signed_tree_head_v2", signed by this log. 1468 Note that no signature is required for the "consistency" output as 1469 it is used to verify the consistency between two STHs, which are 1470 signed. 1472 Error codes: 1474 +===================+======================================+ 1475 | type | detail | 1476 +===================+======================================+ 1477 | firstUnknown | "first" is before the latest known | 1478 | | STH but is not from an existing STH. | 1479 +-------------------+--------------------------------------+ 1480 | secondUnknown | "second" is before the latest known | 1481 | | STH but is not from an existing STH. | 1482 +-------------------+--------------------------------------+ 1483 | secondBeforeFirst | "second" is smaller than "first". | 1484 +-------------------+--------------------------------------+ 1486 Table 3 1488 See Section 2.1.4.2 for an outline of how to use the "consistency" 1489 output. 1491 5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash 1493 GET /ct/v2/get-proof-by-hash 1495 Inputs: hash: A base64 encoded v2 leaf hash. 1497 tree_size: The tree_size of the tree on which to base the 1498 proof, in decimal. 1500 The "hash" must be calculated as defined in Section 4.7. A v2 STH 1501 must exist for the "tree_size". Because of skew, the front-end 1502 may not know the requested tree head. In that case, it will 1503 return the latest STH it knows, along with an inclusion proof to 1504 that STH. If the front-end knows the requested tree head then 1505 only "inclusion" is returned. 1507 Outputs: inclusion: A base64 encoded "TransItem" of type 1508 "inclusion_proof_v2" whose "inclusion_path" array of Merkle 1509 Tree nodes proves the inclusion of the certificate (as 1510 specified by the "hash" parameter) in the selected STH. 1512 sth: A base64 encoded "TransItem" of type 1513 "signed_tree_head_v2", signed by this log. 1515 Note that no signature is required for the "inclusion" output as 1516 it is used to verify inclusion in the selected STH, which is 1517 signed. 1519 Error codes: 1521 +=================+=====================================+ 1522 | type | detail | 1523 +=================+=====================================+ 1524 | hashUnknown | "hash" is not the hash of a known | 1525 | | leaf (may be caused by skew or by a | 1526 | | known certificate not yet merged). | 1527 +-----------------+-------------------------------------+ 1528 | treeSizeUnknown | "hash" is before the latest known | 1529 | | STH but is not from an existing | 1530 | | STH. | 1531 +-----------------+-------------------------------------+ 1533 Table 4 1535 See Section 2.1.3.2 for an outline of how to use the "inclusion" 1536 output. 1538 5.5. Retrieve Merkle Inclusion Proof, Signed Tree Head and Consistency 1539 Proof by Leaf Hash 1541 GET /ct/v2/get-all-by-hash 1543 Inputs: hash: A base64 encoded v2 leaf hash. 1545 tree_size: The tree_size of the tree on which to base the 1546 proofs, in decimal. 1548 The "hash" must be calculated as defined in Section 4.7. A v2 STH 1549 must exist for the "tree_size". 1551 Because of skew, the front-end may not know the requested tree head 1552 or the requested hash, which leads to a number of cases: 1554 +=====================+=====================================+ 1555 | Case | Response | 1556 +=====================+=====================================+ 1557 | latest STH < | Return latest STH | 1558 | requested tree head | | 1559 +---------------------+-------------------------------------+ 1560 | latest STH > | Return latest STH and a consistency | 1561 | requested tree head | proof between it and the requested | 1562 | | tree head (see Section 5.3) | 1563 +---------------------+-------------------------------------+ 1564 | index of requested | Return "inclusion" | 1565 | hash < latest STH | | 1566 +---------------------+-------------------------------------+ 1568 Table 5 1570 Note that more than one case can be true, in which case the returned 1571 data is their union. It is also possible for none to be true, in 1572 which case the front-end MUST return an empty response. 1574 Outputs: inclusion: A base64 encoded "TransItem" of type 1575 "inclusion_proof_v2" whose "inclusion_path" array of Merkle 1576 Tree nodes proves the inclusion of the certificate (as 1577 specified by the "hash" parameter) in the selected STH. 1579 sth: A base64 encoded "TransItem" of type 1580 "signed_tree_head_v2", signed by this log. 1582 consistency: A base64 encoded "TransItem" of type 1583 "consistency_proof_v2" that proves the consistency of the 1584 requested tree head and the returned STH. 1586 Note that no signature is required for the "inclusion" or 1587 "consistency" outputs as they are used to verify inclusion in and 1588 consistency of STHs, which are signed. 1590 Errors are the same as in Section 5.4. 1592 See Section 2.1.3.2 for an outline of how to use the "inclusion" 1593 output, and see Section 2.1.4.2 for an outline of how to use the 1594 "consistency" output. 1596 5.6. Retrieve Entries and STH from Log 1598 GET /ct/v2/get-entries 1600 Inputs: start: 0-based index of first entry to retrieve, in 1601 decimal. 1603 end: 0-based index of last entry to retrieve, in decimal. 1605 Outputs: entries: An array of objects, each consisting of 1607 log_entry: The base64 encoded "TransItem" structure of type 1608 "x509_entry_v2" or "precert_entry_v2" (see Section 4.3). 1610 submitted_entry: JSON object equivalent to inputs that were 1611 submitted to "submit-entry", with the addition of the trust 1612 anchor to the "chain" field if the submission did not 1613 include it. 1615 sct: The base64 encoded "TransItem" of type "x509_sct_v2" or 1616 "precert_sct_v2" corresponding to this log entry. 1618 sth: A base64 encoded "TransItem" of type 1619 "signed_tree_head_v2", signed by this log. 1621 Note that this message is not signed -- the "entries" data can be 1622 verified by constructing the Merkle Tree Hash corresponding to a 1623 retrieved STH. All leaves MUST be v2. However, a compliant v2 1624 client MUST NOT construe an unrecognized TransItem type as an error. 1625 This means it may be unable to parse some entries, but note that each 1626 client can inspect the entries it does recognize as well as verify 1627 the integrity of the data by treating unrecognized leaves as opaque 1628 input to the tree. 1630 The "start" and "end" parameters SHOULD be within the range 0 <= x < 1631 "tree_size" as returned by "get-sth" in Section 5.2. 1633 The "start" parameter MUST be less than or equal to the "end" 1634 parameter. 1636 Each "submitted_entry" output parameter MUST include the trust anchor 1637 that the log used to verify the "submission", even if that trust 1638 anchor was not provided to "submit-entry" (see Section 5.1). If the 1639 "submission" does not certify itself, then the first element of 1640 "chain" MUST be present and MUST certify the "submission". 1642 Log servers MUST honor requests where 0 <= "start" < "tree_size" and 1643 "end" >= "tree_size" by returning a partial response covering only 1644 the valid entries in the specified range. "end" >= "tree_size" could 1645 be caused by skew. Note that the following restriction may also 1646 apply: 1648 Logs MAY restrict the number of entries that can be retrieved per 1649 "get-entries" request. If a client requests more than the permitted 1650 number of entries, the log SHALL return the maximum number of entries 1651 permissible. These entries SHALL be sequential beginning with the 1652 entry specified by "start". Note that limit on the number of entries 1653 is not immutable and therefore the restriction may be changed or 1654 lifted at any time and is not listed with the other Log Parameters in 1655 Section 4.1. 1657 Because of skew, it is possible the log server will not have any 1658 entries between "start" and "end". In this case it MUST return an 1659 empty "entries" array. 1661 In any case, the log server MUST return the latest STH it knows 1662 about. 1664 See Section 2.1.2 for an outline of how to use a complete list of 1665 "log_entry" entries to verify the "root_hash". 1667 Error codes: 1669 +================+====================================+ 1670 | type | detail | 1671 +================+====================================+ 1672 | startUnknown | "start" is greater than the number | 1673 | | of entries in the Merkle tree. | 1674 +----------------+------------------------------------+ 1675 | endBeforeStart | "start" cannot be greater than | 1676 | | "end". | 1677 +----------------+------------------------------------+ 1679 Table 6 1681 5.7. Retrieve Accepted Trust Anchors 1683 GET /ct/v2/get-anchors 1685 No inputs. 1687 Outputs: certificates: An array of JSON strings, each of which is a 1688 base64 encoded CA certificate that is acceptable to the log. 1689 max_chain_length: 1691 If the server has chosen to limit the length of chains it 1692 accepts, this is the maximum number of certificates in the 1693 chain, in decimal. If there is no limit, this is omitted. 1695 This data is not signed and the protocol depends on the security 1696 guarantees of TLS to ensure correctness. 1698 6. TLS Servers 1700 CT-using TLS servers MUST use at least one of the mechanisms 1701 described below to present one or more SCTs from one or more logs to 1702 each TLS client during full TLS handshakes, where each SCT 1703 corresponds to the server certificate. (Of course, a server can only 1704 send a TLS extension if the client has specified it first.) Servers 1705 SHOULD also present corresponding inclusion proofs and STHs. 1707 A server can provide SCTs using a TLS 1.3 extension (Section 4.2 of 1708 [RFC8446]) with type "transparency_info" (see Section 6.5). This 1709 mechanism allows TLS servers to participate in CT without the 1710 cooperation of CAs, unlike the other two mechanisms. It also allows 1711 SCTs and inclusion proofs to be updated on the fly. 1713 The server may also use an Online Certificate Status Protocol (OCSP) 1714 [RFC6960] response extension (see Section 7.1.1), providing the OCSP 1715 response as part of the TLS handshake. Providing a response during a 1716 TLS handshake is popularly known as "OCSP stapling." For TLS 1.3, 1717 the information is encoded as an extension in the "status_request" 1718 extension data; see Section 4.4.2.1 of [RFC8446]. For TLS 1.2 1719 ([RFC5246]), the information is encoded as an extension in the 1720 "CertificateStatus" message; see Section 8 of [RFC6066]. Using 1721 stapling also allows SCTs and inclusion proofs to be updated on the 1722 fly. 1724 CT information can also be encoded as an extension in the X.509v3 1725 certificate (see Section 7.1.2). This mechanism allows the use of 1726 unmodified TLS servers, but the SCTs and inclusion proofs cannot be 1727 updated on the fly. Since the logs from which the SCTs and inclusion 1728 proofs originated won't necessarily be accepted by TLS clients for 1729 the full lifetime of the certificate, there is a risk that TLS 1730 clients may subsequently consider the certificate to be non-compliant 1731 and in need of re-issuance or the use of one of the other two methods 1732 for delivering CT information. 1734 6.1. TLS Client Authentication 1736 This specification includes no description of how a TLS server can 1737 use CT for TLS client certificates. While this may be useful, it is 1738 not documented here for the following reasons: 1740 * The greater security exposure is for clients to end up interacting 1741 with an illegitimate server. 1743 * In general, TLS client certificates are not expected to be 1744 submitted to CT logs, particularly those intended for general 1745 public use. 1747 A future version could include such information. 1749 6.2. Multiple SCTs 1751 CT-using TLS servers SHOULD send SCTs from multiple logs, because: 1753 * One or more logs may not have become acceptable to all CT-using 1754 TLS clients. Note that client discovery, trust, and distrust of 1755 logs is expected to be handled out-of-band and is out of scope of 1756 this document. 1758 * If a CA and a log collude, it is possible to temporarily hide 1759 misissuance from clients. When a TLS client requires SCTs from 1760 multiple logs to be provided, it is more difficult to mount this 1761 attack. 1763 * If a log misbehaves or suffers a key compromise, a consequence may 1764 be that clients cease to trust it. Since the time an SCT may be 1765 in use can be considerable (several years is common in current 1766 practice when embedded in a certificate), including SCTs from 1767 multiple logs reduces the probability of the certificate being 1768 rejected by TLS clients. 1770 * TLS clients may have policies related to the above risks requiring 1771 TLS servers to present multiple SCTs. For example, at the time of 1772 writing, Chromium [Chromium.Log.Policy] requires multiple SCTs to 1773 be presented with EV certificates in order for the EV indicator to 1774 be shown. 1776 To select the logs from which to obtain SCTs, a TLS server can, for 1777 example, examine the set of logs popular TLS clients accept and 1778 recognize. 1780 6.3. TransItemList Structure 1782 Multiple SCTs, inclusion proofs, and indeed "TransItem" structures of 1783 any type, are combined into a list as follows: 1785 opaque SerializedTransItem<1..2^16-1>; 1787 struct { 1788 SerializedTransItem trans_item_list<1..2^16-1>; 1789 } TransItemList; 1791 Here, "SerializedTransItem" is an opaque byte string that contains 1792 the serialized "TransItem" structure. This encoding ensures that TLS 1793 clients can decode each "TransItem" individually (so, for example, if 1794 there is a version upgrade, out-of-date clients can still parse old 1795 "TransItem" structures while skipping over new "TransItem" structures 1796 whose versions they don't understand). 1798 6.4. Presenting SCTs, inclusions proofs and STHs 1800 In each "TransItemList" that is sent during a TLS handshake, the TLS 1801 server MUST include a "TransItem" structure of type "x509_sct_v2" or 1802 "precert_sct_v2". 1804 Presenting inclusion proofs and STHs in the TLS handshake helps to 1805 protect the client's privacy (see Section 8.1.4) and reduces load on 1806 log servers. Therefore, if the TLS server can obtain them, it SHOULD 1807 also include "TransItem"s of type "inclusion_proof_v2" and 1808 "signed_tree_head_v2" in the "TransItemList". 1810 6.5. transparency_info TLS Extension 1812 Provided that a TLS client includes the "transparency_info" extension 1813 type in the ClientHello and the TLS server supports the 1814 "transparency_info" extension: 1816 * The TLS server MUST verify that the received "extension_data" is 1817 empty. 1819 * The TLS server MUST construct a "TransItemList" of relevant 1820 "TransItem"s (see Section 6.4), which SHOULD omit any "TransItem"s 1821 that are already embedded in the server certificate or the stapled 1822 OCSP response (see Section 7.1). If the constructed 1823 "TransItemList" is not empty, then the TLS server MUST include the 1824 "transparency_info" extension with the "extension_data" set to 1825 this "TransItemList". If the list is empty then the server SHOULD 1826 omit the "extension_data" element, but MAY send it with an empty 1827 array. 1829 TLS servers MUST only include this extension in the following 1830 messages: 1832 * the ServerHello message (for TLS 1.2 or earlier). 1834 * the Certificate or CertificateRequest message (for TLS 1.3). 1836 TLS servers MUST NOT process or include this extension when a TLS 1837 session is resumed, since session resumption uses the original 1838 session information. 1840 7. Certification Authorities 1842 7.1. Transparency Information X.509v3 Extension 1844 The Transparency Information X.509v3 extension, which has OID 1845 1.3.101.75 and SHOULD be non-critical, contains one or more 1846 "TransItem" structures in a "TransItemList". This extension MAY be 1847 included in OCSP responses (see Section 7.1.1) and certificates (see 1848 Section 7.1.2). Since RFC5280 requires the "extnValue" field (an 1849 OCTET STRING) of each X.509v3 extension to include the DER encoding 1850 of an ASN.1 value, a "TransItemList" MUST NOT be included directly. 1851 Instead, it MUST be wrapped inside an additional OCTET STRING, which 1852 is then put into the "extnValue" field: 1854 TransparencyInformationSyntax ::= OCTET STRING 1856 "TransparencyInformationSyntax" contains a "TransItemList". 1858 7.1.1. OCSP Response Extension 1860 A certification authority MAY include a Transparency Information 1861 X.509v3 extension in the "singleExtensions" of a "SingleResponse" in 1862 an OCSP response. All included SCTs and inclusion proofs MUST be for 1863 the certificate identified by the "certID" of that "SingleResponse", 1864 or for a precertificate that corresponds to that certificate. 1866 7.1.2. Certificate Extension 1868 A certification authority MAY include a Transparency Information 1869 X.509v3 extension in a certificate. All included SCTs and inclusion 1870 proofs MUST be for a precertificate that corresponds to this 1871 certificate. 1873 7.2. TLS Feature X.509v3 Extension 1875 A certification authority SHOULD NOT issue any certificate that 1876 identifies the "transparency_info" TLS extension in a TLS feature 1877 extension [RFC7633], because TLS servers are not required to support 1878 the "transparency_info" TLS extension in order to participate in CT 1879 (see Section 6). 1881 8. Clients 1883 There are various different functions clients of logs might perform. 1884 We describe here some typical clients and how they should function. 1885 Any inconsistency may be used as evidence that a log has not behaved 1886 correctly, and the signatures on the data structures prevent the log 1887 from denying that misbehavior. 1889 All clients need various parameters in order to communicate with logs 1890 and verify their responses. These parameters are described in 1891 Section 4.1, but note that this document does not describe how the 1892 parameters are obtained, which is implementation-dependent (see, for 1893 example, [Chromium.Policy]). 1895 8.1. TLS Client 1897 8.1.1. Receiving SCTs and inclusion proofs 1899 TLS clients receive SCTs and inclusion proofs alongside or in 1900 certificates. CT-using TLS clients MUST implement all of the three 1901 mechanisms by which TLS servers may present SCTs (see Section 6). 1903 TLS clients that support the "transparency_info" TLS extension (see 1904 Section 6.5) SHOULD include it in ClientHello messages, with empty 1905 "extension_data". If a TLS server includes the "transparency_info" 1906 TLS extension when resuming a TLS session, the TLS client MUST abort 1907 the handshake. 1909 8.1.2. Reconstructing the TBSCertificate 1911 Validation of an SCT for a certificate (where the "type" of the 1912 "TransItem" is "x509_sct_v2") uses the unmodified TBSCertificate 1913 component of the certificate. 1915 Before an SCT for a precertificate (where the "type" of the 1916 "TransItem" is "precert_sct_v2") can be validated, the TBSCertificate 1917 component of the precertificate needs to be reconstructed from the 1918 TBSCertificate component of the certificate as follows: 1920 * Remove the Transparency Information extension (see Section 7.1). 1922 * Remove embedded v1 SCTs, identified by OID 1.3.6.1.4.1.11129.2.4.2 1923 (see section 3.3 of [RFC6962]). This allows embedded v1 and v2 1924 SCTs to co-exist in a certificate (see Appendix A). 1926 8.1.3. Validating SCTs 1928 In order to make use of a received SCT, the TLS client MUST first 1929 validate it as follows: 1931 * Compute the signature input by constructing a "TransItem" of type 1932 "x509_entry_v2" or "precert_entry_v2", depending on the SCT's 1933 "TransItem" type. The "TimestampedCertificateEntryDataV2" 1934 structure is constructed in the following manner: 1936 - "timestamp" is copied from the SCT. 1938 - "tbs_certificate" is the reconstructed TBSCertificate portion 1939 of the server certificate, as described in Section 8.1.2. 1941 - "issuer_key_hash" is computed as described in Section 4.7. 1943 - "sct_extensions" is copied from the SCT. 1945 * Verify the SCT's "signature" against the computed signature input 1946 using the public key of the corresponding log, which is identified 1947 by the "log_id". The required signature algorithm is one of the 1948 log's parameters. 1950 If the TLS client does not have the corresponding log's parameters, 1951 it cannot attempt to validate the SCT. When evaluating compliance 1952 (see Section 8.1.6), the TLS client will consider only those SCTs 1953 that it was able to validate. 1955 Note that SCT validation is not a substitute for the normal 1956 validation of the server certificate and its chain. 1958 8.1.4. Fetching inclusion proofs 1960 When a TLS client has validated a received SCT but does not yet 1961 possess a corresponding inclusion proof, the TLS client MAY request 1962 the inclusion proof directly from a log using "get-proof-by-hash" 1963 (Section 5.4) or "get-all-by-hash" (Section 5.5). 1965 Note that fetching inclusion proofs directly from a log will disclose 1966 to the log which TLS server the client has been communicating with. 1967 This may be regarded as a significant privacy concern, and so it is 1968 preferable for the TLS server to send the inclusion proofs (see 1969 Section 6.4). 1971 8.1.5. Validating inclusion proofs 1973 When a TLS client has received, or fetched, an inclusion proof (and 1974 an STH), it SHOULD proceed to verifying the inclusion proof to the 1975 provided STH. The TLS client SHOULD also verify consistency between 1976 the provided STH and an STH it knows about. 1978 If the TLS client holds an STH that predates the SCT, it MAY, in the 1979 process of auditing, request a new STH from the log (Section 5.2), 1980 then verify it by requesting a consistency proof (Section 5.3). Note 1981 that if the TLS client uses "get-all-by-hash", then it will already 1982 have the new STH. 1984 8.1.6. Evaluating compliance 1986 It is up to a client's local policy to specify the quantity and form 1987 of evidence (SCTs, inclusion proofs or a combination) needed to 1988 achieve compliance and how to handle non-compliance. 1990 A TLS client can only evaluate compliance if it has given the TLS 1991 server the opportunity to send SCTs and inclusion proofs by any of 1992 the three mechanisms that are mandatory to implement for CT-using TLS 1993 clients (see Section 8.1.1). Therefore, a TLS client MUST NOT 1994 evaluate compliance if it did not include both the 1995 "transparency_info" and "status_request" TLS extensions in the 1996 ClientHello. 1998 8.2. Monitor 2000 Monitors watch logs to check that they behave correctly, for 2001 certificates of interest, or both. For example, a monitor may be 2002 configured to report on all certificates that apply to a specific 2003 domain name when fetching new entries for consistency validation. 2005 A monitor MUST at least inspect every new entry in every log it 2006 watches, and it MAY also choose to keep copies of entire logs. 2008 To inspect all of the existing entries, the monitor SHOULD follow 2009 these steps once for each log: 2011 1. Fetch the current STH (Section 5.2). 2013 2. Verify the STH signature. 2015 3. Fetch all the entries in the tree corresponding to the STH 2016 (Section 5.6). 2018 4. If applicable, check each entry to see if it's a certificate of 2019 interest. 2021 5. Confirm that the tree made from the fetched entries produces the 2022 same hash as that in the STH. 2024 To inspect new entries, the monitor SHOULD follow these steps 2025 repeatedly for each log: 2027 1. Fetch the current STH (Section 5.2). Repeat until the STH 2028 changes. This document does not specify the polling frequency, 2029 to allow for experimentation. 2031 2. Verify the STH signature. 2033 3. Fetch all the new entries in the tree corresponding to the STH 2034 (Section 5.6). If they remain unavailable for an extended 2035 period, then this should be viewed as misbehavior on the part of 2036 the log. 2038 4. If applicable, check each entry to see if it's a certificate of 2039 interest. 2041 5. Either: 2043 1. Verify that the updated list of all entries generates a tree 2044 with the same hash as the new STH. 2046 Or, if it is not keeping all log entries: 2048 1. Fetch a consistency proof for the new STH with the previous 2049 STH (Section 5.3). 2051 2. Verify the consistency proof. 2053 3. Verify that the new entries generate the corresponding 2054 elements in the consistency proof. 2056 6. Repeat from step 1. 2058 8.3. Auditing 2060 Auditing ensures that the current published state of a log is 2061 reachable from previously published states that are known to be good, 2062 and that the promises made by the log in the form of SCTs have been 2063 kept. Audits are performed by monitors or TLS clients. 2065 In particular, there are four log behavior properties that should be 2066 checked: 2068 * The Maximum Merge Delay (MMD). 2070 * The STH Frequency Count. 2072 * The append-only property. 2074 * The consistency of the log view presented to all query sources. 2076 A benign, conformant log publishes a series of STHs over time, each 2077 derived from the previous STH and the submitted entries incorporated 2078 into the log since publication of the previous STH. This can be 2079 proven through auditing of STHs. SCTs returned to TLS clients can be 2080 audited by verifying against the accompanying certificate, and using 2081 Merkle Inclusion Proofs, against the log's Merkle tree. 2083 The action taken by the auditor if an audit fails is not specified, 2084 but note that in general if audit fails, the auditor is in possession 2085 of signed proof of the log's misbehavior. 2087 A monitor (Section 8.2) can audit by verifying the consistency of 2088 STHs it receives, ensure that each entry can be fetched and that the 2089 STH is indeed the result of making a tree from all fetched entries. 2091 A TLS client (Section 8.1) can audit by verifying an SCT against any 2092 STH dated after the SCT timestamp + the Maximum Merge Delay by 2093 requesting a Merkle inclusion proof (Section 5.4). It can also 2094 verify that the SCT corresponds to the server certificate it arrived 2095 with (i.e., the log entry is that certificate, or is a precertificate 2096 corresponding to that certificate). 2098 Checking of the consistency of the log view presented to all entities 2099 is more difficult to perform because it requires a way to share log 2100 responses among a set of CT-using entities, and is discussed in 2101 Section 11.3. 2103 9. Algorithm Agility 2105 It is not possible for a log to change any of its algorithms part way 2106 through its lifetime: 2108 Signature algorithm: SCT signatures must remain valid so signature 2109 algorithms can only be added, not removed. 2111 Hash algorithm: A log would have to support the old and new hash 2112 algorithms to allow backwards-compatibility with clients that are 2113 not aware of a hash algorithm change. 2115 Allowing multiple signature or hash algorithms for a log would 2116 require that all data structures support it and would significantly 2117 complicate client implementation, which is why it is not supported by 2118 this document. 2120 If it should become necessary to deprecate an algorithm used by a 2121 live log, then the log MUST be frozen as specified in Section 4.13 2122 and a new log SHOULD be started. Certificates in the frozen log that 2123 have not yet expired and require new SCTs SHOULD be submitted to the 2124 new log and the SCTs from that log used instead. 2126 10. IANA Considerations 2128 The assignment policy criteria mentioned in this section refer to the 2129 policies outlined in [RFC8126]. 2131 10.1. Additions to existing registries 2133 This sub-section defines additions to existing registries. 2135 10.1.1. New Entry to the TLS ExtensionType Registry 2137 IANA is asked to add an entry for "transparency_info(TBD)" to the 2138 "TLS ExtensionType Values" registry defined in [RFC8446], setting the 2139 "Recommended" value to "Y", setting the "TLS 1.3" value to "CH, CR, 2140 CT", and citing this document as the "Reference". 2142 10.1.2. URN Sub-namespace for TRANS errors 2143 (urn:ietf:params:trans:error) 2145 IANA is requested to add a new entry in the "IETF URN Sub-namespace 2146 for Registered Protocol Parameter Identifiers" registry, following 2147 the template in [RFC3553]: 2149 Registry name: trans:error 2151 Specification: RFCXXXX 2153 Repository: https://www.iana.org/assignments/trans 2155 Index value: No transformation needed. 2157 10.2. New CT-Related registries 2159 IANA is requested to add a new protocol registry, "Public Notary 2160 Transparency", to the list that appears at https://www.iana.org/ 2161 assignments/ 2163 The rest of this section defines sub-registries to be created within 2164 the new Public Notary Transparency registry. 2166 10.2.1. Hash Algorithms 2168 IANA is asked to establish a registry of hash algorithm values, named 2169 "Hash Algorithms", that initially consists of: 2171 +========+============+========================+===================+ 2172 | Value | Hash | OID | Reference / | 2173 | | Algorithm | | Assignment Policy | 2174 +========+============+========================+===================+ 2175 | 0x00 | SHA-256 | 2.16.840.1.101.3.4.2.1 | [RFC6234] | 2176 +--------+------------+------------------------+-------------------+ 2177 | 0x01 - | Unassigned | | Specification | 2178 | 0xDF | | | Required | 2179 +--------+------------+------------------------+-------------------+ 2180 | 0xE0 - | Reserved | | Experimental Use | 2181 | 0xEF | | | | 2182 +--------+------------+------------------------+-------------------+ 2183 | 0xF0 - | Reserved | | Private Use | 2184 | 0xFF | | | | 2185 +--------+------------+------------------------+-------------------+ 2187 Table 7 2189 The Designated Expert(s) should ensure that the proposed algorithm 2190 has a public specification and is suitable for use as a cryptographic 2191 hash algorithm with no known preimage or collision attacks. These 2192 attacks can damage the integrity of the log. 2194 10.2.2. Signature Algorithms 2196 IANA is asked to establish a registry of signature algorithm values, 2197 named "Signature Algorithms". 2199 The following notes should be added: 2201 * This is a subset of the TLS SignatureScheme Registry, limited to 2202 those algorithms that are appropriate for CT. A major advantage 2203 of this is leveraging the expertise of the TLS working group and 2204 its Designated Expert(s). 2206 * The value "0x0403" appears twice. While this may be confusing, it 2207 is okay because the verification process is the same for both 2208 algorithms, and the choice of which to use when generating a 2209 signature is purely internal to the log server. 2211 The registry should initially consist of: 2213 +================================+==================+==============+ 2214 | SignatureScheme Value | Signature | Reference / | 2215 | | Algorithm | Assignment | 2216 | | | Policy | 2217 +================================+==================+==============+ 2218 | 0x0000 - 0x0402 | Unassigned | Expert | 2219 | | | Review | 2220 +--------------------------------+------------------+--------------+ 2221 | ecdsa_secp256r1_sha256(0x0403) | ECDSA (NIST | [FIPS186-4] | 2222 | | P-256) with | | 2223 | | SHA-256 | | 2224 +--------------------------------+------------------+--------------+ 2225 | ecdsa_secp256r1_sha256(0x0403) | Deterministic | [RFC6979] | 2226 | | ECDSA (NIST | | 2227 | | P-256) with | | 2228 | | HMAC-SHA256 | | 2229 +--------------------------------+------------------+--------------+ 2230 | 0x0404 - 0x0806 | Unassigned | Expert | 2231 | | | Review | 2232 +--------------------------------+------------------+--------------+ 2233 | ed25519(0x0807) | Ed25519 | [RFC8032] | 2234 | | (PureEdDSA with | | 2235 | | the edwards25519 | | 2236 | | curve) | | 2237 +--------------------------------+------------------+--------------+ 2238 | 0x0808 - 0xFDFF | Unassigned | Expert | 2239 | | | Review | 2240 +--------------------------------+------------------+--------------+ 2241 | 0xFE00 - 0xFEFF | Reserved | Experimental | 2242 | | | Use | 2243 +--------------------------------+------------------+--------------+ 2244 | 0xFF00 - 0xFFFF | Reserved | Private Use | 2245 +--------------------------------+------------------+--------------+ 2247 Table 8 2249 The Designated Expert(s) should ensure that the proposed algorithm 2250 has a public specification, has a value assigned to it in the TLS 2251 SignatureScheme Registry (that IANA was asked to establish in 2252 [RFC8446]) and is suitable for use as a cryptographic signature 2253 algorithm. 2255 10.2.3. VersionedTransTypes 2257 IANA is asked to establish a registry of "VersionedTransType" values, 2258 named "VersionedTransTypes". 2260 The following note should be added: 2262 * The 0x0000 value is reserved so that v1 SCTs are distinguishable 2263 from v2 SCTs and other "TransItem" structures. 2265 The registry should initially consist of: 2267 +==========+======================+===============================+ 2268 | Value | Type and Version | Reference / Assignment Policy | 2269 +==========+======================+===============================+ 2270 | 0x0000 | Reserved | [RFC6962] | 2271 +----------+----------------------+-------------------------------+ 2272 | 0x0001 | x509_entry_v2 | RFCXXXX | 2273 +----------+----------------------+-------------------------------+ 2274 | 0x0002 | precert_entry_v2 | RFCXXXX | 2275 +----------+----------------------+-------------------------------+ 2276 | 0x0003 | x509_sct_v2 | RFCXXXX | 2277 +----------+----------------------+-------------------------------+ 2278 | 0x0004 | precert_sct_v2 | RFCXXXX | 2279 +----------+----------------------+-------------------------------+ 2280 | 0x0005 | signed_tree_head_v2 | RFCXXXX | 2281 +----------+----------------------+-------------------------------+ 2282 | 0x0006 | consistency_proof_v2 | RFCXXXX | 2283 +----------+----------------------+-------------------------------+ 2284 | 0x0007 | inclusion_proof_v2 | RFCXXXX | 2285 +----------+----------------------+-------------------------------+ 2286 | 0x0008 - | Unassigned | Specification Required | 2287 | 0xDFFF | | | 2288 +----------+----------------------+-------------------------------+ 2289 | 0xE000 - | Reserved | Experimental Use | 2290 | 0xEFFF | | | 2291 +----------+----------------------+-------------------------------+ 2292 | 0xF000 - | Reserved | Private Use | 2293 | 0xFFFF | | | 2294 +----------+----------------------+-------------------------------+ 2296 Table 9 2298 The Designated Expert(s) should review the public specification to 2299 ensure that it is detailed enough to ensure implementation 2300 interoperability. 2302 10.2.4. Log Artifact Extension Registry 2304 IANA is asked to establish a registry of "ExtensionType" values, 2305 named "Log Artifact Extensions", that initially consists of: 2307 +===============+============+=====+===============================+ 2308 | ExtensionType | Status | Use | Reference / Assignment Policy | 2309 +===============+============+=====+===============================+ 2310 | 0x0000 - | Unassigned | n/a | Specification Required | 2311 | 0xDFFF | | | | 2312 +---------------+------------+-----+-------------------------------+ 2313 | 0xE000 - | Reserved | n/a | Experimental Use | 2314 | 0xEFFF | | | | 2315 +---------------+------------+-----+-------------------------------+ 2316 | 0xF000 - | Reserved | n/a | Private Use | 2317 | 0xFFFF | | | | 2318 +---------------+------------+-----+-------------------------------+ 2320 Table 10 2322 The "Use" column should contain one or both of the following values: 2324 * "SCT", for extensions specified for use in Signed Certificate 2325 Timestamps. 2327 * "STH", for extensions specified for use in Signed Tree Heads. 2329 The Designated Expert(s) should review the public specification to 2330 ensure that it is detailed enough to ensure implementation 2331 interoperability. They should also verify that the extension is 2332 appropriate to the contexts in which it is specified to be used (SCT, 2333 STH, or both). 2335 10.2.5. Log ID Registry 2337 IANA is asked to establish a registry of Log IDs, named "Log ID 2338 Registry", that initially consists of: 2340 +================+==============+==============+===================+ 2341 | Log ID | Log Base URL | Log Operator | Reference / | 2342 | | | | Assignment Policy | 2343 +================+==============+==============+===================+ 2344 | 1.3.101.8192 - | Unassigned | Unassigned | First Come First | 2345 | 1.3.101.16383 | | | Served | 2346 +----------------+--------------+--------------+-------------------+ 2347 | 1.3.101.80.0 - | Unassigned | Unassigned | First Come First | 2348 | 1.3.101.80.* | | | Served | 2349 +----------------+--------------+--------------+-------------------+ 2351 Table 11 2353 All OIDs in the range from 1.3.101.8192 to 1.3.101.16383 have been 2354 set aside for Log IDs. This is a limited resource of 8,192 OIDs, 2355 each of which has an encoded length of 4 octets. 2357 The 1.3.101.80 arc has also been set assigned for LogIDs. This is an 2358 unlimited resource, but only the 128 OIDs from 1.3.101.80.0 to 2359 1.3.101.80.127 have an encoded length of only 4 octets. 2361 Each application for the allocation of a Log ID MUST be accompanied 2362 by: 2364 * the Log's Base URL (see Section 4.1). 2366 * the Log Operator's contact details. 2368 IANA is asked to reject any request to update a Log ID or Log Base 2369 URL in this registry, because these fields are immutable (see 2370 Section 4.1). 2372 IANA is asked to accept requests from log operators to update their 2373 contact details in this registry. 2375 Since log operators can choose to not use this registry (see 2376 Section 4.4), it is not expected to be a global directory of all 2377 logs. 2379 10.2.6. Error Types Registry 2381 IANA is requested to create a new registry for errors, the "Error 2382 Types" registry. 2384 Requirements for this registry are Specification Required. 2386 This registry should have the following three fields: 2388 +============+========+===========+ 2389 | Field Name | Type | Reference | 2390 +============+========+===========+ 2391 | identifier | string | RFCXXXX | 2392 +------------+--------+-----------+ 2393 | meaning | string | RFCXXXX | 2394 +------------+--------+-----------+ 2395 | reference | string | RFCXXXX | 2396 +------------+--------+-----------+ 2398 Table 12 2400 The initial values are as follows, taken from the text above: 2402 +===================+===============================+===========+ 2403 | Identifier | Meaning | Reference | 2404 +===================+===============================+===========+ 2405 | malformed | The request could not be | RFCXXXX | 2406 | | parsed. | | 2407 +-------------------+-------------------------------+-----------+ 2408 | badSubmission | "submission" is neither a | RFCXXXX | 2409 | | valid certificate nor a valid | | 2410 | | precertificate | | 2411 +-------------------+-------------------------------+-----------+ 2412 | badType | "type" is neither 1 nor 2 | RFCXXXX | 2413 +-------------------+-------------------------------+-----------+ 2414 | badChain | The first element of "chain" | RFCXXXX | 2415 | | is not the certifier of the | | 2416 | | "submission", or the second | | 2417 | | element does not certify the | | 2418 | | first, etc. | | 2419 +-------------------+-------------------------------+-----------+ 2420 | badCertificate | One or more certificates in | RFCXXXX | 2421 | | the "chain" are not valid | | 2422 | | (e.g., not properly encoded) | | 2423 +-------------------+-------------------------------+-----------+ 2424 | unknownAnchor | The last element of "chain" | RFCXXXX | 2425 | | (or, if "chain" is an empty | | 2426 | | array, the "submission") both | | 2427 | | is not, and is not certified | | 2428 | | by, an accepted trust anchor | | 2429 +-------------------+-------------------------------+-----------+ 2430 | shutdown | The log is no longer | RFCXXXX | 2431 | | accepting submissions | | 2432 +-------------------+-------------------------------+-----------+ 2433 | firstUnknown | "first" is before the latest | RFCXXXX | 2434 | | known STH but is not from an | | 2435 | | existing STH. | | 2436 +-------------------+-------------------------------+-----------+ 2437 | secondUnknown | "second" is before the latest | RFCXXXX | 2438 | | known STH but is not from an | | 2439 | | existing STH. | | 2440 +-------------------+-------------------------------+-----------+ 2441 | secondBeforeFirst | "second" is smaller than | RFCXXXX | 2442 | | "first". | | 2443 +-------------------+-------------------------------+-----------+ 2444 | hashUnknown | "hash" is not the hash of a | RFCXXXX | 2445 | | known leaf (may be caused by | | 2446 | | skew or by a known | | 2447 | | certificate not yet merged). | | 2448 +-------------------+-------------------------------+-----------+ 2449 | treeSizeUnknown | "hash" is before the latest | RFCXXXX | 2450 | | known STH but is not from an | | 2451 | | existing STH. | | 2452 +-------------------+-------------------------------+-----------+ 2453 | startUnknown | "start" is greater than the | RFCXXXX | 2454 | | number of entries in the | | 2455 | | Merkle tree. | | 2456 +-------------------+-------------------------------+-----------+ 2457 | endBeforeStart | "start" cannot be greater | RFCXXXX | 2458 | | than "end". | | 2459 +-------------------+-------------------------------+-----------+ 2461 Table 13 2463 11. Security Considerations 2465 With CAs, logs, and servers performing the actions described here, 2466 TLS clients can use logs and signed timestamps to reduce the 2467 likelihood that they will accept misissued certificates. If a server 2468 presents a valid signed timestamp for a certificate, then the client 2469 knows that a log has committed to publishing the certificate. From 2470 this, the client knows that monitors acting for the subject of the 2471 certificate have had some time to notice the misissuance and take 2472 some action, such as asking a CA to revoke a misissued certificate. 2473 A signed timestamp does not guarantee this though, since appropriate 2474 monitors might not have checked the logs or the CA might have refused 2475 to revoke the certificate. 2477 In addition, if TLS clients will not accept unlogged certificates, 2478 then site owners will have a greater incentive to submit certificates 2479 to logs, possibly with the assistance of their CA, increasing the 2480 overall transparency of the system. 2482 11.1. Misissued Certificates 2484 Misissued certificates that have not been publicly logged, and thus 2485 do not have a valid SCT, are not considered compliant. Misissued 2486 certificates that do have an SCT from a log will appear in that 2487 public log within the Maximum Merge Delay, assuming the log is 2488 operating correctly. Since a log is allowed to serve an STH of any 2489 age up to the MMD, the maximum period of time during which a 2490 misissued certificate can be used without being available for audit 2491 is twice the MMD. 2493 11.2. Detection of Misissue 2495 The logs do not themselves detect misissued certificates; they rely 2496 instead on interested parties, such as domain owners, to monitor them 2497 and take corrective action when a misissue is detected. 2499 11.3. Misbehaving Logs 2501 A log can misbehave in several ways. Examples include: failing to 2502 incorporate a certificate with an SCT in the Merkle Tree within the 2503 MMD; presenting different, conflicting views of the Merkle Tree at 2504 different times and/or to different parties; issuing STHs too 2505 frequently; mutating the signature of a logged certificate; and 2506 failing to present a chain containing the certifier of a logged 2507 certificate. 2509 Violation of the MMD contract is detected by log clients requesting a 2510 Merkle inclusion proof (Section 5.4) for each observed SCT. These 2511 checks can be asynchronous and need only be done once per 2512 certificate. However, note that there may be privacy concerns (see 2513 Section 8.1.4). 2515 Violation of the append-only property or the STH issuance rate limit 2516 can be detected by multiple clients comparing their instances of the 2517 Signed Tree Heads. This technique, known as "gossip," is an active 2518 area of research and not defined here. Proof of misbehavior in such 2519 cases would be: a series of STHs that were issued too closely 2520 together, proving violation of the STH issuance rate limit; or an STH 2521 with a root hash that does not match the one calculated from a copy 2522 of the log, proving violation of the append-only property. 2524 Clients that report back SCTs can be tracked or traced if a log 2525 produces multiple STHs or SCTs with the same timestamp and data but 2526 different signatures. Logs SHOULD mitigate this risk by either: 2528 * Using deterministic signature schemes, or 2530 * Producing no more than one SCT for each distinct submission and no 2531 more than one STH for each distinct tree_size. Each of these SCTs 2532 and STHs can be stored by the log and served to other clients that 2533 submit the same certificate or request the same STH. 2535 11.4. Multiple SCTs 2537 By requiring TLS servers to offer multiple SCTs, each from a 2538 different log, TLS clients reduce the effectiveness of an attack 2539 where a CA and a log collude (see Section 6.2). 2541 11.5. Leakage of DNS Information 2543 Malicious monitors can use logs to learn about the existence of 2544 domain names that might not otherwise be easy to discover. Some 2545 subdomain labels may reveal information about the service and 2546 software for which the subdomain is used, which in turn might 2547 facilitate targeted attacks. 2549 12. Acknowledgements 2551 The authors would like to thank Erwann Abelea, Robin Alden, Andrew 2552 Ayer, Richard Barnes, Al Cutter, David Drysdale, Francis Dupont, Adam 2553 Eijdenberg, Stephen Farrell, Daniel Kahn Gillmor, Paul Hadfield, Brad 2554 Hill, Jeff Hodges, Paul Hoffman, Jeffrey Hutzelman, Kat Joyce, 2555 Stephen Kent, SM, Alexey Melnikov, Linus Nordberg, Chris Palmer, 2556 Trevor Perrin, Pierre Phaneuf, Eric Rescorla, Rich Salz, Melinda 2557 Shore, Ryan Sleevi, Martin Smith, Carl Wallace and Paul Wouters for 2558 their valuable contributions. 2560 A big thank you to Symantec for kindly donating the OIDs from the 2561 1.3.101 arc that are used in this document. 2563 13. References 2565 13.1. Normative References 2567 [FIPS186-4] 2568 NIST, "FIPS PUB 186-4", 1 July 2013, 2569 . 2572 [HTML401] Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01 2573 Specification", World Wide Web Consortium Recommendation 2574 REC-html401-19991224, 24 December 1999, 2575 . 2577 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2578 Requirement Levels", BCP 14, RFC 2119, 2579 DOI 10.17487/RFC2119, March 1997, 2580 . 2582 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 2583 IETF URN Sub-namespace for Registered Protocol 2584 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 2585 2003, . 2587 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2588 Resource Identifier (URI): Generic Syntax", STD 66, 2589 RFC 3986, DOI 10.17487/RFC3986, January 2005, 2590 . 2592 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 2593 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 2594 . 2596 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 2597 (TLS) Protocol Version 1.2", RFC 5246, 2598 DOI 10.17487/RFC5246, August 2008, 2599 . 2601 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 2602 Housley, R., and W. Polk, "Internet X.509 Public Key 2603 Infrastructure Certificate and Certificate Revocation List 2604 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 2605 . 2607 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, 2608 RFC 5652, DOI 10.17487/RFC5652, September 2009, 2609 . 2611 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 2612 Extensions: Extension Definitions", RFC 6066, 2613 DOI 10.17487/RFC6066, January 2011, 2614 . 2616 [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms 2617 (SHA and SHA-based HMAC and HKDF)", RFC 6234, 2618 DOI 10.17487/RFC6234, May 2011, 2619 . 2621 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., 2622 Galperin, S., and C. Adams, "X.509 Internet Public Key 2623 Infrastructure Online Certificate Status Protocol - OCSP", 2624 RFC 6960, DOI 10.17487/RFC6960, June 2013, 2625 . 2627 [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature 2628 Algorithm (DSA) and Elliptic Curve Digital Signature 2629 Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2630 2013, . 2632 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 2633 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 2634 DOI 10.17487/RFC7231, June 2014, 2635 . 2637 [RFC7633] Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS) 2638 Feature Extension", RFC 7633, DOI 10.17487/RFC7633, 2639 October 2015, . 2641 [RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP 2642 APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016, 2643 . 2645 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 2646 Signature Algorithm (EdDSA)", RFC 8032, 2647 DOI 10.17487/RFC8032, January 2017, 2648 . 2650 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2651 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2652 May 2017, . 2654 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 2655 Interchange Format", STD 90, RFC 8259, 2656 DOI 10.17487/RFC8259, December 2017, 2657 . 2659 [RFC8391] Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A. 2660 Mohaisen, "XMSS: eXtended Merkle Signature Scheme", 2661 RFC 8391, DOI 10.17487/RFC8391, May 2018, 2662 . 2664 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 2665 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 2666 . 2668 [UNIXTIME] IEEE, "The Open Group Base Specifications Issue 7 IEEE Std 2669 1003.1-2008, 2016 Edition", n.d., 2670 . 2674 [X690] ITU-T, "Information technology - ASN.1 encoding Rules: 2675 Specification of Basic Encoding Rules (BER), Canonical 2676 Encoding Rules (CER) and Distinguished Encoding Rules 2677 (DER)", ISO/IEC 8825-1:2002, November 2015. 2679 13.2. Informative References 2681 [CABBR] CA/Browser Forum, "Baseline Requirements for the Issuance 2682 and Management of Publicly-Trusted Certificates", 2020, 2683 . 2686 [Chromium.Log.Policy] 2687 The Chromium Projects, "Chromium Certificate Transparency 2688 Log Policy", 2014, . 2691 [Chromium.Policy] 2692 The Chromium Projects, "Chromium Certificate 2693 Transparency", 2014, . 2696 [CrosbyWallach] 2697 Crosby, S. and D. Wallach, "Efficient Data Structures for 2698 Tamper-Evident Logging", Proceedings of the 18th USENIX 2699 Security Symposium, Montreal, August 2009, 2700 . 2703 [JSON.Metadata] 2704 The Chromium Projects, "Chromium Log Metadata JSON 2705 Schema", 2014, . 2708 [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate 2709 Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, 2710 . 2712 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2713 Writing an IANA Considerations Section in RFCs", BCP 26, 2714 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2715 . 2717 [RFC8820] Nottingham, M., "URI Design and Ownership", BCP 190, 2718 RFC 8820, DOI 10.17487/RFC8820, June 2020, 2719 . 2721 Appendix A. Supporting v1 and v2 simultaneously (Informative) 2723 Certificate Transparency logs have to be either v1 (conforming to 2724 [RFC6962]) or v2 (conforming to this document), as the data 2725 structures are incompatible and so a v2 log could not issue a valid 2726 v1 SCT. 2728 CT clients, however, can support v1 and v2 SCTs, for the same 2729 certificate, simultaneously, as v1 SCTs are delivered in different 2730 TLS, X.509 and OCSP extensions than v2 SCTs. 2732 v1 and v2 SCTs for X.509 certificates can be validated independently. 2733 For precertificates, v2 SCTs should be embedded in the TBSCertificate 2734 before submission of the TBSCertificate (inside a v1 precertificate, 2735 as described in Section 3.1. of [RFC6962]) to a v1 log so that TLS 2736 clients conforming to [RFC6962] but not this document are oblivious 2737 to the embedded v2 SCTs. An issuer can follow these steps to produce 2738 an X.509 certificate with embedded v1 and v2 SCTs: 2740 * Create a CMS precertificate as described in Section 3.2 and submit 2741 it to v2 logs. 2743 * Embed the obtained v2 SCTs in the TBSCertificate, as described in 2744 Section 7.1.2. 2746 * Use that TBSCertificate to create a v1 precertificate, as 2747 described in Section 3.1. of [RFC6962] and submit it to v1 logs. 2749 * Embed the v1 SCTs in the TBSCertificate, as described in 2750 Section 3.3 of [RFC6962]. 2752 * Sign that TBSCertificate (which now contains v1 and v2 SCTs) to 2753 issue the final X.509 certificate. 2755 Appendix B. An ASN.1 Module (Informative) 2757 The following ASN.1 module may be useful to implementors. 2759 CertificateTransparencyV2Module-2021 2760 -- { OID Needed, but no point in using a short one } 2761 DEFINITIONS IMPLICIT TAGS ::= BEGIN 2763 -- EXPORTS ALL -- 2765 IMPORTS 2766 EXTENSION 2767 FROM PKIX-CommonTypes-2009 -- RFC 5912 2768 { iso(1) identified-organization(3) dod(6) internet(1) 2769 security(5) mechanisms(5) pkix(7) id-mod(0) 2770 id-mod-pkixCommon-02(57) } 2772 CONTENT-TYPE 2773 FROM CryptographicMessageSyntax-2010 -- RFC 6268 2774 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) 2775 pkcs-9(9) smime(16) modules(0) id-mod-cms-2009(58) } 2777 TBSCertificate 2778 FROM PKIX1Explicit-2009 -- RFC 5912 2779 { iso(1) identified-organization(3) dod(6) internet(1) 2780 security(5) mechanisms(5) pkix(7) id-mod(0) 2781 id-mod-pkix1-explicit-02(51) } 2782 ; 2784 -- 2785 -- Section 3.2. Precertificates 2786 -- 2788 ct-tbsCertificate CONTENT-TYPE ::= { 2789 TYPE TBSCertificate 2790 IDENTIFIED BY id-ct-tbsCertificate } 2792 id-ct-tbsCertificate OBJECT IDENTIFIER ::= { 1 3 101 78 } 2794 -- 2795 -- Section 7.1. Transparency Information X.509v3 Extension 2796 -- 2798 ext-transparencyInfo EXTENSION ::= { 2799 SYNTAX TransparencyInformationSyntax 2800 IDENTIFIED BY id-ce-transparencyInfo 2801 CRITICALITY { FALSE } } 2803 id-ce-transparencyInfo OBJECT IDENTIFIER ::= { 1 3 101 75 } 2805 TransparencyInformationSyntax ::= OCTET STRING 2807 -- 2808 -- Section 7.1.1. OCSP Response Extension 2809 -- 2811 ext-ocsp-transparencyInfo EXTENSION ::= { 2812 SYNTAX TransparencyInformationSyntax 2813 IDENTIFIED BY id-pkix-ocsp-transparencyInfo 2814 CRITICALITY { FALSE } } 2816 id-pkix-ocsp-transparencyInfo OBJECT IDENTIFIER ::= 2817 id-ce-transparencyInfo 2819 -- 2820 -- Section 8.1.2. Reconstructing the TBSCertificate 2821 -- 2823 ext-embeddedSCT-CTv1 EXTENSION ::= { 2824 SYNTAX SignedCertificateTimestampList 2825 IDENTIFIED BY id-ce-embeddedSCT-CTv1 2826 CRITICALITY { FALSE } } 2828 id-ce-embeddedSCT-CTv1 OBJECT IDENTIFIER ::= { 2829 1 3 6 1 4 1 11129 2 4 2 } 2831 SignedCertificateTimestampList ::= OCTET STRING 2833 END 2835 Authors' Addresses 2837 Ben Laurie 2838 Google UK Ltd. 2840 Email: benl@google.com 2842 Adam Langley 2843 Google Inc. 2845 Email: agl@google.com 2847 Emilia Kasper 2848 Google Switzerland GmbH 2850 Email: ekasper@google.com 2852 Eran Messeri 2853 Google UK Ltd. 2855 Email: eranm@google.com 2857 Rob Stradling 2858 Sectigo Ltd. 2860 Email: rob@sectigo.com