idnits 2.17.1 draft-ietf-trans-rfc6962-bis-40.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 (28 July 2021) is 974 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: 29 January 2022 Google 7 R. Stradling 8 Sectigo 9 28 July 2021 11 Certificate Transparency Version 2.0 12 draft-ietf-trans-rfc6962-bis-40 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 29 January 2022. 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 STH . . . . . . . . . . . . . . . . . . . 32 102 5.3. Retrieve Merkle Consistency Proof between Two STHs . . . 32 103 5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash . . 33 104 5.5. Retrieve Merkle Inclusion Proof, STH and Consistency Proof 105 by Leaf Hash . . . . . . . . . . . . . . . . . . . . . . 34 106 5.6. Retrieve Entries and STH from Log . . . . . . . . . . . . 35 107 5.7. Retrieve Accepted Trust Anchors . . . . . . . . . . . . . 37 108 6. TLS Servers . . . . . . . . . . . . . . . . . . . . . . . . . 38 109 6.1. TLS Client Authentication . . . . . . . . . . . . . . . . 38 110 6.2. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . . 39 111 6.3. TransItemList Structure . . . . . . . . . . . . . . . . . 39 112 6.4. Presenting SCTs, inclusions proofs and STHs . . . . . . . 40 113 6.5. transparency_info TLS Extension . . . . . . . . . . . . . 40 114 7. Certification Authorities . . . . . . . . . . . . . . . . . . 41 115 7.1. Transparency Information X.509v3 Extension . . . . . . . 41 116 7.1.1. OCSP Response Extension . . . . . . . . . . . . . . . 41 117 7.1.2. Certificate Extension . . . . . . . . . . . . . . . . 41 118 7.2. TLS Feature X.509v3 Extension . . . . . . . . . . . . . . 41 119 8. Clients . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 120 8.1. TLS Client . . . . . . . . . . . . . . . . . . . . . . . 42 121 8.1.1. Receiving SCTs and inclusion proofs . . . . . . . . . 42 122 8.1.2. Reconstructing the TBSCertificate . . . . . . . . . . 42 123 8.1.3. Validating SCTs . . . . . . . . . . . . . . . . . . . 42 124 8.1.4. Fetching inclusion proofs . . . . . . . . . . . . . . 43 125 8.1.5. Validating inclusion proofs . . . . . . . . . . . . . 43 126 8.1.6. Evaluating compliance . . . . . . . . . . . . . . . . 44 127 8.2. Monitor . . . . . . . . . . . . . . . . . . . . . . . . . 44 128 8.3. Auditing . . . . . . . . . . . . . . . . . . . . . . . . 45 129 9. Algorithm Agility . . . . . . . . . . . . . . . . . . . . . . 46 130 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47 131 10.1. Additions to existing registries . . . . . . . . . . . . 47 132 10.1.1. New Entry to the TLS ExtensionType Registry . . . . 47 133 10.1.2. URN Sub-namespace for TRANS errors 134 (urn:ietf:params:trans:error) . . . . . . . . . . . . 47 135 10.2. New CT-Related registries . . . . . . . . . . . . . . . 47 136 10.2.1. Hash Algorithms . . . . . . . . . . . . . . . . . . 48 137 10.2.2. Signature Algorithms . . . . . . . . . . . . . . . . 48 138 10.2.3. VersionedTransTypes . . . . . . . . . . . . . . . . 49 139 10.2.4. Log Artifact Extension Registry . . . . . . . . . . 50 140 10.2.5. Log IDs Registry . . . . . . . . . . . . . . . . . . 51 141 10.2.6. Error Types Registry . . . . . . . . . . . . . . . . 52 142 10.3. OID Assignment . . . . . . . . . . . . . . . . . . . . . 54 143 11. Security Considerations . . . . . . . . . . . . . . . . . . . 54 144 11.1. Misissued Certificates . . . . . . . . . . . . . . . . . 55 145 11.2. Detection of Misissue . . . . . . . . . . . . . . . . . 55 146 11.3. Misbehaving Logs . . . . . . . . . . . . . . . . . . . . 55 147 11.4. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . 56 148 11.5. Leakage of DNS Information . . . . . . . . . . . . . . . 56 149 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 56 150 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 56 151 13.1. Normative References . . . . . . . . . . . . . . . . . . 56 152 13.2. Informative References . . . . . . . . . . . . . . . . . 59 153 Appendix A. Supporting v1 and v2 simultaneously (Informative) . 60 154 Appendix B. An ASN.1 Module (Informative) . . . . . . . . . . . 60 155 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 62 157 1. Introduction 159 Certificate Transparency aims to mitigate the problem of misissued 160 certificates by providing append-only logs of issued certificates. 161 The logs do not themselves prevent misissuance, but they ensure that 162 interested parties (particularly those named in certificates) can 163 detect such misissuance. Note that this is a general mechanism that 164 could be used for transparently logging any form of binary data, 165 subject to some kind of inclusion criteria. In this document, we 166 only describe its use for public TLS server certificates (i.e., where 167 the inclusion criteria is a valid certificate issued by a public 168 certification authority (CA)). A typical definition of "public" can 169 be found in [CABBR]. 171 Each log contains certificate chains, which can be submitted by 172 anyone. It is expected that public CAs will contribute all their 173 newly issued certificates to one or more logs; however, certificate 174 holders can also contribute their own certificate chains, as can 175 third parties. In order to avoid logs being rendered useless by the 176 submission of large numbers of spurious certificates, it is required 177 that each chain ends with a trust anchor that is accepted by the log. 178 A log may also limit the length of the chain it is willing to accept; 179 such chains must also end with an acceptable trust anchor. When a 180 chain is accepted by a log, a signed timestamp is returned, which can 181 later be used to provide evidence to TLS clients that the chain has 182 been submitted. TLS clients can thus require that all certificates 183 they accept as valid are accompanied by signed timestamps. 185 Those who are concerned about misissuance can monitor the logs, 186 asking them regularly for all new entries, and can thus check whether 187 domains for which they are responsible have had certificates issued 188 that they did not expect. What they do with this information, 189 particularly when they find that a misissuance has happened, is 190 beyond the scope of this document. However, broadly speaking, they 191 can invoke existing business mechanisms for dealing with misissued 192 certificates, such as working with the CA to get the certificate 193 revoked, or with maintainers of trust anchor lists to get the CA 194 removed. Of course, anyone who wants can monitor the logs and, if 195 they believe a certificate is incorrectly issued, take action as they 196 see fit. 198 Similarly, those who have seen signed timestamps from a particular 199 log can later demand a proof of inclusion from that log. If the log 200 is unable to provide this (or, indeed, if the corresponding 201 certificate is absent from monitors' copies of that log), that is 202 evidence of the incorrect operation of the log. The checking 203 operation is asynchronous to allow clients to proceed without delay, 204 despite possible issues such as network connectivity and the vagaries 205 of firewalls. 207 The append-only property of each log is achieved using Merkle Trees, 208 which can be used to efficiently prove that any particular instance 209 of the log is a superset of any particular previous instance and to 210 efficiently detect various misbehaviors of the log (e.g., issuing a 211 signed timestamp for a certificate that is not subsequently logged). 213 It is necessary to treat each log as a trusted third party, because 214 the log auditing mechanisms described in this document can be 215 circumvented by a misbehaving log that shows different, inconsistent 216 views of itself to different clients. While mechanisms are being 217 developed to address these shortcomings and thereby avoid the need to 218 blindly trust logs, such mechanisms are outside the scope of this 219 document. 221 1.1. Requirements Language 223 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 224 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 225 "OPTIONAL" in this document are to be interpreted as described in BCP 226 14 [RFC2119] [RFC8174] when, and only when, they appear in all 227 capitals, as shown here. 229 1.2. Data Structures 231 Data structures are defined and encoded according to the conventions 232 laid out in Section 3 of [RFC8446]. 234 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 SCTs (Section 4.8) and STHs (Section 4.10). The 1018 interpretation of the "extension_data" field is determined solely by 1019 the value of the "extension_type" field. 1021 This document does not define any extensions, but it does establish a 1022 registry for future "ExtensionType" values (see Section 10.2.4). 1023 Each document that registers a new "ExtensionType" must specify the 1024 context in which it may be used (e.g., SCT, STH, or both) and 1025 describe how to interpret the corresponding "extension_data". 1027 4.7. Merkle Tree Leaves 1029 The leaves of a log's Merkle Tree correspond to the log's entries 1030 (see Section 4.3). Each leaf is the leaf hash (Section 2.1) of a 1031 "TransItem" structure of type "x509_entry_v2" or "precert_entry_v2", 1032 which encapsulates a "TimestampedCertificateEntryDataV2" structure. 1033 Note that leaf hashes are calculated as HASH(0x00 || TransItem), 1034 where the hash algorithm is one of the log's parameters. 1036 opaque TBSCertificate<1..2^24-1>; 1038 struct { 1039 uint64 timestamp; 1040 opaque issuer_key_hash<32..2^8-1>; 1041 TBSCertificate tbs_certificate; 1042 Extension sct_extensions<0..2^16-1>; 1043 } TimestampedCertificateEntryDataV2; 1045 "timestamp" is the date and time at which the certificate or 1046 precertificate was accepted by the log, in the form of a 64-bit 1047 unsigned number of milliseconds elapsed since the Unix Epoch (1 1048 January 1970 00:00:00 UTC - see [UNIXTIME]), ignoring leap seconds, 1049 in network byte order. Note that the leaves of a log's Merkle Tree 1050 are not required to be in strict chronological order. 1052 "issuer_key_hash" is the HASH of the public key of the CA that issued 1053 the certificate or precertificate, calculated over the DER encoding 1054 of the key represented as SubjectPublicKeyInfo [RFC5280]. This is 1055 needed to bind the CA to the certificate or precertificate, making it 1056 impossible for the corresponding SCT to be valid for any other 1057 certificate or precertificate whose TBSCertificate matches 1058 "tbs_certificate". The length of the "issuer_key_hash" MUST match 1059 HASH_SIZE. 1061 "tbs_certificate" is the DER encoded TBSCertificate from the 1062 submission. (Note that a precertificate's TBSCertificate can be 1063 reconstructed from the corresponding certificate as described in 1064 Section 8.1.2). 1066 "sct_extensions" is byte-for-byte identical to the SCT extensions of 1067 the corresponding SCT. 1069 The type of the "TransItem" corresponds to the value of the "type" 1070 parameter supplied in the Section 5.1 call. 1072 4.8. Signed Certificate Timestamp (SCT) 1074 An SCT is a "TransItem" structure of type "x509_sct_v2" or 1075 "precert_sct_v2", which encapsulates a 1076 "SignedCertificateTimestampDataV2" structure: 1078 struct { 1079 LogID log_id; 1080 uint64 timestamp; 1081 Extension sct_extensions<0..2^16-1>; 1082 opaque signature<1..2^16-1>; 1083 } SignedCertificateTimestampDataV2; 1085 "log_id" is this log's unique ID, encoded in an opaque vector as 1086 described in Section 4.4. 1088 "timestamp" is equal to the timestamp from the corresponding 1089 "TimestampedCertificateEntryDataV2" structure. 1091 "sct_extensions" is a vector of 0 or more SCT extensions. This 1092 vector MUST NOT include more than one extension with the same 1093 "extension_type". The extensions in the vector MUST be ordered by 1094 the value of the "extension_type" field, smallest value first. All 1095 SCT extensions are similar to non-critical X.509v3 extensions (i.e., 1096 the "mustUnderstand" field is not set), and a recipient SHOULD ignore 1097 any extension it does not understand. Furthermore, an implementation 1098 MAY choose to ignore any extension(s) that it does understand. 1100 "signature" is computed over a "TransItem" structure of type 1101 "x509_entry_v2" or "precert_entry_v2" (see Section 4.7) using the 1102 signature algorithm declared in the log's parameters (see 1103 Section 4.1). 1105 4.9. Merkle Tree Head 1107 The log stores information about its Merkle Tree in a 1108 "TreeHeadDataV2": 1110 opaque NodeHash<32..2^8-1>; 1112 struct { 1113 uint64 timestamp; 1114 uint64 tree_size; 1115 NodeHash root_hash; 1116 Extension sth_extensions<0..2^16-1>; 1117 } TreeHeadDataV2; 1119 The length of NodeHash MUST match HASH_SIZE of the log. 1121 "timestamp" is the current date and time, using the format defined in 1122 Section 4.7. 1124 "tree_size" is the number of entries currently in the log's Merkle 1125 Tree. 1127 "root_hash" is the root of the Merkle Hash Tree. 1129 "sth_extensions" is a vector of 0 or more STH extensions. This 1130 vector MUST NOT include more than one extension with the same 1131 "extension_type". The extensions in the vector MUST be ordered by 1132 the value of the "extension_type" field, smallest value first. If an 1133 implementation sees an extension that it does not understand, it 1134 SHOULD ignore that extension. Furthermore, an implementation MAY 1135 choose to ignore any extension(s) that it does understand. 1137 4.10. Signed Tree Head (STH) 1139 Periodically each log SHOULD sign its current tree head information 1140 (see Section 4.9) to produce an STH. When a client requests a log's 1141 latest STH (see Section 5.2), the log MUST return an STH that is no 1142 older than the log's MMD. However, since STHs could be used to mark 1143 individual clients (by producing a new STH for each query), a log 1144 MUST NOT produce STHs more frequently than its parameters declare 1145 (see Section 4.1). In general, there is no need to produce a new STH 1146 unless there are new entries in the log; however, in the event that a 1147 log does not accept any submissions during an MMD period, the log 1148 MUST sign the same Merkle Tree Hash with a fresh timestamp. 1150 An STH is a "TransItem" structure of type "signed_tree_head_v2", 1151 which encapsulates a "SignedTreeHeadDataV2" structure: 1153 struct { 1154 LogID log_id; 1155 TreeHeadDataV2 tree_head; 1156 opaque signature<1..2^16-1>; 1157 } SignedTreeHeadDataV2; 1159 "log_id" is this log's unique ID, encoded in an opaque vector as 1160 described in Section 4.4. 1162 The "timestamp" in "tree_head" MUST be at least as recent as the most 1163 recent SCT timestamp in the tree. Each subsequent timestamp MUST be 1164 more recent than the timestamp of the previous update. 1166 "tree_head" contains the latest tree head information (see 1167 Section 4.9). 1169 "signature" is computed over the "tree_head" field using the 1170 signature algorithm declared in the log's parameters (see 1171 Section 4.1). 1173 4.11. Merkle Consistency Proofs 1175 To prepare a Merkle Consistency Proof for distribution to clients, 1176 the log produces a "TransItem" structure of type 1177 "consistency_proof_v2", which encapsulates a "ConsistencyProofDataV2" 1178 structure: 1180 struct { 1181 LogID log_id; 1182 uint64 tree_size_1; 1183 uint64 tree_size_2; 1184 NodeHash consistency_path<0..2^16-1>; 1185 } ConsistencyProofDataV2; 1187 "log_id" is this log's unique ID, encoded in an opaque vector as 1188 described in Section 4.4. 1190 "tree_size_1" is the size of the older tree. 1192 "tree_size_2" is the size of the newer tree. 1194 "consistency_path" is a vector of Merkle Tree nodes proving the 1195 consistency of two STHs as described in Section 2.1.4. 1197 4.12. Merkle Inclusion Proofs 1199 To prepare a Merkle Inclusion Proof for distribution to clients, the 1200 log produces a "TransItem" structure of type "inclusion_proof_v2", 1201 which encapsulates an "InclusionProofDataV2" structure: 1203 struct { 1204 LogID log_id; 1205 uint64 tree_size; 1206 uint64 leaf_index; 1207 NodeHash inclusion_path<0..2^16-1>; 1208 } InclusionProofDataV2; 1210 "log_id" is this log's unique ID, encoded in an opaque vector as 1211 described in Section 4.4. 1213 "tree_size" is the size of the tree on which this inclusion proof is 1214 based. 1216 "leaf_index" is the 0-based index of the log entry corresponding to 1217 this inclusion proof. 1219 "inclusion_path" is a vector of Merkle Tree nodes proving the 1220 inclusion of the chosen certificate or precertificate as described in 1221 Section 2.1.3. 1223 4.13. Shutting down a log 1225 Log operators may decide to shut down a log for various reasons, such 1226 as deprecation of the signature algorithm. If there are entries in 1227 the log for certificates that have not yet expired, simply making TLS 1228 clients stop recognizing that log will have the effect of 1229 invalidating SCTs from that log. In order to avoid that, the 1230 following actions SHOULD be taken: 1232 * Make it known to clients and monitors that the log will be frozen. 1233 This is not part of the API, so it will have to be done via a 1234 relevant out-of-band mechanism. 1236 * Stop accepting new submissions (the error code "shutdown" should 1237 be returned for such requests). 1239 * Once MMD from the last accepted submission has passed and all 1240 pending submissions are incorporated, issue a final STH and 1241 publish it as one of the log's parameters. Having an STH with a 1242 timestamp that is after the MMD has passed from the last SCT 1243 issuance allows clients to audit this log regularly without 1244 special handling for the final STH. At this point the log's 1245 private key is no longer needed and can be destroyed. 1247 * Keep the log running until the certificates in all of its entries 1248 have expired or exist in other logs (this can be determined by 1249 scanning other logs or connecting to domains mentioned in the 1250 certificates and inspecting the SCTs served). 1252 5. Log Client Messages 1254 Messages are sent as HTTPS GET or POST requests. Parameters for 1255 POSTs and all responses are encoded as JavaScript Object Notation 1256 (JSON) objects [RFC8259]. Parameters for GETs are encoded as order- 1257 independent key/value URL parameters, using the "application/x-www- 1258 form-urlencoded" format described in the "HTML 4.01 Specification" 1259 [HTML401]. Binary data is base64 encoded according to section 4 of 1260 [RFC4648] as specified in the individual messages. 1262 Clients are configured with a log's base URL, which is one of the 1263 log's parameters. Clients construct URLs for requests by appending 1264 suffixes to this base URL. This structure places some degree of 1265 restriction on how log operators can deploy these services, as noted 1266 in [RFC8820]. However, operational experience with version 1 of this 1267 protocol has not indicated that these restrictions are a problem in 1268 practice. 1270 Note that JSON objects and URL parameters may contain fields not 1271 specified here, to allow for experimentation. Any fields that are 1272 not understood SHOULD be ignored. 1274 In practice, log servers may include multiple front-end machines. 1275 Since it is impractical to keep these machines in perfect sync, 1276 errors may occur that are caused by skew between the machines. Where 1277 such errors are possible, the front-end will return additional 1278 information (as specified below) making it possible for clients to 1279 make progress, if progress is possible. Front-ends MUST only serve 1280 data that is free of gaps (that is, for example, no front-end will 1281 respond with an STH unless it is also able to prove consistency from 1282 all log entries logged within that STH). 1284 For example, when a consistency proof between two STHs is requested, 1285 the front-end reached may not yet be aware of one or both STHs. In 1286 the case where it is unaware of both, it will return the latest STH 1287 it is aware of. Where it is aware of the first but not the second, 1288 it will return the latest STH it is aware of and a consistency proof 1289 from the first STH to the returned STH. The case where it knows the 1290 second but not the first should not arise (see the "no gaps" 1291 requirement above). 1293 If the log is unable to process a client's request, it MUST return an 1294 HTTP response code of 4xx/5xx (see [RFC7231]), and, in place of the 1295 responses outlined in the subsections below, the body SHOULD be a 1296 JSON Problem Details Object (see [RFC7807] Section 3), containing: 1298 type: A URN reference identifying the problem. To facilitate 1299 automated response to errors, this document defines a set of 1300 standard tokens for use in the "type" field, within the URN 1301 namespace of: "urn:ietf:params:trans:error:". 1303 detail: A human-readable string describing the error that prevented 1304 the log from processing the request, ideally with sufficient 1305 detail to enable the error to be rectified. 1307 e.g., In response to a request of "/ct/v2/get- 1308 entries?start=100&end=99", the log would return a "400 Bad Request" 1309 response code with a body similar to the following: 1311 { 1312 "type": "urn:ietf:params:trans:error:endBeforeStart", 1313 "detail": "'start' cannot be greater than 'end'" 1314 } 1316 Most error types are specific to the type of request and are defined 1317 in the respective subsections below. The one exception is the 1318 "malformed" error type, which indicates that the log server could not 1319 parse the client's request because it did not comply with this 1320 document: 1322 +===========+==================================+ 1323 | type | detail | 1324 +===========+==================================+ 1325 | malformed | The request could not be parsed. | 1326 +-----------+----------------------------------+ 1328 Table 1 1330 Clients SHOULD treat "500 Internal Server Error" and "503 Service 1331 Unavailable" responses as transient failures and MAY retry the same 1332 request without modification at a later date. Note that as per 1333 [RFC7231], in the case of a 503 response the log MAY include a 1334 "Retry-After:" header field in order to request a minimum time for 1335 the client to wait before retrying the request. In the absence of 1336 this header field, this document does not specify a minimum. 1338 Clients SHOULD treat any 4xx error as a problem with the request and 1339 not attempt to resubmit without some modification to the request. 1340 The full status code MAY provide additional details. 1342 This document deliberately does not provide more specific guidance on 1343 the use of HTTP status codes. 1345 5.1. Submit Entry to Log 1347 POST /ct/v2/submit-entry 1349 Inputs: submission: The base64 encoded certificate or 1350 precertificate. 1352 type: The "VersionedTransType" integer value that indicates 1353 the type of the "submission": 1 for "x509_entry_v2", or 2 for 1354 "precert_entry_v2". 1356 chain: An array of zero or more JSON strings, each of which 1357 is a base64 encoded CA certificate. The first element is the 1358 certifier of the "submission"; the second certifies the first; 1359 etc. The last element of "chain" (or, if "chain" is an empty 1360 array, the "submission") is certified by an accepted trust 1361 anchor. 1363 Outputs: sct: A base64 encoded "TransItem" of type "x509_sct_v2" or 1364 "precert_sct_v2", signed by this log, that corresponds to the 1365 "submission". 1367 If the submitted entry is immediately appended to (or already 1368 exists in) this log's tree, then the log SHOULD also output: 1370 sth: A base64 encoded "TransItem" of type "signed_tree_head_v2", 1371 signed by this log. 1373 inclusion: A base64 encoded "TransItem" of type 1374 "inclusion_proof_v2" whose "inclusion_path" array of Merkle 1375 Tree nodes proves the inclusion of the "submission" in the 1376 returned "sth". 1378 Error codes: 1380 +================+==============================================+ 1381 | type | detail | 1382 +================+==============================================+ 1383 | badSubmission | "submission" is neither a valid certificate | 1384 | | nor a valid precertificate. | 1385 +----------------+----------------------------------------------+ 1386 | badType | "type" is neither 1 nor 2. | 1387 +----------------+----------------------------------------------+ 1388 | badChain | The first element of "chain" is not the | 1389 | | certifier of the "submission", or the second | 1390 | | element does not certify the first, etc. | 1391 +----------------+----------------------------------------------+ 1392 | badCertificate | One or more certificates in the "chain" are | 1393 | | not valid (e.g., not properly encoded). | 1394 +----------------+----------------------------------------------+ 1395 | unknownAnchor | The last element of "chain" (or, if "chain" | 1396 | | is an empty array, the "submission") both is | 1397 | | not, and is not certified by, an accepted | 1398 | | trust anchor. | 1399 +----------------+----------------------------------------------+ 1400 | shutdown | The log is no longer accepting submissions. | 1401 +----------------+----------------------------------------------+ 1403 Table 2 1405 If the version of "sct" is not v2, then a v2 client may be unable to 1406 verify the signature. It MUST NOT construe this as an error. This 1407 is to avoid forcing an upgrade of compliant v2 clients that do not 1408 use the returned SCTs. 1410 If a log detects bad encoding in a chain that otherwise verifies 1411 correctly then the log MUST either log the certificate or return the 1412 "bad certificate" error. If the certificate is logged, an SCT MUST 1413 be issued. Logging the certificate is useful, because monitors 1414 (Section 8.2) can then detect these encoding errors, which may be 1415 accepted by some TLS clients. 1417 If "submission" is an accepted trust anchor whose certifier is 1418 neither an accepted trust anchor nor the first element of "chain", 1419 then the log MUST return the "unknown anchor" error. A log is not 1420 able to generate an SCT for a submission if it does not have access 1421 to the issuer's public key. 1423 If the returned "sct" is intended to be provided to TLS clients, then 1424 "sth" and "inclusion" (if returned) SHOULD also be provided to TLS 1425 clients. For example, if "type" was 2 (indicating "precert_sct_v2") 1426 then all three "TransItem"s could be embedded in the certificate. 1428 5.2. Retrieve Latest STH 1430 GET /ct/v2/get-sth 1432 No inputs. 1434 Outputs: sth: A base64 encoded "TransItem" of type 1435 "signed_tree_head_v2", signed by this log, that is no older 1436 than the log's MMD. 1438 5.3. Retrieve Merkle Consistency Proof between Two STHs 1440 GET /ct/v2/get-sth-consistency 1442 Inputs: first: The tree_size of the older tree, in decimal. 1444 second: The tree_size of the newer tree, in decimal 1445 (optional). 1447 Both tree sizes must be from existing v2 STHs. However, because 1448 of skew, the receiving front-end may not know one or both of the 1449 existing STHs. If both are known, then only the "consistency" 1450 output is returned. If the first is known but the second is not 1451 (or has been omitted), then the latest known STH is returned, 1452 along with a consistency proof between the first STH and the 1453 latest. If neither are known, then the latest known STH is 1454 returned without a consistency proof. 1456 Outputs: consistency: A base64 encoded "TransItem" of type 1457 "consistency_proof_v2", whose "tree_size_1" MUST match the 1458 "first" input. If the "sth" output is omitted, then 1459 "tree_size_2" MUST match the "second" input. If "first" and 1460 "second" are equal and correspond to a known STH, the returned 1461 consistency proof MUST be empty (a "consistency_path" array 1462 with zero elements). 1464 sth: A base64 encoded "TransItem" of type 1465 "signed_tree_head_v2", signed by this log. 1467 Note that no signature is required for the "consistency" output as 1468 it is used to verify the consistency between two STHs, which are 1469 signed. 1471 Error codes: 1473 +===================+======================================+ 1474 | type | detail | 1475 +===================+======================================+ 1476 | firstUnknown | "first" is before the latest known | 1477 | | STH but is not from an existing STH. | 1478 +-------------------+--------------------------------------+ 1479 | secondUnknown | "second" is before the latest known | 1480 | | STH but is not from an existing STH. | 1481 +-------------------+--------------------------------------+ 1482 | secondBeforeFirst | "second" is smaller than "first". | 1483 +-------------------+--------------------------------------+ 1485 Table 3 1487 See Section 2.1.4.2 for an outline of how to use the "consistency" 1488 output. 1490 5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash 1492 GET /ct/v2/get-proof-by-hash 1494 Inputs: hash: A base64 encoded v2 leaf hash. 1496 tree_size: The tree_size of the tree on which to base the 1497 proof, in decimal. 1499 The "hash" must be calculated as defined in Section 4.7. A v2 STH 1500 must exist for the "tree_size". Because of skew, the front-end 1501 may not know the requested tree head. In that case, it will 1502 return the latest STH it knows, along with an inclusion proof to 1503 that STH. If the front-end knows the requested tree head then 1504 only "inclusion" is returned. 1506 Outputs: inclusion: A base64 encoded "TransItem" of type 1507 "inclusion_proof_v2" whose "inclusion_path" array of Merkle 1508 Tree nodes proves the inclusion of the certificate (as 1509 specified by the "hash" parameter) in the selected STH. 1511 sth: A base64 encoded "TransItem" of type 1512 "signed_tree_head_v2", signed by this log. 1514 Note that no signature is required for the "inclusion" output as 1515 it is used to verify inclusion in the selected STH, which is 1516 signed. 1518 Error codes: 1520 +=================+=====================================+ 1521 | type | detail | 1522 +=================+=====================================+ 1523 | hashUnknown | "hash" is not the hash of a known | 1524 | | leaf (may be caused by skew or by a | 1525 | | known certificate not yet merged). | 1526 +-----------------+-------------------------------------+ 1527 | treeSizeUnknown | "hash" is before the latest known | 1528 | | STH but is not from an existing | 1529 | | STH. | 1530 +-----------------+-------------------------------------+ 1532 Table 4 1534 See Section 2.1.3.2 for an outline of how to use the "inclusion" 1535 output. 1537 5.5. Retrieve Merkle Inclusion Proof, STH and Consistency Proof by Leaf 1538 Hash 1540 GET /ct/v2/get-all-by-hash 1542 Inputs: hash: A base64 encoded v2 leaf hash. 1544 tree_size: The tree_size of the tree on which to base the 1545 proofs, in decimal. 1547 The "hash" must be calculated as defined in Section 4.7. A v2 STH 1548 must exist for the "tree_size". 1550 Because of skew, the front-end may not know the requested tree head 1551 or the requested hash, which leads to a number of cases: 1553 +=====================+=====================================+ 1554 | Case | Response | 1555 +=====================+=====================================+ 1556 | latest STH < | Return latest STH | 1557 | requested tree head | | 1558 +---------------------+-------------------------------------+ 1559 | latest STH > | Return latest STH and a consistency | 1560 | requested tree head | proof between it and the requested | 1561 | | tree head (see Section 5.3) | 1562 +---------------------+-------------------------------------+ 1563 | index of requested | Return "inclusion" | 1564 | hash < latest STH | | 1565 +---------------------+-------------------------------------+ 1567 Table 5 1569 Note that more than one case can be true, in which case the returned 1570 data is their union. It is also possible for none to be true, in 1571 which case the front-end MUST return an empty response. 1573 Outputs: inclusion: A base64 encoded "TransItem" of type 1574 "inclusion_proof_v2" whose "inclusion_path" array of Merkle 1575 Tree nodes proves the inclusion of the certificate (as 1576 specified by the "hash" parameter) in the selected STH. 1578 sth: A base64 encoded "TransItem" of type 1579 "signed_tree_head_v2", signed by this log. 1581 consistency: A base64 encoded "TransItem" of type 1582 "consistency_proof_v2" that proves the consistency of the 1583 requested tree head and the returned STH. 1585 Note that no signature is required for the "inclusion" or 1586 "consistency" outputs as they are used to verify inclusion in and 1587 consistency of STHs, which are signed. 1589 Errors are the same as in Section 5.4. 1591 See Section 2.1.3.2 for an outline of how to use the "inclusion" 1592 output, and see Section 2.1.4.2 for an outline of how to use the 1593 "consistency" output. 1595 5.6. Retrieve Entries and STH from Log 1597 GET /ct/v2/get-entries 1599 Inputs: start: 0-based index of first entry to retrieve, in 1600 decimal. 1602 end: 0-based index of last entry to retrieve, in decimal. 1604 Outputs: entries: An array of objects, each consisting of 1606 log_entry: The base64 encoded "TransItem" structure of type 1607 "x509_entry_v2" or "precert_entry_v2" (see Section 4.3). 1609 submitted_entry: JSON object equivalent to inputs that were 1610 submitted to "submit-entry", with the addition of the trust 1611 anchor to the "chain" field if the submission did not 1612 include it. 1614 sct: The base64 encoded "TransItem" of type "x509_sct_v2" or 1615 "precert_sct_v2" corresponding to this log entry. 1617 sth: A base64 encoded "TransItem" of type 1618 "signed_tree_head_v2", signed by this log. 1620 Note that this message is not signed -- the "entries" data can be 1621 verified by constructing the Merkle Tree Hash corresponding to a 1622 retrieved STH. All leaves MUST be v2. However, a compliant v2 1623 client MUST NOT construe an unrecognized TransItem type as an error. 1624 This means it may be unable to parse some entries, but note that each 1625 client can inspect the entries it does recognize as well as verify 1626 the integrity of the data by treating unrecognized leaves as opaque 1627 input to the tree. 1629 The "start" and "end" parameters SHOULD be within the range 0 <= x < 1630 "tree_size" as returned by "get-sth" in Section 5.2. 1632 The "start" parameter MUST be less than or equal to the "end" 1633 parameter. 1635 Each "submitted_entry" output parameter MUST include the trust anchor 1636 that the log used to verify the "submission", even if that trust 1637 anchor was not provided to "submit-entry" (see Section 5.1). If the 1638 "submission" does not certify itself, then the first element of 1639 "chain" MUST be present and MUST certify the "submission". 1641 Log servers MUST honor requests where 0 <= "start" < "tree_size" and 1642 "end" >= "tree_size" by returning a partial response covering only 1643 the valid entries in the specified range. "end" >= "tree_size" could 1644 be caused by skew. Note that the following restriction may also 1645 apply: 1647 Logs MAY restrict the number of entries that can be retrieved per 1648 "get-entries" request. If a client requests more than the permitted 1649 number of entries, the log SHALL return the maximum number of entries 1650 permissible. These entries SHALL be sequential beginning with the 1651 entry specified by "start". Note that limit on the number of entries 1652 is not immutable and therefore the restriction may be changed or 1653 lifted at any time and is not listed with the other Log Parameters in 1654 Section 4.1. 1656 Because of skew, it is possible the log server will not have any 1657 entries between "start" and "end". In this case it MUST return an 1658 empty "entries" array. 1660 In any case, the log server MUST return the latest STH it knows 1661 about. 1663 See Section 2.1.2 for an outline of how to use a complete list of 1664 "log_entry" entries to verify the "root_hash". 1666 Error codes: 1668 +================+====================================+ 1669 | type | detail | 1670 +================+====================================+ 1671 | startUnknown | "start" is greater than the number | 1672 | | of entries in the Merkle tree. | 1673 +----------------+------------------------------------+ 1674 | endBeforeStart | "start" cannot be greater than | 1675 | | "end". | 1676 +----------------+------------------------------------+ 1678 Table 6 1680 5.7. Retrieve Accepted Trust Anchors 1682 GET /ct/v2/get-anchors 1684 No inputs. 1686 Outputs: certificates: An array of JSON strings, each of which is a 1687 base64 encoded CA certificate that is acceptable to the log. 1689 max_chain_length: If the server has chosen to limit the 1690 length of chains it accepts, this is the maximum number of 1691 certificates in the chain, in decimal. If there is no limit, 1692 this is omitted. 1694 This data is not signed and the protocol depends on the security 1695 guarantees of TLS to ensure correctness. 1697 6. TLS Servers 1699 CT-using TLS servers MUST use at least one of the mechanisms 1700 described below to present one or more SCTs from one or more logs to 1701 each TLS client during full TLS handshakes, when requested by the 1702 client, where each SCT corresponds to the server certificate. (Of 1703 course, a server can only send a TLS extension if the client has 1704 specified it first.) Servers SHOULD also present corresponding 1705 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 in the "CertificateStatus" 1720 message; see Section 8 of [RFC6066]. Using stapling also allows SCTs 1721 and inclusion proofs to be updated on the fly. 1723 CT information can also be encoded as an extension in the X.509v3 1724 certificate (see Section 7.1.2). This mechanism allows the use of 1725 unmodified TLS servers, but the SCTs and inclusion proofs cannot be 1726 updated on the fly. Since the logs from which the SCTs and inclusion 1727 proofs originated won't necessarily be accepted by TLS clients for 1728 the full lifetime of the certificate, there is a risk that TLS 1729 clients may subsequently consider the certificate to be non-compliant 1730 and in need of re-issuance or the use of one of the other two methods 1731 for delivering CT information. 1733 6.1. TLS Client Authentication 1735 This specification includes no description of how a TLS server can 1736 use CT for TLS client certificates. While this may be useful, it is 1737 not documented here for the following reasons: 1739 * The greater security exposure is for clients to end up interacting 1740 with an illegitimate server. 1742 * In general, TLS client certificates are not expected to be 1743 submitted to CT logs, particularly those intended for general 1744 public use. 1746 A future version could include such information. 1748 6.2. Multiple SCTs 1750 CT-using TLS servers SHOULD send SCTs from multiple logs, because: 1752 * One or more logs may not have become acceptable to all CT-using 1753 TLS clients. Note that client discovery, trust, and distrust of 1754 logs is expected to be handled out-of-band and is out of scope of 1755 this document. 1757 * If a CA and a log collude, it is possible to temporarily hide 1758 misissuance from clients. When a TLS client requires SCTs from 1759 multiple logs to be provided, it is more difficult to mount this 1760 attack. 1762 * If a log misbehaves or suffers a key compromise, a consequence may 1763 be that clients cease to trust it. Since the time an SCT may be 1764 in use can be considerable (several years is common in current 1765 practice when embedded in a certificate), including SCTs from 1766 multiple logs reduces the probability of the certificate being 1767 rejected by TLS clients. 1769 * TLS clients may have policies related to the above risks requiring 1770 TLS servers to present multiple SCTs. For example, at the time of 1771 writing, Chromium [Chromium.Log.Policy] requires multiple SCTs to 1772 be presented with EV certificates in order for the EV indicator to 1773 be shown. 1775 To select the logs from which to obtain SCTs, a TLS server can, for 1776 example, examine the set of logs popular TLS clients accept and 1777 recognize. 1779 6.3. TransItemList Structure 1781 Multiple SCTs, inclusion proofs, and indeed "TransItem" structures of 1782 any type, are combined into a list as follows: 1784 opaque SerializedTransItem<1..2^16-1>; 1786 struct { 1787 SerializedTransItem trans_item_list<1..2^16-1>; 1788 } TransItemList; 1790 Here, "SerializedTransItem" is an opaque byte string that contains 1791 the serialized "TransItem" structure. This encoding ensures that TLS 1792 clients can decode each "TransItem" individually (so, for example, if 1793 there is a version upgrade, out-of-date clients can still parse old 1794 "TransItem" structures while skipping over new "TransItem" structures 1795 whose versions they don't understand). 1797 6.4. Presenting SCTs, inclusions proofs and STHs 1799 In each "TransItemList" that is sent during a TLS handshake, the TLS 1800 server MUST include a "TransItem" structure of type "x509_sct_v2" or 1801 "precert_sct_v2". 1803 Presenting inclusion proofs and STHs in the TLS handshake helps to 1804 protect the client's privacy (see Section 8.1.4) and reduces load on 1805 log servers. Therefore, if the TLS server can obtain them, it SHOULD 1806 also include "TransItem"s of type "inclusion_proof_v2" and 1807 "signed_tree_head_v2" in the "TransItemList". 1809 6.5. transparency_info TLS Extension 1811 Provided that a TLS client includes the "transparency_info" extension 1812 type in the ClientHello and the TLS server supports the 1813 "transparency_info" extension: 1815 * The TLS server MUST verify that the received "extension_data" is 1816 empty. 1818 * The TLS server MUST construct a "TransItemList" of relevant 1819 "TransItem"s (see Section 6.4), which SHOULD omit any "TransItem"s 1820 that are already embedded in the server certificate or the stapled 1821 OCSP response (see Section 7.1). If the constructed 1822 "TransItemList" is not empty, then the TLS server MUST include the 1823 "transparency_info" extension with the "extension_data" set to 1824 this "TransItemList". If the list is empty then the server SHOULD 1825 omit the "extension_data" element, but MAY send it with an empty 1826 array. 1828 TLS servers MUST only include this extension in the following 1829 messages: 1831 * the ServerHello message (for TLS 1.2 or earlier). 1833 * the Certificate or CertificateRequest message (for TLS 1.3). 1835 TLS servers MUST NOT process or include this extension when a TLS 1836 session is resumed, since session resumption uses the original 1837 session information. 1839 7. Certification Authorities 1841 7.1. Transparency Information X.509v3 Extension 1843 The Transparency Information X.509v3 extension, which has OID 1844 1.3.101.75 and SHOULD be non-critical, contains one or more 1845 "TransItem" structures in a "TransItemList". This extension MAY be 1846 included in OCSP responses (see Section 7.1.1) and certificates (see 1847 Section 7.1.2). Since RFC5280 requires the "extnValue" field (an 1848 OCTET STRING) of each X.509v3 extension to include the DER encoding 1849 of an ASN.1 value, a "TransItemList" MUST NOT be included directly. 1850 Instead, it MUST be wrapped inside an additional OCTET STRING, which 1851 is then put into the "extnValue" field: 1853 TransparencyInformationSyntax ::= OCTET STRING 1855 "TransparencyInformationSyntax" contains a "TransItemList". 1857 7.1.1. OCSP Response Extension 1859 A certification authority MAY include a Transparency Information 1860 X.509v3 extension in the "singleExtensions" of a "SingleResponse" in 1861 an OCSP response. All included SCTs and inclusion proofs MUST be for 1862 the certificate identified by the "certID" of that "SingleResponse", 1863 or for a precertificate that corresponds to that certificate. 1865 7.1.2. Certificate Extension 1867 A certification authority MAY include a Transparency Information 1868 X.509v3 extension in a certificate. All included SCTs and inclusion 1869 proofs MUST be for a precertificate that corresponds to this 1870 certificate. 1872 7.2. TLS Feature X.509v3 Extension 1874 A certification authority SHOULD NOT issue any certificate that 1875 identifies the "transparency_info" TLS extension in a TLS feature 1876 extension [RFC7633], because TLS servers are not required to support 1877 the "transparency_info" TLS extension in order to participate in CT 1878 (see Section 6). 1880 8. Clients 1882 There are various different functions clients of logs might perform. 1883 We describe here some typical clients and how they should function. 1884 Any inconsistency may be used as evidence that a log has not behaved 1885 correctly, and the signatures on the data structures prevent the log 1886 from denying that misbehavior. 1888 All clients need various parameters in order to communicate with logs 1889 and verify their responses. These parameters are described in 1890 Section 4.1, but note that this document does not describe how the 1891 parameters are obtained, which is implementation-dependent (see, for 1892 example, [Chromium.Policy]). 1894 8.1. TLS Client 1896 8.1.1. Receiving SCTs and inclusion proofs 1898 TLS clients receive SCTs and inclusion proofs alongside or in 1899 certificates. CT-using TLS clients MUST implement all of the three 1900 mechanisms by which TLS servers may present SCTs (see Section 6). 1902 TLS clients that support the "transparency_info" TLS extension (see 1903 Section 6.5) SHOULD include it in ClientHello messages, with empty 1904 "extension_data". If a TLS server includes the "transparency_info" 1905 TLS extension when resuming a TLS session, the TLS client MUST abort 1906 the handshake. 1908 8.1.2. Reconstructing the TBSCertificate 1910 Validation of an SCT for a certificate (where the "type" of the 1911 "TransItem" is "x509_sct_v2") uses the unmodified TBSCertificate 1912 component of the certificate. 1914 Before an SCT for a precertificate (where the "type" of the 1915 "TransItem" is "precert_sct_v2") can be validated, the TBSCertificate 1916 component of the precertificate needs to be reconstructed from the 1917 TBSCertificate component of the certificate as follows: 1919 * Remove the Transparency Information extension (see Section 7.1). 1921 * Remove embedded v1 SCTs, identified by OID 1.3.6.1.4.1.11129.2.4.2 1922 (see section 3.3 of [RFC6962]). This allows embedded v1 and v2 1923 SCTs to co-exist in a certificate (see Appendix A). 1925 8.1.3. Validating SCTs 1927 In order to make use of a received SCT, the TLS client MUST first 1928 validate it as follows: 1930 * Compute the signature input by constructing a "TransItem" of type 1931 "x509_entry_v2" or "precert_entry_v2", depending on the SCT's 1932 "TransItem" type. The "TimestampedCertificateEntryDataV2" 1933 structure is constructed in the following manner: 1935 - "timestamp" is copied from the SCT. 1937 - "tbs_certificate" is the reconstructed TBSCertificate portion 1938 of the server certificate, as described in Section 8.1.2. 1940 - "issuer_key_hash" is computed as described in Section 4.7. 1942 - "sct_extensions" is copied from the SCT. 1944 * Verify the SCT's "signature" against the computed signature input 1945 using the public key of the corresponding log, which is identified 1946 by the "log_id". The required signature algorithm is one of the 1947 log's parameters. 1949 If the TLS client does not have the corresponding log's parameters, 1950 it cannot attempt to validate the SCT. When evaluating compliance 1951 (see Section 8.1.6), the TLS client will consider only those SCTs 1952 that it was able to validate. 1954 Note that SCT validation is not a substitute for the normal 1955 validation of the server certificate and its chain. 1957 8.1.4. Fetching inclusion proofs 1959 When a TLS client has validated a received SCT but does not yet 1960 possess a corresponding inclusion proof, the TLS client MAY request 1961 the inclusion proof directly from a log using "get-proof-by-hash" 1962 (Section 5.4) or "get-all-by-hash" (Section 5.5). 1964 Note that fetching inclusion proofs directly from a log will disclose 1965 to the log which TLS server the client has been communicating with. 1966 This may be regarded as a significant privacy concern, and so it is 1967 preferable for the TLS server to send the inclusion proofs (see 1968 Section 6.4). 1970 8.1.5. Validating inclusion proofs 1972 When a TLS client has received, or fetched, an inclusion proof (and 1973 an STH), it SHOULD proceed to verifying the inclusion proof to the 1974 provided STH. The TLS client SHOULD also verify consistency between 1975 the provided STH and an STH it knows about. 1977 If the TLS client holds an STH that predates the SCT, it MAY, in the 1978 process of auditing, request a new STH from the log (Section 5.2), 1979 then verify it by requesting a consistency proof (Section 5.3). Note 1980 that if the TLS client uses "get-all-by-hash", then it will already 1981 have the new STH. 1983 8.1.6. Evaluating compliance 1985 It is up to a client's local policy to specify the quantity and form 1986 of evidence (SCTs, inclusion proofs or a combination) needed to 1987 achieve compliance and how to handle non-compliance. 1989 A TLS client can only evaluate compliance if it has given the TLS 1990 server the opportunity to send SCTs and inclusion proofs by any of 1991 the three mechanisms that are mandatory to implement for CT-using TLS 1992 clients (see Section 8.1.1). Therefore, a TLS client MUST NOT 1993 evaluate compliance if it did not include both the 1994 "transparency_info" and "status_request" TLS extensions in the 1995 ClientHello. 1997 8.2. Monitor 1999 Monitors watch logs to check that they behave correctly, for 2000 certificates of interest, or both. For example, a monitor may be 2001 configured to report on all certificates that apply to a specific 2002 domain name when fetching new entries for consistency validation. 2004 A monitor MUST at least inspect every new entry in every log it 2005 watches, and it MAY also choose to keep copies of entire logs. 2007 To inspect all of the existing entries, the monitor SHOULD follow 2008 these steps once for each log: 2010 1. Fetch the current STH (Section 5.2). 2012 2. Verify the STH signature. 2014 3. Fetch all the entries in the tree corresponding to the STH 2015 (Section 5.6). 2017 4. If applicable, check each entry to see if it's a certificate of 2018 interest. 2020 5. Confirm that the tree made from the fetched entries produces the 2021 same hash as that in the STH. 2023 To inspect new entries, the monitor SHOULD follow these steps 2024 repeatedly for each log: 2026 1. Fetch the current STH (Section 5.2). Repeat until the STH 2027 changes. This document does not specify the polling frequency, 2028 to allow for experimentation. 2030 2. Verify the STH signature. 2032 3. Fetch all the new entries in the tree corresponding to the STH 2033 (Section 5.6). If they remain unavailable for an extended 2034 period, then this should be viewed as misbehavior on the part of 2035 the log. 2037 4. If applicable, check each entry to see if it's a certificate of 2038 interest. 2040 5. Either: 2042 1. Verify that the updated list of all entries generates a tree 2043 with the same hash as the new STH. 2045 Or, if it is not keeping all log entries: 2047 1. Fetch a consistency proof for the new STH with the previous 2048 STH (Section 5.3). 2050 2. Verify the consistency proof. 2052 3. Verify that the new entries generate the corresponding 2053 elements in the consistency proof. 2055 6. Repeat from step 1. 2057 8.3. Auditing 2059 Auditing ensures that the current published state of a log is 2060 reachable from previously published states that are known to be good, 2061 and that the promises made by the log in the form of SCTs have been 2062 kept. Audits are performed by monitors or TLS clients. 2064 In particular, there are four log behavior properties that should be 2065 checked: 2067 * The Maximum Merge Delay (MMD). 2069 * The STH Frequency Count. 2071 * The append-only property. 2073 * The consistency of the log view presented to all query sources. 2075 A benign, conformant log publishes a series of STHs over time, each 2076 derived from the previous STH and the submitted entries incorporated 2077 into the log since publication of the previous STH. This can be 2078 proven through auditing of STHs. SCTs returned to TLS clients can be 2079 audited by verifying against the accompanying certificate, and using 2080 Merkle Inclusion Proofs, against the log's Merkle tree. 2082 The action taken by the auditor if an audit fails is not specified, 2083 but note that in general if audit fails, the auditor is in possession 2084 of signed proof of the log's misbehavior. 2086 A monitor (Section 8.2) can audit by verifying the consistency of 2087 STHs it receives, ensure that each entry can be fetched and that the 2088 STH is indeed the result of making a tree from all fetched entries. 2090 A TLS client (Section 8.1) can audit by verifying an SCT against any 2091 STH dated after the SCT timestamp + the Maximum Merge Delay by 2092 requesting a Merkle inclusion proof (Section 5.4). It can also 2093 verify that the SCT corresponds to the server certificate it arrived 2094 with (i.e., the log entry is that certificate, or is a precertificate 2095 corresponding to that certificate). 2097 Checking of the consistency of the log view presented to all entities 2098 is more difficult to perform because it requires a way to share log 2099 responses among a set of CT-using entities, and is discussed in 2100 Section 11.3. 2102 9. Algorithm Agility 2104 It is not possible for a log to change any of its algorithms part way 2105 through its lifetime: 2107 Signature algorithm: SCT signatures must remain valid so signature 2108 algorithms can only be added, not removed. 2110 Hash algorithm: A log would have to support the old and new hash 2111 algorithms to allow backwards-compatibility with clients that are 2112 not aware of a hash algorithm change. 2114 Allowing multiple signature or hash algorithms for a log would 2115 require that all data structures support it and would significantly 2116 complicate client implementation, which is why it is not supported by 2117 this document. 2119 If it should become necessary to deprecate an algorithm used by a 2120 live log, then the log MUST be frozen as specified in Section 4.13 2121 and a new log SHOULD be started. Certificates in the frozen log that 2122 have not yet expired and require new SCTs SHOULD be submitted to the 2123 new log and the SCTs from that log used instead. 2125 10. IANA Considerations 2127 The assignment policy criteria mentioned in this section refer to the 2128 policies outlined in [RFC8126]. 2130 10.1. Additions to existing registries 2132 This sub-section defines additions to existing registries. 2134 10.1.1. New Entry to the TLS ExtensionType Registry 2136 IANA is asked to add the following entry to the "TLS ExtensionType 2137 Values" registry defined in [RFC8446], with an assigned Value: 2139 +=======+===================+============+=============+===========+ 2140 | Value | Extension Name | TLS 1.3 | Recommended | Reference | 2141 +=======+===================+============+=============+===========+ 2142 | TBD | transparency_info | CH, CR, CT | Y | RFCXXXX | 2143 +-------+-------------------+------------+-------------+-----------+ 2145 Table 7 2147 10.1.2. URN Sub-namespace for TRANS errors 2148 (urn:ietf:params:trans:error) 2150 IANA is requested to add a new entry in the "IETF URN Sub-namespace 2151 for Registered Protocol Parameter Identifiers" registry, following 2152 the template in [RFC3553]: 2154 Registry name: trans:error 2156 Specification: RFCXXXX 2158 Repository: https://www.iana.org/assignments/trans 2160 Index value: No transformation needed. 2162 10.2. New CT-Related registries 2164 IANA is requested to add a new protocol registry, "Public Notary 2165 Transparency", to the list that appears at https://www.iana.org/ 2166 assignments/ 2167 The rest of this section defines sub-registries to be created within 2168 the new Public Notary Transparency registry. 2170 10.2.1. Hash Algorithms 2172 IANA is asked to establish a registry of hash algorithm values, named 2173 "Hash Algorithms", that initially consists of: 2175 +========+============+========================+===================+ 2176 | Value | Hash | OID | Reference / | 2177 | | Algorithm | | Assignment Policy | 2178 +========+============+========================+===================+ 2179 | 0x00 | SHA-256 | 2.16.840.1.101.3.4.2.1 | [RFC6234] | 2180 +--------+------------+------------------------+-------------------+ 2181 | 0x01 - | Unassigned | | Specification | 2182 | 0xDF | | | Required | 2183 +--------+------------+------------------------+-------------------+ 2184 | 0xE0 - | Reserved | | Experimental Use | 2185 | 0xEF | | | | 2186 +--------+------------+------------------------+-------------------+ 2187 | 0xF0 - | Reserved | | Private Use | 2188 | 0xFF | | | | 2189 +--------+------------+------------------------+-------------------+ 2191 Table 8 2193 The Designated Expert(s) should ensure that the proposed algorithm 2194 has a public specification and is suitable for use as a cryptographic 2195 hash algorithm with no known preimage or collision attacks. These 2196 attacks can damage the integrity of the log. 2198 10.2.2. Signature Algorithms 2200 IANA is asked to establish a registry of signature algorithm values, 2201 named "Signature Algorithms". 2203 The following notes should be added: 2205 * This is a subset of the TLS SignatureScheme Registry, limited to 2206 those algorithms that are appropriate for CT. A major advantage 2207 of this is leveraging the expertise of the TLS working group and 2208 its Designated Expert(s). 2210 * The value "0x0403" appears twice. While this may be confusing, it 2211 is okay because the verification process is the same for both 2212 algorithms, and the choice of which to use when generating a 2213 signature is purely internal to the log server. 2215 The registry should initially consist of: 2217 +================================+==================+==============+ 2218 | SignatureScheme Value | Signature | Reference / | 2219 | | Algorithm | Assignment | 2220 | | | Policy | 2221 +================================+==================+==============+ 2222 | 0x0000 - 0x0402 | Unassigned | Expert | 2223 | | | Review | 2224 +--------------------------------+------------------+--------------+ 2225 | ecdsa_secp256r1_sha256(0x0403) | ECDSA (NIST | [FIPS186-4] | 2226 | | P-256) with | | 2227 | | SHA-256 | | 2228 +--------------------------------+------------------+--------------+ 2229 | ecdsa_secp256r1_sha256(0x0403) | Deterministic | [RFC6979] | 2230 | | ECDSA (NIST | | 2231 | | P-256) with | | 2232 | | HMAC-SHA256 | | 2233 +--------------------------------+------------------+--------------+ 2234 | 0x0404 - 0x0806 | Unassigned | Expert | 2235 | | | Review | 2236 +--------------------------------+------------------+--------------+ 2237 | ed25519(0x0807) | Ed25519 | [RFC8032] | 2238 | | (PureEdDSA with | | 2239 | | the edwards25519 | | 2240 | | curve) | | 2241 +--------------------------------+------------------+--------------+ 2242 | 0x0808 - 0xFDFF | Unassigned | Expert | 2243 | | | Review | 2244 +--------------------------------+------------------+--------------+ 2245 | 0xFE00 - 0xFEFF | Reserved | Experimental | 2246 | | | Use | 2247 +--------------------------------+------------------+--------------+ 2248 | 0xFF00 - 0xFFFF | Reserved | Private Use | 2249 +--------------------------------+------------------+--------------+ 2251 Table 9 2253 The Designated Expert(s) should ensure that the proposed algorithm 2254 has a public specification, has a value assigned to it in the TLS 2255 SignatureScheme Registry (that IANA was asked to establish in 2256 [RFC8446]) and is suitable for use as a cryptographic signature 2257 algorithm. 2259 10.2.3. VersionedTransTypes 2261 IANA is asked to establish a registry of "VersionedTransType" values, 2262 named "VersionedTransTypes". 2264 The following note should be added: 2266 * The 0x0000 value is reserved so that v1 SCTs are distinguishable 2267 from v2 SCTs and other "TransItem" structures. 2269 The registry should initially consist of: 2271 +==========+======================+===============================+ 2272 | Value | Type and Version | Reference / Assignment Policy | 2273 +==========+======================+===============================+ 2274 | 0x0000 | Reserved | [RFC6962] | 2275 +----------+----------------------+-------------------------------+ 2276 | 0x0001 | x509_entry_v2 | RFCXXXX | 2277 +----------+----------------------+-------------------------------+ 2278 | 0x0002 | precert_entry_v2 | RFCXXXX | 2279 +----------+----------------------+-------------------------------+ 2280 | 0x0003 | x509_sct_v2 | RFCXXXX | 2281 +----------+----------------------+-------------------------------+ 2282 | 0x0004 | precert_sct_v2 | RFCXXXX | 2283 +----------+----------------------+-------------------------------+ 2284 | 0x0005 | signed_tree_head_v2 | RFCXXXX | 2285 +----------+----------------------+-------------------------------+ 2286 | 0x0006 | consistency_proof_v2 | RFCXXXX | 2287 +----------+----------------------+-------------------------------+ 2288 | 0x0007 | inclusion_proof_v2 | RFCXXXX | 2289 +----------+----------------------+-------------------------------+ 2290 | 0x0008 - | Unassigned | Specification Required | 2291 | 0xDFFF | | | 2292 +----------+----------------------+-------------------------------+ 2293 | 0xE000 - | Reserved | Experimental Use | 2294 | 0xEFFF | | | 2295 +----------+----------------------+-------------------------------+ 2296 | 0xF000 - | Reserved | Private Use | 2297 | 0xFFFF | | | 2298 +----------+----------------------+-------------------------------+ 2300 Table 10 2302 The Designated Expert(s) should review the public specification to 2303 ensure that it is detailed enough to ensure implementation 2304 interoperability. 2306 10.2.4. Log Artifact Extension Registry 2308 IANA is asked to establish a registry of "ExtensionType" values, 2309 named "Log Artifact Extensions", that initially consists of: 2311 +===============+============+=====+===============================+ 2312 | ExtensionType | Status | Use | Reference / Assignment Policy | 2313 +===============+============+=====+===============================+ 2314 | 0x0000 - | Unassigned | n/a | Specification Required | 2315 | 0xDFFF | | | | 2316 +---------------+------------+-----+-------------------------------+ 2317 | 0xE000 - | Reserved | n/a | Experimental Use | 2318 | 0xEFFF | | | | 2319 +---------------+------------+-----+-------------------------------+ 2320 | 0xF000 - | Reserved | n/a | Private Use | 2321 | 0xFFFF | | | | 2322 +---------------+------------+-----+-------------------------------+ 2324 Table 11 2326 The "Use" column should contain one or both of the following values: 2328 * "SCT", for extensions specified for use in Signed Certificate 2329 Timestamps. 2331 * "STH", for extensions specified for use in Signed Tree Heads. 2333 The Designated Expert(s) should review the public specification to 2334 ensure that it is detailed enough to ensure implementation 2335 interoperability. They should also verify that the extension is 2336 appropriate to the contexts in which it is specified to be used (SCT, 2337 STH, or both). 2339 10.2.5. Log IDs Registry 2341 IANA is asked to establish a registry of Log IDs, named "Log IDs", 2342 that initially consists of: 2344 +================+==============+==============+===================+ 2345 | Log ID | Log Base URL | Log Operator | Reference / | 2346 | | | | Assignment Policy | 2347 +================+==============+==============+===================+ 2348 | 1.3.101.8192 - | Unassigned | Unassigned | First Come First | 2349 | 1.3.101.16383 | | | Served | 2350 +----------------+--------------+--------------+-------------------+ 2351 | 1.3.101.80.0 - | Unassigned | Unassigned | First Come First | 2352 | 1.3.101.80.* | | | Served | 2353 +----------------+--------------+--------------+-------------------+ 2355 Table 12 2357 All OIDs in the range from 1.3.101.8192 to 1.3.101.16383 have been 2358 set aside for Log IDs. This is a limited resource of 8,192 OIDs, 2359 each of which has an encoded length of 4 octets. 2361 The 1.3.101.80 arc has also been set assigned for LogIDs. This is an 2362 unlimited resource, but only the 128 OIDs from 1.3.101.80.0 to 2363 1.3.101.80.127 have an encoded length of only 4 octets. 2365 Each application for the allocation of a Log ID MUST be accompanied 2366 by: 2368 * the Log's Base URL (see Section 4.1). 2370 * the Log Operator's contact details. 2372 IANA is asked to reject any request to update a Log ID or Log Base 2373 URL in this registry, because these fields are immutable (see 2374 Section 4.1). 2376 IANA is asked to accept requests from log operators to update their 2377 contact details in this registry. 2379 Since log operators can choose to not use this registry (see 2380 Section 4.4), it is not expected to be a global directory of all 2381 logs. 2383 10.2.6. Error Types Registry 2385 IANA is requested to create a new registry for errors, the "Error 2386 Types" registry. 2388 Requirements for this registry are Specification Required. 2390 This registry should have the following three fields: 2392 +============+========+===========+ 2393 | Field Name | Type | Reference | 2394 +============+========+===========+ 2395 | identifier | string | RFCXXXX | 2396 +------------+--------+-----------+ 2397 | meaning | string | RFCXXXX | 2398 +------------+--------+-----------+ 2399 | reference | string | RFCXXXX | 2400 +------------+--------+-----------+ 2402 Table 13 2404 The initial values are as follows, taken from the text above: 2406 +===================+===============================+===========+ 2407 | Identifier | Meaning | Reference | 2408 +===================+===============================+===========+ 2409 | malformed | The request could not be | RFCXXXX | 2410 | | parsed. | | 2411 +-------------------+-------------------------------+-----------+ 2412 | badSubmission | "submission" is neither a | RFCXXXX | 2413 | | valid certificate nor a valid | | 2414 | | precertificate | | 2415 +-------------------+-------------------------------+-----------+ 2416 | badType | "type" is neither 1 nor 2 | RFCXXXX | 2417 +-------------------+-------------------------------+-----------+ 2418 | badChain | The first element of "chain" | RFCXXXX | 2419 | | is not the certifier of the | | 2420 | | "submission", or the second | | 2421 | | element does not certify the | | 2422 | | first, etc. | | 2423 +-------------------+-------------------------------+-----------+ 2424 | badCertificate | One or more certificates in | RFCXXXX | 2425 | | the "chain" are not valid | | 2426 | | (e.g., not properly encoded) | | 2427 +-------------------+-------------------------------+-----------+ 2428 | unknownAnchor | The last element of "chain" | RFCXXXX | 2429 | | (or, if "chain" is an empty | | 2430 | | array, the "submission") both | | 2431 | | is not, and is not certified | | 2432 | | by, an accepted trust anchor | | 2433 +-------------------+-------------------------------+-----------+ 2434 | shutdown | The log is no longer | RFCXXXX | 2435 | | accepting submissions | | 2436 +-------------------+-------------------------------+-----------+ 2437 | firstUnknown | "first" is before the latest | RFCXXXX | 2438 | | known STH but is not from an | | 2439 | | existing STH. | | 2440 +-------------------+-------------------------------+-----------+ 2441 | secondUnknown | "second" is before the latest | RFCXXXX | 2442 | | known STH but is not from an | | 2443 | | existing STH. | | 2444 +-------------------+-------------------------------+-----------+ 2445 | secondBeforeFirst | "second" is smaller than | RFCXXXX | 2446 | | "first". | | 2447 +-------------------+-------------------------------+-----------+ 2448 | hashUnknown | "hash" is not the hash of a | RFCXXXX | 2449 | | known leaf (may be caused by | | 2450 | | skew or by a known | | 2451 | | certificate not yet merged). | | 2452 +-------------------+-------------------------------+-----------+ 2453 | treeSizeUnknown | "hash" is before the latest | RFCXXXX | 2454 | | known STH but is not from an | | 2455 | | existing STH. | | 2456 +-------------------+-------------------------------+-----------+ 2457 | startUnknown | "start" is greater than the | RFCXXXX | 2458 | | number of entries in the | | 2459 | | Merkle tree. | | 2460 +-------------------+-------------------------------+-----------+ 2461 | endBeforeStart | "start" cannot be greater | RFCXXXX | 2462 | | than "end". | | 2463 +-------------------+-------------------------------+-----------+ 2465 Table 14 2467 10.3. OID Assignment 2469 IANA is asked to assign one object identifier from the "SMI Security 2470 for PKIX Module Identifier" registry to identify the ASN.1 module in 2471 Appendix B of this document with an assigned Decimal value. 2473 +=========+=========================+============+ 2474 | Decimal | Description | References | 2475 +=========+=========================+============+ 2476 | TBD | id-mod-public-notary-v2 | RFCXXXX | 2477 +---------+-------------------------+------------+ 2479 Table 15 2481 11. Security Considerations 2483 With CAs, logs, and servers performing the actions described here, 2484 TLS clients can use logs and signed timestamps to reduce the 2485 likelihood that they will accept misissued certificates. If a server 2486 presents a valid signed timestamp for a certificate, then the client 2487 knows that a log has committed to publishing the certificate. From 2488 this, the client knows that monitors acting for the subject of the 2489 certificate have had some time to notice the misissuance and take 2490 some action, such as asking a CA to revoke a misissued certificate. 2491 A signed timestamp does not guarantee this though, since appropriate 2492 monitors might not have checked the logs or the CA might have refused 2493 to revoke the certificate. 2495 In addition, if TLS clients will not accept unlogged certificates, 2496 then site owners will have a greater incentive to submit certificates 2497 to logs, possibly with the assistance of their CA, increasing the 2498 overall transparency of the system. 2500 11.1. Misissued Certificates 2502 Misissued certificates that have not been publicly logged, and thus 2503 do not have a valid SCT, are not considered compliant. Misissued 2504 certificates that do have an SCT from a log will appear in that 2505 public log within the Maximum Merge Delay, assuming the log is 2506 operating correctly. Since a log is allowed to serve an STH of any 2507 age up to the MMD, the maximum period of time during which a 2508 misissued certificate can be used without being available for audit 2509 is twice the MMD. 2511 11.2. Detection of Misissue 2513 The logs do not themselves detect misissued certificates; they rely 2514 instead on interested parties, such as domain owners, to monitor them 2515 and take corrective action when a misissue is detected. 2517 11.3. Misbehaving Logs 2519 A log can misbehave in several ways. Examples include: failing to 2520 incorporate a certificate with an SCT in the Merkle Tree within the 2521 MMD; presenting different, conflicting views of the Merkle Tree at 2522 different times and/or to different parties; issuing STHs too 2523 frequently; mutating the signature of a logged certificate; and 2524 failing to present a chain containing the certifier of a logged 2525 certificate. 2527 Violation of the MMD contract is detected by log clients requesting a 2528 Merkle inclusion proof (Section 5.4) for each observed SCT. These 2529 checks can be asynchronous and need only be done once per 2530 certificate. However, note that there may be privacy concerns (see 2531 Section 8.1.4). 2533 Violation of the append-only property or the STH issuance rate limit 2534 can be detected by multiple clients comparing their instances of the 2535 STHs. This technique, known as "gossip," is an active area of 2536 research and not defined here. Proof of misbehavior in such cases 2537 would be: a series of STHs that were issued too closely together, 2538 proving violation of the STH issuance rate limit; or an STH with a 2539 root hash that does not match the one calculated from a copy of the 2540 log, proving violation of the append-only property. 2542 Clients that report back SCTs can be tracked or traced if a log 2543 produces multiple STHs or SCTs with the same timestamp and data but 2544 different signatures. Logs SHOULD mitigate this risk by either: 2546 * Using deterministic signature schemes, or 2547 * Producing no more than one SCT for each distinct submission and no 2548 more than one STH for each distinct tree_size. Each of these SCTs 2549 and STHs can be stored by the log and served to other clients that 2550 submit the same certificate or request the same STH. 2552 11.4. Multiple SCTs 2554 By requiring TLS servers to offer multiple SCTs, each from a 2555 different log, TLS clients reduce the effectiveness of an attack 2556 where a CA and a log collude (see Section 6.2). 2558 11.5. Leakage of DNS Information 2560 Malicious monitors can use logs to learn about the existence of 2561 domain names that might not otherwise be easy to discover. Some 2562 subdomain labels may reveal information about the service and 2563 software for which the subdomain is used, which in turn might 2564 facilitate targeted attacks. 2566 12. Acknowledgements 2568 The authors would like to thank Erwann Abelea, Robin Alden, Andrew 2569 Ayer, Richard Barnes, Al Cutter, David Drysdale, Francis Dupont, Adam 2570 Eijdenberg, Stephen Farrell, Daniel Kahn Gillmor, Paul Hadfield, Brad 2571 Hill, Jeff Hodges, Paul Hoffman, Jeffrey Hutzelman, Kat Joyce, 2572 Stephen Kent, SM, Alexey Melnikov, Linus Nordberg, Chris Palmer, 2573 Trevor Perrin, Pierre Phaneuf, Eric Rescorla, Rich Salz, Melinda 2574 Shore, Ryan Sleevi, Martin Smith, Carl Wallace and Paul Wouters for 2575 their valuable contributions. 2577 A big thank you to Symantec for kindly donating the OIDs from the 2578 1.3.101 arc that are used in this document. 2580 13. References 2582 13.1. Normative References 2584 [FIPS186-4] 2585 NIST, "FIPS PUB 186-4", 1 July 2013, 2586 . 2589 [HTML401] Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01 2590 Specification", World Wide Web Consortium Recommendation 2591 REC-html401-19991224, 24 December 1999, 2592 . 2594 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2595 Requirement Levels", BCP 14, RFC 2119, 2596 DOI 10.17487/RFC2119, March 1997, 2597 . 2599 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 2600 IETF URN Sub-namespace for Registered Protocol 2601 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 2602 2003, . 2604 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2605 Resource Identifier (URI): Generic Syntax", STD 66, 2606 RFC 3986, DOI 10.17487/RFC3986, January 2005, 2607 . 2609 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 2610 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 2611 . 2613 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 2614 (TLS) Protocol Version 1.2", RFC 5246, 2615 DOI 10.17487/RFC5246, August 2008, 2616 . 2618 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 2619 Housley, R., and W. Polk, "Internet X.509 Public Key 2620 Infrastructure Certificate and Certificate Revocation List 2621 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 2622 . 2624 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, 2625 RFC 5652, DOI 10.17487/RFC5652, September 2009, 2626 . 2628 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 2629 Extensions: Extension Definitions", RFC 6066, 2630 DOI 10.17487/RFC6066, January 2011, 2631 . 2633 [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms 2634 (SHA and SHA-based HMAC and HKDF)", RFC 6234, 2635 DOI 10.17487/RFC6234, May 2011, 2636 . 2638 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., 2639 Galperin, S., and C. Adams, "X.509 Internet Public Key 2640 Infrastructure Online Certificate Status Protocol - OCSP", 2641 RFC 6960, DOI 10.17487/RFC6960, June 2013, 2642 . 2644 [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature 2645 Algorithm (DSA) and Elliptic Curve Digital Signature 2646 Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2647 2013, . 2649 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 2650 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 2651 DOI 10.17487/RFC7231, June 2014, 2652 . 2654 [RFC7633] Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS) 2655 Feature Extension", RFC 7633, DOI 10.17487/RFC7633, 2656 October 2015, . 2658 [RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP 2659 APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016, 2660 . 2662 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 2663 Signature Algorithm (EdDSA)", RFC 8032, 2664 DOI 10.17487/RFC8032, January 2017, 2665 . 2667 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2668 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2669 May 2017, . 2671 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 2672 Interchange Format", STD 90, RFC 8259, 2673 DOI 10.17487/RFC8259, December 2017, 2674 . 2676 [RFC8391] Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A. 2677 Mohaisen, "XMSS: eXtended Merkle Signature Scheme", 2678 RFC 8391, DOI 10.17487/RFC8391, May 2018, 2679 . 2681 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 2682 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 2683 . 2685 [UNIXTIME] IEEE, "The Open Group Base Specifications Issue 7 IEEE Std 2686 1003.1-2008, 2016 Edition", n.d., 2687 . 2691 [X690] ITU-T, "Information technology - ASN.1 encoding Rules: 2692 Specification of Basic Encoding Rules (BER), Canonical 2693 Encoding Rules (CER) and Distinguished Encoding Rules 2694 (DER)", ISO/IEC 8825-1:2002, November 2015. 2696 13.2. Informative References 2698 [CABBR] CA/Browser Forum, "Baseline Requirements for the Issuance 2699 and Management of Publicly-Trusted Certificates", 2020, 2700 . 2703 [Chromium.Log.Policy] 2704 The Chromium Projects, "Chromium Certificate Transparency 2705 Log Policy", 2014, . 2708 [Chromium.Policy] 2709 The Chromium Projects, "Chromium Certificate 2710 Transparency", 2014, . 2713 [CrosbyWallach] 2714 Crosby, S. and D. Wallach, "Efficient Data Structures for 2715 Tamper-Evident Logging", Proceedings of the 18th USENIX 2716 Security Symposium, Montreal, August 2009, 2717 . 2720 [JSON.Metadata] 2721 The Chromium Projects, "Chromium Log Metadata JSON 2722 Schema", 2014, . 2725 [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate 2726 Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, 2727 . 2729 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2730 Writing an IANA Considerations Section in RFCs", BCP 26, 2731 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2732 . 2734 [RFC8820] Nottingham, M., "URI Design and Ownership", BCP 190, 2735 RFC 8820, DOI 10.17487/RFC8820, June 2020, 2736 . 2738 Appendix A. Supporting v1 and v2 simultaneously (Informative) 2740 Certificate Transparency logs have to be either v1 (conforming to 2741 [RFC6962]) or v2 (conforming to this document), as the data 2742 structures are incompatible and so a v2 log could not issue a valid 2743 v1 SCT. 2745 CT clients, however, can support v1 and v2 SCTs, for the same 2746 certificate, simultaneously, as v1 SCTs are delivered in different 2747 TLS, X.509 and OCSP extensions than v2 SCTs. 2749 v1 and v2 SCTs for X.509 certificates can be validated independently. 2750 For precertificates, v2 SCTs should be embedded in the TBSCertificate 2751 before submission of the TBSCertificate (inside a v1 precertificate, 2752 as described in Section 3.1. of [RFC6962]) to a v1 log so that TLS 2753 clients conforming to [RFC6962] but not this document are oblivious 2754 to the embedded v2 SCTs. An issuer can follow these steps to produce 2755 an X.509 certificate with embedded v1 and v2 SCTs: 2757 * Create a CMS precertificate as described in Section 3.2 and submit 2758 it to v2 logs. 2760 * Embed the obtained v2 SCTs in the TBSCertificate, as described in 2761 Section 7.1.2. 2763 * Use that TBSCertificate to create a v1 precertificate, as 2764 described in Section 3.1. of [RFC6962] and submit it to v1 logs. 2766 * Embed the v1 SCTs in the TBSCertificate, as described in 2767 Section 3.3 of [RFC6962]. 2769 * Sign that TBSCertificate (which now contains v1 and v2 SCTs) to 2770 issue the final X.509 certificate. 2772 Appendix B. An ASN.1 Module (Informative) 2774 The following ASN.1 module may be useful to implementors. 2776 CertificateTransparencyV2Module-2021 2777 -- { OID Needed, but no point in using a short one } 2778 DEFINITIONS IMPLICIT TAGS ::= BEGIN 2780 -- EXPORTS ALL -- 2781 IMPORTS 2782 EXTENSION 2783 FROM PKIX-CommonTypes-2009 -- RFC 5912 2784 { iso(1) identified-organization(3) dod(6) internet(1) 2785 security(5) mechanisms(5) pkix(7) id-mod(0) 2786 id-mod-pkixCommon-02(57) } 2788 CONTENT-TYPE 2789 FROM CryptographicMessageSyntax-2010 -- RFC 6268 2790 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) 2791 pkcs-9(9) smime(16) modules(0) id-mod-cms-2009(58) } 2793 TBSCertificate 2794 FROM PKIX1Explicit-2009 -- RFC 5912 2795 { iso(1) identified-organization(3) dod(6) internet(1) 2796 security(5) mechanisms(5) pkix(7) id-mod(0) 2797 id-mod-pkix1-explicit-02(51) } 2798 ; 2800 -- 2801 -- Section 3.2. Precertificates 2802 -- 2804 ct-tbsCertificate CONTENT-TYPE ::= { 2805 TYPE TBSCertificate 2806 IDENTIFIED BY id-ct-tbsCertificate } 2808 id-ct-tbsCertificate OBJECT IDENTIFIER ::= { 1 3 101 78 } 2810 -- 2811 -- Section 7.1. Transparency Information X.509v3 Extension 2812 -- 2814 ext-transparencyInfo EXTENSION ::= { 2815 SYNTAX TransparencyInformationSyntax 2816 IDENTIFIED BY id-ce-transparencyInfo 2817 CRITICALITY { FALSE } } 2819 id-ce-transparencyInfo OBJECT IDENTIFIER ::= { 1 3 101 75 } 2821 TransparencyInformationSyntax ::= OCTET STRING 2823 -- 2824 -- Section 7.1.1. OCSP Response Extension 2825 -- 2827 ext-ocsp-transparencyInfo EXTENSION ::= { 2828 SYNTAX TransparencyInformationSyntax 2829 IDENTIFIED BY id-pkix-ocsp-transparencyInfo 2830 CRITICALITY { FALSE } } 2832 id-pkix-ocsp-transparencyInfo OBJECT IDENTIFIER ::= 2833 id-ce-transparencyInfo 2835 -- 2836 -- Section 8.1.2. Reconstructing the TBSCertificate 2837 -- 2839 ext-embeddedSCT-CTv1 EXTENSION ::= { 2840 SYNTAX SignedCertificateTimestampList 2841 IDENTIFIED BY id-ce-embeddedSCT-CTv1 2842 CRITICALITY { FALSE } } 2844 id-ce-embeddedSCT-CTv1 OBJECT IDENTIFIER ::= { 2845 1 3 6 1 4 1 11129 2 4 2 } 2847 SignedCertificateTimestampList ::= OCTET STRING 2849 END 2851 Authors' Addresses 2853 Ben Laurie 2854 Google UK Ltd. 2856 Email: benl@google.com 2858 Adam Langley 2859 Google Inc. 2861 Email: agl@google.com 2863 Emilia Kasper 2864 Google Switzerland GmbH 2866 Email: ekasper@google.com 2868 Eran Messeri 2869 Google UK Ltd. 2871 Email: eranm@google.com 2872 Rob Stradling 2873 Sectigo Ltd. 2875 Email: rob@sectigo.com