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2 TRANS (Public Notary Transparency) B. Laurie
3 Internet-Draft A. Langley
4 Obsoletes: 6962 (if approved) E. Kasper
5 Intended status: Experimental E. Messeri
6 Expires: 14 November 2021 Google
7 R. Stradling
8 Sectigo
9 13 May 2021
11 Certificate Transparency Version 2.0
12 draft-ietf-trans-rfc6962-bis-37
14 Abstract
16 This document describes version 2.0 of the Certificate Transparency
17 (CT) protocol for publicly logging the existence of Transport Layer
18 Security (TLS) server certificates as they are issued or observed, in
19 a manner that allows anyone to audit certification authority (CA)
20 activity and notice the issuance of suspect certificates as well as
21 to audit the certificate logs themselves. The intent is that
22 eventually clients would refuse to honor certificates that do not
23 appear in a log, effectively forcing CAs to add all issued
24 certificates to the logs.
26 This document obsoletes RFC 6962. It also specifies a new TLS
27 extension that is used to send various CT log artifacts.
29 Logs are network services that implement the protocol operations for
30 submissions and queries that are defined in this document.
32 [RFC Editor: please update 'RFCXXXX' to refer to this document, once
33 its RFC number is known, through the document.]
35 Status of This Memo
37 This Internet-Draft is submitted in full conformance with the
38 provisions of BCP 78 and BCP 79.
40 Internet-Drafts are working documents of the Internet Engineering
41 Task Force (IETF). Note that other groups may also distribute
42 working documents as Internet-Drafts. The list of current Internet-
43 Drafts is at https://datatracker.ietf.org/drafts/current/.
45 Internet-Drafts are draft documents valid for a maximum of six months
46 and may be updated, replaced, or obsoleted by other documents at any
47 time. It is inappropriate to use Internet-Drafts as reference
48 material or to cite them other than as "work in progress."
49 This Internet-Draft will expire on 14 November 2021.
51 Copyright Notice
53 Copyright (c) 2021 IETF Trust and the persons identified as the
54 document authors. All rights reserved.
56 This document is subject to BCP 78 and the IETF Trust's Legal
57 Provisions Relating to IETF Documents (https://trustee.ietf.org/
58 license-info) in effect on the date of publication of this document.
59 Please review these documents carefully, as they describe your rights
60 and restrictions with respect to this document. Code Components
61 extracted from this document must include Simplified BSD License text
62 as described in Section 4.e of the Trust Legal Provisions and are
63 provided without warranty as described in the Simplified BSD License.
65 Table of Contents
67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
68 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
69 1.2. Data Structures . . . . . . . . . . . . . . . . . . . . . 5
70 1.3. Major Differences from CT 1.0 . . . . . . . . . . . . . . 5
71 2. Cryptographic Components . . . . . . . . . . . . . . . . . . 7
72 2.1. Merkle Hash Trees . . . . . . . . . . . . . . . . . . . . 7
73 2.1.1. Definition of the Merkle Tree . . . . . . . . . . . . 7
74 2.1.2. Verifying a Tree Head Given Entries . . . . . . . . . 8
75 2.1.3. Merkle Inclusion Proofs . . . . . . . . . . . . . . . 9
76 2.1.4. Merkle Consistency Proofs . . . . . . . . . . . . . . 10
77 2.1.5. Example . . . . . . . . . . . . . . . . . . . . . . . 13
78 2.2. Signatures . . . . . . . . . . . . . . . . . . . . . . . 14
79 3. Submitters . . . . . . . . . . . . . . . . . . . . . . . . . 15
80 3.1. Certificates . . . . . . . . . . . . . . . . . . . . . . 15
81 3.2. Precertificates . . . . . . . . . . . . . . . . . . . . . 15
82 3.2.1. Binding Intent to Issue . . . . . . . . . . . . . . . 17
83 4. Log Format and Operation . . . . . . . . . . . . . . . . . . 17
84 4.1. Log Parameters . . . . . . . . . . . . . . . . . . . . . 18
85 4.2. Evaluating Submissions . . . . . . . . . . . . . . . . . 19
86 4.2.1. Minimum Acceptance Criteria . . . . . . . . . . . . . 19
87 4.2.2. Discretionary Acceptance Criteria . . . . . . . . . . 20
88 4.3. Log Entries . . . . . . . . . . . . . . . . . . . . . . . 20
89 4.4. Log ID . . . . . . . . . . . . . . . . . . . . . . . . . 21
90 4.5. TransItem Structure . . . . . . . . . . . . . . . . . . . 21
91 4.6. Log Artifact Extensions . . . . . . . . . . . . . . . . . 22
92 4.7. Merkle Tree Leaves . . . . . . . . . . . . . . . . . . . 23
93 4.8. Signed Certificate Timestamp (SCT) . . . . . . . . . . . 24
94 4.9. Merkle Tree Head . . . . . . . . . . . . . . . . . . . . 25
95 4.10. Signed Tree Head (STH) . . . . . . . . . . . . . . . . . 26
96 4.11. Merkle Consistency Proofs . . . . . . . . . . . . . . . . 26
97 4.12. Merkle Inclusion Proofs . . . . . . . . . . . . . . . . . 27
98 4.13. Shutting down a log . . . . . . . . . . . . . . . . . . . 28
99 5. Log Client Messages . . . . . . . . . . . . . . . . . . . . . 28
100 5.1. Submit Entry to Log . . . . . . . . . . . . . . . . . . . 30
101 5.2. Retrieve Latest Signed Tree Head . . . . . . . . . . . . 32
102 5.3. Retrieve Merkle Consistency Proof between Two Signed Tree
103 Heads . . . . . . . . . . . . . . . . . . . . . . . . . . 32
104 5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash . . 33
105 5.5. Retrieve Merkle Inclusion Proof, Signed Tree Head and
106 Consistency Proof by Leaf Hash . . . . . . . . . . . . . 34
107 5.6. Retrieve Entries and STH from Log . . . . . . . . . . . . 35
108 5.7. Retrieve Accepted Trust Anchors . . . . . . . . . . . . . 37
109 6. TLS Servers . . . . . . . . . . . . . . . . . . . . . . . . . 38
110 6.1. TLS Client Authentication . . . . . . . . . . . . . . . . 38
111 6.2. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . . 39
112 6.3. TransItemList Structure . . . . . . . . . . . . . . . . . 39
113 6.4. Presenting SCTs, inclusions proofs and STHs . . . . . . . 40
114 6.5. transparency_info TLS Extension . . . . . . . . . . . . . 40
115 7. Certification Authorities . . . . . . . . . . . . . . . . . . 41
116 7.1. Transparency Information X.509v3 Extension . . . . . . . 41
117 7.1.1. OCSP Response Extension . . . . . . . . . . . . . . . 41
118 7.1.2. Certificate Extension . . . . . . . . . . . . . . . . 41
119 7.2. TLS Feature X.509v3 Extension . . . . . . . . . . . . . . 41
120 8. Clients . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
121 8.1. TLS Client . . . . . . . . . . . . . . . . . . . . . . . 42
122 8.1.1. Receiving SCTs and inclusion proofs . . . . . . . . . 42
123 8.1.2. Reconstructing the TBSCertificate . . . . . . . . . . 42
124 8.1.3. Validating SCTs . . . . . . . . . . . . . . . . . . . 42
125 8.1.4. Fetching inclusion proofs . . . . . . . . . . . . . . 43
126 8.1.5. Validating inclusion proofs . . . . . . . . . . . . . 43
127 8.1.6. Evaluating compliance . . . . . . . . . . . . . . . . 44
128 8.2. Monitor . . . . . . . . . . . . . . . . . . . . . . . . . 44
129 8.3. Auditing . . . . . . . . . . . . . . . . . . . . . . . . 45
130 9. Algorithm Agility . . . . . . . . . . . . . . . . . . . . . . 46
131 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
132 10.1. Additions to existing registries . . . . . . . . . . . . 47
133 10.1.1. New Entry to the TLS ExtensionType Registry . . . . 47
134 10.1.2. URN Sub-namespace for TRANS errors
135 (urn:ietf:params:trans:error) . . . . . . . . . . . . 47
136 10.2. New CT-Related registries . . . . . . . . . . . . . . . 47
137 10.2.1. Hash Algorithms . . . . . . . . . . . . . . . . . . 47
138 10.2.2. Signature Algorithms . . . . . . . . . . . . . . . . 48
139 10.2.3. VersionedTransTypes . . . . . . . . . . . . . . . . 49
140 10.2.4. Log Artifact Extension Registry . . . . . . . . . . 50
141 10.2.5. Object Identifiers . . . . . . . . . . . . . . . . . 51
142 10.2.6. CT Error Types Registry . . . . . . . . . . . . . . 52
143 11. Security Considerations . . . . . . . . . . . . . . . . . . . 54
144 11.1. Misissued Certificates . . . . . . . . . . . . . . . . . 55
145 11.2. Detection of Misissue . . . . . . . . . . . . . . . . . 55
146 11.3. Misbehaving Logs . . . . . . . . . . . . . . . . . . . . 55
147 11.4. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . 56
148 11.5. Leakage of DNS Information . . . . . . . . . . . . . . . 56
149 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 56
150 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 56
151 13.1. Normative References . . . . . . . . . . . . . . . . . . 56
152 13.2. Informative References . . . . . . . . . . . . . . . . . 59
153 Appendix A. Supporting v1 and v2 simultaneously (Informative) . 60
154 Appendix B. An ASN.1 Module (Informative) . . . . . . . . . . . 60
155 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 62
157 1. Introduction
159 Certificate Transparency aims to mitigate the problem of misissued
160 certificates by providing append-only logs of issued certificates.
161 The logs do not themselves prevent misissuance, but they ensure that
162 interested parties (particularly those named in certificates) can
163 detect such misissuance. Note that this is a general mechanism that
164 could be used for transparently logging any form of binary data,
165 subject to some kind of inclusion criteria. In this document, we
166 only describe its use for public TLS server certificates (i.e., where
167 the inclusion criteria is a valid certificate issued by a public
168 certification authority (CA)). A typical definition of "public" can
169 be found in [CABBR].
171 Each log contains certificate chains, which can be submitted by
172 anyone. It is expected that public CAs will contribute all their
173 newly issued certificates to one or more logs; however certificate
174 holders can also contribute their own certificate chains, as can
175 third parties. In order to avoid logs being rendered useless by the
176 submission of large numbers of spurious certificates, it is required
177 that each chain ends with a trust anchor that is accepted by the log.
178 A log may also limit the length of the chain it is willing to accept;
179 such chains must also end with an acceptable trust anchor. When a
180 chain is accepted by a log, a signed timestamp is returned, which can
181 later be used to provide evidence to TLS clients that the chain has
182 been submitted. TLS clients can thus require that all certificates
183 they accept as valid are accompanied by signed timestamps.
185 Those who are concerned about misissuance can monitor the logs,
186 asking them regularly for all new entries, and can thus check whether
187 domains for which they are responsible have had certificates issued
188 that they did not expect. What they do with this information,
189 particularly when they find that a misissuance has happened, is
190 beyond the scope of this document. However, broadly speaking, they
191 can invoke existing business mechanisms for dealing with misissued
192 certificates, such as working with the CA to get the certificate
193 revoked, or with maintainers of trust anchor lists to get the CA
194 removed. Of course, anyone who wants can monitor the logs and, if
195 they believe a certificate is incorrectly issued, take action as they
196 see fit.
198 Similarly, those who have seen signed timestamps from a particular
199 log can later demand a proof of inclusion from that log. If the log
200 is unable to provide this (or, indeed, if the corresponding
201 certificate is absent from monitors' copies of that log), that is
202 evidence of the incorrect operation of the log. The checking
203 operation is asynchronous to allow clients to proceed without delay,
204 despite possible issues such as network connectivity and the vagaries
205 of firewalls.
207 The append-only property of each log is achieved using Merkle Trees,
208 which can be used to efficiently prove that any particular instance
209 of the log is a superset of any particular previous instance and to
210 efficiently detect various misbehaviors of the log (e.g., issuing a
211 signed timestamp for a certificate that is not subsequently logged).
213 It is necessary to treat each log as a trusted third party, because
214 the log auditing mechanisms described in this document can be
215 circumvented by a misbehaving log that shows different, inconsistent
216 views of itself to different clients. While mechanisms are being
217 developed to address these shortcomings and thereby avoid the need to
218 blindly trust logs, such mechanisms are outside the scope of this
219 document.
221 1.1. Requirements Language
223 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
224 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
225 "OPTIONAL" in this document are to be interpreted as described in BCP
226 14 [RFC2119] [RFC8174] when, and only when, they appear in all
227 capitals, as shown here.
229 1.2. Data Structures
231 Data structures are defined and encoded according to the conventions
232 laid out in Section 3 of [RFC8446].
234 1.3. Major Differences from CT 1.0
236 This document revises and obsoletes the CT 1.0 [RFC6962] protocol,
237 drawing on insights gained from CT 1.0 deployments and on feedback
238 from the community. The major changes are:
240 * Hash and signature algorithm agility: permitted algorithms are now
241 specified in IANA registries.
243 * Precertificate format: precertificates are now CMS objects rather
244 than X.509 certificates, which avoids violating the certificate
245 serial number uniqueness requirement in Section 4.1.2.2 of
246 [RFC5280].
248 * Removed precertificate signing certificates and the precertificate
249 poison extension: the change of precertificate format means that
250 these are no longer needed.
252 * Logs IDs: each log is now identified by an OID rather than by the
253 hash of its public key. OID allocations are managed by an IANA
254 registry.
256 * "TransItem" structure: this new data structure is used to
257 encapsulate most types of CT data. A "TransItemList", consisting
258 of one or more "TransItem" structures, can be used anywhere that
259 "SignedCertificateTimestampList" was used in [RFC6962].
261 * Merkle tree leaves: the "MerkleTreeLeaf" structure has been
262 replaced by the "TransItem" structure, which eases extensibility
263 and simplifies the leaf structure by removing one layer of
264 abstraction.
266 * Unified leaf format: the structure for both certificate and
267 precertificate entries now includes only the TBSCertificate
268 (whereas certificate entries in [RFC6962] included the entire
269 certificate).
271 * Log Artifact Extensions: these are now typed and managed by an
272 IANA registry, and they can now appear not only in SCTs but also
273 in STHs.
275 * API outputs: complete "TransItem" structures are returned, rather
276 than the constituent parts of each structure.
278 * get-all-by-hash: new client API for obtaining an inclusion proof
279 and the corresponding consistency proof at the same time.
281 * submit-entry: new client API, replacing add-chain and add-pre-
282 chain.
284 * Presenting SCTs with proofs: TLS servers may present SCTs together
285 with the corresponding inclusion proofs using any of the
286 mechanisms that [RFC6962] defined for presenting SCTs only.
287 (Presenting SCTs only is still supported).
289 * CT TLS extension: the "signed_certificate_timestamp" TLS extension
290 has been replaced by the "transparency_info" TLS extension.
292 * Verification algorithms: added detailed algorithms for verifying
293 inclusion proofs, for verifying consistency between two STHs, and
294 for verifying a root hash given a complete list of the relevant
295 leaf input entries.
297 * Extensive clarifications and editorial work.
299 2. Cryptographic Components
301 2.1. Merkle Hash Trees
303 A full description of Merkle Hash Tree is beyond the scope of this
304 document. Briefly, it is a binary tree where each non-leaf node is a
305 hash of its children. For CT, the number of children is at most two.
306 Additional information can be found in the Introduction and Reference
307 section of [RFC8391].
309 2.1.1. Definition of the Merkle Tree
311 The log uses a binary Merkle Hash Tree for efficient auditing. The
312 hash algorithm used is one of the log's parameters (see Section 4.1).
313 This document establishes a registry of acceptable hash algorithms
314 (see Section 10.2.1). Throughout this document, the hash algorithm
315 in use is referred to as HASH and the size of its output in bytes as
316 HASH_SIZE. The input to the Merkle Tree Hash is a list of data
317 entries; these entries will be hashed to form the leaves of the
318 Merkle Hash Tree. The output is a single HASH_SIZE Merkle Tree Hash.
319 Given an ordered list of n inputs, D_n = {d[0], d[1], ..., d[n-1]},
320 the Merkle Tree Hash (MTH) is thus defined as follows:
322 The hash of an empty list is the hash of an empty string:
324 MTH({}) = HASH().
326 The hash of a list with one entry (also known as a leaf hash) is:
328 MTH({d[0]}) = HASH(0x00 || d[0]).
330 For n > 1, let k be the largest power of two smaller than n (i.e., k
331 < n <= 2k). The Merkle Tree Hash of an n-element list D_n is then
332 defined recursively as
334 MTH(D_n) = HASH(0x01 || MTH(D[0:k]) || MTH(D[k:n])),
336 where:
338 * || denotes concatenation
340 * : denotes concatenation of lists
342 * D[k1:k2] = D'_(k2-k1) denotes the list {d'[0] = d[k1], d'[1] =
343 d[k1+1], ..., d'[k2-k1-1] = d[k2-1]} of length (k2 - k1).
345 Note that the hash calculations for leaves and nodes differ; this
346 domain separation is required to give second preimage resistance.
348 Note that we do not require the length of the input list to be a
349 power of two. The resulting Merkle Tree may thus not be balanced;
350 however, its shape is uniquely determined by the number of leaves.
351 (Note: This Merkle Tree is essentially the same as the history tree
352 [CrosbyWallach] proposal, except our definition handles non-full
353 trees differently).
355 2.1.2. Verifying a Tree Head Given Entries
357 When a client has a complete list of "entries" from "0" up to
358 "tree_size - 1" and wishes to verify this list against a tree head
359 "root_hash" returned by the log for the same "tree_size", the
360 following algorithm may be used:
362 1. Set "stack" to an empty stack.
364 2. For each "i" from "0" up to "tree_size - 1":
366 1. Push "HASH(0x00 || entries[i])" to "stack".
368 2. Set "merge_count" to the lowest value ("0" included) such
369 that "LSB(i >> merge_count)" is not set, where "LSB" means
370 the least significant bit. In other words, set "merge_count"
371 to the number of consecutive "1"s found starting at the least
372 significant bit of "i".
374 3. Repeat "merge_count" times:
376 1. Pop "right" from "stack".
378 2. Pop "left" from "stack".
380 3. Push "HASH(0x01 || left || right)" to "stack".
382 3. If there is more than one element in the "stack", repeat the same
383 merge procedure (the sub-items of Step 2.3 above) until only a
384 single element remains.
386 4. The remaining element in "stack" is the Merkle Tree hash for the
387 given "tree_size" and should be compared by equality against the
388 supplied "root_hash".
390 2.1.3. Merkle Inclusion Proofs
392 A Merkle inclusion proof for a leaf in a Merkle Hash Tree is the
393 shortest list of additional nodes in the Merkle Tree required to
394 compute the Merkle Tree Hash for that tree. Each node in the tree is
395 either a leaf node or is computed from the two nodes immediately
396 below it (i.e., towards the leaves). At each step up the tree
397 (towards the root), a node from the inclusion proof is combined with
398 the node computed so far. In other words, the inclusion proof
399 consists of the list of missing nodes required to compute the nodes
400 leading from a leaf to the root of the tree. If the root computed
401 from the inclusion proof matches the true root, then the inclusion
402 proof proves that the leaf exists in the tree.
404 2.1.3.1. Generating an Inclusion Proof
406 Given an ordered list of n inputs to the tree, D_n = {d[0], d[1],
407 ..., d[n-1]}, the Merkle inclusion proof PATH(m, D_n) for the (m+1)th
408 input d[m], 0 <= m < n, is defined as follows:
410 The proof for the single leaf in a tree with a one-element input list
411 D[1] = {d[0]} is empty:
413 PATH(0, {d[0]}) = {}
415 For n > 1, let k be the largest power of two smaller than n. The
416 proof for the (m+1)th element d[m] in a list of n > m elements is
417 then defined recursively as
419 PATH(m, D_n) = PATH(m, D[0:k]) : MTH(D[k:n]) for m < k; and
421 PATH(m, D_n) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for m >= k,
423 The : operator and D[k1:k2] are defined the same as in Section 2.1.1.
425 2.1.3.2. Verifying an Inclusion Proof
427 When a client has received an inclusion proof (e.g., in a "TransItem"
428 of type "inclusion_proof_v2") and wishes to verify inclusion of an
429 input "hash" for a given "tree_size" and "root_hash", the following
430 algorithm may be used to prove the "hash" was included in the
431 "root_hash":
433 1. Compare "leaf_index" from the "inclusion_proof_v2" structure
434 against "tree_size". If "leaf_index" is greater than or equal to
435 "tree_size" then fail the proof verification.
437 2. Set "fn" to "leaf_index" and "sn" to "tree_size - 1".
439 3. Set "r" to "hash".
441 4. For each value "p" in the "inclusion_path" array:
443 If "sn" is 0, stop the iteration and fail the proof verification.
445 If "LSB(fn)" is set, or if "fn" is equal to "sn", then:
447 1. Set "r" to "HASH(0x01 || p || r)"
449 2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn"
450 equally until either "LSB(fn)" is set or "fn" is "0".
452 Otherwise:
454 1. Set "r" to "HASH(0x01 || r || p)"
456 Finally, right-shift both "fn" and "sn" one time.
458 5. Compare "sn" to 0. Compare "r" against the "root_hash". If "sn"
459 is equal to 0, and "r" and the "root_hash" are equal, then the
460 log has proven the inclusion of "hash". Otherwise, fail the
461 proof verification.
463 2.1.4. Merkle Consistency Proofs
465 Merkle consistency proofs prove the append-only property of the tree.
466 A Merkle consistency proof for a Merkle Tree Hash MTH(D_n) and a
467 previously advertised hash MTH(D[0:m]) of the first m leaves, m <= n,
468 is the list of nodes in the Merkle Tree required to verify that the
469 first m inputs D[0:m] are equal in both trees. Thus, a consistency
470 proof must contain a set of intermediate nodes (i.e., commitments to
471 inputs) sufficient to verify MTH(D_n), such that (a subset of) the
472 same nodes can be used to verify MTH(D[0:m]). We define an algorithm
473 that outputs the (unique) minimal consistency proof.
475 2.1.4.1. Generating a Consistency Proof
477 Given an ordered list of n inputs to the tree, D_n = {d[0], d[1],
478 ..., d[n-1]}, the Merkle consistency proof PROOF(m, D_n) for a
479 previous Merkle Tree Hash MTH(D[0:m]), 0 < m < n, is defined as:
481 PROOF(m, D_n) = SUBPROOF(m, D_n, true)
483 In SUBPROOF, the boolean value represents whether the subtree created
484 from D[0:m] is a complete subtree of the Merkle Tree created from
485 D_n, and, consequently, whether the subtree Merkle Tree Hash
486 MTH(D[0:m]) is known. The initial call to SUBPROOF sets this to be
487 true, and SUBPROOF is then defined as follows:
489 The subproof for m = n is empty if m is the value for which PROOF was
490 originally requested (meaning that the subtree created from D[0:m] is
491 a complete subtree of the Merkle Tree created from the original D_n
492 for which PROOF was requested, and the subtree Merkle Tree Hash
493 MTH(D[0:m]) is known):
495 SUBPROOF(m, D_m, true) = {}
497 Otherwise, the subproof for m = n is the Merkle Tree Hash committing
498 inputs D[0:m]:
500 SUBPROOF(m, D_m, false) = {MTH(D_m)}
502 For m < n, let k be the largest power of two smaller than n. The
503 subproof is then defined recursively, using the appropriate step
504 below:
506 If m <= k, the right subtree entries D[k:n] only exist in the current
507 tree. We prove that the left subtree entries D[0:k] are consistent
508 and add a commitment to D[k:n]:
510 SUBPROOF(m, D_n, b) = SUBPROOF(m, D[0:k], b) : MTH(D[k:n])
512 If m > k, the left subtree entries D[0:k] are identical in both
513 trees. We prove that the right subtree entries D[k:n] are consistent
514 and add a commitment to D[0:k].
516 SUBPROOF(m, D_n, b) = SUBPROOF(m - k, D[k:n], false) : MTH(D[0:k])
518 The number of nodes in the resulting proof is bounded above by
519 ceil(log2(n)) + 1.
521 The : operator and D[k1:k2] are defined the same as in Section 2.1.1.
523 2.1.4.2. Verifying Consistency between Two Tree Heads
525 When a client has a tree head "first_hash" for tree size "first", a
526 tree head "second_hash" for tree size "second" where "0 < first <
527 second", and has received a consistency proof between the two (e.g.,
528 in a "TransItem" of type "consistency_proof_v2"), the following
529 algorithm may be used to verify the consistency proof:
531 1. If "consistency_path" is an empty array, stop and fail the proof
532 verification.
534 2. If "first" is an exact power of 2, then prepend "first_hash" to
535 the "consistency_path" array.
537 3. Set "fn" to "first - 1" and "sn" to "second - 1".
539 4. If "LSB(fn)" is set, then right-shift both "fn" and "sn" equally
540 until "LSB(fn)" is not set.
542 5. Set both "fr" and "sr" to the first value in the
543 "consistency_path" array.
545 6. For each subsequent value "c" in the "consistency_path" array:
547 If "sn" is 0, stop the iteration and fail the proof verification.
549 If "LSB(fn)" is set, or if "fn" is equal to "sn", then:
551 1. Set "fr" to "HASH(0x01 || c || fr)"
553 Set "sr" to "HASH(0x01 || c || sr)"
555 2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn"
556 equally until either "LSB(fn)" is set or "fn" is "0".
558 Otherwise:
560 1. Set "sr" to "HASH(0x01 || sr || c)"
562 Finally, right-shift both "fn" and "sn" one time.
564 7. After completing iterating through the "consistency_path" array
565 as described above, verify that the "fr" calculated is equal to
566 the "first_hash" supplied, that the "sr" calculated is equal to
567 the "second_hash" supplied and that "sn" is 0.
569 2.1.5. Example
571 The binary Merkle Tree with 7 leaves:
573 hash
574 / \
575 / \
576 / \
577 / \
578 / \
579 k l
580 / \ / \
581 / \ / \
582 / \ / \
583 g h i j
584 / \ / \ / \ |
585 a b c d e f d6
586 | | | | | |
587 d0 d1 d2 d3 d4 d5
589 The inclusion proof for d0 is [b, h, l].
591 The inclusion proof for d3 is [c, g, l].
593 The inclusion proof for d4 is [f, j, k].
595 The inclusion proof for d6 is [i, k].
597 The same tree, built incrementally in four steps:
599 hash0 hash1=k
600 / \ / \
601 / \ / \
602 / \ / \
603 g c g h
604 / \ | / \ / \
605 a b d2 a b c d
606 | | | | | |
607 d0 d1 d0 d1 d2 d3
609 hash2 hash
610 / \ / \
611 / \ / \
612 / \ / \
613 / \ / \
614 / \ / \
615 k i k l
616 / \ / \ / \ / \
617 / \ e f / \ / \
618 / \ | | / \ / \
619 g h d4 d5 g h i j
620 / \ / \ / \ / \ / \ |
621 a b c d a b c d e f d6
622 | | | | | | | | | |
623 d0 d1 d2 d3 d0 d1 d2 d3 d4 d5
625 The consistency proof between hash0 and hash is PROOF(3, D[7]) = [c,
626 d, g, l]. c, g are used to verify hash0, and d, l are additionally
627 used to show hash is consistent with hash0.
629 The consistency proof between hash1 and hash is PROOF(4, D[7]) = [l].
630 hash can be verified using hash1=k and l.
632 The consistency proof between hash2 and hash is PROOF(6, D[7]) = [i,
633 j, k]. k, i are used to verify hash2, and j is additionally used to
634 show hash is consistent with hash2.
636 2.2. Signatures
638 When signing data structures, a log MUST use one of the signature
639 algorithms from the IANA CT Signature Algorithms registry, described
640 in Section 10.2.2.
642 3. Submitters
644 Submitters submit certificates or preannouncements of certificates
645 prior to issuance (precertificates) to logs for public auditing, as
646 described below. In order to enable attribution of each logged
647 certificate or precertificate to its issuer, each submission MUST be
648 accompanied by all additional certificates required to verify the
649 chain up to an accepted trust anchor (Section 5.7). The trust anchor
650 (a root or intermediate CA certificate) MAY be omitted from the
651 submission.
653 If a log accepts a submission, it will return a Signed Certificate
654 Timestamp (SCT) (see Section 4.8). The submitter SHOULD validate the
655 returned SCT as described in Section 8.1 if they understand its
656 format and they intend to use it directly in a TLS handshake or to
657 construct a certificate. If the submitter does not need the SCT (for
658 example, the certificate is being submitted simply to make it
659 available in the log), it MAY validate the SCT.
661 3.1. Certificates
663 Any entity can submit a certificate (Section 5.1) to a log. Since it
664 is anticipated that TLS clients will reject certificates that are not
665 logged, it is expected that certificate issuers and subjects will be
666 strongly motivated to submit them.
668 3.2. Precertificates
670 CAs may preannounce a certificate prior to issuance by submitting a
671 precertificate (Section 5.1) that the log can use to create an entry
672 that will be valid against the issued certificate. The CA MAY
673 incorporate the returned SCT in the issued certificate. One example
674 of where the returned SCT is not incorporated in the issued
675 certificate is when a CA sends the precertificate to multiple logs,
676 but only incorporates the SCTs that are returned first.
678 A precertificate is a CMS [RFC5652] "signed-data" object that
679 conforms to the following profile:
681 * It MUST be DER encoded as described in [X690].
683 * "SignedData.version" MUST be v3(3).
685 * "SignedData.digestAlgorithms" MUST be the same as the
686 "SignerInfo.digestAlgorithm" OID value (see below).
688 * "SignedData.encapContentInfo":
690 - "eContentType" MUST be the OID 1.3.101.78.
692 - "eContent" MUST contain a TBSCertificate [RFC5280] that will be
693 identical to the TBSCertificate in the issued certificate,
694 except that the Transparency Information (Section 7.1)
695 extension MUST be omitted.
697 * "SignedData.certificates" MUST be omitted.
699 * "SignedData.crls" MUST be omitted.
701 * "SignedData.signerInfos" MUST contain one "SignerInfo":
703 - "version" MUST be v3(3).
705 - "sid" MUST use the "subjectKeyIdentifier" option.
707 - "digestAlgorithm" MUST be one of the hash algorithm OIDs listed
708 in the IANA CT Hash Algorithms Registry, described in
709 Section 10.2.1.
711 - "signedAttrs" MUST be present and MUST contain two attributes:
713 o A content-type attribute whose value is the same as
714 "SignedData.encapContentInfo.eContentType".
716 o A message-digest attribute whose value is the message digest
717 of "SignedData.encapContentInfo.eContent".
719 - "signatureAlgorithm" MUST be the same OID as
720 "TBSCertificate.signature".
722 - "signature" MUST be from the same (root or intermediate) CA
723 that intends to issue the corresponding certificate (see
724 Section 3.2.1).
726 - "unsignedAttrs" MUST be omitted.
728 "SignerInfo.signedAttrs" is included in the message digest
729 calculation process (see Section 5.4 of [RFC5652]), which ensures
730 that the "SignerInfo.signature" value will not be a valid X.509v3
731 signature that could be used in conjunction with the TBSCertificate
732 (from "SignedData.encapContentInfo.eContent") to construct a valid
733 certificate.
735 3.2.1. Binding Intent to Issue
737 Under normal circumstances, there will be a short delay between
738 precertificate submission and issuance of the corresponding
739 certificate. Longer delays are to be expected occasionally (e.g.,
740 due to log server downtime), and in some cases the CA might not
741 actually issue the corresponding certificate. Nevertheless, a
742 precertificate's "signature" indicates the CA's binding intent to
743 issue the corresponding certificate, which means that:
745 * Misissuance of a precertificate is considered equivalent to
746 misissuance of the corresponding certificate. The CA should
747 expect to be held to account, even if the corresponding
748 certificate has not actually been issued.
750 * Upon observing a precertificate, a client can reasonably presume
751 that the corresponding certificate has been issued. A client may
752 wish to obtain status information (e.g., by using the Online
753 Certificate Status Protocol [RFC6960] or by checking a Certificate
754 Revocation List [RFC5280]) about a certificate that is presumed to
755 exist, especially if there is evidence or suspicion that the
756 corresponding precertificate was misissued.
758 * TLS clients may have policies that require CAs to be able to
759 revoke, and to provide certificate status services for, each
760 certificate that is presumed to exist based on the existence of a
761 corresponding precertificate.
763 4. Log Format and Operation
765 A log is a single, append-only Merkle Tree of submitted certificate
766 and precertificate entries.
768 When it receives and accepts a valid submission, the log MUST return
769 an SCT that corresponds to the submitted certificate or
770 precertificate. If the log has previously seen this valid
771 submission, it SHOULD return the same SCT as it returned before, as
772 discussed in Section 11.3. If different SCTs are produced for the
773 same submission, multiple log entries will have to be created, one
774 for each SCT (as the timestamp is a part of the leaf structure).
775 Note that if a certificate was previously logged as a precertificate,
776 then the precertificate's SCT of type "precert_sct_v2" would not be
777 appropriate; instead, a fresh SCT of type "x509_sct_v2" should be
778 generated.
780 An SCT is the log's promise to append to its Merkle Tree an entry for
781 the accepted submission. Upon producing an SCT, the log MUST fulfil
782 this promise by performing the following actions within a fixed
783 amount of time known as the Maximum Merge Delay (MMD), which is one
784 of the log's parameters (see Section 4.1):
786 * Allocate a tree index to the entry representing the accepted
787 submission.
789 * Calculate the root of the tree.
791 * Sign the root of the tree (see Section 4.10).
793 The log may append multiple entries before signing the root of the
794 tree.
796 Log operators SHOULD NOT impose any conditions on retrieving or
797 sharing data from the log.
799 4.1. Log Parameters
801 A log is defined by a collection of immutable parameters, which are
802 used by clients to communicate with the log and to verify log
803 artifacts. Except for the Final Signed Tree Head (STH), each of
804 these parameters MUST be established before the log operator begins
805 to operate the log.
807 Base URL: The prefix used to construct URLs ([RFC3986]) for client
808 messages (see Section 5). The base URL MUST be an "https" URL,
809 MAY contain a port, MAY contain a path with any number of path
810 segments, but MUST NOT contain a query string, fragment, or
811 trailing "/". Example: https://ct.example.org/blue
813 Hash Algorithm: The hash algorithm used for the Merkle Tree (see
814 Section 10.2.1).
816 Signature Algorithm: The signature algorithm used (see Section 2.2).
818 Public Key: The public key used to verify signatures generated by
819 the log. A log MUST NOT use the same keypair as any other log.
821 Log ID: The OID that uniquely identifies the log.
823 Maximum Merge Delay: The MMD the log has committed to. This
824 document deliberately does not specify any limits on the value, to
825 allow for experimentation.
827 Version: The version of the protocol supported by the log (currently
828 1 or 2).
830 Maximum Chain Length: The longest certificate chain submission the
831 log is willing to accept, if the log imposes any limit.
833 STH Frequency Count: The maximum number of STHs the log may produce
834 in any period equal to the "Maximum Merge Delay" (see
835 Section 4.10).
837 Final STH: If a log has been closed down (i.e., no longer accepts
838 new entries), existing entries may still be valid. In this case,
839 the client should know the final valid STH in the log to ensure no
840 new entries can be added without detection. This value MUST be
841 provided in the form of a TransItem of type "signed_tree_head_v2".
842 If a log is still accepting entries, this value should not be
843 provided.
845 [JSON.Metadata] is an example of a metadata format which includes the
846 above elements.
848 4.2. Evaluating Submissions
850 A log determines whether to accept or reject a submission by
851 evaluating it against the minimum acceptance criteria (see
852 Section 4.2.1) and against the log's discretionary acceptance
853 criteria (see Section 4.2.2).
855 If the acceptance criteria are met, the log SHOULD accept the
856 submission. (A log may decide, for example, to temporarily reject
857 acceptable submissions to protect itself against denial-of-service
858 attacks).
860 The log SHALL allow retrieval of its list of accepted trust anchors
861 (see Section 5.7), each of which is a root or intermediate CA
862 certificate. This list might usefully be the union of root
863 certificates trusted by major browser vendors.
865 4.2.1. Minimum Acceptance Criteria
867 To ensure that logged certificates and precertificates are
868 attributable to an accepted trust anchor, and to set clear
869 expectations for what monitors would find in the log, and to avoid
870 being overloaded by invalid submissions, the log MUST reject a
871 submission if any of the following conditions are not met:
873 * The "submission", "type" and "chain" inputs MUST be set as
874 described in Section 5.1. The log MUST NOT accommodate misordered
875 CA certificates or use any other source of intermediate CA
876 certificates to attempt certification path construction.
878 * Each of the zero or more intermediate CA certificates in the chain
879 MUST have one or both of the following features:
881 - The Basic Constraints extension with the cA boolean asserted.
883 - The Key Usage extension with the keyCertSign bit asserted.
885 * Each certificate in the chain MUST fall within the limits imposed
886 by the zero or more Basic Constraints pathLenConstraint values
887 found higher up the chain.
889 * Precertificate submissions MUST conform to all of the requirements
890 in Section 3.2.
892 4.2.2. Discretionary Acceptance Criteria
894 If the minimum acceptance criteria are met but the submission is not
895 fully valid according to [RFC5280] verification rules (e.g., the
896 certificate or precertificate has expired, is not yet valid, has been
897 revoked, exhibits ASN.1 DER encoding errors but the log can still
898 parse it, etc), then the acceptability of the submission is left to
899 the log's discretion. It is useful for logs to accept such
900 submissions in order to accommodate quirks of CA certificate-issuing
901 software and to facilitate monitoring of CA compliance with
902 applicable policies and technical standards. However, it is
903 impractical for this document to enumerate, and for logs to consider,
904 all of the ways that a submission might fail to comply with
905 [RFC5280].
907 Logs SHOULD limit the length of chain they will accept. The maximum
908 chain length is one of the log's parameters (see Section 4.1).
910 4.3. Log Entries
912 If a submission is accepted and an SCT issued, the accepting log MUST
913 store the entire chain used for verification. This chain MUST
914 include the certificate or precertificate itself, the zero or more
915 intermediate CA certificates provided by the submitter, and the trust
916 anchor used to verify the chain (even if it was omitted from the
917 submission). The log MUST provide this chain for auditing upon
918 request (see Section 5.6) so that the CA cannot avoid blame by
919 logging a partial or empty chain. Each log entry is a "TransItem"
920 structure of type "x509_entry_v2" or "precert_entry_v2". However, a
921 log may store its entries in any format. If a log does not store
922 this "TransItem" in full, it must store the "timestamp" and
923 "sct_extensions" of the corresponding
924 "TimestampedCertificateEntryDataV2" structure. The "TransItem" can
925 be reconstructed from these fields and the entire chain that the log
926 used to verify the submission.
928 4.4. Log ID
930 Each log is identified by an OID, which is one of the log's
931 parameters (see Section 4.1) and which MUST NOT be used to identify
932 any other log. A log's operator MUST either allocate the OID
933 themselves or request an OID from the Log ID Registry (see
934 Section 10.2.5.1). The only advantage of the registry is that the
935 DER encoding can be small. (Recall that OID allocations do not
936 require a central registration, although logs will most likely want
937 to make themselves known to potential clients through out of band
938 means.) Various data structures include the DER encoding of this
939 OID, excluding the ASN.1 tag and length bytes, in an opaque vector:
941 opaque LogID<2..127>;
943 Note that the ASN.1 length and the opaque vector length are identical
944 in size (1 byte) and value, so the full DER encoding (including the
945 tag and length) of the OID can be reproduced simply by prepending an
946 OBJECT IDENTIFIER tag (0x06) to the opaque vector length and
947 contents.
949 The OID used to identify a log is limited such that the DER encoding
950 of its value, excluding the tag and length, MUST be no longer than
951 127 octets.
953 4.5. TransItem Structure
955 Various data structures are encapsulated in the "TransItem" structure
956 to ensure that the type and version of each one is identified in a
957 common fashion:
959 enum {
960 reserved(0),
961 x509_entry_v2(1), precert_entry_v2(2),
962 x509_sct_v2(3), precert_sct_v2(4),
963 signed_tree_head_v2(5), consistency_proof_v2(6),
964 inclusion_proof_v2(7),
965 (65535)
966 } VersionedTransType;
968 struct {
969 VersionedTransType versioned_type;
970 select (versioned_type) {
971 case x509_entry_v2: TimestampedCertificateEntryDataV2;
972 case precert_entry_v2: TimestampedCertificateEntryDataV2;
973 case x509_sct_v2: SignedCertificateTimestampDataV2;
974 case precert_sct_v2: SignedCertificateTimestampDataV2;
975 case signed_tree_head_v2: SignedTreeHeadDataV2;
976 case consistency_proof_v2: ConsistencyProofDataV2;
977 case inclusion_proof_v2: InclusionProofDataV2;
978 } data;
979 } TransItem;
981 "versioned_type" is a value from the IANA registry in Section 10.2.3
982 that identifies the type of the encapsulated data structure and the
983 earliest version of this protocol to which it conforms. This
984 document is v2.
986 "data" is the encapsulated data structure. The various structures
987 named with the "DataV2" suffix are defined in later sections of this
988 document.
990 Note that "VersionedTransType" combines the v1 [RFC6962] type
991 enumerations "Version", "LogEntryType", "SignatureType" and
992 "MerkleLeafType". Note also that v1 did not define "TransItem", but
993 this document provides guidelines (see Appendix A) on how v2
994 implementations can co-exist with v1 implementations.
996 Future versions of this protocol may reuse "VersionedTransType"
997 values defined in this document as long as the corresponding data
998 structures are not modified, and may add new "VersionedTransType"
999 values for new or modified data structures.
1001 4.6. Log Artifact Extensions
1002 enum {
1003 reserved(65535)
1004 } ExtensionType;
1006 struct {
1007 ExtensionType extension_type;
1008 opaque extension_data<0..2^16-1>;
1009 } Extension;
1011 The "Extension" structure provides a generic extensibility for log
1012 artifacts, including Signed Certificate Timestamps (Section 4.8) and
1013 Signed Tree Heads (Section 4.10). The interpretation of the
1014 "extension_data" field is determined solely by the value of the
1015 "extension_type" field.
1017 This document does not define any extensions, but it does establish a
1018 registry for future "ExtensionType" values (see Section 10.2.4).
1019 Each document that registers a new "ExtensionType" must specify the
1020 context in which it may be used (e.g., SCT, STH, or both) and
1021 describe how to interpret the corresponding "extension_data".
1023 4.7. Merkle Tree Leaves
1025 The leaves of a log's Merkle Tree correspond to the log's entries
1026 (see Section 4.3). Each leaf is the leaf hash (Section 2.1) of a
1027 "TransItem" structure of type "x509_entry_v2" or "precert_entry_v2",
1028 which encapsulates a "TimestampedCertificateEntryDataV2" structure.
1029 Note that leaf hashes are calculated as HASH(0x00 || TransItem),
1030 where the hash algorithm is one of the log's parameters.
1032 opaque TBSCertificate<1..2^24-1>;
1034 struct {
1035 uint64 timestamp;
1036 opaque issuer_key_hash<32..2^8-1>;
1037 TBSCertificate tbs_certificate;
1038 Extension sct_extensions<0..2^16-1>;
1039 } TimestampedCertificateEntryDataV2;
1041 "timestamp" is the date and time at which the certificate or
1042 precertificate was accepted by the log, in the form of a 64-bit
1043 unsigned number of milliseconds elapsed since the Unix Epoch (1
1044 January 1970 00:00:00 UTC - see [UNIXTIME]), ignoring leap seconds,
1045 in network byte order. Note that the leaves of a log's Merkle Tree
1046 are not required to be in strict chronological order.
1048 "issuer_key_hash" is the HASH of the public key of the CA that issued
1049 the certificate or precertificate, calculated over the DER encoding
1050 of the key represented as SubjectPublicKeyInfo [RFC5280]. This is
1051 needed to bind the CA to the certificate or precertificate, making it
1052 impossible for the corresponding SCT to be valid for any other
1053 certificate or precertificate whose TBSCertificate matches
1054 "tbs_certificate". The length of the "issuer_key_hash" MUST match
1055 HASH_SIZE.
1057 "tbs_certificate" is the DER encoded TBSCertificate from the
1058 submission. (Note that a precertificate's TBSCertificate can be
1059 reconstructed from the corresponding certificate as described in
1060 Section 8.1.2).
1062 "sct_extensions" is byte-for-byte identical to the SCT extensions of
1063 the corresponding SCT.
1065 The type of the "TransItem" corresponds to the value of the "type"
1066 parameter supplied in the Section 5.1 call.
1068 4.8. Signed Certificate Timestamp (SCT)
1070 An SCT is a "TransItem" structure of type "x509_sct_v2" or
1071 "precert_sct_v2", which encapsulates a
1072 "SignedCertificateTimestampDataV2" structure:
1074 struct {
1075 LogID log_id;
1076 uint64 timestamp;
1077 Extension sct_extensions<0..2^16-1>;
1078 opaque signature<1..2^16-1>;
1079 } SignedCertificateTimestampDataV2;
1081 "log_id" is this log's unique ID, encoded in an opaque vector as
1082 described in Section 4.4.
1084 "timestamp" is equal to the timestamp from the corresponding
1085 "TimestampedCertificateEntryDataV2" structure.
1087 "sct_extensions" is a vector of 0 or more SCT extensions. This
1088 vector MUST NOT include more than one extension with the same
1089 "extension_type". The extensions in the vector MUST be ordered by
1090 the value of the "extension_type" field, smallest value first. All
1091 SCT extensions are similar to non-critical X.509v3 extensions (i.e.,
1092 the "mustUnderstand" field is not set), and a recipient SHOULD ignore
1093 any extension it does not understand. Furthermore, an implementation
1094 MAY choose to ignore any extension(s) that it does understand.
1096 "signature" is computed over a "TransItem" structure of type
1097 "x509_entry_v2" or "precert_entry_v2" (see Section 4.7) using the
1098 signature algorithm declared in the log's parameters (see
1099 Section 4.1).
1101 4.9. Merkle Tree Head
1103 The log stores information about its Merkle Tree in a
1104 "TreeHeadDataV2":
1106 opaque NodeHash<32..2^8-1>;
1108 struct {
1109 uint64 timestamp;
1110 uint64 tree_size;
1111 NodeHash root_hash;
1112 Extension sth_extensions<0..2^16-1>;
1113 } TreeHeadDataV2;
1115 The length of NodeHash MUST match HASH_SIZE of the log.
1117 "timestamp" is the current date and time, using the format defined in
1118 {tree_leaves}.
1120 "tree_size" is the number of entries currently in the log's Merkle
1121 Tree.
1123 "root_hash" is the root of the Merkle Hash Tree.
1125 "sth_extensions" is a vector of 0 or more STH extensions. This
1126 vector MUST NOT include more than one extension with the same
1127 "extension_type". The extensions in the vector MUST be ordered by
1128 the value of the "extension_type" field, smallest value first. If an
1129 implementation sees an extension that it does not understand, it
1130 SHOULD ignore that extension. Furthermore, an implementation MAY
1131 choose to ignore any extension(s) that it does understand.
1133 4.10. Signed Tree Head (STH)
1135 Periodically each log SHOULD sign its current tree head information
1136 (see Section 4.9) to produce an STH. When a client requests a log's
1137 latest STH (see Section 5.2), the log MUST return an STH that is no
1138 older than the log's MMD. However, since STHs could be used to mark
1139 individual clients (by producing a new STH for each query), a log
1140 MUST NOT produce STHs more frequently than its parameters declare
1141 (see Section 4.1). In general, there is no need to produce a new STH
1142 unless there are new entries in the log; however, in the event that a
1143 log does not accept any submissions during an MMD period, the log
1144 MUST sign the same Merkle Tree Hash with a fresh timestamp.
1146 An STH is a "TransItem" structure of type "signed_tree_head_v2",
1147 which encapsulates a "SignedTreeHeadDataV2" structure:
1149 struct {
1150 LogID log_id;
1151 TreeHeadDataV2 tree_head;
1152 opaque signature<0..2^16-1>;
1153 } SignedTreeHeadDataV2;
1155 "log_id" is this log's unique ID, encoded in an opaque vector as
1156 described in Section 4.4.
1158 The "timestamp" in "tree_head" MUST be at least as recent as the most
1159 recent SCT timestamp in the tree. Each subsequent timestamp MUST be
1160 more recent than the timestamp of the previous update.
1162 "tree_head" contains the latest tree head information (see
1163 Section 4.9).
1165 "signature" is computed over the "tree_head" field using the
1166 signature algorithm declared in the log's parameters (see
1167 Section 4.1).
1169 4.11. Merkle Consistency Proofs
1171 To prepare a Merkle Consistency Proof for distribution to clients,
1172 the log produces a "TransItem" structure of type
1173 "consistency_proof_v2", which encapsulates a "ConsistencyProofDataV2"
1174 structure:
1176 struct {
1177 LogID log_id;
1178 uint64 tree_size_1;
1179 uint64 tree_size_2;
1180 NodeHash consistency_path<0..2^16-1>;
1181 } ConsistencyProofDataV2;
1183 "log_id" is this log's unique ID, encoded in an opaque vector as
1184 described in Section 4.4.
1186 "tree_size_1" is the size of the older tree.
1188 "tree_size_2" is the size of the newer tree.
1190 "consistency_path" is a vector of Merkle Tree nodes proving the
1191 consistency of two STHs as described in {consistency}.
1193 4.12. Merkle Inclusion Proofs
1195 To prepare a Merkle Inclusion Proof for distribution to clients, the
1196 log produces a "TransItem" structure of type "inclusion_proof_v2",
1197 which encapsulates an "InclusionProofDataV2" structure:
1199 struct {
1200 LogID log_id;
1201 uint64 tree_size;
1202 uint64 leaf_index;
1203 NodeHash inclusion_path<0..2^16-1>;
1204 } InclusionProofDataV2;
1206 "log_id" is this log's unique ID, encoded in an opaque vector as
1207 described in Section 4.4.
1209 "tree_size" is the size of the tree on which this inclusion proof is
1210 based.
1212 "leaf_index" is the 0-based index of the log entry corresponding to
1213 this inclusion proof.
1215 "inclusion_path" is a vector of Merkle Tree nodes proving the
1216 inclusion of the chosen certificate or precertificate as described in
1217 {merkle_inclusion_proof}.
1219 4.13. Shutting down a log
1221 Log operators may decide to shut down a log for various reasons, such
1222 as deprecation of the signature algorithm. If there are entries in
1223 the log for certificates that have not yet expired, simply making TLS
1224 clients stop recognizing that log will have the effect of
1225 invalidating SCTs from that log. In order to avoid that, the
1226 following actions SHOULD be taken:
1228 * Make it known to clients and monitors that the log will be frozen.
1229 This is not part of the API, so it will have to be done via a
1230 relevant out-of-band mechanism.
1232 * Stop accepting new submissions (the error code "shutdown" should
1233 be returned for such requests).
1235 * Once MMD from the last accepted submission has passed and all
1236 pending submissions are incorporated, issue a final STH and
1237 publish it as one of the log's parameters. Having an STH with a
1238 timestamp that is after the MMD has passed from the last SCT
1239 issuance allows clients to audit this log regularly without
1240 special handling for the final STH. At this point the log's
1241 private key is no longer needed and can be destroyed.
1243 * Keep the log running until the certificates in all of its entries
1244 have expired or exist in other logs (this can be determined by
1245 scanning other logs or connecting to domains mentioned in the
1246 certificates and inspecting the SCTs served).
1248 5. Log Client Messages
1250 Messages are sent as HTTPS GET or POST requests. Parameters for
1251 POSTs and all responses are encoded as JavaScript Object Notation
1252 (JSON) objects [RFC8259]. Parameters for GETs are encoded as order-
1253 independent key/value URL parameters, using the "application/x-www-
1254 form-urlencoded" format described in the "HTML 4.01 Specification"
1255 [HTML401]. Binary data is base64 encoded according to section 4 of
1256 [RFC4648] as specified in the individual messages.
1258 Clients are configured with a log's base URL, which is one of the
1259 log's parameters. Clients construct URLs for requests by appending
1260 suffixes to this base URL. This structure places some degree of
1261 restriction on how log operators can deploy these services, as noted
1262 in [RFC8820]. However, operational experience with version 1 of this
1263 protocol has not indicated that these restrictions are a problem in
1264 practice.
1266 Note that JSON objects and URL parameters may contain fields not
1267 specified here, to allow for experimentation. Any fields that are
1268 not understood SHOULD be ignored.
1270 In practice, log servers may include multiple front-end machines.
1271 Since it is impractical to keep these machines in perfect sync,
1272 errors may occur that are caused by skew between the machines. Where
1273 such errors are possible, the front-end will return additional
1274 information (as specified below) making it possible for clients to
1275 make progress, if progress is possible. Front-ends MUST only serve
1276 data that is free of gaps (that is, for example, no front-end will
1277 respond with an STH unless it is also able to prove consistency from
1278 all log entries logged within that STH).
1280 For example, when a consistency proof between two STHs is requested,
1281 the front-end reached may not yet be aware of one or both STHs. In
1282 the case where it is unaware of both, it will return the latest STH
1283 it is aware of. Where it is aware of the first but not the second,
1284 it will return the latest STH it is aware of and a consistency proof
1285 from the first STH to the returned STH. The case where it knows the
1286 second but not the first should not arise (see the "no gaps"
1287 requirement above).
1289 If the log is unable to process a client's request, it MUST return an
1290 HTTP response code of 4xx/5xx (see [RFC7231]), and, in place of the
1291 responses outlined in the subsections below, the body SHOULD be a
1292 JSON Problem Details Object (see [RFC7807] Section 3), containing:
1294 type: A URN reference identifying the problem. To facilitate
1295 automated response to errors, this document defines a set of
1296 standard tokens for use in the "type" field, within the URN
1297 namespace of: "urn:ietf:params:trans:error:".
1299 detail: A human-readable string describing the error that prevented
1300 the log from processing the request, ideally with sufficient
1301 detail to enable the error to be rectified.
1303 e.g., In response to a request of "/ct/v2/get-
1304 entries?start=100&end=99", the log would return a "400 Bad Request"
1305 response code with a body similar to the following:
1307 {
1308 "type": "urn:ietf:params:trans:error:endBeforeStart",
1309 "detail": "'start' cannot be greater than 'end'"
1310 }
1312 Most error types are specific to the type of request and are defined
1313 in the respective subsections below. The one exception is the
1314 "malformed" error type, which indicates that the log server could not
1315 parse the client's request because it did not comply with this
1316 document:
1318 +===========+==================================+
1319 | type | detail |
1320 +===========+==================================+
1321 | malformed | The request could not be parsed. |
1322 +-----------+----------------------------------+
1324 Table 1
1326 Clients SHOULD treat "500 Internal Server Error" and "503 Service
1327 Unavailable" responses as transient failures and MAY retry the same
1328 request without modification at a later date. Note that as per
1329 [RFC7231], in the case of a 503 response the log MAY include a
1330 "Retry-After:" header in order to request a minimum time for the
1331 client to wait before retrying the request. In the absence of this
1332 header, this document does not specify a minimum.
1334 Clients SHOULD treat any 4xx error as a problem with the request and
1335 not attempt to resubmit without some modification to the request.
1336 The full status code MAY provide additional details.
1338 This document deliberately does not provide more specific guidance on
1339 the use of HTTP status codes.
1341 5.1. Submit Entry to Log
1343 POST /ct/v2/submit-entry
1345 Inputs: submission: The base64 encoded certificate or
1346 precertificate.
1348 type: The "VersionedTransType" integer value that indicates
1349 the type of the "submission": 1 for "x509_entry_v2", or 2 for
1350 "precert_entry_v2".
1352 chain: An array of zero or more JSON strings, each of which
1353 is a base64 encoded CA certificate. The first element is the
1354 certifier of the "submission"; the second certifies the first;
1355 etc. The last element of "chain" (or, if "chain" is an empty
1356 array, the "submission") is certified by an accepted trust
1357 anchor.
1359 Outputs: sct: A base64 encoded "TransItem" of type "x509_sct_v2" or
1360 "precert_sct_v2", signed by this log, that corresponds to the
1361 "submission".
1363 If the submitted entry is immediately appended to (or already
1364 exists in) this log's tree, then the log SHOULD also output:
1366 sth: A base64 encoded "TransItem" of type "signed_tree_head_v2",
1367 signed by this log.
1369 inclusion: A base64 encoded "TransItem" of type
1370 "inclusion_proof_v2" whose "inclusion_path" array of Merkle
1371 Tree nodes proves the inclusion of the "submission" in the
1372 returned "sth".
1374 Error codes:
1376 +================+==============================================+
1377 | type | detail |
1378 +================+==============================================+
1379 | badSubmission | "submission" is neither a valid certificate |
1380 | | nor a valid precertificate. |
1381 +----------------+----------------------------------------------+
1382 | badType | "type" is neither 1 nor 2. |
1383 +----------------+----------------------------------------------+
1384 | badChain | The first element of "chain" is not the |
1385 | | certifier of the "submission", or the second |
1386 | | element does not certify the first, etc. |
1387 +----------------+----------------------------------------------+
1388 | badCertificate | One or more certificates in the "chain" are |
1389 | | not valid (e.g., not properly encoded). |
1390 +----------------+----------------------------------------------+
1391 | unknownAnchor | The last element of "chain" (or, if "chain" |
1392 | | is an empty array, the "submission") both is |
1393 | | not, and is not certified by, an accepted |
1394 | | trust anchor. |
1395 +----------------+----------------------------------------------+
1396 | shutdown | The log is no longer accepting submissions. |
1397 +----------------+----------------------------------------------+
1399 Table 2
1401 If the version of "sct" is not v2, then a v2 client may be unable to
1402 verify the signature. It MUST NOT construe this as an error. This
1403 is to avoid forcing an upgrade of compliant v2 clients that do not
1404 use the returned SCTs.
1406 If a log detects bad encoding in a chain that otherwise verifies
1407 correctly then the log MUST either log the certificate or return the
1408 "bad certificate" error. If the certificate is logged, an SCT MUST
1409 be issued. Logging the certificate is useful, because monitors
1410 (Section 8.2) can then detect these encoding errors, which may be
1411 accepted by some TLS clients.
1413 If "submission" is an accepted trust anchor whose certifier is
1414 neither an accepted trust anchor nor the first element of "chain",
1415 then the log MUST return the "unknown anchor" error. A log is not
1416 able to generate an SCT for a submission if it does not have access
1417 to the issuer's public key.
1419 If the returned "sct" is intended to be provided to TLS clients, then
1420 "sth" and "inclusion" (if returned) SHOULD also be provided to TLS
1421 clients. For example, if "type" was 2 (indicating "precert_sct_v2")
1422 then all three "TransItem"s could be embedded in the certificate.
1424 5.2. Retrieve Latest Signed Tree Head
1426 GET /ct/v2/get-sth
1428 No inputs.
1430 Outputs: sth: A base64 encoded "TransItem" of type
1431 "signed_tree_head_v2", signed by this log, that is no older
1432 than the log's MMD.
1434 5.3. Retrieve Merkle Consistency Proof between Two Signed Tree Heads
1436 GET /ct/v2/get-sth-consistency
1438 Inputs: first: The tree_size of the older tree, in decimal.
1440 second: The tree_size of the newer tree, in decimal
1441 (optional).
1443 Both tree sizes must be from existing v2 STHs. However, because
1444 of skew, the receiving front-end may not know one or both of the
1445 existing STHs. If both are known, then only the "consistency"
1446 output is returned. If the first is known but the second is not
1447 (or has been omitted), then the latest known STH is returned,
1448 along with a consistency proof between the first STH and the
1449 latest. If neither are known, then the latest known STH is
1450 returned without a consistency proof.
1452 Outputs: consistency: A base64 encoded "TransItem" of type
1453 "consistency_proof_v2", whose "tree_size_1" MUST match the
1454 "first" input. If the "sth" output is omitted, then
1455 "tree_size_2" MUST match the "second" input. If "first" and
1456 "second" are equal and correspond to a known STH, the returned
1457 consistency proof MUST be empty (a "consistency_path" array
1458 with zero elements).
1460 sth: A base64 encoded "TransItem" of type
1461 "signed_tree_head_v2", signed by this log.
1463 Note that no signature is required for the "consistency" output as
1464 it is used to verify the consistency between two STHs, which are
1465 signed.
1467 Error codes:
1469 +===================+======================================+
1470 | type | detail |
1471 +===================+======================================+
1472 | firstUnknown | "first" is before the latest known |
1473 | | STH but is not from an existing STH. |
1474 +-------------------+--------------------------------------+
1475 | secondUnknown | "second" is before the latest known |
1476 | | STH but is not from an existing STH. |
1477 +-------------------+--------------------------------------+
1478 | secondBeforeFirst | "second" is smaller than "first". |
1479 +-------------------+--------------------------------------+
1481 Table 3
1483 See Section 2.1.4.2 for an outline of how to use the "consistency"
1484 output.
1486 5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash
1488 GET /ct/v2/get-proof-by-hash
1490 Inputs: hash: A base64 encoded v2 leaf hash.
1492 tree_size: The tree_size of the tree on which to base the
1493 proof, in decimal.
1495 The "hash" must be calculated as defined in Section 4.7. A v2 STH
1496 must exist for the "tree_size". Because of skew, the front-end
1497 may not know the requested tree head. In that case, it will
1498 return the latest STH it knows, along with an inclusion proof to
1499 that STH. If the front-end knows the requested tree head then
1500 only "inclusion" is returned.
1502 Outputs: inclusion: A base64 encoded "TransItem" of type
1503 "inclusion_proof_v2" whose "inclusion_path" array of Merkle
1504 Tree nodes proves the inclusion of the certificate (as
1505 specified by the "hash" parameter) in the selected STH.
1507 sth: A base64 encoded "TransItem" of type
1508 "signed_tree_head_v2", signed by this log.
1510 Note that no signature is required for the "inclusion" output as
1511 it is used to verify inclusion in the selected STH, which is
1512 signed.
1514 Error codes:
1516 +=================+=====================================+
1517 | type | detail |
1518 +=================+=====================================+
1519 | hashUnknown | "hash" is not the hash of a known |
1520 | | leaf (may be caused by skew or by a |
1521 | | known certificate not yet merged). |
1522 +-----------------+-------------------------------------+
1523 | treeSizeUnknown | "hash" is before the latest known |
1524 | | STH but is not from an existing |
1525 | | STH. |
1526 +-----------------+-------------------------------------+
1528 Table 4
1530 See Section 2.1.3.2 for an outline of how to use the "inclusion"
1531 output.
1533 5.5. Retrieve Merkle Inclusion Proof, Signed Tree Head and Consistency
1534 Proof by Leaf Hash
1536 GET /ct/v2/get-all-by-hash
1538 Inputs: hash: A base64 encoded v2 leaf hash.
1540 tree_size: The tree_size of the tree on which to base the
1541 proofs, in decimal.
1543 The "hash" must be calculated as defined in Section 4.7. A v2 STH
1544 must exist for the "tree_size".
1546 Because of skew, the front-end may not know the requested tree head
1547 or the requested hash, which leads to a number of cases:
1549 +=====================+=====================================+
1550 | Case | Response |
1551 +=====================+=====================================+
1552 | latest STH < | Return latest STH |
1553 | requested tree head | |
1554 +---------------------+-------------------------------------+
1555 | latest STH > | Return latest STH and a consistency |
1556 | requested tree head | proof between it and the requested |
1557 | | tree head (see Section 5.3) |
1558 +---------------------+-------------------------------------+
1559 | index of requested | Return "inclusion" |
1560 | hash < latest STH | |
1561 +---------------------+-------------------------------------+
1563 Table 5
1565 Note that more than one case can be true, in which case the returned
1566 data is their union. It is also possible for none to be true, in
1567 which case the front-end MUST return an empty response.
1569 Outputs: inclusion: A base64 encoded "TransItem" of type
1570 "inclusion_proof_v2" whose "inclusion_path" array of Merkle
1571 Tree nodes proves the inclusion of the certificate (as
1572 specified by the "hash" parameter) in the selected STH.
1574 sth: A base64 encoded "TransItem" of type
1575 "signed_tree_head_v2", signed by this log.
1577 consistency: A base64 encoded "TransItem" of type
1578 "consistency_proof_v2" that proves the consistency of the
1579 requested tree head and the returned STH.
1581 Note that no signature is required for the "inclusion" or
1582 "consistency" outputs as they are used to verify inclusion in and
1583 consistency of STHs, which are signed.
1585 Errors are the same as in Section 5.4.
1587 See Section 2.1.3.2 for an outline of how to use the "inclusion"
1588 output, and see Section 2.1.4.2 for an outline of how to use the
1589 "consistency" output.
1591 5.6. Retrieve Entries and STH from Log
1593 GET /ct/v2/get-entries
1595 Inputs: start: 0-based index of first entry to retrieve, in
1596 decimal.
1598 end: 0-based index of last entry to retrieve, in decimal.
1600 Outputs: entries: An array of objects, each consisting of
1602 log_entry: The base64 encoded "TransItem" structure of type
1603 "x509_entry_v2" or "precert_entry_v2" (see Section 4.3).
1605 submitted_entry: JSON object equivalent to inputs that were
1606 submitted to "submit-entry", with the addition of the trust
1607 anchor to the "chain" field if the submission did not
1608 include it.
1610 sct: The base64 encoded "TransItem" of type "x509_sct_v2" or
1611 "precert_sct_v2" corresponding to this log entry.
1613 sth: A base64 encoded "TransItem" of type
1614 "signed_tree_head_v2", signed by this log.
1616 Note that this message is not signed -- the "entries" data can be
1617 verified by constructing the Merkle Tree Hash corresponding to a
1618 retrieved STH. All leaves MUST be v2. However, a compliant v2
1619 client MUST NOT construe an unrecognized TransItem type as an error.
1620 This means it may be unable to parse some entries, but note that each
1621 client can inspect the entries it does recognize as well as verify
1622 the integrity of the data by treating unrecognized leaves as opaque
1623 input to the tree.
1625 The "start" and "end" parameters SHOULD be within the range 0 <= x <
1626 "tree_size" as returned by "get-sth" in Section 5.2.
1628 The "start" parameter MUST be less than or equal to the "end"
1629 parameter.
1631 Each "submitted_entry" output parameter MUST include the trust anchor
1632 that the log used to verify the "submission", even if that trust
1633 anchor was not provided to "submit-entry" (see Section 5.1). If the
1634 "submission" does not certify itself, then the first element of
1635 "chain" MUST be present and MUST certify the "submission".
1637 Log servers MUST honor requests where 0 <= "start" < "tree_size" and
1638 "end" >= "tree_size" by returning a partial response covering only
1639 the valid entries in the specified range. "end" >= "tree_size" could
1640 be caused by skew. Note that the following restriction may also
1641 apply:
1643 Logs MAY restrict the number of entries that can be retrieved per
1644 "get-entries" request. If a client requests more than the permitted
1645 number of entries, the log SHALL return the maximum number of entries
1646 permissible. These entries SHALL be sequential beginning with the
1647 entry specified by "start". Note that limit on the number of entries
1648 is not immutable and therefore the restriction may be changed or
1649 lifted at any time and is not listed with the other Log Parameters in
1650 Section 4.1.
1652 Because of skew, it is possible the log server will not have any
1653 entries between "start" and "end". In this case it MUST return an
1654 empty "entries" array.
1656 In any case, the log server MUST return the latest STH it knows
1657 about.
1659 See Section 2.1.2 for an outline of how to use a complete list of
1660 "log_entry" entries to verify the "root_hash".
1662 Error codes:
1664 +================+====================================+
1665 | type | detail |
1666 +================+====================================+
1667 | startUnknown | "start" is greater than the number |
1668 | | of entries in the Merkle tree. |
1669 +----------------+------------------------------------+
1670 | endBeforeStart | "start" cannot be greater than |
1671 | | "end". |
1672 +----------------+------------------------------------+
1674 Table 6
1676 5.7. Retrieve Accepted Trust Anchors
1678 GET /ct/v2/get-anchors
1680 No inputs.
1682 Outputs: certificates: An array of JSON strings, each of which is a
1683 base64 encoded CA certificate that is acceptable to the log.
1684 max_chain_length:
1686 If the server has chosen to limit the length of chains it
1687 accepts, this is the maximum number of certificates in the
1688 chain, in decimal. If there is no limit, this is omitted.
1690 This data is not signed and the protocol depends on the security
1691 guarantees of TLS to ensure correctness.
1693 6. TLS Servers
1695 CT-using TLS servers MUST use at least one of the mechanisms
1696 described below to present one or more SCTs from one or more logs to
1697 each TLS client during full TLS handshakes, where each SCT
1698 corresponds to the server certificate. (Of course, a server can only
1699 send a TLS extension if the client has specified it first.) Servers
1700 SHOULD also present corresponding inclusion proofs and STHs.
1702 A server can provide SCTs using a TLS 1.3 extension (Section 4.2 of
1703 [RFC8446]) with type "transparency_info" (see Section 6.5). This
1704 mechanism allows TLS servers to participate in CT without the
1705 cooperation of CAs, unlike the other two mechanisms. It also allows
1706 SCTs and inclusion proofs to be updated on the fly.
1708 The server may also use an Online Certificate Status Protocol (OCSP)
1709 [RFC6960] response extension (see Section 7.1.1), providing the OCSP
1710 response as part of the TLS handshake. Providing a response during a
1711 TLS handshake is popularly known as "OCSP stapling." For TLS 1.3,
1712 the information is encoded as an extension in the "status_request"
1713 extension data; see Section 4.4.2.1 of [RFC8446]. For TLS 1.2
1714 ([RFC5246]), the information is encoded as an extension in the
1715 "CertificateStatus" message; see Section 8 of [RFC6066]. Using
1716 stapling also allows SCTs and inclusion proofs to be updated on the
1717 fly.
1719 CT information can also be encoded as an extension in the X.509v3
1720 certificate (see Section 7.1.2). This mechanism allows the use of
1721 unmodified TLS servers, but the SCTs and inclusion proofs cannot be
1722 updated on the fly. Since the logs from which the SCTs and inclusion
1723 proofs originated won't necessarily be accepted by TLS clients for
1724 the full lifetime of the certificate, there is a risk that TLS
1725 clients may subsequently consider the certificate to be non-compliant
1726 and in need of re-issuance or the use of one of the other two methods
1727 for delivering CT information.
1729 6.1. TLS Client Authentication
1731 This specification includes no description of how a TLS server can
1732 use CT for TLS client certificates. While this may be useful, it is
1733 not documented here for the following reasons:
1735 * The greater security exposure is for clients to end up interacting
1736 with an illegitimate server.
1738 * In general, TLS client certificates are not expected to be
1739 submitted to CT logs, particularly those intended for general
1740 public use.
1742 A future version could include such information.
1744 6.2. Multiple SCTs
1746 CT-using TLS servers SHOULD send SCTs from multiple logs, because:
1748 * One or more logs may not have become acceptable to all CT-using
1749 TLS clients. Note that client discovery, trust, and distrust of
1750 logs is expected to be handled out-of-band and is out of scope of
1751 this document.
1753 * If a CA and a log collude, it is possible to temporarily hide
1754 misissuance from clients. When a TLS client requires SCTs from
1755 multiple logs to be provided, it is more difficult to mount this
1756 attack.
1758 * If a log misbehaves or suffers a key compromise, a consequence may
1759 be that clients cease to trust it. Since the time an SCT may be
1760 in use can be considerable (several years is common in current
1761 practice when embedded in a certificate), including SCTs from
1762 multiple logs reduces the probability of the certificate being
1763 rejected by TLS clients.
1765 * TLS clients may have policies related to the above risks requiring
1766 TLS servers to present multiple SCTs. For example, at the time of
1767 writing, Chromium [Chromium.Log.Policy] requires multiple SCTs to
1768 be presented with EV certificates in order for the EV indicator to
1769 be shown.
1771 To select the logs from which to obtain SCTs, a TLS server can, for
1772 example, examine the set of logs popular TLS clients accept and
1773 recognize.
1775 6.3. TransItemList Structure
1777 Multiple SCTs, inclusion proofs, and indeed "TransItem" structures of
1778 any type, are combined into a list as follows:
1780 opaque SerializedTransItem<1..2^16-1>;
1782 struct {
1783 SerializedTransItem trans_item_list<1..2^16-1>;
1784 } TransItemList;
1786 Here, "SerializedTransItem" is an opaque byte string that contains
1787 the serialized "TransItem" structure. This encoding ensures that TLS
1788 clients can decode each "TransItem" individually (so, for example, if
1789 there is a version upgrade, out-of-date clients can still parse old
1790 "TransItem" structures while skipping over new "TransItem" structures
1791 whose versions they don't understand).
1793 6.4. Presenting SCTs, inclusions proofs and STHs
1795 In each "TransItemList" that is sent during a TLS handshake, the TLS
1796 server MUST include a "TransItem" structure of type "x509_sct_v2" or
1797 "precert_sct_v2".
1799 Presenting inclusion proofs and STHs in the TLS handshake helps to
1800 protect the client's privacy (see Section 8.1.4) and reduces load on
1801 log servers. Therefore, if the TLS server can obtain them, it SHOULD
1802 also include "TransItem"s of type "inclusion_proof_v2" and
1803 "signed_tree_head_v2" in the "TransItemList".
1805 6.5. transparency_info TLS Extension
1807 Provided that a TLS client includes the "transparency_info" extension
1808 type in the ClientHello and the TLS server supports the
1809 "transparency_info" extension:
1811 * The TLS server MUST verify that the received "extension_data" is
1812 empty.
1814 * The TLS server MUST construct a "TransItemList" of relevant
1815 "TransItem"s (see Section 6.4), which SHOULD omit any "TransItem"s
1816 that are already embedded in the server certificate or the stapled
1817 OCSP response (see Section 7.1). If the constructed
1818 "TransItemList" is not empty, then the TLS server MUST include the
1819 "transparency_info" extension with the "extension_data" set to
1820 this "TransItemList". If the list is empty then the server SHOULD
1821 omit the "extension_data" element, but MAY send it with an empty
1822 array.
1824 TLS servers MUST only include this extension in the following
1825 messages:
1827 * the ServerHello message (for TLS 1.2 or earlier).
1829 * the Certificate or CertificateRequest message (for TLS 1.3).
1831 TLS servers MUST NOT process or include this extension when a TLS
1832 session is resumed, since session resumption uses the original
1833 session information.
1835 7. Certification Authorities
1837 7.1. Transparency Information X.509v3 Extension
1839 The Transparency Information X.509v3 extension, which has OID
1840 1.3.101.75 and SHOULD be non-critical, contains one or more
1841 "TransItem" structures in a "TransItemList". This extension MAY be
1842 included in OCSP responses (see Section 7.1.1) and certificates (see
1843 Section 7.1.2). Since RFC5280 requires the "extnValue" field (an
1844 OCTET STRING) of each X.509v3 extension to include the DER encoding
1845 of an ASN.1 value, a "TransItemList" MUST NOT be included directly.
1846 Instead, it MUST be wrapped inside an additional OCTET STRING, which
1847 is then put into the "extnValue" field:
1849 TransparencyInformationSyntax ::= OCTET STRING
1851 "TransparencyInformationSyntax" contains a "TransItemList".
1853 7.1.1. OCSP Response Extension
1855 A certification authority MAY include a Transparency Information
1856 X.509v3 extension in the "singleExtensions" of a "SingleResponse" in
1857 an OCSP response. All included SCTs and inclusion proofs MUST be for
1858 the certificate identified by the "certID" of that "SingleResponse",
1859 or for a precertificate that corresponds to that certificate.
1861 7.1.2. Certificate Extension
1863 A certification authority MAY include a Transparency Information
1864 X.509v3 extension in a certificate. All included SCTs and inclusion
1865 proofs MUST be for a precertificate that corresponds to this
1866 certificate.
1868 7.2. TLS Feature X.509v3 Extension
1870 A certification authority SHOULD NOT issue any certificate that
1871 identifies the "transparency_info" TLS extension in a TLS feature
1872 extension [RFC7633], because TLS servers are not required to support
1873 the "transparency_info" TLS extension in order to participate in CT
1874 (see Section 6).
1876 8. Clients
1878 There are various different functions clients of logs might perform.
1879 We describe here some typical clients and how they should function.
1880 Any inconsistency may be used as evidence that a log has not behaved
1881 correctly, and the signatures on the data structures prevent the log
1882 from denying that misbehavior.
1884 All clients need various parameters in order to communicate with logs
1885 and verify their responses. These parameters are described in
1886 Section 4.1, but note that this document does not describe how the
1887 parameters are obtained, which is implementation-dependent (see, for
1888 example, [Chromium.Policy]).
1890 8.1. TLS Client
1892 8.1.1. Receiving SCTs and inclusion proofs
1894 TLS clients receive SCTs and inclusion proofs alongside or in
1895 certificates. CT-using TLS clients MUST implement all of the three
1896 mechanisms by which TLS servers may present SCTs (see Section 6).
1898 TLS clients that support the "transparency_info" TLS extension (see
1899 Section 6.5) SHOULD include it in ClientHello messages, with empty
1900 "extension_data". If a TLS server includes the "transparency_info"
1901 TLS extension when resuming a TLS session, the TLS client MUST abort
1902 the handshake.
1904 8.1.2. Reconstructing the TBSCertificate
1906 Validation of an SCT for a certificate (where the "type" of the
1907 "TransItem" is "x509_sct_v2") uses the unmodified TBSCertificate
1908 component of the certificate.
1910 Before an SCT for a precertificate (where the "type" of the
1911 "TransItem" is "precert_sct_v2") can be validated, the TBSCertificate
1912 component of the precertificate needs to be reconstructed from the
1913 TBSCertificate component of the certificate as follows:
1915 * Remove the Transparency Information extension (see Section 7.1).
1917 * Remove embedded v1 SCTs, identified by OID 1.3.6.1.4.1.11129.2.4.2
1918 (see section 3.3 of [RFC6962]). This allows embedded v1 and v2
1919 SCTs to co-exist in a certificate (see Appendix A).
1921 8.1.3. Validating SCTs
1923 In order to make use of a received SCT, the TLS client MUST first
1924 validate it as follows:
1926 * Compute the signature input by constructing a "TransItem" of type
1927 "x509_entry_v2" or "precert_entry_v2", depending on the SCT's
1928 "TransItem" type. The "TimestampedCertificateEntryDataV2"
1929 structure is constructed in the following manner:
1931 - "timestamp" is copied from the SCT.
1933 - "tbs_certificate" is the reconstructed TBSCertificate portion
1934 of the server certificate, as described in Section 8.1.2.
1936 - "issuer_key_hash" is computed as described in Section 4.7.
1938 - "sct_extensions" is copied from the SCT.
1940 * Verify the SCT's "signature" against the computed signature input
1941 using the public key of the corresponding log, which is identified
1942 by the "log_id". The required signature algorithm is one of the
1943 log's parameters.
1945 If the TLS client does not have the corresponding log's parameters,
1946 it cannot attempt to validate the SCT. When evaluating compliance
1947 (see Section 8.1.6), the TLS client will consider only those SCTs
1948 that it was able to validate.
1950 Note that SCT validation is not a substitute for the normal
1951 validation of the server certificate and its chain.
1953 8.1.4. Fetching inclusion proofs
1955 When a TLS client has validated a received SCT but does not yet
1956 possess a corresponding inclusion proof, the TLS client MAY request
1957 the inclusion proof directly from a log using "get-proof-by-hash"
1958 (Section 5.4) or "get-all-by-hash" (Section 5.5).
1960 Note that fetching inclusion proofs directly from a log will disclose
1961 to the log which TLS server the client has been communicating with.
1962 This may be regarded as a significant privacy concern, and so it is
1963 preferable for the TLS server to send the inclusion proofs (see
1964 Section 6.4).
1966 8.1.5. Validating inclusion proofs
1968 When a TLS client has received, or fetched, an inclusion proof (and
1969 an STH), it SHOULD proceed to verifying the inclusion proof to the
1970 provided STH. The TLS client SHOULD also verify consistency between
1971 the provided STH and an STH it knows about.
1973 If the TLS client holds an STH that predates the SCT, it MAY, in the
1974 process of auditing, request a new STH from the log (Section 5.2),
1975 then verify it by requesting a consistency proof (Section 5.3). Note
1976 that if the TLS client uses "get-all-by-hash", then it will already
1977 have the new STH.
1979 8.1.6. Evaluating compliance
1981 It is up to a client's local policy to specify the quantity and form
1982 of evidence (SCTs, inclusion proofs or a combination) needed to
1983 achieve compliance and how to handle non-compliance.
1985 A TLS client can only evaluate compliance if it has given the TLS
1986 server the opportunity to send SCTs and inclusion proofs by any of
1987 the three mechanisms that are mandatory to implement for CT-using TLS
1988 clients (see Section 8.1.1). Therefore, a TLS client MUST NOT
1989 evaluate compliance if it did not include both the
1990 "transparency_info" and "status_request" TLS extensions in the
1991 ClientHello.
1993 8.2. Monitor
1995 Monitors watch logs to check that they behave correctly, for
1996 certificates of interest, or both. For example, a monitor may be
1997 configured to report on all certificates that apply to a specific
1998 domain name when fetching new entries for consistency validation.
2000 A monitor MUST at least inspect every new entry in every log it
2001 watches, and it MAY also choose to keep copies of entire logs.
2003 To inspect all of the existing entries, the monitor SHOULD follow
2004 these steps once for each log:
2006 1. Fetch the current STH (Section 5.2).
2008 2. Verify the STH signature.
2010 3. Fetch all the entries in the tree corresponding to the STH
2011 (Section 5.6).
2013 4. If applicable, check each entry to see if it's a certificate of
2014 interest.
2016 5. Confirm that the tree made from the fetched entries produces the
2017 same hash as that in the STH.
2019 To inspect new entries, the monitor SHOULD follow these steps
2020 repeatedly for each log:
2022 1. Fetch the current STH (Section 5.2). Repeat until the STH
2023 changes. This document does not specify the polling frequency,
2024 to allow for experimentation.
2026 2. Verify the STH signature.
2028 3. Fetch all the new entries in the tree corresponding to the STH
2029 (Section 5.6). If they remain unavailable for an extended
2030 period, then this should be viewed as misbehavior on the part of
2031 the log.
2033 4. If applicable, check each entry to see if it's a certificate of
2034 interest.
2036 5. Either:
2038 1. Verify that the updated list of all entries generates a tree
2039 with the same hash as the new STH.
2041 Or, if it is not keeping all log entries:
2043 1. Fetch a consistency proof for the new STH with the previous
2044 STH (Section 5.3).
2046 2. Verify the consistency proof.
2048 3. Verify that the new entries generate the corresponding
2049 elements in the consistency proof.
2051 6. Repeat from step 1.
2053 8.3. Auditing
2055 Auditing ensures that the current published state of a log is
2056 reachable from previously published states that are known to be good,
2057 and that the promises made by the log in the form of SCTs have been
2058 kept. Audits are performed by monitors or TLS clients.
2060 In particular, there are four log behavior properties that should be
2061 checked:
2063 * The Maximum Merge Delay (MMD).
2065 * The STH Frequency Count.
2067 * The append-only property.
2069 * The consistency of the log view presented to all query sources.
2071 A benign, conformant log publishes a series of STHs over time, each
2072 derived from the previous STH and the submitted entries incorporated
2073 into the log since publication of the previous STH. This can be
2074 proven through auditing of STHs. SCTs returned to TLS clients can be
2075 audited by verifying against the accompanying certificate, and using
2076 Merkle Inclusion Proofs, against the log's Merkle tree.
2078 The action taken by the auditor if an audit fails is not specified,
2079 but note that in general if audit fails, the auditor is in possession
2080 of signed proof of the log's misbehavior.
2082 A monitor (Section 8.2) can audit by verifying the consistency of
2083 STHs it receives, ensure that each entry can be fetched and that the
2084 STH is indeed the result of making a tree from all fetched entries.
2086 A TLS client (Section 8.1) can audit by verifying an SCT against any
2087 STH dated after the SCT timestamp + the Maximum Merge Delay by
2088 requesting a Merkle inclusion proof (Section 5.4). It can also
2089 verify that the SCT corresponds to the server certificate it arrived
2090 with (i.e., the log entry is that certificate, or is a precertificate
2091 corresponding to that certificate).
2093 Checking of the consistency of the log view presented to all entities
2094 is more difficult to perform because it requires a way to share log
2095 responses among a set of CT-using entities, and is discussed in
2096 Section 11.3.
2098 9. Algorithm Agility
2100 It is not possible for a log to change any of its algorithms part way
2101 through its lifetime:
2103 Signature algorithm: SCT signatures must remain valid so signature
2104 algorithms can only be added, not removed.
2106 Hash algorithm: A log would have to support the old and new hash
2107 algorithms to allow backwards-compatibility with clients that are
2108 not aware of a hash algorithm change.
2110 Allowing multiple signature or hash algorithms for a log would
2111 require that all data structures support it and would significantly
2112 complicate client implementation, which is why it is not supported by
2113 this document.
2115 If it should become necessary to deprecate an algorithm used by a
2116 live log, then the log MUST be frozen as specified in Section 4.13
2117 and a new log SHOULD be started. Certificates in the frozen log that
2118 have not yet expired and require new SCTs SHOULD be submitted to the
2119 new log and the SCTs from that log used instead.
2121 10. IANA Considerations
2123 The assignment policy criteria mentioned in this section refer to the
2124 policies outlined in [RFC8126].
2126 10.1. Additions to existing registries
2128 This sub-section defines additions to existing registries.
2130 10.1.1. New Entry to the TLS ExtensionType Registry
2132 IANA is asked to add an entry for "transparency_info(TBD)" to the
2133 "TLS ExtensionType Values" registry defined in [RFC8446], setting the
2134 "Recommended" value to "Y", setting the "TLS 1.3" value to "CH, CR,
2135 CT", and citing this document as the "Reference".
2137 10.1.2. URN Sub-namespace for TRANS errors
2138 (urn:ietf:params:trans:error)
2140 IANA is requested to add a new entry in the "IETF URN Sub-namespace
2141 for Registered Protocol Parameter Identifiers" registry, following
2142 the template in [RFC3553]:
2144 Registry name: trans:error
2146 Specification: RFCXXXX
2148 Repository: https://www.iana.org/assignments/trans
2150 Index value: No transformation needed.
2152 10.2. New CT-Related registries
2154 This sub-section defines new registries for CT. They should be made
2155 available at https://www.iana.org/assignments/
2157 10.2.1. Hash Algorithms
2159 IANA is asked to establish a registry of hash algorithm values, named
2160 "CT Hash Algorithms", that initially consists of:
2162 +========+============+========================+===================+
2163 | Value | Hash | OID | Reference / |
2164 | | Algorithm | | Assignment Policy |
2165 +========+============+========================+===================+
2166 | 0x00 | SHA-256 | 2.16.840.1.101.3.4.2.1 | [RFC6234] |
2167 +--------+------------+------------------------+-------------------+
2168 | 0x01 - | Unassigned | | Specification |
2169 | 0xDF | | | Required |
2170 +--------+------------+------------------------+-------------------+
2171 | 0xE0 - | Reserved | | Experimental Use |
2172 | 0xEF | | | |
2173 +--------+------------+------------------------+-------------------+
2174 | 0xF0 - | Reserved | | Private Use |
2175 | 0xFF | | | |
2176 +--------+------------+------------------------+-------------------+
2178 Table 7
2180 The Designated Expert(s) should ensure that the proposed algorithm
2181 has a public specification and is suitable for use as a cryptographic
2182 hash algorithm with no known preimage or collision attacks. These
2183 attacks can damage the integrity of the log.
2185 10.2.2. Signature Algorithms
2187 IANA is asked to establish a registry of signature algorithm values,
2188 named "CT Signature Algorithms".
2190 The following notes should be added:
2192 * This is a subset of the TLS SignatureScheme Registry, limited to
2193 those algorithms that are appropriate for CT. A major advantage
2194 of this is leveraging the expertise of the TLS working group and
2195 its Designated Expert(s).
2197 * The value "0x0403" appears twice. While this may be confusing, it
2198 is okay because the verification process is the same for both
2199 algorithms, and the choice of which to use when generating a
2200 signature is purely internal to the log server.
2202 The registry should initially consist of:
2204 +================================+==================+==============+
2205 | SignatureScheme Value | Signature | Reference / |
2206 | | Algorithm | Assignment |
2207 | | | Policy |
2208 +================================+==================+==============+
2209 | 0x0000 - 0x0402 | Unassigned | Expert |
2210 | | | Review |
2211 +--------------------------------+------------------+--------------+
2212 | ecdsa_secp256r1_sha256(0x0403) | ECDSA (NIST | [FIPS186-4] |
2213 | | P-256) with | |
2214 | | SHA-256 | |
2215 +--------------------------------+------------------+--------------+
2216 | ecdsa_secp256r1_sha256(0x0403) | Deterministic | [RFC6979] |
2217 | | ECDSA (NIST | |
2218 | | P-256) with | |
2219 | | HMAC-SHA256 | |
2220 +--------------------------------+------------------+--------------+
2221 | 0x0404 - 0x0806 | Unassigned | Expert |
2222 | | | Review |
2223 +--------------------------------+------------------+--------------+
2224 | ed25519(0x0807) | Ed25519 | [RFC8032] |
2225 | | (PureEdDSA with | |
2226 | | the edwards25519 | |
2227 | | curve) | |
2228 +--------------------------------+------------------+--------------+
2229 | 0x0808 - 0xFDFF | Unassigned | Expert |
2230 | | | Review |
2231 +--------------------------------+------------------+--------------+
2232 | 0xFE00 - 0xFEFF | Reserved | Experimental |
2233 | | | Use |
2234 +--------------------------------+------------------+--------------+
2235 | 0xFF00 - 0xFFFF | Reserved | Private Use |
2236 +--------------------------------+------------------+--------------+
2238 Table 8
2240 The Designated Expert(s) should ensure that the proposed algorithm
2241 has a public specification, has a value assigned to it in the TLS
2242 SignatureScheme Registry (that IANA is asked to establish in
2243 [RFC8446]) and is suitable for use as a cryptographic signature
2244 algorithm.
2246 10.2.3. VersionedTransTypes
2248 IANA is asked to establish a registry of "VersionedTransType" values,
2249 named "CT VersionedTransTypes", that initially consists of:
2251 +==========+======================+===============================+
2252 | Value | Type and Version | Reference / Assignment Policy |
2253 +==========+======================+===============================+
2254 | 0x0000 | Reserved | [RFC6962] * |
2255 +----------+----------------------+-------------------------------+
2256 | 0x0001 | x509_entry_v2 | RFCXXXX |
2257 +----------+----------------------+-------------------------------+
2258 | 0x0002 | precert_entry_v2 | RFCXXXX |
2259 +----------+----------------------+-------------------------------+
2260 | 0x0003 | x509_sct_v2 | RFCXXXX |
2261 +----------+----------------------+-------------------------------+
2262 | 0x0004 | precert_sct_v2 | RFCXXXX |
2263 +----------+----------------------+-------------------------------+
2264 | 0x0005 | signed_tree_head_v2 | RFCXXXX |
2265 +----------+----------------------+-------------------------------+
2266 | 0x0006 | consistency_proof_v2 | RFCXXXX |
2267 +----------+----------------------+-------------------------------+
2268 | 0x0007 | inclusion_proof_v2 | RFCXXXX |
2269 +----------+----------------------+-------------------------------+
2270 | 0x0008 - | Unassigned | Specification Required |
2271 | 0xDFFF | | |
2272 +----------+----------------------+-------------------------------+
2273 | 0xE000 - | Reserved | Experimental Use |
2274 | 0xEFFF | | |
2275 +----------+----------------------+-------------------------------+
2276 | 0xF000 - | Reserved | Private Use |
2277 | 0xFFFF | | |
2278 +----------+----------------------+-------------------------------+
2280 Table 9
2282 * The 0x0000 value is reserved so that v1 SCTs are distinguishable
2283 from v2 SCTs and other "TransItem" structures.
2285 The Designated Expert(s) should review the public specification to
2286 ensure that it is detailed enough to ensure implementation
2287 interoperability.
2289 10.2.4. Log Artifact Extension Registry
2291 IANA is asked to establish a registry of "ExtensionType" values,
2292 named "CT Log Artifact Extensions", that initially consists of:
2294 +===============+============+=====+===============================+
2295 | ExtensionType | Status | Use | Reference / Assignment Policy |
2296 +===============+============+=====+===============================+
2297 | 0x0000 - | Unassigned | n/a | Specification Required |
2298 | 0xDFFF | | | |
2299 +---------------+------------+-----+-------------------------------+
2300 | 0xE000 - | Reserved | n/a | Experimental Use |
2301 | 0xEFFF | | | |
2302 +---------------+------------+-----+-------------------------------+
2303 | 0xF000 - | Reserved | n/a | Private Use |
2304 | 0xFFFF | | | |
2305 +---------------+------------+-----+-------------------------------+
2307 Table 10
2309 The "Use" column should contain one or both of the following values:
2311 * "SCT", for extensions specified for use in Signed Certificate
2312 Timestamps.
2314 * "STH", for extensions specified for use in Signed Tree Heads.
2316 The Designated Expert(s) should review the public specification to
2317 ensure that it is detailed enough to ensure implementation
2318 interoperability. They should also verify that the extension is
2319 appropriate to the contexts in which it is specified to be used (SCT,
2320 STH, or both).
2322 10.2.5. Object Identifiers
2324 This document uses object identifiers (OIDs) to identify Log IDs (see
2325 Section 4.4), the precertificate CMS "eContentType" (see
2326 Section 3.2), and X.509v3 extensions in certificates (see
2327 Section 7.1.2) and OCSP responses (see Section 7.1.1). The OIDs are
2328 defined in an arc that was selected due to its short encoding.
2330 10.2.5.1. Log ID Registry
2332 IANA is asked to establish a registry of Log IDs, named "CT Log ID
2333 Registry", that initially consists of:
2335 +================+==============+==============+===================+
2336 | Log ID | Log Base URL | Log Operator | Reference / |
2337 | | | | Assignment Policy |
2338 +================+==============+==============+===================+
2339 | 1.3.101.8192 - | Unassigned | Unassigned | First Come First |
2340 | 1.3.101.16383 | | | Served |
2341 +----------------+--------------+--------------+-------------------+
2342 | 1.3.101.80.0 - | Unassigned | Unassigned | First Come First |
2343 | 1.3.101.80.* | | | Served |
2344 +----------------+--------------+--------------+-------------------+
2346 Table 11
2348 All OIDs in the range from 1.3.101.8192 to 1.3.101.16383 have been
2349 set aside for Log IDs. This is a limited resource of 8,192 OIDs,
2350 each of which has an encoded length of 4 octets.
2352 The 1.3.101.80 arc has also been set assigned for LogIDs. This is an
2353 unlimited resource, but only the 128 OIDs from 1.3.101.80.0 to
2354 1.3.101.80.127 have an encoded length of only 4 octets.
2356 Each application for the allocation of a Log ID MUST be accompanied
2357 by:
2359 * the Log's Base URL (see Section 4.1).
2361 * the Log Operator's contact details.
2363 IANA is asked to reject any request to update a Log ID or Log Base
2364 URL in this registry, because these fields are immutable (see
2365 Section 4.1).
2367 IANA is asked to accept requests from log operators to update their
2368 contact details in this registry.
2370 Since log operators can choose to not use this registry (see
2371 Section 4.4), it is not expected to be a global directory of all
2372 logs.
2374 10.2.6. CT Error Types Registry
2376 IANA is requested to create a new registry for errors, the "CT Error
2377 Types" registry.
2379 Requirements for this registry are Specification Required.
2381 This registry should have the following three fields:
2383 +============+========+===========+
2384 | Field Name | Type | Reference |
2385 +============+========+===========+
2386 | identifier | string | RFCXXXX |
2387 +------------+--------+-----------+
2388 | meaning | string | RFCXXXX |
2389 +------------+--------+-----------+
2390 | reference | string | RFCXXXX |
2391 +------------+--------+-----------+
2393 Table 12
2395 The initial values are as follows, taken from the text above:
2397 +===================+===============================+===========+
2398 | Identifier | Meaning | Reference |
2399 +===================+===============================+===========+
2400 | malformed | The request could not be | RFCXXXX |
2401 | | parsed. | |
2402 +-------------------+-------------------------------+-----------+
2403 | badSubmission | "submission" is neither a | RFCXXXX |
2404 | | valid certificate nor a valid | |
2405 | | precertificate | |
2406 +-------------------+-------------------------------+-----------+
2407 | badType | "type" is neither 1 nor 2 | RFCXXXX |
2408 +-------------------+-------------------------------+-----------+
2409 | badChain | The first element of "chain" | RFCXXXX |
2410 | | is not the certifier of the | |
2411 | | "submission", or the second | |
2412 | | element does not certify the | |
2413 | | first, etc. | |
2414 +-------------------+-------------------------------+-----------+
2415 | badCertificate | One or more certificates in | RFCXXXX |
2416 | | the "chain" are not valid | |
2417 | | (e.g., not properly encoded) | |
2418 +-------------------+-------------------------------+-----------+
2419 | unknownAnchor | The last element of "chain" | RFCXXXX |
2420 | | (or, if "chain" is an empty | |
2421 | | array, the "submission") both | |
2422 | | is not, and is not certified | |
2423 | | by, an accepted trust anchor | |
2424 +-------------------+-------------------------------+-----------+
2425 | shutdown | The log is no longer | RFCXXXX |
2426 | | accepting submissions | |
2427 +-------------------+-------------------------------+-----------+
2428 | firstUnknown | "first" is before the latest | RFCXXXX |
2429 | | known STH but is not from an | |
2430 | | existing STH. | |
2431 +-------------------+-------------------------------+-----------+
2432 | secondUnknown | "second" is before the latest | RFCXXXX |
2433 | | known STH but is not from an | |
2434 | | existing STH. | |
2435 +-------------------+-------------------------------+-----------+
2436 | secondBeforeFirst | "second" is smaller than | RFCXXXX |
2437 | | "first". | |
2438 +-------------------+-------------------------------+-----------+
2439 | hashUnknown | "hash" is not the hash of a | RFCXXXX |
2440 | | known leaf (may be caused by | |
2441 | | skew or by a known | |
2442 | | certificate not yet merged). | |
2443 +-------------------+-------------------------------+-----------+
2444 | treeSizeUnknown | "hash" is before the latest | RFCXXXX |
2445 | | known STH but is not from an | |
2446 | | existing STH. | |
2447 +-------------------+-------------------------------+-----------+
2448 | startUnknown | "start" is greater than the | RFCXXXX |
2449 | | number of entries in the | |
2450 | | Merkle tree. | |
2451 +-------------------+-------------------------------+-----------+
2452 | endBeforeStart | "start" cannot be greater | RFCXXXX |
2453 | | than "end". | |
2454 +-------------------+-------------------------------+-----------+
2456 Table 13
2458 11. Security Considerations
2460 With CAs, logs, and servers performing the actions described here,
2461 TLS clients can use logs and signed timestamps to reduce the
2462 likelihood that they will accept misissued certificates. If a server
2463 presents a valid signed timestamp for a certificate, then the client
2464 knows that a log has committed to publishing the certificate. From
2465 this, the client knows that monitors acting for the subject of the
2466 certificate have had some time to notice the misissuance and take
2467 some action, such as asking a CA to revoke a misissued certificate.
2468 A signed timestamp does not guarantee this though, since appropriate
2469 monitors might not have checked the logs or the CA might have refused
2470 to revoke the certificate.
2472 In addition, if TLS clients will not accept unlogged certificates,
2473 then site owners will have a greater incentive to submit certificates
2474 to logs, possibly with the assistance of their CA, increasing the
2475 overall transparency of the system.
2477 11.1. Misissued Certificates
2479 Misissued certificates that have not been publicly logged, and thus
2480 do not have a valid SCT, are not considered compliant. Misissued
2481 certificates that do have an SCT from a log will appear in that
2482 public log within the Maximum Merge Delay, assuming the log is
2483 operating correctly. Since a log is allowed to serve an STH of any
2484 age up to the MMD, the maximum period of time during which a
2485 misissued certificate can be used without being available for audit
2486 is twice the MMD.
2488 11.2. Detection of Misissue
2490 The logs do not themselves detect misissued certificates; they rely
2491 instead on interested parties, such as domain owners, to monitor them
2492 and take corrective action when a misissue is detected.
2494 11.3. Misbehaving Logs
2496 A log can misbehave in several ways. Examples include: failing to
2497 incorporate a certificate with an SCT in the Merkle Tree within the
2498 MMD; presenting different, conflicting views of the Merkle Tree at
2499 different times and/or to different parties; issuing STHs too
2500 frequently; mutating the signature of a logged certificate; and
2501 failing to present a chain containing the certifier of a logged
2502 certificate.
2504 Violation of the MMD contract is detected by log clients requesting a
2505 Merkle inclusion proof (Section 5.4) for each observed SCT. These
2506 checks can be asynchronous and need only be done once per
2507 certificate. However, note that there may be privacy concerns (see
2508 Section 8.1.4).
2510 Violation of the append-only property or the STH issuance rate limit
2511 can be detected by multiple clients comparing their instances of the
2512 Signed Tree Heads. This technique, known as "gossip," is an active
2513 area of research and not defined here. Proof of misbehavior in such
2514 cases would be: a series of STHs that were issued too closely
2515 together, proving violation of the STH issuance rate limit; or an STH
2516 with a root hash that does not match the one calculated from a copy
2517 of the log, proving violation of the append-only property.
2519 Clients that report back SCTs can be tracked or traced if a log
2520 produces multiple STHs or SCTs with the same timestamp and data but
2521 different signatures. Logs SHOULD mitigate this risk by either:
2523 * Using deterministic signature schemes, or
2524 * Producing no more than one SCT for each distinct submission and no
2525 more than one STH for each distinct tree_size. Each of these SCTs
2526 and STHs can be stored by the log and served to other clients that
2527 submit the same certificate or request the same STH.
2529 11.4. Multiple SCTs
2531 By requiring TLS servers to offer multiple SCTs, each from a
2532 different log, TLS clients reduce the effectiveness of an attack
2533 where a CA and a log collude (see Section 6.2).
2535 11.5. Leakage of DNS Information
2537 Malicious monitors can use logs to learn about the existence of
2538 domain names that might not otherwise be easy to discover. Some
2539 subdomain labels may reveal information about the service and
2540 software for which the subdomain is used, which in turn might
2541 facilitate targeted attacks.
2543 12. Acknowledgements
2545 The authors would like to thank Erwann Abelea, Robin Alden, Andrew
2546 Ayer, Richard Barnes, Al Cutter, David Drysdale, Francis Dupont, Adam
2547 Eijdenberg, Stephen Farrell, Daniel Kahn Gillmor, Paul Hadfield, Brad
2548 Hill, Jeff Hodges, Paul Hoffman, Jeffrey Hutzelman, Kat Joyce,
2549 Stephen Kent, SM, Alexey Melnikov, Linus Nordberg, Chris Palmer,
2550 Trevor Perrin, Pierre Phaneuf, Eric Rescorla, Rich Salz, Melinda
2551 Shore, Ryan Sleevi, Martin Smith, Carl Wallace and Paul Wouters for
2552 their valuable contributions.
2554 A big thank you to Symantec for kindly donating the OIDs from the
2555 1.3.101 arc that are used in this document.
2557 13. References
2559 13.1. Normative References
2561 [FIPS186-4]
2562 NIST, "FIPS PUB 186-4", 1 July 2013,
2563 .
2566 [HTML401] Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
2567 Specification", World Wide Web Consortium Recommendation
2568 REC-html401-19991224, 24 December 1999,
2569 .
2571 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
2572 Requirement Levels", BCP 14, RFC 2119,
2573 DOI 10.17487/RFC2119, March 1997,
2574 .
2576 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
2577 IETF URN Sub-namespace for Registered Protocol
2578 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
2579 2003, .
2581 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
2582 Resource Identifier (URI): Generic Syntax", STD 66,
2583 RFC 3986, DOI 10.17487/RFC3986, January 2005,
2584 .
2586 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
2587 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
2588 .
2590 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
2591 (TLS) Protocol Version 1.2", RFC 5246,
2592 DOI 10.17487/RFC5246, August 2008,
2593 .
2595 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
2596 Housley, R., and W. Polk, "Internet X.509 Public Key
2597 Infrastructure Certificate and Certificate Revocation List
2598 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
2599 .
2601 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
2602 RFC 5652, DOI 10.17487/RFC5652, September 2009,
2603 .
2605 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
2606 Extensions: Extension Definitions", RFC 6066,
2607 DOI 10.17487/RFC6066, January 2011,
2608 .
2610 [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
2611 (SHA and SHA-based HMAC and HKDF)", RFC 6234,
2612 DOI 10.17487/RFC6234, May 2011,
2613 .
2615 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
2616 Galperin, S., and C. Adams, "X.509 Internet Public Key
2617 Infrastructure Online Certificate Status Protocol - OCSP",
2618 RFC 6960, DOI 10.17487/RFC6960, June 2013,
2619 .
2621 [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
2622 Algorithm (DSA) and Elliptic Curve Digital Signature
2623 Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2624 2013, .
2626 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
2627 Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
2628 DOI 10.17487/RFC7231, June 2014,
2629 .
2631 [RFC7633] Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS)
2632 Feature Extension", RFC 7633, DOI 10.17487/RFC7633,
2633 October 2015, .
2635 [RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP
2636 APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
2637 .
2639 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
2640 Signature Algorithm (EdDSA)", RFC 8032,
2641 DOI 10.17487/RFC8032, January 2017,
2642 .
2644 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2645 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
2646 May 2017, .
2648 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
2649 Interchange Format", STD 90, RFC 8259,
2650 DOI 10.17487/RFC8259, December 2017,
2651 .
2653 [RFC8391] Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A.
2654 Mohaisen, "XMSS: eXtended Merkle Signature Scheme",
2655 RFC 8391, DOI 10.17487/RFC8391, May 2018,
2656 .
2658 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
2659 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
2660 .
2662 [UNIXTIME] IEEE, "The Open Group Base Specifications Issue 7 IEEE Std
2663 1003.1-2008, 2016 Edition", n.d.,
2664 .
2668 [X690] ITU-T, "Information technology - ASN.1 encoding Rules:
2669 Specification of Basic Encoding Rules (BER), Canonical
2670 Encoding Rules (CER) and Distinguished Encoding Rules
2671 (DER)", ISO/IEC 8825-1:2002, November 2015.
2673 13.2. Informative References
2675 [CABBR] CA/Browser Forum, "Baseline Requirements for the Issuance
2676 and Management of Publicly-Trusted Certificates", 2020,
2677 .
2680 [Chromium.Log.Policy]
2681 The Chromium Projects, "Chromium Certificate Transparency
2682 Log Policy", 2014, .
2685 [Chromium.Policy]
2686 The Chromium Projects, "Chromium Certificate
2687 Transparency", 2014, .
2690 [CrosbyWallach]
2691 Crosby, S. and D. Wallach, "Efficient Data Structures for
2692 Tamper-Evident Logging", Proceedings of the 18th USENIX
2693 Security Symposium, Montreal, August 2009,
2694 .
2697 [JSON.Metadata]
2698 The Chromium Projects, "Chromium Log Metadata JSON
2699 Schema", 2014, .
2702 [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
2703 Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
2704 .
2706 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
2707 Writing an IANA Considerations Section in RFCs", BCP 26,
2708 RFC 8126, DOI 10.17487/RFC8126, June 2017,
2709 .
2711 [RFC8820] Nottingham, M., "URI Design and Ownership", BCP 190,
2712 RFC 8820, DOI 10.17487/RFC8820, June 2020,
2713 .
2715 Appendix A. Supporting v1 and v2 simultaneously (Informative)
2717 Certificate Transparency logs have to be either v1 (conforming to
2718 [RFC6962]) or v2 (conforming to this document), as the data
2719 structures are incompatible and so a v2 log could not issue a valid
2720 v1 SCT.
2722 CT clients, however, can support v1 and v2 SCTs, for the same
2723 certificate, simultaneously, as v1 SCTs are delivered in different
2724 TLS, X.509 and OCSP extensions than v2 SCTs.
2726 v1 and v2 SCTs for X.509 certificates can be validated independently.
2727 For precertificates, v2 SCTs should be embedded in the TBSCertificate
2728 before submission of the TBSCertificate (inside a v1 precertificate,
2729 as described in Section 3.1. of [RFC6962]) to a v1 log so that TLS
2730 clients conforming to [RFC6962] but not this document are oblivious
2731 to the embedded v2 SCTs. An issuer can follow these steps to produce
2732 an X.509 certificate with embedded v1 and v2 SCTs:
2734 * Create a CMS precertificate as described in Section 3.2 and submit
2735 it to v2 logs.
2737 * Embed the obtained v2 SCTs in the TBSCertificate, as described in
2738 Section 7.1.2.
2740 * Use that TBSCertificate to create a v1 precertificate, as
2741 described in Section 3.1. of [RFC6962] and submit it to v1 logs.
2743 * Embed the v1 SCTs in the TBSCertificate, as described in
2744 Section 3.3 of [RFC6962].
2746 * Sign that TBSCertificate (which now contains v1 and v2 SCTs) to
2747 issue the final X.509 certificate.
2749 Appendix B. An ASN.1 Module (Informative)
2751 The following ASN.1 module may be useful to implementors.
2753 CertificateTransparencyV2Module-2021
2754 -- { OID Needed, but no point in using a short one }
2755 DEFINITIONS IMPLICIT TAGS ::= BEGIN
2757 -- EXPORTS ALL --
2758 IMPORTS
2759 EXTENSION
2760 FROM PKIX-CommonTypes-2009 -- RFC 5912
2761 { iso(1) identified-organization(3) dod(6) internet(1)
2762 security(5) mechanisms(5) pkix(7) id-mod(0)
2763 id-mod-pkixCommon-02(57) }
2765 CONTENT-TYPE
2766 FROM CryptographicMessageSyntax-2010 -- RFC 6268
2767 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
2768 pkcs-9(9) smime(16) modules(0) id-mod-cms-2009(58) }
2770 TBSCertificate
2771 FROM PKIX1Explicit-2009 -- RFC 5912
2772 { iso(1) identified-organization(3) dod(6) internet(1)
2773 security(5) mechanisms(5) pkix(7) id-mod(0)
2774 id-mod-pkix1-explicit-02(51) }
2775 ;
2777 --
2778 -- Section 3.2. Precertificates
2779 --
2781 ct-tbsCertificate CONTENT-TYPE ::= {
2782 TYPE TBSCertificate
2783 IDENTIFIED BY id-ct-tbsCertificate }
2785 id-ct-tbsCertificate OBJECT IDENTIFIER ::= { 1 3 101 78 }
2787 --
2788 -- Section 7.1. Transparency Information X.509v3 Extension
2789 --
2791 ext-transparencyInfo EXTENSION ::= {
2792 SYNTAX TransparencyInformationSyntax
2793 IDENTIFIED BY id-ce-transparencyInfo
2794 CRITICALITY { FALSE } }
2796 id-ce-transparencyInfo OBJECT IDENTIFIER ::= { 1 3 101 75 }
2798 TransparencyInformationSyntax ::= OCTET STRING
2800 --
2801 -- Section 7.1.1. OCSP Response Extension
2802 --
2804 ext-ocsp-transparencyInfo EXTENSION ::= {
2805 SYNTAX TransparencyInformationSyntax
2806 IDENTIFIED BY id-pkix-ocsp-transparencyInfo
2807 CRITICALITY { FALSE } }
2809 id-pkix-ocsp-transparencyInfo OBJECT IDENTIFIER ::=
2810 id-ce-transparencyInfo
2812 --
2813 -- Section 8.1.2. Reconstructing the TBSCertificate
2814 --
2816 ext-embeddedSCT-CTv1 EXTENSION ::= {
2817 SYNTAX SignedCertificateTimestampList
2818 IDENTIFIED BY id-ce-embeddedSCT-CTv1
2819 CRITICALITY { FALSE } }
2821 id-ce-embeddedSCT-CTv1 OBJECT IDENTIFIER ::= {
2822 1 3 6 1 4 1 11129 2 4 2 }
2824 SignedCertificateTimestampList ::= OCTET STRING
2826 END
2828 Authors' Addresses
2830 Ben Laurie
2831 Google UK Ltd.
2833 Email: benl@google.com
2835 Adam Langley
2836 Google Inc.
2838 Email: agl@google.com
2840 Emilia Kasper
2841 Google Switzerland GmbH
2843 Email: ekasper@google.com
2845 Eran Messeri
2846 Google UK Ltd.
2848 Email: eranm@google.com
2849 Rob Stradling
2850 Sectigo Ltd.
2852 Email: rob@sectigo.com