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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: May 7, 2020 Google
7 R. Stradling
8 Sectigo
9 November 04, 2019
11 Certificate Transparency Version 2.0
12 draft-ietf-trans-rfc6962-bis-34
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 Status of This Memo
34 This Internet-Draft is submitted in full conformance with the
35 provisions of BCP 78 and BCP 79.
37 Internet-Drafts are working documents of the Internet Engineering
38 Task Force (IETF). Note that other groups may also distribute
39 working documents as Internet-Drafts. The list of current Internet-
40 Drafts is at https://datatracker.ietf.org/drafts/current/.
42 Internet-Drafts are draft documents valid for a maximum of six months
43 and may be updated, replaced, or obsoleted by other documents at any
44 time. It is inappropriate to use Internet-Drafts as reference
45 material or to cite them other than as "work in progress."
47 This Internet-Draft will expire on May 7, 2020.
49 Copyright Notice
51 Copyright (c) 2019 IETF Trust and the persons identified as the
52 document authors. All rights reserved.
54 This document is subject to BCP 78 and the IETF Trust's Legal
55 Provisions Relating to IETF Documents
56 (https://trustee.ietf.org/license-info) in effect on the date of
57 publication of this document. Please review these documents
58 carefully, as they describe your rights and restrictions with respect
59 to this document. Code Components extracted from this document must
60 include Simplified BSD License text as described in Section 4.e of
61 the Trust Legal Provisions and are provided without warranty as
62 described in the Simplified BSD License.
64 Table of Contents
66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
67 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
68 1.2. Data Structures . . . . . . . . . . . . . . . . . . . . . 5
69 1.3. Major Differences from CT 1.0 . . . . . . . . . . . . . . 5
70 2. Cryptographic Components . . . . . . . . . . . . . . . . . . 7
71 2.1. Merkle Hash Trees . . . . . . . . . . . . . . . . . . . . 7
72 2.1.1. Definition of the Merkle Tree . . . . . . . . . . . . 7
73 2.1.2. Verifying a Tree Head Given Entries . . . . . . . . . 8
74 2.1.3. Merkle Inclusion Proofs . . . . . . . . . . . . . . . 9
75 2.1.4. Merkle Consistency Proofs . . . . . . . . . . . . . . 10
76 2.1.5. Example . . . . . . . . . . . . . . . . . . . . . . . 12
77 2.2. Signatures . . . . . . . . . . . . . . . . . . . . . . . 14
78 3. Submitters . . . . . . . . . . . . . . . . . . . . . . . . . 14
79 3.1. Certificates . . . . . . . . . . . . . . . . . . . . . . 14
80 3.2. Precertificates . . . . . . . . . . . . . . . . . . . . . 14
81 3.2.1. Binding Intent to Issue . . . . . . . . . . . . . . . 16
82 4. Log Format and Operation . . . . . . . . . . . . . . . . . . 16
83 4.1. Log Parameters . . . . . . . . . . . . . . . . . . . . . 17
84 4.2. Evaluating Submissions . . . . . . . . . . . . . . . . . 18
85 4.2.1. Minimum Acceptance Criteria . . . . . . . . . . . . . 18
86 4.2.2. Discretionary Acceptance Criteria . . . . . . . . . . 19
87 4.3. Log Entries . . . . . . . . . . . . . . . . . . . . . . . 19
88 4.4. Log ID . . . . . . . . . . . . . . . . . . . . . . . . . 20
89 4.5. TransItem Structure . . . . . . . . . . . . . . . . . . . 20
90 4.6. Log Artifact Extensions . . . . . . . . . . . . . . . . . 21
91 4.7. Merkle Tree Leaves . . . . . . . . . . . . . . . . . . . 21
92 4.8. Signed Certificate Timestamp (SCT) . . . . . . . . . . . 22
93 4.9. Merkle Tree Head . . . . . . . . . . . . . . . . . . . . 23
94 4.10. Signed Tree Head (STH) . . . . . . . . . . . . . . . . . 24
95 4.11. Merkle Consistency Proofs . . . . . . . . . . . . . . . . 25
96 4.12. Merkle Inclusion Proofs . . . . . . . . . . . . . . . . . 25
97 4.13. Shutting down a log . . . . . . . . . . . . . . . . . . . 26
98 5. Log Client Messages . . . . . . . . . . . . . . . . . . . . . 26
99 5.1. Submit Entry to Log . . . . . . . . . . . . . . . . . . . 28
100 5.2. Retrieve Latest Signed Tree Head . . . . . . . . . . . . 30
101 5.3. Retrieve Merkle Consistency Proof between Two Signed Tree
102 Heads . . . . . . . . . . . . . . . . . . . . . . . . . . 30
103 5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash . . 31
104 5.5. Retrieve Merkle Inclusion Proof, Signed Tree Head and
105 Consistency Proof by Leaf Hash . . . . . . . . . . . . . 32
106 5.6. Retrieve Entries and STH from Log . . . . . . . . . . . . 33
107 5.7. Retrieve Accepted Trust Anchors . . . . . . . . . . . . . 35
108 6. TLS Servers . . . . . . . . . . . . . . . . . . . . . . . . . 35
109 6.1. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . . 36
110 6.2. TransItemList Structure . . . . . . . . . . . . . . . . . 37
111 6.3. Presenting SCTs, inclusions proofs and STHs . . . . . . . 37
112 6.4. transparency_info TLS Extension . . . . . . . . . . . . . 37
113 7. Certification Authorities . . . . . . . . . . . . . . . . . . 38
114 7.1. Transparency Information X.509v3 Extension . . . . . . . 38
115 7.1.1. OCSP Response Extension . . . . . . . . . . . . . . . 38
116 7.1.2. Certificate Extension . . . . . . . . . . . . . . . . 38
117 7.2. TLS Feature X.509v3 Extension . . . . . . . . . . . . . . 39
118 8. Clients . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
119 8.1. TLS Client . . . . . . . . . . . . . . . . . . . . . . . 39
120 8.1.1. Receiving SCTs and inclusion proofs . . . . . . . . . 39
121 8.1.2. Reconstructing the TBSCertificate . . . . . . . . . . 39
122 8.1.3. Validating SCTs . . . . . . . . . . . . . . . . . . . 40
123 8.1.4. Fetching inclusion proofs . . . . . . . . . . . . . . 40
124 8.1.5. Validating inclusion proofs . . . . . . . . . . . . . 41
125 8.1.6. Evaluating compliance . . . . . . . . . . . . . . . . 41
126 8.2. Monitor . . . . . . . . . . . . . . . . . . . . . . . . . 41
127 8.3. Auditing . . . . . . . . . . . . . . . . . . . . . . . . 42
128 9. Algorithm Agility . . . . . . . . . . . . . . . . . . . . . . 43
129 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
130 10.1. New Entry to the TLS ExtensionType Registry . . . . . . 44
131 10.2. Hash Algorithms . . . . . . . . . . . . . . . . . . . . 44
132 10.2.1. Specification Required guidance . . . . . . . . . . 45
133 10.3. Signature Algorithms . . . . . . . . . . . . . . . . . . 45
134 10.3.1. Expert Review guidelines . . . . . . . . . . . . . . 46
135 10.4. VersionedTransTypes . . . . . . . . . . . . . . . . . . 46
136 10.4.1. Specification Required guidance . . . . . . . . . . 47
137 10.5. Log Artifact Extension Registry . . . . . . . . . . . . 47
138 10.5.1. Specification Required guidance . . . . . . . . . . 47
139 10.6. Object Identifiers . . . . . . . . . . . . . . . . . . . 47
140 10.6.1. Log ID Registry . . . . . . . . . . . . . . . . . . 47
141 11. Security Considerations . . . . . . . . . . . . . . . . . . . 48
142 11.1. Misissued Certificates . . . . . . . . . . . . . . . . . 49
143 11.2. Detection of Misissue . . . . . . . . . . . . . . . . . 49
144 11.3. Misbehaving Logs . . . . . . . . . . . . . . . . . . . . 49
145 11.4. Preventing Tracking Clients . . . . . . . . . . . . . . 50
146 11.5. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . 50
147 11.6. Leakage of DNS Information . . . . . . . . . . . . . . . 50
148 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 50
149 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 51
150 13.1. Normative References . . . . . . . . . . . . . . . . . . 51
151 13.2. Informative References . . . . . . . . . . . . . . . . . 52
152 Appendix A. Supporting v1 and v2 simultaneously . . . . . . . . 54
153 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 54
155 1. Introduction
157 Certificate Transparency aims to mitigate the problem of misissued
158 certificates by providing append-only logs of issued certificates.
159 The logs do not themselves prevent misissuance, but they ensure that
160 interested parties (particularly those named in certificates) can
161 detect such misissuance. Note that this is a general mechanism that
162 could be used for transparently logging any form of binary data,
163 subject to some kind of inclusion criteria. In this document, we
164 only describe its use for public TLS server certificates (i.e., where
165 the inclusion criteria is a valid certificate issued by a public
166 certification authority (CA)).
168 Each log contains certificate chains, which can be submitted by
169 anyone. It is expected that public CAs will contribute all their
170 newly issued certificates to one or more logs; however certificate
171 holders can also contribute their own certificate chains, as can
172 third parties. In order to avoid logs being rendered useless by the
173 submission of large numbers of spurious certificates, it is required
174 that each chain ends with a trust anchor that is accepted by the log.
175 When a chain is accepted by a log, a signed timestamp is returned,
176 which can later be used to provide evidence to TLS clients that the
177 chain has been submitted. TLS clients can thus require that all
178 certificates they accept as valid are accompanied by signed
179 timestamps.
181 Those who are concerned about misissuance can monitor the logs,
182 asking them regularly for all new entries, and can thus check whether
183 domains for which they are responsible have had certificates issued
184 that they did not expect. What they do with this information,
185 particularly when they find that a misissuance has happened, is
186 beyond the scope of this document. However, broadly speaking, they
187 can invoke existing business mechanisms for dealing with misissued
188 certificates, such as working with the CA to get the certificate
189 revoked, or with maintainers of trust anchor lists to get the CA
190 removed. Of course, anyone who wants can monitor the logs and, if
191 they believe a certificate is incorrectly issued, take action as they
192 see fit.
194 Similarly, those who have seen signed timestamps from a particular
195 log can later demand a proof of inclusion from that log. If the log
196 is unable to provide this (or, indeed, if the corresponding
197 certificate is absent from monitors' copies of that log), that is
198 evidence of the incorrect operation of the log. The checking
199 operation is asynchronous to allow clients to proceed without delay,
200 despite possible issues such as network connectivity and the vagaries
201 of firewalls.
203 The append-only property of each log is achieved using Merkle Trees,
204 which can be used to efficiently prove that any particular instance
205 of the log is a superset of any particular previous instance and to
206 efficiently detect various misbehaviors of the log (e.g., issuing a
207 signed timestamp for a certificate that is not subsequently logged).
209 It is necessary to treat each log as a trusted third party, because
210 the log auditing mechanisms described in this document can be
211 circumvented by a misbehaving log that shows different, inconsistent
212 views of itself to different clients. Whilst it is anticipated that
213 additional mechanisms could be developed to address these
214 shortcomings and thereby avoid the need to blindly trust logs, such
215 mechanisms are outside the scope of this document.
217 1.1. Requirements Language
219 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
220 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
221 "OPTIONAL" in this document are to be interpreted as described in BCP
222 14 [RFC2119] [RFC8174] when, and only when, they appear in all
223 capitals, as shown here.
225 1.2. Data Structures
227 Data structures are defined and encoded according to the conventions
228 laid out in Section 3 of [RFC8446].
230 1.3. Major Differences from CT 1.0
232 This document revises and obsoletes the CT 1.0 [RFC6962] protocol,
233 drawing on insights gained from CT 1.0 deployments and on feedback
234 from the community. The major changes are:
236 o Hash and signature algorithm agility: permitted algorithms are now
237 specified in IANA registries.
239 o Precertificate format: precertificates are now CMS objects rather
240 than X.509 certificates, which avoids violating the certificate
241 serial number uniqueness requirement in Section 4.1.2.2 of
242 [RFC5280].
244 o Removed precertificate signing certificates and the precertificate
245 poison extension: the change of precertificate format means that
246 these are no longer needed.
248 o Logs IDs: each log is now identified by an OID rather than by the
249 hash of its public key. OID allocations are managed by an IANA
250 registry.
252 o "TransItem" structure: this new data structure is used to
253 encapsulate most types of CT data. A "TransItemList", consisting
254 of one or more "TransItem" structures, can be used anywhere that
255 "SignedCertificateTimestampList" was used in [RFC6962].
257 o Merkle tree leaves: the "MerkleTreeLeaf" structure has been
258 replaced by the "TransItem" structure, which eases extensibility
259 and simplifies the leaf structure by removing one layer of
260 abstraction.
262 o Unified leaf format: the structure for both certificate and
263 precertificate entries now includes only the TBSCertificate
264 (whereas certificate entries in [RFC6962] included the entire
265 certificate).
267 o Log Artifact Extensions: these are now typed and managed by an
268 IANA registry, and they can now appear not only in SCTs but also
269 in STHs.
271 o API outputs: complete "TransItem" structures are returned, rather
272 than the constituent parts of each structure.
274 o get-all-by-hash: new client API for obtaining an inclusion proof
275 and the corresponding consistency proof at the same time.
277 o submit-entry: new client API, replacing add-chain and add-pre-
278 chain.
280 o Presenting SCTs with proofs: TLS servers may present SCTs together
281 with the corresponding inclusion proofs using any of the
282 mechanisms that [RFC6962] defined for presenting SCTs only.
283 (Presenting SCTs only is still supported).
285 o CT TLS extension: the "signed_certificate_timestamp" TLS extension
286 has been replaced by the "transparency_info" TLS extension.
288 o Verification algorithms: added detailed algorithms for verifying
289 inclusion proofs, for verifying consistency between two STHs, and
290 for verifying a root hash given a complete list of the relevant
291 leaf input entries.
293 o Extensive clarifications and editorial work.
295 2. Cryptographic Components
297 2.1. Merkle Hash Trees
299 2.1.1. Definition of the Merkle Tree
301 The log uses a binary Merkle Hash Tree for efficient auditing. The
302 hash algorithm used is one of the log's parameters (see Section 4.1).
303 This document establishes a registry of acceptable hash algorithms
304 (see Section 10.2). Throughout this document, the hash algorithm in
305 use is referred to as HASH and the size of its output in bytes as
306 HASH_SIZE. The input to the Merkle Tree Hash is a list of data
307 entries; these entries will be hashed to form the leaves of the
308 Merkle Hash Tree. The output is a single HASH_SIZE Merkle Tree Hash.
309 Given an ordered list of n inputs, D_n = {d[0], d[1], ..., d[n-1]},
310 the Merkle Tree Hash (MTH) is thus defined as follows:
312 The hash of an empty list is the hash of an empty string:
314 MTH({}) = HASH().
316 The hash of a list with one entry (also known as a leaf hash) is:
318 MTH({d[0]}) = HASH(0x00 || d[0]).
320 For n > 1, let k be the largest power of two smaller than n (i.e., k
321 < n <= 2k). The Merkle Tree Hash of an n-element list D_n is then
322 defined recursively as
324 MTH(D_n) = HASH(0x01 || MTH(D[0:k]) || MTH(D[k:n])),
326 where:
328 o || denotes concatenation
330 o : denotes concatenation of lists
332 o D[k1:k2] = D'_(k2-k1) denotes the list {d'[0] = d[k1], d'[1] =
333 d[k1+1], ..., d'[k2-k1-1] = d[k2-1]} of length (k2 - k1).
335 Note that the hash calculations for leaves and nodes differ; this
336 domain separation is required to give second preimage resistance.
338 Note that we do not require the length of the input list to be a
339 power of two. The resulting Merkle Tree may thus not be balanced;
340 however, its shape is uniquely determined by the number of leaves.
341 (Note: This Merkle Tree is essentially the same as the history tree
342 [CrosbyWallach] proposal, except our definition handles non-full
343 trees differently).
345 2.1.2. Verifying a Tree Head Given Entries
347 When a client has a complete list of n input "entries" from "0" up to
348 "tree_size - 1" and wishes to verify this list against a tree head
349 "root_hash" returned by the log for the same "tree_size", the
350 following algorithm may be used:
352 1. Set "stack" to an empty stack.
354 2. For each "i" from "0" up to "tree_size - 1":
356 1. Push "HASH(0x00 || entries[i])" to "stack".
358 2. Set "merge_count" to the lowest value ("0" included) such
359 that "LSB(i >> merge_count)" is not set. In other words, set
360 "merge_count" to the number of consecutive "1"s found
361 starting at the least significant bit of "i".
363 3. Repeat "merge_count" times:
365 1. Pop "right" from "stack".
367 2. Pop "left" from "stack".
369 3. Push "HASH(0x01 || left || right)" to "stack".
371 3. If there is more than one element in the "stack", repeat the same
372 merge procedure (Step 2.3 above) until only a single element
373 remains.
375 4. The remaining element in "stack" is the Merkle Tree hash for the
376 given "tree_size" and should be compared by equality against the
377 supplied "root_hash".
379 2.1.3. Merkle Inclusion Proofs
381 A Merkle inclusion proof for a leaf in a Merkle Hash Tree is the
382 shortest list of additional nodes in the Merkle Tree required to
383 compute the Merkle Tree Hash for that tree. Each node in the tree is
384 either a leaf node or is computed from the two nodes immediately
385 below it (i.e., towards the leaves). At each step up the tree
386 (towards the root), a node from the inclusion proof is combined with
387 the node computed so far. In other words, the inclusion proof
388 consists of the list of missing nodes required to compute the nodes
389 leading from a leaf to the root of the tree. If the root computed
390 from the inclusion proof matches the true root, then the inclusion
391 proof proves that the leaf exists in the tree.
393 2.1.3.1. Generating an Inclusion Proof
395 Given an ordered list of n inputs to the tree, D_n = {d[0], d[1],
396 ..., d[n-1]}, the Merkle inclusion proof PATH(m, D_n) for the (m+1)th
397 input d[m], 0 <= m < n, is defined as follows:
399 The proof for the single leaf in a tree with a one-element input list
400 D[1] = {d[0]} is empty:
402 PATH(0, {d[0]}) = {}
404 For n > 1, let k be the largest power of two smaller than n. The
405 proof for the (m+1)th element d[m] in a list of n > m elements is
406 then defined recursively as
408 PATH(m, D_n) = PATH(m, D[0:k]) : MTH(D[k:n]) for m < k; and
410 PATH(m, D_n) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for m >= k,
412 The : operator and D[k1:k2] are defined the same as in Section 2.1.1.
414 2.1.3.2. Verifying an Inclusion Proof
416 When a client has received an inclusion proof (e.g., in a "TransItem"
417 of type "inclusion_proof_v2") and wishes to verify inclusion of an
418 input "hash" for a given "tree_size" and "root_hash", the following
419 algorithm may be used to prove the "hash" was included in the
420 "root_hash":
422 1. Compare "leaf_index" against "tree_size". If "leaf_index" is
423 greater than or equal to "tree_size" then fail the proof
424 verification.
426 2. Set "fn" to "leaf_index" and "sn" to "tree_size - 1".
428 3. Set "r" to "hash".
430 4. For each value "p" in the "inclusion_path" array:
432 If "sn" is 0, stop the iteration and fail the proof verification.
434 If "LSB(fn)" is set, or if "fn" is equal to "sn", then:
436 1. Set "r" to "HASH(0x01 || p || r)"
438 2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn"
439 equally until either "LSB(fn)" is set or "fn" is "0".
441 Otherwise:
443 1. Set "r" to "HASH(0x01 || r || p)"
445 Finally, right-shift both "fn" and "sn" one time.
447 5. Compare "sn" to 0. Compare "r" against the "root_hash". If "sn"
448 is equal to 0, and "r" and the "root_hash" are equal, then the
449 log has proven the inclusion of "hash". Otherwise, fail the
450 proof verification.
452 2.1.4. Merkle Consistency Proofs
454 Merkle consistency proofs prove the append-only property of the tree.
455 A Merkle consistency proof for a Merkle Tree Hash MTH(D_n) and a
456 previously advertised hash MTH(D[0:m]) of the first m leaves, m <= n,
457 is the list of nodes in the Merkle Tree required to verify that the
458 first m inputs D[0:m] are equal in both trees. Thus, a consistency
459 proof must contain a set of intermediate nodes (i.e., commitments to
460 inputs) sufficient to verify MTH(D_n), such that (a subset of) the
461 same nodes can be used to verify MTH(D[0:m]). We define an algorithm
462 that outputs the (unique) minimal consistency proof.
464 2.1.4.1. Generating a Consistency Proof
466 Given an ordered list of n inputs to the tree, D_n = {d[0], d[1],
467 ..., d[n-1]}, the Merkle consistency proof PROOF(m, D_n) for a
468 previous Merkle Tree Hash MTH(D[0:m]), 0 < m < n, is defined as:
470 PROOF(m, D_n) = SUBPROOF(m, D_n, true)
472 In SUBPROOF, the boolean value represents whether the subtree created
473 from D[0:m] is a complete subtree of the Merkle Tree created from
474 D_n, and, consequently, whether the subtree Merkle Tree Hash
475 MTH(D[0:m]) is known. The initial call to SUBPROOF sets this to be
476 true, and SUBPROOF is then defined as follows:
478 The subproof for m = n is empty if m is the value for which PROOF was
479 originally requested (meaning that the subtree created from D[0:m] is
480 a complete subtree of the Merkle Tree created from the original D_n
481 for which PROOF was requested, and the subtree Merkle Tree Hash
482 MTH(D[0:m]) is known):
484 SUBPROOF(m, D[m], true) = {}
486 Otherwise, the subproof for m = n is the Merkle Tree Hash committing
487 inputs D[0:m]:
489 SUBPROOF(m, D[m], false) = {MTH(D[m])}
491 For m < n, let k be the largest power of two smaller than n. The
492 subproof is then defined recursively.
494 If m <= k, the right subtree entries D[k:n] only exist in the current
495 tree. We prove that the left subtree entries D[0:k] are consistent
496 and add a commitment to D[k:n]:
498 SUBPROOF(m, D_n, b) = SUBPROOF(m, D[0:k], b) : MTH(D[k:n])
500 If m > k, the left subtree entries D[0:k] are identical in both
501 trees. We prove that the right subtree entries D[k:n] are consistent
502 and add a commitment to D[0:k].
504 SUBPROOF(m, D_n, b) = SUBPROOF(m - k, D[k:n], false) : MTH(D[0:k])
506 The number of nodes in the resulting proof is bounded above by
507 ceil(log2(n)) + 1.
509 The : operator and D[k1:k2] are defined the same as in Section 2.1.1.
511 2.1.4.2. Verifying Consistency between Two Tree Heads
513 When a client has a tree head "first_hash" for tree size "first", a
514 tree head "second_hash" for tree size "second" where "0 < first <
515 second", and has received a consistency proof between the two (e.g.,
516 in a "TransItem" of type "consistency_proof_v2"), the following
517 algorithm may be used to verify the consistency proof:
519 1. If "first" is an exact power of 2, then prepend "first_hash" to
520 the "consistency_path" array.
522 2. Set "fn" to "first - 1" and "sn" to "second - 1".
524 3. If "LSB(fn)" is set, then right-shift both "fn" and "sn" equally
525 until "LSB(fn)" is not set.
527 4. Set both "fr" and "sr" to the first value in the
528 "consistency_path" array.
530 5. For each subsequent value "c" in the "consistency_path" array:
532 If "sn" is 0, stop the iteration and fail the proof verification.
534 If "LSB(fn)" is set, or if "fn" is equal to "sn", then:
536 1. Set "fr" to "HASH(0x01 || c || fr)"
537 Set "sr" to "HASH(0x01 || c || sr)"
539 2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn"
540 equally until either "LSB(fn)" is set or "fn" is "0".
542 Otherwise:
544 1. Set "sr" to "HASH(0x01 || sr || c)"
546 Finally, right-shift both "fn" and "sn" one time.
548 6. After completing iterating through the "consistency_path" array
549 as described above, verify that the "fr" calculated is equal to
550 the "first_hash" supplied, that the "sr" calculated is equal to
551 the "second_hash" supplied and that "sn" is 0.
553 2.1.5. Example
555 The binary Merkle Tree with 7 leaves:
557 hash
558 / \
559 / \
560 / \
561 / \
562 / \
563 k l
564 / \ / \
565 / \ / \
566 / \ / \
567 g h i j
568 / \ / \ / \ |
569 a b c d e f d6
570 | | | | | |
571 d0 d1 d2 d3 d4 d5
572 The inclusion proof for d0 is [b, h, l].
574 The inclusion proof for d3 is [c, g, l].
576 The inclusion proof for d4 is [f, j, k].
578 The inclusion proof for d6 is [i, k].
580 The same tree, built incrementally in four steps:
582 hash0 hash1=k
583 / \ / \
584 / \ / \
585 / \ / \
586 g c g h
587 / \ | / \ / \
588 a b d2 a b c d
589 | | | | | |
590 d0 d1 d0 d1 d2 d3
592 hash2 hash
593 / \ / \
594 / \ / \
595 / \ / \
596 / \ / \
597 / \ / \
598 k i k l
599 / \ / \ / \ / \
600 / \ e f / \ / \
601 / \ | | / \ / \
602 g h d4 d5 g h i j
603 / \ / \ / \ / \ / \ |
604 a b c d a b c d e f d6
605 | | | | | | | | | |
606 d0 d1 d2 d3 d0 d1 d2 d3 d4 d5
608 The consistency proof between hash0 and hash is PROOF(3, D[7]) = [c,
609 d, g, l]. c, g are used to verify hash0, and d, l are additionally
610 used to show hash is consistent with hash0.
612 The consistency proof between hash1 and hash is PROOF(4, D[7]) = [l].
613 hash can be verified using hash1=k and l.
615 The consistency proof between hash2 and hash is PROOF(6, D[7]) = [i,
616 j, k]. k, i are used to verify hash2, and j is additionally used to
617 show hash is consistent with hash2.
619 2.2. Signatures
621 Various data structures Section 1.2 are signed. A log MUST use one
622 of the signature algorithms defined in Section 10.3.
624 3. Submitters
626 Submitters submit certificates or preannouncements of certificates
627 prior to issuance (precertificates) to logs for public auditing, as
628 described below. In order to enable attribution of each logged
629 certificate or precertificate to its issuer, each submission MUST be
630 accompanied by all additional certificates required to verify the
631 chain up to an accepted trust anchor (Section 5.7). The trust anchor
632 (a root or intermediate CA certificate) MAY be omitted from the
633 submission.
635 If a log accepts a submission, it will return a Signed Certificate
636 Timestamp (SCT) (see Section 4.8). The submitter SHOULD validate the
637 returned SCT as described in Section 8.1 if they understand its
638 format and they intend to use it directly in a TLS handshake or to
639 construct a certificate. If the submitter does not need the SCT (for
640 example, the certificate is being submitted simply to make it
641 available in the log), it MAY validate the SCT.
643 3.1. Certificates
645 Any entity can submit a certificate (Section 5.1) to a log. Since it
646 is anticipated that TLS clients will reject certificates that are not
647 logged, it is expected that certificate issuers and subjects will be
648 strongly motivated to submit them.
650 3.2. Precertificates
652 CAs may preannounce a certificate prior to issuance by submitting a
653 precertificate (Section 5.1) that the log can use to create an entry
654 that will be valid against the issued certificate. The CA MAY
655 incorporate the returned SCT in the issued certificate. One example
656 of where the returned SCT is not incorporated in the issued
657 certificate is when a CA sends the precertificate to multiple logs,
658 but only incorporates the SCTs that are returned first.
660 A precertificate is a CMS [RFC5652] "signed-data" object that
661 conforms to the following profile:
663 o It MUST be DER encoded.
665 o "SignedData.version" MUST be v3(3).
667 o "SignedData.digestAlgorithms" MUST only include the
668 "SignerInfo.digestAlgorithm" OID value (see below).
670 o "SignedData.encapContentInfo":
672 * "eContentType" MUST be the OID 1.3.101.78.
674 * "eContent" MUST contain a TBSCertificate [RFC5280] that will be
675 identical to the TBSCertificate in the issued certificate,
676 except that the Transparency Information (Section 7.1)
677 extension MUST be omitted.
679 o "SignedData.certificates" MUST be omitted.
681 o "SignedData.crls" MUST be omitted.
683 o "SignedData.signerInfos" MUST contain one "SignerInfo":
685 * "version" MUST be v3(3).
687 * "sid" MUST use the "subjectKeyIdentifier" option.
689 * "digestAlgorithm" MUST be one of the hash algorithm OIDs listed
690 in Section 10.2.
692 * "signedAttrs" MUST be present and MUST contain two attributes:
694 + A content-type attribute whose value is the same as
695 "SignedData.encapContentInfo.eContentType".
697 + A message-digest attribute whose value is the message digest
698 of "SignedData.encapContentInfo.eContent".
700 * "signatureAlgorithm" MUST be the same OID as
701 "TBSCertificate.signature".
703 * "signature" MUST be from the same (root or intermediate) CA
704 that intends to issue the corresponding certificate (see
705 Section 3.2.1).
707 * "unsignedAttrs" MUST be omitted.
709 "SignerInfo.signedAttrs" is included in the message digest
710 calculation process (see Section 5.4 of [RFC5652]), which ensures
711 that the "SignerInfo.signature" value will not be a valid X.509v3
712 signature that could be used in conjunction with the TBSCertificate
713 (from "SignedData.encapContentInfo.eContent") to construct a valid
714 certificate.
716 3.2.1. Binding Intent to Issue
718 Under normal circumstances, there will be a short delay between
719 precertificate submission and issuance of the corresponding
720 certificate. Longer delays are to be expected occasionally (e.g.,
721 due to log server downtime), and in some cases the CA might not
722 actually issue the corresponding certificate. Nevertheless, a
723 precertificate's "signature" indicates the CA's binding intent to
724 issue the corresponding certificate, which means that:
726 o Misissuance of a precertificate is considered equivalent to
727 misissuance of the corresponding certificate. The CA should
728 expect to be held to account, even if the corresponding
729 certificate has not actually been issued.
731 o Upon observing a precertificate, a client can reasonably presume
732 that the corresponding certificate has been issued. A client may
733 wish to obtain status information (e.g., by using the Online
734 Certificate Status Protocol [RFC6960] or by checking a Certificate
735 Revocation List [RFC5280]) about a certificate that is presumed to
736 exist, especially if there is evidence or suspicion that the
737 corresponding precertificate was misissued.
739 o TLS clients may have policies that require CAs to be able to
740 revoke, and to provide certificate status services for, each
741 certificate that is presumed to exist based on the existence of a
742 corresponding precertificate.
744 4. Log Format and Operation
746 A log is a single, append-only Merkle Tree of submitted certificate
747 and precertificate entries.
749 When it receives and accepts a valid submission, the log MUST return
750 an SCT that corresponds to the submitted certificate or
751 precertificate. If the log has previously seen this valid
752 submission, it SHOULD return the same SCT as it returned before (to
753 reduce the ability to track clients as described in Section 11.4).
754 If different SCTs are produced for the same submission, multiple log
755 entries will have to be created, one for each SCT (as the timestamp
756 is a part of the leaf structure). Note that if a certificate was
757 previously logged as a precertificate, then the precertificate's SCT
758 of type "precert_sct_v2" would not be appropriate; instead, a fresh
759 SCT of type "x509_sct_v2" should be generated.
761 An SCT is the log's promise to append to its Merkle Tree an entry for
762 the accepted submission. Upon producing an SCT, the log MUST fulfil
763 this promise by performing the following actions within a fixed
764 amount of time known as the Maximum Merge Delay (MMD), which is one
765 of the log's parameters (see Section 4.1):
767 o Allocate a tree index to the entry representing the accepted
768 submission.
770 o Calculate the root of the tree.
772 o Sign the root of the tree (see Section 4.10).
774 The log may append multiple entries before signing the root of the
775 tree.
777 Log operators SHOULD NOT impose any conditions on retrieving or
778 sharing data from the log.
780 4.1. Log Parameters
782 A log is defined by a collection of immutable parameters, which are
783 used by clients to communicate with the log and to verify log
784 artifacts. Except for the Final Signed Tree Head (STH), each of
785 these parameters MUST be established before the log operator begins
786 to operate the log.
788 Base URL: The prefix used to construct URLs for client messages (see
789 Section 5). The base URL MUST be an "https" URL, MAY contain a
790 port, MAY contain a path with any number of path segments, but
791 MUST NOT contain a query string, fragment, or trailing "/".
792 Example: https://ct.example.org/blue
794 Hash Algorithm: The hash algorithm used for the Merkle Tree (see
795 Section 10.2).
797 Signature Algorithm: The signature algorithm used (see Section 2.2).
799 Public Key: The public key used to verify signatures generated by
800 the log. A log MUST NOT use the same keypair as any other log.
802 Log ID: The OID that uniquely identifies the log.
804 Maximum Merge Delay: The MMD the log has committed to.
806 Version: The version of the protocol supported by the log (currently
807 1 or 2).
809 Maximum Chain Length: The longest chain submission the log is
810 willing to accept, if the log imposes any limit.
812 STH Frequency Count: The maximum number of STHs the log may produce
813 in any period equal to the "Maximum Merge Delay" (see
814 Section 4.10).
816 Final STH: If a log has been closed down (i.e., no longer accepts
817 new entries), existing entries may still be valid. In this case,
818 the client should know the final valid STH in the log to ensure no
819 new entries can be added without detection. The final STH should
820 be provided in the form of a TransItem of type
821 "signed_tree_head_v2".
823 [JSON.Metadata] is an example of a metadata format which includes the
824 above elements.
826 4.2. Evaluating Submissions
828 A log determines whether to accept or reject a submission by
829 evaluating it against the minimum acceptance criteria (see
830 Section 4.2.1) and against the log's discretionary acceptance
831 criteria (see Section 4.2.2).
833 If the acceptance criteria are met, the log SHOULD accept the
834 submission. (A log may decide, for example, to temporarily reject
835 acceptable submissions to protect itself against denial-of-service
836 attacks).
838 The log SHALL allow retrieval of its list of accepted trust anchors
839 (see Section 5.7), each of which is a root or intermediate CA
840 certificate. This list might usefully be the union of root
841 certificates trusted by major browser vendors.
843 4.2.1. Minimum Acceptance Criteria
845 To ensure that logged certificates and precertificates are
846 attributable to an accepted trust anchor, and to set clear
847 expectations for what monitors would find in the log, and to avoid
848 being overloaded by invalid submissions, the log MUST reject a
849 submission if any of the following conditions are not met:
851 o The "submission", "type" and "chain" inputs MUST be set as
852 described in Section 5.1. The log MUST NOT accommodate misordered
853 CA certificates or use any other source of intermediate CA
854 certificates to attempt certification path construction.
856 o Each of the zero or more intermediate CA certificates in the chain
857 MUST have one or both of the following features:
859 * The Basic Constraints extension with the cA boolean asserted.
861 * The Key Usage extension with the keyCertSign bit asserted.
863 o Each certificate in the chain MUST fall within the limits imposed
864 by the zero or more Basic Constraints pathLenConstraint values
865 found higher up the chain.
867 o Precertificate submissions MUST conform to all of the requirements
868 in Section 3.2.
870 4.2.2. Discretionary Acceptance Criteria
872 If the minimum acceptance criteria are met but the submission is not
873 fully valid according to [RFC5280] verification rules (e.g., the
874 certificate or precertificate has expired, is not yet valid, has been
875 revoked, exhibits ASN.1 DER encoding errors but the log can still
876 parse it, etc), then the acceptability of the submission is left to
877 the log's discretion. It is useful for logs to accept such
878 submissions in order to accommodate quirks of CA certificate-issuing
879 software and to facilitate monitoring of CA compliance with
880 applicable policies and technical standards. However, it is
881 impractical for this document to enumerate, and for logs to consider,
882 all of the ways that a submission might fail to comply with
883 [RFC5280].
885 Logs SHOULD limit the length of chain they will accept. The maximum
886 chain length is one of the log's parameters (see Section 4.1).
888 4.3. Log Entries
890 If a submission is accepted and an SCT issued, the accepting log MUST
891 store the entire chain used for verification. This chain MUST
892 include the certificate or precertificate itself, the zero or more
893 intermediate CA certificates provided by the submitter, and the trust
894 anchor used to verify the chain (even if it was omitted from the
895 submission). The log MUST present this chain for auditing upon
896 request (see Section 5.6). This prevents the CA from avoiding blame
897 by logging a partial or empty chain. Each log entry is a "TransItem"
898 structure of type "x509_entry_v2" or "precert_entry_v2". However, a
899 log may store its entries in any format. If a log does not store
900 this "TransItem" in full, it must store the "timestamp" and
901 "sct_extensions" of the corresponding
902 "TimestampedCertificateEntryDataV2" structure. The "TransItem" can
903 be reconstructed from these fields and the entire chain that the log
904 used to verify the submission.
906 4.4. Log ID
908 Each log is identified by an OID, which is one of the log's
909 parameters (see Section 4.1) and which MUST NOT be used to identify
910 any other log. A log's operator MUST either allocate the OID
911 themselves or request an OID from the Log ID Registry (see
912 Section 10.6.1). Various data structures include the DER encoding of
913 this OID, excluding the ASN.1 tag and length bytes, in an opaque
914 vector:
916 opaque LogID<2..127>;
918 Note that the ASN.1 length and the opaque vector length are identical
919 in size (1 byte) and value, so the DER encoding of the OID can be
920 reproduced simply by prepending an OBJECT IDENTIFIER tag (0x06) to
921 the opaque vector length and contents.
923 OIDs used to identify logs are limited such that the DER encoding of
924 their value is less than or equal to 127 octets.
926 4.5. TransItem Structure
928 Various data structures are encapsulated in the "TransItem" structure
929 to ensure that the type and version of each one is identified in a
930 common fashion:
932 enum {
933 reserved(0),
934 x509_entry_v2(1), precert_entry_v2(2),
935 x509_sct_v2(3), precert_sct_v2(4),
936 signed_tree_head_v2(5), consistency_proof_v2(6),
937 inclusion_proof_v2(7),
938 (65535)
939 } VersionedTransType;
941 struct {
942 VersionedTransType versioned_type;
943 select (versioned_type) {
944 case x509_entry_v2: TimestampedCertificateEntryDataV2;
945 case precert_entry_v2: TimestampedCertificateEntryDataV2;
946 case x509_sct_v2: SignedCertificateTimestampDataV2;
947 case precert_sct_v2: SignedCertificateTimestampDataV2;
948 case signed_tree_head_v2: SignedTreeHeadDataV2;
949 case consistency_proof_v2: ConsistencyProofDataV2;
950 case inclusion_proof_v2: InclusionProofDataV2;
951 } data;
952 } TransItem;
954 "versioned_type" is a value from the IANA registry in Section 10.4
955 that identifies the type of the encapsulated data structure and the
956 earliest version of this protocol to which it conforms. This
957 document is v2.
959 "data" is the encapsulated data structure. The various structures
960 named with the "DataV2" suffix are defined in later sections of this
961 document.
963 Note that "VersionedTransType" combines the v1 [RFC6962] type
964 enumerations "Version", "LogEntryType", "SignatureType" and
965 "MerkleLeafType". Note also that v1 did not define "TransItem", but
966 this document provides guidelines (see Appendix A) on how v2
967 implementations can co-exist with v1 implementations.
969 Future versions of this protocol may reuse "VersionedTransType"
970 values defined in this document as long as the corresponding data
971 structures are not modified, and may add new "VersionedTransType"
972 values for new or modified data structures.
974 4.6. Log Artifact Extensions
976 enum {
977 reserved(65535)
978 } ExtensionType;
980 struct {
981 ExtensionType extension_type;
982 opaque extension_data<0..2^16-1>;
983 } Extension;
985 The "Extension" structure provides a generic extensibility for log
986 artifacts, including Signed Certificate Timestamps (Section 4.8) and
987 Signed Tree Heads (Section 4.10). The interpretation of the
988 "extension_data" field is determined solely by the value of the
989 "extension_type" field.
991 This document does not define any extensions, but it does establish a
992 registry for future "ExtensionType" values (see Section 10.5). Each
993 document that registers a new "ExtensionType" must specify the
994 context in which it may be used (e.g., SCT, STH, or both) and
995 describe how to interpret the corresponding "extension_data".
997 4.7. Merkle Tree Leaves
999 The leaves of a log's Merkle Tree correspond to the log's entries
1000 (see Section 4.3). Each leaf is the leaf hash (Section 2.1) of a
1001 "TransItem" structure of type "x509_entry_v2" or "precert_entry_v2",
1002 which encapsulates a "TimestampedCertificateEntryDataV2" structure.
1003 Note that leaf hashes are calculated as HASH(0x00 || TransItem),
1004 where the hash algorithm is one of the log's parameters.
1006 opaque TBSCertificate<1..2^24-1>;
1008 struct {
1009 uint64 timestamp;
1010 opaque issuer_key_hash<32..2^8-1>;
1011 TBSCertificate tbs_certificate;
1012 Extension sct_extensions<0..2^16-1>;
1013 } TimestampedCertificateEntryDataV2;
1015 "timestamp" is the date and time at which the certificate or
1016 precertificate was accepted by the log, in the form of a 64-bit
1017 unsigned number of milliseconds elapsed since the Unix Epoch (1
1018 January 1970 00:00:00 UTC - see [UNIXTIME]), ignoring leap seconds,
1019 in network byte order. Note that the leaves of a log's Merkle Tree
1020 are not required to be in strict chronological order.
1022 "issuer_key_hash" is the HASH of the public key of the CA that issued
1023 the certificate or precertificate, calculated over the DER encoding
1024 of the key represented as SubjectPublicKeyInfo [RFC5280]. This is
1025 needed to bind the CA to the certificate or precertificate, making it
1026 impossible for the corresponding SCT to be valid for any other
1027 certificate or precertificate whose TBSCertificate matches
1028 "tbs_certificate". The length of the "issuer_key_hash" MUST match
1029 HASH_SIZE.
1031 "tbs_certificate" is the DER encoded TBSCertificate from the
1032 submission. (Note that a precertificate's TBSCertificate can be
1033 reconstructed from the corresponding certificate as described in
1034 Section 8.1.2).
1036 "sct_extensions" matches the SCT extensions of the corresponding SCT.
1038 The type of the "TransItem" corresponds to the value of the "type"
1039 parameter supplied in the Section 5.1 call.
1041 4.8. Signed Certificate Timestamp (SCT)
1043 An SCT is a "TransItem" structure of type "x509_sct_v2" or
1044 "precert_sct_v2", which encapsulates a
1045 "SignedCertificateTimestampDataV2" structure:
1047 struct {
1048 LogID log_id;
1049 uint64 timestamp;
1050 Extension sct_extensions<0..2^16-1>;
1051 opaque signature<0..2^16-1>;
1052 } SignedCertificateTimestampDataV2;
1054 "log_id" is this log's unique ID, encoded in an opaque vector as
1055 described in Section 4.4.
1057 "timestamp" is equal to the timestamp from the corresponding
1058 "TimestampedCertificateEntryDataV2" structure.
1060 "sct_extensions" is a vector of 0 or more SCT extensions. This
1061 vector MUST NOT include more than one extension with the same
1062 "extension_type". The extensions in the vector MUST be ordered by
1063 the value of the "extension_type" field, smallest value first. If an
1064 implementation sees an extension that it does not understand, it
1065 SHOULD ignore that extension. Furthermore, an implementation MAY
1066 choose to ignore any extension(s) that it does understand.
1068 "signature" is computed over a "TransItem" structure of type
1069 "x509_entry_v2" or "precert_entry_v2" (see Section 4.7) using the
1070 signature algorithm declared in the log's parameters (see
1071 Section 4.1).
1073 4.9. Merkle Tree Head
1075 The log stores information about its Merkle Tree in a
1076 "TreeHeadDataV2":
1078 opaque NodeHash<32..2^8-1>;
1080 struct {
1081 uint64 timestamp;
1082 uint64 tree_size;
1083 NodeHash root_hash;
1084 Extension sth_extensions<0..2^16-1>;
1085 } TreeHeadDataV2;
1087 The length of NodeHash MUST match HASH_SIZE of the log.
1089 "timestamp" is the current date and time, in the form of a 64-bit
1090 unsigned number of milliseconds elapsed since the Unix Epoch (1
1091 January 1970 00:00:00 UTC - see [UNIXTIME]), ignoring leap seconds,
1092 in network byte order.
1094 "tree_size" is the number of entries currently in the log's Merkle
1095 Tree.
1097 "root_hash" is the root of the Merkle Hash Tree.
1099 "sth_extensions" is a vector of 0 or more STH extensions. This
1100 vector MUST NOT include more than one extension with the same
1101 "extension_type". The extensions in the vector MUST be ordered by
1102 the value of the "extension_type" field, smallest value first. If an
1103 implementation sees an extension that it does not understand, it
1104 SHOULD ignore that extension. Furthermore, an implementation MAY
1105 choose to ignore any extension(s) that it does understand.
1107 4.10. Signed Tree Head (STH)
1109 Periodically each log SHOULD sign its current tree head information
1110 (see Section 4.9) to produce an STH. When a client requests a log's
1111 latest STH (see Section 5.2), the log MUST return an STH that is no
1112 older than the log's MMD. However, since STHs could be used to mark
1113 individual clients (by producing a new STH for each query), a log
1114 MUST NOT produce STHs more frequently than its parameters declare
1115 (see Section 4.1). In general, there is no need to produce a new STH
1116 unless there are new entries in the log; however, in the event that a
1117 log does not accept any submissions during an MMD period, the log
1118 MUST sign the same Merkle Tree Hash with a fresh timestamp.
1120 An STH is a "TransItem" structure of type "signed_tree_head_v2",
1121 which encapsulates a "SignedTreeHeadDataV2" structure:
1123 struct {
1124 LogID log_id;
1125 TreeHeadDataV2 tree_head;
1126 opaque signature<0..2^16-1>;
1127 } SignedTreeHeadDataV2;
1129 "log_id" is this log's unique ID, encoded in an opaque vector as
1130 described in Section 4.4.
1132 The "timestamp" in "tree_head" MUST be at least as recent as the most
1133 recent SCT timestamp in the tree. Each subsequent timestamp MUST be
1134 more recent than the timestamp of the previous update.
1136 "tree_head" contains the latest tree head information (see
1137 Section 4.9).
1139 "signature" is computed over the "tree_head" field using the
1140 signature algorithm declared in the log's parameters (see
1141 Section 4.1).
1143 4.11. Merkle Consistency Proofs
1145 To prepare a Merkle Consistency Proof for distribution to clients,
1146 the log produces a "TransItem" structure of type
1147 "consistency_proof_v2", which encapsulates a "ConsistencyProofDataV2"
1148 structure:
1150 struct {
1151 LogID log_id;
1152 uint64 tree_size_1;
1153 uint64 tree_size_2;
1154 NodeHash consistency_path<1..2^16-1>;
1155 } ConsistencyProofDataV2;
1157 "log_id" is this log's unique ID, encoded in an opaque vector as
1158 described in Section 4.4.
1160 "tree_size_1" is the size of the older tree.
1162 "tree_size_2" is the size of the newer tree.
1164 "consistency_path" is a vector of Merkle Tree nodes proving the
1165 consistency of two STHs.
1167 4.12. Merkle Inclusion Proofs
1169 To prepare a Merkle Inclusion Proof for distribution to clients, the
1170 log produces a "TransItem" structure of type "inclusion_proof_v2",
1171 which encapsulates an "InclusionProofDataV2" structure:
1173 struct {
1174 LogID log_id;
1175 uint64 tree_size;
1176 uint64 leaf_index;
1177 NodeHash inclusion_path<1..2^16-1>;
1178 } InclusionProofDataV2;
1180 "log_id" is this log's unique ID, encoded in an opaque vector as
1181 described in Section 4.4.
1183 "tree_size" is the size of the tree on which this inclusion proof is
1184 based.
1186 "leaf_index" is the 0-based index of the log entry corresponding to
1187 this inclusion proof.
1189 "inclusion_path" is a vector of Merkle Tree nodes proving the
1190 inclusion of the chosen certificate or precertificate.
1192 4.13. Shutting down a log
1194 Log operators may decide to shut down a log for various reasons, such
1195 as deprecation of the signature algorithm. If there are entries in
1196 the log for certificates that have not yet expired, simply making TLS
1197 clients stop recognizing that log will have the effect of
1198 invalidating SCTs from that log. To avoid that, the following
1199 actions are suggested:
1201 o Make it known to clients and monitors that the log will be frozen.
1203 o Stop accepting new submissions (the error code "shutdown" should
1204 be returned for such requests).
1206 o Once MMD from the last accepted submission has passed and all
1207 pending submissions are incorporated, issue a final STH and
1208 publish it as one of the log's parameters. Having an STH with a
1209 timestamp that is after the MMD has passed from the last SCT
1210 issuance allows clients to audit this log regularly without
1211 special handling for the final STH. At this point the log's
1212 private key is no longer needed and can be destroyed.
1214 o Keep the log running until the certificates in all of its entries
1215 have expired or exist in other logs (this can be determined by
1216 scanning other logs or connecting to domains mentioned in the
1217 certificates and inspecting the SCTs served).
1219 5. Log Client Messages
1221 Messages are sent as HTTPS GET or POST requests. Parameters for
1222 POSTs and all responses are encoded as JavaScript Object Notation
1223 (JSON) objects [RFC8259]. Parameters for GETs are encoded as order-
1224 independent key/value URL parameters, using the "application/x-www-
1225 form-urlencoded" format described in the "HTML 4.01 Specification"
1226 [HTML401]. Binary data is base64 encoded [RFC4648] as specified in
1227 the individual messages.
1229 Clients are configured with a log's base URL, which is one of the
1230 log's parameters. Clients construct URLs for requests by appending
1231 suffixes to this base URL. This structure places some degree of
1232 restriction on how log operators can deploy these services, as noted
1233 in [RFC7320]. However, operational experience with version 1 of this
1234 protocol has not indicated that these restrictions are a problem in
1235 practice.
1237 Note that JSON objects and URL parameters may contain fields not
1238 specified here. These extra fields SHOULD be ignored.
1240 In practice, log servers may include multiple front-end machines.
1241 Since it is impractical to keep these machines in perfect sync,
1242 errors may occur that are caused by skew between the machines. Where
1243 such errors are possible, the front-end will return additional
1244 information (as specified below) making it possible for clients to
1245 make progress, if progress is possible. Front-ends MUST only serve
1246 data that is free of gaps (that is, for example, no front-end will
1247 respond with an STH unless it is also able to prove consistency from
1248 all log entries logged within that STH).
1250 For example, when a consistency proof between two STHs is requested,
1251 the front-end reached may not yet be aware of one or both STHs. In
1252 the case where it is unaware of both, it will return the latest STH
1253 it is aware of. Where it is aware of the first but not the second,
1254 it will return the latest STH it is aware of and a consistency proof
1255 from the first STH to the returned STH. The case where it knows the
1256 second but not the first should not arise (see the "no gaps"
1257 requirement above).
1259 If the log is unable to process a client's request, it MUST return an
1260 HTTP response code of 4xx/5xx (see [RFC7231]), and, in place of the
1261 responses outlined in the subsections below, the body SHOULD be a
1262 JSON Problem Details Object (see [RFC7807] Section 3), containing:
1264 type: A URN reference identifying the problem. To facilitate
1265 automated response to errors, this document defines a set of
1266 standard tokens for use in the "type" field, within the URN
1267 namespace of: "urn:ietf:params:trans:error:".
1269 detail: A human-readable string describing the error that prevented
1270 the log from processing the request, ideally with sufficient
1271 detail to enable the error to be rectified.
1273 e.g., In response to a request of "/ct/v2/get-
1274 entries?start=100&end=99", the log would return a "400 Bad Request"
1275 response code with a body similar to the following:
1277 {
1278 "type": "urn:ietf:params:trans:error:endBeforeStart",
1279 "detail": "'start' cannot be greater than 'end'"
1280 }
1282 Most error types are specific to the type of request and are defined
1283 in the respective subsections below. The one exception is the
1284 "malformed" error type, which indicates that the log server could not
1285 parse the client's request because it did not comply with this
1286 document:
1288 +-----------+----------------------------------+
1289 | type | detail |
1290 +-----------+----------------------------------+
1291 | malformed | The request could not be parsed. |
1292 +-----------+----------------------------------+
1294 Clients SHOULD treat "500 Internal Server Error" and "503 Service
1295 Unavailable" responses as transient failures and MAY retry the same
1296 request without modification at a later date. Note that as per
1297 [RFC7231], in the case of a 503 response the log MAY include a
1298 "Retry-After:" header in order to request a minimum time for the
1299 client to wait before retrying the request.
1301 5.1. Submit Entry to Log
1303 POST /ct/v2/submit-entry
1305 Inputs:
1307 submission: The base64 encoded certificate or precertificate.
1309 type: The "VersionedTransType" integer value that indicates the
1310 type of the "submission": 1 for "x509_entry_v2", or 2 for
1311 "precert_entry_v2".
1313 chain: An array of zero or more base64 encoded CA certificates.
1314 The first element is the certifier of the "submission"; the
1315 second certifies the first; etc. The last element of "chain"
1316 (or, if "chain" is an empty array, the "submission") is
1317 certified by an accepted trust anchor.
1319 Outputs:
1321 sct: A base64 encoded "TransItem" of type "x509_sct_v2" or
1322 "precert_sct_v2", signed by this log, that corresponds to the
1323 "submission".
1325 If the submitted entry is immediately appended to (or already
1326 exists in) this log's tree, then the log SHOULD also output:
1328 sth: A base64 encoded "TransItem" of type "signed_tree_head_v2",
1329 signed by this log.
1331 inclusion: A base64 encoded "TransItem" of type
1332 "inclusion_proof_v2" whose "inclusion_path" array of Merkle
1333 Tree nodes proves the inclusion of the "submission" in the
1334 returned "sth".
1336 Error codes:
1338 +----------------+--------------------------------------------------+
1339 | type | detail |
1340 +----------------+--------------------------------------------------+
1341 | badSubmission | "submission" is neither a valid certificate nor |
1342 | | a valid precertificate. |
1343 | | |
1344 | badType | "type" is neither 1 nor 2. |
1345 | | |
1346 | badChain | The first element of "chain" is not the |
1347 | | certifier of the "submission", or the second |
1348 | | element does not certify the first, etc. |
1349 | | |
1350 | badCertificate | One or more certificates in the "chain" are not |
1351 | | valid (e.g., not properly encoded). |
1352 | | |
1353 | unknownAnchor | The last element of "chain" (or, if "chain" is |
1354 | | an empty array, the "submission") both is not, |
1355 | | and is not certified by, an accepted trust |
1356 | | anchor. |
1357 | | |
1358 | shutdown | The log is no longer accepting submissions. |
1359 +----------------+--------------------------------------------------+
1361 If the version of "sct" is not v2, then a v2 client may be unable to
1362 verify the signature. It MUST NOT construe this as an error. This
1363 is to avoid forcing an upgrade of compliant v2 clients that do not
1364 use the returned SCTs.
1366 If a log detects bad encoding in a chain that otherwise verifies
1367 correctly then the log MUST either log the certificate or return the
1368 "bad certificate" error. If the certificate is logged, an SCT MUST
1369 be issued. Logging the certificate is useful, because monitors
1370 (Section 8.2) can then detect these encoding errors, which may be
1371 accepted by some TLS clients.
1373 If "submission" is an accepted trust anchor whose certifier is
1374 neither an accepted trust anchor nor the first element of "chain",
1375 then the log MUST return the "unknown anchor" error. A log cannot
1376 generate an SCT for a submission if it does not have access to the
1377 issuer's public key.
1379 If the returned "sct" is intended to be provided to TLS clients, then
1380 "sth" and "inclusion" (if returned) SHOULD also be provided to TLS
1381 clients (e.g., if "type" was 2 (for "precert_sct_v2") then all three
1382 "TransItem"s could be embedded in the certificate).
1384 5.2. Retrieve Latest Signed Tree Head
1386 GET /ct/v2/get-sth
1388 No inputs.
1390 Outputs:
1392 sth: A base64 encoded "TransItem" of type "signed_tree_head_v2",
1393 signed by this log, that is no older than the log's MMD.
1395 5.3. Retrieve Merkle Consistency Proof between Two Signed Tree Heads
1397 GET /ct/v2/get-sth-consistency
1399 Inputs:
1401 first: The tree_size of the older tree, in decimal.
1403 second: The tree_size of the newer tree, in decimal (optional).
1405 Both tree sizes must be from existing v2 STHs. However, because
1406 of skew, the receiving front-end may not know one or both of the
1407 existing STHs. If both are known, then only the "consistency"
1408 output is returned. If the first is known but the second is not
1409 (or has been omitted), then the latest known STH is returned,
1410 along with a consistency proof between the first STH and the
1411 latest. If neither are known, then the latest known STH is
1412 returned without a consistency proof.
1414 Outputs:
1416 consistency: A base64 encoded "TransItem" of type
1417 "consistency_proof_v2", whose "tree_size_1" MUST match the
1418 "first" input. If the "sth" output is omitted, then
1419 "tree_size_2" MUST match the "second" input. If "first" and
1420 "second" are equal and correspond to a known STH, the returned
1421 consistency proof MUST be empty (a "consistency_path" array
1422 with zero elements).
1424 sth: A base64 encoded "TransItem" of type "signed_tree_head_v2",
1425 signed by this log.
1427 Note that no signature is required for the "consistency" output as
1428 it is used to verify the consistency between two STHs, which are
1429 signed.
1431 Error codes:
1433 +-------------------+-----------------------------------------------+
1434 | type | detail |
1435 +-------------------+-----------------------------------------------+
1436 | firstUnknown | "first" is before the latest known STH but is |
1437 | | not from an existing STH. |
1438 | | |
1439 | secondUnknown | "second" is before the latest known STH but |
1440 | | is not from an existing STH. |
1441 | | |
1442 | secondBeforeFirst | "second" is smaller than "first". |
1443 +-------------------+-----------------------------------------------+
1445 See Section 2.1.4.2 for an outline of how to use the "consistency"
1446 output.
1448 5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash
1450 GET /ct/v2/get-proof-by-hash
1452 Inputs:
1454 hash: A base64 encoded v2 leaf hash.
1456 tree_size: The tree_size of the tree on which to base the proof,
1457 in decimal.
1459 The "hash" must be calculated as defined in Section 4.7. The
1460 "tree_size" must designate an existing v2 STH. Because of skew,
1461 the front-end may not know the requested STH. In that case, it
1462 will return the latest STH it knows, along with an inclusion proof
1463 to that STH. If the front-end knows the requested STH then only
1464 "inclusion" is returned.
1466 Outputs:
1468 inclusion: A base64 encoded "TransItem" of type
1469 "inclusion_proof_v2" whose "inclusion_path" array of Merkle
1470 Tree nodes proves the inclusion of the chosen certificate in
1471 the selected STH.
1473 sth: A base64 encoded "TransItem" of type "signed_tree_head_v2",
1474 signed by this log.
1476 Note that no signature is required for the "inclusion" output as
1477 it is used to verify inclusion in the selected STH, which is
1478 signed.
1480 Error codes:
1482 +-----------------+-------------------------------------------------+
1483 | type | detail |
1484 +-----------------+-------------------------------------------------+
1485 | hashUnknown | "hash" is not the hash of a known leaf (may be |
1486 | | caused by skew or by a known certificate not |
1487 | | yet merged). |
1488 | | |
1489 | treeSizeUnknown | "hash" is before the latest known STH but is |
1490 | | not from an existing STH. |
1491 +-----------------+-------------------------------------------------+
1493 See Section 2.1.3.2 for an outline of how to use the "inclusion"
1494 output.
1496 5.5. Retrieve Merkle Inclusion Proof, Signed Tree Head and Consistency
1497 Proof by Leaf Hash
1499 GET /ct/v2/get-all-by-hash
1501 Inputs:
1503 hash: A base64 encoded v2 leaf hash.
1505 tree_size: The tree_size of the tree on which to base the proofs,
1506 in decimal.
1508 The "hash" must be calculated as defined in Section 4.7. The
1509 "tree_size" must designate an existing v2 STH.
1511 Because of skew, the front-end may not know the requested STH or the
1512 requested hash, which leads to a number of cases:
1514 +--------------------+----------------------------------------------+
1515 | Case | Response |
1516 +--------------------+----------------------------------------------+
1517 | latest STH < | Return latest STH |
1518 | requested STH | |
1519 | | |
1520 | latest STH > | Return latest STH and a consistency proof |
1521 | requested STH | between it and the requested STH (see |
1522 | | Section 5.3) |
1523 | | |
1524 | index of requested | Return "inclusion" |
1525 | hash < latest STH | |
1526 +--------------------+----------------------------------------------+
1527 Note that more than one case can be true, in which case the returned
1528 data is their union. It is also possible for none to be true, in
1529 which case the front-end MUST return an empty response.
1531 Outputs:
1533 inclusion: A base64 encoded "TransItem" of type
1534 "inclusion_proof_v2" whose "inclusion_path" array of Merkle
1535 Tree nodes proves the inclusion of the chosen certificate in
1536 the returned STH.
1538 sth: A base64 encoded "TransItem" of type "signed_tree_head_v2",
1539 signed by this log.
1541 consistency: A base64 encoded "TransItem" of type
1542 "consistency_proof_v2" that proves the consistency of the
1543 requested STH and the returned STH.
1545 Note that no signature is required for the "inclusion" or
1546 "consistency" outputs as they are used to verify inclusion in and
1547 consistency of STHs, which are signed.
1549 Errors are the same as in Section 5.4.
1551 See Section 2.1.3.2 for an outline of how to use the "inclusion"
1552 output, and see Section 2.1.4.2 for an outline of how to use the
1553 "consistency" output.
1555 5.6. Retrieve Entries and STH from Log
1557 GET /ct/v2/get-entries
1559 Inputs:
1561 start: 0-based index of first entry to retrieve, in decimal.
1563 end: 0-based index of last entry to retrieve, in decimal.
1565 Outputs:
1567 entries: An array of objects, each consisting of
1569 log_entry: The base64 encoded "TransItem" structure of type
1570 "x509_entry_v2" or "precert_entry_v2" (see Section 4.3).
1572 submitted_entry: JSON object representing the inputs that were
1573 submitted to "submit-entry", with the addition of the trust
1574 anchor to the "chain" field if the submission did not
1575 include it.
1577 sct: The base64 encoded "TransItem" of type "x509_sct_v2" or
1578 "precert_sct_v2" corresponding to this log entry.
1580 sth: A base64 encoded "TransItem" of type "signed_tree_head_v2",
1581 signed by this log.
1583 Note that this message is not signed -- the "entries" data can be
1584 verified by constructing the Merkle Tree Hash corresponding to a
1585 retrieved STH. All leaves MUST be v2. However, a compliant v2
1586 client MUST NOT construe an unrecognized TransItem type as an error.
1587 This means it may be unable to parse some entries, but note that each
1588 client can inspect the entries it does recognize as well as verify
1589 the integrity of the data by treating unrecognized leaves as opaque
1590 input to the tree.
1592 The "start" and "end" parameters SHOULD be within the range 0 <= x <
1593 "tree_size" as returned by "get-sth" in Section 5.2.
1595 The "start" parameter MUST be less than or equal to the "end"
1596 parameter.
1598 Each "submitted_entry" output parameter MUST include the trust anchor
1599 that the log used to verify the "submission", even if that trust
1600 anchor was not provided to "submit-entry" (see Section 5.1). If the
1601 "submission" does not certify itself, then the first element of
1602 "chain" MUST be present and MUST certify the "submission".
1604 Log servers MUST honor requests where 0 <= "start" < "tree_size" and
1605 "end" >= "tree_size" by returning a partial response covering only
1606 the valid entries in the specified range. "end" >= "tree_size" could
1607 be caused by skew. Note that the following restriction may also
1608 apply:
1610 Logs MAY restrict the number of entries that can be retrieved per
1611 "get-entries" request. If a client requests more than the permitted
1612 number of entries, the log SHALL return the maximum number of entries
1613 permissible. These entries SHALL be sequential beginning with the
1614 entry specified by "start".
1616 Because of skew, it is possible the log server will not have any
1617 entries between "start" and "end". In this case it MUST return an
1618 empty "entries" array.
1620 In any case, the log server MUST return the latest STH it knows
1621 about.
1623 See Section 2.1.2 for an outline of how to use a complete list of
1624 "log_entry" entries to verify the "root_hash".
1626 Error codes:
1628 +----------------+--------------------------------------------------+
1629 | type | detail |
1630 +----------------+--------------------------------------------------+
1631 | startUnknown | "start" is greater than the number of entries in |
1632 | | the Merkle tree. |
1633 | | |
1634 | endBeforeStart | "start" cannot be greater than "end". |
1635 +----------------+--------------------------------------------------+
1637 5.7. Retrieve Accepted Trust Anchors
1639 GET /ct/v2/get-anchors
1641 No inputs.
1643 Outputs:
1645 certificates: An array of base64 encoded trust anchors that are
1646 acceptable to the log.
1648 max_chain_length: If the server has chosen to limit the length of
1649 chains it accepts, this is the maximum number of certificates
1650 in the chain, in decimal. If there is no limit, this is
1651 omitted.
1653 6. TLS Servers
1655 CT-using TLS servers MUST use at least one of the three mechanisms
1656 listed below to present one or more SCTs from one or more logs to
1657 each TLS client during full TLS handshakes, where each SCT
1658 corresponds to the server certificate. They SHOULD also present
1659 corresponding inclusion proofs and STHs.
1661 Three mechanisms are provided because they have different tradeoffs.
1663 o A TLS extension (Section 4.2 of [RFC8446]) with type
1664 "transparency_info" (see Section 6.4). This mechanism allows TLS
1665 servers to participate in CT without the cooperation of CAs,
1666 unlike the other two mechanisms. It also allows SCTs and
1667 inclusion proofs to be updated on the fly.
1669 o An Online Certificate Status Protocol (OCSP) [RFC6960] response
1670 extension (see Section 7.1.1), where the OCSP response is provided
1671 in the "CertificateStatus" message, provided that the TLS client
1672 included the "status_request" extension in the (extended)
1673 "ClientHello" (Section 8 of [RFC6066]). This mechanism, popularly
1674 known as OCSP stapling, is already widely (but not universally)
1675 implemented. It also allows SCTs and inclusion proofs to be
1676 updated on the fly.
1678 o An X509v3 certificate extension (see Section 7.1.2). This
1679 mechanism allows the use of unmodified TLS servers, but the SCTs
1680 and inclusion proofs cannot be updated on the fly. Since the logs
1681 from which the SCTs and inclusion proofs originated won't
1682 necessarily be accepted by TLS clients for the full lifetime of
1683 the certificate, there is a risk that TLS clients will
1684 subsequently consider the certificate to be non-compliant and in
1685 need of re-issuance.
1687 6.1. Multiple SCTs
1689 CT-using TLS servers SHOULD send SCTs from multiple logs, because:
1691 o One or more logs may not have become acceptable to all CT-using
1692 TLS clients.
1694 o If a CA and a log collude, it is possible to temporarily hide
1695 misissuance from clients. When a TLS client requires SCTs from
1696 multiple logs to be provided, it is more difficult to mount this
1697 attack.
1699 o If a log misbehaves or suffers a key compromise, a consequence may
1700 be that clients cease to trust it. Since the time an SCT may be
1701 in use can be considerable (several years is common in current
1702 practice when embedded in a certificate), including SCTs from
1703 multiple logs reduces the probability of the certificate being
1704 rejected by TLS clients.
1706 o TLS clients may have policies related to the above risks requiring
1707 TLS servers to present multiple SCTs. For example, at the time of
1708 writing, Chromium [Chromium.Log.Policy] requires multiple SCTs to
1709 be presented with EV certificates in order for the EV indicator to
1710 be shown.
1712 To select the logs from which to obtain SCTs, a TLS server can, for
1713 example, examine the set of logs popular TLS clients accept and
1714 recognize.
1716 6.2. TransItemList Structure
1718 Multiple SCTs, inclusion proofs, and indeed "TransItem" structures of
1719 any type, are combined into a list as follows:
1721 opaque SerializedTransItem<1..2^16-1>;
1723 struct {
1724 SerializedTransItem trans_item_list<1..2^16-1>;
1725 } TransItemList;
1727 Here, "SerializedTransItem" is an opaque byte string that contains
1728 the serialized "TransItem" structure. This encoding ensures that TLS
1729 clients can decode each "TransItem" individually (so, for example, if
1730 there is a version upgrade, out-of-date clients can still parse old
1731 "TransItem" structures while skipping over new "TransItem" structures
1732 whose versions they don't understand).
1734 6.3. Presenting SCTs, inclusions proofs and STHs
1736 In each "TransItemList" that is sent to a client during a TLS
1737 handshake, the TLS server MUST include a "TransItem" structure of
1738 type "x509_sct_v2" or "precert_sct_v2".
1740 Presenting inclusion proofs and STHs in the TLS handshake helps to
1741 protect the client's privacy (see Section 8.1.4) and reduces load on
1742 log servers. Therefore, if the TLS server can obtain them, it SHOULD
1743 also include "TransItem"s of type "inclusion_proof_v2" and
1744 "signed_tree_head_v2" in the "TransItemList".
1746 6.4. transparency_info TLS Extension
1748 Provided that a TLS client includes the "transparency_info" extension
1749 type in the ClientHello and the TLS server supports the
1750 "transparency_info" extension:
1752 o The TLS server MUST verify that the received "extension_data" is
1753 empty.
1755 o The TLS server MUST construct a "TransItemList" of relevant
1756 "TransItem"s (see Section 6.3), which SHOULD omit any "TransItem"s
1757 that are already embedded in the server certificate or the stapled
1758 OCSP response (see Section 7.1). If the constructed
1759 "TransItemList" is not empty, then the TLS server MUST include the
1760 "transparency_info" extension with the "extension_data" set to
1761 this "TransItemList".
1763 TLS servers MUST only include this extension in the following
1764 messages:
1766 o the ServerHello message (for TLS 1.2 or earlier).
1768 o the Certificate or CertificateRequest message (for TLS 1.3).
1770 TLS servers MUST NOT process or include this extension when a TLS
1771 session is resumed, since session resumption uses the original
1772 session information.
1774 7. Certification Authorities
1776 7.1. Transparency Information X.509v3 Extension
1778 The Transparency Information X.509v3 extension, which has OID
1779 1.3.101.75 and SHOULD be non-critical, contains one or more
1780 "TransItem" structures in a "TransItemList". This extension MAY be
1781 included in OCSP responses (see Section 7.1.1) and certificates (see
1782 Section 7.1.2). Since RFC5280 requires the "extnValue" field (an
1783 OCTET STRING) of each X.509v3 extension to include the DER encoding
1784 of an ASN.1 value, a "TransItemList" MUST NOT be included directly.
1785 Instead, it MUST be wrapped inside an additional OCTET STRING, which
1786 is then put into the "extnValue" field:
1788 TransparencyInformationSyntax ::= OCTET STRING
1790 "TransparencyInformationSyntax" contains a "TransItemList".
1792 7.1.1. OCSP Response Extension
1794 A certification authority MAY include a Transparency Information
1795 X.509v3 extension in the "singleExtensions" of a "SingleResponse" in
1796 an OCSP response. All included SCTs and inclusion proofs MUST be for
1797 the certificate identified by the "certID" of that "SingleResponse",
1798 or for a precertificate that corresponds to that certificate.
1800 7.1.2. Certificate Extension
1802 A certification authority MAY include a Transparency Information
1803 X.509v3 extension in a certificate. All included SCTs and inclusion
1804 proofs MUST be for a precertificate that corresponds to this
1805 certificate.
1807 7.2. TLS Feature X.509v3 Extension
1809 A certification authority SHOULD NOT issue any certificate that
1810 identifies the "transparency_info" TLS extension in a TLS feature
1811 extension [RFC7633], because TLS servers are not required to support
1812 the "transparency_info" TLS extension in order to participate in CT
1813 (see Section 6).
1815 8. Clients
1817 There are various different functions clients of logs might perform.
1818 We describe here some typical clients and how they should function.
1819 Any inconsistency may be used as evidence that a log has not behaved
1820 correctly, and the signatures on the data structures prevent the log
1821 from denying that misbehavior.
1823 All clients need various parameters in order to communicate with logs
1824 and verify their responses. These parameters are described in
1825 Section 4.1, but note that this document does not describe how the
1826 parameters are obtained, which is implementation-dependent (see, for
1827 example, [Chromium.Policy]).
1829 8.1. TLS Client
1831 8.1.1. Receiving SCTs and inclusion proofs
1833 TLS clients receive SCTs and inclusion proofs alongside or in
1834 certificates. CT-using TLS clients MUST implement all of the three
1835 mechanisms by which TLS servers may present SCTs (see Section 6).
1837 TLS clients that support the "transparency_info" TLS extension (see
1838 Section 6.4) SHOULD include it in ClientHello messages, with empty
1839 "extension_data". If a TLS server includes the "transparency_info"
1840 TLS extension when resuming a TLS session, the TLS client MUST abort
1841 the handshake.
1843 8.1.2. Reconstructing the TBSCertificate
1845 Validation of an SCT for a certificate (where the "type" of the
1846 "TransItem" is "x509_sct_v2") uses the unmodified TBSCertificate
1847 component of the certificate.
1849 Before an SCT for a precertificate (where the "type" of the
1850 "TransItem" is "precert_sct_v2") can be validated, the TBSCertificate
1851 component of the precertificate needs to be reconstructed from the
1852 TBSCertificate component of the certificate as follows:
1854 o Remove the Transparency Information extension (see Section 7.1).
1856 o Remove embedded v1 SCTs, identified by OID 1.3.6.1.4.1.11129.2.4.2
1857 (see section 3.3 of [RFC6962]). This allows embedded v1 and v2
1858 SCTs to co-exist in a certificate (see Appendix A).
1860 8.1.3. Validating SCTs
1862 In order to make use of a received SCT, the TLS client MUST first
1863 validate it as follows:
1865 o Compute the signature input by constructing a "TransItem" of type
1866 "x509_entry_v2" or "precert_entry_v2", depending on the SCT's
1867 "TransItem" type. The "TimestampedCertificateEntryDataV2"
1868 structure is constructed in the following manner:
1870 * "timestamp" is copied from the SCT.
1872 * "tbs_certificate" is the reconstructed TBSCertificate portion
1873 of the server certificate, as described in Section 8.1.2.
1875 * "issuer_key_hash" is computed as described in Section 4.7.
1877 * "sct_extensions" is copied from the SCT.
1879 o Verify the SCT's "signature" against the computed signature input
1880 using the public key of the corresponding log, which is identified
1881 by the "log_id". The required signature algorithm is one of the
1882 log's parameters.
1884 If the TLS client does not have the corresponding log's parameters,
1885 it cannot attempt to validate the SCT. When evaluating compliance
1886 (see Section 8.1.6), the TLS client will consider only those SCTs
1887 that it was able to validate.
1889 Note that SCT validation is not a substitute for the normal
1890 validation of the server certificate and its chain.
1892 8.1.4. Fetching inclusion proofs
1894 When a TLS client has validated a received SCT but does not yet
1895 possess a corresponding inclusion proof, the TLS client MAY request
1896 the inclusion proof directly from a log using "get-proof-by-hash"
1897 (Section 5.4) or "get-all-by-hash" (Section 5.5).
1899 Note that fetching inclusion proofs directly from a log will disclose
1900 to the log which TLS server the client has been communicating with.
1901 This may be regarded as a significant privacy concern, and so it is
1902 preferable for the TLS server to send the inclusion proofs (see
1903 Section 6.3).
1905 8.1.5. Validating inclusion proofs
1907 When a TLS client has received, or fetched, an inclusion proof (and
1908 an STH), it SHOULD proceed to verifying the inclusion proof to the
1909 provided STH. The TLS client SHOULD also verify consistency between
1910 the provided STH and an STH it knows about.
1912 If the TLS client holds an STH that predates the SCT, it MAY, in the
1913 process of auditing, request a new STH from the log (Section 5.2),
1914 then verify it by requesting a consistency proof (Section 5.3). Note
1915 that if the TLS client uses "get-all-by-hash", then it will already
1916 have the new STH.
1918 8.1.6. Evaluating compliance
1920 It is up to a client's local policy to specify the quantity and form
1921 of evidence (SCTs, inclusion proofs or a combination) needed to
1922 achieve compliance and how to handle non-compliance.
1924 A TLS client can only evaluate compliance if it has given the TLS
1925 server the opportunity to send SCTs and inclusion proofs by any of
1926 the three mechanisms that are mandatory to implement for CT-using TLS
1927 clients (see Section 8.1.1). Therefore, a TLS client MUST NOT
1928 evaluate compliance if it did not include both the
1929 "transparency_info" and "status_request" TLS extensions in the
1930 ClientHello.
1932 8.2. Monitor
1934 Monitors watch logs to check that they behave correctly, for
1935 certificates of interest, or both. For example, a monitor may be
1936 configured to report on all certificates that apply to a specific
1937 domain name when fetching new entries for consistency validation.
1939 A monitor MUST at least inspect every new entry in every log it
1940 watches, and it MAY also choose to keep copies of entire logs.
1942 To inspect all of the existing entries, the monitor SHOULD follow
1943 these steps once for each log:
1945 1. Fetch the current STH (Section 5.2).
1947 2. Verify the STH signature.
1949 3. Fetch all the entries in the tree corresponding to the STH
1950 (Section 5.6).
1952 4. If applicable, check each entry to see if it's a certificate of
1953 interest.
1955 5. Confirm that the tree made from the fetched entries produces the
1956 same hash as that in the STH.
1958 To inspect new entries, the monitor SHOULD follow these steps
1959 repeatedly for each log:
1961 1. Fetch the current STH (Section 5.2). Repeat until the STH
1962 changes.
1964 2. Verify the STH signature.
1966 3. Fetch all the new entries in the tree corresponding to the STH
1967 (Section 5.6). If they remain unavailable for an extended
1968 period, then this should be viewed as misbehavior on the part of
1969 the log.
1971 4. If applicable, check each entry to see if it's a certificate of
1972 interest.
1974 5. Either:
1976 1. Verify that the updated list of all entries generates a tree
1977 with the same hash as the new STH.
1979 Or, if it is not keeping all log entries:
1981 1. Fetch a consistency proof for the new STH with the previous
1982 STH (Section 5.3).
1984 2. Verify the consistency proof.
1986 3. Verify that the new entries generate the corresponding
1987 elements in the consistency proof.
1989 6. Repeat from step 1.
1991 8.3. Auditing
1993 Auditing ensures that the current published state of a log is
1994 reachable from previously published states that are known to be good,
1995 and that the promises made by the log in the form of SCTs have been
1996 kept. Audits are performed by monitors or TLS clients.
1998 In particular, there are four log behavior properties that should be
1999 checked:
2001 o The Maximum Merge Delay (MMD).
2003 o The STH Frequency Count.
2005 o The append-only property.
2007 o The consistency of the log view presented to all query sources.
2009 A benign, conformant log publishes a series of STHs over time, each
2010 derived from the previous STH and the submitted entries incorporated
2011 into the log since publication of the previous STH. This can be
2012 proven through auditing of STHs. SCTs returned to TLS clients can be
2013 audited by verifying against the accompanying certificate, and using
2014 Merkle Inclusion Proofs, against the log's Merkle tree.
2016 The action taken by the auditor if an audit fails is not specified,
2017 but note that in general if audit fails, the auditor is in possession
2018 of signed proof of the log's misbehavior.
2020 A monitor (Section 8.2) can audit by verifying the consistency of
2021 STHs it receives, ensure that each entry can be fetched and that the
2022 STH is indeed the result of making a tree from all fetched entries.
2024 A TLS client (Section 8.1) can audit by verifying an SCT against any
2025 STH dated after the SCT timestamp + the Maximum Merge Delay by
2026 requesting a Merkle inclusion proof (Section 5.4). It can also
2027 verify that the SCT corresponds to the server certificate it arrived
2028 with (i.e., the log entry is that certificate, or is a precertificate
2029 corresponding to that certificate).
2031 Checking of the consistency of the log view presented to all entities
2032 is more difficult to perform because it requires a way to share log
2033 responses among a set of CT-using entities, and is discussed in
2034 Section 11.3.
2036 9. Algorithm Agility
2038 It is not possible for a log to change any of its algorithms part way
2039 through its lifetime:
2041 Signature algorithm: SCT signatures must remain valid so signature
2042 algorithms can only be added, not removed.
2044 Hash algorithm: A log would have to support the old and new hash
2045 algorithms to allow backwards-compatibility with clients that are
2046 not aware of a hash algorithm change.
2048 Allowing multiple signature or hash algorithms for a log would
2049 require that all data structures support it and would significantly
2050 complicate client implementation, which is why it is not supported by
2051 this document.
2053 If it should become necessary to deprecate an algorithm used by a
2054 live log, then the log MUST be frozen as specified in Section 4.13
2055 and a new log SHOULD be started. Certificates in the frozen log that
2056 have not yet expired and require new SCTs SHOULD be submitted to the
2057 new log and the SCTs from that log used instead.
2059 10. IANA Considerations
2061 The assignment policy criteria mentioned in this section refer to the
2062 policies outlined in [RFC8126].
2064 10.1. New Entry to the TLS ExtensionType Registry
2066 IANA is asked to add an entry for "transparency_info(TBD)" to the
2067 "TLS ExtensionType Values" registry defined in [RFC8446], setting the
2068 "Recommended" value to "Y", setting the "TLS 1.3" value to "CH, CR,
2069 CT", and citing this document as the "Reference".
2071 10.2. Hash Algorithms
2073 IANA is asked to establish a registry of hash algorithm values, named
2074 "CT Hash Algorithms", that initially consists of:
2076 +---------+------------+------------------------+-------------------+
2077 | Value | Hash | OID | Reference / |
2078 | | Algorithm | | Assignment Policy |
2079 +---------+------------+------------------------+-------------------+
2080 | 0x00 | SHA-256 | 2.16.840.1.101.3.4.2.1 | [RFC6234] |
2081 | | | | |
2082 | 0x01 - | Unassigned | | Specification |
2083 | 0xDF | | | Required |
2084 | | | | |
2085 | 0xE0 - | Reserved | | Experimental Use |
2086 | 0xEF | | | |
2087 | | | | |
2088 | 0xF0 - | Reserved | | Private Use |
2089 | 0xFF | | | |
2090 +---------+------------+------------------------+-------------------+
2092 10.2.1. Specification Required guidance
2094 The appointed Expert should ensure that the proposed algorithm has a
2095 public specification and is suitable for use as a cryptographic hash
2096 algorithm with no known preimage or collision attacks. These attacks
2097 can damage the integrity of the log.
2099 10.3. Signature Algorithms
2101 IANA is asked to establish a registry of signature algorithm values,
2102 named "CT Signature Algorithms", that initially consists of:
2104 +--------------------------------+-------------------+--------------+
2105 | SignatureScheme Value | Signature | Reference / |
2106 | | Algorithm | Assignment |
2107 | | | Policy |
2108 +--------------------------------+-------------------+--------------+
2109 | 0x0000 - 0x0402 | Unassigned | Expert |
2110 | | | Review |
2111 | | | |
2112 | ecdsa_secp256r1_sha256(0x0403) | ECDSA (NIST | [FIPS186-4] |
2113 | | P-256) with | |
2114 | | SHA-256 | |
2115 | | | |
2116 | ecdsa_secp256r1_sha256(0x0403) | Deterministic | [RFC6979] |
2117 | | ECDSA (NIST | |
2118 | | P-256) with HMAC- | |
2119 | | SHA256 | |
2120 | | | |
2121 | 0x0404 - 0x0806 | Unassigned | Expert |
2122 | | | Review |
2123 | | | |
2124 | ed25519(0x0807) | Ed25519 | [RFC8032] |
2125 | | (PureEdDSA with | |
2126 | | the edwards25519 | |
2127 | | curve) | |
2128 | | | |
2129 | 0x0808 - 0xFDFF | Unassigned | Expert |
2130 | | | Review |
2131 | | | |
2132 | 0xFE00 - 0xFEFF | Reserved | Experimental |
2133 | | | Use |
2134 | | | |
2135 | 0xFF00 - 0xFFFF | Reserved | Private Use |
2136 +--------------------------------+-------------------+--------------+
2138 10.3.1. Expert Review guidelines
2140 The appointed Expert should ensure that the proposed algorithm has a
2141 public specification, has a value assigned to it in the TLS
2142 SignatureScheme Registry (that IANA is asked to establish in
2143 [RFC8446]) and is suitable for use as a cryptographic signature
2144 algorithm.
2146 10.4. VersionedTransTypes
2148 IANA is asked to establish a registry of "VersionedTransType" values,
2149 named "CT VersionedTransTypes", that initially consists of:
2151 +----------------+----------------------+---------------------------+
2152 | Value | Type and Version | Reference / Assignment |
2153 | | | Policy |
2154 +----------------+----------------------+---------------------------+
2155 | 0x0000 | Reserved | [RFC6962] (*) |
2156 | | | |
2157 | 0x0001 | x509_entry_v2 | RFCXXXX |
2158 | | | |
2159 | 0x0002 | precert_entry_v2 | RFCXXXX |
2160 | | | |
2161 | 0x0003 | x509_sct_v2 | RFCXXXX |
2162 | | | |
2163 | 0x0004 | precert_sct_v2 | RFCXXXX |
2164 | | | |
2165 | 0x0005 | signed_tree_head_v2 | RFCXXXX |
2166 | | | |
2167 | 0x0006 | consistency_proof_v2 | RFCXXXX |
2168 | | | |
2169 | 0x0007 | inclusion_proof_v2 | RFCXXXX |
2170 | | | |
2171 | 0x0008 - | Unassigned | Specification Required |
2172 | 0xDFFF | | |
2173 | | | |
2174 | 0xE000 - | Reserved | Experimental Use |
2175 | 0xEFFF | | |
2176 | | | |
2177 | 0xF000 - | Reserved | Private Use |
2178 | 0xFFFF | | |
2179 +----------------+----------------------+---------------------------+
2181 (*) The 0x0000 value is reserved so that v1 SCTs are distinguishable
2182 from v2 SCTs and other "TransItem" structures.
2184 [RFC Editor: please update 'RFCXXXX' to refer to this document, once
2185 its RFC number is known.]
2187 10.4.1. Specification Required guidance
2189 The appointed Expert should review the public specification to ensure
2190 that it is detailed enough to ensure implementation interoperability.
2192 10.5. Log Artifact Extension Registry
2194 IANA is asked to establish a registry of "ExtensionType" values,
2195 named "CT Log Artifact Extensions", that initially consists of:
2197 +-----------------+------------+-----+------------------------------+
2198 | ExtensionType | Status | Use | Reference / Assignment |
2199 | | | | Policy |
2200 +-----------------+------------+-----+------------------------------+
2201 | 0x0000 - 0xDFFF | Unassigned | n/a | Specification Required |
2202 | | | | |
2203 | 0xE000 - 0xEFFF | Reserved | n/a | Experimental Use |
2204 | | | | |
2205 | 0xF000 - 0xFFFF | Reserved | n/a | Private Use |
2206 +-----------------+------------+-----+------------------------------+
2208 The "Use" column should contain one or both of the following values:
2210 o "SCT", for extensions specified for use in Signed Certificate
2211 Timestamps.
2213 o "STH", for extensions specified for use in Signed Tree Heads.
2215 10.5.1. Specification Required guidance
2217 The appointed Expert should review the public specification to ensure
2218 that it is detailed enough to ensure implementation interoperability.
2219 The Expert should also verify that the extension is appropriate to
2220 the contexts in which it is specified to be used (SCT, STH, or both).
2222 10.6. Object Identifiers
2224 This document uses object identifiers (OIDs) to identify Log IDs (see
2225 Section 4.4), the precertificate CMS "eContentType" (see
2226 Section 3.2), and X.509v3 extensions in certificates (see
2227 Section 7.1.2) and OCSP responses (see Section 7.1.1). The OIDs are
2228 defined in an arc that was selected due to its short encoding.
2230 10.6.1. Log ID Registry
2232 IANA is asked to establish a registry of Log IDs, named "CT Log ID
2233 Registry", that initially consists of:
2235 +---------------------+------------+------------+-------------------+
2236 | Log ID | Log Base | Log | Reference / |
2237 | | URL | Operator | Assignment Policy |
2238 +---------------------+------------+------------+-------------------+
2239 | 1.3.101.8192 - | Unassigned | Unassigned | First Come First |
2240 | 1.3.101.16383 | | | Served |
2241 | | | | |
2242 | 1.3.101.80.0 - | Unassigned | Unassigned | First Come First |
2243 | 1.3.101.80.* | | | Served |
2244 +---------------------+------------+------------+-------------------+
2246 All OIDs in the range from 1.3.101.8192 to 1.3.101.16383 have been
2247 reserved. This is a limited resource of 8,192 OIDs, each of which
2248 has an encoded length of 4 octets.
2250 The 1.3.101.80 arc has been delegated. This is an unlimited
2251 resource, but only the 128 OIDs from 1.3.101.80.0 to 1.3.101.80.127
2252 have an encoded length of only 4 octets.
2254 Each application for the allocation of a Log ID MUST be accompanied
2255 by:
2257 o the Log's Base URL (see Section 4.1).
2259 o the Log Operator's contact details.
2261 IANA is asked to reject any request to update a Log ID or Log Base
2262 URL in this registry, because these fields are immutable (see
2263 Section 4.1).
2265 IANA is asked to accept requests from log operators to update their
2266 contact details in this registry.
2268 Since log operators can choose to not use this registry (see
2269 Section 4.4), it is not expected to be a global directory of all
2270 logs.
2272 11. Security Considerations
2274 With CAs, logs, and servers performing the actions described here,
2275 TLS clients can use logs and signed timestamps to reduce the
2276 likelihood that they will accept misissued certificates. If a server
2277 presents a valid signed timestamp for a certificate, then the client
2278 knows that a log has committed to publishing the certificate. From
2279 this, the client knows that monitors acting for the subject of the
2280 certificate have had some time to notice the misissuance and take
2281 some action, such as asking a CA to revoke a misissued certificate.
2282 A signed timestamp does not guarantee this though, since appropriate
2283 monitors might not have checked the logs or the CA might have refused
2284 to revoke the certificate.
2286 In addition, if TLS clients will not accept unlogged certificates,
2287 then site owners will have a greater incentive to submit certificates
2288 to logs, possibly with the assistance of their CA, increasing the
2289 overall transparency of the system.
2291 [I-D.ietf-trans-threat-analysis] provides a more detailed threat
2292 analysis of the Certificate Transparency architecture.
2294 11.1. Misissued Certificates
2296 Misissued certificates that have not been publicly logged, and thus
2297 do not have a valid SCT, are not considered compliant. Misissued
2298 certificates that do have an SCT from a log will appear in that
2299 public log within the Maximum Merge Delay, assuming the log is
2300 operating correctly. Since a log is allowed to serve an STH of any
2301 age up to the MMD, the maximum period of time during which a
2302 misissued certificate can be used without being available for audit
2303 is twice the MMD.
2305 11.2. Detection of Misissue
2307 The logs do not themselves detect misissued certificates; they rely
2308 instead on interested parties, such as domain owners, to monitor them
2309 and take corrective action when a misissue is detected.
2311 11.3. Misbehaving Logs
2313 A log can misbehave in several ways. Examples include: failing to
2314 incorporate a certificate with an SCT in the Merkle Tree within the
2315 MMD; presenting different, conflicting views of the Merkle Tree at
2316 different times and/or to different parties; issuing STHs too
2317 frequently; mutating the signature of a logged certificate; and
2318 failing to present a chain containing the certifier of a logged
2319 certificate. Such misbehavior is detectable and
2320 [I-D.ietf-trans-threat-analysis] provides more details on how this
2321 can be done.
2323 Violation of the MMD contract is detected by log clients requesting a
2324 Merkle inclusion proof (Section 5.4) for each observed SCT. These
2325 checks can be asynchronous and need only be done once per
2326 certificate. However, note that there may be privacy concerns (see
2327 Section 8.1.4).
2329 Violation of the append-only property or the STH issuance rate limit
2330 can be detected by clients comparing their instances of the Signed
2331 Tree Heads. There are various ways this could be done, for example
2332 via gossip (see [I-D.ietf-trans-gossip]) or peer-to-peer
2333 communications or by sending STHs to monitors (who could then
2334 directly check against their own copy of the relevant log). Proof of
2335 misbehavior in such cases would be: a series of STHs that were issued
2336 too closely together, proving violation of the STH issuance rate
2337 limit; or an STH with a root hash that does not match the one
2338 calculated from a copy of the log, proving violation of the append-
2339 only property.
2341 11.4. Preventing Tracking Clients
2343 Clients that gossip STHs or report back SCTs can be tracked or traced
2344 if a log produces multiple STHs or SCTs with the same timestamp and
2345 data but different signatures. Logs SHOULD mitigate this risk by
2346 either:
2348 o Using deterministic signature schemes, or
2350 o Producing no more than one SCT for each distinct submission and no
2351 more than one STH for each distinct tree_size. Each of these SCTs
2352 and STHs can be stored by the log and served to other clients that
2353 submit the same certificate or request the same STH.
2355 11.5. Multiple SCTs
2357 By requiring TLS servers to offer multiple SCTs, each from a
2358 different log, TLS clients reduce the effectiveness of an attack
2359 where a CA and a log collude (see Section 6.1).
2361 11.6. Leakage of DNS Information
2363 Malicious monitors can use logs to learn about the existence of
2364 domain names that might not otherwise be easy to discover. Some
2365 subdomain labels may reveal information about the service and
2366 software for which the subdomain is used, which in turn might
2367 facilitate targeted attacks.
2369 12. Acknowledgements
2371 The authors would like to thank Erwann Abelea, Robin Alden, Andrew
2372 Ayer, Richard Barnes, Al Cutter, David Drysdale, Francis Dupont, Adam
2373 Eijdenberg, Stephen Farrell, Daniel Kahn Gillmor, Paul Hadfield, Brad
2374 Hill, Jeff Hodges, Paul Hoffman, Jeffrey Hutzelman, Kat Joyce,
2375 Stephen Kent, SM, Alexey Melnikov, Linus Nordberg, Chris Palmer,
2376 Trevor Perrin, Pierre Phaneuf, Eric Rescorla, Melinda Shore, Ryan
2377 Sleevi, Martin Smith, Carl Wallace and Paul Wouters for their
2378 valuable contributions.
2380 A big thank you to Symantec for kindly donating the OIDs from the
2381 1.3.101 arc that are used in this document.
2383 13. References
2385 13.1. Normative References
2387 [FIPS186-4]
2388 NIST, "FIPS PUB 186-4", July 2013,
2389 .
2392 [HTML401] Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
2393 Specification", World Wide Web Consortium Recommendation
2394 REC-html401-19991224, December 1999,
2395 .
2397 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
2398 Requirement Levels", BCP 14, RFC 2119,
2399 DOI 10.17487/RFC2119, March 1997,
2400 .
2402 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
2403 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
2404 .
2406 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
2407 Housley, R., and W. Polk, "Internet X.509 Public Key
2408 Infrastructure Certificate and Certificate Revocation List
2409 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
2410 .
2412 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
2413 RFC 5652, DOI 10.17487/RFC5652, September 2009,
2414 .
2416 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
2417 Extensions: Extension Definitions", RFC 6066,
2418 DOI 10.17487/RFC6066, January 2011,
2419 .
2421 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
2422 Galperin, S., and C. Adams, "X.509 Internet Public Key
2423 Infrastructure Online Certificate Status Protocol - OCSP",
2424 RFC 6960, DOI 10.17487/RFC6960, June 2013,
2425 .
2427 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
2428 Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
2429 DOI 10.17487/RFC7231, June 2014,
2430 .
2432 [RFC7633] Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS)
2433 Feature Extension", RFC 7633, DOI 10.17487/RFC7633,
2434 October 2015, .
2436 [RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP
2437 APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
2438 .
2440 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
2441 Signature Algorithm (EdDSA)", RFC 8032,
2442 DOI 10.17487/RFC8032, January 2017,
2443 .
2445 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2446 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
2447 May 2017, .
2449 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
2450 Interchange Format", STD 90, RFC 8259,
2451 DOI 10.17487/RFC8259, December 2017,
2452 .
2454 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
2455 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
2456 .
2458 [UNIXTIME]
2459 IEEE, "The Open Group Base Specifications Issue 7 IEEE Std
2460 1003.1-2008, 2016 Edition", n.d., .
2464 13.2. Informative References
2466 [Chromium.Log.Policy]
2467 The Chromium Projects, "Chromium Certificate Transparency
2468 Log Policy", 2014, .
2471 [Chromium.Policy]
2472 The Chromium Projects, "Chromium Certificate
2473 Transparency", 2014, .
2476 [CrosbyWallach]
2477 Crosby, S. and D. Wallach, "Efficient Data Structures for
2478 Tamper-Evident Logging", Proceedings of the 18th USENIX
2479 Security Symposium, Montreal, August 2009,
2480 .
2483 [I-D.ietf-trans-gossip]
2484 Nordberg, L., Gillmor, D., and T. Ritter, "Gossiping in
2485 CT", draft-ietf-trans-gossip-05 (work in progress),
2486 January 2018.
2488 [I-D.ietf-trans-threat-analysis]
2489 Kent, S., "Attack and Threat Model for Certificate
2490 Transparency", draft-ietf-trans-threat-analysis-16 (work
2491 in progress), October 2018.
2493 [JSON.Metadata]
2494 The Chromium Projects, "Chromium Log Metadata JSON
2495 Schema", 2014, .
2498 [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
2499 (SHA and SHA-based HMAC and HKDF)", RFC 6234,
2500 DOI 10.17487/RFC6234, May 2011,
2501 .
2503 [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
2504 Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
2505 .
2507 [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
2508 Algorithm (DSA) and Elliptic Curve Digital Signature
2509 Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2510 2013, .
2512 [RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190,
2513 RFC 7320, DOI 10.17487/RFC7320, July 2014,
2514 .
2516 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
2517 Writing an IANA Considerations Section in RFCs", BCP 26,
2518 RFC 8126, DOI 10.17487/RFC8126, June 2017,
2519 .
2521 Appendix A. Supporting v1 and v2 simultaneously
2523 Certificate Transparency logs have to be either v1 (conforming to
2524 [RFC6962]) or v2 (conforming to this document), as the data
2525 structures are incompatible and so a v2 log could not issue a valid
2526 v1 SCT.
2528 CT clients, however, can support v1 and v2 SCTs, for the same
2529 certificate, simultaneously, as v1 SCTs are delivered in different
2530 TLS, X.509 and OCSP extensions than v2 SCTs.
2532 v1 and v2 SCTs for X.509 certificates can be validated independently.
2533 For precertificates, v2 SCTs should be embedded in the TBSCertificate
2534 before submission of the TBSCertificate (inside a v1 precertificate,
2535 as described in Section 3.1. of [RFC6962]) to a v1 log so that TLS
2536 clients conforming to [RFC6962] but not this document are oblivious
2537 to the embedded v2 SCTs. An issuer can follow these steps to produce
2538 an X.509 certificate with embedded v1 and v2 SCTs:
2540 o Create a CMS precertificate as described in Section 3.2 and submit
2541 it to v2 logs.
2543 o Embed the obtained v2 SCTs in the TBSCertificate, as described in
2544 Section 7.1.2.
2546 o Use that TBSCertificate to create a v1 precertificate, as
2547 described in Section 3.1. of [RFC6962] and submit it to v1 logs.
2549 o Embed the v1 SCTs in the TBSCertificate, as described in
2550 Section 3.3 of [RFC6962].
2552 o Sign that TBSCertificate (which now contains v1 and v2 SCTs) to
2553 issue the final X.509 certificate.
2555 Authors' Addresses
2557 Ben Laurie
2558 Google UK Ltd.
2560 Email: benl@google.com
2562 Adam Langley
2563 Google Inc.
2565 Email: agl@google.com
2566 Emilia Kasper
2567 Google Switzerland GmbH
2569 Email: ekasper@google.com
2571 Eran Messeri
2572 Google UK Ltd.
2574 Email: eranm@google.com
2576 Rob Stradling
2577 Sectigo Ltd.
2579 Email: rob@sectigo.com