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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: 15 November 2021 Google
7 R. Stradling
8 Sectigo
9 14 May 2021
11 Certificate Transparency Version 2.0
12 draft-ietf-trans-rfc6962-bis-38
14 Abstract
16 This document describes version 2.0 of the Certificate Transparency
17 (CT) protocol for publicly logging the existence of Transport Layer
18 Security (TLS) server certificates as they are issued or observed, in
19 a manner that allows anyone to audit certification authority (CA)
20 activity and notice the issuance of suspect certificates as well as
21 to audit the certificate logs themselves. The intent is that
22 eventually clients would refuse to honor certificates that do not
23 appear in a log, effectively forcing CAs to add all issued
24 certificates to the logs.
26 This document obsoletes RFC 6962. It also specifies a new TLS
27 extension that is used to send various CT log artifacts.
29 Logs are network services that implement the protocol operations for
30 submissions and queries that are defined in this document.
32 [RFC Editor: please update 'RFCXXXX' to refer to this document, once
33 its RFC number is known, through the document.]
35 Status of This Memo
37 This Internet-Draft is submitted in full conformance with the
38 provisions of BCP 78 and BCP 79.
40 Internet-Drafts are working documents of the Internet Engineering
41 Task Force (IETF). Note that other groups may also distribute
42 working documents as Internet-Drafts. The list of current Internet-
43 Drafts is at https://datatracker.ietf.org/drafts/current/.
45 Internet-Drafts are draft documents valid for a maximum of six months
46 and may be updated, replaced, or obsoleted by other documents at any
47 time. It is inappropriate to use Internet-Drafts as reference
48 material or to cite them other than as "work in progress."
49 This Internet-Draft will expire on 15 November 2021.
51 Copyright Notice
53 Copyright (c) 2021 IETF Trust and the persons identified as the
54 document authors. All rights reserved.
56 This document is subject to BCP 78 and the IETF Trust's Legal
57 Provisions Relating to IETF Documents (https://trustee.ietf.org/
58 license-info) in effect on the date of publication of this document.
59 Please review these documents carefully, as they describe your rights
60 and restrictions with respect to this document. Code Components
61 extracted from this document must include Simplified BSD License text
62 as described in Section 4.e of the Trust Legal Provisions and are
63 provided without warranty as described in the Simplified BSD License.
65 Table of Contents
67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
68 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
69 1.2. Data Structures . . . . . . . . . . . . . . . . . . . . . 5
70 1.3. Major Differences from CT 1.0 . . . . . . . . . . . . . . 6
71 2. Cryptographic Components . . . . . . . . . . . . . . . . . . 7
72 2.1. Merkle Hash Trees . . . . . . . . . . . . . . . . . . . . 7
73 2.1.1. Definition of the Merkle Tree . . . . . . . . . . . . 7
74 2.1.2. Verifying a Tree Head Given Entries . . . . . . . . . 8
75 2.1.3. Merkle Inclusion Proofs . . . . . . . . . . . . . . . 9
76 2.1.4. Merkle Consistency Proofs . . . . . . . . . . . . . . 11
77 2.1.5. Example . . . . . . . . . . . . . . . . . . . . . . . 13
78 2.2. Signatures . . . . . . . . . . . . . . . . . . . . . . . 14
79 3. Submitters . . . . . . . . . . . . . . . . . . . . . . . . . 15
80 3.1. Certificates . . . . . . . . . . . . . . . . . . . . . . 15
81 3.2. Precertificates . . . . . . . . . . . . . . . . . . . . . 15
82 3.2.1. Binding Intent to Issue . . . . . . . . . . . . . . . 17
83 4. Log Format and Operation . . . . . . . . . . . . . . . . . . 17
84 4.1. Log Parameters . . . . . . . . . . . . . . . . . . . . . 18
85 4.2. Evaluating Submissions . . . . . . . . . . . . . . . . . 19
86 4.2.1. Minimum Acceptance Criteria . . . . . . . . . . . . . 19
87 4.2.2. Discretionary Acceptance Criteria . . . . . . . . . . 20
88 4.3. Log Entries . . . . . . . . . . . . . . . . . . . . . . . 20
89 4.4. Log ID . . . . . . . . . . . . . . . . . . . . . . . . . 21
90 4.5. TransItem Structure . . . . . . . . . . . . . . . . . . . 21
91 4.6. Log Artifact Extensions . . . . . . . . . . . . . . . . . 22
92 4.7. Merkle Tree Leaves . . . . . . . . . . . . . . . . . . . 23
93 4.8. Signed Certificate Timestamp (SCT) . . . . . . . . . . . 24
94 4.9. Merkle Tree Head . . . . . . . . . . . . . . . . . . . . 25
95 4.10. Signed Tree Head (STH) . . . . . . . . . . . . . . . . . 26
96 4.11. Merkle Consistency Proofs . . . . . . . . . . . . . . . . 26
97 4.12. Merkle Inclusion Proofs . . . . . . . . . . . . . . . . . 27
98 4.13. Shutting down a log . . . . . . . . . . . . . . . . . . . 28
99 5. Log Client Messages . . . . . . . . . . . . . . . . . . . . . 28
100 5.1. Submit Entry to Log . . . . . . . . . . . . . . . . . . . 30
101 5.2. Retrieve Latest Signed Tree Head . . . . . . . . . . . . 32
102 5.3. Retrieve Merkle Consistency Proof between Two Signed Tree
103 Heads . . . . . . . . . . . . . . . . . . . . . . . . . . 32
104 5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash . . 33
105 5.5. Retrieve Merkle Inclusion Proof, Signed Tree Head and
106 Consistency Proof by Leaf Hash . . . . . . . . . . . . . 34
107 5.6. Retrieve Entries and STH from Log . . . . . . . . . . . . 35
108 5.7. Retrieve Accepted Trust Anchors . . . . . . . . . . . . . 37
109 6. TLS Servers . . . . . . . . . . . . . . . . . . . . . . . . . 38
110 6.1. TLS Client Authentication . . . . . . . . . . . . . . . . 38
111 6.2. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . . 39
112 6.3. TransItemList Structure . . . . . . . . . . . . . . . . . 39
113 6.4. Presenting SCTs, inclusions proofs and STHs . . . . . . . 40
114 6.5. transparency_info TLS Extension . . . . . . . . . . . . . 40
115 7. Certification Authorities . . . . . . . . . . . . . . . . . . 41
116 7.1. Transparency Information X.509v3 Extension . . . . . . . 41
117 7.1.1. OCSP Response Extension . . . . . . . . . . . . . . . 41
118 7.1.2. Certificate Extension . . . . . . . . . . . . . . . . 41
119 7.2. TLS Feature X.509v3 Extension . . . . . . . . . . . . . . 41
120 8. Clients . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
121 8.1. TLS Client . . . . . . . . . . . . . . . . . . . . . . . 42
122 8.1.1. Receiving SCTs and inclusion proofs . . . . . . . . . 42
123 8.1.2. Reconstructing the TBSCertificate . . . . . . . . . . 42
124 8.1.3. Validating SCTs . . . . . . . . . . . . . . . . . . . 42
125 8.1.4. Fetching inclusion proofs . . . . . . . . . . . . . . 43
126 8.1.5. Validating inclusion proofs . . . . . . . . . . . . . 43
127 8.1.6. Evaluating compliance . . . . . . . . . . . . . . . . 44
128 8.2. Monitor . . . . . . . . . . . . . . . . . . . . . . . . . 44
129 8.3. Auditing . . . . . . . . . . . . . . . . . . . . . . . . 45
130 9. Algorithm Agility . . . . . . . . . . . . . . . . . . . . . . 46
131 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
132 10.1. Additions to existing registries . . . . . . . . . . . . 47
133 10.1.1. New Entry to the TLS ExtensionType Registry . . . . 47
134 10.1.2. URN Sub-namespace for TRANS errors
135 (urn:ietf:params:trans:error) . . . . . . . . . . . . 47
136 10.2. New CT-Related registries . . . . . . . . . . . . . . . 47
137 10.2.1. Hash Algorithms . . . . . . . . . . . . . . . . . . 48
138 10.2.2. Signature Algorithms . . . . . . . . . . . . . . . . 48
139 10.2.3. VersionedTransTypes . . . . . . . . . . . . . . . . 49
140 10.2.4. Log Artifact Extension Registry . . . . . . . . . . 50
141 10.2.5. Log ID Registry . . . . . . . . . . . . . . . . . . 51
142 10.2.6. Error Types Registry . . . . . . . . . . . . . . . . 52
143 11. Security Considerations . . . . . . . . . . . . . . . . . . . 54
144 11.1. Misissued Certificates . . . . . . . . . . . . . . . . . 54
145 11.2. Detection of Misissue . . . . . . . . . . . . . . . . . 54
146 11.3. Misbehaving Logs . . . . . . . . . . . . . . . . . . . . 55
147 11.4. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . 55
148 11.5. Leakage of DNS Information . . . . . . . . . . . . . . . 56
149 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 56
150 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 56
151 13.1. Normative References . . . . . . . . . . . . . . . . . . 56
152 13.2. Informative References . . . . . . . . . . . . . . . . . 58
153 Appendix A. Supporting v1 and v2 simultaneously (Informative) . 59
154 Appendix B. An ASN.1 Module (Informative) . . . . . . . . . . . 60
155 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 62
157 1. Introduction
159 Certificate Transparency aims to mitigate the problem of misissued
160 certificates by providing append-only logs of issued certificates.
161 The logs do not themselves prevent misissuance, but they ensure that
162 interested parties (particularly those named in certificates) can
163 detect such misissuance. Note that this is a general mechanism that
164 could be used for transparently logging any form of binary data,
165 subject to some kind of inclusion criteria. In this document, we
166 only describe its use for public TLS server certificates (i.e., where
167 the inclusion criteria is a valid certificate issued by a public
168 certification authority (CA)). A typical definition of "public" can
169 be found in [CABBR].
171 Each log contains certificate chains, which can be submitted by
172 anyone. It is expected that public CAs will contribute all their
173 newly issued certificates to one or more logs; however certificate
174 holders can also contribute their own certificate chains, as can
175 third parties. In order to avoid logs being rendered useless by the
176 submission of large numbers of spurious certificates, it is required
177 that each chain ends with a trust anchor that is accepted by the log.
178 A log may also limit the length of the chain it is willing to accept;
179 such chains must also end with an acceptable trust anchor. When a
180 chain is accepted by a log, a signed timestamp is returned, which can
181 later be used to provide evidence to TLS clients that the chain has
182 been submitted. TLS clients can thus require that all certificates
183 they accept as valid are accompanied by signed timestamps.
185 Those who are concerned about misissuance can monitor the logs,
186 asking them regularly for all new entries, and can thus check whether
187 domains for which they are responsible have had certificates issued
188 that they did not expect. What they do with this information,
189 particularly when they find that a misissuance has happened, is
190 beyond the scope of this document. However, broadly speaking, they
191 can invoke existing business mechanisms for dealing with misissued
192 certificates, such as working with the CA to get the certificate
193 revoked, or with maintainers of trust anchor lists to get the CA
194 removed. Of course, anyone who wants can monitor the logs and, if
195 they believe a certificate is incorrectly issued, take action as they
196 see fit.
198 Similarly, those who have seen signed timestamps from a particular
199 log can later demand a proof of inclusion from that log. If the log
200 is unable to provide this (or, indeed, if the corresponding
201 certificate is absent from monitors' copies of that log), that is
202 evidence of the incorrect operation of the log. The checking
203 operation is asynchronous to allow clients to proceed without delay,
204 despite possible issues such as network connectivity and the vagaries
205 of firewalls.
207 The append-only property of each log is achieved using Merkle Trees,
208 which can be used to efficiently prove that any particular instance
209 of the log is a superset of any particular previous instance and to
210 efficiently detect various misbehaviors of the log (e.g., issuing a
211 signed timestamp for a certificate that is not subsequently logged).
213 It is necessary to treat each log as a trusted third party, because
214 the log auditing mechanisms described in this document can be
215 circumvented by a misbehaving log that shows different, inconsistent
216 views of itself to different clients. While mechanisms are being
217 developed to address these shortcomings and thereby avoid the need to
218 blindly trust logs, such mechanisms are outside the scope of this
219 document.
221 1.1. Requirements Language
223 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
224 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
225 "OPTIONAL" in this document are to be interpreted as described in BCP
226 14 [RFC2119] [RFC8174] when, and only when, they appear in all
227 capitals, as shown here.
229 1.2. Data Structures
231 Data structures are defined and encoded according to the conventions
232 laid out in Section 3 of [RFC8446].
234 This document uses object identifiers (OIDs) to identify Log IDs (see
235 Section 4.4), the precertificate CMS "eContentType" (see
236 Section 3.2), and X.509v3 extensions in certificates (see
237 Section 7.1.2) and OCSP responses (see Section 7.1.1). The OIDs are
238 defined in an arc that was selected due to its short encoding.
240 1.3. Major Differences from CT 1.0
242 This document revises and obsoletes the CT 1.0 [RFC6962] protocol,
243 drawing on insights gained from CT 1.0 deployments and on feedback
244 from the community. The major changes are:
246 * Hash and signature algorithm agility: permitted algorithms are now
247 specified in IANA registries.
249 * Precertificate format: precertificates are now CMS objects rather
250 than X.509 certificates, which avoids violating the certificate
251 serial number uniqueness requirement in Section 4.1.2.2 of
252 [RFC5280].
254 * Removed precertificate signing certificates and the precertificate
255 poison extension: the change of precertificate format means that
256 these are no longer needed.
258 * Logs IDs: each log is now identified by an OID rather than by the
259 hash of its public key. OID allocations are managed by an IANA
260 registry.
262 * "TransItem" structure: this new data structure is used to
263 encapsulate most types of CT data. A "TransItemList", consisting
264 of one or more "TransItem" structures, can be used anywhere that
265 "SignedCertificateTimestampList" was used in [RFC6962].
267 * Merkle tree leaves: the "MerkleTreeLeaf" structure has been
268 replaced by the "TransItem" structure, which eases extensibility
269 and simplifies the leaf structure by removing one layer of
270 abstraction.
272 * Unified leaf format: the structure for both certificate and
273 precertificate entries now includes only the TBSCertificate
274 (whereas certificate entries in [RFC6962] included the entire
275 certificate).
277 * Log Artifact Extensions: these are now typed and managed by an
278 IANA registry, and they can now appear not only in SCTs but also
279 in STHs.
281 * API outputs: complete "TransItem" structures are returned, rather
282 than the constituent parts of each structure.
284 * get-all-by-hash: new client API for obtaining an inclusion proof
285 and the corresponding consistency proof at the same time.
287 * submit-entry: new client API, replacing add-chain and add-pre-
288 chain.
290 * Presenting SCTs with proofs: TLS servers may present SCTs together
291 with the corresponding inclusion proofs using any of the
292 mechanisms that [RFC6962] defined for presenting SCTs only.
293 (Presenting SCTs only is still supported).
295 * CT TLS extension: the "signed_certificate_timestamp" TLS extension
296 has been replaced by the "transparency_info" TLS extension.
298 * Verification algorithms: added detailed algorithms for verifying
299 inclusion proofs, for verifying consistency between two STHs, and
300 for verifying a root hash given a complete list of the relevant
301 leaf input entries.
303 * Extensive clarifications and editorial work.
305 2. Cryptographic Components
307 2.1. Merkle Hash Trees
309 A full description of Merkle Hash Tree is beyond the scope of this
310 document. Briefly, it is a binary tree where each non-leaf node is a
311 hash of its children. For CT, the number of children is at most two.
312 Additional information can be found in the Introduction and Reference
313 section of [RFC8391].
315 2.1.1. Definition of the Merkle Tree
317 The log uses a binary Merkle Hash Tree for efficient auditing. The
318 hash algorithm used is one of the log's parameters (see Section 4.1).
319 This document establishes a registry of acceptable hash algorithms
320 (see Section 10.2.1). Throughout this document, the hash algorithm
321 in use is referred to as HASH and the size of its output in bytes as
322 HASH_SIZE. The input to the Merkle Tree Hash is a list of data
323 entries; these entries will be hashed to form the leaves of the
324 Merkle Hash Tree. The output is a single HASH_SIZE Merkle Tree Hash.
325 Given an ordered list of n inputs, D_n = {d[0], d[1], ..., d[n-1]},
326 the Merkle Tree Hash (MTH) is thus defined as follows:
328 The hash of an empty list is the hash of an empty string:
330 MTH({}) = HASH().
332 The hash of a list with one entry (also known as a leaf hash) is:
334 MTH({d[0]}) = HASH(0x00 || d[0]).
336 For n > 1, let k be the largest power of two smaller than n (i.e., k
337 < n <= 2k). The Merkle Tree Hash of an n-element list D_n is then
338 defined recursively as
340 MTH(D_n) = HASH(0x01 || MTH(D[0:k]) || MTH(D[k:n])),
342 where:
344 * || denotes concatenation
346 * : denotes concatenation of lists
348 * D[k1:k2] = D'_(k2-k1) denotes the list {d'[0] = d[k1], d'[1] =
349 d[k1+1], ..., d'[k2-k1-1] = d[k2-1]} of length (k2 - k1).
351 Note that the hash calculations for leaves and nodes differ; this
352 domain separation is required to give second preimage resistance.
354 Note that we do not require the length of the input list to be a
355 power of two. The resulting Merkle Tree may thus not be balanced;
356 however, its shape is uniquely determined by the number of leaves.
357 (Note: This Merkle Tree is essentially the same as the history tree
358 [CrosbyWallach] proposal, except our definition handles non-full
359 trees differently).
361 2.1.2. Verifying a Tree Head Given Entries
363 When a client has a complete list of "entries" from "0" up to
364 "tree_size - 1" and wishes to verify this list against a tree head
365 "root_hash" returned by the log for the same "tree_size", the
366 following algorithm may be used:
368 1. Set "stack" to an empty stack.
370 2. For each "i" from "0" up to "tree_size - 1":
372 1. Push "HASH(0x00 || entries[i])" to "stack".
374 2. Set "merge_count" to the lowest value ("0" included) such
375 that "LSB(i >> merge_count)" is not set, where "LSB" means
376 the least significant bit. In other words, set "merge_count"
377 to the number of consecutive "1"s found starting at the least
378 significant bit of "i".
380 3. Repeat "merge_count" times:
382 1. Pop "right" from "stack".
384 2. Pop "left" from "stack".
386 3. Push "HASH(0x01 || left || right)" to "stack".
388 3. If there is more than one element in the "stack", repeat the same
389 merge procedure (the sub-items of Step 2.3 above) until only a
390 single element remains.
392 4. The remaining element in "stack" is the Merkle Tree hash for the
393 given "tree_size" and should be compared by equality against the
394 supplied "root_hash".
396 2.1.3. Merkle Inclusion Proofs
398 A Merkle inclusion proof for a leaf in a Merkle Hash Tree is the
399 shortest list of additional nodes in the Merkle Tree required to
400 compute the Merkle Tree Hash for that tree. Each node in the tree is
401 either a leaf node or is computed from the two nodes immediately
402 below it (i.e., towards the leaves). At each step up the tree
403 (towards the root), a node from the inclusion proof is combined with
404 the node computed so far. In other words, the inclusion proof
405 consists of the list of missing nodes required to compute the nodes
406 leading from a leaf to the root of the tree. If the root computed
407 from the inclusion proof matches the true root, then the inclusion
408 proof proves that the leaf exists in the tree.
410 2.1.3.1. Generating an Inclusion Proof
412 Given an ordered list of n inputs to the tree, D_n = {d[0], d[1],
413 ..., d[n-1]}, the Merkle inclusion proof PATH(m, D_n) for the (m+1)th
414 input d[m], 0 <= m < n, is defined as follows:
416 The proof for the single leaf in a tree with a one-element input list
417 D[1] = {d[0]} is empty:
419 PATH(0, {d[0]}) = {}
421 For n > 1, let k be the largest power of two smaller than n. The
422 proof for the (m+1)th element d[m] in a list of n > m elements is
423 then defined recursively as
425 PATH(m, D_n) = PATH(m, D[0:k]) : MTH(D[k:n]) for m < k; and
427 PATH(m, D_n) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for m >= k,
429 The : operator and D[k1:k2] are defined the same as in Section 2.1.1.
431 2.1.3.2. Verifying an Inclusion Proof
433 When a client has received an inclusion proof (e.g., in a "TransItem"
434 of type "inclusion_proof_v2") and wishes to verify inclusion of an
435 input "hash" for a given "tree_size" and "root_hash", the following
436 algorithm may be used to prove the "hash" was included in the
437 "root_hash":
439 1. Compare "leaf_index" from the "inclusion_proof_v2" structure
440 against "tree_size". If "leaf_index" is greater than or equal to
441 "tree_size" then fail the proof verification.
443 2. Set "fn" to "leaf_index" and "sn" to "tree_size - 1".
445 3. Set "r" to "hash".
447 4. For each value "p" in the "inclusion_path" array:
449 If "sn" is 0, stop the iteration and fail the proof verification.
451 If "LSB(fn)" is set, or if "fn" is equal to "sn", then:
453 1. Set "r" to "HASH(0x01 || p || r)"
455 2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn"
456 equally until either "LSB(fn)" is set or "fn" is "0".
458 Otherwise:
460 1. Set "r" to "HASH(0x01 || r || p)"
462 Finally, right-shift both "fn" and "sn" one time.
464 5. Compare "sn" to 0. Compare "r" against the "root_hash". If "sn"
465 is equal to 0, and "r" and the "root_hash" are equal, then the
466 log has proven the inclusion of "hash". Otherwise, fail the
467 proof verification.
469 2.1.4. Merkle Consistency Proofs
471 Merkle consistency proofs prove the append-only property of the tree.
472 A Merkle consistency proof for a Merkle Tree Hash MTH(D_n) and a
473 previously advertised hash MTH(D[0:m]) of the first m leaves, m <= n,
474 is the list of nodes in the Merkle Tree required to verify that the
475 first m inputs D[0:m] are equal in both trees. Thus, a consistency
476 proof must contain a set of intermediate nodes (i.e., commitments to
477 inputs) sufficient to verify MTH(D_n), such that (a subset of) the
478 same nodes can be used to verify MTH(D[0:m]). We define an algorithm
479 that outputs the (unique) minimal consistency proof.
481 2.1.4.1. Generating a Consistency Proof
483 Given an ordered list of n inputs to the tree, D_n = {d[0], d[1],
484 ..., d[n-1]}, the Merkle consistency proof PROOF(m, D_n) for a
485 previous Merkle Tree Hash MTH(D[0:m]), 0 < m < n, is defined as:
487 PROOF(m, D_n) = SUBPROOF(m, D_n, true)
489 In SUBPROOF, the boolean value represents whether the subtree created
490 from D[0:m] is a complete subtree of the Merkle Tree created from
491 D_n, and, consequently, whether the subtree Merkle Tree Hash
492 MTH(D[0:m]) is known. The initial call to SUBPROOF sets this to be
493 true, and SUBPROOF is then defined as follows:
495 The subproof for m = n is empty if m is the value for which PROOF was
496 originally requested (meaning that the subtree created from D[0:m] is
497 a complete subtree of the Merkle Tree created from the original D_n
498 for which PROOF was requested, and the subtree Merkle Tree Hash
499 MTH(D[0:m]) is known):
501 SUBPROOF(m, D_m, true) = {}
503 Otherwise, the subproof for m = n is the Merkle Tree Hash committing
504 inputs D[0:m]:
506 SUBPROOF(m, D_m, false) = {MTH(D_m)}
508 For m < n, let k be the largest power of two smaller than n. The
509 subproof is then defined recursively, using the appropriate step
510 below:
512 If m <= k, the right subtree entries D[k:n] only exist in the current
513 tree. We prove that the left subtree entries D[0:k] are consistent
514 and add a commitment to D[k:n]:
516 SUBPROOF(m, D_n, b) = SUBPROOF(m, D[0:k], b) : MTH(D[k:n])
517 If m > k, the left subtree entries D[0:k] are identical in both
518 trees. We prove that the right subtree entries D[k:n] are consistent
519 and add a commitment to D[0:k].
521 SUBPROOF(m, D_n, b) = SUBPROOF(m - k, D[k:n], false) : MTH(D[0:k])
523 The number of nodes in the resulting proof is bounded above by
524 ceil(log2(n)) + 1.
526 The : operator and D[k1:k2] are defined the same as in Section 2.1.1.
528 2.1.4.2. Verifying Consistency between Two Tree Heads
530 When a client has a tree head "first_hash" for tree size "first", a
531 tree head "second_hash" for tree size "second" where "0 < first <
532 second", and has received a consistency proof between the two (e.g.,
533 in a "TransItem" of type "consistency_proof_v2"), the following
534 algorithm may be used to verify the consistency proof:
536 1. If "consistency_path" is an empty array, stop and fail the proof
537 verification.
539 2. If "first" is an exact power of 2, then prepend "first_hash" to
540 the "consistency_path" array.
542 3. Set "fn" to "first - 1" and "sn" to "second - 1".
544 4. If "LSB(fn)" is set, then right-shift both "fn" and "sn" equally
545 until "LSB(fn)" is not set.
547 5. Set both "fr" and "sr" to the first value in the
548 "consistency_path" array.
550 6. For each subsequent value "c" in the "consistency_path" array:
552 If "sn" is 0, stop the iteration and fail the proof verification.
554 If "LSB(fn)" is set, or if "fn" is equal to "sn", then:
556 1. Set "fr" to "HASH(0x01 || c || fr)"
558 Set "sr" to "HASH(0x01 || c || sr)"
560 2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn"
561 equally until either "LSB(fn)" is set or "fn" is "0".
563 Otherwise:
565 1. Set "sr" to "HASH(0x01 || sr || c)"
567 Finally, right-shift both "fn" and "sn" one time.
569 7. After completing iterating through the "consistency_path" array
570 as described above, verify that the "fr" calculated is equal to
571 the "first_hash" supplied, that the "sr" calculated is equal to
572 the "second_hash" supplied and that "sn" is 0.
574 2.1.5. Example
576 The binary Merkle Tree with 7 leaves:
578 hash
579 / \
580 / \
581 / \
582 / \
583 / \
584 k l
585 / \ / \
586 / \ / \
587 / \ / \
588 g h i j
589 / \ / \ / \ |
590 a b c d e f d6
591 | | | | | |
592 d0 d1 d2 d3 d4 d5
594 The inclusion proof for d0 is [b, h, l].
596 The inclusion proof for d3 is [c, g, l].
598 The inclusion proof for d4 is [f, j, k].
600 The inclusion proof for d6 is [i, k].
602 The same tree, built incrementally in four steps:
604 hash0 hash1=k
605 / \ / \
606 / \ / \
607 / \ / \
608 g c g h
609 / \ | / \ / \
610 a b d2 a b c d
611 | | | | | |
612 d0 d1 d0 d1 d2 d3
614 hash2 hash
615 / \ / \
616 / \ / \
617 / \ / \
618 / \ / \
619 / \ / \
620 k i k l
621 / \ / \ / \ / \
622 / \ e f / \ / \
623 / \ | | / \ / \
624 g h d4 d5 g h i j
625 / \ / \ / \ / \ / \ |
626 a b c d a b c d e f d6
627 | | | | | | | | | |
628 d0 d1 d2 d3 d0 d1 d2 d3 d4 d5
630 The consistency proof between hash0 and hash is PROOF(3, D[7]) = [c,
631 d, g, l]. c, g are used to verify hash0, and d, l are additionally
632 used to show hash is consistent with hash0.
634 The consistency proof between hash1 and hash is PROOF(4, D[7]) = [l].
635 hash can be verified using hash1=k and l.
637 The consistency proof between hash2 and hash is PROOF(6, D[7]) = [i,
638 j, k]. k, i are used to verify hash2, and j is additionally used to
639 show hash is consistent with hash2.
641 2.2. Signatures
643 When signing data structures, a log MUST use one of the signature
644 algorithms from the IANA CT Signature Algorithms registry, described
645 in Section 10.2.2.
647 3. Submitters
649 Submitters submit certificates or preannouncements of certificates
650 prior to issuance (precertificates) to logs for public auditing, as
651 described below. In order to enable attribution of each logged
652 certificate or precertificate to its issuer, each submission MUST be
653 accompanied by all additional certificates required to verify the
654 chain up to an accepted trust anchor (Section 5.7). The trust anchor
655 (a root or intermediate CA certificate) MAY be omitted from the
656 submission.
658 If a log accepts a submission, it will return a Signed Certificate
659 Timestamp (SCT) (see Section 4.8). The submitter SHOULD validate the
660 returned SCT as described in Section 8.1 if they understand its
661 format and they intend to use it directly in a TLS handshake or to
662 construct a certificate. If the submitter does not need the SCT (for
663 example, the certificate is being submitted simply to make it
664 available in the log), it MAY validate the SCT.
666 3.1. Certificates
668 Any entity can submit a certificate (Section 5.1) to a log. Since it
669 is anticipated that TLS clients will reject certificates that are not
670 logged, it is expected that certificate issuers and subjects will be
671 strongly motivated to submit them.
673 3.2. Precertificates
675 CAs may preannounce a certificate prior to issuance by submitting a
676 precertificate (Section 5.1) that the log can use to create an entry
677 that will be valid against the issued certificate. The CA MAY
678 incorporate the returned SCT in the issued certificate. One example
679 of where the returned SCT is not incorporated in the issued
680 certificate is when a CA sends the precertificate to multiple logs,
681 but only incorporates the SCTs that are returned first.
683 A precertificate is a CMS [RFC5652] "signed-data" object that
684 conforms to the following profile:
686 * It MUST be DER encoded as described in [X690].
688 * "SignedData.version" MUST be v3(3).
690 * "SignedData.digestAlgorithms" MUST be the same as the
691 "SignerInfo.digestAlgorithm" OID value (see below).
693 * "SignedData.encapContentInfo":
695 - "eContentType" MUST be the OID 1.3.101.78.
697 - "eContent" MUST contain a TBSCertificate [RFC5280] that will be
698 identical to the TBSCertificate in the issued certificate,
699 except that the Transparency Information (Section 7.1)
700 extension MUST be omitted.
702 * "SignedData.certificates" MUST be omitted.
704 * "SignedData.crls" MUST be omitted.
706 * "SignedData.signerInfos" MUST contain one "SignerInfo":
708 - "version" MUST be v3(3).
710 - "sid" MUST use the "subjectKeyIdentifier" option.
712 - "digestAlgorithm" MUST be one of the hash algorithm OIDs listed
713 in the IANA CT Hash Algorithms Registry, described in
714 Section 10.2.1.
716 - "signedAttrs" MUST be present and MUST contain two attributes:
718 o A content-type attribute whose value is the same as
719 "SignedData.encapContentInfo.eContentType".
721 o A message-digest attribute whose value is the message digest
722 of "SignedData.encapContentInfo.eContent".
724 - "signatureAlgorithm" MUST be the same OID as
725 "TBSCertificate.signature".
727 - "signature" MUST be from the same (root or intermediate) CA
728 that intends to issue the corresponding certificate (see
729 Section 3.2.1).
731 - "unsignedAttrs" MUST be omitted.
733 "SignerInfo.signedAttrs" is included in the message digest
734 calculation process (see Section 5.4 of [RFC5652]), which ensures
735 that the "SignerInfo.signature" value will not be a valid X.509v3
736 signature that could be used in conjunction with the TBSCertificate
737 (from "SignedData.encapContentInfo.eContent") to construct a valid
738 certificate.
740 3.2.1. Binding Intent to Issue
742 Under normal circumstances, there will be a short delay between
743 precertificate submission and issuance of the corresponding
744 certificate. Longer delays are to be expected occasionally (e.g.,
745 due to log server downtime), and in some cases the CA might not
746 actually issue the corresponding certificate. Nevertheless, a
747 precertificate's "signature" indicates the CA's binding intent to
748 issue the corresponding certificate, which means that:
750 * Misissuance of a precertificate is considered equivalent to
751 misissuance of the corresponding certificate. The CA should
752 expect to be held to account, even if the corresponding
753 certificate has not actually been issued.
755 * Upon observing a precertificate, a client can reasonably presume
756 that the corresponding certificate has been issued. A client may
757 wish to obtain status information (e.g., by using the Online
758 Certificate Status Protocol [RFC6960] or by checking a Certificate
759 Revocation List [RFC5280]) about a certificate that is presumed to
760 exist, especially if there is evidence or suspicion that the
761 corresponding precertificate was misissued.
763 * TLS clients may have policies that require CAs to be able to
764 revoke, and to provide certificate status services for, each
765 certificate that is presumed to exist based on the existence of a
766 corresponding precertificate.
768 4. Log Format and Operation
770 A log is a single, append-only Merkle Tree of submitted certificate
771 and precertificate entries.
773 When it receives and accepts a valid submission, the log MUST return
774 an SCT that corresponds to the submitted certificate or
775 precertificate. If the log has previously seen this valid
776 submission, it SHOULD return the same SCT as it returned before, as
777 discussed in Section 11.3. If different SCTs are produced for the
778 same submission, multiple log entries will have to be created, one
779 for each SCT (as the timestamp is a part of the leaf structure).
780 Note that if a certificate was previously logged as a precertificate,
781 then the precertificate's SCT of type "precert_sct_v2" would not be
782 appropriate; instead, a fresh SCT of type "x509_sct_v2" should be
783 generated.
785 An SCT is the log's promise to append to its Merkle Tree an entry for
786 the accepted submission. Upon producing an SCT, the log MUST fulfil
787 this promise by performing the following actions within a fixed
788 amount of time known as the Maximum Merge Delay (MMD), which is one
789 of the log's parameters (see Section 4.1):
791 * Allocate a tree index to the entry representing the accepted
792 submission.
794 * Calculate the root of the tree.
796 * Sign the root of the tree (see Section 4.10).
798 The log may append multiple entries before signing the root of the
799 tree.
801 Log operators SHOULD NOT impose any conditions on retrieving or
802 sharing data from the log.
804 4.1. Log Parameters
806 A log is defined by a collection of immutable parameters, which are
807 used by clients to communicate with the log and to verify log
808 artifacts. Except for the Final Signed Tree Head (STH), each of
809 these parameters MUST be established before the log operator begins
810 to operate the log.
812 Base URL: The prefix used to construct URLs ([RFC3986]) for client
813 messages (see Section 5). The base URL MUST be an "https" URL,
814 MAY contain a port, MAY contain a path with any number of path
815 segments, but MUST NOT contain a query string, fragment, or
816 trailing "/". Example: https://ct.example.org/blue
818 Hash Algorithm: The hash algorithm used for the Merkle Tree (see
819 Section 10.2.1).
821 Signature Algorithm: The signature algorithm used (see Section 2.2).
823 Public Key: The public key used to verify signatures generated by
824 the log. A log MUST NOT use the same keypair as any other log.
826 Log ID: The OID that uniquely identifies the log.
828 Maximum Merge Delay: The MMD the log has committed to. This
829 document deliberately does not specify any limits on the value, to
830 allow for experimentation.
832 Version: The version of the protocol supported by the log (currently
833 1 or 2).
835 Maximum Chain Length: The longest certificate chain submission the
836 log is willing to accept, if the log imposes any limit.
838 STH Frequency Count: The maximum number of STHs the log may produce
839 in any period equal to the "Maximum Merge Delay" (see
840 Section 4.10).
842 Final STH: If a log has been closed down (i.e., no longer accepts
843 new entries), existing entries may still be valid. In this case,
844 the client should know the final valid STH in the log to ensure no
845 new entries can be added without detection. This value MUST be
846 provided in the form of a TransItem of type "signed_tree_head_v2".
847 If a log is still accepting entries, this value should not be
848 provided.
850 [JSON.Metadata] is an example of a metadata format which includes the
851 above elements.
853 4.2. Evaluating Submissions
855 A log determines whether to accept or reject a submission by
856 evaluating it against the minimum acceptance criteria (see
857 Section 4.2.1) and against the log's discretionary acceptance
858 criteria (see Section 4.2.2).
860 If the acceptance criteria are met, the log SHOULD accept the
861 submission. (A log may decide, for example, to temporarily reject
862 acceptable submissions to protect itself against denial-of-service
863 attacks).
865 The log SHALL allow retrieval of its list of accepted trust anchors
866 (see Section 5.7), each of which is a root or intermediate CA
867 certificate. This list might usefully be the union of root
868 certificates trusted by major browser vendors.
870 4.2.1. Minimum Acceptance Criteria
872 To ensure that logged certificates and precertificates are
873 attributable to an accepted trust anchor, and to set clear
874 expectations for what monitors would find in the log, and to avoid
875 being overloaded by invalid submissions, the log MUST reject a
876 submission if any of the following conditions are not met:
878 * The "submission", "type" and "chain" inputs MUST be set as
879 described in Section 5.1. The log MUST NOT accommodate misordered
880 CA certificates or use any other source of intermediate CA
881 certificates to attempt certification path construction.
883 * Each of the zero or more intermediate CA certificates in the chain
884 MUST have one or both of the following features:
886 - The Basic Constraints extension with the cA boolean asserted.
888 - The Key Usage extension with the keyCertSign bit asserted.
890 * Each certificate in the chain MUST fall within the limits imposed
891 by the zero or more Basic Constraints pathLenConstraint values
892 found higher up the chain.
894 * Precertificate submissions MUST conform to all of the requirements
895 in Section 3.2.
897 4.2.2. Discretionary Acceptance Criteria
899 If the minimum acceptance criteria are met but the submission is not
900 fully valid according to [RFC5280] verification rules (e.g., the
901 certificate or precertificate has expired, is not yet valid, has been
902 revoked, exhibits ASN.1 DER encoding errors but the log can still
903 parse it, etc), then the acceptability of the submission is left to
904 the log's discretion. It is useful for logs to accept such
905 submissions in order to accommodate quirks of CA certificate-issuing
906 software and to facilitate monitoring of CA compliance with
907 applicable policies and technical standards. However, it is
908 impractical for this document to enumerate, and for logs to consider,
909 all of the ways that a submission might fail to comply with
910 [RFC5280].
912 Logs SHOULD limit the length of chain they will accept. The maximum
913 chain length is one of the log's parameters (see Section 4.1).
915 4.3. Log Entries
917 If a submission is accepted and an SCT issued, the accepting log MUST
918 store the entire chain used for verification. This chain MUST
919 include the certificate or precertificate itself, the zero or more
920 intermediate CA certificates provided by the submitter, and the trust
921 anchor used to verify the chain (even if it was omitted from the
922 submission). The log MUST provide this chain for auditing upon
923 request (see Section 5.6) so that the CA cannot avoid blame by
924 logging a partial or empty chain. Each log entry is a "TransItem"
925 structure of type "x509_entry_v2" or "precert_entry_v2". However, a
926 log may store its entries in any format. If a log does not store
927 this "TransItem" in full, it must store the "timestamp" and
928 "sct_extensions" of the corresponding
929 "TimestampedCertificateEntryDataV2" structure. The "TransItem" can
930 be reconstructed from these fields and the entire chain that the log
931 used to verify the submission.
933 4.4. Log ID
935 Each log is identified by an OID, which is one of the log's
936 parameters (see Section 4.1) and which MUST NOT be used to identify
937 any other log. A log's operator MUST either allocate the OID
938 themselves or request an OID from the Log ID Registry (see
939 Section 10.2.5). The only advantage of the registry is that the DER
940 encoding can be small. (Recall that OID allocations do not require a
941 central registration, although logs will most likely want to make
942 themselves known to potential clients through out of band means.)
943 Various data structures include the DER encoding of this OID,
944 excluding the ASN.1 tag and length bytes, in an opaque vector:
946 opaque LogID<2..127>;
948 Note that the ASN.1 length and the opaque vector length are identical
949 in size (1 byte) and value, so the full DER encoding (including the
950 tag and length) of the OID can be reproduced simply by prepending an
951 OBJECT IDENTIFIER tag (0x06) to the opaque vector length and
952 contents.
954 The OID used to identify a log is limited such that the DER encoding
955 of its value, excluding the tag and length, MUST be no longer than
956 127 octets.
958 4.5. TransItem Structure
960 Various data structures are encapsulated in the "TransItem" structure
961 to ensure that the type and version of each one is identified in a
962 common fashion:
964 enum {
965 reserved(0),
966 x509_entry_v2(1), precert_entry_v2(2),
967 x509_sct_v2(3), precert_sct_v2(4),
968 signed_tree_head_v2(5), consistency_proof_v2(6),
969 inclusion_proof_v2(7),
970 (65535)
971 } VersionedTransType;
973 struct {
974 VersionedTransType versioned_type;
975 select (versioned_type) {
976 case x509_entry_v2: TimestampedCertificateEntryDataV2;
977 case precert_entry_v2: TimestampedCertificateEntryDataV2;
978 case x509_sct_v2: SignedCertificateTimestampDataV2;
979 case precert_sct_v2: SignedCertificateTimestampDataV2;
980 case signed_tree_head_v2: SignedTreeHeadDataV2;
981 case consistency_proof_v2: ConsistencyProofDataV2;
982 case inclusion_proof_v2: InclusionProofDataV2;
983 } data;
984 } TransItem;
986 "versioned_type" is a value from the IANA registry in Section 10.2.3
987 that identifies the type of the encapsulated data structure and the
988 earliest version of this protocol to which it conforms. This
989 document is v2.
991 "data" is the encapsulated data structure. The various structures
992 named with the "DataV2" suffix are defined in later sections of this
993 document.
995 Note that "VersionedTransType" combines the v1 [RFC6962] type
996 enumerations "Version", "LogEntryType", "SignatureType" and
997 "MerkleLeafType". Note also that v1 did not define "TransItem", but
998 this document provides guidelines (see Appendix A) on how v2
999 implementations can co-exist with v1 implementations.
1001 Future versions of this protocol may reuse "VersionedTransType"
1002 values defined in this document as long as the corresponding data
1003 structures are not modified, and may add new "VersionedTransType"
1004 values for new or modified data structures.
1006 4.6. Log Artifact Extensions
1007 enum {
1008 reserved(65535)
1009 } ExtensionType;
1011 struct {
1012 ExtensionType extension_type;
1013 opaque extension_data<0..2^16-1>;
1014 } Extension;
1016 The "Extension" structure provides a generic extensibility for log
1017 artifacts, including Signed Certificate Timestamps (Section 4.8) and
1018 Signed Tree Heads (Section 4.10). The interpretation of the
1019 "extension_data" field is determined solely by the value of the
1020 "extension_type" field.
1022 This document does not define any extensions, but it does establish a
1023 registry for future "ExtensionType" values (see Section 10.2.4).
1024 Each document that registers a new "ExtensionType" must specify the
1025 context in which it may be used (e.g., SCT, STH, or both) and
1026 describe how to interpret the corresponding "extension_data".
1028 4.7. Merkle Tree Leaves
1030 The leaves of a log's Merkle Tree correspond to the log's entries
1031 (see Section 4.3). Each leaf is the leaf hash (Section 2.1) of a
1032 "TransItem" structure of type "x509_entry_v2" or "precert_entry_v2",
1033 which encapsulates a "TimestampedCertificateEntryDataV2" structure.
1034 Note that leaf hashes are calculated as HASH(0x00 || TransItem),
1035 where the hash algorithm is one of the log's parameters.
1037 opaque TBSCertificate<1..2^24-1>;
1039 struct {
1040 uint64 timestamp;
1041 opaque issuer_key_hash<32..2^8-1>;
1042 TBSCertificate tbs_certificate;
1043 Extension sct_extensions<0..2^16-1>;
1044 } TimestampedCertificateEntryDataV2;
1046 "timestamp" is the date and time at which the certificate or
1047 precertificate was accepted by the log, in the form of a 64-bit
1048 unsigned number of milliseconds elapsed since the Unix Epoch (1
1049 January 1970 00:00:00 UTC - see [UNIXTIME]), ignoring leap seconds,
1050 in network byte order. Note that the leaves of a log's Merkle Tree
1051 are not required to be in strict chronological order.
1053 "issuer_key_hash" is the HASH of the public key of the CA that issued
1054 the certificate or precertificate, calculated over the DER encoding
1055 of the key represented as SubjectPublicKeyInfo [RFC5280]. This is
1056 needed to bind the CA to the certificate or precertificate, making it
1057 impossible for the corresponding SCT to be valid for any other
1058 certificate or precertificate whose TBSCertificate matches
1059 "tbs_certificate". The length of the "issuer_key_hash" MUST match
1060 HASH_SIZE.
1062 "tbs_certificate" is the DER encoded TBSCertificate from the
1063 submission. (Note that a precertificate's TBSCertificate can be
1064 reconstructed from the corresponding certificate as described in
1065 Section 8.1.2).
1067 "sct_extensions" is byte-for-byte identical to the SCT extensions of
1068 the corresponding SCT.
1070 The type of the "TransItem" corresponds to the value of the "type"
1071 parameter supplied in the Section 5.1 call.
1073 4.8. Signed Certificate Timestamp (SCT)
1075 An SCT is a "TransItem" structure of type "x509_sct_v2" or
1076 "precert_sct_v2", which encapsulates a
1077 "SignedCertificateTimestampDataV2" structure:
1079 struct {
1080 LogID log_id;
1081 uint64 timestamp;
1082 Extension sct_extensions<0..2^16-1>;
1083 opaque signature<1..2^16-1>;
1084 } SignedCertificateTimestampDataV2;
1086 "log_id" is this log's unique ID, encoded in an opaque vector as
1087 described in Section 4.4.
1089 "timestamp" is equal to the timestamp from the corresponding
1090 "TimestampedCertificateEntryDataV2" structure.
1092 "sct_extensions" is a vector of 0 or more SCT extensions. This
1093 vector MUST NOT include more than one extension with the same
1094 "extension_type". The extensions in the vector MUST be ordered by
1095 the value of the "extension_type" field, smallest value first. All
1096 SCT extensions are similar to non-critical X.509v3 extensions (i.e.,
1097 the "mustUnderstand" field is not set), and a recipient SHOULD ignore
1098 any extension it does not understand. Furthermore, an implementation
1099 MAY choose to ignore any extension(s) that it does understand.
1101 "signature" is computed over a "TransItem" structure of type
1102 "x509_entry_v2" or "precert_entry_v2" (see Section 4.7) using the
1103 signature algorithm declared in the log's parameters (see
1104 Section 4.1).
1106 4.9. Merkle Tree Head
1108 The log stores information about its Merkle Tree in a
1109 "TreeHeadDataV2":
1111 opaque NodeHash<32..2^8-1>;
1113 struct {
1114 uint64 timestamp;
1115 uint64 tree_size;
1116 NodeHash root_hash;
1117 Extension sth_extensions<0..2^16-1>;
1118 } TreeHeadDataV2;
1120 The length of NodeHash MUST match HASH_SIZE of the log.
1122 "timestamp" is the current date and time, using the format defined in
1123 {tree_leaves}.
1125 "tree_size" is the number of entries currently in the log's Merkle
1126 Tree.
1128 "root_hash" is the root of the Merkle Hash Tree.
1130 "sth_extensions" is a vector of 0 or more STH extensions. This
1131 vector MUST NOT include more than one extension with the same
1132 "extension_type". The extensions in the vector MUST be ordered by
1133 the value of the "extension_type" field, smallest value first. If an
1134 implementation sees an extension that it does not understand, it
1135 SHOULD ignore that extension. Furthermore, an implementation MAY
1136 choose to ignore any extension(s) that it does understand.
1138 4.10. Signed Tree Head (STH)
1140 Periodically each log SHOULD sign its current tree head information
1141 (see Section 4.9) to produce an STH. When a client requests a log's
1142 latest STH (see Section 5.2), the log MUST return an STH that is no
1143 older than the log's MMD. However, since STHs could be used to mark
1144 individual clients (by producing a new STH for each query), a log
1145 MUST NOT produce STHs more frequently than its parameters declare
1146 (see Section 4.1). In general, there is no need to produce a new STH
1147 unless there are new entries in the log; however, in the event that a
1148 log does not accept any submissions during an MMD period, the log
1149 MUST sign the same Merkle Tree Hash with a fresh timestamp.
1151 An STH is a "TransItem" structure of type "signed_tree_head_v2",
1152 which encapsulates a "SignedTreeHeadDataV2" structure:
1154 struct {
1155 LogID log_id;
1156 TreeHeadDataV2 tree_head;
1157 opaque signature<0..2^16-1>;
1158 } SignedTreeHeadDataV2;
1160 "log_id" is this log's unique ID, encoded in an opaque vector as
1161 described in Section 4.4.
1163 The "timestamp" in "tree_head" MUST be at least as recent as the most
1164 recent SCT timestamp in the tree. Each subsequent timestamp MUST be
1165 more recent than the timestamp of the previous update.
1167 "tree_head" contains the latest tree head information (see
1168 Section 4.9).
1170 "signature" is computed over the "tree_head" field using the
1171 signature algorithm declared in the log's parameters (see
1172 Section 4.1).
1174 4.11. Merkle Consistency Proofs
1176 To prepare a Merkle Consistency Proof for distribution to clients,
1177 the log produces a "TransItem" structure of type
1178 "consistency_proof_v2", which encapsulates a "ConsistencyProofDataV2"
1179 structure:
1181 struct {
1182 LogID log_id;
1183 uint64 tree_size_1;
1184 uint64 tree_size_2;
1185 NodeHash consistency_path<0..2^16-1>;
1186 } ConsistencyProofDataV2;
1188 "log_id" is this log's unique ID, encoded in an opaque vector as
1189 described in Section 4.4.
1191 "tree_size_1" is the size of the older tree.
1193 "tree_size_2" is the size of the newer tree.
1195 "consistency_path" is a vector of Merkle Tree nodes proving the
1196 consistency of two STHs as described in {consistency}.
1198 4.12. Merkle Inclusion Proofs
1200 To prepare a Merkle Inclusion Proof for distribution to clients, the
1201 log produces a "TransItem" structure of type "inclusion_proof_v2",
1202 which encapsulates an "InclusionProofDataV2" structure:
1204 struct {
1205 LogID log_id;
1206 uint64 tree_size;
1207 uint64 leaf_index;
1208 NodeHash inclusion_path<0..2^16-1>;
1209 } InclusionProofDataV2;
1211 "log_id" is this log's unique ID, encoded in an opaque vector as
1212 described in Section 4.4.
1214 "tree_size" is the size of the tree on which this inclusion proof is
1215 based.
1217 "leaf_index" is the 0-based index of the log entry corresponding to
1218 this inclusion proof.
1220 "inclusion_path" is a vector of Merkle Tree nodes proving the
1221 inclusion of the chosen certificate or precertificate as described in
1222 {merkle_inclusion_proof}.
1224 4.13. Shutting down a log
1226 Log operators may decide to shut down a log for various reasons, such
1227 as deprecation of the signature algorithm. If there are entries in
1228 the log for certificates that have not yet expired, simply making TLS
1229 clients stop recognizing that log will have the effect of
1230 invalidating SCTs from that log. In order to avoid that, the
1231 following actions SHOULD be taken:
1233 * Make it known to clients and monitors that the log will be frozen.
1234 This is not part of the API, so it will have to be done via a
1235 relevant out-of-band mechanism.
1237 * Stop accepting new submissions (the error code "shutdown" should
1238 be returned for such requests).
1240 * Once MMD from the last accepted submission has passed and all
1241 pending submissions are incorporated, issue a final STH and
1242 publish it as one of the log's parameters. Having an STH with a
1243 timestamp that is after the MMD has passed from the last SCT
1244 issuance allows clients to audit this log regularly without
1245 special handling for the final STH. At this point the log's
1246 private key is no longer needed and can be destroyed.
1248 * Keep the log running until the certificates in all of its entries
1249 have expired or exist in other logs (this can be determined by
1250 scanning other logs or connecting to domains mentioned in the
1251 certificates and inspecting the SCTs served).
1253 5. Log Client Messages
1255 Messages are sent as HTTPS GET or POST requests. Parameters for
1256 POSTs and all responses are encoded as JavaScript Object Notation
1257 (JSON) objects [RFC8259]. Parameters for GETs are encoded as order-
1258 independent key/value URL parameters, using the "application/x-www-
1259 form-urlencoded" format described in the "HTML 4.01 Specification"
1260 [HTML401]. Binary data is base64 encoded according to section 4 of
1261 [RFC4648] as specified in the individual messages.
1263 Clients are configured with a log's base URL, which is one of the
1264 log's parameters. Clients construct URLs for requests by appending
1265 suffixes to this base URL. This structure places some degree of
1266 restriction on how log operators can deploy these services, as noted
1267 in [RFC8820]. However, operational experience with version 1 of this
1268 protocol has not indicated that these restrictions are a problem in
1269 practice.
1271 Note that JSON objects and URL parameters may contain fields not
1272 specified here, to allow for experimentation. Any fields that are
1273 not understood SHOULD be ignored.
1275 In practice, log servers may include multiple front-end machines.
1276 Since it is impractical to keep these machines in perfect sync,
1277 errors may occur that are caused by skew between the machines. Where
1278 such errors are possible, the front-end will return additional
1279 information (as specified below) making it possible for clients to
1280 make progress, if progress is possible. Front-ends MUST only serve
1281 data that is free of gaps (that is, for example, no front-end will
1282 respond with an STH unless it is also able to prove consistency from
1283 all log entries logged within that STH).
1285 For example, when a consistency proof between two STHs is requested,
1286 the front-end reached may not yet be aware of one or both STHs. In
1287 the case where it is unaware of both, it will return the latest STH
1288 it is aware of. Where it is aware of the first but not the second,
1289 it will return the latest STH it is aware of and a consistency proof
1290 from the first STH to the returned STH. The case where it knows the
1291 second but not the first should not arise (see the "no gaps"
1292 requirement above).
1294 If the log is unable to process a client's request, it MUST return an
1295 HTTP response code of 4xx/5xx (see [RFC7231]), and, in place of the
1296 responses outlined in the subsections below, the body SHOULD be a
1297 JSON Problem Details Object (see [RFC7807] Section 3), containing:
1299 type: A URN reference identifying the problem. To facilitate
1300 automated response to errors, this document defines a set of
1301 standard tokens for use in the "type" field, within the URN
1302 namespace of: "urn:ietf:params:trans:error:".
1304 detail: A human-readable string describing the error that prevented
1305 the log from processing the request, ideally with sufficient
1306 detail to enable the error to be rectified.
1308 e.g., In response to a request of "/ct/v2/get-
1309 entries?start=100&end=99", the log would return a "400 Bad Request"
1310 response code with a body similar to the following:
1312 {
1313 "type": "urn:ietf:params:trans:error:endBeforeStart",
1314 "detail": "'start' cannot be greater than 'end'"
1315 }
1317 Most error types are specific to the type of request and are defined
1318 in the respective subsections below. The one exception is the
1319 "malformed" error type, which indicates that the log server could not
1320 parse the client's request because it did not comply with this
1321 document:
1323 +===========+==================================+
1324 | type | detail |
1325 +===========+==================================+
1326 | malformed | The request could not be parsed. |
1327 +-----------+----------------------------------+
1329 Table 1
1331 Clients SHOULD treat "500 Internal Server Error" and "503 Service
1332 Unavailable" responses as transient failures and MAY retry the same
1333 request without modification at a later date. Note that as per
1334 [RFC7231], in the case of a 503 response the log MAY include a
1335 "Retry-After:" header in order to request a minimum time for the
1336 client to wait before retrying the request. In the absence of this
1337 header, this document does not specify a minimum.
1339 Clients SHOULD treat any 4xx error as a problem with the request and
1340 not attempt to resubmit without some modification to the request.
1341 The full status code MAY provide additional details.
1343 This document deliberately does not provide more specific guidance on
1344 the use of HTTP status codes.
1346 5.1. Submit Entry to Log
1348 POST /ct/v2/submit-entry
1350 Inputs: submission: The base64 encoded certificate or
1351 precertificate.
1353 type: The "VersionedTransType" integer value that indicates
1354 the type of the "submission": 1 for "x509_entry_v2", or 2 for
1355 "precert_entry_v2".
1357 chain: An array of zero or more JSON strings, each of which
1358 is a base64 encoded CA certificate. The first element is the
1359 certifier of the "submission"; the second certifies the first;
1360 etc. The last element of "chain" (or, if "chain" is an empty
1361 array, the "submission") is certified by an accepted trust
1362 anchor.
1364 Outputs: sct: A base64 encoded "TransItem" of type "x509_sct_v2" or
1365 "precert_sct_v2", signed by this log, that corresponds to the
1366 "submission".
1368 If the submitted entry is immediately appended to (or already
1369 exists in) this log's tree, then the log SHOULD also output:
1371 sth: A base64 encoded "TransItem" of type "signed_tree_head_v2",
1372 signed by this log.
1374 inclusion: A base64 encoded "TransItem" of type
1375 "inclusion_proof_v2" whose "inclusion_path" array of Merkle
1376 Tree nodes proves the inclusion of the "submission" in the
1377 returned "sth".
1379 Error codes:
1381 +================+==============================================+
1382 | type | detail |
1383 +================+==============================================+
1384 | badSubmission | "submission" is neither a valid certificate |
1385 | | nor a valid precertificate. |
1386 +----------------+----------------------------------------------+
1387 | badType | "type" is neither 1 nor 2. |
1388 +----------------+----------------------------------------------+
1389 | badChain | The first element of "chain" is not the |
1390 | | certifier of the "submission", or the second |
1391 | | element does not certify the first, etc. |
1392 +----------------+----------------------------------------------+
1393 | badCertificate | One or more certificates in the "chain" are |
1394 | | not valid (e.g., not properly encoded). |
1395 +----------------+----------------------------------------------+
1396 | unknownAnchor | The last element of "chain" (or, if "chain" |
1397 | | is an empty array, the "submission") both is |
1398 | | not, and is not certified by, an accepted |
1399 | | trust anchor. |
1400 +----------------+----------------------------------------------+
1401 | shutdown | The log is no longer accepting submissions. |
1402 +----------------+----------------------------------------------+
1404 Table 2
1406 If the version of "sct" is not v2, then a v2 client may be unable to
1407 verify the signature. It MUST NOT construe this as an error. This
1408 is to avoid forcing an upgrade of compliant v2 clients that do not
1409 use the returned SCTs.
1411 If a log detects bad encoding in a chain that otherwise verifies
1412 correctly then the log MUST either log the certificate or return the
1413 "bad certificate" error. If the certificate is logged, an SCT MUST
1414 be issued. Logging the certificate is useful, because monitors
1415 (Section 8.2) can then detect these encoding errors, which may be
1416 accepted by some TLS clients.
1418 If "submission" is an accepted trust anchor whose certifier is
1419 neither an accepted trust anchor nor the first element of "chain",
1420 then the log MUST return the "unknown anchor" error. A log is not
1421 able to generate an SCT for a submission if it does not have access
1422 to the issuer's public key.
1424 If the returned "sct" is intended to be provided to TLS clients, then
1425 "sth" and "inclusion" (if returned) SHOULD also be provided to TLS
1426 clients. For example, if "type" was 2 (indicating "precert_sct_v2")
1427 then all three "TransItem"s could be embedded in the certificate.
1429 5.2. Retrieve Latest Signed Tree Head
1431 GET /ct/v2/get-sth
1433 No inputs.
1435 Outputs: sth: A base64 encoded "TransItem" of type
1436 "signed_tree_head_v2", signed by this log, that is no older
1437 than the log's MMD.
1439 5.3. Retrieve Merkle Consistency Proof between Two Signed Tree Heads
1441 GET /ct/v2/get-sth-consistency
1443 Inputs: first: The tree_size of the older tree, in decimal.
1445 second: The tree_size of the newer tree, in decimal
1446 (optional).
1448 Both tree sizes must be from existing v2 STHs. However, because
1449 of skew, the receiving front-end may not know one or both of the
1450 existing STHs. If both are known, then only the "consistency"
1451 output is returned. If the first is known but the second is not
1452 (or has been omitted), then the latest known STH is returned,
1453 along with a consistency proof between the first STH and the
1454 latest. If neither are known, then the latest known STH is
1455 returned without a consistency proof.
1457 Outputs: consistency: A base64 encoded "TransItem" of type
1458 "consistency_proof_v2", whose "tree_size_1" MUST match the
1459 "first" input. If the "sth" output is omitted, then
1460 "tree_size_2" MUST match the "second" input. If "first" and
1461 "second" are equal and correspond to a known STH, the returned
1462 consistency proof MUST be empty (a "consistency_path" array
1463 with zero elements).
1465 sth: A base64 encoded "TransItem" of type
1466 "signed_tree_head_v2", signed by this log.
1468 Note that no signature is required for the "consistency" output as
1469 it is used to verify the consistency between two STHs, which are
1470 signed.
1472 Error codes:
1474 +===================+======================================+
1475 | type | detail |
1476 +===================+======================================+
1477 | firstUnknown | "first" is before the latest known |
1478 | | STH but is not from an existing STH. |
1479 +-------------------+--------------------------------------+
1480 | secondUnknown | "second" is before the latest known |
1481 | | STH but is not from an existing STH. |
1482 +-------------------+--------------------------------------+
1483 | secondBeforeFirst | "second" is smaller than "first". |
1484 +-------------------+--------------------------------------+
1486 Table 3
1488 See Section 2.1.4.2 for an outline of how to use the "consistency"
1489 output.
1491 5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash
1493 GET /ct/v2/get-proof-by-hash
1495 Inputs: hash: A base64 encoded v2 leaf hash.
1497 tree_size: The tree_size of the tree on which to base the
1498 proof, in decimal.
1500 The "hash" must be calculated as defined in Section 4.7. A v2 STH
1501 must exist for the "tree_size". Because of skew, the front-end
1502 may not know the requested tree head. In that case, it will
1503 return the latest STH it knows, along with an inclusion proof to
1504 that STH. If the front-end knows the requested tree head then
1505 only "inclusion" is returned.
1507 Outputs: inclusion: A base64 encoded "TransItem" of type
1508 "inclusion_proof_v2" whose "inclusion_path" array of Merkle
1509 Tree nodes proves the inclusion of the certificate (as
1510 specified by the "hash" parameter) in the selected STH.
1512 sth: A base64 encoded "TransItem" of type
1513 "signed_tree_head_v2", signed by this log.
1515 Note that no signature is required for the "inclusion" output as
1516 it is used to verify inclusion in the selected STH, which is
1517 signed.
1519 Error codes:
1521 +=================+=====================================+
1522 | type | detail |
1523 +=================+=====================================+
1524 | hashUnknown | "hash" is not the hash of a known |
1525 | | leaf (may be caused by skew or by a |
1526 | | known certificate not yet merged). |
1527 +-----------------+-------------------------------------+
1528 | treeSizeUnknown | "hash" is before the latest known |
1529 | | STH but is not from an existing |
1530 | | STH. |
1531 +-----------------+-------------------------------------+
1533 Table 4
1535 See Section 2.1.3.2 for an outline of how to use the "inclusion"
1536 output.
1538 5.5. Retrieve Merkle Inclusion Proof, Signed Tree Head and Consistency
1539 Proof by Leaf Hash
1541 GET /ct/v2/get-all-by-hash
1543 Inputs: hash: A base64 encoded v2 leaf hash.
1545 tree_size: The tree_size of the tree on which to base the
1546 proofs, in decimal.
1548 The "hash" must be calculated as defined in Section 4.7. A v2 STH
1549 must exist for the "tree_size".
1551 Because of skew, the front-end may not know the requested tree head
1552 or the requested hash, which leads to a number of cases:
1554 +=====================+=====================================+
1555 | Case | Response |
1556 +=====================+=====================================+
1557 | latest STH < | Return latest STH |
1558 | requested tree head | |
1559 +---------------------+-------------------------------------+
1560 | latest STH > | Return latest STH and a consistency |
1561 | requested tree head | proof between it and the requested |
1562 | | tree head (see Section 5.3) |
1563 +---------------------+-------------------------------------+
1564 | index of requested | Return "inclusion" |
1565 | hash < latest STH | |
1566 +---------------------+-------------------------------------+
1568 Table 5
1570 Note that more than one case can be true, in which case the returned
1571 data is their union. It is also possible for none to be true, in
1572 which case the front-end MUST return an empty response.
1574 Outputs: inclusion: A base64 encoded "TransItem" of type
1575 "inclusion_proof_v2" whose "inclusion_path" array of Merkle
1576 Tree nodes proves the inclusion of the certificate (as
1577 specified by the "hash" parameter) in the selected STH.
1579 sth: A base64 encoded "TransItem" of type
1580 "signed_tree_head_v2", signed by this log.
1582 consistency: A base64 encoded "TransItem" of type
1583 "consistency_proof_v2" that proves the consistency of the
1584 requested tree head and the returned STH.
1586 Note that no signature is required for the "inclusion" or
1587 "consistency" outputs as they are used to verify inclusion in and
1588 consistency of STHs, which are signed.
1590 Errors are the same as in Section 5.4.
1592 See Section 2.1.3.2 for an outline of how to use the "inclusion"
1593 output, and see Section 2.1.4.2 for an outline of how to use the
1594 "consistency" output.
1596 5.6. Retrieve Entries and STH from Log
1598 GET /ct/v2/get-entries
1600 Inputs: start: 0-based index of first entry to retrieve, in
1601 decimal.
1603 end: 0-based index of last entry to retrieve, in decimal.
1605 Outputs: entries: An array of objects, each consisting of
1607 log_entry: The base64 encoded "TransItem" structure of type
1608 "x509_entry_v2" or "precert_entry_v2" (see Section 4.3).
1610 submitted_entry: JSON object equivalent to inputs that were
1611 submitted to "submit-entry", with the addition of the trust
1612 anchor to the "chain" field if the submission did not
1613 include it.
1615 sct: The base64 encoded "TransItem" of type "x509_sct_v2" or
1616 "precert_sct_v2" corresponding to this log entry.
1618 sth: A base64 encoded "TransItem" of type
1619 "signed_tree_head_v2", signed by this log.
1621 Note that this message is not signed -- the "entries" data can be
1622 verified by constructing the Merkle Tree Hash corresponding to a
1623 retrieved STH. All leaves MUST be v2. However, a compliant v2
1624 client MUST NOT construe an unrecognized TransItem type as an error.
1625 This means it may be unable to parse some entries, but note that each
1626 client can inspect the entries it does recognize as well as verify
1627 the integrity of the data by treating unrecognized leaves as opaque
1628 input to the tree.
1630 The "start" and "end" parameters SHOULD be within the range 0 <= x <
1631 "tree_size" as returned by "get-sth" in Section 5.2.
1633 The "start" parameter MUST be less than or equal to the "end"
1634 parameter.
1636 Each "submitted_entry" output parameter MUST include the trust anchor
1637 that the log used to verify the "submission", even if that trust
1638 anchor was not provided to "submit-entry" (see Section 5.1). If the
1639 "submission" does not certify itself, then the first element of
1640 "chain" MUST be present and MUST certify the "submission".
1642 Log servers MUST honor requests where 0 <= "start" < "tree_size" and
1643 "end" >= "tree_size" by returning a partial response covering only
1644 the valid entries in the specified range. "end" >= "tree_size" could
1645 be caused by skew. Note that the following restriction may also
1646 apply:
1648 Logs MAY restrict the number of entries that can be retrieved per
1649 "get-entries" request. If a client requests more than the permitted
1650 number of entries, the log SHALL return the maximum number of entries
1651 permissible. These entries SHALL be sequential beginning with the
1652 entry specified by "start". Note that limit on the number of entries
1653 is not immutable and therefore the restriction may be changed or
1654 lifted at any time and is not listed with the other Log Parameters in
1655 Section 4.1.
1657 Because of skew, it is possible the log server will not have any
1658 entries between "start" and "end". In this case it MUST return an
1659 empty "entries" array.
1661 In any case, the log server MUST return the latest STH it knows
1662 about.
1664 See Section 2.1.2 for an outline of how to use a complete list of
1665 "log_entry" entries to verify the "root_hash".
1667 Error codes:
1669 +================+====================================+
1670 | type | detail |
1671 +================+====================================+
1672 | startUnknown | "start" is greater than the number |
1673 | | of entries in the Merkle tree. |
1674 +----------------+------------------------------------+
1675 | endBeforeStart | "start" cannot be greater than |
1676 | | "end". |
1677 +----------------+------------------------------------+
1679 Table 6
1681 5.7. Retrieve Accepted Trust Anchors
1683 GET /ct/v2/get-anchors
1685 No inputs.
1687 Outputs: certificates: An array of JSON strings, each of which is a
1688 base64 encoded CA certificate that is acceptable to the log.
1689 max_chain_length:
1691 If the server has chosen to limit the length of chains it
1692 accepts, this is the maximum number of certificates in the
1693 chain, in decimal. If there is no limit, this is omitted.
1695 This data is not signed and the protocol depends on the security
1696 guarantees of TLS to ensure correctness.
1698 6. TLS Servers
1700 CT-using TLS servers MUST use at least one of the mechanisms
1701 described below to present one or more SCTs from one or more logs to
1702 each TLS client during full TLS handshakes, where each SCT
1703 corresponds to the server certificate. (Of course, a server can only
1704 send a TLS extension if the client has specified it first.) Servers
1705 SHOULD also present corresponding inclusion proofs and STHs.
1707 A server can provide SCTs using a TLS 1.3 extension (Section 4.2 of
1708 [RFC8446]) with type "transparency_info" (see Section 6.5). This
1709 mechanism allows TLS servers to participate in CT without the
1710 cooperation of CAs, unlike the other two mechanisms. It also allows
1711 SCTs and inclusion proofs to be updated on the fly.
1713 The server may also use an Online Certificate Status Protocol (OCSP)
1714 [RFC6960] response extension (see Section 7.1.1), providing the OCSP
1715 response as part of the TLS handshake. Providing a response during a
1716 TLS handshake is popularly known as "OCSP stapling." For TLS 1.3,
1717 the information is encoded as an extension in the "status_request"
1718 extension data; see Section 4.4.2.1 of [RFC8446]. For TLS 1.2
1719 ([RFC5246]), the information is encoded as an extension in the
1720 "CertificateStatus" message; see Section 8 of [RFC6066]. Using
1721 stapling also allows SCTs and inclusion proofs to be updated on the
1722 fly.
1724 CT information can also be encoded as an extension in the X.509v3
1725 certificate (see Section 7.1.2). This mechanism allows the use of
1726 unmodified TLS servers, but the SCTs and inclusion proofs cannot be
1727 updated on the fly. Since the logs from which the SCTs and inclusion
1728 proofs originated won't necessarily be accepted by TLS clients for
1729 the full lifetime of the certificate, there is a risk that TLS
1730 clients may subsequently consider the certificate to be non-compliant
1731 and in need of re-issuance or the use of one of the other two methods
1732 for delivering CT information.
1734 6.1. TLS Client Authentication
1736 This specification includes no description of how a TLS server can
1737 use CT for TLS client certificates. While this may be useful, it is
1738 not documented here for the following reasons:
1740 * The greater security exposure is for clients to end up interacting
1741 with an illegitimate server.
1743 * In general, TLS client certificates are not expected to be
1744 submitted to CT logs, particularly those intended for general
1745 public use.
1747 A future version could include such information.
1749 6.2. Multiple SCTs
1751 CT-using TLS servers SHOULD send SCTs from multiple logs, because:
1753 * One or more logs may not have become acceptable to all CT-using
1754 TLS clients. Note that client discovery, trust, and distrust of
1755 logs is expected to be handled out-of-band and is out of scope of
1756 this document.
1758 * If a CA and a log collude, it is possible to temporarily hide
1759 misissuance from clients. When a TLS client requires SCTs from
1760 multiple logs to be provided, it is more difficult to mount this
1761 attack.
1763 * If a log misbehaves or suffers a key compromise, a consequence may
1764 be that clients cease to trust it. Since the time an SCT may be
1765 in use can be considerable (several years is common in current
1766 practice when embedded in a certificate), including SCTs from
1767 multiple logs reduces the probability of the certificate being
1768 rejected by TLS clients.
1770 * TLS clients may have policies related to the above risks requiring
1771 TLS servers to present multiple SCTs. For example, at the time of
1772 writing, Chromium [Chromium.Log.Policy] requires multiple SCTs to
1773 be presented with EV certificates in order for the EV indicator to
1774 be shown.
1776 To select the logs from which to obtain SCTs, a TLS server can, for
1777 example, examine the set of logs popular TLS clients accept and
1778 recognize.
1780 6.3. TransItemList Structure
1782 Multiple SCTs, inclusion proofs, and indeed "TransItem" structures of
1783 any type, are combined into a list as follows:
1785 opaque SerializedTransItem<1..2^16-1>;
1787 struct {
1788 SerializedTransItem trans_item_list<1..2^16-1>;
1789 } TransItemList;
1791 Here, "SerializedTransItem" is an opaque byte string that contains
1792 the serialized "TransItem" structure. This encoding ensures that TLS
1793 clients can decode each "TransItem" individually (so, for example, if
1794 there is a version upgrade, out-of-date clients can still parse old
1795 "TransItem" structures while skipping over new "TransItem" structures
1796 whose versions they don't understand).
1798 6.4. Presenting SCTs, inclusions proofs and STHs
1800 In each "TransItemList" that is sent during a TLS handshake, the TLS
1801 server MUST include a "TransItem" structure of type "x509_sct_v2" or
1802 "precert_sct_v2".
1804 Presenting inclusion proofs and STHs in the TLS handshake helps to
1805 protect the client's privacy (see Section 8.1.4) and reduces load on
1806 log servers. Therefore, if the TLS server can obtain them, it SHOULD
1807 also include "TransItem"s of type "inclusion_proof_v2" and
1808 "signed_tree_head_v2" in the "TransItemList".
1810 6.5. transparency_info TLS Extension
1812 Provided that a TLS client includes the "transparency_info" extension
1813 type in the ClientHello and the TLS server supports the
1814 "transparency_info" extension:
1816 * The TLS server MUST verify that the received "extension_data" is
1817 empty.
1819 * The TLS server MUST construct a "TransItemList" of relevant
1820 "TransItem"s (see Section 6.4), which SHOULD omit any "TransItem"s
1821 that are already embedded in the server certificate or the stapled
1822 OCSP response (see Section 7.1). If the constructed
1823 "TransItemList" is not empty, then the TLS server MUST include the
1824 "transparency_info" extension with the "extension_data" set to
1825 this "TransItemList". If the list is empty then the server SHOULD
1826 omit the "extension_data" element, but MAY send it with an empty
1827 array.
1829 TLS servers MUST only include this extension in the following
1830 messages:
1832 * the ServerHello message (for TLS 1.2 or earlier).
1834 * the Certificate or CertificateRequest message (for TLS 1.3).
1836 TLS servers MUST NOT process or include this extension when a TLS
1837 session is resumed, since session resumption uses the original
1838 session information.
1840 7. Certification Authorities
1842 7.1. Transparency Information X.509v3 Extension
1844 The Transparency Information X.509v3 extension, which has OID
1845 1.3.101.75 and SHOULD be non-critical, contains one or more
1846 "TransItem" structures in a "TransItemList". This extension MAY be
1847 included in OCSP responses (see Section 7.1.1) and certificates (see
1848 Section 7.1.2). Since RFC5280 requires the "extnValue" field (an
1849 OCTET STRING) of each X.509v3 extension to include the DER encoding
1850 of an ASN.1 value, a "TransItemList" MUST NOT be included directly.
1851 Instead, it MUST be wrapped inside an additional OCTET STRING, which
1852 is then put into the "extnValue" field:
1854 TransparencyInformationSyntax ::= OCTET STRING
1856 "TransparencyInformationSyntax" contains a "TransItemList".
1858 7.1.1. OCSP Response Extension
1860 A certification authority MAY include a Transparency Information
1861 X.509v3 extension in the "singleExtensions" of a "SingleResponse" in
1862 an OCSP response. All included SCTs and inclusion proofs MUST be for
1863 the certificate identified by the "certID" of that "SingleResponse",
1864 or for a precertificate that corresponds to that certificate.
1866 7.1.2. Certificate Extension
1868 A certification authority MAY include a Transparency Information
1869 X.509v3 extension in a certificate. All included SCTs and inclusion
1870 proofs MUST be for a precertificate that corresponds to this
1871 certificate.
1873 7.2. TLS Feature X.509v3 Extension
1875 A certification authority SHOULD NOT issue any certificate that
1876 identifies the "transparency_info" TLS extension in a TLS feature
1877 extension [RFC7633], because TLS servers are not required to support
1878 the "transparency_info" TLS extension in order to participate in CT
1879 (see Section 6).
1881 8. Clients
1883 There are various different functions clients of logs might perform.
1884 We describe here some typical clients and how they should function.
1885 Any inconsistency may be used as evidence that a log has not behaved
1886 correctly, and the signatures on the data structures prevent the log
1887 from denying that misbehavior.
1889 All clients need various parameters in order to communicate with logs
1890 and verify their responses. These parameters are described in
1891 Section 4.1, but note that this document does not describe how the
1892 parameters are obtained, which is implementation-dependent (see, for
1893 example, [Chromium.Policy]).
1895 8.1. TLS Client
1897 8.1.1. Receiving SCTs and inclusion proofs
1899 TLS clients receive SCTs and inclusion proofs alongside or in
1900 certificates. CT-using TLS clients MUST implement all of the three
1901 mechanisms by which TLS servers may present SCTs (see Section 6).
1903 TLS clients that support the "transparency_info" TLS extension (see
1904 Section 6.5) SHOULD include it in ClientHello messages, with empty
1905 "extension_data". If a TLS server includes the "transparency_info"
1906 TLS extension when resuming a TLS session, the TLS client MUST abort
1907 the handshake.
1909 8.1.2. Reconstructing the TBSCertificate
1911 Validation of an SCT for a certificate (where the "type" of the
1912 "TransItem" is "x509_sct_v2") uses the unmodified TBSCertificate
1913 component of the certificate.
1915 Before an SCT for a precertificate (where the "type" of the
1916 "TransItem" is "precert_sct_v2") can be validated, the TBSCertificate
1917 component of the precertificate needs to be reconstructed from the
1918 TBSCertificate component of the certificate as follows:
1920 * Remove the Transparency Information extension (see Section 7.1).
1922 * Remove embedded v1 SCTs, identified by OID 1.3.6.1.4.1.11129.2.4.2
1923 (see section 3.3 of [RFC6962]). This allows embedded v1 and v2
1924 SCTs to co-exist in a certificate (see Appendix A).
1926 8.1.3. Validating SCTs
1928 In order to make use of a received SCT, the TLS client MUST first
1929 validate it as follows:
1931 * Compute the signature input by constructing a "TransItem" of type
1932 "x509_entry_v2" or "precert_entry_v2", depending on the SCT's
1933 "TransItem" type. The "TimestampedCertificateEntryDataV2"
1934 structure is constructed in the following manner:
1936 - "timestamp" is copied from the SCT.
1938 - "tbs_certificate" is the reconstructed TBSCertificate portion
1939 of the server certificate, as described in Section 8.1.2.
1941 - "issuer_key_hash" is computed as described in Section 4.7.
1943 - "sct_extensions" is copied from the SCT.
1945 * Verify the SCT's "signature" against the computed signature input
1946 using the public key of the corresponding log, which is identified
1947 by the "log_id". The required signature algorithm is one of the
1948 log's parameters.
1950 If the TLS client does not have the corresponding log's parameters,
1951 it cannot attempt to validate the SCT. When evaluating compliance
1952 (see Section 8.1.6), the TLS client will consider only those SCTs
1953 that it was able to validate.
1955 Note that SCT validation is not a substitute for the normal
1956 validation of the server certificate and its chain.
1958 8.1.4. Fetching inclusion proofs
1960 When a TLS client has validated a received SCT but does not yet
1961 possess a corresponding inclusion proof, the TLS client MAY request
1962 the inclusion proof directly from a log using "get-proof-by-hash"
1963 (Section 5.4) or "get-all-by-hash" (Section 5.5).
1965 Note that fetching inclusion proofs directly from a log will disclose
1966 to the log which TLS server the client has been communicating with.
1967 This may be regarded as a significant privacy concern, and so it is
1968 preferable for the TLS server to send the inclusion proofs (see
1969 Section 6.4).
1971 8.1.5. Validating inclusion proofs
1973 When a TLS client has received, or fetched, an inclusion proof (and
1974 an STH), it SHOULD proceed to verifying the inclusion proof to the
1975 provided STH. The TLS client SHOULD also verify consistency between
1976 the provided STH and an STH it knows about.
1978 If the TLS client holds an STH that predates the SCT, it MAY, in the
1979 process of auditing, request a new STH from the log (Section 5.2),
1980 then verify it by requesting a consistency proof (Section 5.3). Note
1981 that if the TLS client uses "get-all-by-hash", then it will already
1982 have the new STH.
1984 8.1.6. Evaluating compliance
1986 It is up to a client's local policy to specify the quantity and form
1987 of evidence (SCTs, inclusion proofs or a combination) needed to
1988 achieve compliance and how to handle non-compliance.
1990 A TLS client can only evaluate compliance if it has given the TLS
1991 server the opportunity to send SCTs and inclusion proofs by any of
1992 the three mechanisms that are mandatory to implement for CT-using TLS
1993 clients (see Section 8.1.1). Therefore, a TLS client MUST NOT
1994 evaluate compliance if it did not include both the
1995 "transparency_info" and "status_request" TLS extensions in the
1996 ClientHello.
1998 8.2. Monitor
2000 Monitors watch logs to check that they behave correctly, for
2001 certificates of interest, or both. For example, a monitor may be
2002 configured to report on all certificates that apply to a specific
2003 domain name when fetching new entries for consistency validation.
2005 A monitor MUST at least inspect every new entry in every log it
2006 watches, and it MAY also choose to keep copies of entire logs.
2008 To inspect all of the existing entries, the monitor SHOULD follow
2009 these steps once for each log:
2011 1. Fetch the current STH (Section 5.2).
2013 2. Verify the STH signature.
2015 3. Fetch all the entries in the tree corresponding to the STH
2016 (Section 5.6).
2018 4. If applicable, check each entry to see if it's a certificate of
2019 interest.
2021 5. Confirm that the tree made from the fetched entries produces the
2022 same hash as that in the STH.
2024 To inspect new entries, the monitor SHOULD follow these steps
2025 repeatedly for each log:
2027 1. Fetch the current STH (Section 5.2). Repeat until the STH
2028 changes. This document does not specify the polling frequency,
2029 to allow for experimentation.
2031 2. Verify the STH signature.
2033 3. Fetch all the new entries in the tree corresponding to the STH
2034 (Section 5.6). If they remain unavailable for an extended
2035 period, then this should be viewed as misbehavior on the part of
2036 the log.
2038 4. If applicable, check each entry to see if it's a certificate of
2039 interest.
2041 5. Either:
2043 1. Verify that the updated list of all entries generates a tree
2044 with the same hash as the new STH.
2046 Or, if it is not keeping all log entries:
2048 1. Fetch a consistency proof for the new STH with the previous
2049 STH (Section 5.3).
2051 2. Verify the consistency proof.
2053 3. Verify that the new entries generate the corresponding
2054 elements in the consistency proof.
2056 6. Repeat from step 1.
2058 8.3. Auditing
2060 Auditing ensures that the current published state of a log is
2061 reachable from previously published states that are known to be good,
2062 and that the promises made by the log in the form of SCTs have been
2063 kept. Audits are performed by monitors or TLS clients.
2065 In particular, there are four log behavior properties that should be
2066 checked:
2068 * The Maximum Merge Delay (MMD).
2070 * The STH Frequency Count.
2072 * The append-only property.
2074 * The consistency of the log view presented to all query sources.
2076 A benign, conformant log publishes a series of STHs over time, each
2077 derived from the previous STH and the submitted entries incorporated
2078 into the log since publication of the previous STH. This can be
2079 proven through auditing of STHs. SCTs returned to TLS clients can be
2080 audited by verifying against the accompanying certificate, and using
2081 Merkle Inclusion Proofs, against the log's Merkle tree.
2083 The action taken by the auditor if an audit fails is not specified,
2084 but note that in general if audit fails, the auditor is in possession
2085 of signed proof of the log's misbehavior.
2087 A monitor (Section 8.2) can audit by verifying the consistency of
2088 STHs it receives, ensure that each entry can be fetched and that the
2089 STH is indeed the result of making a tree from all fetched entries.
2091 A TLS client (Section 8.1) can audit by verifying an SCT against any
2092 STH dated after the SCT timestamp + the Maximum Merge Delay by
2093 requesting a Merkle inclusion proof (Section 5.4). It can also
2094 verify that the SCT corresponds to the server certificate it arrived
2095 with (i.e., the log entry is that certificate, or is a precertificate
2096 corresponding to that certificate).
2098 Checking of the consistency of the log view presented to all entities
2099 is more difficult to perform because it requires a way to share log
2100 responses among a set of CT-using entities, and is discussed in
2101 Section 11.3.
2103 9. Algorithm Agility
2105 It is not possible for a log to change any of its algorithms part way
2106 through its lifetime:
2108 Signature algorithm: SCT signatures must remain valid so signature
2109 algorithms can only be added, not removed.
2111 Hash algorithm: A log would have to support the old and new hash
2112 algorithms to allow backwards-compatibility with clients that are
2113 not aware of a hash algorithm change.
2115 Allowing multiple signature or hash algorithms for a log would
2116 require that all data structures support it and would significantly
2117 complicate client implementation, which is why it is not supported by
2118 this document.
2120 If it should become necessary to deprecate an algorithm used by a
2121 live log, then the log MUST be frozen as specified in Section 4.13
2122 and a new log SHOULD be started. Certificates in the frozen log that
2123 have not yet expired and require new SCTs SHOULD be submitted to the
2124 new log and the SCTs from that log used instead.
2126 10. IANA Considerations
2128 The assignment policy criteria mentioned in this section refer to the
2129 policies outlined in [RFC8126].
2131 10.1. Additions to existing registries
2133 This sub-section defines additions to existing registries.
2135 10.1.1. New Entry to the TLS ExtensionType Registry
2137 IANA is asked to add an entry for "transparency_info(TBD)" to the
2138 "TLS ExtensionType Values" registry defined in [RFC8446], setting the
2139 "Recommended" value to "Y", setting the "TLS 1.3" value to "CH, CR,
2140 CT", and citing this document as the "Reference".
2142 10.1.2. URN Sub-namespace for TRANS errors
2143 (urn:ietf:params:trans:error)
2145 IANA is requested to add a new entry in the "IETF URN Sub-namespace
2146 for Registered Protocol Parameter Identifiers" registry, following
2147 the template in [RFC3553]:
2149 Registry name: trans:error
2151 Specification: RFCXXXX
2153 Repository: https://www.iana.org/assignments/trans
2155 Index value: No transformation needed.
2157 10.2. New CT-Related registries
2159 IANA is requested to add a new protocol registry, "Public Notary
2160 Transparency", to the list that appears at https://www.iana.org/
2161 assignments/
2163 The rest of this section defines sub-registries to be created within
2164 the new Public Notary Transparency registry.
2166 10.2.1. Hash Algorithms
2168 IANA is asked to establish a registry of hash algorithm values, named
2169 "Hash Algorithms", that initially consists of:
2171 +========+============+========================+===================+
2172 | Value | Hash | OID | Reference / |
2173 | | Algorithm | | Assignment Policy |
2174 +========+============+========================+===================+
2175 | 0x00 | SHA-256 | 2.16.840.1.101.3.4.2.1 | [RFC6234] |
2176 +--------+------------+------------------------+-------------------+
2177 | 0x01 - | Unassigned | | Specification |
2178 | 0xDF | | | Required |
2179 +--------+------------+------------------------+-------------------+
2180 | 0xE0 - | Reserved | | Experimental Use |
2181 | 0xEF | | | |
2182 +--------+------------+------------------------+-------------------+
2183 | 0xF0 - | Reserved | | Private Use |
2184 | 0xFF | | | |
2185 +--------+------------+------------------------+-------------------+
2187 Table 7
2189 The Designated Expert(s) should ensure that the proposed algorithm
2190 has a public specification and is suitable for use as a cryptographic
2191 hash algorithm with no known preimage or collision attacks. These
2192 attacks can damage the integrity of the log.
2194 10.2.2. Signature Algorithms
2196 IANA is asked to establish a registry of signature algorithm values,
2197 named "Signature Algorithms".
2199 The following notes should be added:
2201 * This is a subset of the TLS SignatureScheme Registry, limited to
2202 those algorithms that are appropriate for CT. A major advantage
2203 of this is leveraging the expertise of the TLS working group and
2204 its Designated Expert(s).
2206 * The value "0x0403" appears twice. While this may be confusing, it
2207 is okay because the verification process is the same for both
2208 algorithms, and the choice of which to use when generating a
2209 signature is purely internal to the log server.
2211 The registry should initially consist of:
2213 +================================+==================+==============+
2214 | SignatureScheme Value | Signature | Reference / |
2215 | | Algorithm | Assignment |
2216 | | | Policy |
2217 +================================+==================+==============+
2218 | 0x0000 - 0x0402 | Unassigned | Expert |
2219 | | | Review |
2220 +--------------------------------+------------------+--------------+
2221 | ecdsa_secp256r1_sha256(0x0403) | ECDSA (NIST | [FIPS186-4] |
2222 | | P-256) with | |
2223 | | SHA-256 | |
2224 +--------------------------------+------------------+--------------+
2225 | ecdsa_secp256r1_sha256(0x0403) | Deterministic | [RFC6979] |
2226 | | ECDSA (NIST | |
2227 | | P-256) with | |
2228 | | HMAC-SHA256 | |
2229 +--------------------------------+------------------+--------------+
2230 | 0x0404 - 0x0806 | Unassigned | Expert |
2231 | | | Review |
2232 +--------------------------------+------------------+--------------+
2233 | ed25519(0x0807) | Ed25519 | [RFC8032] |
2234 | | (PureEdDSA with | |
2235 | | the edwards25519 | |
2236 | | curve) | |
2237 +--------------------------------+------------------+--------------+
2238 | 0x0808 - 0xFDFF | Unassigned | Expert |
2239 | | | Review |
2240 +--------------------------------+------------------+--------------+
2241 | 0xFE00 - 0xFEFF | Reserved | Experimental |
2242 | | | Use |
2243 +--------------------------------+------------------+--------------+
2244 | 0xFF00 - 0xFFFF | Reserved | Private Use |
2245 +--------------------------------+------------------+--------------+
2247 Table 8
2249 The Designated Expert(s) should ensure that the proposed algorithm
2250 has a public specification, has a value assigned to it in the TLS
2251 SignatureScheme Registry (that IANA was asked to establish in
2252 [RFC8446]) and is suitable for use as a cryptographic signature
2253 algorithm.
2255 10.2.3. VersionedTransTypes
2257 IANA is asked to establish a registry of "VersionedTransType" values,
2258 named "VersionedTransTypes".
2260 The following note should be added:
2262 * The 0x0000 value is reserved so that v1 SCTs are distinguishable
2263 from v2 SCTs and other "TransItem" structures.
2265 The registry should initially consist of:
2267 +==========+======================+===============================+
2268 | Value | Type and Version | Reference / Assignment Policy |
2269 +==========+======================+===============================+
2270 | 0x0000 | Reserved | [RFC6962] |
2271 +----------+----------------------+-------------------------------+
2272 | 0x0001 | x509_entry_v2 | RFCXXXX |
2273 +----------+----------------------+-------------------------------+
2274 | 0x0002 | precert_entry_v2 | RFCXXXX |
2275 +----------+----------------------+-------------------------------+
2276 | 0x0003 | x509_sct_v2 | RFCXXXX |
2277 +----------+----------------------+-------------------------------+
2278 | 0x0004 | precert_sct_v2 | RFCXXXX |
2279 +----------+----------------------+-------------------------------+
2280 | 0x0005 | signed_tree_head_v2 | RFCXXXX |
2281 +----------+----------------------+-------------------------------+
2282 | 0x0006 | consistency_proof_v2 | RFCXXXX |
2283 +----------+----------------------+-------------------------------+
2284 | 0x0007 | inclusion_proof_v2 | RFCXXXX |
2285 +----------+----------------------+-------------------------------+
2286 | 0x0008 - | Unassigned | Specification Required |
2287 | 0xDFFF | | |
2288 +----------+----------------------+-------------------------------+
2289 | 0xE000 - | Reserved | Experimental Use |
2290 | 0xEFFF | | |
2291 +----------+----------------------+-------------------------------+
2292 | 0xF000 - | Reserved | Private Use |
2293 | 0xFFFF | | |
2294 +----------+----------------------+-------------------------------+
2296 Table 9
2298 The Designated Expert(s) should review the public specification to
2299 ensure that it is detailed enough to ensure implementation
2300 interoperability.
2302 10.2.4. Log Artifact Extension Registry
2304 IANA is asked to establish a registry of "ExtensionType" values,
2305 named "Log Artifact Extensions", that initially consists of:
2307 +===============+============+=====+===============================+
2308 | ExtensionType | Status | Use | Reference / Assignment Policy |
2309 +===============+============+=====+===============================+
2310 | 0x0000 - | Unassigned | n/a | Specification Required |
2311 | 0xDFFF | | | |
2312 +---------------+------------+-----+-------------------------------+
2313 | 0xE000 - | Reserved | n/a | Experimental Use |
2314 | 0xEFFF | | | |
2315 +---------------+------------+-----+-------------------------------+
2316 | 0xF000 - | Reserved | n/a | Private Use |
2317 | 0xFFFF | | | |
2318 +---------------+------------+-----+-------------------------------+
2320 Table 10
2322 The "Use" column should contain one or both of the following values:
2324 * "SCT", for extensions specified for use in Signed Certificate
2325 Timestamps.
2327 * "STH", for extensions specified for use in Signed Tree Heads.
2329 The Designated Expert(s) should review the public specification to
2330 ensure that it is detailed enough to ensure implementation
2331 interoperability. They should also verify that the extension is
2332 appropriate to the contexts in which it is specified to be used (SCT,
2333 STH, or both).
2335 10.2.5. Log ID Registry
2337 IANA is asked to establish a registry of Log IDs, named "Log ID
2338 Registry", that initially consists of:
2340 +================+==============+==============+===================+
2341 | Log ID | Log Base URL | Log Operator | Reference / |
2342 | | | | Assignment Policy |
2343 +================+==============+==============+===================+
2344 | 1.3.101.8192 - | Unassigned | Unassigned | First Come First |
2345 | 1.3.101.16383 | | | Served |
2346 +----------------+--------------+--------------+-------------------+
2347 | 1.3.101.80.0 - | Unassigned | Unassigned | First Come First |
2348 | 1.3.101.80.* | | | Served |
2349 +----------------+--------------+--------------+-------------------+
2351 Table 11
2353 All OIDs in the range from 1.3.101.8192 to 1.3.101.16383 have been
2354 set aside for Log IDs. This is a limited resource of 8,192 OIDs,
2355 each of which has an encoded length of 4 octets.
2357 The 1.3.101.80 arc has also been set assigned for LogIDs. This is an
2358 unlimited resource, but only the 128 OIDs from 1.3.101.80.0 to
2359 1.3.101.80.127 have an encoded length of only 4 octets.
2361 Each application for the allocation of a Log ID MUST be accompanied
2362 by:
2364 * the Log's Base URL (see Section 4.1).
2366 * the Log Operator's contact details.
2368 IANA is asked to reject any request to update a Log ID or Log Base
2369 URL in this registry, because these fields are immutable (see
2370 Section 4.1).
2372 IANA is asked to accept requests from log operators to update their
2373 contact details in this registry.
2375 Since log operators can choose to not use this registry (see
2376 Section 4.4), it is not expected to be a global directory of all
2377 logs.
2379 10.2.6. Error Types Registry
2381 IANA is requested to create a new registry for errors, the "Error
2382 Types" registry.
2384 Requirements for this registry are Specification Required.
2386 This registry should have the following three fields:
2388 +============+========+===========+
2389 | Field Name | Type | Reference |
2390 +============+========+===========+
2391 | identifier | string | RFCXXXX |
2392 +------------+--------+-----------+
2393 | meaning | string | RFCXXXX |
2394 +------------+--------+-----------+
2395 | reference | string | RFCXXXX |
2396 +------------+--------+-----------+
2398 Table 12
2400 The initial values are as follows, taken from the text above:
2402 +===================+===============================+===========+
2403 | Identifier | Meaning | Reference |
2404 +===================+===============================+===========+
2405 | malformed | The request could not be | RFCXXXX |
2406 | | parsed. | |
2407 +-------------------+-------------------------------+-----------+
2408 | badSubmission | "submission" is neither a | RFCXXXX |
2409 | | valid certificate nor a valid | |
2410 | | precertificate | |
2411 +-------------------+-------------------------------+-----------+
2412 | badType | "type" is neither 1 nor 2 | RFCXXXX |
2413 +-------------------+-------------------------------+-----------+
2414 | badChain | The first element of "chain" | RFCXXXX |
2415 | | is not the certifier of the | |
2416 | | "submission", or the second | |
2417 | | element does not certify the | |
2418 | | first, etc. | |
2419 +-------------------+-------------------------------+-----------+
2420 | badCertificate | One or more certificates in | RFCXXXX |
2421 | | the "chain" are not valid | |
2422 | | (e.g., not properly encoded) | |
2423 +-------------------+-------------------------------+-----------+
2424 | unknownAnchor | The last element of "chain" | RFCXXXX |
2425 | | (or, if "chain" is an empty | |
2426 | | array, the "submission") both | |
2427 | | is not, and is not certified | |
2428 | | by, an accepted trust anchor | |
2429 +-------------------+-------------------------------+-----------+
2430 | shutdown | The log is no longer | RFCXXXX |
2431 | | accepting submissions | |
2432 +-------------------+-------------------------------+-----------+
2433 | firstUnknown | "first" is before the latest | RFCXXXX |
2434 | | known STH but is not from an | |
2435 | | existing STH. | |
2436 +-------------------+-------------------------------+-----------+
2437 | secondUnknown | "second" is before the latest | RFCXXXX |
2438 | | known STH but is not from an | |
2439 | | existing STH. | |
2440 +-------------------+-------------------------------+-----------+
2441 | secondBeforeFirst | "second" is smaller than | RFCXXXX |
2442 | | "first". | |
2443 +-------------------+-------------------------------+-----------+
2444 | hashUnknown | "hash" is not the hash of a | RFCXXXX |
2445 | | known leaf (may be caused by | |
2446 | | skew or by a known | |
2447 | | certificate not yet merged). | |
2448 +-------------------+-------------------------------+-----------+
2449 | treeSizeUnknown | "hash" is before the latest | RFCXXXX |
2450 | | known STH but is not from an | |
2451 | | existing STH. | |
2452 +-------------------+-------------------------------+-----------+
2453 | startUnknown | "start" is greater than the | RFCXXXX |
2454 | | number of entries in the | |
2455 | | Merkle tree. | |
2456 +-------------------+-------------------------------+-----------+
2457 | endBeforeStart | "start" cannot be greater | RFCXXXX |
2458 | | than "end". | |
2459 +-------------------+-------------------------------+-----------+
2461 Table 13
2463 11. Security Considerations
2465 With CAs, logs, and servers performing the actions described here,
2466 TLS clients can use logs and signed timestamps to reduce the
2467 likelihood that they will accept misissued certificates. If a server
2468 presents a valid signed timestamp for a certificate, then the client
2469 knows that a log has committed to publishing the certificate. From
2470 this, the client knows that monitors acting for the subject of the
2471 certificate have had some time to notice the misissuance and take
2472 some action, such as asking a CA to revoke a misissued certificate.
2473 A signed timestamp does not guarantee this though, since appropriate
2474 monitors might not have checked the logs or the CA might have refused
2475 to revoke the certificate.
2477 In addition, if TLS clients will not accept unlogged certificates,
2478 then site owners will have a greater incentive to submit certificates
2479 to logs, possibly with the assistance of their CA, increasing the
2480 overall transparency of the system.
2482 11.1. Misissued Certificates
2484 Misissued certificates that have not been publicly logged, and thus
2485 do not have a valid SCT, are not considered compliant. Misissued
2486 certificates that do have an SCT from a log will appear in that
2487 public log within the Maximum Merge Delay, assuming the log is
2488 operating correctly. Since a log is allowed to serve an STH of any
2489 age up to the MMD, the maximum period of time during which a
2490 misissued certificate can be used without being available for audit
2491 is twice the MMD.
2493 11.2. Detection of Misissue
2495 The logs do not themselves detect misissued certificates; they rely
2496 instead on interested parties, such as domain owners, to monitor them
2497 and take corrective action when a misissue is detected.
2499 11.3. Misbehaving Logs
2501 A log can misbehave in several ways. Examples include: failing to
2502 incorporate a certificate with an SCT in the Merkle Tree within the
2503 MMD; presenting different, conflicting views of the Merkle Tree at
2504 different times and/or to different parties; issuing STHs too
2505 frequently; mutating the signature of a logged certificate; and
2506 failing to present a chain containing the certifier of a logged
2507 certificate.
2509 Violation of the MMD contract is detected by log clients requesting a
2510 Merkle inclusion proof (Section 5.4) for each observed SCT. These
2511 checks can be asynchronous and need only be done once per
2512 certificate. However, note that there may be privacy concerns (see
2513 Section 8.1.4).
2515 Violation of the append-only property or the STH issuance rate limit
2516 can be detected by multiple clients comparing their instances of the
2517 Signed Tree Heads. This technique, known as "gossip," is an active
2518 area of research and not defined here. Proof of misbehavior in such
2519 cases would be: a series of STHs that were issued too closely
2520 together, proving violation of the STH issuance rate limit; or an STH
2521 with a root hash that does not match the one calculated from a copy
2522 of the log, proving violation of the append-only property.
2524 Clients that report back SCTs can be tracked or traced if a log
2525 produces multiple STHs or SCTs with the same timestamp and data but
2526 different signatures. Logs SHOULD mitigate this risk by either:
2528 * Using deterministic signature schemes, or
2530 * Producing no more than one SCT for each distinct submission and no
2531 more than one STH for each distinct tree_size. Each of these SCTs
2532 and STHs can be stored by the log and served to other clients that
2533 submit the same certificate or request the same STH.
2535 11.4. Multiple SCTs
2537 By requiring TLS servers to offer multiple SCTs, each from a
2538 different log, TLS clients reduce the effectiveness of an attack
2539 where a CA and a log collude (see Section 6.2).
2541 11.5. Leakage of DNS Information
2543 Malicious monitors can use logs to learn about the existence of
2544 domain names that might not otherwise be easy to discover. Some
2545 subdomain labels may reveal information about the service and
2546 software for which the subdomain is used, which in turn might
2547 facilitate targeted attacks.
2549 12. Acknowledgements
2551 The authors would like to thank Erwann Abelea, Robin Alden, Andrew
2552 Ayer, Richard Barnes, Al Cutter, David Drysdale, Francis Dupont, Adam
2553 Eijdenberg, Stephen Farrell, Daniel Kahn Gillmor, Paul Hadfield, Brad
2554 Hill, Jeff Hodges, Paul Hoffman, Jeffrey Hutzelman, Kat Joyce,
2555 Stephen Kent, SM, Alexey Melnikov, Linus Nordberg, Chris Palmer,
2556 Trevor Perrin, Pierre Phaneuf, Eric Rescorla, Rich Salz, Melinda
2557 Shore, Ryan Sleevi, Martin Smith, Carl Wallace and Paul Wouters for
2558 their valuable contributions.
2560 A big thank you to Symantec for kindly donating the OIDs from the
2561 1.3.101 arc that are used in this document.
2563 13. References
2565 13.1. Normative References
2567 [FIPS186-4]
2568 NIST, "FIPS PUB 186-4", 1 July 2013,
2569 .
2572 [HTML401] Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
2573 Specification", World Wide Web Consortium Recommendation
2574 REC-html401-19991224, 24 December 1999,
2575 .
2577 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
2578 Requirement Levels", BCP 14, RFC 2119,
2579 DOI 10.17487/RFC2119, March 1997,
2580 .
2582 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
2583 IETF URN Sub-namespace for Registered Protocol
2584 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
2585 2003, .
2587 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
2588 Resource Identifier (URI): Generic Syntax", STD 66,
2589 RFC 3986, DOI 10.17487/RFC3986, January 2005,
2590 .
2592 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
2593 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
2594 .
2596 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
2597 (TLS) Protocol Version 1.2", RFC 5246,
2598 DOI 10.17487/RFC5246, August 2008,
2599 .
2601 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
2602 Housley, R., and W. Polk, "Internet X.509 Public Key
2603 Infrastructure Certificate and Certificate Revocation List
2604 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
2605 .
2607 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
2608 RFC 5652, DOI 10.17487/RFC5652, September 2009,
2609 .
2611 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
2612 Extensions: Extension Definitions", RFC 6066,
2613 DOI 10.17487/RFC6066, January 2011,
2614 .
2616 [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
2617 (SHA and SHA-based HMAC and HKDF)", RFC 6234,
2618 DOI 10.17487/RFC6234, May 2011,
2619 .
2621 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
2622 Galperin, S., and C. Adams, "X.509 Internet Public Key
2623 Infrastructure Online Certificate Status Protocol - OCSP",
2624 RFC 6960, DOI 10.17487/RFC6960, June 2013,
2625 .
2627 [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
2628 Algorithm (DSA) and Elliptic Curve Digital Signature
2629 Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2630 2013, .
2632 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
2633 Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
2634 DOI 10.17487/RFC7231, June 2014,
2635 .
2637 [RFC7633] Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS)
2638 Feature Extension", RFC 7633, DOI 10.17487/RFC7633,
2639 October 2015, .
2641 [RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP
2642 APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
2643 .
2645 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
2646 Signature Algorithm (EdDSA)", RFC 8032,
2647 DOI 10.17487/RFC8032, January 2017,
2648 .
2650 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2651 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
2652 May 2017, .
2654 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
2655 Interchange Format", STD 90, RFC 8259,
2656 DOI 10.17487/RFC8259, December 2017,
2657 .
2659 [RFC8391] Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A.
2660 Mohaisen, "XMSS: eXtended Merkle Signature Scheme",
2661 RFC 8391, DOI 10.17487/RFC8391, May 2018,
2662 .
2664 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
2665 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
2666 .
2668 [UNIXTIME] IEEE, "The Open Group Base Specifications Issue 7 IEEE Std
2669 1003.1-2008, 2016 Edition", n.d.,
2670 .
2674 [X690] ITU-T, "Information technology - ASN.1 encoding Rules:
2675 Specification of Basic Encoding Rules (BER), Canonical
2676 Encoding Rules (CER) and Distinguished Encoding Rules
2677 (DER)", ISO/IEC 8825-1:2002, November 2015.
2679 13.2. Informative References
2681 [CABBR] CA/Browser Forum, "Baseline Requirements for the Issuance
2682 and Management of Publicly-Trusted Certificates", 2020,
2683 .
2686 [Chromium.Log.Policy]
2687 The Chromium Projects, "Chromium Certificate Transparency
2688 Log Policy", 2014, .
2691 [Chromium.Policy]
2692 The Chromium Projects, "Chromium Certificate
2693 Transparency", 2014, .
2696 [CrosbyWallach]
2697 Crosby, S. and D. Wallach, "Efficient Data Structures for
2698 Tamper-Evident Logging", Proceedings of the 18th USENIX
2699 Security Symposium, Montreal, August 2009,
2700 .
2703 [JSON.Metadata]
2704 The Chromium Projects, "Chromium Log Metadata JSON
2705 Schema", 2014, .
2708 [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
2709 Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
2710 .
2712 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
2713 Writing an IANA Considerations Section in RFCs", BCP 26,
2714 RFC 8126, DOI 10.17487/RFC8126, June 2017,
2715 .
2717 [RFC8820] Nottingham, M., "URI Design and Ownership", BCP 190,
2718 RFC 8820, DOI 10.17487/RFC8820, June 2020,
2719 .
2721 Appendix A. Supporting v1 and v2 simultaneously (Informative)
2723 Certificate Transparency logs have to be either v1 (conforming to
2724 [RFC6962]) or v2 (conforming to this document), as the data
2725 structures are incompatible and so a v2 log could not issue a valid
2726 v1 SCT.
2728 CT clients, however, can support v1 and v2 SCTs, for the same
2729 certificate, simultaneously, as v1 SCTs are delivered in different
2730 TLS, X.509 and OCSP extensions than v2 SCTs.
2732 v1 and v2 SCTs for X.509 certificates can be validated independently.
2733 For precertificates, v2 SCTs should be embedded in the TBSCertificate
2734 before submission of the TBSCertificate (inside a v1 precertificate,
2735 as described in Section 3.1. of [RFC6962]) to a v1 log so that TLS
2736 clients conforming to [RFC6962] but not this document are oblivious
2737 to the embedded v2 SCTs. An issuer can follow these steps to produce
2738 an X.509 certificate with embedded v1 and v2 SCTs:
2740 * Create a CMS precertificate as described in Section 3.2 and submit
2741 it to v2 logs.
2743 * Embed the obtained v2 SCTs in the TBSCertificate, as described in
2744 Section 7.1.2.
2746 * Use that TBSCertificate to create a v1 precertificate, as
2747 described in Section 3.1. of [RFC6962] and submit it to v1 logs.
2749 * Embed the v1 SCTs in the TBSCertificate, as described in
2750 Section 3.3 of [RFC6962].
2752 * Sign that TBSCertificate (which now contains v1 and v2 SCTs) to
2753 issue the final X.509 certificate.
2755 Appendix B. An ASN.1 Module (Informative)
2757 The following ASN.1 module may be useful to implementors.
2759 CertificateTransparencyV2Module-2021
2760 -- { OID Needed, but no point in using a short one }
2761 DEFINITIONS IMPLICIT TAGS ::= BEGIN
2763 -- EXPORTS ALL --
2765 IMPORTS
2766 EXTENSION
2767 FROM PKIX-CommonTypes-2009 -- RFC 5912
2768 { iso(1) identified-organization(3) dod(6) internet(1)
2769 security(5) mechanisms(5) pkix(7) id-mod(0)
2770 id-mod-pkixCommon-02(57) }
2772 CONTENT-TYPE
2773 FROM CryptographicMessageSyntax-2010 -- RFC 6268
2774 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
2775 pkcs-9(9) smime(16) modules(0) id-mod-cms-2009(58) }
2777 TBSCertificate
2778 FROM PKIX1Explicit-2009 -- RFC 5912
2779 { iso(1) identified-organization(3) dod(6) internet(1)
2780 security(5) mechanisms(5) pkix(7) id-mod(0)
2781 id-mod-pkix1-explicit-02(51) }
2782 ;
2784 --
2785 -- Section 3.2. Precertificates
2786 --
2788 ct-tbsCertificate CONTENT-TYPE ::= {
2789 TYPE TBSCertificate
2790 IDENTIFIED BY id-ct-tbsCertificate }
2792 id-ct-tbsCertificate OBJECT IDENTIFIER ::= { 1 3 101 78 }
2794 --
2795 -- Section 7.1. Transparency Information X.509v3 Extension
2796 --
2798 ext-transparencyInfo EXTENSION ::= {
2799 SYNTAX TransparencyInformationSyntax
2800 IDENTIFIED BY id-ce-transparencyInfo
2801 CRITICALITY { FALSE } }
2803 id-ce-transparencyInfo OBJECT IDENTIFIER ::= { 1 3 101 75 }
2805 TransparencyInformationSyntax ::= OCTET STRING
2807 --
2808 -- Section 7.1.1. OCSP Response Extension
2809 --
2811 ext-ocsp-transparencyInfo EXTENSION ::= {
2812 SYNTAX TransparencyInformationSyntax
2813 IDENTIFIED BY id-pkix-ocsp-transparencyInfo
2814 CRITICALITY { FALSE } }
2816 id-pkix-ocsp-transparencyInfo OBJECT IDENTIFIER ::=
2817 id-ce-transparencyInfo
2819 --
2820 -- Section 8.1.2. Reconstructing the TBSCertificate
2821 --
2823 ext-embeddedSCT-CTv1 EXTENSION ::= {
2824 SYNTAX SignedCertificateTimestampList
2825 IDENTIFIED BY id-ce-embeddedSCT-CTv1
2826 CRITICALITY { FALSE } }
2828 id-ce-embeddedSCT-CTv1 OBJECT IDENTIFIER ::= {
2829 1 3 6 1 4 1 11129 2 4 2 }
2831 SignedCertificateTimestampList ::= OCTET STRING
2833 END
2835 Authors' Addresses
2837 Ben Laurie
2838 Google UK Ltd.
2840 Email: benl@google.com
2842 Adam Langley
2843 Google Inc.
2845 Email: agl@google.com
2847 Emilia Kasper
2848 Google Switzerland GmbH
2850 Email: ekasper@google.com
2852 Eran Messeri
2853 Google UK Ltd.
2855 Email: eranm@google.com
2857 Rob Stradling
2858 Sectigo Ltd.
2860 Email: rob@sectigo.com