draft-ietf-trans-rfc6962-bis.md

---

title: "Certificate Transparency Version 2.0" docname: draft-ietf-trans-rfc6962-bis-25 obsoletes: 6962 category: "std"

ipr: "trust200902" area: Security wg: TRANS (Public Notary Transparency)

stand_alone: yes smart_quotes: off pi: [toc, sortrefs, symrefs]

author:

name: Ben Laurie
org: Google UK Ltd.
abbrev: Google
email: [email protected]

• name: Adam Langley org: Google Inc. abbrev: Google email: [email protected]
• name: Emilia Kasper org: Google Switzerland GmbH abbrev: Google email: [email protected]
• name: Eran Messeri org: Google UK Ltd. abbrev: Google email: [email protected]
• name: Rob Stradling org: Comodo CA, Ltd. abbrev: Comodo email: [email protected]

normative: RFC2119: RFC4648: RFC5246: RFC5280: RFC5652: RFC5905: RFC6066: RFC6960: RFC6961: RFC7159: RFC7231: RFC7633: RFC7924: RFC8032: HTML401: target: http://www.w3.org/TR/1999/REC-html401-19991224 title: HTML 4.01 Specification author: - ins: D. Raggett name: David Raggett - ins: A. Le Hors name: Arnaud Le Hors - ins: I. Jacobs name: Ian Jacobs date: 1999-12-24 seriesinfo: "World Wide Web Consortium Recommendation": REC-html401-19991224 FIPS186-4: target: http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf title: FIPS PUB 186-4 author: org: NIST date: 2013-07-01 I-D.ietf-tls-tls13:

informative: RFC6234: RFC5226: RFC6962: RFC6979: RFC7320: I-D.ietf-trans-gossip: CrosbyWallach: target: http://static.usenix.org/event/sec09/tech/full_papers/crosby.pdf title: Efficient Data Structures for Tamper-Evident Logging author: - ins: S. Crosby name: Scott A. Crosby - ins: D. Wallach name: Dan S. Wallach date: 2009-08 seriesinfo: "Proceedings of the 18th USENIX Security Symposium,": Montreal Chromium.Policy: target: http://www.chromium.org/Home/chromium-security/certificate-transparency title: Chromium Certificate Transparency author: org: The Chromium Projects date: 2014 JSON.Metadata: target: http://www.certificate-transparency.org/known-logs/log_list_schema.json title: Chromium Log Metadata JSON Schema author: org: The Chromium Projects date: 2014 Chromium.Log.Policy: target: http://www.chromium.org/Home/chromium-security/certificate-transparency/log-policy title: Chromium Certificate Transparency Log Policy author: org: The Chromium Projects date: 2014 I-D.ietf-trans-threat-analysis:

--- abstract

This document describes version 2.0 of the Certificate Transparency (CT) protocol for publicly logging the existence of Transport Layer Security (TLS) server certificates as they are issued or observed, in a manner that allows anyone to audit certification authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs.

Logs are network services that implement the protocol operations for submissions and queries that are defined in this document.

--- middle

Introduction

Certificate Transparency aims to mitigate the problem of misissued certificates by providing append-only logs of issued certificates. The logs do not need to be trusted because they are publicly auditable. Anyone may verify the correctness of each log and monitor when new certificates are added to it. The logs do not themselves prevent misissue, but they ensure that interested parties (particularly those named in certificates) can detect such misissuance. Note that this is a general mechanism that could be used for transparently logging any form of binary data, subject to some kind of inclusion criteria. In this document, we only describe its use for public TLS server certificates (i.e., where the inclusion criteria is a valid certificate issued by a public certification authority (CA)).

Each log contains certificate chains, which can be submitted by anyone. It is expected that public CAs will contribute all their newly issued certificates to one or more logs; however certificate holders can also contribute their own certificate chains, as can third parties. In order to avoid logs being rendered useless by the submission of large numbers of spurious certificates, it is required that each chain ends with a trust anchor that is accepted by the log. When a chain is accepted by a log, a signed timestamp is returned, which can later be used to provide evidence to TLS clients that the chain has been submitted. TLS clients can thus require that all certificates they accept as valid are accompanied by signed timestamps.

Those who are concerned about misissuance can monitor the logs, asking them regularly for all new entries, and can thus check whether domains for which they are responsible have had certificates issued that they did not expect. What they do with this information, particularly when they find that a misissuance has happened, is beyond the scope of this document. However, broadly speaking, they can invoke existing business mechanisms for dealing with misissued certificates, such as working with the CA to get the certificate revoked, or with maintainers of trust anchor lists to get the CA removed. Of course, anyone who wants can monitor the logs and, if they believe a certificate is incorrectly issued, take action as they see fit.

Similarly, those who have seen signed timestamps from a particular log can later demand a proof of inclusion from that log. If the log is unable to provide this (or, indeed, if the corresponding certificate is absent from monitors' copies of that log), that is evidence of the incorrect operation of the log. The checking operation is asynchronous to allow clients to proceed without delay, despite possible issues such as network connectivity and the vagaries of firewalls.

The append-only property of each log is achieved using Merkle Trees, which can be used to show that any particular instance of the log is a superset of any particular previous instance. Likewise, Merkle Trees avoid the need to blindly trust logs: if a log attempts to show different things to different people, this can be efficiently detected by comparing tree roots and consistency proofs. Similarly, other misbehaviors of any log (e.g., issuing signed timestamps for certificates they then don't log) can be efficiently detected and proved to the world at large.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

Data Structures {#data_structures}

Data structures are defined and encoded according to the conventions laid out in Section 4 of [RFC5246].

Major Differences from CT 1.0

This document revises and obsoletes the experimental CT 1.0 [RFC6962] protocol, drawing on insights gained from CT 1.0 deployments and on feedback from the community. The major changes are:

• Hash and signature algorithm agility: permitted algorithms are now specified in IANA registries.

• Precertificate format: precertificates are now CMS objects rather than X.509 certificates, which avoids violating the certificate serial number uniqueness requirement in Section 4.1.2.2 of [RFC5280].

• Removed precertificate signing certificates and the precertificate poison extension: the change of precertificate format means that these are no longer needed.

• Logs IDs: each log is now identified by an OID rather than by the hash of its public key. OID allocations are managed by an IANA registry.

• TransItem structure: this new data structure is used to encapsulate most types of CT data. A TransItemList, consisting of one or more TransItem structures, can be used anywhere that SignedCertificateTimestampList was used in [RFC6962].

• Merkle tree leaves: the MerkleTreeLeaf structure has been replaced by the TransItem structure, which eases extensibility and simplifies the leaf structure by removing one layer of abstraction.

• Unified leaf format: the structure for both certificate and precertificate entries now includes only the TBSCertificate (whereas certificate entries in [RFC6962] included the entire certificate).

• Log Artifact Extensions: these are now typed and managed by an IANA registry, and they can now appear not only in SCTs but also in STHs.

• API outputs: complete TransItem structures are returned, rather than the constituent parts of each structure.

• get-all-by-hash: new client API for obtaining an inclusion proof and the corresponding consistency proof at the same time.

• Presenting SCTs with proofs: TLS servers may present SCTs together with the corresponding inclusion proofs using any of the mechanisms that [RFC6962] defined for presenting SCTs only. (Presenting SCTs only is still supported).

• CT TLS extension: the signed_certificate_timestamp TLS extension has been replaced by the transparency_info TLS extension.

• Other TLS extensions: status_request_v2 may be used (in the same manner as status_request); cached_info may be used to avoid sending the same complete SCTs and inclusion proofs to the same TLS clients multiple times.

• Verification algorithms: added detailed algorithms for verifying inclusion proofs, for verifying consistency between two STHs, and for verifying a root hash given a complete list of the relevant leaf input entries.

• Extensive clarifications and editorial work.

Cryptographic Components

Merkle Hash Trees {#mht}

Definition of the Merkle Tree {#mht_definition}

The log uses a binary Merkle Hash Tree for efficient auditing. The hash algorithm used is one of the log's parameters (see {{log_parameters}}). We have established a registry of acceptable hash algorithms (see {{hash_algorithms}}). Throughout this document, the hash algorithm in use is referred to as HASH and the size of its output in bytes as HASH_SIZE. The input to the Merkle Tree Hash is a list of data entries; these entries will be hashed to form the leaves of the Merkle Hash Tree. The output is a single HASH_SIZE Merkle Tree Hash. Given an ordered list of n inputs, D_n = {d[0], d[1], ..., d[n-1]}, the Merkle Tree Hash (MTH) is thus defined as follows:

The hash of an empty list is the hash of an empty string:

MTH({}) = HASH().

The hash of a list with one entry (also known as a leaf hash) is:

MTH({d[0]}) = HASH(0x00 || d[0]).

For n > 1, let k be the largest power of two smaller than n (i.e., k < n <= 2k). The Merkle Tree Hash of an n-element list D_n is then defined recursively as

MTH(D_n) = HASH(0x01 || MTH(D[0:k]) || MTH(D[k:n])),

Where || is concatenation and D[k1:k2] = D'_(k2-k1) denotes the list {d'[0] = d[k1], d'[1] = d[k1+1], ..., d'[k2-k1-1] = d[k2-1]} of length (k2 - k1). (Note that the hash calculations for leaves and nodes differ; this domain separation is required to give second preimage resistance).

Note that we do not require the length of the input list to be a power of two. The resulting Merkle Tree may thus not be balanced; however, its shape is uniquely determined by the number of leaves. (Note: This Merkle Tree is essentially the same as the history tree [CrosbyWallach] proposal, except our definition handles non-full trees differently).

Verifying a Tree Head Given Entries {#verify_hash}

When a client has a complete list of n input entries from 0 up to tree_size - 1 and wishes to verify this list against a tree head root_hash returned by the log for the same tree_size, the following algorithm may be used:

1. Set stack to an empty stack.

2. For each i from 0 up to tree_size - 1:

   1. Push HASH(0x00 || entries[i]) to stack.

   2. Set merge_count to the lowest value (0 included) such that LSB(i >> merge_count) is not set. In other words, set merge_count to the number of consecutive 1s found starting at the least significant bit of i.

   3. Repeat merge_count times:

      1. Pop right from stack.

      2. Pop left from stack.

      3. Push HASH(0x01 || left || right) to stack.

3. If there is more than one element in the stack, repeat the same merge procedure (Step 2.3 above) until only a single element remains.

4. The remaining element in stack is the Merkle Tree hash for the given tree_size and should be compared by equality against the supplied root_hash.

Merkle Inclusion Proofs {#merkle_inclusion_proof}

A Merkle inclusion proof for a leaf in a Merkle Hash Tree is the shortest list of additional nodes in the Merkle Tree required to compute the Merkle Tree Hash for that tree. Each node in the tree is either a leaf node or is computed from the two nodes immediately below it (i.e., towards the leaves). At each step up the tree (towards the root), a node from the inclusion proof is combined with the node computed so far. In other words, the inclusion proof consists of the list of missing nodes required to compute the nodes leading from a leaf to the root of the tree. If the root computed from the inclusion proof matches the true root, then the inclusion proof proves that the leaf exists in the tree.

Generating an Inclusion Proof

Given an ordered list of n inputs to the tree, D_n = {d[0], d[1], ..., d[n-1]}, the Merkle inclusion proof PATH(m, D_n) for the (m+1)th input d[m], 0 <= m < n, is defined as follows:

The proof for the single leaf in a tree with a one-element input list D[1] = {d[0]} is empty:

PATH(0, {d[0]}) = {}

For n > 1, let k be the largest power of two smaller than n. The proof for the (m+1)th element d[m] in a list of n > m elements is then defined recursively as

PATH(m, D_n) = PATH(m, D[0:k]) : MTH(D[k:n]) for m < k; and

PATH(m, D_n) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for m >= k,

The : operator and D[k1:k2] are defined the same as in {{mht_definition}}.

Verifying an Inclusion Proof {#verify_inclusion}

When a client has received an inclusion proof (e.g., in a TransItem of type inclusion_proof_v2) and wishes to verify inclusion of an input hash for a given tree_size and root_hash, the following algorithm may be used to prove the hash was included in the root_hash:

1. Compare leaf_index against tree_size. If leaf_index is greater than or equal to tree_size then fail the proof verification.

2. Set fn to leaf_index and sn to tree_size - 1.

3. Set r to hash.

4. For each value p in the inclusion_path array:

   If sn is 0, stop the iteration and fail the proof verification.

   If LSB(fn) is set, or if fn is equal to sn, then:

   1. Set r to HASH(0x01 || p || r)

   2. If LSB(fn) is not set, then right-shift both fn and sn equally until either LSB(fn) is set or fn is 0.

   Otherwise:

   1. Set r to HASH(0x01 || r || p)

   Finally, right-shift both fn and sn one time.

5. Compare sn to 0. Compare r against the root_hash. If sn is equal to 0, and r and the root_hash are equal, then the log has proven the inclusion of hash. Otherwise, fail the proof verification.

Merkle Consistency Proofs {#consistency}

Merkle consistency proofs prove the append-only property of the tree. A Merkle consistency proof for a Merkle Tree Hash MTH(D_n) and a previously advertised hash MTH(D[0:m]) of the first m leaves, m <= n, is the list of nodes in the Merkle Tree required to verify that the first m inputs D[0:m] are equal in both trees. Thus, a consistency proof must contain a set of intermediate nodes (i.e., commitments to inputs) sufficient to verify MTH(D_n), such that (a subset of) the same nodes can be used to verify MTH(D[0:m]). We define an algorithm that outputs the (unique) minimal consistency proof.

Generating a Consistency Proof

Given an ordered list of n inputs to the tree, D_n = {d[0], d[1], ..., d[n-1]}, the Merkle consistency proof PROOF(m, D_n) for a previous Merkle Tree Hash MTH(D[0:m]), 0 < m < n, is defined as:

PROOF(m, D_n) = SUBPROOF(m, D_n, true)

In SUBPROOF, the boolean value represents whether the subtree created from D[0:m] is a complete subtree of the Merkle Tree created from D_n, and, consequently, whether the subtree Merkle Tree Hash MTH(D[0:m]) is known. The initial call to SUBPROOF sets this to be true, and SUBPROOF is then defined as follows:

The subproof for m = n is empty if m is the value for which PROOF was originally requested (meaning that the subtree created from D[0:m] is a complete subtree of the Merkle Tree created from the original D_n for which PROOF was requested, and the subtree Merkle Tree Hash MTH(D[0:m]) is known):

SUBPROOF(m, D[m], true) = {}

Otherwise, the subproof for m = n is the Merkle Tree Hash committing inputs D[0:m]:

SUBPROOF(m, D[m], false) = {MTH(D[m])}

For m < n, let k be the largest power of two smaller than n. The subproof is then defined recursively.

If m <= k, the right subtree entries D[k:n] only exist in the current tree. We prove that the left subtree entries D[0:k] are consistent and add a commitment to D[k:n]:

SUBPROOF(m, D_n, b) = SUBPROOF(m, D[0:k], b) : MTH(D[k:n])

If m > k, the left subtree entries D[0:k] are identical in both trees. We prove that the right subtree entries D[k:n] are consistent and add a commitment to D[0:k].

SUBPROOF(m, D_n, b) = SUBPROOF(m - k, D[k:n], false) : MTH(D[0:k])

The number of nodes in the resulting proof is bounded above by ceil(log2(n)) + 1.

The : operator and D[k1:k2] are defined the same as in {{mht_definition}}.

Verifying Consistency between Two Tree Heads {#verify_consistency}

When a client has a tree head first_hash for tree size first, a tree head second_hash for tree size second where 0 < first < second, and has received a consistency proof between the two (e.g., in a TransItem of type consistency_proof_v2), the following algorithm may be used to verify the consistency proof:

1. If first is an exact power of 2, then prepend first_hash to the consistency_path array.

2. Set fn to first - 1 and sn to second - 1.

3. If LSB(fn) is set, then right-shift both fn and sn equally until LSB(fn) is not set.

4. Set both fr and sr to the first value in the consistency_path array.

5. For each subsequent value c in the consistency_path array:

   If sn is 0, stop the iteration and fail the proof verification.

   If LSB(fn) is set, or if fn is equal to sn, then:

   1. Set fr to HASH(0x01 || c || fr)\ Set sr to HASH(0x01 || c || sr)

   2. If LSB(fn) is not set, then right-shift both fn and sn equally until either LSB(fn) is set or fn is 0.

   Otherwise:

   1. Set sr to HASH(0x01 || sr || c)

   Finally, right-shift both fn and sn one time.

6. After completing iterating through the consistency_path array as described above, verify that the fr calculated is equal to the first_hash supplied, that the sr calculated is equal to the second_hash supplied and that sn is 0.

Example

The binary Merkle Tree with 7 leaves:

            hash
           /    \
          /      \
         /        \
        /          \
       /            \
      k              l
     / \            / \
    /   \          /   \
   /     \        /     \
  g       h      i      j
 / \     / \    / \     |
 a b     c d    e f     d6
 | |     | |    | |
d0 d1   d2 d3  d4 d5

The inclusion proof for d0 is [b, h, l].

The inclusion proof for d3 is [c, g, l].

The inclusion proof for d4 is [f, j, k].

The inclusion proof for d6 is [i, k].

The same tree, built incrementally in four steps:

    hash0          hash1=k
    / \              /  \
   /   \            /    \
  /     \          /      \
  g      c         g       h
 / \     |        / \     / \
 a b     d2       a b     c d
 | |              | |     | |
d0 d1            d0 d1   d2 d3

          hash2                    hash
          /  \                    /    \
         /    \                  /      \
        /      \                /        \
       /        \              /          \
      /          \            /            \
     k            i          k              l
    / \          / \        / \            / \
   /   \         e f       /   \          /   \
  /     \        | |      /     \        /     \
 g       h      d4 d5    g       h      i      j
/ \     / \             / \     / \    / \     |
a b     c d             a b     c d    e f     d6
| |     | |             | |     | |    | |
d0 d1   d2 d3           d0 d1   d2 d3  d4 d5

The consistency proof between hash0 and hash is PROOF(3, D[7]) = [c, d, g, l]. c, g are used to verify hash0, and d, l are additionally used to show hash is consistent with hash0.

The consistency proof between hash1 and hash is PROOF(4, D[7]) = [l]. hash can be verified using hash1=k and l.

The consistency proof between hash2 and hash is PROOF(6, D[7]) = [i, j, k]. k, i are used to verify hash2, and j is additionally used to show hash is consistent with hash2.

Signatures {#signatures}

Various data structures {{data_structures}} are signed. A log MUST use one of the signature algorithms defined in {{signature_algorithms}}.

Submitters

Submitters submit certificates or preannouncements of certificates prior to issuance (precertificates) to logs for public auditing, as described below. In order to enable attribution of each logged certificate or precertificate to its issuer, each submission MUST be accompanied by all additional certificates required to verify the chain up to an accepted trust anchor. The trust anchor (a root or intermediate CA certificate) MAY be omitted from the submission.

If a log accepts a submission, it will return a Signed Certificate Timestamp (SCT) (see {{sct}}). The submitter SHOULD validate the returned SCT as described in {{tls_clients}} if they understand its format and they intend to use it directly in a TLS handshake or to construct a certificate. If the submitter does not need the SCT (for example, the certificate is being submitted simply to make it available in the log), it MAY validate the SCT.

Certificates

Any entity can submit a certificate ({{submit-entry}}) to a log. Since it is anticipated that TLS clients will reject certificates that are not logged, it is expected that certificate issuers and subjects will be strongly motivated to submit them.

Precertificates {#precertificates}

CAs may preannounce a certificate prior to issuance by submitting a precertificate ({{submit-entry}}) that the log can use to create an entry that will be valid against the issued certificate. The CA MAY incorporate the returned SCT in the issued certificate. One example of where the returned SCT is not incorporated in the issued certificate is when a CA sends the precertificate to multiple logs, but only incorporates the SCTs that are returned first.

A precertificate is a CMS [RFC5652] signed-data object that conforms to the following profile:

• It MUST be DER encoded.

• SignedData.version MUST be v3(3).

• SignedData.digestAlgorithms MUST only include the SignerInfo.digestAlgorithm OID value (see below).

• SignedData.encapContentInfo:

  • eContentType MUST be the OID 1.3.101.78.
  • eContent MUST contain a TBSCertificate [RFC5280] that will be identical to the TBSCertificate in the issued certificate, except that the Transparency Information ({{x509v3_transinfo_extension}}) extension MUST be omitted.

• SignedData.certificates MUST be omitted.

• SignedData.crls MUST be omitted.

• SignedData.signerInfos MUST contain one SignerInfo:

  • version MUST be v3(3).
  • sid MUST use the subjectKeyIdentifier option.
  • digestAlgorithm MUST be one of the hash algorithm OIDs listed in {{hash_algorithms}}.
  • signedAttrs MUST be present and MUST contain two attributes:
    • A content-type attribute whose value is the same as SignedData.encapContentInfo.eContentType.
    • A message-digest attribute whose value is the message digest of SignedData.encapContentInfo.eContent.
  • signatureAlgorithm MUST be the same OID as TBSCertificate.signature.
  • signature MUST be from the same (root or intermediate) CA that will ultimately issue the certificate. This signature indicates the CA's intent to issue the certificate. This intent is considered binding (i.e., misissuance of the precertificate is considered equivalent to misissuance of the corresponding certificate).
  • unsignedAttrs MUST be omitted.

SignerInfo.signedAttrs is included in the message digest calculation process (see Section 5.4 of [RFC5652]), which ensures that the SignerInfo.signature value will not be a valid X.509v3 signature that could be used in conjunction with the TBSCertificate (from SignedData.encapContentInfo.eContent) to construct a valid certificate.

Log Format and Operation

A log is a single, append-only Merkle Tree of submitted certificate and precertificate entries.

When it receives and accepts a valid submission, the log MUST return an SCT that corresponds to the submitted certificate or precertificate. If the log has previously seen this valid submission, it SHOULD return the same SCT as it returned before (to reduce the ability to track clients as described in {{prevent_tracking_clients}}). If different SCTs are produced for the same submission, multiple log entries will have to be created, one for each SCT (as the timestamp is a part of the leaf structure). Note that if a certificate was previously logged as a precertificate, then the precertificate's SCT of type precert_sct_v2 would not be appropriate; instead, a fresh SCT of type x509_sct_v2 should be generated.

An SCT is the log's promise to append to its Merkle Tree an entry for the accepted submission. Upon producing an SCT, the log MUST fulfil this promise by performing the following actions within a fixed amount of time known as the Maximum Merge Delay (MMD), which is one of the log's parameters (see {{log_parameters}}):

• Allocate a tree index to the entry representing the accepted submission.
• Calculate the root of the tree.
• Sign the root of the tree (see {{sth}}). The log may append multiple entries before signing the root of the tree.

Log operators SHOULD NOT impose any conditions on retrieving or sharing data from the log.

Log Parameters {#log_parameters}

A log is defined by a collection of parameters, which are used by clients to communicate with the log and to verify log artifacts.

Base URL: : The URL to substitute for <log server> in {{client_messages}}.

Hash Algorithm: : The hash algorithm used for the Merkle Tree (see {{hash_algorithms}}).

Signature Algorithm: : The signature algorithm used (see {{signatures}}).

Public Key: : The public key used to verify signatures generated by the log. A log MUST NOT use the same keypair as any other log.

Log ID: : The OID that uniquely identifies the log.

Maximum Merge Delay: : The MMD the log has committed to.

Version: : The version of the protocol supported by the log (currently 1 or 2).

Maximum Chain Length: : The longest chain submission the log is willing to accept, if the log chose to limit it.

STH Frequency Count: : The maximum number of STHs the log may produce in any period equal to the Maximum Merge Delay (see {{sth}}).

Final STH: : If a log has been closed down (i.e., no longer accepts new entries), existing entries may still be valid. In this case, the client should know the final valid STH in the log to ensure no new entries can be added without detection. The final STH should be provided in the form of a TransItem of type signed_tree_head_v2.

[JSON.Metadata] is an example of a metadata format which includes the above elements.

Accepting Submissions

To avoid being overloaded by invalid submissions, the log MUST NOT accept any submission until it has verified that the submitted certificate or precertificate has a valid signature chain to an accepted trust anchor, using only the chain of intermediate CA certificates provided by the submitter.

Logs SHOULD accept certificates and precertificates that are fully valid according to RFC 5280 [RFC5280] verification rules and are submitted with such a chain. (A log may decide, for example, to temporarily reject valid submissions to protect itself against denial-of-service attacks).

Logs MAY accept certificates and precertificates that have expired, are not yet valid, have been revoked, or are otherwise not fully valid according to RFC 5280 verification rules in order to accommodate quirks of CA certificate-issuing software. However, logs MUST reject submissions without a valid signature chain to an accepted trust anchor. Logs MUST also reject precertificates that do not conform to the requirements in {{precertificates}}.

Logs SHOULD limit the length of chain they will accept. The maximum chain length is one of the log's parameters (see {{log_parameters}}).

The log SHALL allow retrieval of its list of accepted trust anchors (see {{get-anchors}}), each of which is a root or intermediate CA certificate. This list might usefully be the union of root certificates trusted by major browser vendors.

Log Entries {#log_entries}

If a submission is accepted and an SCT issued, the accepting log MUST store the entire chain used for verification. This chain MUST include the certificate or precertificate itself, the zero or more intermediate CA certificates provided by the submitter, and the trust anchor used to verify the chain (even if it was omitted from the submission). The log MUST present this chain for auditing upon request (see {{get-entries}}). This prevents the CA from avoiding blame by logging a partial or empty chain. Each log entry is a TransItem structure of type x509_entry_v2 or precert_entry_v2. However, a log may store its entries in any format. If a log does not store this TransItem in full, it must store the timestamp and sct_extensions of the corresponding TimestampedCertificateEntryDataV2 structure. The TransItem can be reconstructed from these fields and the entire chain that the log used to verify the submission.

Log ID {#log_id}

Each log is identified by an OID, which is one of the log's parameters (see {{log_parameters}}) and which MUST NOT be used to identify any other log. A log's operator MUST either allocate the OID themselves or request an OID from the Log ID Registry (see {{log_id_registry}}). Various data structures include the DER encoding of this OID, excluding the ASN.1 tag and length bytes, in an opaque vector:

    opaque LogID<2..127>;

Note that the ASN.1 length and the opaque vector length are identical in size (1 byte) and value, so the DER encoding of the OID can be reproduced simply by prepending an OBJECT IDENTIFIER tag (0x06) to the opaque vector length and contents.

OIDs used to identify logs are limited such that the DER encoding of their value is less than or equal to 127 octets.

TransItem Structure

Various data structures are encapsulated in the TransItem structure to ensure that the type and version of each one is identified in a common fashion:

    enum {
        reserved(0),
        x509_entry_v2(1), precert_entry_v2(2),
        x509_sct_v2(3), precert_sct_v2(4),
        signed_tree_head_v2(5), consistency_proof_v2(6),
        inclusion_proof_v2(7),
        (65535)
    } VersionedTransType;

    struct {
        VersionedTransType versioned_type;
        select (versioned_type) {
            case x509_entry_v2: TimestampedCertificateEntryDataV2;
            case precert_entry_v2: TimestampedCertificateEntryDataV2;
            case x509_sct_v2: SignedCertificateTimestampDataV2;
            case precert_sct_v2: SignedCertificateTimestampDataV2;
            case signed_tree_head_v2: SignedTreeHeadDataV2;
            case consistency_proof_v2: ConsistencyProofDataV2;
            case inclusion_proof_v2: InclusionProofDataV2;
        } data;
    } TransItem;

versioned_type is a value from the IANA registry in {{versioned_trans_types}} that identifies the type of the encapsulated data structure and the earliest version of this protocol to which it conforms. This document is v2.

data is the encapsulated data structure. The various structures named with the DataV2 suffix are defined in later sections of this document.

Note that VersionedTransType combines the v1 [RFC6962] type enumerations Version, LogEntryType, SignatureType and MerkleLeafType. Note also that v1 did not define TransItem, but this document provides guidelines (see {{v1_coexistence}}) on how v2 implementations can co-exist with v1 implementations.

Future versions of this protocol may reuse VersionedTransType values defined in this document as long as the corresponding data structures are not modified, and may add new VersionedTransType values for new or modified data structures.

Log Artifact Extensions

    enum {
        reserved(65535)
    } ExtensionType;

    struct {
        ExtensionType extension_type;
        opaque extension_data<0..2^16-1>;
    } Extension;

The Extension structure provides a generic extensibility for log artifacts, including Signed Certificate Timestamps ({{sct}}) and Signed Tree Heads ({{sth}}). The interpretation of the extension_data field is determined solely by the value of the extension_type field.

This document does not define any extensions, but it does establish a registry for future ExtensionType values (see {{log_artifact_extension_registry}}). Each document that registers a new ExtensionType must specify the context in which it may be used (e.g., SCT, STH, or both) and describe how to interpret the corresponding extension_data.

Merkle Tree Leaves {#tree_leaves}

The leaves of a log's Merkle Tree correspond to the log's entries (see {{log_entries}}). Each leaf is the leaf hash ({{mht}}) of a TransItem structure of type x509_entry_v2 or precert_entry_v2, which encapsulates a TimestampedCertificateEntryDataV2 structure. Note that leaf hashes are calculated as HASH(0x00 || TransItem), where the hash algorithm is one of the log's parameters.

    opaque SubjectPublicKeyInfo<1..2^24-1>;
    opaque TBSCertificate<1..2^24-1>;

    struct {
        uint64 timestamp;
        SubjectPublicKeyInfo issuer_key;
        TBSCertificate tbs_certificate;
        Extension sct_extensions<0..2^16-1>;
    } TimestampedCertificateEntryDataV2;

timestamp is the NTP Time [RFC5905] at which the certificate or precertificate was accepted by the log, measured in milliseconds since the epoch (January 1, 1970, 00:00 UTC), ignoring leap seconds. Note that the leaves of a log's Merkle Tree are not required to be in strict chronological order.

issuer_key is the DER encoded public key of the CA that issued the certificate or precertificate, represented as SubjectPublicKeyInfo [RFC5280]. This is needed (1) to bind the CA to the certificate or precertificate, making it impossible for the corresponding SCT to be valid for any other certificate or precertificate whose TBSCertificate matches tbs_certificate, and (2) to make it possible to verify the certificate signature to prevent a malicious log from mutating it.

tbs_certificate is the DER encoded TBSCertificate from the submission. (Note that a precertificate's TBSCertificate can be reconstructed from the corresponding certificate as described in {{reconstructing_tbscertificate}}).

sct_extensions matches the SCT extensions of the corresponding SCT.

The type of the TransItem corresponds to the value of the type parameter supplied in the {{submit-entry}} call.

Signed Certificate Timestamp (SCT) {#sct}

An SCT is a TransItem structure of type x509_sct_v2 or precert_sct_v2, which encapsulates a SignedCertificateTimestampDataV2 structure:

    struct {
        LogID log_id;
        uint64 timestamp;
        Extension sct_extensions<0..2^16-1>;
        opaque signature<0..2^16-1>;
    } SignedCertificateTimestampDataV2;

log_id is this log's unique ID, encoded in an opaque vector as described in {{log_id}}.

timestamp is equal to the timestamp from the corresponding TimestampedCertificateEntryDataV2 structure.

sct_extensions is a vector of 0 or more SCT extensions. This vector MUST NOT include more than one extension with the same extension_type. The extensions in the vector MUST be ordered by the value of the extension_type field, smallest value first. If an implementation sees an extension that it does not understand, it SHOULD ignore that extension. Furthermore, an implementation MAY choose to ignore any extension(s) that it does understand.

signature is computed over a TransItem structure of type x509_entry_v2 or precert_entry_v2 (see {{tree_leaves}}) using the signature algorithm declared in the log's parameters (see {{log_parameters}}).

Merkle Tree Head {#tree_head}

The log stores information about its Merkle Tree in a TreeHeadDataV2:

    opaque NodeHash<32..2^8-1>;

    struct {
        uint64 timestamp;
        uint64 tree_size;
        NodeHash root_hash;
        Extension sth_extensions<0..2^16-1>;
    } TreeHeadDataV2;

The length of NodeHash MUST match HASH_SIZE of the log.

timestamp is the current NTP Time [RFC5905], measured in milliseconds since the epoch (January 1, 1970, 00:00 UTC), ignoring leap seconds.

tree_size is the number of entries currently in the log's Merkle Tree.

root_hash is the root of the Merkle Hash Tree.

sth_extensions is a vector of 0 or more STH extensions. This vector MUST NOT include more than one extension with the same extension_type. The extensions in the vector MUST be ordered by the value of the extension_type field, smallest value first. If an implementation sees an extension that it does not understand, it SHOULD ignore that extension. Furthermore, an implementation MAY choose to ignore any extension(s) that it does understand.

Signed Tree Head (STH) {#sth}

Periodically each log SHOULD sign its current tree head information (see {{tree_head}}) to produce an STH. When a client requests a log's latest STH (see {{get-sth}}), the log MUST return an STH that is no older than the log's MMD. However, since STHs could be used to mark individual clients (by producing a new STH for each query), a log MUST NOT produce STHs more frequently than its parameters declare (see {{log_parameters}}). In general, there is no need to produce a new STH unless there are new entries in the log; however, in the event that a log does not accept any submissions during an MMD period, the log MUST sign the same Merkle Tree Hash with a fresh timestamp.

An STH is a TransItem structure of type signed_tree_head_v2, which encapsulates a SignedTreeHeadDataV2 structure:

    struct {
        LogID log_id;
        TreeHeadDataV2 tree_head;
        opaque signature<0..2^16-1>;
    } SignedTreeHeadDataV2;

log_id is this log's unique ID, encoded in an opaque vector as described in {{log_id}}.

The timestamp in tree_head MUST be at least as recent as the most recent SCT timestamp in the tree. Each subsequent timestamp MUST be more recent than the timestamp of the previous update.

tree_head contains the latest tree head information (see {{tree_head}}).

signature is computed over the tree_head field using the signature algorithm declared in the log's parameters (see {{log_parameters}}).

Merkle Consistency Proofs

To prepare a Merkle Consistency Proof for distribution to clients, the log produces a TransItem structure of type consistency_proof_v2, which encapsulates a ConsistencyProofDataV2 structure:

    struct {
        LogID log_id;
        uint64 tree_size_1;
        uint64 tree_size_2;
        NodeHash consistency_path<1..2^16-1>;
    } ConsistencyProofDataV2;

log_id is this log's unique ID, encoded in an opaque vector as described in {{log_id}}.

tree_size_1 is the size of the older tree.

tree_size_2 is the size of the newer tree.

consistency_path is a vector of Merkle Tree nodes proving the consistency of two STHs.

Merkle Inclusion Proofs

To prepare a Merkle Inclusion Proof for distribution to clients, the log produces a TransItem structure of type inclusion_proof_v2, which encapsulates an InclusionProofDataV2 structure:

    struct {
        LogID log_id;
        uint64 tree_size;
        uint64 leaf_index;
        NodeHash inclusion_path<1..2^16-1>;
    } InclusionProofDataV2;

log_id is this log's unique ID, encoded in an opaque vector as described in {{log_id}}.

tree_size is the size of the tree on which this inclusion proof is based.

leaf_index is the 0-based index of the log entry corresponding to this inclusion proof.

inclusion_path is a vector of Merkle Tree nodes proving the inclusion of the chosen certificate or precertificate.

Shutting down a log {#log_shutdown}

Log operators may decide to shut down a log for various reasons, such as deprecation of the signature algorithm. If there are entries in the log for certificates that have not yet expired, simply making TLS clients stop recognizing that log will have the effect of invalidating SCTs from that log. To avoid that, the following actions are suggested:

• Make it known to clients and monitors that the log will be frozen.

• Stop accepting new submissions (the error code "shutdown" should be returned for such requests).

• Once MMD from the last accepted submission has passed and all pending submissions are incorporated, issue a final STH and publish it as one of the log's parameters. Having an STH with a timestamp that is after the MMD has passed from the last SCT issuance allows clients to audit this log regularly without special handling for the final STH. At this point the log's private key is no longer needed and can be destroyed.

• Keep the log running until the certificates in all of its entries have expired or exist in other logs (this can be determined by scanning other logs or connecting to domains mentioned in the certificates and inspecting the SCTs served).

Log Client Messages {#client_messages}

Messages are sent as HTTPS GET or POST requests. Parameters for POSTs and all responses are encoded as JavaScript Object Notation (JSON) objects [RFC7159]. Parameters for GETs are encoded as order-independent key/value URL parameters, using the "application/x-www-form-urlencoded" format described in the "HTML 4.01 Specification" [HTML401]. Binary data is base64 encoded [RFC4648] as specified in the individual messages.

Clients are configured with a base URL for a log and construct URLs for requests by appending suffixes to this base URL. This structure places some degree of restriction on how log operators can deploy these services, as noted in [RFC7320]. However, operational experience with version 1 of this protocol has not indicated that these restrictions are a problem in practice.

Note that JSON objects and URL parameters may contain fields not specified here. These extra fields should be ignored.

The <log server> prefix, which is one of the log's parameters, MAY include a path as well as a server name and a port.

In practice, log servers may include multiple front-end machines. Since it is impractical to keep these machines in perfect sync, errors may occur that are caused by skew between the machines. Where such errors are possible, the front-end will return additional information (as specified below) making it possible for clients to make progress, if progress is possible. Front-ends MUST only serve data that is free of gaps (that is, for example, no front-end will respond with an STH unless it is also able to prove consistency from all log entries logged within that STH).

For example, when a consistency proof between two STHs is requested, the front-end reached may not yet be aware of one or both STHs. In the case where it is unaware of both, it will return the latest STH it is aware of. Where it is aware of the first but not the second, it will return the latest STH it is aware of and a consistency proof from the first STH to the returned STH. The case where it knows the second but not the first should not arise (see the "no gaps" requirement above).

If the log is unable to process a client's request, it MUST return an HTTP response code of 4xx/5xx (see [RFC7231]), and, in place of the responses outlined in the subsections below, the body SHOULD be a JSON structure containing at least the following field:

error_message: : A human-readable string describing the error which prevented the log from processing the request.

: In the case of a malformed request, the string SHOULD provide sufficient detail for the error to be rectified.

error_code: : An error code readable by the client. Other than the generic codes detailed here, each error code is specific to the type of request. Specific errors are specified in the respective sections below. Error codes are fixed text strings.

|---------------+---------------------------------------------| | Error Code | Meaning | |---------------+---------------------------------------------| | not compliant | The request is not compliant with this RFC. | |---------------+---------------------------------------------|

e.g., In response to a request of /ct/v2/get-entries?start=100&end=99, the log would return a 400 Bad Request response code with a body similar to the following:

    {
        "error_message": "'start' cannot be greater than 'end'",
        "error_code": "not compliant",
    }

Clients SHOULD treat 500 Internal Server Error and 503 Service Unavailable responses as transient failures and MAY retry the same request without modification at a later date. Note that as per [RFC7231], in the case of a 503 response the log MAY include a Retry-After: header in order to request a minimum time for the client to wait before retrying the request.

Submit Entry to Log {#submit-entry}

POST https://<log server>/ct/v2/submit-entry

Inputs:

: submission: : The base64 encoded certificate or precertificate.

type: : The VersionedTransType integer value that indicates the type of the submission: 1 for x509_entry_v2, or 2 for precert_entry_v2.

chain: : An array of zero or more base64 encoded CA certificates. The first element is the signer of the submission; the second certifies the first; etc. The last element of chain (or, if chain is an empty array, the submission) either is, or is certified by, an accepted trust anchor.

Outputs:

: sct: : A base64 encoded TransItem of type x509_sct_v2 or precert_sct_v2, signed by this log, that corresponds to the submission.

If the submitted entry is immediately appended to (or already exists in) this log's tree, then the log SHOULD also output:

sth: : A base64 encoded TransItem of type signed_tree_head_v2, signed by this log.

inclusion: : A base64 encoded TransItem of type inclusion_proof_v2 whose inclusion_path array of Merkle Tree nodes proves the inclusion of the submission in the returned sth.

Error codes:

|-----------------+--------------------------------------------------------------------------------------------------------------------------------------------------| | Error Code | Meaning | |-----------------+--------------------------------------------------------------------------------------------------------------------------------------------------| | bad submission | submission is neither a valid certificate nor a valid precertificate. | | bad type | type is neither 1 nor 2. | | bad chain | The first element of chain is not the signer of the submission, or the second element does not certify the first, etc. | | bad certificate | One or more certificates in the chain are not valid (e.g., not properly encoded). | | unknown anchor | The last element of chain (or, if chain is an empty array, the submission) both is not, and is not certified by, an accepted trust anchor. | | shutdown | The log is no longer accepting submissions. | |-----------------+--------------------------------------------------------------------------------------------------------------------------------------------------|

If the version of sct is not v2, then a v2 client may be unable to verify the signature. It MUST NOT construe this as an error. This is to avoid forcing an upgrade of compliant v2 clients that do not use the returned SCTs.

If a log detects bad encoding in a chain that otherwise verifies correctly then the log MUST either log the certificate or return the "bad certificate" error. If the certificate is logged, an SCT MUST be issued. Logging the certificate is useful, because monitors ({{monitor}}) can then detect these encoding errors, which may be accepted by some TLS clients.

If the returned sct is intended to be provided to clients, then sth and inclusion (if returned) SHOULD also be provided to clients (e.g., if type was 1 then all three TransItems could be embedded in the certificate).

Retrieve Latest Signed Tree Head {#get-sth}

GET https://<log server>/ct/v2/get-sth

No inputs.

Outputs:

: sth: : A base64 encoded TransItem of type signed_tree_head_v2, signed by this log, that is no older than the log's MMD.

Retrieve Merkle Consistency Proof between Two Signed Tree Heads {#get-sth-consistency}

GET https://<log server>/ct/v2/get-sth-consistency

Inputs:

: first: : The tree_size of the older tree, in decimal.

second: : The tree_size of the newer tree, in decimal (optional).

Both tree sizes must be from existing v2 STHs. However, because of skew, the receiving front-end may not know one or both of the existing STHs. If both are known, then only the consistency output is returned. If the first is known but the second is not (or has been omitted), then the latest known STH is returned, along with a consistency proof between the first STH and the latest. If neither are known, then the latest known STH is returned without a consistency proof.

Outputs:

: consistency: : A base64 encoded TransItem of type consistency_proof_v2, whose tree_size_1 MUST match the first input. If the sth output is omitted, then tree_size_2 MUST match the second input. If first and second are equal and correspond to a known STH, the returned consistency proof MUST be empty (a consistency_path array with zero elements).

sth: : A base64 encoded TransItem of type signed_tree_head_v2, signed by this log.

Note that no signature is required for the consistency output as it is used to verify the consistency between two STHs, which are signed.

Error codes:

|----------------+--------------------------------------------------------------------------| | Error Code | Meaning | |----------------+--------------------------------------------------------------------------| | first unknown | first is before the latest known STH but is not from an existing STH. | | second unknown | second is before the latest known STH but is not from an existing STH. | |----------------+--------------------------------------------------------------------------|

See {{verify_consistency}} for an outline of how to use the consistency output.

Retrieve Merkle Inclusion Proof from Log by Leaf Hash {#get-proof-by-hash}

GET https://<log server>/ct/v2/get-proof-by-hash

Inputs:

: hash: : A base64 encoded v2 leaf hash.

tree_size: : The tree_size of the tree on which to base the proof, in decimal.

The hash must be calculated as defined in {{tree_leaves}}. The tree_size must designate an existing v2 STH. Because of skew, the front-end may not know the requested STH. In that case, it will return the latest STH it knows, along with an inclusion proof to that STH. If the front-end knows the requested STH then only inclusion is returned.

Outputs:

: inclusion: : A base64 encoded TransItem of type inclusion_proof_v2 whose inclusion_path array of Merkle Tree nodes proves the inclusion of the chosen certificate in the selected STH.

sth: : A base64 encoded TransItem of type signed_tree_head_v2, signed by this log.

Note that no signature is required for the inclusion output as it is used to verify inclusion in the selected STH, which is signed.

Error codes:

|-------------------+----------------------------------------------------------------------------------------------------------| | Error Code | Meaning | |-------------------+----------------------------------------------------------------------------------------------------------| | hash unknown | hash is not the hash of a known leaf (may be caused by skew or by a known certificate not yet merged). | | tree_size unknown | hash is before the latest known STH but is not from an existing STH. | |-------------------+----------------------------------------------------------------------------------------------------------|

See {{verify_inclusion}} for an outline of how to use the inclusion output.

Retrieve Merkle Inclusion Proof, Signed Tree Head and Consistency Proof by Leaf Hash {#get-all-by-hash}

GET https://<log server>/ct/v2/get-all-by-hash

Inputs:

: hash: : A base64 encoded v2 leaf hash.

tree_size: : The tree_size of the tree on which to base the proofs, in decimal.

The hash must be calculated as defined in {{tree_leaves}}. The tree_size must designate an existing v2 STH.

Because of skew, the front-end may not know the requested STH or the requested hash, which leads to a number of cases.

latest STH < requested STH : Return latest STH.

latest STH > requested STH : Return latest STH and a consistency proof between it and the requested STH (see {{get-sth-consistency}}).

index of requested hash < latest STH : Return inclusion.

Note that more than one case can be true, in which case the returned data is their concatenation. It is also possible for none to be true, in which case the front-end MUST return an empty response.

Outputs:

: inclusion: : A base64 encoded TransItem of type inclusion_proof_v2 whose inclusion_path array of Merkle Tree nodes proves the inclusion of the chosen certificate in the returned STH.

sth: : A base64 encoded TransItem of type signed_tree_head_v2, signed by this log.

consistency: : A base64 encoded TransItem of type consistency_proof_v2 that proves the consistency of the requested STH and the returned STH.

Note that no signature is required for the inclusion or consistency outputs as they are used to verify inclusion in and consistency of STHs, which are signed.

Errors are the same as in {{get-proof-by-hash}}.

See {{verify_inclusion}} for an outline of how to use the inclusion output, and see {{verify_consistency}} for an outline of how to use the consistency output.

Retrieve Entries and STH from Log {#get-entries}

GET https://<log server>/ct/v2/get-entries

Inputs:

: start: : 0-based index of first entry to retrieve, in decimal.

end: : 0-based index of last entry to retrieve, in decimal.

Outputs:

: entries: : An array of objects, each consisting of

log_entry:
: The base64 encoded `TransItem` structure of type `x509_entry_v2` or
  `precert_entry_v2` (see {{log_entries}}).

submitted_entry:
: JSON object representing the inputs that were submitted to
  `submit-entry`, with the addition of the trust anchor to the `chain`
  field if the submission did not include it.

sct:
: The base64 encoded `TransItem` of type `x509_sct_v2` or `precert_sct_v2`
  corresponding to this log entry.

sth: : A base64 encoded TransItem of type signed_tree_head_v2, signed by this log.

Note that this message is not signed -- the entries data can be verified by constructing the Merkle Tree Hash corresponding to a retrieved STH. All leaves MUST be v2. However, a compliant v2 client MUST NOT construe an unrecognized TransItem type as an error. This means it may be unable to parse some entries, but note that each client can inspect the entries it does recognize as well as verify the integrity of the data by treating unrecognized leaves as opaque input to the tree.

The start and end parameters SHOULD be within the range 0 <= x < tree_size as returned by get-sth in {{get-sth}}.

The start parameter MUST be less than or equal to the end parameter.

The chain field in the submission output parameter MUST include the trust anchor that the log used to verify the submission, even if it was omitted in the original submission.

Log servers MUST honor requests where 0 <= start < tree_size and end >= tree_size by returning a partial response covering only the valid entries in the specified range. end >= tree_size could be caused by skew. Note that the following restriction may also apply:

Logs MAY restrict the number of entries that can be retrieved per get-entries request. If a client requests more than the permitted number of entries, the log SHALL return the maximum number of entries permissible. These entries SHALL be sequential beginning with the entry specified by start.

Because of skew, it is possible the log server will not have any entries between start and end. In this case it MUST return an empty entries array.

In any case, the log server MUST return the latest STH it knows about.

See {{verify_hash}} for an outline of how to use a complete list of log_entry entries to verify the root_hash.

Retrieve Accepted Trust Anchors {#get-anchors}

GET https://<log server>/ct/v2/get-anchors

No inputs.

Outputs:

: certificates: : An array of base64 encoded trust anchors that are acceptable to the log.

max_chain_length: : If the server has chosen to limit the length of chains it accepts, this is the maximum number of certificates in the chain, in decimal. If there is no limit, this is omitted.

TLS Servers {#tls_servers}

TLS servers MUST use at least one of the three mechanisms listed below to present one or more SCTs from one or more logs to each TLS client during full TLS handshakes, where each SCT corresponds to the server certificate. TLS servers SHOULD also present corresponding inclusion proofs and STHs.

Three mechanisms are provided because they have different tradeoffs.

• A TLS extension (Section 7.4.1.4 of [RFC5246]) with type transparency_info (see {{tls_transinfo_extension}}). This mechanism allows TLS servers to participate in CT without the cooperation of CAs, unlike the other two mechanisms. It also allows SCTs and inclusion proofs to be updated on the fly.

• An Online Certificate Status Protocol (OCSP) [RFC6960] response extension (see {{ocsp_transinfo_extension}}), where the OCSP response is provided in the CertificateStatus message, provided that the TLS client included the status_request extension in the (extended) ClientHello (Section 8 of [RFC6066]). This mechanism, popularly known as OCSP stapling, is already widely (but not universally) implemented. It also allows SCTs and inclusion proofs to be updated on the fly.

• An X509v3 certificate extension (see {{cert_transinfo_extension}}). This mechanism allows the use of unmodified TLS servers, but the SCTs and inclusion proofs cannot be updated on the fly. Since the logs from which the SCTs and inclusion proofs originated won't necessarily be accepted by TLS clients for the full lifetime of the certificate, there is a risk that TLS clients will subsequently consider the certificate to be non-compliant and in need of re-issuance.

Additionally, a TLS server which supports presenting SCTs using an OCSP response MAY provide it when the TLS client included the status_request_v2 extension ([RFC6961]) in the (extended) ClientHello, but only in addition to at least one of the three mechanisms listed above.

Multiple SCTs {#multiple-scts}

TLS servers SHOULD send SCTs from multiple logs in case one or more logs are not acceptable to the TLS client (for example, if a log has been struck off for misbehavior, has had a key compromise, or is not known to the TLS client). For example:

• If a CA and a log collude, it is possible to temporarily hide misissuance from clients. Including SCTs from different logs makes it more difficult to mount this attack.

• If a log misbehaves, a consequence may be that clients cease to trust it. Since the time an SCT may be in use can be considerable (several years is common in current practice when embedded in a certificate), servers may wish to reduce the probability of their certificates being rejected as a result by including SCTs from different logs.

• TLS clients may have policies related to the above risks requiring servers to present multiple SCTs. For example, at the time of writing, Chromium [Chromium.Log.Policy] requires multiple SCTs to be presented with EV certificates in order for the EV indicator to be shown.

To select the logs from which to obtain SCTs, a TLS server can, for example, examine the set of logs popular TLS clients accept and recognize.

TransItemList Structure

Multiple SCTs, inclusion proofs, and indeed TransItem structures of any type, are combined into a list as follows:

      opaque SerializedTransItem<1..2^16-1>;

      struct {
          SerializedTransItem trans_item_list<1..2^16-1>;
      } TransItemList;

Here, SerializedTransItem is an opaque byte string that contains the serialized TransItem structure. This encoding ensures that TLS clients can decode each TransItem individually (so, for example, if there is a version upgrade, out-of-date clients can still parse old TransItem structures while skipping over new TransItem structures whose versions they don't understand).

Presenting SCTs, inclusions proofs and STHs

In each TransItemList that is sent to a client during a TLS handshake, the TLS server MUST include a TransItem structure of type x509_sct_v2 or precert_sct_v2 (except as described in {{cached_info}}).

Presenting inclusion proofs and STHs in the TLS handshake helps to protect the client's privacy (see {{validating_inclusion_proofs}}) and reduces load on log servers. Therefore, if the TLS server can obtain them, it SHOULD also include TransItems of type inclusion_proof_v2 and signed_tree_head_v2 in the TransItemList.

transparency_info TLS Extension {#tls_transinfo_extension}

Provided that a TLS client includes the transparency_info extension type in the ClientHello, the TLS server SHOULD include the transparency_info extension in the ServerHello with extension_data set to a TransItemList. The TLS server SHOULD ignore any extension_data sent by the TLS client. Additionally, the TLS server MUST NOT process or include this extension when a TLS session is resumed, since session resumption uses the original session information.

cached_info TLS Extension {#cached_info}

When a TLS server includes the transparency_info extension in the ServerHello, it SHOULD NOT include any TransItem structures of type x509_sct_v2 or precert_sct_v2 in the TransItemList if all of the following conditions are met:

• The TLS client includes the transparency_info extension type in the ClientHello.

• The TLS client includes the cached_info ([RFC7924]) extension type in the ClientHello, with a CachedObject of type ct_compliant (see {{tls_cachedinfo_extension}}) and at least one CachedObject of type cert.

• The TLS server sends a modified Certificate message (as described in section 4.1 of [RFC7924]).

TLS servers SHOULD ignore the hash_value fields of each CachedObject of type ct_compliant sent by TLS clients.

Certification Authorities

Transparency Information X.509v3 Extension {#x509v3_transinfo_extension}

The Transparency Information X.509v3 extension, which has OID 1.3.101.75 and SHOULD be non-critical, contains one or more TransItem structures in a TransItemList. This extension MAY be included in OCSP responses (see {{ocsp_transinfo_extension}}) and certificates (see {{cert_transinfo_extension}}). Since RFC5280 requires the extnValue field (an OCTET STRING) of each X.509v3 extension to include the DER encoding of an ASN.1 value, a TransItemList MUST NOT be included directly. Instead, it MUST be wrapped inside an additional OCTET STRING, which is then put into the extnValue field:

    TransparencyInformationSyntax ::= OCTET STRING

TransparencyInformationSyntax contains a TransItemList.

OCSP Response Extension {#ocsp_transinfo_extension}

A certification authority MAY include a Transparency Information X.509v3 extension in the singleExtensions of a SingleResponse in an OCSP response. All included SCTs and inclusion proofs MUST be for the certificate identified by the certID of that SingleResponse, or for a precertificate that corresponds to that certificate.

Certificate Extension {#cert_transinfo_extension}

A certification authority MAY include a Transparency Information X.509v3 extension in a certificate. All included SCTs and inclusion proofs MUST be for a precertificate that corresponds to this certificate.

TLS Feature X.509v3 Extension

A certification authority SHOULD NOT issue any certificate that identifies the transparency_info TLS extension in a TLS feature extension [RFC7633], because TLS servers are not required to support the transparency_info TLS extension in order to participate in CT (see {{tls_servers}}).

Clients

There are various different functions clients of logs might perform. We describe here some typical clients and how they should function. Any inconsistency may be used as evidence that a log has not behaved correctly, and the signatures on the data structures prevent the log from denying that misbehavior.

All clients need various parameters in order to communicate with logs and verify their responses. These parameters are described in {{log_parameters}}, but note that this document does not describe how the parameters are obtained, which is implementation-dependent (see, for example, [Chromium.Policy]).

Clients should somehow exchange STHs they see, or make them available for scrutiny, in order to ensure that they all have a consistent view. The exact mechanisms will be in separate documents, but it is expected there will be a variety.

TLS Client {#tls_clients}

Receiving SCTs and inclusion proofs

TLS clients receive SCTs alongside or in certificates. TLS clients MUST implement all of the three mechanisms by which TLS servers may present SCTs (see {{tls_servers}}). TLS clients MAY also accept SCTs via the status_request_v2 extension ([RFC6961]). TLS clients that support the transparency_info TLS extension SHOULD include it in ClientHello messages, with empty extension_data. TLS clients may also receive inclusion proofs in addition to SCTs, which should be checked once the SCTs are validated.

Reconstructing the TBSCertificate {#reconstructing_tbscertificate}

To reconstruct the TBSCertificate component of a precertificate from a certificate, TLS clients should remove the Transparency Information extension described in {{x509v3_transinfo_extension}}.

If the SCT checked is for a precertificate (where the type of the TransItem is precert_sct_v2), then the client SHOULD also remove embedded v1 SCTs, identified by OID 1.3.6.1.4.1.11129.2.4.2 (See Section 3.3. of [RFC6962]), in the process of reconstructing the TBSCertificate. That is to allow embedded v1 and v2 SCTs to co-exist in a certificate (See {{v1_coexistence}}).

Validating SCTs

In addition to normal validation of the server certificate and its chain, TLS clients SHOULD validate each received SCT for which they have the corresponding log's parameters. To validate an SCT, a TLS client computes the signature input by constructing a TransItem of type x509_entry_v2 or precert_entry_v2, depending on the SCT's TransItem type. The TimestampedCertificateEntryDataV2 structure is constructed in the following manner:

• timestamp is copied from the SCT.
• tbs_certificate is the reconstructed TBSCertificate portion of the server certificate, as described in {{reconstructing_tbscertificate}}.
• issuer_key is computed as described in {{tree_leaves}}.
• sct_extensions is copied from the SCT.

The SCT's signature is then verified using the public key of the corresponding log, which is identified by the log_id. The required signature algorithm is one of the log's parameters.

TLS clients MUST NOT consider valid any SCT whose timestamp is in the future.

Fetching inclusion proofs {#fetching_inclusion_proofs}

When a TLS client has validated a received SCT but does not yet possess a corresponding inclusion proof, the TLS client MAY request the inclusion proof directly from a log using get-proof-by-hash ({{get-proof-by-hash}}) or get-all-by-hash ({{get-all-by-hash}}). Note that this will disclose to the log which TLS server the client has been communicating with.

Validating inclusion proofs {#validating_inclusion_proofs}

When a TLS client has received, or fetched, an inclusion proof (and an STH), it SHOULD proceed to verifying the inclusion proof to the provided STH. The TLS client SHOULD also verify consistency between the provided STH and an STH it knows about.

If the TLS client holds an STH that predates the SCT, it MAY, in the process of auditing, request a new STH from the log ({{get-sth}}), then verify it by requesting a consistency proof ({{get-sth-consistency}}). Note that if the TLS client uses get-all-by-hash, then it will already have the new STH.

Evaluating compliance

It is up to a client's local policy to specify the quantity and form of evidence (SCTs, inclusion proofs or a combination) needed to achieve compliance and how to handle non-compliance.

A TLS client MUST NOT evaluate compliance if it did not send both the transparency_info and status_request TLS extensions in the ClientHello.

cached_info TLS Extension {#tls_cachedinfo_extension}

If a TLS client uses the cached_info TLS extension ([RFC7924]) to indicate 1 or more cached certificates, all of which it already considers to be CT compliant, the TLS client MAY also include a CachedObject of type ct_compliant in the cached_info extension. The hash_value field MUST be 1 byte long with the value 0.

Monitor {#monitor}

Monitors watch logs to check that they behave correctly, for certificates of interest, or both. For example, a monitor may be configured to report on all certificates that apply to a specific domain name when fetching new entries for consistency validation.

A monitor needs to, at least, inspect every new entry in each log it watches. It may also want to keep copies of entire logs. In order to do this, it should follow these steps for each log:

1. Fetch the current STH ({{get-sth}}).

2. Verify the STH signature.

3. Fetch all the entries in the tree corresponding to the STH ({{get-entries}}).

4. Confirm that the tree made from the fetched entries produces the same hash as that in the STH.

5. Fetch the current STH ({{get-sth}}). Repeat until the STH changes.

6. Verify the STH signature.

7. Fetch all the new entries in the tree corresponding to the STH ({{get-entries}}). If they remain unavailable for an extended period, then this should be viewed as misbehavior on the part of the log.

8. Either:

   1. Verify that the updated list of all entries generates a tree with the same hash as the new STH.

   Or, if it is not keeping all log entries:

   1. Fetch a consistency proof for the new STH with the previous STH ({{get-sth-consistency}}).

   2. Verify the consistency proof.

   3. Verify that the new entries generate the corresponding elements in the consistency proof.

9. Go to Step 5.

Verifying the Submitted Entry

To prevent a malicious log from mutating a certificate's signature (which would allow a CA to avoid responsibility for misissuing a certificate), monitors should perform the following checks for each entry they download:

1. Verify that the type field of submitted_entry matches the type of log_entry.

2. Verify that the TBSCertificate portion of the certificate or precertificate in the submission field of submitted_entry is byte-for-byte identical to the tbs_certificate field of log_entry.

3. Verify the signature of the certificate or precertificate in the submission field of submitted_entry, using the public key contained in the issuer_key field of log_entry.

If an entry fails these checks, it should be considered misbehavior by the log equivalent to violating the append-only property.

Auditing

Auditing ensures that the current published state of a log is reachable from previously published states that are known to be good, and that the promises made by the log in the form of SCTs have been kept. Audits are performed by monitors or TLS clients.

In particular, there are four log behavior properties that should be checked:

• The Maximum Merge Delay (MMD).

• The STH Frequency Count.

• The append-only property.

• The consistency of the log view presented to all query sources.

A benign, conformant log publishes a series of STHs over time, each derived from the previous STH and the submitted entries incorporated into the log since publication of the previous STH. This can be proven through auditing of STHs. SCTs returned to TLS clients can be audited by verifying against the accompanying certificate, and using Merkle Inclusion Proofs, against the log's Merkle tree.

The action taken by the auditor if an audit fails is not specified, but note that in general if audit fails, the auditor is in possession of signed proof of the log's misbehavior.

A monitor ({{monitor}}) can audit by verifying the consistency of STHs it receives, ensure that each entry can be fetched and that the STH is indeed the result of making a tree from all fetched entries.

A TLS client ({{tls_clients}}) can audit by verifying an SCT against any STH dated after the SCT timestamp + the Maximum Merge Delay by requesting a Merkle inclusion proof ({{get-proof-by-hash}}). It can also verify that the SCT corresponds to the server certificate it arrived with (i.e., the log entry is that certificate, or is a precertificate corresponding to that certificate).

Checking of the consistency of the log view presented to all entities is more difficult to perform because it requires a way to share log responses among a set of CT-aware entities, and is discussed in {{misbehaving_logs}}.

Algorithm Agility

It is not possible for a log to change any of its algorithms part way through its lifetime:

Signature algorithm: : SCT signatures must remain valid so signature algorithms can only be added, not removed.

Hash algorithm: : A log would have to support the old and new hash algorithms to allow backwards-compatibility with clients that are not aware of a hash algorithm change.

Allowing multiple signature or hash algorithms for a log would require that all data structures support it and would significantly complicate client implementation, which is why it is not supported by this document.

If it should become necessary to deprecate an algorithm used by a live log, then the log should be frozen as specified in {{log_shutdown}} and a new log should be started. Certificates in the frozen log that have not yet expired and require new SCTs SHOULD be submitted to the new log and the SCTs from that log used instead.

IANA Considerations

The assignment policy criteria mentioned in this section refer to the policies outlined in [RFC5226].

TLS Extension Type

IANA is asked to allocate an RFC 5246 ExtensionType value for the transparency_info TLS extension. IANA should update this extension type to point at this document.

New Entry to the TLS CachedInformationType registry

IANA is asked to add an entry for ct_compliant(TBD) to the "TLS CachedInformationType Values" registry that was defined in [RFC7924].

Hash Algorithms {#hash_algorithms}

IANA is asked to establish a registry of hash algorithm values, named "CT Hash Algorithms", that initially consists of:

|-------------+----------------+------------------------+------------------------------------------| | Value | Hash Algorithm | OID | Reference / Assignment Policy | |-------------+----------------+------------------------|------------------------------------------| | 0x00 | SHA-256 | 2.16.840.1.101.3.4.2.1 | [RFC6234] | | 0x01 - 0xDF | Unassigned | | Specification Required and Expert Review | | 0xE0 - 0xEF | Reserved | | Experimental Use | | 0xF0 - 0xFF | Reserved | | Private Use | |-------------+----------------+------------------------+------------------------------------------|

Expert Review guidelines

The appointed Expert should ensure that the proposed algorithm has a public specification and is suitable for use as a cryptographic hash algorithm with no known preimage or collision attacks. These attacks can damage the integrity of the log.

Signature Algorithms {#signature_algorithms}

IANA is asked to establish a registry of signature algorithm values, named "CT Signature Algorithms", that initially consists of:

|--------------------------------+----------------------------------------------------+--------------------------------| | SignatureScheme Value | Signature Algorithm | Reference / Assignment Policy | |--------------------------------+----------------------------------------------------+--------------------------------| | ecdsa_secp256r1_sha256(0x0403) | ECDSA (NIST P-256) with SHA-256 | [FIPS186-4] | | ecdsa_secp256r1_sha256(0x0403) | Deterministic ECDSA (NIST P-256) with HMAC-SHA256 | [RFC6979] | | ed25519(0x0807) | Ed25519 (PureEdDSA with the edwards25519 curve) | [RFC8032] | | private_use(0xFE00..0xFFFF) | Reserved | Private Use | |--------------------------------+----------------------------------------------------+--------------------------------|

Expert Review guidelines

The appointed Expert should ensure that the proposed algorithm has a public specification, has a value assigned to it in the TLS SignatureScheme Registry (that IANA is asked to establish in [I-D.ietf-tls-tls13]) and is suitable for use as a cryptographic signature algorithm.

VersionedTransTypes {#versioned_trans_types}

IANA is asked to establish a registry of VersionedTransType values, named "CT VersionedTransTypes", that initially consists of:

|-----------------+---------------------------+------------------------------------------| | Value | Type and Version | Reference / Assignment Policy | |-----------------+---------------------------+------------------------------------------| | 0x0000 | Reserved | [RFC6962] (*) | | 0x0001 | x509_entry_v2 | RFCXXXX | | 0x0002 | precert_entry_v2 | RFCXXXX | | 0x0003 | x509_sct_v2 | RFCXXXX | | 0x0004 | precert_sct_v2 | RFCXXXX | | 0x0005 | signed_tree_head_v2 | RFCXXXX | | 0x0006 | consistency_proof_v2 | RFCXXXX | | 0x0007 | inclusion_proof_v2 | RFCXXXX | | 0x0008 - 0xDFFF | Unassigned | Specification Required and Expert Review | | 0xE000 - 0xEFFF | Reserved | Experimental Use | | 0xF000 - 0xFFFF | Reserved | Private Use | |-----------------+---------------------------+------------------------------------------|

(*) The 0x0000 value is reserved so that v1 SCTs are distinguishable from v2 SCTs and other TransItem structures.

[RFC Editor: please update 'RFCXXXX' to refer to this document, once its RFC number is known.]

Expert Review guidelines

The appointed Expert should review the public specification to ensure that it is detailed enough to ensure implementation interoperability.

Log Artifact Extension Registry {#log_artifact_extension_registry}

IANA is asked to establish a registry of ExtensionType values, named "CT Log Artifact Extensions", that initially consists of:

|-----------------+------------+-----+------------------------------------------| | ExtensionType | Status | Use | Reference / Assignment Policy | |-----------------+------------+-----+------------------------------------------| | 0x0000 - 0xDFFF | Unassigned | n/a | Specification Required and Expert Review | | 0xE000 - 0xEFFF | Reserved | n/a | Experimental Use | | 0xF000 - 0xFFFF | Reserved | n/a | Private Use | |-----------------+------------+-----+------------------------------------------|

The "Use" column should contain one or both of the following values:

• "SCT", for extensions specified for use in Signed Certificate Timestamps.

• "STH", for extensions specified for use in Signed Tree Heads.

Expert Review guidelines

The appointed Expert should review the public specification to ensure that it is detailed enough to ensure implementation interoperability. The Expert should also verify that the extension is appropriate to the contexts in which it is specified to be used (SCT, STH, or both).

Object Identifiers

This document uses object identifiers (OIDs) to identify Log IDs (see {{log_id}}), the precertificate CMS eContentType (see {{precertificates}}), and X.509v3 extensions in certificates (see {{cert_transinfo_extension}}) and OCSP responses (see {{ocsp_transinfo_extension}}). The OIDs are defined in an arc that was selected due to its short encoding.

Log ID Registry {#log_id_registry}

IANA is asked to establish a registry of Log IDs, named "CT Log ID Registry", that initially consists of:

|-------------------------------+------------+---------------------------------------| | Value | Log | Reference / Assignment Policy | |-------------------------------+------------+---------------------------------------| | 1.3.101.8192 - 1.3.101.16383 | Unassigned | Parameters Required and Expert Review | | 1.3.101.80.0 - 1.3.101.80.127 | Unassigned | Parameters Required and Expert Review | | 1.3.101.80.128 - 1.3.101.80.* | Unassigned | First Come First Served | |-------------------------------+------------+---------------------------------------|

All OIDs in the range from 1.3.101.8192 to 1.3.101.16383 have been reserved. This is a limited resource of 8,192 OIDs, each of which has an encoded length of 4 octets.

The 1.3.101.80 arc has been delegated. This is an unlimited resource, but only the 128 OIDs from 1.3.101.80.0 to 1.3.101.80.127 have an encoded length of only 4 octets.

Each application for the allocation of a Log ID should be accompanied by all of the required parameters (except for the Log ID) listed in {{log_parameters}}.

Expert Review guidelines

Since the Log IDs with the shortest encodings are a limited resource, the appointed Expert should review the submitted parameters and judge whether or not the applicant is requesting a Log ID in good faith (with the intention of actually running a CT log that will be identified by the allocated Log ID).

Security Considerations

With CAs, logs, and servers performing the actions described here, TLS clients can use logs and signed timestamps to reduce the likelihood that they will accept misissued certificates. If a server presents a valid signed timestamp for a certificate, then the client knows that a log has committed to publishing the certificate. From this, the client knows that monitors acting for the subject of the certificate have had some time to notice the misissue and take some action, such as asking a CA to revoke a misissued certificate, or that the log has misbehaved, which will be discovered when the SCT is audited. A signed timestamp is not a guarantee that the certificate is not misissued, since appropriate monitors might not have checked the logs or the CA might have refused to revoke the certificate.

In addition, if TLS clients will not accept unlogged certificates, then site owners will have a greater incentive to submit certificates to logs, possibly with the assistance of their CA, increasing the overall transparency of the system.

[I-D.ietf-trans-threat-analysis] provides a more detailed threat analysis of the Certificate Transparency architecture.

Misissued Certificates

Misissued certificates that have not been publicly logged, and thus do not have a valid SCT, are not considered compliant. Misissued certificates that do have an SCT from a log will appear in that public log within the Maximum Merge Delay, assuming the log is operating correctly. As a log is allowed to serve an STH that's up to MMD old, the maximum period of time during which a misissued certificate can be used without being available for audit is twice the MMD.

Detection of Misissue

The logs do not themselves detect misissued certificates; they rely instead on interested parties, such as domain owners, to monitor them and take corrective action when a misissue is detected.

Misbehaving Logs {#misbehaving_logs}

A log can misbehave in several ways. Examples include: failing to incorporate a certificate with an SCT in the Merkle Tree within the MMD; presenting different, conflicting views of the Merkle Tree at different times and/or to different parties; and issuing STHs too frequently. Such misbehavior is detectable and [I-D.ietf-trans-threat-analysis] provides more details on how this can be done.

Violation of the MMD contract is detected by log clients requesting a Merkle inclusion proof ({{get-proof-by-hash}}) for each observed SCT. These checks can be asynchronous and need only be done once per certificate. In order to protect the clients' privacy, these checks need not reveal the exact certificate to the log. Instead, clients can request the proof from a trusted auditor (since anyone can compute the proofs from the log) or communicate with the log via proxies.

Violation of the append-only property or the STH issuance rate limit can be detected by clients comparing their instances of the Signed Tree Heads. There are various ways this could be done, for example via gossip (see [I-D.ietf-trans-gossip]) or peer-to-peer communications or by sending STHs to monitors (who could then directly check against their own copy of the relevant log). Proof of misbehavior in such cases would be: a series of STHs that were issued too closely together, proving violation of the STH issuance rate limit; or an STH with a root hash that does not match the one calculated from a copy of the log, proving violation of the append-only property.

Preventing Tracking Clients {#prevent_tracking_clients}

Clients that gossip STHs or report back SCTs can be tracked or traced if a log produces multiple STHs or SCTs with the same timestamp and data but different signatures. Logs SHOULD mitigate this risk by either:

• Using deterministic signature schemes, or

• Producing no more than one SCT for each distinct submission and no more than one STH for each distinct tree_size. Each of these SCTs and STHs can be stored by the log and served to other clients that submit the same certificate or request the same STH.

Multiple SCTs {#offering_multiple_scts}

By offering multiple SCTs, each from a different log, TLS servers reduce the effectiveness of an attack where a CA and a log collude (see {{multiple-scts}}).

Acknowledgements

The authors would like to thank Erwann Abelea, Robin Alden, Andrew Ayer, Richard Barnes, Al Cutter, David Drysdale, Francis Dupont, Adam Eijdenberg, Stephen Farrell, Daniel Kahn Gillmor, Paul Hadfield, Brad Hill, Jeff Hodges, Paul Hoffman, Jeffrey Hutzelman, Kat Joyce, Stephen Kent, SM, Alexey Melnikov, Linus Nordberg, Chris Palmer, Trevor Perrin, Pierre Phaneuf, Eric Rescorla, Melinda Shore, Ryan Sleevi, Martin Smith, Carl Wallace and Paul Wouters for their valuable contributions.

A big thank you to Symantec for kindly donating the OIDs from the 1.3.101 arc that are used in this document.

--- back

Supporting v1 and v2 simultaneously {#v1_coexistence}

Certificate Transparency logs have to be either v1 (conforming to [RFC6962]) or v2 (conforming to this document), as the data structures are incompatible and so a v2 log could not issue a valid v1 SCT.

CT clients, however, can support v1 and v2 SCTs, for the same certificate, simultaneously, as v1 SCTs are delivered in different TLS, X.509 and OCSP extensions than v2 SCTs.

v1 and v2 SCTs for X.509 certificates can be validated independently. For precertificates, v2 SCTs should be embedded in the TBSCertificate before submission of the TBSCertificate (inside a v1 precertificate, as described in Section 3.1. of [RFC6962]) to a v1 log so that TLS clients conforming to [RFC6962] but not this document are oblivious to the embedded v2 SCTs. An issuer can follow these steps to produce an X.509 certificate with embedded v1 and v2 SCTs:

• Create a CMS precertificate as described in {{precertificates}} and submit it to v2 logs.

• Embed the obtained v2 SCTs in the TBSCertificate, as described in {{cert_transinfo_extension}}.

• Use that TBSCertificate to create a v1 precertificate, as described in Section 3.1. of [RFC6962] and submit it to v1 logs.

• Embed the v1 SCTs in the TBSCertificate, as described in Section 3.3 of [RFC6962].

• Sign that TBSCertificate (which now contains v1 and v2 SCTs) to issue the final X.509 certificate.