Internet Engineering Task Force (IETF)                         B. Laurie
Request for Comments: 9162                                    E. Messeri
Obsoletes: 6962                                                   Google
Category: Experimental                                      R. Stradling
ISSN: 2070-1721                                                  Sectigo
                                                           December 2021


                  Certificate Transparency Version 2.0

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.

   This document obsoletes RFC 6962.  It also specifies a new TLS
   extension that is used to send various CT log artifacts.

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

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9162.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
     1.2.  Data Structures
     1.3.  Major Differences from CT 1.0
   2.  Cryptographic Components
     2.1.  Merkle Trees
       2.1.1.  Definition of the Merkle Tree
       2.1.2.  Verifying a Tree Head Given Entries
       2.1.3.  Merkle Inclusion Proofs
       2.1.4.  Merkle Consistency Proofs
       2.1.5.  Example
     2.2.  Signatures
   3.  Submitters
     3.1.  Certificates
     3.2.  Precertificates
       3.2.1.  Binding Intent to Issue
   4.  Log Format and Operation
     4.1.  Log Parameters
     4.2.  Evaluating Submissions
       4.2.1.  Minimum Acceptance Criteria
       4.2.2.  Discretionary Acceptance Criteria
     4.3.  Log Entries
     4.4.  Log ID
     4.5.  TransItem Structure
     4.6.  Log Artifact Extensions
     4.7.  Merkle Tree Leaves
     4.8.  Signed Certificate Timestamp (SCT)
     4.9.  Merkle Tree Head
     4.10. Signed Tree Head (STH)
     4.11. Merkle Consistency Proofs
     4.12. Merkle Inclusion Proofs
     4.13. Shutting Down a Log
   5.  Log Client Messages
     5.1.  Submit Entry to Log
     5.2.  Retrieve Latest STH
     5.3.  Retrieve Merkle Consistency Proof between Two STHs
     5.4.  Retrieve Merkle Inclusion Proof from Log by Leaf Hash
     5.5.  Retrieve Merkle Inclusion Proof, STH, and Consistency Proof
           by Leaf Hash
     5.6.  Retrieve Entries and STH from Log
     5.7.  Retrieve Accepted Trust Anchors
   6.  TLS Servers
     6.1.  TLS Client Authentication
     6.2.  Multiple SCTs
     6.3.  TransItemList Structure
     6.4.  Presenting SCTs, Inclusions Proofs, and STHs
     6.5.  transparency_info TLS Extension
   7.  Certification Authorities
     7.1.  Transparency Information X.509v3 Extension
       7.1.1.  OCSP Response Extension
       7.1.2.  Certificate Extension
     7.2.  TLS Feature X.509v3 Extension
   8.  Clients
     8.1.  TLS Client
       8.1.1.  Receiving SCTs and Inclusion Proofs
       8.1.2.  Reconstructing the TBSCertificate
       8.1.3.  Validating SCTs
       8.1.4.  Fetching Inclusion Proofs
       8.1.5.  Validating Inclusion Proofs
       8.1.6.  Evaluating Compliance
     8.2.  Monitor
     8.3.  Auditing
   9.  Algorithm Agility
   10. IANA Considerations
     10.1.  Additions to Existing Registries
       10.1.1.  New Entry to the TLS ExtensionType Registry
       10.1.2.  URN Sub-namespace for TRANS (urn:ietf:params:trans)
     10.2.  New CT-Related Registries
       10.2.1.  Hash Algorithms
       10.2.2.  Signature Algorithms
       10.2.3.  VersionedTransTypes
       10.2.4.  Log Artifact Extensions
       10.2.5.  Log IDs
       10.2.6.  Error Types
     10.3.  OID Assignment
   11. Security Considerations
     11.1.  Misissued Certificates
     11.2.  Detection of Misissue
     11.3.  Misbehaving Logs
     11.4.  Multiple SCTs
     11.5.  Leakage of DNS Information
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Appendix A.  Supporting v1 and v2 Simultaneously (Informative)
   Appendix B.  An ASN.1 Module (Informative)
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Certificate Transparency aims to mitigate the problem of misissued
   certificates by providing append-only logs of issued certificates.
   The logs do not themselves prevent misissuance, 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)).  A typical definition of "public" can
   be found in [CABBR].

   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.
   A log may also limit the length of the chain it is willing to accept;
   such chains must also end with an acceptable trust anchor.  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 efficiently prove that any particular instance
   of the log is a superset of any particular previous instance and to
   efficiently detect various misbehaviors of the log (e.g., issuing a
   signed timestamp for a certificate that is not subsequently logged).

   The log auditing mechanisms described in this document can be
   circumvented by a misbehaving log that shows different, inconsistent
   views of itself to different clients.  Therefore, it is necessary to
   treat each log as a trusted third party.  While mechanisms are being
   developed to address these shortcomings and thereby avoid the need to
   blindly trust logs, such mechanisms are outside the scope of this
   document.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Data Structures

   Data structures are defined and encoded according to the conventions
   laid out in Section 3 of [RFC8446].

   This document uses object identifiers (OIDs) to identify Log IDs (see
   Section 4.4), the precertificate Cryptographic Message Syntax (CMS)
   eContentType (see Section 3.2), X.509v3 extensions in certificates
   (see Section 7.1.2), and Online Certificate Status Protocol (OCSP)
   responses (see Section 7.1.1).  The OIDs are defined in an arc that
   was selected due to its short encoding.

1.3.  Major Differences from CT 1.0

   This document revises and obsoletes the CT 1.0 protocol [RFC6962],
   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].

   *  Removal of 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 available from 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 Signed
      Certificate Timestamps (SCTs) but also in Signed Tree Heads
      (STHs).

   *  API outputs: Complete TransItem structures are returned rather
      than the constituent parts of each structure.

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

   *  submit-entry: This is a new client API, replacing add-chain and
      add-pre-chain.

   *  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.

   *  Verification algorithms: 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 have been added.

   *  Extensive clarifications and editorial work.

2.  Cryptographic Components

2.1.  Merkle Trees

   A full description of the Merkle Tree is beyond the scope of this
   document.  Briefly, it is a binary tree where each non-leaf node is a
   hash of its children.  For CT, the number of children is at most two.
   Additional information can be found in the Introduction and Reference
   sections of [RFC8391].

2.1.1.  Definition of the Merkle Tree

   The log uses a binary Merkle Tree for efficient auditing.  The hash
   algorithm used is one of the log's parameters (see Section 4.1).
   This document establishes a registry of acceptable hash algorithms
   (see Section 10.2.1).  Throughout this document, the hash algorithm
   in use is referred to as HASH and the size of its output in bytes is
   referred to 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 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:

   *  || denotes concatenation

   *  : denotes concatenation of lists

   *  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
   proposed by [CrosbyWallach], except our definition handles non-full
   trees differently.)

2.1.2.  Verifying a Tree Head Given Entries

   When a client has a complete list of 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:

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

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

       c.  Repeat merge_count times:

           i.    Pop right from stack.

           ii.   Pop left from stack.

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

   3.  If there is more than one element in the stack, repeat the same
       merge procedure (the sub-items of Step 2(c) 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.

2.1.3.  Merkle Inclusion Proofs

   A Merkle inclusion proof for a leaf in a Merkle 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.

2.1.3.1.  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 Section 2.1.1.

2.1.3.2.  Verifying an Inclusion Proof

   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 from the inclusion_proof_v2 structure 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:

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

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

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

           ii.  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:

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

       c.  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.

2.1.4.  Merkle Consistency Proofs

   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.

2.1.4.1.  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, using the appropriate step
   below:

   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 Section 2.1.1.

2.1.4.2.  Verifying Consistency between Two Tree Heads

   When a client has a tree head first_hash for tree size first, has 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 consistency_path is an empty array, stop and fail the proof
       verification.

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

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

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

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

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

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

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

           i.    Set fr to HASH(0x01 || c || fr).

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

           iii.  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:

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

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

   7.  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.

2.1.5.  Example

   The following is a 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].  Non-leaf nodes c, g are used to verify hash0, and non-leaf
   nodes 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].  Non-leaf nodes k, i are used to verify hash2, and non-leaf
   node j is additionally used to show hash is consistent with hash2.

2.2.  Signatures

   When signing data structures, a log MUST use one of the signature
   algorithms from the IANA "Signature Algorithms" registry, described
   in Section 10.2.2.

3.  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 (Section 5.7).  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 Section 4.8).  The submitter SHOULD validate the
   returned SCT, as described in Section 8.1, 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.

3.1.  Certificates

   Any entity can submit a certificate (Section 5.1) 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.

3.2.  Precertificates

   CAs may preannounce a certificate prior to issuance by submitting a
   precertificate (Section 5.1) 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, as described in [X690].

   *  SignedData.version MUST be v3(3).

   *  SignedData.digestAlgorithms MUST be the same as 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 (Section 7.1)
         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 the IANA "Hash Algorithms" registry, described in
         Section 10.2.1.

      -  signedAttrs MUST be present and MUST contain two attributes:

         o  a content-type attribute whose value is the same as
            SignedData.encapContentInfo.eContentType and

         o  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
         intends to issue the corresponding certificate (see
         Section 3.2.1).

      -  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.

3.2.1.  Binding Intent to Issue

   Under normal circumstances, there will be a short delay between
   precertificate submission and issuance of the corresponding
   certificate.  Longer delays are to be expected occasionally (e.g.,
   due to log server downtime); in some cases, the CA might not actually
   issue the corresponding certificate.  Nevertheless, a
   precertificate's signature indicates the CA's binding intent to issue
   the corresponding certificate, which means that:

   *  Misissuance of a precertificate is considered equivalent to
      misissuance of the corresponding certificate.  The CA should
      expect to be held accountable, even if the corresponding
      certificate has not actually been issued.

   *  Upon observing a precertificate, a client can reasonably presume
      that the corresponding certificate has been issued.  A client may
      wish to obtain status information (e.g., by using the Online
      Certificate Status Protocol [RFC6960] or by checking a Certificate
      Revocation List [RFC5280]) about a certificate that is presumed to
      exist, especially if there is evidence or suspicion that the
      corresponding precertificate was misissued.

   *  TLS clients may have policies that require CAs to be able to
      revoke and to provide certificate status services for each
      certificate that is presumed to exist based on the existence of a
      corresponding precertificate.

4.  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, as
   discussed in Section 11.3.  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 fulfill
   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 Section 4.1):

   *  Allocate a tree index to the entry representing the accepted
      submission.

   *  Calculate the root of the tree.

   *  Sign the root of the tree (see Section 4.10).

   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.

4.1.  Log Parameters

   A log is defined by a collection of immutable parameters, which are
   used by clients to communicate with the log and to verify log
   artifacts.  Except for the Final STH, each of these parameters MUST
   be established before the log operator begins to operate the log.

   Base URL:  The prefix used to construct URLs [RFC3986] for client
      messages (see Section 5).  The base URL MUST be an "https" URL,
      MAY contain a port, and MAY contain a path with any number of path
      segments but MUST NOT contain a query string, fragment, or
      trailing "/".  Example: https://ct.example.org/blue.

   Hash Algorithm:  The hash algorithm used for the Merkle Tree (see
      Section 10.2.1).

   Signature Algorithm:  The signature algorithm used (see Section 2.2).

   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.  This
      document deliberately does not specify any limits on the value to
      allow for experimentation.

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

   Maximum Chain Length:  The longest certificate chain submission the
      log is willing to accept, if the log imposes any limit.

   STH Frequency Count:  The maximum number of STHs the log may produce
      in any period equal to the Maximum Merge Delay (see Section 4.10).

   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.  This value MUST be
      provided in the form of a TransItem of type signed_tree_head_v2.
      If a log is still accepting entries, this value should not be
      provided.

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

4.2.  Evaluating Submissions

   A log determines whether to accept or reject a submission by
   evaluating it against the minimum acceptance criteria (see
   Section 4.2.1) and against the log's discretionary acceptance
   criteria (see Section 4.2.2).

   If the acceptance criteria are met, the log SHOULD accept the
   submission.  (A log may decide, for example, to temporarily reject
   acceptable submissions to protect itself against denial-of-service
   attacks.)

   The log SHALL allow retrieval of its list of accepted trust anchors
   (see Section 5.7), 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.

4.2.1.  Minimum Acceptance Criteria

   To ensure that logged certificates and precertificates are
   attributable to an accepted trust anchor, to set clear expectations
   for what monitors would find in the log, and to avoid being
   overloaded by invalid submissions, the log MUST reject a submission
   if any of the following conditions are not met:

   *  The submission, type, and chain inputs MUST be set as described in
      Section 5.1.  The log MUST NOT accommodate misordered CA
      certificates or use any other source of intermediate CA
      certificates to attempt certification path construction.

   *  Each of the zero or more intermediate CA certificates in the chain
      MUST have one or both of the following features:

      -  The Basic Constraints extension with the cA boolean asserted.

      -  The Key Usage extension with the keyCertSign bit asserted.

   *  Each certificate in the chain MUST fall within the limits imposed
      by the zero or more Basic Constraints pathLenConstraint values
      found higher up the chain.

   *  Precertificate submissions MUST conform to all of the requirements
      in Section 3.2.

4.2.2.  Discretionary Acceptance Criteria

   If the minimum acceptance criteria are met but the submission is not
   fully valid according to [RFC5280] verification rules (e.g., the
   certificate or precertificate has expired, is not yet valid, has been
   revoked, exhibits ASN.1 DER encoding errors but the log can still
   parse it, etc.), then the acceptability of the submission is left to
   the log's discretion.  It is useful for logs to accept such
   submissions in order to accommodate quirks of CA certificate-issuing
   software and to facilitate monitoring of CA compliance with
   applicable policies and technical standards.  However, it is
   impractical for this document to enumerate, and for logs to consider,
   all of the ways that a submission might fail to comply with
   [RFC5280].

   Logs SHOULD limit the length of chain they will accept.  The maximum
   chain length is one of the log's parameters (see Section 4.1).

4.3.  Log Entries

   If a submission is accepted and an SCT is 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 provide this chain for auditing upon
   request (see Section 5.6) so that the CA cannot avoid 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.

4.4.  Log ID

   Each log is identified by an OID, which is one of the log's
   parameters (see Section 4.1) 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
   Section 10.2.5).  One way to get an OID arc, from which OIDs can be
   allocated, is to request a Private Enterprise Number from IANA by
   completing the registration form (https://pen.iana.org/pen/
   PenApplication.page).  The only advantage of the registry is that the
   DER encoding can be small.  (Recall that OID allocations do not
   require a central registration, although logs will most likely want
   to make themselves known to potential clients through out-of-band
   means.)  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 full DER encoding (including the
   tag and length) of the OID can be reproduced simply by prepending an
   OBJECT IDENTIFIER tag (0x06) to the opaque vector length and
   contents.

   The OID used to identify a log is limited such that the DER encoding
   of its value, excluding the tag and length, MUST be no longer than
   127 octets.

4.5.  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 {
           x509_entry_v2(0x0100), precert_entry_v2(0x0101),
           x509_sct_v2(0x0102), precert_sct_v2(0x0103),
           signed_tree_head_v2(0x0104), consistency_proof_v2(0x0105),
           inclusion_proof_v2(0x0106),

           /* Reserved Code Points */
           reserved_rfc6962(0x0000..0x00FF),
           reserved_experimentaluse(0xE000..0xEFFF),
           reserved_privateuse(0xF000..0xFFFF),
           (0xFFFF)
       } 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 Section 10.2.3
   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 type enumerations
   Version, LogEntryType, SignatureType, and MerkleLeafType [RFC6962].
   Note also that v1 did not define TransItem, but this document
   provides guidelines (see Appendix A) on how v2 implementations can
   coexist 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.

4.6.  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 SCTs (Section 4.8) and STHs (Section 4.10).  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 Section 10.2.4).  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.

4.7.  Merkle Tree Leaves

   The leaves of a log's Merkle Tree correspond to the log's entries
   (see Section 4.3).  Each leaf is the leaf hash (Section 2.1) 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 TBSCertificate<1..2^24-1>;

       struct {
           uint64 timestamp;
           opaque issuer_key_hash<32..2^8-1>;
           TBSCertificate tbs_certificate;
           Extension sct_extensions<0..2^16-1>;
       } TimestampedCertificateEntryDataV2;

   timestamp is the date and time at which the certificate or
   precertificate was accepted by the log, in the form of a 64-bit
   unsigned number of milliseconds elapsed since the Unix Epoch (1
   January 1970 00:00:00 UTC -- see [UNIXTIME]), ignoring leap seconds,
   in network byte order.  Note that the leaves of a log's Merkle Tree
   are not required to be in strict chronological order.

   issuer_key_hash is the HASH of the public key of the CA that issued
   the certificate or precertificate, calculated over the DER encoding
   of the key represented as SubjectPublicKeyInfo [RFC5280].  This is
   needed 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.  The length of the issuer_key_hash MUST match
   HASH_SIZE.

   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
   Section 8.1.2).

   sct_extensions is byte-for-byte identical to the SCT extensions of
   the corresponding SCT.

   The type of the TransItem corresponds to the value of the type
   parameter supplied in the Section 5.1 call.

4.8.  Signed Certificate Timestamp (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<1..2^16-1>;
       } SignedCertificateTimestampDataV2;

   log_id is this log's unique ID, encoded in an opaque vector, as
   described in Section 4.4.

   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.  All SCT
   extensions are similar to noncritical X.509v3 extensions (i.e., the
   mustUnderstand field is not set), and a recipient SHOULD ignore any
   extension it does not understand.  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 Section 4.7) using the
   signature algorithm declared in the log's parameters (see
   Section 4.1).

4.9.  Merkle 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 date and time, using the format defined in
   Section 4.7.

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

   root_hash is the root of the Merkle 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.

4.10.  Signed Tree Head (STH)

   Periodically, each log SHOULD sign its current tree head information
   (see Section 4.9) to produce an STH.  When a client requests a log's
   latest STH (see Section 5.2), 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 Section 4.1).  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<1..2^16-1>;
       } SignedTreeHeadDataV2;

   log_id is this log's unique ID encoded in an opaque vector, as
   described in Section 4.4.

   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
   Section 4.9).

   signature is computed over the tree_head field using the signature
   algorithm declared in the log's parameters (see Section 4.1).

4.11.  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<0..2^16-1>;
       } ConsistencyProofDataV2;

   log_id is this log's unique ID encoded in an opaque vector, as
   described in Section 4.4.

   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, as described in Section 2.1.4.

4.12.  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<0..2^16-1>;
       } InclusionProofDataV2;

   log_id is this log's unique ID encoded in an opaque vector, as
   described in Section 4.4.

   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, as described in
   Section 2.1.3.

4.13.  Shutting Down a Log

   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.  In order to avoid that, the
   following actions SHOULD be taken:

   *  Make it known to clients and monitors that the log will be frozen.
      This is not part of the API, so it will have to be done via a
      relevant out-of-band mechanism.

   *  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).

5.  Log 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 [RFC8259].  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 according to Section 4 of
   [RFC4648], as specified in the individual messages.

   Clients are configured with a log's base URL, which is one of the
   log's parameters.  Clients 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 [RFC8820].  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 to allow for experimentation.  Any fields that are not
   understood SHOULD be ignored.

   In practice, log servers may include multiple front-end machines.
   Since it is impractical to keep these machines in perfect sync,
   errors that are caused by skew between the machines may occur.  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 problem details object (see Section 3 of [RFC7807]) containing:

   type:  A URN reference identifying the problem.  To facilitate
      automated response to errors, this document defines a set of
      standard tokens for use in the type field within the URN namespace
      of: "urn:ietf:params:trans:error:".

   detail:  A human-readable string describing the error that prevented
      the log from processing the request, ideally with sufficient
      detail to enable the error to be rectified.

   For example, in response to a request of <Base URL>/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:

       {
           "type": "urn:ietf:params:trans:error:endBeforeStart",
           "detail": "'start' cannot be greater than 'end'"
       }

   Most error types are specific to the type of request and are defined
   in the respective subsections below.  The one exception is the
   "malformed" error type, which indicates that the log server could not
   parse the client's request because it did not comply with this
   document:

             +===========+==================================+
             | type      | detail                           |
             +===========+==================================+
             | malformed | The request could not be parsed. |
             +-----------+----------------------------------+

                                 Table 1

   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 in the case
   of a 503 response, the log MAY include a Retry-After header field per
   [RFC7231] in order to request a minimum time for the client to wait
   before retrying the request.  In the absence of this header field,
   this document does not specify a minimum.

   Clients SHOULD treat any 4xx error as a problem with the request and
   not attempt to resubmit without some modification to the request.
   The full status code MAY provide additional details.

   This document deliberately does not provide more specific guidance on
   the use of HTTP status codes.

5.1.  Submit Entry to Log

   POST <Base URL>/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 JSON strings, each of which is a
         base64-encoded CA certificate.  The first element is the
         certifier of the submission, the second certifies the first,
         etc.  The last element of chain (or, if chain is an empty
         array, the submission) 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:

    +================+===============================================+
    | type           | detail                                        |
    +================+===============================================+
    | badSubmission  | submission is neither a valid certificate nor |
    |                | a valid precertificate.                       |
    +----------------+-----------------------------------------------+
    | badType        | type is neither 1 nor 2.                      |
    +----------------+-----------------------------------------------+
    | badChain       | The first element of chain is not the         |
    |                | certifier of the submission, or the second    |
    |                | element does not certify the first, etc.      |
    +----------------+-----------------------------------------------+
    | badCertificate | One or more certificates in chain are not     |
    |                | valid (e.g., not properly encoded).           |
    +----------------+-----------------------------------------------+
    | unknownAnchor  | The last element of chain (or, if chain is an |
    |                | empty array, the submission) is not, nor is   |
    |                | it certified by, an accepted trust anchor.    |
    +----------------+-----------------------------------------------+
    | shutdown       | The log is no longer accepting submissions.   |
    +----------------+-----------------------------------------------+

                                 Table 2

   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
   "badCertificate" error.  If the certificate is logged, an SCT MUST be
   issued.  Logging the certificate is useful, because monitors
   (Section 8.2) can then detect these encoding errors, which may be
   accepted by some TLS clients.

   If submission is an accepted trust anchor whose certifier is neither
   an accepted trust anchor nor the first element of chain, then the log
   MUST return the "unknownAnchor" error.  A log is not able to generate
   an SCT for a submission if it does not have access to the issuer's
   public key.

   If the returned sct is intended to be provided to TLS clients, then
   sth and inclusion (if returned) SHOULD also be provided to TLS
   clients.  For example, if type was 2 (indicating precert_sct_v2),
   then all three TransItems could be embedded in the certificate.

5.2.  Retrieve Latest STH

   GET <Base URL>/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.

5.3.  Retrieve Merkle Consistency Proof between Two STHs

   GET <Base URL>/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 signed STHs.

   Error codes:

       +===================+======================================+
       | type              | detail                               |
       +===================+======================================+
       | firstUnknown      | first is before the latest known STH |
       |                   | but is not from an existing STH.     |
       +-------------------+--------------------------------------+
       | secondUnknown     | second is before the latest known    |
       |                   | STH but is not from an existing STH. |
       +-------------------+--------------------------------------+
       | secondBeforeFirst | second is smaller than first.        |
       +-------------------+--------------------------------------+

                                 Table 3

   See Section 2.1.4.2 for an outline of how to use the consistency
   output.

5.4.  Retrieve Merkle Inclusion Proof from Log by Leaf Hash

   GET <Base URL>/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 Section 4.7.  A v2 STH
      must exist for the tree_size.  Because of skew, the front end may
      not know the requested tree head.  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 tree head, 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 certificate (as specified by the hash
         parameter) 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:

         +=================+=====================================+
         | type            | detail                              |
         +=================+=====================================+
         | hashUnknown     | hash is not the hash of a known     |
         |                 | leaf (may be caused by skew or by a |
         |                 | known certificate not yet merged).  |
         +-----------------+-------------------------------------+
         | treeSizeUnknown | hash is before the latest known STH |
         |                 | but is not from an existing STH.    |
         +-----------------+-------------------------------------+

                                  Table 4

   See Section 2.1.3.2 for an outline of how to use the inclusion
   output.

5.5.  Retrieve Merkle Inclusion Proof, STH, and Consistency Proof by
      Leaf Hash

   GET <Base URL>/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 Section 4.7.  A v2 STH
      must exist for the tree_size.

   Because of skew, the front end may not know the requested tree head
   or the requested hash, which leads to a number of cases:

       +=====================+=====================================+
       | Case                | Response                            |
       +=====================+=====================================+
       | latest STH <        | Return latest STH.                  |
       | requested tree head |                                     |
       +---------------------+-------------------------------------+
       | latest STH >        | Return latest STH and a consistency |
       | requested tree head | proof between it and the requested  |
       |                     | tree head (see Section 5.3).        |
       +---------------------+-------------------------------------+
       | index of requested  | Return inclusion.                   |
       | hash < latest STH   |                                     |
       +---------------------+-------------------------------------+

                                  Table 5

   Note that more than one case can be true; in which case, the returned
   data is their union.  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 certificate (as specified by the hash
         parameter) in the selected 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 tree head 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 signed STHs.

   Errors are the same as in Section 5.4.

   See Section 2.1.3.2 for an outline of how to use the inclusion
   output, and see Section 2.1.4.2 for an outline of how to use the
   consistency output.

5.6.  Retrieve Entries and STH from Log

   GET <Base URL>/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 Section 4.3).

         submitted_entry:  JSON object equivalent to 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 Section 5.2.

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

   Each submitted_entry output parameter MUST include the trust anchor
   that the log used to verify the submission, even if that trust anchor
   was not provided to submit-entry (see Section 5.1).  If the
   submission does not certify itself, then the first element of chain
   MUST be present and MUST certify the 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.  Note that a limit on the number of entries
   is not immutable, and therefore the restriction may be changed or
   lifted at any time and is not listed with the other Log Parameters in
   Section 4.1.

   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 Section 2.1.2 for an outline of how to use a complete list of
   log_entry entries to verify the root_hash.

   Error codes:

           +================+==================================+
           | type           | detail                           |
           +================+==================================+
           | startUnknown   | start is greater than the number |
           |                | of entries in the Merkle Tree.   |
           +----------------+----------------------------------+
           | endBeforeStart | start cannot be greater than     |
           |                | end.                             |
           +----------------+----------------------------------+

                                  Table 6

5.7.  Retrieve Accepted Trust Anchors

   GET <Base URL>/ct/v2/get-anchors

   No inputs.

   Outputs:
      certificates:  An array of JSON strings, each of which is a
         base64-encoded CA certificate that is 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.

      This data is not signed, and the protocol depends on the security
      guarantees of TLS to ensure correctness.

6.  TLS Servers

   CT-using TLS servers MUST use at least one of the mechanisms
   described below to present one or more SCTs from one or more logs to
   each TLS client during full TLS handshakes, when requested by the
   client, where each SCT corresponds to the server certificate.  (Of
   course, a server can only send a TLS extension if the client has
   specified it first.)  Servers SHOULD also present corresponding
   inclusion proofs and STHs.

   A server can provide SCTs using a TLS 1.3 extension (Section 4.2 of
   [RFC8446]) with type transparency_info (see Section 6.5).  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.

   The server may also use an Online Certificate Status Protocol (OCSP)
   [RFC6960] response extension (see Section 7.1.1), providing the OCSP
   response as part of the TLS handshake.  Providing a response during a
   TLS handshake is popularly known as "OCSP stapling".  For TLS 1.3,
   the information is encoded as an extension in the status_request
   extension data; see Section 4.4.2.1 of [RFC8446].  For TLS 1.2
   [RFC5246], the information is encoded in the CertificateStatus
   message; see Section 8 of [RFC6066].  Using stapling also allows SCTs
   and inclusion proofs to be updated on the fly.

   CT information can also be encoded as an extension in the X.509v3
   certificate (see Section 7.1.2).  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 may subsequently consider the certificate to be noncompliant.
   In such an event, one of the other two mechanisms will need to be
   used to deliver CT information, or, if this is not possible, the
   certificate will need to be reissued.

6.1.  TLS Client Authentication

   This specification includes no description of how a TLS server can
   use CT for TLS client certificates.  While this may be useful, it is
   not documented here for the following reasons:

   *  The greater security exposure is for clients to end up interacting
      with an illegitimate server.

   *  In general, TLS client certificates are not expected to be
      submitted to CT logs, particularly those intended for general
      public use.

   A future version could include such information.

6.2.  Multiple SCTs

   CT-using TLS servers SHOULD send SCTs from multiple logs because:

   *  The set of logs trusted by TLS clients is neither unified nor
      static; each client vendor may maintain an independent list of
      trusted logs, and, over time, new logs may become trusted and
      current logs may become distrusted.  Note that client discovery,
      trust, and distrust of logs are expected to be handled out of band
      and are out of scope of this document.

   *  If a CA and a log collude, it is possible to temporarily hide
      misissuance from clients.  When a TLS client requires SCTs from
      multiple logs to be provided, it is more difficult to mount this
      attack.

   *  If a log misbehaves or suffers a key compromise, 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), including SCTs from
      multiple logs reduces the probability of the certificate being
      rejected by TLS clients.

   *  TLS clients may have policies related to the above risks requiring
      TLS servers to present multiple SCTs.  For example, at the time of
      writing, Chromium [Chromium.Log.Policy] requires multiple SCTs to
      be presented with Extended Validation (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.

6.3.  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).

6.4.  Presenting SCTs, Inclusions Proofs, and STHs

   In each TransItemList that is sent during a TLS handshake, the TLS
   server MUST include a TransItem structure of type x509_sct_v2 or
   precert_sct_v2.

   Presenting inclusion proofs and STHs in the TLS handshake helps to
   protect the client's privacy (see Section 8.1.4) 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.

6.5.  transparency_info TLS Extension

   Provided that a TLS client includes the transparency_info extension
   type in the ClientHello and the TLS server supports the
   transparency_info extension:

   *  The TLS server MUST verify that the received extension_data is
      empty.

   *  The TLS server MUST construct a TransItemList of relevant
      TransItems (see Section 6.4), which SHOULD omit any TransItems
      that are already embedded in the server certificate or the stapled
      OCSP response (see Section 7.1).  If the constructed TransItemList
      is not empty, then the TLS server MUST include the
      transparency_info extension with the extension_data set to this
      TransItemList.  If the list is empty, then the server SHOULD omit
      the extension_data element but MAY send it with an empty array.

   TLS servers MUST only include this extension in the following
   messages:

   *  the ServerHello message (for TLS 1.2 or earlier)

   *  the Certificate or CertificateRequest message (for TLS 1.3)

   TLS servers MUST NOT process or include this extension when a TLS
   session is resumed, since session resumption uses the original
   session information.

7.  Certification Authorities

7.1.  Transparency Information X.509v3 Extension

   The Transparency Information X.509v3 extension, which has OID
   1.3.101.75 and SHOULD be noncritical, contains one or more TransItem
   structures in a TransItemList.  This extension MAY be included in
   OCSP responses (see Section 7.1.1) and certificates (see
   Section 7.1.2).  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.

7.1.1.  OCSP Response 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.

7.1.2.  Certificate 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.

7.2.  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 Section 6).

8.  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
   Section 4.1, but note that this document does not describe how the
   parameters are obtained, which is implementation dependent (for
   example, see [Chromium.Policy]).

8.1.  TLS Client

8.1.1.  Receiving SCTs and Inclusion Proofs

   TLS clients receive SCTs and inclusion proofs alongside or in
   certificates.  CT-using TLS clients MUST implement all of the three
   mechanisms by which TLS servers may present SCTs (see Section 6).

   TLS clients that support the transparency_info TLS extension (see
   Section 6.5) SHOULD include it in ClientHello messages, with empty
   extension_data.  If a TLS server includes the transparency_info TLS
   extension when resuming a TLS session, the TLS client MUST abort the
   handshake.

8.1.2.  Reconstructing the TBSCertificate

   Validation of an SCT for a certificate (where the type of the
   TransItem is x509_sct_v2) uses the unmodified TBSCertificate
   component of the certificate.

   Before an SCT for a precertificate (where the type of the TransItem
   is precert_sct_v2) can be validated, the TBSCertificate component of
   the precertificate needs to be reconstructed from the TBSCertificate
   component of the certificate as follows:

   *  Remove the Transparency Information extension (see Section 7.1).

   *  Remove embedded v1 SCTs, identified by OID 1.3.6.1.4.1.11129.2.4.2
      (see Section 3.3 of [RFC6962]).  This allows embedded v1 and v2
      SCTs to co-exist in a certificate (see Appendix A).

8.1.3.  Validating SCTs

   In order to make use of a received SCT, the TLS client MUST first
   validate it as follows:

   *  Compute 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 Section 8.1.2.

      -  issuer_key_hash is computed as described in Section 4.7.

      -  sct_extensions is copied from the SCT.

   *  Verify the SCT's signature against the computed signature input
      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.

   If the TLS client does not have the corresponding log's parameters,
   it cannot attempt to validate the SCT.  When evaluating compliance
   (see Section 8.1.6), the TLS client will consider only those SCTs
   that it was able to validate.

   Note that SCT validation is not a substitute for the normal
   validation of the server certificate and its chain.

8.1.4.  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
   (Section 5.4) or get-all-by-hash (Section 5.5).

   Note that fetching inclusion proofs directly from a log will disclose
   to the log which TLS server the client has been communicating with.
   This may be regarded as a significant privacy concern, and so it is
   preferable for the TLS server to send the inclusion proofs (see
   Section 6.4).

8.1.5.  Validating Inclusion Proofs

   When a TLS client has received, or fetched, an inclusion proof (and
   an STH), it SHOULD proceed to verify 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 (Section 5.2) and
   then verify it by requesting a consistency proof (Section 5.3).  Note
   that if the TLS client uses get-all-by-hash, then it will already
   have the new STH.

8.1.6.  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 noncompliance.

   A TLS client can only evaluate compliance if it has given the TLS
   server the opportunity to send SCTs and inclusion proofs by any of
   the three mechanisms that are mandatory to implement for CT-using TLS
   clients (see Section 8.1.1).  Therefore, a TLS client MUST NOT
   evaluate compliance if it did not include both the transparency_info
   and status_request TLS extensions in the ClientHello.

8.2.  Monitor

   Monitors watch logs to check for correct behavior, for certificates
   of interest, or for 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 MUST at least inspect every new entry in every log it
   watches, and it MAY also choose to keep copies of entire logs.

   To inspect all of the existing entries, the monitor SHOULD follow
   these steps once for each log:

   1.  Fetch the current STH (Section 5.2).

   2.  Verify the STH signature.

   3.  Fetch all the entries in the tree corresponding to the STH
       (Section 5.6).

   4.  If applicable, check each entry to see if it's a certificate of
       interest.

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

   To inspect new entries, the monitor SHOULD follow these steps
   repeatedly for each log:

   1.  Fetch the current STH (Section 5.2).  Repeat until the STH
       changes.  To allow for experimentation, this document does not
       specify the polling frequency.

   2.  Verify the STH signature.

   3.  Fetch all the new entries in the tree corresponding to the STH
       (Section 5.6).  If they remain unavailable for an extended
       period, then this should be viewed as misbehavior on the part of
       the log.

   4.  If applicable, check each entry to see if it's a certificate of
       interest.

   5.  Either:

       a.  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:

       a.  Fetch a consistency proof for the new STH with the previous
           STH (Section 5.3).

       b.  Verify the consistency proof.

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

   6.  Repeat from Step 1.

8.3.  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 properties of log behavior 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 an audit fails, the auditor is in
   possession of signed proof of the log's misbehavior.

   A monitor (Section 8.2) can audit by verifying the consistency of
   STHs it receives, ensuring 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 (Section 8.1) can audit by verifying an SCT against any
   STH dated after the SCT timestamp + the Maximum Merge Delay by
   requesting a Merkle inclusion proof (Section 5.4).  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-using entities and is discussed in
   Section 11.3.

9.  Algorithm Agility

   It is not possible for a log to change either 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 MUST be frozen, as specified in Section 4.13,
   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.

10.  IANA Considerations

   The assignment policy criteria mentioned in this section refer to the
   policies outlined in [RFC8126].

10.1.  Additions to Existing Registries

   This subsection defines additions to existing registries.

10.1.1.  New Entry to the TLS ExtensionType Registry

   IANA has added the following entry to the "TLS ExtensionType Values"
   registry defined in [RFC8446], with an assigned Value:

   +=====+===================+===+===========+=============+===========+
   |Value| Extension Name    |TLS| DTLS-Only | Recommended | Reference |
   |     |                   |1.3|           |             |           |
   +=====+===================+===+===========+=============+===========+
   |52   | transparency_info |CH,| N         | Y           | RFC 9162  |
   |     |                   |CR,|           |             |           |
   |     |                   |CT |           |             |           |
   +-----+-------------------+---+-----------+-------------+-----------+

                                  Table 7

10.1.2.  URN Sub-namespace for TRANS (urn:ietf:params:trans)

   IANA has added a new entry in the "IETF URN Sub-namespace for
   Registered Protocol Parameter Identifiers" registry, following the
   template in [RFC3553]:

   Registry name:  trans
   Specification:  RFC 9162
   Repository:  <https://www.iana.org/assignments/trans>
   Index value:  No transformation needed.

10.2.  New CT-Related Registries

   IANA has added a new protocol registry, "Public Notary Transparency",
   to the list that appears at <https://www.iana.org/assignments/>

   The rest of this section defines the subregistries that have been
   created within the new "Public Notary Transparency" registry.

10.2.1.  Hash Algorithms

   IANA has established a registry of hash algorithm values, named "Hash
   Algorithms", with the following registration procedures:

                  +===========+=========================+
                  | Range     | Registration Procedures |
                  +===========+=========================+
                  | 0x00-0xDF | Specification Required  |
                  +-----------+-------------------------+
                  | 0xE0-0xEF | Experimental Use        |
                  +-----------+-------------------------+
                  | 0xF0-0xFF | Private Use             |
                  +-----------+-------------------------+

                                  Table 8

   The "Hash Algorithms" registry initially consists of:

    +========+==================+========================+===========+
    | Value  | Hash Algorithm   | OID                    | Reference |
    +========+==================+========================+===========+
    | 0x00   | SHA-256          | 2.16.840.1.101.3.4.2.1 | [RFC6234] |
    +--------+------------------+------------------------+-----------+
    | 0x01 - | Unassigned       |                        | RFC 9162  |
    | 0xDF   |                  |                        |           |
    +--------+------------------+------------------------+-----------+
    | 0xE0 - | Reserved for     |                        | RFC 9162  |
    | 0xEF   | Experimental Use |                        |           |
    +--------+------------------+------------------------+-----------+
    | 0xF0 - | Reserved for     |                        | RFC 9162  |
    | 0xFF   | Private Use      |                        |           |
    +--------+------------------+------------------------+-----------+

                                 Table 9

   The designated expert(s) 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.

10.2.2.  Signature Algorithms

   IANA has established a registry of signature algorithm values, named
   "Signature Algorithms".

   The following notes have been added to the registry:

   |  *Note:*
   |     This is a subset of the "TLS SignatureScheme" registry, limited
   |     to those algorithms that are appropriate for CT.  A major
   |     advantage of this is leveraging the expertise of the TLS
   |     Working Group and its designated expert(s).

   |  *Note:*
   |     The value 0x0403 appears twice.  While this may be confusing,
   |     it is okay because the verification process is the same for
   |     both algorithms, and the choice of which to use when generating
   |     a signature is purely internal to the log server.

   The "Signature Algorithms" registry has the following registration
   procedures:

                +===============+=========================+
                | Range         | Registration Procedures |
                +===============+=========================+
                | 0x0000-0x0807 | Specification Required  |
                +---------------+-------------------------+
                | 0x0808-0xFDFF | Expert Review           |
                +---------------+-------------------------+
                | 0xFE00-0xFEFF | Experimental Use        |
                +---------------+-------------------------+
                | 0xFF00-0xFFFF | Private Use             |
                +---------------+-------------------------+

                                  Table 10

   The "Signature Algorithms" registry initially consists of:

   +========================+===========================+=============+
   | SignatureScheme Value  | Signature Algorithm       | Reference   |
   +========================+===========================+=============+
   | 0x0000 - 0x0402        | Unassigned                |             |
   +------------------------+---------------------------+-------------+
   | ecdsa_secp256r1_sha256 | ECDSA (NIST P-256) with   | [FIPS186-4] |
   | (0x0403)               | SHA-256                   |             |
   +------------------------+---------------------------+-------------+
   | ecdsa_secp256r1_sha256 | Deterministic ECDSA (NIST | [RFC6979]   |
   | (0x0403)               | P-256) with HMAC-SHA256   |             |
   +------------------------+---------------------------+-------------+
   | 0x0404 - 0x0806        | Unassigned                |             |
   +------------------------+---------------------------+-------------+
   | ed25519 (0x0807)       | Ed25519 (PureEdDSA with   | [RFC8032]   |
   |                        | the edwards25519 curve)   |             |
   +------------------------+---------------------------+-------------+
   | 0x0808 - 0xFDFF        | Unassigned                |             |
   +------------------------+---------------------------+-------------+
   | 0xFE00 - 0xFEFF        | Reserved for Experimental | RFC 9162    |
   |                        | Use                       |             |
   +------------------------+---------------------------+-------------+
   | 0xFF00 - 0xFFFF        | Reserved for Private Use  | RFC 9162    |
   +------------------------+---------------------------+-------------+

                                 Table 11

   The designated expert(s) should ensure that the proposed algorithm
   has a public specification, has a value assigned to it in the "TLS
   SignatureScheme" registry (which was established by [RFC8446]), and
   is suitable for use as a cryptographic signature algorithm.

10.2.3.  VersionedTransTypes

   IANA has established a registry of VersionedTransType values, named
   "VersionedTransTypes".

   The following note has been added:

   |  *Note:*
   |     The range 0x0000..0x00FF is reserved so that v1 SCTs are
   |     distinguishable from v2 SCTs and other TransItem structures.

   The registration procedures for the "VersionedTransTypes" registry
   are the following:

                +===============+=========================+
                | Range         | Registration Procedures |
                +===============+=========================+
                | 0x0100-0xDFFF | Specification Required  |
                +---------------+-------------------------+
                | 0xE000-0xEFFF | Experimental Use        |
                +---------------+-------------------------+
                | 0xF000-0xFFFF | Private Use             |
                +---------------+-------------------------+

                                  Table 12

   The "VersionedTransTypes" registry initially consists of:

      +=================+===============================+===========+
      | Value           | Type and Version              | Reference |
      +=================+===============================+===========+
      | 0x0000 - 0x00FF | Reserved                      | [RFC6962] |
      +-----------------+-------------------------------+-----------+
      | 0x0100          | x509_entry_v2                 | RFC 9162  |
      +-----------------+-------------------------------+-----------+
      | 0x0101          | precert_entry_v2              | RFC 9162  |
      +-----------------+-------------------------------+-----------+
      | 0x0102          | x509_sct_v2                   | RFC 9162  |
      +-----------------+-------------------------------+-----------+
      | 0x0103          | precert_sct_v2                | RFC 9162  |
      +-----------------+-------------------------------+-----------+
      | 0x0104          | signed_tree_head_v2           | RFC 9162  |
      +-----------------+-------------------------------+-----------+
      | 0x0105          | consistency_proof_v2          | RFC 9162  |
      +-----------------+-------------------------------+-----------+
      | 0x0106          | inclusion_proof_v2            | RFC 9162  |
      +-----------------+-------------------------------+-----------+
      | 0x0107 - 0xDFFF | Unassigned                    |           |
      +-----------------+-------------------------------+-----------+
      | 0xE000 - 0xEFFF | Reserved for Experimental Use | RFC 9162  |
      +-----------------+-------------------------------+-----------+
      | 0xF000 - 0xFFFF | Reserved for Private Use      | RFC 9162  |
      +-----------------+-------------------------------+-----------+

                                  Table 13

   The designated expert(s) should review the public specification to
   ensure that it is detailed enough to ensure implementation
   interoperability.

10.2.4.  Log Artifact Extensions

   IANA has established a registry of ExtensionType values, named "Log
   Artifact Extensions".

   The registration procedures for the "Log Artifact Extensions"
   registry are the following:

                +===============+=========================+
                | Range         | Registration Procedures |
                +===============+=========================+
                | 0x0000-0xDFFF | Specification Required  |
                +---------------+-------------------------+
                | 0xE000-0xEFFF | Experimental Use        |
                +---------------+-------------------------+
                | 0xF000-0xFFFF | Private Use             |
                +---------------+-------------------------+

                                  Table 14

   The "Log Artifact Extensions" registry initially consists of:

   +=================+===============================+=====+===========+
   | ExtensionType   | Status                        | Use | Reference |
   +=================+===============================+=====+===========+
   | 0x0000 - 0xDFFF | Unassigned                    | n/a |           |
   +-----------------+-------------------------------+-----+-----------+
   | 0xE000 - 0xEFFF | Reserved for                  | n/a | RFC 9162  |
   |                 | Experimental Use              |     |           |
   +-----------------+-------------------------------+-----+-----------+
   | 0xF000 - 0xFFFF | Reserved for                  | n/a | RFC 9162  |
   |                 | Private Use                   |     |           |
   +-----------------+-------------------------------+-----+-----------+

                                  Table 15

   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.

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

10.2.5.  Log IDs

   IANA has established a registry of Log IDs, named "Log IDs".

   The registry's registration procedure is First Come First Served.

   The "Log IDs" registry initially consists of:

       +================+==============+==============+===========+
       | Log ID         | Log Base URL | Log Operator | Reference |
       +================+==============+==============+===========+
       | 1.3.101.8192 - | Unassigned   | Unassigned   |           |
       | 1.3.101.16383  |              |              |           |
       +----------------+--------------+--------------+-----------+
       | 1.3.101.80.0 - | Unassigned   | Unassigned   |           |
       | 1.3.101.80.*   |              |              |           |
       +----------------+--------------+--------------+-----------+

                                 Table 16

   The following notes have been added to the registry:

   |  *Note:*
   |     All OIDs in the range from 1.3.101.8192 to 1.3.101.16383 have
   |     been set aside for Log IDs.  This is a limited resource of
   |     8,192 OIDs, each of which has an encoded length of 4 octets.

   |  *Note:*
   |     The 1.3.101.80 arc has also been set aside for Log IDs.  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 MUST be accompanied
   by:

   *  the Log's Base URL (see Section 4.1) and

   *  the Log Operator's contact details.

   IANA is asked to reject any request to update a Log ID or Log Base
   URL in this registry because these fields are immutable (see
   Section 4.1).

   IANA is asked to accept requests from log operators to update their
   contact details in this registry.

   Since log operators can choose to not use this registry (see
   Section 4.4), it is not expected to be a global directory of all
   logs.

10.2.6.  Error Types

   IANA has created a new registry for errors, the "Error Types"
   registry.

   The registration procedure for this registry is Specification
   Required.

   This registry has the following three fields:

                    +============+========+===========+
                    | Field Name | Type   | Reference |
                    +============+========+===========+
                    | Identifier | string | RFC 9162  |
                    +------------+--------+-----------+
                    | Meaning    | string | RFC 9162  |
                    +------------+--------+-----------+
                    | Reference  | string | RFC 9162  |
                    +------------+--------+-----------+

                                  Table 17

   The initial values of the "Error Types" registry, which are taken
   from the text in Section 5, are as follows:

   +===================+===================================+===========+
   | Identifier        | Meaning                           | Reference |
   +===================+===================================+===========+
   | malformed         | The request could not be          | RFC 9162  |
   |                   | parsed.                           |           |
   +-------------------+-----------------------------------+-----------+
   | badSubmission     | submission is neither a           | RFC 9162  |
   |                   | valid certificate nor a           |           |
   |                   | valid precertificate.             |           |
   +-------------------+-----------------------------------+-----------+
   | badType           | type is neither 1 nor 2.          | RFC 9162  |
   +-------------------+-----------------------------------+-----------+
   | badChain          | The first element of chain        | RFC 9162  |
   |                   | is not the certifier of the       |           |
   |                   | submission, or the second         |           |
   |                   | element does not certify the      |           |
   |                   | first, etc.                       |           |
   +-------------------+-----------------------------------+-----------+
   | badCertificate    | One or more certificates in       | RFC 9162  |
   |                   | chain are not valid (e.g.,        |           |
   |                   | not properly encoded).            |           |
   +-------------------+-----------------------------------+-----------+
   | unknownAnchor     | The last element of chain         | RFC 9162  |
   |                   | (or, if chain is an empty         |           |
   |                   | array, the submission) is         |           |
   |                   | not, nor is it certified by,      |           |
   |                   | an accepted trust anchor.         |           |
   +-------------------+-----------------------------------+-----------+
   | shutdown          | The log is no longer              | RFC 9162  |
   |                   | accepting submissions.            |           |
   +-------------------+-----------------------------------+-----------+
   | firstUnknown      | first is before the latest        | RFC 9162  |
   |                   | known STH but is not from an      |           |
   |                   | existing STH.                     |           |
   +-------------------+-----------------------------------+-----------+
   | secondUnknown     | second is before the latest       | RFC 9162  |
   |                   | known STH but is not from an      |           |
   |                   | existing STH.                     |           |
   +-------------------+-----------------------------------+-----------+
   | secondBeforeFirst | second is smaller than            | RFC 9162  |
   |                   | first.                            |           |
   +-------------------+-----------------------------------+-----------+
   | hashUnknown       | hash is not the hash of a         | RFC 9162  |
   |                   | known leaf (may be caused by      |           |
   |                   | skew or by a known                |           |
   |                   | certificate not yet merged).      |           |
   +-------------------+-----------------------------------+-----------+
   | treeSizeUnknown   | hash is before the latest         | RFC 9162  |
   |                   | known STH but is not from an      |           |
   |                   | existing STH.                     |           |
   +-------------------+-----------------------------------+-----------+
   | startUnknown      | start is greater than the         | RFC 9162  |
   |                   | number of entries in the          |           |
   |                   | Merkle Tree.                      |           |
   +-------------------+-----------------------------------+-----------+
   | endBeforeStart    | start cannot be greater than      | RFC 9162  |
   |                   | end.                              |           |
   +-------------------+-----------------------------------+-----------+

                                  Table 18

10.3.  OID Assignment

   IANA has assigned an object identifier from the "SMI Security for
   PKIX Module Identifier" registry to identify the ASN.1 module in
   Appendix B of this document.

            +=========+=========================+============+
            | Decimal | Description             | References |
            +=========+=========================+============+
            | 102     | id-mod-public-notary-v2 | RFC 9162   |
            +---------+-------------------------+------------+

                                 Table 19

11.  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 misissuance and take
   some action, such as asking a CA to revoke a misissued certificate.
   A signed timestamp does not guarantee this, though, 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.

11.1.  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.  Since a log is allowed to serve an STH of any
   age up to the MMD, the maximum period of time during which a
   misissued certificate can be used without being available for audit
   is twice the MMD.

11.2.  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.

11.3.  Misbehaving Logs

   A log can misbehave in several ways.  Examples include the following:
   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; issuing STHs too
   frequently; mutating the signature of a logged certificate; and
   failing to present a chain containing the certifier of a logged
   certificate.

   Violation of the MMD contract is detected by log clients requesting a
   Merkle inclusion proof (Section 5.4) for each observed SCT.  These
   checks can be asynchronous and need only be done once per
   certificate.  However, note that there may be privacy concerns (see
   Section 8.1.4).

   Violation of the append-only property or the STH issuance rate limit
   can be detected by multiple clients comparing their instances of the
   STHs.  This technique, known as "gossip", is an active area of
   research and not defined here.  Proof of misbehavior in such cases
   would be either 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.

   Clients that 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.

11.4.  Multiple SCTs

   By requiring TLS servers to offer multiple SCTs, each from a
   different log, TLS clients reduce the effectiveness of an attack
   where a CA and a log collude (see Section 6.2).

11.5.  Leakage of DNS Information

   Malicious monitors can use logs to learn about the existence of
   domain names that might not otherwise be easy to discover.  Some
   subdomain labels may reveal information about the service and
   software for which the subdomain is used, which in turn might
   facilitate targeted attacks.

12.  References

12.1.  Normative References

   [FIPS186-4]
              National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", FIPS PUB 186-4, July 2013,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.186-4.pdf>.

   [HTML401]  Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
              Specification", W3C Recommendation SPSD-html401-20180327,
              March 2018,
              <https://www.w3.org/TR/2018/SPSD-html401-20180327>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
              2003, <https://www.rfc-editor.org/info/rfc3553>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,
              <https://www.rfc-editor.org/info/rfc6960>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <https://www.rfc-editor.org/info/rfc6979>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7633]  Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS)
              Feature Extension", RFC 7633, DOI 10.17487/RFC7633,
              October 2015, <https://www.rfc-editor.org/info/rfc7633>.

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
              <https://www.rfc-editor.org/info/rfc7807>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8391]  Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A.
              Mohaisen, "XMSS: eXtended Merkle Signature Scheme",
              RFC 8391, DOI 10.17487/RFC8391, May 2018,
              <https://www.rfc-editor.org/info/rfc8391>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [UNIXTIME] IEEE, "The Open Group Base Specifications Issue 7",
              Section 4.16 Seconds Since the Epoch, IEEE
              Std 1003.1-2008, 2016, <http://pubs.opengroup.org/
              onlinepubs/9699919799.2016edition/basedefs/
              V1_chap04.html#tag_04_16>.

   [X690]     ITU-T, "Information technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1,
              February 2021.

12.2.  Informative References

   [CABBR]    CA/Browser Forum, "Baseline Requirements for the Issuance
              and Management of Publicly-Trusted Certificates",
              Version 1.7.3, October 2020, <https://cabforum.org/wp-
              content/uploads/CA-Browser-Forum-BR-1.7.3.pdf>.

   [Chromium.Log.Policy]
              The Chromium Projects, "Chromium Certificate Transparency
              Log Policy",
              <https://googlechrome.github.io/CertificateTransparency/
              log_policy.html>.

   [Chromium.Policy]
              The Chromium Projects, "Chromium Certificate Transparency
              Policy",
              <https://googlechrome.github.io/CertificateTransparency/
              ct_policy.html>.

   [CrosbyWallach]
              Crosby, S. and D. Wallach, "Efficient Data Structures for
              Tamper-Evident Logging", Proceedings of the 18th USENIX
              Security Symposium, Montreal, August 2009,
              <http://static.usenix.org/event/sec09/tech/full_papers/
              crosby.pdf>.

   [JSON.Metadata]
              The Chromium Projects, "Chromium Log Metadata JSON
              Schema", <https://www.gstatic.com/ct/log_list/
              log_list_schema.json>.

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              DOI 10.17487/RFC5912, June 2010,
              <https://www.rfc-editor.org/info/rfc5912>.

   [RFC6268]  Schaad, J. and S. Turner, "Additional New ASN.1 Modules
              for the Cryptographic Message Syntax (CMS) and the Public
              Key Infrastructure Using X.509 (PKIX)", RFC 6268,
              DOI 10.17487/RFC6268, July 2011,
              <https://www.rfc-editor.org/info/rfc6268>.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
              <https://www.rfc-editor.org/info/rfc6962>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8820]  Nottingham, M., "URI Design and Ownership", BCP 190,
              RFC 8820, DOI 10.17487/RFC8820, June 2020,
              <https://www.rfc-editor.org/info/rfc8820>.

   [X.680]    ITU-T, "Information technology - Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, February 2021.

Appendix A.  Supporting v1 and v2 Simultaneously (Informative)

   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 Section 3.2, and
      submit it to v2 logs.

   *  Embed the obtained v2 SCTs in the TBSCertificate, as described in
      Section 7.1.2.

   *  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.

Appendix B.  An ASN.1 Module (Informative)

   The following ASN.1 [X.680] module may be useful to implementors.
   This module references [RFC5912] and [RFC6268].

   CertificateTransparencyV2Module-2021
    -- { id-mod-public-notary-v2 from above, in
           iso(1) identified-organization(3) ...
       form }
   DEFINITIONS IMPLICIT TAGS ::= BEGIN

   -- EXPORTS ALL --

   IMPORTS
     EXTENSION
     FROM PKIX-CommonTypes-2009 -- RFC 5912
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkixCommon-02(57) }

     CONTENT-TYPE
     FROM CryptographicMessageSyntax-2010  -- RFC 6268
       { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
         pkcs-9(9) smime(16) modules(0) id-mod-cms-2009(58) }

     TBSCertificate
     FROM PKIX1Explicit-2009 -- RFC 5912
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkix1-explicit-02(51) }
   ;

   --
   -- Section 3.2.  Precertificates
   --

   ct-tbsCertificate CONTENT-TYPE ::= {
     TYPE TBSCertificate
     IDENTIFIED BY id-ct-tbsCertificate }

   id-ct-tbsCertificate OBJECT IDENTIFIER ::= { 1 3 101 78 }

   --
   -- Section 7.1.  Transparency Information X.509v3 Extension
   --

   ext-transparencyInfo EXTENSION ::= {
      SYNTAX TransparencyInformationSyntax
      IDENTIFIED BY id-ce-transparencyInfo
      CRITICALITY { FALSE } }

   id-ce-transparencyInfo OBJECT IDENTIFIER ::= { 1 3 101 75 }

   TransparencyInformationSyntax ::= OCTET STRING

   --
   -- Section 7.1.1.  OCSP Response Extension
   --

   ext-ocsp-transparencyInfo EXTENSION ::= {
      SYNTAX TransparencyInformationSyntax
      IDENTIFIED BY id-pkix-ocsp-transparencyInfo
      CRITICALITY { FALSE } }

   id-pkix-ocsp-transparencyInfo OBJECT IDENTIFIER ::=
      id-ce-transparencyInfo

   --
   -- Section 8.1.2.  Reconstructing the TBSCertificate
   --

   ext-embeddedSCT-CTv1 EXTENSION ::= {
      SYNTAX SignedCertificateTimestampList
      IDENTIFIED BY id-ce-embeddedSCT-CTv1
      CRITICALITY { FALSE } }

   id-ce-embeddedSCT-CTv1 OBJECT IDENTIFIER ::= {
      1 3 6 1 4 1 11129 2 4 2 }

   SignedCertificateTimestampList ::= OCTET STRING

   END

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, Emilia
   Kasper, Stephen Kent, Adam Langley, SM, Alexey Melnikov, Linus
   Nordberg, Chris Palmer, Trevor Perrin, Pierre Phaneuf, Eric Rescorla,
   Rich Salz, 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.

Authors' Addresses

   Ben Laurie
   Google UK Ltd.

   Email: benl@google.com


   Eran Messeri
   Google UK Ltd.

   Email: eranm@google.com


   Rob Stradling
   Sectigo Ltd.

   Email: rob@sectigo.com