draft-ietf-trans-rfc6962-bis-34.txt

TRANS (Public Notary Transparency)                             B. Laurie
Internet-Draft                                                A. Langley
Obsoletes: 6962 (if approved)                                  E. Kasper
Intended status: Experimental                                 E. Messeri
Expires: May 7, 2020                                              Google
                                                            R. Stradling
                                                                 Sectigo
                                                       November 04, 2019


                  Certificate Transparency Version 2.0
                    draft-ietf-trans-rfc6962-bis-34

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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 7, 2020.




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Copyright Notice

   Copyright (c) 2019 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
     1.2.  Data Structures . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Major Differences from CT 1.0 . . . . . . . . . . . . . .   5
   2.  Cryptographic Components  . . . . . . . . . . . . . . . . . .   7
     2.1.  Merkle Hash Trees . . . . . . . . . . . . . . . . . . . .   7
       2.1.1.  Definition of the Merkle Tree . . . . . . . . . . . .   7
       2.1.2.  Verifying a Tree Head Given Entries . . . . . . . . .   8
       2.1.3.  Merkle Inclusion Proofs . . . . . . . . . . . . . . .   9
       2.1.4.  Merkle Consistency Proofs . . . . . . . . . . . . . .  10
       2.1.5.  Example . . . . . . . . . . . . . . . . . . . . . . .  12
     2.2.  Signatures  . . . . . . . . . . . . . . . . . . . . . . .  14
   3.  Submitters  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     3.1.  Certificates  . . . . . . . . . . . . . . . . . . . . . .  14
     3.2.  Precertificates . . . . . . . . . . . . . . . . . . . . .  14
       3.2.1.  Binding Intent to Issue . . . . . . . . . . . . . . .  16
   4.  Log Format and Operation  . . . . . . . . . . . . . . . . . .  16
     4.1.  Log Parameters  . . . . . . . . . . . . . . . . . . . . .  17
     4.2.  Evaluating Submissions  . . . . . . . . . . . . . . . . .  18
       4.2.1.  Minimum Acceptance Criteria . . . . . . . . . . . . .  18
       4.2.2.  Discretionary Acceptance Criteria . . . . . . . . . .  19
     4.3.  Log Entries . . . . . . . . . . . . . . . . . . . . . . .  19
     4.4.  Log ID  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     4.5.  TransItem Structure . . . . . . . . . . . . . . . . . . .  20
     4.6.  Log Artifact Extensions . . . . . . . . . . . . . . . . .  21
     4.7.  Merkle Tree Leaves  . . . . . . . . . . . . . . . . . . .  21
     4.8.  Signed Certificate Timestamp (SCT)  . . . . . . . . . . .  22
     4.9.  Merkle Tree Head  . . . . . . . . . . . . . . . . . . . .  23
     4.10. Signed Tree Head (STH)  . . . . . . . . . . . . . . . . .  24
     4.11. Merkle Consistency Proofs . . . . . . . . . . . . . . . .  25
     4.12. Merkle Inclusion Proofs . . . . . . . . . . . . . . . . .  25



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     4.13. Shutting down a log . . . . . . . . . . . . . . . . . . .  26
   5.  Log Client Messages . . . . . . . . . . . . . . . . . . . . .  26
     5.1.  Submit Entry to Log . . . . . . . . . . . . . . . . . . .  28
     5.2.  Retrieve Latest Signed Tree Head  . . . . . . . . . . . .  30
     5.3.  Retrieve Merkle Consistency Proof between Two Signed Tree
           Heads . . . . . . . . . . . . . . . . . . . . . . . . . .  30
     5.4.  Retrieve Merkle Inclusion Proof from Log by Leaf Hash . .  31
     5.5.  Retrieve Merkle Inclusion Proof, Signed Tree Head and
           Consistency Proof by Leaf Hash  . . . . . . . . . . . . .  32
     5.6.  Retrieve Entries and STH from Log . . . . . . . . . . . .  33
     5.7.  Retrieve Accepted Trust Anchors . . . . . . . . . . . . .  35
   6.  TLS Servers . . . . . . . . . . . . . . . . . . . . . . . . .  35
     6.1.  Multiple SCTs . . . . . . . . . . . . . . . . . . . . . .  36
     6.2.  TransItemList Structure . . . . . . . . . . . . . . . . .  37
     6.3.  Presenting SCTs, inclusions proofs and STHs . . . . . . .  37
     6.4.  transparency_info TLS Extension . . . . . . . . . . . . .  37
   7.  Certification Authorities . . . . . . . . . . . . . . . . . .  38
     7.1.  Transparency Information X.509v3 Extension  . . . . . . .  38
       7.1.1.  OCSP Response Extension . . . . . . . . . . . . . . .  38
       7.1.2.  Certificate Extension . . . . . . . . . . . . . . . .  38
     7.2.  TLS Feature X.509v3 Extension . . . . . . . . . . . . . .  39
   8.  Clients . . . . . . . . . . . . . . . . . . . . . . . . . . .  39
     8.1.  TLS Client  . . . . . . . . . . . . . . . . . . . . . . .  39
       8.1.1.  Receiving SCTs and inclusion proofs . . . . . . . . .  39
       8.1.2.  Reconstructing the TBSCertificate . . . . . . . . . .  39
       8.1.3.  Validating SCTs . . . . . . . . . . . . . . . . . . .  40
       8.1.4.  Fetching inclusion proofs . . . . . . . . . . . . . .  40
       8.1.5.  Validating inclusion proofs . . . . . . . . . . . . .  41
       8.1.6.  Evaluating compliance . . . . . . . . . . . . . . . .  41
     8.2.  Monitor . . . . . . . . . . . . . . . . . . . . . . . . .  41
     8.3.  Auditing  . . . . . . . . . . . . . . . . . . . . . . . .  42
   9.  Algorithm Agility . . . . . . . . . . . . . . . . . . . . . .  43
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  44
     10.1.  New Entry to the TLS ExtensionType Registry  . . . . . .  44
     10.2.  Hash Algorithms  . . . . . . . . . . . . . . . . . . . .  44
       10.2.1.  Specification Required guidance  . . . . . . . . . .  45
     10.3.  Signature Algorithms . . . . . . . . . . . . . . . . . .  45
       10.3.1.  Expert Review guidelines . . . . . . . . . . . . . .  46
     10.4.  VersionedTransTypes  . . . . . . . . . . . . . . . . . .  46
       10.4.1.  Specification Required guidance  . . . . . . . . . .  47
     10.5.  Log Artifact Extension Registry  . . . . . . . . . . . .  47
       10.5.1.  Specification Required guidance  . . . . . . . . . .  47
     10.6.  Object Identifiers . . . . . . . . . . . . . . . . . . .  47
       10.6.1.  Log ID Registry  . . . . . . . . . . . . . . . . . .  47
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  48
     11.1.  Misissued Certificates . . . . . . . . . . . . . . . . .  49
     11.2.  Detection of Misissue  . . . . . . . . . . . . . . . . .  49
     11.3.  Misbehaving Logs . . . . . . . . . . . . . . . . . . . .  49



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     11.4.  Preventing Tracking Clients  . . . . . . . . . . . . . .  50
     11.5.  Multiple SCTs  . . . . . . . . . . . . . . . . . . . . .  50
     11.6.  Leakage of DNS Information . . . . . . . . . . . . . . .  50
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  50
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  51
     13.2.  Informative References . . . . . . . . . . . . . . . . .  52
   Appendix A.  Supporting v1 and v2 simultaneously  . . . . . . . .  54
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54

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)).

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

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



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   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).

   It is necessary to treat each log as a trusted third party, because
   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.  Whilst it is anticipated that
   additional mechanisms could be 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].

1.3.  Major Differences from CT 1.0

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

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

   o  Precertificate format: precertificates are now CMS objects rather
      than X.509 certificates, which avoids violating the certificate




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      serial number uniqueness requirement in Section 4.1.2.2 of
      [RFC5280].

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

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

   o  "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].

   o  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.

   o  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).

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

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

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

   o  submit-entry: new client API, replacing add-chain and add-pre-
      chain.

   o  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).

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





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

   o  Extensive clarifications and editorial work.

2.  Cryptographic Components

2.1.  Merkle Hash Trees

2.1.1.  Definition of the Merkle Tree

   The log uses a binary Merkle Hash 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).  Throughout this document, the hash algorithm in
   use is referred to as HASH and the size of its output in bytes as
   HASH_SIZE.  The input to the Merkle Tree Hash is a list of data
   entries; these entries will be hashed to form the leaves of the
   Merkle Hash Tree.  The output is a single HASH_SIZE Merkle Tree Hash.
   Given an ordered list of n inputs, D_n = {d[0], d[1], ..., d[n-1]},
   the Merkle Tree Hash (MTH) is thus defined as follows:

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

   MTH({}) = HASH().

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

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

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

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

   where:

   o  || denotes concatenation

   o  : denotes concatenation of lists

   o  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).





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   Note that the hash calculations for leaves and nodes differ; this
   domain separation is required to give second preimage resistance.

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

2.1.2.  Verifying a Tree Head Given Entries

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

   1.  Set "stack" to an empty stack.

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

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

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

       3.  Repeat "merge_count" times:

           1.  Pop "right" from "stack".

           2.  Pop "left" from "stack".

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

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

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








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2.1.3.  Merkle Inclusion Proofs

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

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" 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".



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   3.  Set "r" to "hash".

   4.  For each value "p" in the "inclusion_path" array:

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

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

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

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

       Otherwise:

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

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

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

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




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   MTH(D[0:m]) is known.  The initial call to SUBPROOF sets this to be
   true, and SUBPROOF is then defined as follows:

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

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

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

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

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

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

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

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

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

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

   The : operator and D[k1:k2] are defined the same as in 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", a
   tree head "second_hash" for tree size "second" where "0 < first <
   second", and has received a consistency proof between the two (e.g.,
   in a "TransItem" of type "consistency_proof_v2"), the following
   algorithm may be used to verify the consistency proof:

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

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



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

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

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

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

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

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

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

       Otherwise:

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

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

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

2.1.5.  Example

   The binary Merkle Tree with 7 leaves:

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



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

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

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

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

   The same tree, built incrementally in four steps:

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

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

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

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

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





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2.2.  Signatures

   Various data structures Section 1.2 are signed.  A log MUST use one
   of the signature algorithms defined in Section 10.3.

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:

   o  It MUST be DER encoded.

   o  "SignedData.version" MUST be v3(3).




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

   o  "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.

   o  "SignedData.certificates" MUST be omitted.

   o  "SignedData.crls" MUST be omitted.

   o  "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 Section 10.2.

      *  "signedAttrs" MUST be present and MUST contain two attributes:

         +  A content-type attribute whose value is the same as
            "SignedData.encapContentInfo.eContentType".

         +  A message-digest attribute whose value is the message digest
            of "SignedData.encapContentInfo.eContent".

      *  "signatureAlgorithm" MUST be the same OID as
         "TBSCertificate.signature".

      *  "signature" MUST be from the same (root or intermediate) CA
         that 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.



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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), and 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:

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

   o  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.

   o  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 (to
   reduce the ability to track clients as described in Section 11.4).
   If different SCTs are produced for the same submission, multiple log
   entries will have to be created, one for each SCT (as the timestamp
   is a part of the leaf structure).  Note that if a certificate was
   previously logged as a precertificate, then the precertificate's SCT
   of type "precert_sct_v2" would not be appropriate; instead, a fresh
   SCT of type "x509_sct_v2" should be generated.

   An SCT is the log's promise to append to its Merkle Tree an entry for
   the accepted submission.  Upon producing an SCT, the log MUST fulfil
   this promise by performing the following actions within a fixed



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   amount of time known as the Maximum Merge Delay (MMD), which is one
   of the log's parameters (see Section 4.1):

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

   o  Calculate the root of the tree.

   o  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 Signed Tree Head (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 for client messages (see
      Section 5).  The base URL MUST be an "https" URL, MAY contain a
      port, 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).

   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.

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

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




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   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.  The final STH should
      be provided in the form of a TransItem of type
      "signed_tree_head_v2".

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

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, and 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:

   o  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.

   o  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.



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      *  The Key Usage extension with the keyCertSign bit asserted.

   o  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.

   o  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 issued, the accepting log MUST
   store the entire chain used for verification.  This chain MUST
   include the certificate or precertificate itself, the zero or more
   intermediate CA certificates provided by the submitter, and the trust
   anchor used to verify the chain (even if it was omitted from the
   submission).  The log MUST present this chain for auditing upon
   request (see Section 5.6).  This prevents the CA from avoiding blame
   by logging a partial or empty chain.  Each log entry is a "TransItem"
   structure of type "x509_entry_v2" or "precert_entry_v2".  However, a
   log may store its entries in any format.  If a log does not store
   this "TransItem" in full, it must store the "timestamp" and
   "sct_extensions" of the corresponding
   "TimestampedCertificateEntryDataV2" structure.  The "TransItem" can
   be reconstructed from these fields and the entire chain that the log
   used to verify the submission.







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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 10.6.1).  Various data structures include the DER encoding of
   this OID, excluding the ASN.1 tag and length bytes, in an opaque
   vector:

       opaque LogID<2..127>;

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

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

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

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




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   "versioned_type" is a value from the IANA registry in Section 10.4
   that identifies the type of the encapsulated data structure and the
   earliest version of this protocol to which it conforms.  This
   document is v2.

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

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

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

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 Signed Certificate Timestamps (Section 4.8) and
   Signed Tree Heads (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 10.5).  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",



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   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" matches 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:







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

   "log_id" is this log's unique ID, encoded in an opaque vector as
   described in 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.  If an
   implementation sees an extension that it does not understand, it
   SHOULD ignore that extension.  Furthermore, an implementation MAY
   choose to ignore any extension(s) that it does understand.

   "signature" is computed over a "TransItem" structure of type
   "x509_entry_v2" or "precert_entry_v2" (see 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, 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.





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

   "root_hash" is the root of the Merkle Hash Tree.

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

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<0..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).



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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<1..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.

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<1..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.



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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.  To avoid that, the following
   actions are suggested:

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

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

   o  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.

   o  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 [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 [RFC7320].  However, operational experience with version 1 of this
   protocol has not indicated that these restrictions are a problem in
   practice.

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




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

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

   If the log is unable to process a client's request, it MUST return an
   HTTP response code of 4xx/5xx (see [RFC7231]), and, in place of the
   responses outlined in the subsections below, the body SHOULD be a
   JSON Problem Details Object (see [RFC7807] Section 3), 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.

   e.g., 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:




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

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

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 base64 encoded CA certificates.
         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".




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   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 the "chain" are not  |
   |                | valid (e.g., not properly encoded).              |
   |                |                                                  |
   | unknownAnchor  | The last element of "chain" (or, if "chain" is   |
   |                | an empty array, the "submission") both is not,   |
   |                | and is not certified by, an accepted trust       |
   |                | anchor.                                          |
   |                |                                                  |
   | shutdown       | The log is no longer accepting submissions.      |
   +----------------+--------------------------------------------------+

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

   If a log detects bad encoding in a chain that otherwise verifies
   correctly then the log MUST either log the certificate or return the
   "bad certificate" error.  If the certificate is logged, an SCT MUST
   be issued.  Logging the certificate is useful, because monitors
   (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 "unknown anchor" error.  A log cannot
   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 (e.g., if "type" was 2 (for "precert_sct_v2") then all three
   "TransItem"s could be embedded in the certificate).




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5.2.  Retrieve Latest Signed Tree Head

   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 Signed Tree Heads

   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 STHs, which are
      signed.

   Error codes:



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   +-------------------+-----------------------------------------------+
   | 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".             |
   +-------------------+-----------------------------------------------+

   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.  The
      "tree_size" must designate an existing v2 STH.  Because of skew,
      the front-end may not know the requested STH.  In that case, it
      will return the latest STH it knows, along with an inclusion proof
      to that STH.  If the front-end knows the requested STH then only
      "inclusion" is returned.

   Outputs:

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

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

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

   Error codes:



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   +-----------------+-------------------------------------------------+
   | 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.                       |
   +-----------------+-------------------------------------------------+

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

5.5.  Retrieve Merkle Inclusion Proof, Signed Tree Head 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.  The
      "tree_size" must designate an existing v2 STH.

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

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






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   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 chosen certificate in
         the returned STH.

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

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

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

   Errors are the same as in 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 representing the inputs that were
            submitted to "submit-entry", with the addition of the trust




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            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".

   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.



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   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".            |
   +----------------+--------------------------------------------------+

5.7.  Retrieve Accepted Trust Anchors

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

   No inputs.

   Outputs:

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

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

6.  TLS Servers

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

   Three mechanisms are provided because they have different tradeoffs.

   o  A TLS extension (Section 4.2 of [RFC8446]) with type
      "transparency_info" (see Section 6.4).  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.

   o  An Online Certificate Status Protocol (OCSP) [RFC6960] response
      extension (see Section 7.1.1), where the OCSP response is provided



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      in the "CertificateStatus" message, provided that the TLS client
      included the "status_request" extension in the (extended)
      "ClientHello" (Section 8 of [RFC6066]).  This mechanism, popularly
      known as OCSP stapling, is already widely (but not universally)
      implemented.  It also allows SCTs and inclusion proofs to be
      updated on the fly.

   o  An X509v3 certificate extension (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 will
      subsequently consider the certificate to be non-compliant and in
      need of re-issuance.

6.1.  Multiple SCTs

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

   o  One or more logs may not have become acceptable to all CT-using
      TLS clients.

   o  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.

   o  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.

   o  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 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.







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6.2.  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).

6.3.  Presenting SCTs, inclusions proofs and STHs

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

   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 "TransItem"s of type "inclusion_proof_v2" and
   "signed_tree_head_v2" in the "TransItemList".

6.4.  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:

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

   o  The TLS server MUST construct a "TransItemList" of relevant
      "TransItem"s (see Section 6.3), which SHOULD omit any "TransItem"s
      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".





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

   o  the ServerHello message (for TLS 1.2 or earlier).

   o  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 non-critical, 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.








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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 (see, for
   example, [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 6.4) 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:

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



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   o  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:

   o  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.

   o  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 6.3).



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8.1.5.  Validating inclusion proofs

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

   If the TLS client holds an STH that predates the SCT, it MAY, in the
   process of auditing, request a new STH from the log (Section 5.2),
   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 non-compliance.

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

   A monitor 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).





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   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.

   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:

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

       Or, if it is not keeping all log entries:

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

       2.  Verify the consistency proof.

       3.  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 log behavior properties that should be
   checked:



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   o  The Maximum Merge Delay (MMD).

   o  The STH Frequency Count.

   o  The append-only property.

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

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

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

   A monitor (Section 8.2) can audit by verifying the consistency of
   STHs it receives, ensure that each entry can be fetched and that the
   STH is indeed the result of making a tree from all fetched entries.

   A TLS client (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 any of its algorithms part way
   through its lifetime:

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

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





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   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.  New Entry to the TLS ExtensionType Registry

   IANA is asked to add an entry for "transparency_info(TBD)" to the
   "TLS ExtensionType Values" registry defined in [RFC8446], setting the
   "Recommended" value to "Y", setting the "TLS 1.3" value to "CH, CR,
   CT", and citing this document as the "Reference".

10.2.  Hash Algorithms

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

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








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10.2.1.  Specification Required guidance

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

10.3.  Signature Algorithms

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

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






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10.3.1.  Expert Review guidelines

   The appointed Expert should ensure that the proposed algorithm has a
   public specification, has a value assigned to it in the TLS
   SignatureScheme Registry (that IANA is asked to establish in
   [RFC8446]) and is suitable for use as a cryptographic signature
   algorithm.

10.4.  VersionedTransTypes

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

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

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

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



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10.4.1.  Specification Required guidance

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

10.5.  Log Artifact Extension Registry

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

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

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

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

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

10.5.1.  Specification Required guidance

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

10.6.  Object Identifiers

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

10.6.1.  Log ID Registry

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




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   +---------------------+------------+------------+-------------------+
   | Log ID              | Log Base   | Log        | Reference /       |
   |                     | URL        | Operator   | Assignment Policy |
   +---------------------+------------+------------+-------------------+
   | 1.3.101.8192 -      | Unassigned | Unassigned | First Come First  |
   | 1.3.101.16383       |            |            | Served            |
   |                     |            |            |                   |
   | 1.3.101.80.0 -      | Unassigned | Unassigned | First Come First  |
   | 1.3.101.80.*        |            |            | Served            |
   +---------------------+------------+------------+-------------------+

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

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

   Each application for the allocation of a Log ID MUST be accompanied
   by:

   o  the Log's Base URL (see Section 4.1).

   o  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.

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



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   monitors might not have checked the logs or the CA might have refused
   to revoke the certificate.

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

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

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: 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.  Such misbehavior is detectable and
   [I-D.ietf-trans-threat-analysis] provides more details on how this
   can be done.

   Violation of the MMD contract is detected by log clients requesting a
   Merkle inclusion proof (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 clients comparing their instances of the Signed



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

11.4.  Preventing Tracking Clients

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

   o  Using deterministic signature schemes, or

   o  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.

11.5.  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 6.1).

11.6.  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.  Acknowledgements

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



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

13.  References

13.1.  Normative References

   [FIPS186-4]
              NIST, "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", World Wide Web Consortium Recommendation
              REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

   [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>.

   [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>.

   [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>.

   [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>.





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   [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>.

   [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 IEEE Std
              1003.1-2008, 2016 Edition", n.d., <http://pubs.opengroup.o
              rg/onlinepubs/9699919799.2016edition/basedefs/
              V1_chap04.html#tag_04_16>.

13.2.  Informative References

   [Chromium.Log.Policy]
              The Chromium Projects, "Chromium Certificate Transparency
              Log Policy", 2014, <http://www.chromium.org/Home/chromium-
              security/certificate-transparency/log-policy>.

   [Chromium.Policy]
              The Chromium Projects, "Chromium Certificate
              Transparency", 2014, <http://www.chromium.org/Home/
              chromium-security/certificate-transparency>.



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   [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>.

   [I-D.ietf-trans-gossip]
              Nordberg, L., Gillmor, D., and T. Ritter, "Gossiping in
              CT", draft-ietf-trans-gossip-05 (work in progress),
              January 2018.

   [I-D.ietf-trans-threat-analysis]
              Kent, S., "Attack and Threat Model for Certificate
              Transparency", draft-ietf-trans-threat-analysis-16 (work
              in progress), October 2018.

   [JSON.Metadata]
              The Chromium Projects, "Chromium Log Metadata JSON
              Schema", 2014, <https://www.gstatic.com/ct/log_list/
              log_list_schema.json>.

   [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>.

   [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>.

   [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>.

   [RFC7320]  Nottingham, M., "URI Design and Ownership", BCP 190,
              RFC 7320, DOI 10.17487/RFC7320, July 2014,
              <https://www.rfc-editor.org/info/rfc7320>.

   [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>.







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Appendix A.  Supporting v1 and v2 simultaneously

   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:

   o  Create a CMS precertificate as described in Section 3.2 and submit
      it to v2 logs.

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

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

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

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

Authors' Addresses

   Ben Laurie
   Google UK Ltd.

   Email: benl@google.com


   Adam Langley
   Google Inc.

   Email: agl@google.com





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   Emilia Kasper
   Google Switzerland GmbH

   Email: ekasper@google.com


   Eran Messeri
   Google UK Ltd.

   Email: eranm@google.com


   Rob Stradling
   Sectigo Ltd.

   Email: rob@sectigo.com



































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