draft-ietf-trans-rfc6962-bis-22.txt

TRANS (Public Notary Transparency)                             B. Laurie
Internet-Draft                                                A. Langley
Obsoletes: 6962 (if approved)                                  E. Kasper
Intended status: Standards Track                              E. Messeri
Expires: June 17, 2017                                            Google
                                                            R. Stradling
                                                                  Comodo
                                                       December 14, 2016


                  Certificate Transparency Version 2.0
                    draft-ietf-trans-rfc6962-bis-22

Abstract

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

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

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
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   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 June 17, 2017.







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Copyright Notice

   Copyright (c) 2016 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
   (http://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.  Merkle Inclusion Proofs . . . . . . . . . . . . . . .   8
       2.1.2.  Merkle Consistency Proofs . . . . . . . . . . . . . .   9
       2.1.3.  Example . . . . . . . . . . . . . . . . . . . . . . .  10
       2.1.4.  Signatures  . . . . . . . . . . . . . . . . . . . . .  11
   3.  Submitters  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.1.  Certificates  . . . . . . . . . . . . . . . . . . . . . .  12
     3.2.  Precertificates . . . . . . . . . . . . . . . . . . . . .  12
   4.  Private Domain Name Labels  . . . . . . . . . . . . . . . . .  13
     4.1.  Wildcard Certificates . . . . . . . . . . . . . . . . . .  13
     4.2.  Using a Name-Constrained Intermediate CA  . . . . . . . .  13
   5.  Log Format and Operation  . . . . . . . . . . . . . . . . . .  14
     5.1.  Accepting Submissions . . . . . . . . . . . . . . . . . .  15
     5.2.  Log Entries . . . . . . . . . . . . . . . . . . . . . . .  15
     5.3.  Log ID  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     5.4.  TransItem Structure . . . . . . . . . . . . . . . . . . .  17
     5.5.  Merkle Tree Leaves  . . . . . . . . . . . . . . . . . . .  18
     5.6.  Signed Certificate Timestamp (SCT)  . . . . . . . . . . .  19
     5.7.  Merkle Tree Head  . . . . . . . . . . . . . . . . . . . .  20
     5.8.  Signed Tree Head (STH)  . . . . . . . . . . . . . . . . .  21
     5.9.  Merkle Consistency Proofs . . . . . . . . . . . . . . . .  22
     5.10. Merkle Inclusion Proofs . . . . . . . . . . . . . . . . .  22
     5.11. Shutting down a log . . . . . . . . . . . . . . . . . . .  23
   6.  Log Client Messages . . . . . . . . . . . . . . . . . . . . .  23
     6.1.  Add Chain to Log  . . . . . . . . . . . . . . . . . . . .  25
     6.2.  Add PreCertChain to Log . . . . . . . . . . . . . . . . .  26



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     6.3.  Retrieve Latest Signed Tree Head  . . . . . . . . . . . .  26
     6.4.  Retrieve Merkle Consistency Proof between Two Signed Tree
           Heads . . . . . . . . . . . . . . . . . . . . . . . . . .  26
     6.5.  Retrieve Merkle Inclusion Proof from Log by Leaf Hash . .  27
     6.6.  Retrieve Merkle Inclusion Proof, Signed Tree Head and
           Consistency Proof by Leaf Hash  . . . . . . . . . . . . .  28
     6.7.  Retrieve Entries and STH from Log . . . . . . . . . . . .  30
     6.8.  Retrieve Accepted Trust Anchors . . . . . . . . . . . . .  31
   7.  TLS Servers . . . . . . . . . . . . . . . . . . . . . . . . .  31
     7.1.  Multiple SCTs . . . . . . . . . . . . . . . . . . . . . .  32
     7.2.  TransItemList Structure . . . . . . . . . . . . . . . . .  33
     7.3.  Presenting SCTs, inclusion proofs and STHs  . . . . . . .  33
     7.4.  Presenting SCTs only  . . . . . . . . . . . . . . . . . .  34
     7.5.  transparency_info TLS Extension . . . . . . . . . . . . .  34
     7.6.  cached_info TLS Extension . . . . . . . . . . . . . . . .  34
   8.  Certification Authorities . . . . . . . . . . . . . . . . . .  34
     8.1.  Transparency Information X.509v3 Extension  . . . . . . .  35
       8.1.1.  OCSP Response Extension . . . . . . . . . . . . . . .  35
       8.1.2.  Certificate Extension . . . . . . . . . . . . . . . .  35
     8.2.  TLS Feature Extension . . . . . . . . . . . . . . . . . .  35
   9.  Clients . . . . . . . . . . . . . . . . . . . . . . . . . . .  35
     9.1.  Metadata  . . . . . . . . . . . . . . . . . . . . . . . .  36
     9.2.  TLS Client  . . . . . . . . . . . . . . . . . . . . . . .  37
       9.2.1.  Receiving SCTs  . . . . . . . . . . . . . . . . . . .  37
       9.2.2.  Reconstructing the TBSCertificate . . . . . . . . . .  37
       9.2.3.  Validating SCTs . . . . . . . . . . . . . . . . . . .  37
       9.2.4.  Validating inclusion proofs . . . . . . . . . . . . .  38
       9.2.5.  Evaluating compliance . . . . . . . . . . . . . . . .  38
       9.2.6.  TLS Feature Extension . . . . . . . . . . . . . . . .  38
       9.2.7.  cached_info TLS Extension . . . . . . . . . . . . . .  38
       9.2.8.  Handling of Non-compliance  . . . . . . . . . . . . .  39
     9.3.  Monitor . . . . . . . . . . . . . . . . . . . . . . . . .  39
     9.4.  Auditing  . . . . . . . . . . . . . . . . . . . . . . . .  40
       9.4.1.  Verifying an inclusion proof  . . . . . . . . . . . .  41
       9.4.2.  Verifying consistency between two STHs  . . . . . . .  42
       9.4.3.  Verifying root hash given entries . . . . . . . . . .  42
   10. Algorithm Agility . . . . . . . . . . . . . . . . . . . . . .  43
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  44
     11.1.  TLS Extension Type . . . . . . . . . . . . . . . . . . .  44
     11.2.  New Entry to the TLS CachedInformationType registry  . .  44
     11.3.  Hash Algorithms  . . . . . . . . . . . . . . . . . . . .  44
       11.3.1.  Expert Review guidelines . . . . . . . . . . . . . .  44
     11.4.  Signature Algorithms . . . . . . . . . . . . . . . . . .  45
       11.4.1.  Expert Review guidelines . . . . . . . . . . . . . .  45
     11.5.  VersionedTransTypes  . . . . . . . . . . . . . . . . . .  45
       11.5.1.  Expert Review guidelines . . . . . . . . . . . . . .  46
     11.6.  SCT Extensions . . . . . . . . . . . . . . . . . . . . .  47
       11.6.1.  Expert Review guidelines . . . . . . . . . . . . . .  47



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     11.7.  STH Extensions . . . . . . . . . . . . . . . . . . . . .  47
       11.7.1.  Expert Review guidelines . . . . . . . . . . . . . .  47
     11.8.  Object Identifiers . . . . . . . . . . . . . . . . . . .  48
       11.8.1.  Log ID Registry  . . . . . . . . . . . . . . . . . .  48
       11.8.2.  Expert Review guidelines . . . . . . . . . . . . . .  48
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  49
     12.1.  Misissued Certificates . . . . . . . . . . . . . . . . .  49
     12.2.  Detection of Misissue  . . . . . . . . . . . . . . . . .  49
     12.3.  Misbehaving Logs . . . . . . . . . . . . . . . . . . . .  49
     12.4.  Deterministic Signatures . . . . . . . . . . . . . . . .  50
     12.5.  Multiple SCTs  . . . . . . . . . . . . . . . . . . . . .  50
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  50
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  51
     14.2.  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 need to be trusted because they are publicly
   auditable.  Anyone may verify the correctness of each log and monitor
   when new certificates are added to it.  The logs do not themselves
   prevent misissue, but they ensure that interested parties
   (particularly those named in certificates) can detect such
   misissuance.  Note that this is a general mechanism that could be
   used for transparently logging any form of binary data, subject to
   some kind of inclusion criteria.  In this document, we only describe
   its use for public TLS server certificates (i.e., where the inclusion
   criteria is a valid certificate issued by a public certification
   authority (CA)).

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





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

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

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

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Data Structures

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

1.3.  Major Differences from CT 1.0

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



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   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
      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  Private domain name labels: added a mechanism for logging a name-
      constrained intermediate in place of end-entity certificates
      issued by that CA.

   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  SCT extensions: these are now typed and managed by an IANA
      registry.

   o  STH extensions: STHs can now contain extensions, which are typed
      and managed by an IANA registry.

   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.





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   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.

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

   o  TLS Feature extension: this certificate extension may be used by a
      CA to indicate that CT compliance is required.

   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

   Logs use a binary Merkle Hash Tree for efficient auditing.  The
   hashing algorithm used by each log is expected to be specified as
   part of the metadata relating to that log (see Section 9.1).  We have
   established a registry of acceptable algorithms, see Section 11.3.
   The hashing algorithm in use is referred to as HASH throughout this
   document 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)).





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

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

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

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

2.1.1.  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.

   Given an ordered list of n inputs to the tree, D[n] = {d(0), ...,
   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,



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   where : is concatenation of lists and D[k1:k2] denotes the length (k2
   - k1) list {d(k1), d(k1+1),..., d(k2-1)} as before.

2.1.2.  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.

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

   PROOF(m, D[n]) = SUBPROOF(m, D[n], true)

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

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

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

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

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

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

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

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



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   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])

   Here, : is a concatenation of lists, and D[k1:k2] denotes the length
   (k2 - k1) list {d(k1), d(k1+1),..., d(k2-1)} as before.

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

2.1.3.  Example

   The binary Merkle Tree with 7 leaves:

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

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

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

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

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

   The same tree, built incrementally in four steps:










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       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.

2.1.4.  Signatures

   Various data structures are signed.  A log MUST use one of the
   signature algorithms defined in Section 11.4.

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



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   accompanied by all additional certificates required to verify the
   chain up to an accepted trust anchor.  The trust anchor (a root or
   intermediate CA certificate) MAY be omitted from the submission.

   If a log accepts a submission, it will return a Signed Certificate
   Timestamp (SCT) (see Section 5.6).  The submitter SHOULD validate the
   returned SCT as described in Section 9.2 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 6.1) 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 6.2) 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 requirements:

   o  It MUST be DER encoded.

   o  "SignedData.encapContentInfo.eContentType" MUST be the OID
      1.3.101.78.

   o  "SignedData.encapContentInfo.eContent" MUST contain a
      TBSCertificate [RFC5280] that will be identical to the
      TBSCertificate in the issued certificate, except that the
      Transparency Information (Section 8.1) extension MUST be omitted.

   o  "SignedData.signerInfos" MUST contain a signature from the same
      (root or intermediate) CA that will ultimately issue the
      certificate.  This signature indicates the CA's intent to issue
      the certificate.  This intent is considered binding (i.e.,
      misissuance of the precertificate is considered equivalent to
      misissuance of the certificate).  (Note that, because of the



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      structure of CMS, the signature on the CMS object will not be a
      valid X.509v3 signature and so cannot be used to construct a
      certificate from the precertificate).

   o  "SignedData.certificates" SHOULD be omitted.

4.  Private Domain Name Labels

   Some regard certain DNS domain name labels within their registered
   domain space as private and security sensitive.  Even though these
   domains are often only accessible within the domain owner's private
   network, it's common for them to be secured using publicly trusted
   TLS server certificates.

4.1.  Wildcard Certificates

   A certificate containing a DNS-ID [RFC6125] of "*.example.com" could
   be used to secure the domain "topsecret.example.com", without
   revealing the string "topsecret" publicly.

   Since TLS clients only match the wildcard character to the complete
   leftmost label of the DNS domain name (see Section 6.4.3 of
   [RFC6125]), a different approach is needed when any label other than
   the leftmost label in a DNS-ID is considered private (e.g.,
   "top.secret.example.com").  Also, wildcard certificates are
   prohibited in some cases, such as Extended Validation Certificates
   [EVSSLGuidelines].

4.2.  Using a Name-Constrained Intermediate CA

   An intermediate CA certificate or intermediate CA precertificate that
   contains the Name Constraints [RFC5280] extension MAY be logged in
   place of end-entity certificates issued by that intermediate CA, as
   long as all of the following conditions are met:

   o  there MUST be a non-critical extension (OID 1.3.101.76, whose
      extnValue OCTET STRING contains ASN.1 NULL data (0x05 0x00)).
      This extension is an explicit indication that it is acceptable to
      not log certificates issued by this intermediate CA.

   o  there MUST be a Name Constraints extension, in which:

      *  permittedSubtrees MUST specify one or more dNSNames.

      *  excludedSubtrees MUST specify the entire IPv4 and IPv6 address
         ranges.





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   Below is an example Name Constraints extension that meets these
   conditions:

       SEQUENCE {
         OBJECT IDENTIFIER '2 5 29 30'
         OCTET STRING, encapsulates {
           SEQUENCE {
             [0] {
               SEQUENCE {
                 [2] 'example.com'
                 }
               }
             [1] {
               SEQUENCE {
                 [7] 00 00 00 00 00 00 00 00
                 }
               SEQUENCE {
                 [7]
                   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
                   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
                 }
               }
             }
           }
         }

5.  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 12.4).
   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 incorporate the submitted entry in its
   Merkle Tree no later than a fixed amount of time, known as the
   Maximum Merge Delay (MMD), after the issuance of the SCT.
   Periodically, the log MUST append all its new entries to its Merkle
   Tree and sign the root of the tree.



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

5.1.  Accepting Submissions

   Before accepting a submitted certificate or precertificate, the log
   MUST verify that it has a valid signature chain to an accepted trust
   anchor, using the chain of intermediate CA certificates provided by
   the submitter.  Logs SHOULD accept certificates and precertificates
   that are fully valid according to RFC 5280 [RFC5280] verification
   rules and are submitted with such a chain (A log may decide, for
   example, to temporarily reject valid submissions to protect itself
   against denial-of-service attacks).

   Logs MAY accept certificates and precertificates that have expired,
   are not yet valid, have been revoked, or are otherwise not fully
   valid according to RFC 5280 verification rules in order to
   accommodate quirks of CA certificate-issuing software.  However, logs
   MUST reject submissions without a valid signature chain to an
   accepted trust anchor.  Logs MUST also reject precertificates that do
   not conform to the requirements in Section 3.2.

   Logs SHOULD limit the length of chain they will accept.  The maximum
   chain length is specified in the log's metadata.

   The log SHALL allow retrieval of its list of accepted trust anchors
   (see Section 6.8), 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.

5.2.  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 6.7).  This chain is required to prevent a CA
   from avoiding blame by logging a partial or empty chain.

   Each certificate entry in a log MUST include a "X509ChainEntry"
   structure, and each precertificate entry MUST include a
   "PrecertChainEntryV2" structure:







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       opaque ASN.1Cert<1..2^24-1>;

       struct {
           ASN.1Cert leaf_certificate;
           ASN.1Cert certificate_chain<0..2^24-1>;
       } X509ChainEntry;

       opaque CMSPrecert<1..2^24-1>;

       struct {
           CMSPrecert pre_certificate;
           ASN.1Cert precertificate_chain<1..2^24-1>;
       } PrecertChainEntryV2;

   "leaf_certificate" is a submitted certificate that has been accepted
   by the log.

   "certificate_chain" is a vector of 0 or more additional certificates
   required to verify "leaf_certificate".  The first certificate MUST
   certify "leaf_certificate".  Each following certificate MUST directly
   certify the one preceding it.  The final certificate MUST be a trust
   anchor accepted by the log.  If "leaf_certificate" is an accepted
   trust anchor, then this vector is empty.

   "pre_certificate" is a submitted precertificate that has been
   accepted by the log.

   "precertificate_chain" is a vector of 1 or more additional
   certificates required to verify "pre_certificate".  The first
   certificate MUST certify "pre_certificate".  Each following
   certificate MUST directly certify the one preceding it.  The final
   certificate MUST be a trust anchor accepted by the log.

5.3.  Log ID

   Each log is identified by an OID, which is specified in the log's
   metadata 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 11.8.1.  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.



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

5.4.  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), x509_sct_with_proof_v2(8),
           precert_sct_with_proof_v2(9),
           (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;
               case x509_sct_with_proof_v2: SCTWithProofDataV2;
               case precert_sct_with_proof_v2: SCTWithProofDataV2;
           } data;
       } TransItem;

   "versioned_type" is a value from the IANA registry in Section 11.5
   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.



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   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.

5.5.  Merkle Tree Leaves

   The leaves of a log's Merkle Tree correspond to the log's entries
   (see Section 5.2).  Each leaf is the leaf hash (Section 2.1) of a
   "TransItem" structure of type "x509_entry_v2" or "precert_entry_v2",
   which encapsulates a "TimestampedCertificateEntryDataV2" structure.
   Note that leaf hashes are calculated as HASH(0x00 || TransItem),
   where the hashing algorithm is specified in the log's metadata.

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

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

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

   "issuer_key_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 either the
   "leaf_certificate" (in the case of an "X509ChainEntry") or the
   "pre_certificate" (in the case of a "PrecertChainEntryV2").  (Note
   that a precertificate's TBSCertificate can be reconstructed from the
   corresponding certificate as described in Section 9.2.2).

   "sct_extensions" matches the SCT extensions of the corresponding SCT.






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5.6.  Signed Certificate Timestamp (SCT)

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

       enum {
           reserved(65535)
       } SctExtensionType;

       struct {
           SctExtensionType sct_extension_type;
           opaque sct_extension_data<0..2^16-1>;
       } SctExtension;

       struct {
           LogID log_id;
           uint64 timestamp;
           SctExtension sct_extensions<0..2^16-1>;
           digitally-signed struct {
               TransItem timestamped_entry;
           } signature;
       } SignedCertificateTimestampDataV2;

   "log_id" is this log's unique ID, encoded in an opaque vector as
   described in Section 5.3.

   "timestamp" is equal to the timestamp from the
   "TimestampedCertificateEntryDataV2" structure encapsulated in the
   "timestamped_entry".

   "sct_extension_type" identifies a single extension from the IANA
   registry in Section 11.6.  At the time of writing, no extensions are
   specified.

   The interpretation of the "sct_extension_data" field is determined
   solely by the value of the "sct_extension_type" field.  Each document
   that registers a new "sct_extension_type" must describe how to
   interpret the corresponding "sct_extension_data".

   "sct_extensions" is a vector of 0 or more SCT extensions.  This
   vector MUST NOT include more than one extension with the same
   "sct_extension_type".  The extensions in the vector MUST be ordered
   by the value of the "sct_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.




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   The encoding of the digitally-signed element is defined in [RFC5246].

   "timestamped_entry" is a "TransItem" structure that MUST be of type
   "x509_entry_v2" or "precert_entry_v2" (see Section 5.5).

5.7.  Merkle Tree Head

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

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

       enum {
           reserved(65535)
       } SthExtensionType;

       struct {
           SthExtensionType sth_extension_type;
           opaque sth_extension_data<0..2^16-1>;
       } SthExtension;

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

   The length of NodeHash MUST match HASH_SIZE of the log.

   "sth_extension_type" identifies a single extension from the IANA
   registry in Section 11.7.  At the time of writing, no extensions are
   specified.

   The interpretation of the "sth_extension_data" field is determined
   solely by the value of the "sth_extension_type" field.  Each document
   that registers a new "sth_extension_type" must describe how to
   interpret the corresponding "sth_extension_data".

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

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

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




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   "sth_extensions" is a vector of 0 or more STH extensions.  This
   vector MUST NOT include more than one extension with the same
   "sth_extension_type".  The extensions in the vector MUST be ordered
   by the value of the "sth_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.

5.8.  Signed Tree Head (STH)

   Periodically each log SHOULD sign its current tree head information
   (see Section 5.7) to produce an STH.  When a client requests a log's
   latest STH (see Section 6.3), the log MUST return an STH that is no
   older than the log's MMD.  However, STHs could be used to mark
   individual clients (by producing a new one for each query), so logs
   MUST NOT produce them more frequently than is declared in their
   metadata.  In general, there is no need to produce a new STH unless
   there are new entries in the log; however, in the unlikely event that
   it receives no new submissions during an MMD period, the log SHALL
   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;
           digitally-signed struct {
               TreeHeadDataV2 tree_head;
           } signature;
       } SignedTreeHeadDataV2;

   "log_id" is this log's unique ID, encoded in an opaque vector as
   described in Section 5.3.

   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 5.7).

   "signature" is a signature over the encoded "tree_head" field.








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5.9.  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 5.3.

   "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.

5.10.  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 5.3.

   "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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5.11.  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 a part of the log's metadata.  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).

6.  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 [RFC7159].  Parameters for GETs are encoded as order-
   independent key/value URL parameters, using the "application/x-www-
   form-urlencoded" format described in the "HTML 4.01 Specification"
   [HTML401].  Binary data is base64 encoded [RFC4648] as specified in
   the individual messages.

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

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

   In practice, log servers may include multiple front-end machines.
   Since it is impractical to keep these machines in perfect sync,
   errors may occur that are caused by skew between the machines.  Where
   such errors are possible, the front-end will return additional
   information (as specified below) making it possible for clients to



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   make progress, if progress is possible.  Front-ends MUST only serve
   data that is free of gaps (that is, for example, no front-end will
   respond with an STH unless it is also able to prove consistency from
   all log entries logged within that STH).

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

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

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

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

   error_code:  An error code readable by the client.  Some codes are
      generic and are detailed here.  Others are detailed in the
      individual requests.  Error codes are fixed text strings.

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

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

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

   Clients SHOULD treat "500 Internal Server Error" and "503 Service
   Unavailable" responses as transient failures and MAY retry the same
   request without modification at a later date.  Note that as per
   [RFC7231], in the case of a 503 response the log MAY include a



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   "Retry-After:" header in order to request a minimum time for the
   client to wait before retrying the request.

6.1.  Add Chain to Log

   POST https://<log server>/ct/v2/add-chain

   Inputs:

      chain:  An array of base64 encoded certificates.  The first
         element is the certificate for which the submitter desires an
         SCT; the second certifies the first and so on to the last,
         which either is, or is certified by, an accepted trust anchor.

   Outputs:

      sct:  A base64 encoded "TransItem" of type "x509_sct_v2", signed
         by this log, that corresponds to the submitted certificate.

   Error codes:

   +-------------+-----------------------------------------------------+
   | Error Code  | Meaning                                             |
   +-------------+-----------------------------------------------------+
   | unknown     | The last certificate in the chain both is not, and  |
   | anchor      | is not certified by, an accepted trust anchor.      |
   |             |                                                     |
   | bad chain   | The alleged chain is not actually a chain of        |
   |             | certificates.                                       |
   |             |                                                     |
   | bad         | One or more certificates in the chain are not valid |
   | certificate | (e.g., not properly encoded).                       |
   |             |                                                     |
   | shutdown    | The log has ceased operation and is not accepting   |
   |             | new 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 9.3) can then detect these encoding errors, which may be
   accepted by some TLS clients.



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6.2.  Add PreCertChain to Log

   POST https://<log server>/ct/v2/add-pre-chain

   Inputs:

      precertificate:  The base64 encoded precertificate.

      chain:  An array of base64 encoded CA certificates.  The first
         element is the signer of the precertificate; the second
         certifies the first and so on to the last, which either is, or
         is certified by, an accepted trust anchor.

   Outputs:

      sct:  A base64 encoded "TransItem" of type "precert_sct_v2",
         signed by this log, that corresponds to the submitted
         precertificate.

   Errors are the same as in Section 6.1.

6.3.  Retrieve Latest Signed Tree Head

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

   No inputs.

   Outputs:

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

6.4.  Retrieve Merkle Consistency Proof between Two Signed Tree Heads

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

   Inputs:

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

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

      Both tree sizes must be from existing v2 STHs.  However, because
      of skew, the receiving front-end may not know one or both of the
      existing STHs.  If both are known, then only the "consistency"
      output is returned.  If the first is known but the second is not
      (or has been omitted), then the latest known STH is returned,
      along with a consistency proof between the first STH and the



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      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.

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

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

   Error codes:

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

   See Section 9.4.2 for an outline of how to use the "consistency"
   output.

6.5.  Retrieve Merkle Inclusion Proof from Log by Leaf Hash

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

   Inputs:

      hash:  A base64 encoded v2 leaf hash.

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

      The "hash" must be calculated as defined in Section 5.5.  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




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      to that STH.  If the front-end knows the requested STH then only
      "inclusion" is returned.

   Outputs:

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

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

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

   Error codes:

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

   See Section 9.4.1 for an outline of how to use the "inclusion"
   output.

6.6.  Retrieve Merkle Inclusion Proof, Signed Tree Head and Consistency
      Proof by Leaf Hash

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

   Inputs:

      hash:  A base64 encoded v2 leaf hash.

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

      The "hash" must be calculated as defined in Section 5.5.  The
      "tree_size" must designate an existing v2 STH.




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



      latest STH < requested STH  Return latest STH.



      latest STH > requested STH  Return latest STH and a consistency
         proof between it and the requested STH (see Section 6.4).



      index of requested hash < latest STH  Return "inclusion".

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

   Outputs:

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

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

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

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

   Errors are the same as in Section 6.5.

   See Section 9.4.1 for an outline of how to use the "inclusion"
   output, and see Section 9.4.2 for an outline of how to use the
   "consistency" output.







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6.7.  Retrieve Entries and STH from Log

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

   Inputs:

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

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

   Outputs:

      entries:  An array of objects, each consisting of

         leaf_input:  The base64 encoded "TransItem" structure of type
            "x509_entry_v2" or "precert_entry_v2" (see Section 5.5).

         log_entry:  The base64 encoded log entry (see Section 5.2).  In
            the case of an "x509_entry_v2" entry, this is the whole
            "X509ChainEntry"; and in the case of a "precert_entry_v2",
            this is the whole "PrecertChainEntryV2".

         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 6.3.

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

   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:



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

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

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

   See Section 9.4.3 for an outline of how to use a complete list of
   "leaf_input" entries to verify the "root_hash".

6.8.  Retrieve Accepted Trust Anchors

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

   No inputs.

   Outputs:

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

      max_chain:  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.

7.  TLS Servers

   TLS servers MUST use at least one of the three mechanisms listed
   below to present one or more SCTs from one or more logs to each TLS
   client during full TLS handshakes, where each SCT corresponds to the
   server certificate or to a name-constrained intermediate the server
   certificate chains to.  TLS servers SHOULD also present corresponding
   inclusion proofs and STHs (see Section 7.3).

   Three mechanisms are provided because they have different tradeoffs.

   o  A TLS extension (Section 7.4.1.4 of [RFC5246]) with type
      "transparency_info" (see Section 7.5).  This mechanism allows TLS
      servers to participate in CT without the cooperation of CAs,
      unlike the other two mechanisms.  It also allows SCTs and
      inclusion proofs to be updated on the fly.




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   o  An Online Certificate Status Protocol (OCSP) [RFC6960] response
      extension (see Section 8.1.1), where the OCSP response is provided
      in the "CertificateStatus" message, provided that the TLS client
      included the "status_request" extension in the (extended)
      "ClientHello" (Section 8 of [RFC6066]).  This mechanism, popularly
      known as OCSP stapling, is already widely (but not universally)
      implemented.  It also allows SCTs and inclusion proofs to be
      updated on the fly.

   o  An X509v3 certificate extension (see Section 8.1.2).  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.

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

7.1.  Multiple SCTs

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

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

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

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




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   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.

7.2.  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).

7.3.  Presenting SCTs, inclusion proofs and STHs

   When constructing a "TransItemList" structure, a TLS server SHOULD
   construct and include "TransItem" structures of type
   "x509_sct_with_proof_v2" (for an SCT of type "x509_sct_v2") or
   "precert_sct_with_proof_v2" (for an SCT of type "precert_sct_v2"),
   both of which encapsulate a "SCTWithProofDataV2" structure:

       struct {
           SignedCertificateTimestampDataV2 sct;
           SignedTreeHeadDataV2 sth;
           InclusionProofDataV2 inclusion_proof;
       } SCTWithProofDataV2;

   "sct" is the encapsulated data structure from an SCT that corresponds
   to the server certificate or to a name-constrained intermediate the
   server certificate chains to.

   "sth" is the encapsulated data structure from an STH that was signed
   by the same log as "sct".

   "inclusion_proof" is the encapsulated data structure from an
   inclusion proof that corresponds to "sct" and can be used to compute
   the root in "sth".





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7.4.  Presenting SCTs only

   Presenting inclusion proofs and STHs in the TLS handshake helps to
   protect the client's privacy (see Section 9.2.4) and reduces load on
   log servers.  However, if a TLS server is unable to obtain an
   inclusion proof and STH that correspond to an SCT, then it MUST
   include "TransItem" structures of type "x509_sct_v2" or
   "precert_sct_v2" in the "TransItemList".

7.5.  transparency_info TLS Extension

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

7.6.  cached_info TLS Extension

   When a TLS server includes the "transparency_info" extension in the
   ServerHello, it SHOULD NOT include any "TransItem" structures of type
   "x509_sct_with_proof_v2", "x509_sct_v2", "precert_sct_with_proof_v2"
   or "precert_sct_v2" in the "TransItemList" if all of the following
   conditions are met:

   o  The TLS client includes the "transparency_info" extension type in
      the ClientHello.

   o  The TLS client includes the "cached_info" ([RFC7924]) extension
      type in the ClientHello, with a "CachedObject" of type
      "ct_compliant" (see Section 9.2.7) and at least one "CachedObject"
      of type "cert".

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

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

8.  Certification Authorities








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8.1.  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 8.1.1) and certificates (see
   Section 8.1.2).  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".

8.1.1.  OCSP Response Extension

   A certification authority MAY include a Transparency Information
   X.509v3 extension in the "singleExtensions" of a "SingleResponse" in
   an OCSP response.  The included SCTs or inclusion proofs MUST be for
   the certificate identified by the "certID" of that "SingleResponse",
   or for a precertificate that corresponds to that certificate, or for
   a name-constrained intermediate to which that certificate chains.

8.1.2.  Certificate Extension

   A certification authority MAY include a Transparency Information
   X.509v3 extension in a certificate.  Any included SCTs or inclusion
   proofs MUST be either for a precertificate that corresponds to this
   certificate, or for a name-constrained intermediate to which this
   certificate chains.

8.2.  TLS Feature Extension

   A certification authority MAY include the transparency_info
   (Section 7.5) TLS extension identifier in the TLS Feature [RFC7633]
   certificate extension in root, intermediate and end-entity
   certificates.  When a certificate chain includes such a certificate,
   this indicates that CT compliance is required.

9.  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.



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

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

9.1.  Metadata

   In order to communicate with and verify a log, clients need metadata
   about the log.

   Base URL:  The URL to substitute for <log server> in Section 6.

   Hash Algorithm:  The hash algorithm used for the Merkle Tree (see
      Section 11.3).

   Signing Algorithm:  The signing algorithm used (see Section 2.1.4).

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

   Log ID:  The OID that uniquely identifies the log.

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

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

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

   STH Frequency Count:  The maximum number of STHs the log may produce
      in any period equal to the "Maximum Merge Delay" (see
      Section 5.8).

   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".





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

9.2.  TLS Client

9.2.1.  Receiving SCTs

   TLS clients receive SCTs alongside or in certificates.  TLS clients
   MUST implement all of the three mechanisms by which TLS servers may
   present SCTs (see Section 7).  TLS clients MAY also accept SCTs via
   the "status_request_v2" extension ([RFC6961]).  TLS clients that
   support the "transparency_info" TLS extension SHOULD include it in
   ClientHello messages, with empty "extension_data".  TLS clients may
   also receive inclusion proofs in addition to SCTs, which should be
   checked once the SCTs are validated.

9.2.2.  Reconstructing the TBSCertificate

   To reconstruct the TBSCertificate component of a precertificate from
   a certificate, TLS clients should remove the Transparency Information
   extension described in Section 8.1.

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

9.2.3.  Validating SCTs

   In addition to normal validation of the server certificate and its
   chain, TLS clients SHOULD validate each received SCT for which they
   have the corresponding log's metadata.  To validate an SCT, a TLS
   client computes the signature input from the SCT data and the
   corresponding certificate, and then verifies the signature using the
   corresponding log's public key.  TLS clients MUST NOT consider valid
   any SCT whose timestamp is in the future.

   Before considering any SCT to be invalid, the TLS client MUST attempt
   to validate it against the server certificate and against each of the
   zero or more suitable name-constrained intermediates (Section 4.2) in
   the chain.  These certificates may be evaluated in the order they
   appear in the chain, or, indeed, in any order.







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9.2.4.  Validating inclusion proofs

   After validating a received SCT, a TLS client MAY request a
   corresponding inclusion proof (if one is not already available) and
   then verify it.  An inclusion proof can be requested directly from a
   log using "get-proof-by-hash" (Section 6.5) or "get-all-by-hash"
   (Section 6.6), but note that this will disclose to the log which TLS
   server the client has been communicating with.

   Alternatively, if the TLS client has received an inclusion proof (and
   an STH) alongside the SCT, it can proceed to verifying the inclusion
   proof to the provided STH.  The client then has to verify consistency
   between the provided STH and an STH it knows about, which is less
   sensitive from a privacy perspective.

   TLS clients SHOULD also verify each received inclusion proof (see
   Section 9.4.1) for which they have the corresponding log's metadata,
   to audit the log and gain confidence that the certificate is logged.

   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 6.3),
   then verify it by requesting a consistency proof (Section 6.4).  Note
   that if the TLS client uses "get-all-by-hash", then it will already
   have the new STH.

9.2.5.  Evaluating compliance

   To be considered compliant, a certificate MUST be accompanied by at
   least one valid SCT.  A certificate not accompanied by any valid SCTs
   MUST NOT be considered compliant by TLS clients.

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

9.2.6.  TLS Feature Extension

   If any certificate in a chain includes the transparency_info
   (Section 7.5) TLS extension identifier in the TLS Feature [RFC7633]
   certificate extension, then CT compliance (using any of the
   mechanisms from Section 7) is required.

9.2.7.  cached_info TLS Extension

   If a TLS client uses the "cached_info" TLS extension ([RFC7924]) to
   indicate 1 or more cached certificates, all of which it already
   considers to be CT compliant, the TLS client MAY also include a




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   "CachedObject" of type "ct_compliant" in the "cached_info" extension.
   The "hash_value" field MUST be 1 byte long with the value 0.

9.2.8.  Handling of Non-compliance

   If a TLS server presents a certificate chain that is non-compliant,
   and the use of a compliant certificate is mandated by an explicit
   security policy, application protocol specification, the TLS Feature
   extension or any other means, the TLS client MUST refuse the
   connection.

9.3.  Monitor

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

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

   1.  Fetch the current STH (Section 6.3).

   2.  Verify the STH signature.

   3.  Fetch all the entries in the tree corresponding to the STH
       (Section 6.7).

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

   5.  Fetch the current STH (Section 6.3).  Repeat until the STH
       changes.

   6.  Verify the STH signature.

   7.  Fetch all the new entries in the tree corresponding to the STH
       (Section 6.7).  If they remain unavailable for an extended
       period, then this should be viewed as misbehavior on the part of
       the log.

   8.  Either:

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

       Or, if it is not keeping all log entries:



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       1.  Fetch a consistency proof for the new STH with the previous
           STH (Section 6.4).

       2.  Verify the consistency proof.

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

   9.  Go to Step 5.

9.4.  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 behaviour properties that should be
   checked:

   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 9.3) 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 9.2) 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 6.5).  It can also
   verify that the SCT corresponds to the certificate it arrived with
   (i.e., the log entry is that certificate, is a precertificate for



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   that certificate or is an appropriate name-constrained intermediate
   (Section 4.2).

   Checking of the consistency of the log view presented to all entities
   is more difficult to perform because it requires a way to share log
   responses among a set of CT-aware entities, and is discussed in
   Section 12.3.

   The following algorithm outlines may be useful for clients that wish
   to perform various audit operations.

9.4.1.  Verifying an inclusion proof

   When a client has received a "TransItem" of type "inclusion_proof_v2"
   and wishes to verify inclusion of an input "hash" for an STH with 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" fail the proof verification.

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

   3.  Set "r" to "hash".

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

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

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

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

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

       Otherwise:

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

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

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





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9.4.2.  Verifying consistency between two STHs

   When a client has an STH "first_hash" for tree size "first", an STH
   "second_hash" for tree size "second" where "0 < first < second", and
   has received a "TransItem" of type "consistency_proof_v2" that they
   wish to use to verify both hashes, the following algorithm may be
   used:

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

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

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

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

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

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

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

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

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

       Otherwise:

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

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

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

9.4.3.  Verifying root hash given entries

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



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   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".

10.  Algorithm Agility

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

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

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

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

   If it should become necessary to deprecate an algorithm used by a
   live log, then the log should be frozen as specified in Section 9.1
   and a new log should be started.  Certificates in the frozen log that




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   have not yet expired and require new SCTs SHOULD be submitted to the
   new log and the SCTs from that log used instead.

11.  IANA Considerations

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

11.1.  TLS Extension Type

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

11.2.  New Entry to the TLS CachedInformationType registry

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

11.3.  Hash Algorithms

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

   +------------+---------------+--------------------------------------+
   | Value      | Hash          | Reference / Assignment Policy        |
   |            | Algorithm     |                                      |
   +------------+---------------+--------------------------------------+
   | 0x00       | SHA-256       | [RFC4634]                            |
   |            |               |                                      |
   | 0x01 -     | Unassigned    | Specification Required and Expert    |
   | 0xDF       |               | Review                               |
   |            |               |                                      |
   | 0xE0 -     | Reserved      | Experimental Use                     |
   | 0xEF       |               |                                      |
   |            |               |                                      |
   | 0xF0 -     | Reserved      | Private Use                          |
   | 0xFF       |               |                                      |
   +------------+---------------+--------------------------------------+

11.3.1.  Expert Review guidelines

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





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11.4.  Signature Algorithms

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

   +---------+-------------------------------+-------------------------+
   | Value   | Signature Algorithm           | Reference / Assignment  |
   |         |                               | Policy                  |
   +---------+-------------------------------+-------------------------+
   | 0x00    | Deterministic ECDSA (NIST     | [RFC6979]               |
   |         | P-256) with HMAC-SHA256       |                         |
   |         |                               |                         |
   | 0x01    | RSA (RSASSA-PKCS1-v1_5, key   | [RFC8017]               |
   |         | >= 2048 bits) with SHA-256    |                         |
   |         |                               |                         |
   | 0x02 -  | Unassigned                    | Specification Required  |
   | 0xDF    |                               | and Expert Review       |
   |         |                               |                         |
   | 0xE0 -  | Reserved                      | Experimental Use        |
   | 0xEF    |                               |                         |
   |         |                               |                         |
   | 0xF0 -  | Reserved                      | Private Use             |
   | 0xFF    |                               |                         |
   +---------+-------------------------------+-------------------------+

11.4.1.  Expert Review guidelines

   The appointed Expert should ensure that the proposed algorithm has a
   public specification and is suitable for use as a cryptographic
   signature algorithm that always generates signatures
   deterministically (for the reasons listed in Section 12.4).

11.5.  VersionedTransTypes

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















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   +------------+---------------------------+--------------------------+
   | 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     | x509_sct_with_proof_v2    | RFCXXXX                  |
   |            |                           |                          |
   | 0x0009     | precert_sct_with_proof_v2 | RFCXXXX                  |
   |            |                           |                          |
   | 0x0010 -   | Unassigned                | Specification Required   |
   | 0xDFFF     |                           | and Expert Review        |
   |            |                           |                          |
   | 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.]

11.5.1.  Expert Review guidelines

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







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11.6.  SCT Extensions

   IANA is asked to establish a registry of SCT extensions, named "CT
   Extension Types for SCT", that initially consists of:

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

11.6.1.  Expert Review guidelines

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

11.7.  STH Extensions

   IANA is asked to establish a registry of STH extensions, named "CT
   Extension Types for STH", that initially consists of:

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

11.7.1.  Expert Review guidelines

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






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11.8.  Object Identifiers

   This document uses object identifiers (OIDs) to identify Log IDs (see
   Section 5.3), the precertificate CMS "eContentType" (see
   Section 3.2), and X.509v3 extensions in certificates (see Section 4.2
   and Section 8.1.2) and OCSP responses (see Section 8.1.1).  The OIDs
   are defined in an arc that was selected due to its short encoding.

11.8.1.  Log ID Registry

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

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

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

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

   Each application for the allocation of a Log ID should be accompanied
   by all of the required metadata (except for the Log ID) listed in
   Section 9.1.

11.8.2.  Expert Review guidelines

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





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12.  Security Considerations

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

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

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

12.1.  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.  Thus, the maximum period of time during which a
   misissued certificate can be used without being available for audit
   is the MMD.

12.2.  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.

12.3.  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 and issuing STHs too
   frequently.  Such misbehavior is detectable and the
   [I-D.ietf-trans-threat-analysis] provides more details on how this
   can be done.



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   Violation of the MMD contract is detected by log clients requesting a
   Merkle inclusion proof (Section 6.5) for each observed SCT.  These
   checks can be asynchronous and need only be done once per each
   certificate.  In order to protect the clients' privacy, these checks
   need not reveal the exact certificate to the log.  Instead, clients
   can request the proof from a trusted auditor (since anyone can
   compute the proofs from the log) or communicate with the log via
   proxies.

   Violation of the append-only property or the STH issuance rate limit
   can be detected by clients comparing their instances of the Signed
   Tree Heads.  There are various ways this could be done, for example
   via gossip (see [I-D.ietf-trans-gossip]) or peer-to-peer
   communications or by sending STHs to monitors (who could then
   directly check against their own copy of the relevant log).  A 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.

12.4.  Deterministic Signatures

   Logs are required to use deterministic signatures for the following
   reasons:

   o  Using non-deterministic ECDSA with a predictable source of
      randomness means that each signature can potentially expose the
      secret material of the signing key.

   o  Clients that gossip STHs or report back SCTs can be tracked or
      traced if a log was to produce multiple STHs or SCTs with the same
      timestamp and data but different signatures.

12.5.  Multiple SCTs

   By offering multiple SCTs, each from a different log, TLS servers
   reduce the effectiveness of an attack where a CA and a log collude
   (see Section 7.1).

13.  Acknowledgements

   The authors would like to thank Erwann Abelea, Robin Alden, Andrew
   Ayer, 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




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   Phaneuf, Melinda Shore, Ryan Sleevi, Martin Smith, Carl Wallace and
   Paul Wouters for their valuable contributions.

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

14.  References

14.1.  Normative References

   [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,
              <http://www.rfc-editor.org/info/rfc2119>.

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

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

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

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

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <http://www.rfc-editor.org/info/rfc5905>.

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



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   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <http://www.rfc-editor.org/info/rfc6125>.

   [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,
              <http://www.rfc-editor.org/info/rfc6960>.

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              DOI 10.17487/RFC6961, June 2013,
              <http://www.rfc-editor.org/info/rfc6961>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [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,
              <http://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, <http://www.rfc-editor.org/info/rfc7633>.

   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,
              <http://www.rfc-editor.org/info/rfc7924>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <http://www.rfc-editor.org/info/rfc8017>.

14.2.  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>.




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

   [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>.

   [EVSSLGuidelines]
              CA/Browser Forum, "Guidelines For The Issuance And
              Management Of Extended Validation Certificates", 2007,
              <https://cabforum.org/wp-content/uploads/
              EV_Certificate_Guidelines.pdf>.

   [I-D.ietf-trans-gossip]
              Nordberg, L., Gillmor, D., and T. Ritter, "Gossiping in
              CT", draft-ietf-trans-gossip-03 (work in progress), July
              2016.

   [I-D.ietf-trans-threat-analysis]
              Kent, S., "Attack and Threat Model for Certificate
              Transparency", draft-ietf-trans-threat-analysis-10 (work
              in progress), October 2016.

   [JSON.Metadata]
              The Chromium Projects, "Chromium Log Metadata JSON
              Schema", 2014, <http://www.certificate-transparency.org/
              known-logs/log_list_schema.json>.

   [RFC4634]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and HMAC-SHA)", RFC 4634, DOI 10.17487/RFC4634, July
              2006, <http://www.rfc-editor.org/info/rfc4634>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

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






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   [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, <http://www.rfc-editor.org/info/rfc6979>.

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 8.1.2.

   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






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   Adam Langley
   Google Inc.

   Email: agl@google.com


   Emilia Kasper
   Google Switzerland GmbH

   Email: ekasper@google.com


   Eran Messeri
   Google UK Ltd.

   Email: eranm@google.com


   Rob Stradling
   Comodo CA, Ltd.

   Email: rob.stradling@comodo.com





























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