draft-ietf-trans-rfc6962-bis-07.txt

Public Notary Transparency Working Group                       B. Laurie
Internet-Draft                                                A. Langley
Intended status: Standards Track                               E. Kasper
Expires: December 11, 2015                                    E. Messeri
                                                                  Google
                                                            R. Stradling
                                                                  Comodo
                                                            June 9, 2015


                        Certificate Transparency
                    draft-ietf-trans-rfc6962-bis-07

Abstract

   This document describes a protocol for publicly logging the existence
   of Transport Layer Security (TLS) 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on December 11, 2015.








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Copyright Notice

   Copyright (c) 2015 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Data Structures . . . . . . . . . . . . . . . . . . . . .   4
   2.  Cryptographic Components  . . . . . . . . . . . . . . . . . .   4
     2.1.  Merkle Hash Trees . . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Merkle Inclusion Proofs . . . . . . . . . . . . . . .   5
       2.1.2.  Merkle Consistency Proofs . . . . . . . . . . . . . .   6
       2.1.3.  Example . . . . . . . . . . . . . . . . . . . . . . .   7
       2.1.4.  Signatures  . . . . . . . . . . . . . . . . . . . . .   9
   3.  Log Format and Operation  . . . . . . . . . . . . . . . . . .   9
     3.1.  Log Entries . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Private Domain Name Labels  . . . . . . . . . . . . . . .  12
       3.2.1.  Wildcard Certificates . . . . . . . . . . . . . . . .  12
       3.2.2.  Redacting Domain Name Labels in Precertificates . . .  12
       3.2.3.  Using a Name-Constrained Intermediate CA  . . . . . .  13
     3.3.  Structure of the Signed Certificate Timestamp . . . . . .  14
     3.4.  Including the Signed Certificate Timestamp in the TLS
           Handshake . . . . . . . . . . . . . . . . . . . . . . . .  15
       3.4.1.  TLS Extension . . . . . . . . . . . . . . . . . . . .  16
       3.4.2.  X.509v3 Extension . . . . . . . . . . . . . . . . . .  17
     3.5.  Merkle Tree . . . . . . . . . . . . . . . . . . . . . . .  17
     3.6.  Signed Tree Head  . . . . . . . . . . . . . . . . . . . .  18
   4.  Log Client Messages . . . . . . . . . . . . . . . . . . . . .  19
     4.1.  Add Chain to Log  . . . . . . . . . . . . . . . . . . . .  21
     4.2.  Add PreCertChain to Log . . . . . . . . . . . . . . . . .  22
     4.3.  Retrieve Latest Signed Tree Head  . . . . . . . . . . . .  22
     4.4.  Retrieve Merkle Consistency Proof between Two Signed Tree
           Heads . . . . . . . . . . . . . . . . . . . . . . . . . .  23
     4.5.  Retrieve Merkle Inclusion Proof from Log by Leaf Hash . .  23
     4.6.  Retrieve Merkle Inclusion Proof, Signed Tree Head and
           Consistency Proof by Leaf Hash  . . . . . . . . . . . . .  24



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     4.7.  Retrieve Entries from Log . . . . . . . . . . . . . . . .  25
     4.8.  Retrieve Accepted Root Certificates . . . . . . . . . . .  26
   5.  Clients . . . . . . . . . . . . . . . . . . . . . . . . . . .  27
     5.1.  Metadata  . . . . . . . . . . . . . . . . . . . . . . . .  27
     5.2.  Submitters  . . . . . . . . . . . . . . . . . . . . . . .  28
     5.3.  TLS Client  . . . . . . . . . . . . . . . . . . . . . . .  28
     5.4.  Monitor . . . . . . . . . . . . . . . . . . . . . . . . .  28
     5.5.  Auditing  . . . . . . . . . . . . . . . . . . . . . . . .  29
   6.  Algorithm Agility . . . . . . . . . . . . . . . . . . . . . .  30
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
     7.1.  TLS Extension Type  . . . . . . . . . . . . . . . . . . .  30
     7.2.  Hash Algorithms . . . . . . . . . . . . . . . . . . . . .  30
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
     8.1.  Misissued Certificates  . . . . . . . . . . . . . . . . .  31
     8.2.  Detection of Misissue . . . . . . . . . . . . . . . . . .  31
     8.3.  Redaction of Public Domain Name Labels  . . . . . . . . .  31
     8.4.  Misbehaving Logs  . . . . . . . . . . . . . . . . . . . .  31
     8.5.  Multiple SCTs . . . . . . . . . . . . . . . . . . . . . .  32
   9.  Efficiency Considerations . . . . . . . . . . . . . . . . . .  32
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  33
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  33
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  33
     11.2.  Informative References . . . . . . . . . . . . . . . . .  34

1.  Introduction

   Certificate transparency aims to mitigate the problem of misissued
   certificates by providing publicly auditable, append-only, untrusted
   logs of all issued certificates.  The logs are publicly auditable so
   that it is possible for anyone to verify the correctness of each log
   and to 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, but in this
   document, we only describe its use for public TLS server certificates
   issued by public certification authorities (CAs).

   Each log consists of 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
   submitting large numbers of spurious certificates, it is required
   that each chain is rooted in a CA certificate accepted by the log.
   When a chain is submitted to 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 have been logged.



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   Those who are concerned about misissue can monitor the logs, asking
   them regularly for all new entries, and can thus check whether
   domains they are responsible for 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, but 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 TLS connections to proceed without
   delay, despite network connectivity issues and the vagaries of
   firewalls.

   The append-only property of each log is technically 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].

2.  Cryptographic Components







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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.  We have established a
   registry of acceptable algorithms, see Section 7.2.  The hashing
   algorithm in use is referred to as HASH throughout this document.
   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 32-byte Merkle Tree Hash.  Given an ordered
   list of n inputs, D[n] = {d(0), d(1), ..., d(n-1)}, the Merkle Tree
   Hash (MTH) is thus defined as follows:

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

   MTH({}) = HASH().

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

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

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

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

   where || is concatenation and D[k1:k2] 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



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   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,

   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)

   The subproof for m = n is empty if m is the value for which PROOF was
   originally requested (meaning that the subtree Merkle Tree Hash
   MTH(D[0:m]) is known):

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



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   The subproof for m = n is the Merkle Tree Hash committing inputs
   D[0:m]; otherwise:

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

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

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

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

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

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

   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].




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

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

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

   The same tree, built incrementally in four steps:

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

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

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

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

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







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2.1.4.  Signatures

   Various data structures are signed.  A log MUST use either elliptic
   curve signatures using the NIST P-256 curve (Section D.1.2.3 of the
   Digital Signature Standard [DSS]) or RSA signatures (RSASSA-
   PKCS1-v1_5 with SHA-256, Section 8.2 of [RFC3447]) using a key of at
   least 2048 bits.

3.  Log Format and Operation

   Anyone can submit certificates to certificate logs for public
   auditing; however, since certificates will not be accepted by TLS
   clients unless logged, it is expected that certificate owners or
   their CAs will usually submit them.  A log is a single, ever-growing,
   append-only Merkle Tree of such certificates.

   When a valid certificate is submitted to a log, the log MUST return a
   Signed Certificate Timestamp (SCT).  The SCT is the log's promise to
   incorporate the certificate in the Merkle Tree within a fixed amount
   of time known as the Maximum Merge Delay (MMD).  If the log has
   previously seen the certificate, it MAY return the same SCT as it
   returned before (note that if a certificate was previously logged as
   a precertificate, then the precertificate's SCT would not be
   appropriate, instead a fresh SCT of type x509_entry should be
   generated).  TLS servers MUST present an SCT from one or more logs to
   the TLS client together with the certificate.  A certificate not
   accompanied by an SCT (either for the end-entity certificate or for a
   name-constrained intermediate the end-entity certificate chains to)
   MUST NOT be considered compliant by TLS clients.

   Periodically, each log appends all its new entries to the Merkle Tree
   and signs the root of the tree.  The log MUST incorporate a
   certificate in its Merkle Tree within the Maximum Merge Delay period
   after the issuance of the SCT.  When encountering an SCT, an Auditor
   can verify that the certificate was added to the Merkle Tree within
   that timeframe.

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

3.1.  Log Entries

   In order to enable attribution of each logged certificate to its
   issuer, each submitted certificate MUST be accompanied by all
   additional certificates required to verify the certificate chain up
   to an accepted root certificate.  The root certificate itself MAY be
   omitted from the chain submitted to the log server.  The log SHALL
   allow retrieval of a list of accepted root certificates (this list



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   might usefully be the union of root certificates trusted by major
   browser vendors).

   Alternatively, (root as well as intermediate) certification
   authorities may preannounce a certificate to logs prior to issuance
   in order to incorporate the SCT in the issued certificate.  To do
   this, the CA submits a precertificate that the log can use to create
   an entry that will be valid against the issued certificate.  A
   precertificate is a CMS [RFC5652] "signed-data" object that contains
   a TBSCertificate [RFC5280] in its
   "SignedData.encapContentInfo.eContent" field, identified by the OID
   <TBD> in the "SignedData.encapContentInfo.eContentType" field.  This
   TBSCertificate MAY redact certain domain name labels that will be
   present in the issued certificate (see Section 3.2.2) and MUST NOT
   contain any SCTs, but it will be otherwise identical to the
   TBSCertificate in the issued certificate.  "SignedData.signerInfos"
   MUST contain a signature from the same (root or intermediate) CA that
   will ultimately issue the certificate.  This signature indicates the
   certification authority'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).  As
   above, the precertificate submission MUST be accompanied by all the
   additional certificates required to verify the chain up to an
   accepted root certificate.  This does not involve using the
   "SignedData.certificates" field, so that field SHOULD be omitted.

   Logs MUST verify that the submitted certificate or precertificate has
   a valid signature chain to an accepted root certificate, using the
   chain of intermediate CA certificates provided by the submitter.
   Logs MUST accept certificates that are fully valid according to X.509
   verification rules and are submitted with such a chain.  Logs MAY
   accept certificates and precertificates that have expired, are not
   yet valid, have been revoked, or are otherwise not fully valid
   according to X.509 verification rules in order to accommodate quirks
   of CA certificate-issuing software.  However, logs MUST reject
   certificates without a valid signature chain to an accepted root
   certificate.  If a certificate is accepted and an SCT issued, the
   accepting log MUST store the entire chain used for verification,
   including the certificate or precertificate itself and including the
   root certificate used to verify the chain (even if it was omitted
   from the submission), and MUST present this chain for auditing upon
   request.  This chain is required to prevent a CA from avoiding blame
   by logging a partial or empty chain.  (Note: This effectively
   excludes self-signed and DANE-based certificates until some mechanism
   to limit the submission of spurious certificates is found.  The
   authors welcome suggestions.)





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   Each certificate or precertificate entry in a log MUST include the
   following components:

      enum { x509_entry(0), precert_entry_V2(3), (65535) } LogEntryType;

      struct {
          LogEntryType entry_type;
          select (entry_type) {
              case x509_entry: X509ChainEntry;
              case precert_entry_V2: PrecertChainEntryV2;
          } entry;
      } LogEntry;

      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<0..2^24-1>;
      } PrecertChainEntryV2;

   Logs SHOULD limit the length of chain they will accept.

   "entry_type" is the type of this entry.  Future revisions of this
   protocol may add new LogEntryType values.  Section 4 explains how
   clients should handle unknown entry types.

   "leaf_certificate" is the end-entity certificate submitted for
   auditing.

   "certificate_chain" is a chain of additional certificates required to
   verify the end-entity certificate.  The first certificate MUST
   certify the end-entity certificate.  Each following certificate MUST
   directly certify the one preceding it.  The final certificate MUST
   either be, or be issued by, a root certificate accepted by the log.

   "pre_certificate" is the precertificate submitted for auditing.

   "precertificate_chain" is a chain of additional certificates required
   to verify the precertificate submission.  The first certificate MUST
   certify the precertificate.  Each following certificate MUST directly




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   certify the one preceding it.  The final certificate MUST be a root
   certificate accepted by the log.

3.2.  Private Domain Name Labels

   Some regard some 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.  We define a mechanism to allow these
   private labels to not appear in public logs.

3.2.1.  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]), this approach would not work for a DNS-ID such as
   "top.secret.example.com".  Also, wildcard certificates are prohibited
   in some cases, such as Extended Validation Certificates
   [EVSSLGuidelines].

3.2.2.  Redacting Domain Name Labels in Precertificates

   When creating a precertificate, the CA MAY substitute one or more
   labels in each DNS-ID with a corresponding number of "?" labels.
   Every label to the left of a "?" label MUST also be redacted.  For
   example, if a certificate contains a DNS-ID of
   "top.secret.example.com", then the corresponding precertificate could
   contain "?.?.example.com" instead, but not "top.?.example.com"
   instead.

   Wildcard "*" labels MUST NOT be redacted.  However, if the complete
   leftmost label of a DNS-ID is "*", it is considered redacted for the
   purposes of determining if the label to the right may be redacted.
   For example, if a certificate contains a DNS-ID of
   "*.top.secret.example.com", then the corresponding precertificate
   could contain "*.?.?.example.com" instead, but not
   "?.?.?.example.com" instead.

   When a precertificate contains one or more redacted labels, a non-
   critical extension (OID 1.3.6.1.4.1.11129.2.4.6, whose extnValue
   OCTET STRING contains an ASN.1 SEQUENCE OF INTEGERs) MUST be added to
   the corresponding certificate: the first INTEGER indicates the total
   number of redacted labels and wildcard "*" labels in the



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   precertificate's first DNS-ID; the second INTEGER does the same for
   the precertificate's second DNS-ID; etc.  There MUST NOT be more
   INTEGERs than there are DNS-IDs.  If there are fewer INTEGERs than
   there are DNS-IDs, the shortfall is made up by implicitly repeating
   the last INTEGER.  Each INTEGER MUST have a value of zero or more.
   The purpose of this extension is to enable TLS clients to accurately
   reconstruct the TBSCertificate component of the precertificate from
   the certificate without having to perform any guesswork.

   When a precertificate contains that extension and contains a CN-ID
   [RFC6125], the CN-ID MUST match the first DNS-ID and have the same
   labels redacted.  TLS clients will use the first entry in the
   SEQUENCE OF INTEGERs to reconstruct both the first DNS-ID and the CN-
   ID.

3.2.3.  Using a Name-Constrained Intermediate CA

   An intermediate CA certificate or intermediate CA precertificate that
   contains the critical or non-critical 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
      1.3.6.1.4.1.11129.2.4.7, 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  permittedSubtrees MUST specify one or more dNSNames.

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

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















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   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
             }
           }
         }
       }
     }

3.3.  Structure of the Signed Certificate Timestamp

       enum { certificate_timestamp(0), tree_hash(1), (255) }
         SignatureType;

       enum { v1(0), v2(1), (255) }
         Version;

         struct {
             opaque key_id[32];
         } LogID;

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

         opaque CtExtensions<0..2^16-1>;

   "key_id" is the SHA-256 hash of the log's public key, calculated over
   the DER encoding of the key represented as SubjectPublicKeyInfo.

   "tbs_certificate" is the DER-encoded TBSCertificate component of the
   precertificate.  Note that it is also possible to reconstruct this
   TBSCertificate from the issued certificate by extracting the
   TBSCertificate from it, redacting the domain name labels indicated by
   the redacted labels extension, and deleting the SCT list extension
   and redacted labels extension.



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       struct {
           Version sct_version;
           LogID id;
           uint64 timestamp;
           CtExtensions extensions;
           digitally-signed struct {
               Version sct_version;
               SignatureType signature_type = certificate_timestamp;
               uint64 timestamp;
               LogEntryType entry_type;
               select(entry_type) {
                   case x509_entry: ASN.1Cert;
                   case precert_entry_V2: TBSCertificate;
               } signed_entry;
              CtExtensions extensions;
           };
       } SignedCertificateTimestamp;

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

   "sct_version" is the version of the protocol to which the SCT
   conforms.  This version is v2.

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

   "entry_type" may be implicit from the context in which the SCT is
   presented.

   "signed_entry" is the "leaf_certificate" (in the case of an
   X509ChainEntry) or is the TBSCertificate (in the case of a
   PrecertChainEntryV2), as described above.

   "extensions" are future extensions to SignedCertificateTimestamp v2.
   Currently, no extensions are specified.

3.4.  Including the Signed Certificate Timestamp in the TLS Handshake

   The SCT data corresponding to at least one certificate in the chain
   from at least one log must be included in the TLS handshake by using
   one or more of the mechanisms listed below.  Three mechanisms are
   provided because they have different tradeoffs.  TLS clients MUST
   implement all three mechanisms.  TLS servers MUST present SCTs using
   at least one of the three mechanisms.

   o  A TLS extension (Section 7.4.1.4 of [RFC5246]) with type
      "signed_certificate_timestamp" (see Section 3.4.1).  This



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      mechanism allows TLS servers to participate in CT without the
      cooperation of CAs, unlike the other two mechanisms.  It also
      allows SCTs to be updated on the fly.

   o  An Online Certificate Status Protocol (OCSP) [RFC6960] response
      extension (see Section 3.4.2.1), where the OCSP response is
      provided in the "certificate_status" TLS extension (Section 8 of
      [RFC6066]), also known as OCSP stapling.  This mechanism is
      already widely (but not universally) implemented.  It also allows
      SCTs to be updated on the fly.

   o  An X509v3 certificate extension (see Section 3.4.2.2).  This
      mechanism allows the use of unmodified TLS servers, but, because
      the included SCTs cannot be changed without re-issuing the
      certificate, increases the risk that the certificate will be
      refused if any of the SCTs become invalid.

   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).  Multiple SCTs are combined into an SCT
   list as follows:

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

        struct {
            SerializedSCT sct_list <1..2^16-1>;
        } SignedCertificateTimestampList;

   Here, "SerializedSCT" is an opaque byte string that contains the
   serialized SCT structure.  This encoding ensures that TLS clients can
   decode each SCT individually (i.e., if there is a version upgrade,
   out-of-date clients can still parse old SCTs while skipping over new
   SCTs whose versions they don't understand).

3.4.1.  TLS Extension

   One or more SCTs can be sent during the TLS handshake using a TLS
   extension with type "signed_certificate_timestamp".

   TLS clients that support the extension SHOULD send a ClientHello
   extension with the appropriate type and empty "extension_data".

   TLS servers MUST only send SCTs in this TLS extension to TLS clients
   that have indicated support for the extension in the ClientHello, in
   which case the SCTs are sent by setting the "extension_data" to a
   "SignedCertificateTimestampList".




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   Session resumption uses the original session information: TLS clients
   SHOULD include the extension type in the ClientHello, but if the
   session is resumed, the TLS server is not expected to process it or
   include the extension in the ServerHello.

3.4.2.  X.509v3 Extension

   One or more SCTs can be embedded in an X.509v3 extension that is
   included in a certificate or an OCSP response.  Since RFC5280
   requires the "extnValue" field (an OCTET STRING) of each X.509v3
   extension to include the DER encoding of an ASN.1 value, we cannot
   embed a "SignedCertificateTimestampList" directly.  Instead, we have
   to wrap it inside an additional OCTET STRING (see below), which we
   then put into the "extnValue" field.

3.4.2.1.  OCSP Response Extension

   A certification authority may embed one or more SCTs in OCSP
   responses pertaining to the end-entity certificate, by including a
   non-critical "singleExtensions" extension with OID
   1.3.6.1.4.1.11129.2.4.5 whose "extnValue" contains:

        CertificateSCTList ::= OCTET STRING

   "CertificateSCTList" contains a "SignedCertificateTimestampList"
   whose SCTs all have the "x509_entry" "LogEntryType".

3.4.2.2.  Certificate Extension

   A certification authority that has submitted a precertificate to one
   or more logs may embed the obtained SCTs in the "TBSCertificate" that
   will be signed to produce the certificate, by including a non-
   critical X.509v3 extension with OID 1.3.6.1.4.1.11129.2.4.2 whose
   "extnValue" contains:

        PrecertificateSCTList ::= OCTET STRING

   "PrecertificateSCTList" contains a "SignedCertificateTimestampList"
   whose SCTs all have the "precert_entry_V2" "LogEntryType".

   Upon receiving the certificate, clients can reconstruct the original
   "TBSCertificate" to verify the SCT signatures.

3.5.  Merkle Tree

   The hashing algorithm for the Merkle Tree Hash is specified in the
   log's metadata.




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   Structure of the Merkle Tree input:

       enum { v1(0), v2(1), (255) }
         LeafVersion;

       struct {
           uint64 timestamp;
           LogEntryType entry_type;
           select(entry_type) {
               case x509_entry: ASN.1Cert;
               case precert_entry_V2: TBSCertificate;
           } signed_entry;
           CtExtensions extensions;
       } TimestampedEntry;

       struct {
           LeafVersion version;
           TimestampedEntry timestamped_entry;
       } MerkleTreeLeaf;

   Here, "version" is the version of the MerkleTreeLeaf structure.  This
   version is v2.  Note that MerkleTreeLeaf v1 [RFC6962] had another
   layer of indirection which is removed in v2.

   "timestamp" is the timestamp of the corresponding SCT issued for this
   certificate.

   "entry_type" is the type of entry stored in "signed_entry".  New
   "LogEntryType" values may be added to "signed_entry" without
   increasing the "MerkleTreeLeaf" version.  Section 4 explains how
   clients should handle unknown entry types.

   "signed_entry" is the "signed_entry" of the corresponding SCT.

   "extensions" are "extensions" of the corresponding SCT.

   The leaves of the Merkle Tree are the leaf hashes of the
   corresponding "MerkleTreeLeaf" structures.

3.6.  Signed Tree Head

   Every time a log appends new entries to the tree, the log SHOULD sign
   the corresponding tree hash and tree information (see the
   corresponding Signed Tree Head client message in Section 4.3).  The
   signature for that data is structured as follows:






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           enum { v1(0), (255) } TreeHeadVersion;

           digitally-signed struct {
                 TreeHeadVersion version;
                 SignatureType signature_type = tree_hash;
                 uint64 timestamp;
                 uint64 tree_size;
                 opaque sha256_root_hash[32];
           } TreeHeadSignature;

   "version" is the version of the TreeHeadSignature structure.  This
   version is v1.

   "timestamp" is the current time.  The timestamp 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_size" equals the number of entries in the new tree.

   "sha256_root_hash" is the root of the Merkle Hash Tree.

   Each log MUST produce on demand a Signed Tree Head that is no older
   than the Maximum Merge Delay.  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.

4.  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 [RFC4627].  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 MAY include a path as well as a server name
   and a port.

   In general, where needed, the "version" is v1 and the "id" is the log
   id for the log server queried.

   In practice, log servers may include multiple front-end machines.
   Since it is impractical to keep these machines in perfect sync,



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

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

   If the log is unable to process a client's request, it MUST return an
   HTTP response code of 4xx/5xx (see [RFC2616]), 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.

      not compliant  The request is not compliant with this RFC.

   e.g. In response to a request of "/ct/v1/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
   [RFC2616], 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.

4.1.  Add Chain to Log

   POST https://<log server>/ct/v1/add-chain

   Inputs:

      chain:  An array of base64-encoded certificates.  The first
         element is the end-entity certificate; the second chains to the
         first and so on to the last, which is either the root
         certificate or a certificate that chains to a known root
         certificate.

   Outputs:

      sct_version:  The version of the SignedCertificateTimestamp
         structure, in decimal.  A compliant v1 implementation MUST NOT
         expect this to be 0 (i.e., v1).

      id:  The log ID, base64 encoded.

      timestamp:  The SCT timestamp, in decimal.

      extensions:  An opaque type for future expansion.  It is likely
         that not all participants will need to understand data in this
         field.  Logs should set this to the empty string.  Clients
         should decode the base64-encoded data and include it in the
         SCT.

      signature:  The SCT signature, base64 encoded.

   Error codes:

      unknown root  The root of the chain is not one accepted by the
         log.

      bad chain  The alleged chain is not actually a chain of
         certificates.

      bad certificate  One or more certificates in the chain are not
         valid (e.g. not properly encoded).

   If the "sct_version" is not v1, then a v1 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 v1 clients that do not
   use the returned SCTs.



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   If a log detects bad encoding in a chain that otherwise verifies
   correctly (e.g. some software will accept BER instead of DER
   encodings in certificates, or incorrect character encodings, even
   though these are technically incorrect) then the log MAY still log
   the certificate but SHOULD NOT return an SCT.  It should instead
   return the "bad certificate" error.  Logging the certificate is
   useful, because monitors can then detect these encoding errors, which
   may be accepted by some TLS clients.

   Note that not all certificate handling software is capable of
   detecting all encoding errors.

4.2.  Add PreCertChain to Log

   POST https://<log server>/ct/v1/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 chains
         to the first and so on to the last, which is either the root
         certificate or a certificate that chains to an accepted root
         certificate.

   Outputs and errors are the same as in Section 4.1.

4.3.  Retrieve Latest Signed Tree Head

   GET https://<log server>/ct/v1/get-sth

   No inputs.

   Outputs:

      tree_size:  The size of the tree, in entries, in decimal.

      timestamp:  The timestamp, in decimal.

      sha256_root_hash:  The Merkle Tree Hash of the tree, in base64.

      tree_head_signature:  A TreeHeadSignature for the above data.








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4.4.  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 v1 STHs (Signed Tree Heads).
      However, because of skew, the receiving front-end may not know one
      or both of the existing STHs.  If both are known, then only the
      "consistency" output is returned.  If the first is known but the
      second is not (or has been omitted), then the latest known STH is
      returned, along with a consistency proof between the first STH and
      the latest.  If neither are known, then the latest known STH is
      returned without a consistency proof.

   Outputs:

      consistency:  An array of Merkle Tree nodes, base64 encoded.

      tree_size:  The size of the tree, in entries, in decimal.

      timestamp:  The timestamp, in decimal.

      sha256_root_hash:  The Merkle Tree Hash of the tree, in base64.

      tree_head_signature:  A TreeHeadSignature for the above data.

      Note that no signature is required on this data, as it is used to
      verify an STH, which is signed.

   Error codes:

      first unknown  "first" is before the latest known STH but is not
         from an existing STH.

      second unknown  "second" is before the latest known STH but is not
         from an existing STH.

4.5.  Retrieve Merkle Inclusion Proof from Log by Leaf Hash

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

   Inputs:




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      hash:  A base64-encoded v1 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 3.5.  The
      "tree_size" must designate an existing v1 STH.  Because of skew,
      the front-end may not know the requested STH.  In that case, it
      will return the latest STH it knows, along with an inclusion proof
      to that STH.  If the front-end knows the requested STH then only
      "leaf_index" and "audit_path" are returned.

   Outputs:

      leaf_index:  The 0-based index of the entry corresponding to the
         "hash" parameter.

      audit_path:  An array of base64-encoded Merkle Tree nodes proving
         the inclusion of the chosen certificate.

      tree_size:  The size of the tree, in entries, in decimal.

      timestamp:  The timestamp, in decimal.

      sha256_root_hash:  The Merkle Tree Hash of the tree, in base64.

      tree_head_signature:  A TreeHeadSignature for the above data.

   Error codes:

      hash unknown  "hash" is not the hash of a known leaf (may be
         caused by skew or by a known certificate not yet merged).

      tree_size unknown  "hash" is before the latest known STH but is
         not from an existing STH.

4.6.  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 v1 leaf hash.

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




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      The "hash" must be calculated as defined in Section 3.5.  The
      "tree_size" must designate an existing v1 STH.

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

      latest STH < requested STH  Return latest STH.

      latest STH > requested STH  Return latest STH and a consistency
         proof between it and the requested STH (see Section 4.4).

      index of requested hash < latest STH  Return "leaf_index" and
         "audit_path".

      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:

      leaf_index:  The 0-based index of the entry corresponding to the
         "hash" parameter.

      audit_path:  An array of base64-encoded Merkle Tree nodes proving
         the inclusion of the chosen certificate.

      tree_size:  The size of the tree, in entries, in decimal.

      timestamp:  The timestamp, in decimal.

      sha256_root_hash:  The Merkle Tree Hash of the tree, in base64.

      tree_head_signature:  A TreeHeadSignature for the above data.

      consistency:  An array of base64-encoded Merkle Tree nodes proving
         the consistency of the requested STH and the returned STH.

      Errors are the same as in Section 4.5.

4.7.  Retrieve Entries from Log

   GET https://<log server>/ct/v1/get-entries

   Inputs:

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




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      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 MerkleTreeLeaf structure.

         extra_data:  The base64-encoded unsigned data pertaining to the
            log entry.  In the case of an X509ChainEntry, this is the
            "certificate_chain".  In the case of a PrecertChainEntryV2,
            this is the whole "PrecertChainEntryV2".

   Note that this message is not signed -- the retrieved data can be
   verified by constructing the Merkle Tree Hash corresponding to a
   retrieved STH.  All leaves MUST be v1 or v2.  However, a compliant v1
   client MUST NOT construe an unrecognized LogEntryType value 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 4.3.

   Logs MAY 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.  Note that the following restriction
   may also apply:

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

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

4.8.  Retrieve Accepted Root Certificates

   GET https://<log server>/ct/v1/get-roots

   No inputs.

   Outputs:




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      certificates:  An array of base64-encoded root certificates 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.

5.  Clients

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

   All clients need various 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.

5.1.  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 4.

   Hash Algorithm  The hash algorithm used for the Merkle Tree (see
      Section 7.2).

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

   Public Key  The public key used for signing.

   Maximum Merge Delay  The MMD the log has committed to.

   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.





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

5.2.  Submitters

   Submitters submit certificates or precertificates to the log as
   described above.  When a Submitter intends to use the returned SCT
   directly in a TLS handshake or to construct a certificate, they
   SHOULD validate the SCT as described in Section 5.3 if they
   understand its format.

5.3.  TLS Client

   TLS clients receive SCTs alongside or in certificates, either for the
   server certificate itself or for intermediate CA precertificates.  In
   addition to normal validation of the certificate and its chain, TLS
   clients SHOULD validate the SCT by computing the signature input from
   the SCT data as well as the certificate and verifying the signature,
   using the corresponding log's public key.

   A TLS client MAY audit the corresponding log by requesting, and
   verifying, a Merkle audit proof for said certificate.  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 4.3), then verify
   it by requesting a consistency proof (Section 4.4).

   TLS clients MUST reject SCTs whose timestamp is in the future.

5.4.  Monitor

   Monitors watch logs and check that they behave correctly.  Monitors
   may additionally watch for certificates of interest.  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 4.3).

   2.  Verify the STH signature.

   3.  Fetch all the entries in the tree corresponding to the STH
       (Section 4.7).





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   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 4.3).  Repeat until the STH
       changes.

   6.  Verify the STH signature.

   7.  Fetch all the new entries in the tree corresponding to the STH
       (Section 4.7).  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:

       1.  Fetch a consistency proof for the new STH with the previous
           STH (Section 4.4).

       2.  Verify the consistency proof.

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

   9.  Go to Step 5.

5.5.  Auditing

   Auditing is taking partial information about a log as input and
   verifying that this information is consistent with other partial
   information held.  All clients described above may perform auditing
   as an additional function.  The action taken by the client if audit
   fails is not specified, but note that in general if audit fails, the
   client is in possession of signed proof of the log's misbehavior.

   A monitor (Section 5.4) 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 5.3) 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 4.5).  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 that



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   certificate or is an appropriate name-constrained intermediate [see
   Section 3.2.3]).

6.  Algorithm Agility

   It is not possible for a log to change any of its algorithms part way
   through its lifetime.  If it should become necessary to deprecate an
   algorithm used by a live log, then the log should be frozen as
   specified in Section 5.1 and a new log should be started.  If
   necessary, the new log can contain existing entries from the frozen
   log, which monitors can verify are an exact match.

7.  IANA Considerations

7.1.  TLS Extension Type

   IANA has allocated an RFC 5246 ExtensionType value (18) for the SCT
   TLS extension.  The extension name is "signed_certificate_timestamp".
   IANA should update this extension type to point at this document.

7.2.  Hash Algorithms

   IANA is asked to establish a registry of hash values, initially
   consisting of:

                     +-------+----------------------+
                     | Index | Hash                 |
                     +-------+----------------------+
                     | 0     | SHA-256 [FIPS.180-4] |
                     +-------+----------------------+

8.  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 the subject of the certificate has 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 the subject of the certificate might not have
   checked the logs or the CA might have refused to revoke the
   certificate.





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   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.

8.1.  Misissued Certificates

   Misissued certificates that have not been publicly logged, and thus
   do not have a valid SCT, will be rejected by TLS clients.  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.

8.2.  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.

8.3.  Redaction of Public Domain Name Labels

   CAs SHOULD NOT redact domain name labels in precertificates such that
   the entirety of the domain space below the unredacted part of the
   domain name is not owned or controlled by a single entity (e.g.
   "?.com" and "?.co.uk" would both be problematic).  Logs MUST NOT
   reject any precertificate that is overly redacted but which is
   otherwise considered compliant.  It is expected that monitors will
   treat overly redacted precertificates as potentially misissued.  TLS
   clients MAY reject a certificate whose corresponding precertificate
   would be overly redacted, perhaps using the same mechanism for
   determining whether a wildcard in a domain name of a certificate is
   too broad.

8.4.  Misbehaving Logs

   A log can misbehave in two ways: (1) by failing to incorporate a
   certificate with an SCT in the Merkle Tree within the MMD and (2) by
   violating its append-only property by presenting two different,
   conflicting views of the Merkle Tree at different times and/or to
   different parties.  Both forms of violation will be promptly and
   publicly detectable.

   Violation of the MMD contract is detected by log clients requesting a
   Merkle audit proof 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



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   the exact certificate to the log.  Clients can instead request the
   proof from a trusted auditor (since anyone can compute the audit
   proofs from the log) or request Merkle proofs for a batch of
   certificates around the SCT timestamp.

   Violation of the append-only property can be detected by clients
   comparing their instances of the Signed Tree Heads.  As soon as two
   conflicting Signed Tree Heads for the same log are detected, this is
   cryptographic proof of that log's misbehavior.  There are various
   ways this could be done, for example via gossip (see http://
   trac.tools.ietf.org/id/draft-linus-trans-gossip-00.txt) or peer-to-
   peer communications or by sending STHs to monitors (who could then
   directly check against their own copy of the relevant log).

8.5.  Multiple SCTs

   TLS servers may wish to offer multiple SCTs, each from a different
   log.

   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 the SCT is
      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 Chromium
      [Chromium.Log.Policy] currently requires multiple SCTs to be
      presented with EV certificates in order for the EV indicator to be
      shown.

9.  Efficiency Considerations

   The Merkle Tree design serves the purpose of keeping communication
   overhead low.

   Auditing logs for integrity does not require third parties to
   maintain a copy of each entire log.  The Signed Tree Heads can be
   updated as new entries become available, without recomputing entire
   trees.  Third-party auditors need only fetch the Merkle consistency
   proofs against a log's existing STH to efficiently verify the append-
   only property of updates to their Merkle Trees, without auditing the
   entire tree.



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10.  Acknowledgements

   The authors would like to thank Erwann Abelea, Robin Alden, Al
   Cutter, Francis Dupont, Stephen Farrell, Brad Hill, Jeff Hodges, Paul
   Hoffman, Jeffrey Hutzelman, SM, Alexey Melnikov, Chris Palmer, Trevor
   Perrin, Ryan Sleevi and Carl Wallace for their valuable
   contributions.

11.  References

11.1.  Normative References

   [DSS]      National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", FIPS 186-3, June 2009,
              <http://csrc.nist.gov/publications/fips/fips186-3/
              fips_186-3.pdf>.

   [FIPS.180-4]
              National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS PUB 180-4, March 2012,
              <http://csrc.nist.gov/publications/fips/fips180-4/
              fips-180-4.pdf>.

   [HTML401]  Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium Recommendation
              REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.




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   [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, May 2008.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, September 2009.

   [RFC5905]  Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [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, March 2011.

   [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, June 2013.

11.2.  Informative References

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

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

   [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>.







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   [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>.

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

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, June 2013.

Authors' Addresses

   Ben Laurie
   Google UK Ltd.

   EMail: benl@google.com


   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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