<pre>Internet Engineering Task Force (IETF)                         B. Laurie
Request for Comments: 6962                                    A. Langley
Category: Experimental                                         E. Kasper
ISSN: 2070-1721                                                   Google
                                                               June 2013


                        <span class="h1">Certificate Transparency</span>

Abstract

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

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc6962">http://www.rfc-editor.org/info/rfc6962</a>.











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Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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

   <a href="#section-1">1</a>. Informal Introduction ...........................................<a href="#page-3">3</a>
      <a href="#section-1.1">1.1</a>. Requirements Language ......................................<a href="#page-4">4</a>
      <a href="#section-1.2">1.2</a>. Data Structures ............................................<a href="#page-4">4</a>
   <a href="#section-2">2</a>. Cryptographic Components ........................................<a href="#page-4">4</a>
      <a href="#section-2.1">2.1</a>. Merkle Hash Trees ..........................................<a href="#page-4">4</a>
           <a href="#section-2.1.1">2.1.1</a>. Merkle Audit Paths ..................................<a href="#page-5">5</a>
           <a href="#section-2.1.2">2.1.2</a>. Merkle Consistency Proofs ...........................<a href="#page-6">6</a>
           <a href="#section-2.1.3">2.1.3</a>. Example .............................................<a href="#page-7">7</a>
           <a href="#section-2.1.4">2.1.4</a>. Signatures ..........................................<a href="#page-8">8</a>
   <a href="#section-3">3</a>. Log Format and Operation ........................................<a href="#page-9">9</a>
      <a href="#section-3.1">3.1</a>. Log Entries ................................................<a href="#page-9">9</a>
      <a href="#section-3.2">3.2</a>. Structure of the Signed Certificate Timestamp .............<a href="#page-12">12</a>
      3.3. Including the Signed Certificate Timestamp in the
           TLS Handshake .............................................<a href="#page-13">13</a>
           <a href="#section-3.3.1">3.3.1</a>. TLS Extension ......................................<a href="#page-15">15</a>
      <a href="#section-3.4">3.4</a>. Merkle Tree ...............................................<a href="#page-15">15</a>
      <a href="#section-3.5">3.5</a>. Signed Tree Head ..........................................<a href="#page-16">16</a>
   <a href="#section-4">4</a>. Log Client Messages ............................................<a href="#page-17">17</a>
      <a href="#section-4.1">4.1</a>. Add Chain to Log ..........................................<a href="#page-17">17</a>
      <a href="#section-4.2">4.2</a>. Add PreCertChain to Log ...................................<a href="#page-18">18</a>
      <a href="#section-4.3">4.3</a>. Retrieve Latest Signed Tree Head ..........................<a href="#page-18">18</a>
      4.4. Retrieve Merkle Consistency Proof between Two
           Signed Tree Heads .........................................<a href="#page-19">19</a>
      <a href="#section-4.5">4.5</a>. Retrieve Merkle Audit Proof from Log by Leaf Hash .........<a href="#page-19">19</a>
      <a href="#section-4.6">4.6</a>. Retrieve Entries from Log .................................<a href="#page-20">20</a>
      <a href="#section-4.7">4.7</a>. Retrieve Accepted Root Certificates .......................<a href="#page-21">21</a>
      <a href="#section-4.8">4.8</a>. Retrieve Entry+Merkle Audit Proof from Log ................<a href="#page-21">21</a>
   <a href="#section-5">5</a>. Clients ........................................................<a href="#page-21">21</a>
      <a href="#section-5.1">5.1</a>. Submitters ................................................<a href="#page-22">22</a>
      <a href="#section-5.2">5.2</a>. TLS Client ................................................<a href="#page-22">22</a>
      <a href="#section-5.3">5.3</a>. Monitor ...................................................<a href="#page-22">22</a>



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      <a href="#section-5.4">5.4</a>. Auditor ...................................................<a href="#page-23">23</a>
   <a href="#section-6">6</a>. IANA Considerations ............................................<a href="#page-23">23</a>
   <a href="#section-7">7</a>. Security Considerations ........................................<a href="#page-23">23</a>
      <a href="#section-7.1">7.1</a>. Misissued Certificates ....................................<a href="#page-24">24</a>
      <a href="#section-7.2">7.2</a>. Detection of Misissue .....................................<a href="#page-24">24</a>
      <a href="#section-7.3">7.3</a>. Misbehaving Logs ..........................................<a href="#page-24">24</a>
   <a href="#section-8">8</a>. Efficiency Considerations ......................................<a href="#page-25">25</a>
   <a href="#section-9">9</a>. Future Changes .................................................<a href="#page-25">25</a>
   <a href="#section-10">10</a>. Acknowledgements ..............................................<a href="#page-25">25</a>
   <a href="#section-11">11</a>. References ....................................................<a href="#page-25">25</a>
      <a href="#section-11.1">11.1</a>. Normative Reference ......................................<a href="#page-25">25</a>
      <a href="#section-11.2">11.2</a>. Informative References ...................................<a href="#page-26">26</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Informal Introduction</span>

   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 certificate 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; it is also expected
   that certificate holders will contribute their own certificate
   chains.  In order to avoid logs being spammed into uselessness, it is
   required that each chain is rooted in a known CA certificate.  When a
   chain is submitted to a log, a signed timestamp is returned, which
   can later be used to provide evidence to clients that the chain has
   been submitted.  TLS clients can thus require that all certificates
   they see have been logged.

   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.  Of course, anyone who wants can monitor the logs and,
   if they believe a certificate is incorrectly issued, take action as
   they see fit.




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   Similarly, those who have seen signed timestamps from a particular
   log can later demand a proof of inclusion from that log.  If the log
   is unable to provide this (or, indeed, if the corresponding
   certificate is absent from monitors' copies of that log), that is
   evidence of the incorrect operation of the log.  The checking
   operation is asynchronous to allow 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 version
   of the log is a superset of any particular previous version.
   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.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Requirements Language</span>

   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 <a href="./rfc2119">RFC 2119</a> [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Data Structures</span>

   Data structures are defined according to the conventions laid out in
   <a href="./rfc5246#section-4">Section&nbsp;4 of [RFC5246]</a>.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Cryptographic Components</span>

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  Merkle Hash Trees</span>

   Logs use a binary Merkle Hash Tree for efficient auditing.  The
   hashing algorithm is SHA-256 [<a href="#ref-FIPS.180-4" title="&quot;Secure Hash Standard&quot;">FIPS.180-4</a>] (note that this is fixed
   for this experiment, but it is anticipated that each log would be
   able to specify a hash algorithm).  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({}) = SHA-256().




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

   MTH({d(0)}) = SHA-256(0x00 || d(0)).

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

   MTH(D[n]) = SHA-256(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
   [<a href="#ref-CrosbyWallach" title="&quot;Efficient Data Structures for Tamper-Evident Logging&quot;">CrosbyWallach</a>] proposal, except our definition handles non-full
   trees differently.)

<span class="h4"><a class="selflink" id="section-2.1.1" href="#section-2.1.1">2.1.1</a>.  Merkle Audit Paths</span>

   A Merkle audit path 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 audit path is combined with the node computed
   so far.  In other words, the audit path 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 audit path
   matches the true root, then the audit path is proof 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 audit path PATH(m, D[n]) for the (m+1)th input
   d(m), 0 &lt;= m &lt; n, is defined as follows:

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

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







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

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

   PATH(m, D[n]) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for m &gt;= 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.

<span class="h4"><a class="selflink" id="section-2.1.2" href="#section-2.1.2">2.1.2</a>.  Merkle Consistency Proofs</span>

   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 &lt;= 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 &lt; m &lt; 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) = {}

   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 &lt; n, let k be the largest power of two smaller than n.  The
   subproof is then defined recursively.

   If m &lt;= 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 &gt; 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.

<span class="h4"><a class="selflink" id="section-2.1.3" href="#section-2.1.3">2.1.3</a>.  Example</span>

   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 audit path for d0 is [b, h, l].

   The audit path for d3 is [c, g, l].

   The audit path for d4 is [f, j, k].

   The audit path for d6 is [i, k].












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   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.

<span class="h4"><a class="selflink" id="section-2.1.4" href="#section-2.1.4">2.1.4</a>.  Signatures</span>

   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 [<a href="#ref-DSS" title="&quot;Digital Signature Standard (DSS)&quot;">DSS</a>]) or RSA signatures (RSASSA-PKCS1-
   V1_5 with SHA-256, <a href="./rfc3447#section-8.2">Section&nbsp;8.2 of [RFC3447]</a>) using a key of at least
   2048 bits.





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<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Log Format and Operation</span>

   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
   immediately 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.  TLS servers MUST present an SCT from
   one or more logs to the TLS client together with the certificate.
   TLS clients MUST reject certificates that do not have a valid SCT for
   the end-entity certificate.

   Periodically, each log appends all its new entries to the Merkle Tree
   and signs the root of the tree.  Auditors can thus verify that each
   certificate for which an SCT has been issued indeed appears in the
   log.  The log MUST incorporate a certificate in its Merkle Tree
   within the Maximum Merge Delay period after the issuance of the SCT.

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

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Log Entries</span>

   Anyone can submit a certificate to any log.  In order to enable
   attribution of each logged certificate to its issuer, the log SHALL
   publish a list of acceptable root certificates (this list might
   usefully be the union of root certificates trusted by major browser
   vendors).  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.

   Alternatively, (root as well as intermediate) certificate authorities
   may submit a certificate to logs prior to issuance.  To do so, the CA
   submits a Precertificate that the log can use to create an entry that
   will be valid against the issued certificate.  The Precertificate is
   constructed from the certificate to be issued by adding a special
   critical poison extension (OID 1.3.6.1.4.1.11129.2.4.3, whose
   extnValue OCTET STRING contains ASN.1 NULL data (0x05 0x00)) to the
   end-entity TBSCertificate (this extension is to ensure that the
   Precertificate cannot be validated by a standard X.509v3 client) and
   signing the resulting TBSCertificate [<a href="./rfc5280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC5280</a>] with either



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   o  a special-purpose (CA:true, Extended Key Usage: Certificate
      Transparency, OID 1.3.6.1.4.1.11129.2.4.4) Precertificate Signing
      Certificate.  The Precertificate Signing Certificate MUST be
      directly certified by the (root or intermediate) CA certificate
      that will ultimately sign the end-entity TBSCertificate yielding
      the end-entity certificate (note that the log may relax standard
      validation rules to allow this, so long as the issued certificate
      will be valid),

   o  or, the CA certificate that will sign the final certificate.

   As above, the Precertificate submission MUST be accompanied by the
   Precertificate Signing Certificate, if used, and all additional
   certificates required to verify the chain up to an accepted root
   certificate.  The signature on the TBSCertificate indicates the
   certificate authority's intent to issue a certificate.  This intent
   is considered binding (i.e., misissuance of the Precertificate is
   considered equal to misissuance of the final certificate).  Each log
   verifies the Precertificate signature chain and issues a Signed
   Certificate Timestamp on the corresponding TBSCertificate.

   Logs MUST verify that the submitted end-entity certificate or
   Precertificate has a valid signature chain leading back to a trusted
   root CA certificate, using the chain of intermediate CA certificates
   provided by the submitter.  Logs MAY accept certificates 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 refuse to publish certificates without a valid chain to a known
   root CA.  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 control spam for those certificates is found.  The authors welcome
   suggestions.)












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

       enum { x509_entry(0), precert_entry(1), (65535) } LogEntryType;

       struct {
           LogEntryType entry_type;
           select (entry_type) {
               case x509_entry: X509ChainEntry;
               case precert_entry: PrecertChainEntry;
           } entry;
       } LogEntry;

       opaque ASN.1Cert&lt;1..2^24-1&gt;;

       struct {
           ASN.1Cert leaf_certificate;
           ASN.1Cert certificate_chain&lt;0..2^24-1&gt;;
       } X509ChainEntry;

       struct {
           ASN.1Cert pre_certificate;
           ASN.1Cert precertificate_chain&lt;0..2^24-1&gt;;
       } PrecertChainEntry;

   Logs MAY limit the length of chain they will accept.

   "entry_type" is the type of this entry.  Future revisions of this
   protocol version may add new LogEntryType values.  <a href="#section-4">Section 4</a> 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 be
   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 MAY
   be a valid Precertificate Signing Certificate and MUST certify the
   first certificate.  Each following certificate MUST directly certify
   the one preceding it.  The final certificate MUST be a root
   certificate accepted by the log.



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<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Structure of the Signed Certificate Timestamp</span>

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

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

         struct {
             opaque key_id[32];
         } LogID;

         opaque TBSCertificate&lt;1..2^24-1&gt;;

         struct {
           opaque issuer_key_hash[32];
           TBSCertificate tbs_certificate;
         } PreCert;

         opaque CtExtensions&lt;0..2^16-1&gt;;

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

   "issuer_key_hash" is the SHA-256 hash of the certificate issuer's
   public key, calculated over the DER encoding of the key represented
   as SubjectPublicKeyInfo.  This is needed to bind the issuer to the
   final certificate.

   "tbs_certificate" is the DER-encoded TBSCertificate (see [<a href="./rfc5280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC5280</a>])
   component of the Precertificate -- that is, without the signature and
   the poison extension.  If the Precertificate is not signed with the
   CA certificate that will issue the final certificate, then the
   TBSCertificate also has its issuer changed to that of the CA that
   will issue the final certificate.  Note that it is also possible to
   reconstruct this TBSCertificate from the final certificate by
   extracting the TBSCertificate from it and deleting the SCT extension.
   Also note that since the TBSCertificate contains an
   AlgorithmIdentifier that must match both the Precertificate signature
   algorithm and final certificate signature algorithm, they must be
   signed with the same algorithm and parameters.  If the Precertificate
   is issued using a Precertificate Signing Certificate and an Authority
   Key Identifier extension is present in the TBSCertificate, the
   corresponding extension must also be present in the Precertificate
   Signing Certificate -- in this case, the TBSCertificate also has its
   Authority Key Identifier changed to match the final issuer.





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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: PreCert;
               } signed_entry;
              CtExtensions extensions;
           };
       } SignedCertificateTimestamp;

   The encoding of the digitally-signed element is defined in [<a href="./rfc5246" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">RFC5246</a>].

   "sct_version" is the version of the protocol to which the SCT
   conforms.  This version is v1.

   "timestamp" is the current NTP Time [<a href="./rfc5905" title="&quot;Network Time Protocol Version 4: Protocol and Algorithms Specification&quot;">RFC5905</a>], 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 PreCert (in the case of a
   PrecertChainEntry), as described above.

   "extensions" are future extensions to this protocol version (v1).
   Currently, no extensions are specified.

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Including the Signed Certificate Timestamp in the TLS Handshake</span>

   The SCT data corresponding to the end-entity certificate from at
   least one log must be included in the TLS handshake, either by using
   an X509v3 certificate extension as described below, by using a TLS
   extension (<a href="./rfc5246#section-7.4.1.4">Section&nbsp;7.4.1.4 of [RFC5246]</a>) with type
   "signed_certificate_timestamp", or by using Online Certificate Status
   Protocol (OCSP) Stapling (also known as the "Certificate Status






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   Request" TLS extension; see [<a href="./rfc6066" title="&quot;Transport Layer Security (TLS) Extensions: Extension Definitions&quot;">RFC6066</a>]), where the response includes
   an OCSP extension with OID 1.3.6.1.4.1.11129.2.4.5 (see [<a href="./rfc2560" title="&quot;X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP&quot;">RFC2560</a>])
   and body:

       SignedCertificateTimestampList ::= OCTET STRING

   At least one SCT MUST be included.  Server operators MAY include more
   than one SCT.

   Similarly, a certificate authority MAY submit a Precertificate to
   more than one log, and all obtained SCTs can be directly embedded in
   the final certificate, by encoding the SignedCertificateTimestampList
   structure as an ASN.1 OCTET STRING and inserting the resulting data
   in the TBSCertificate as an X.509v3 certificate extension (OID
   1.3.6.1.4.1.11129.2.4.2).  Upon receiving the certificate, clients
   can reconstruct the original TBSCertificate to verify the SCT
   signature.

   The contents of the ASN.1 OCTET STRING embedded in an OCSP extension
   or X509v3 certificate extension are as follows:

        opaque SerializedSCT&lt;1..2^16-1&gt;;

        struct {
            SerializedSCT sct_list &lt;1..2^16-1&gt;;
        } SignedCertificateTimestampList;

   Here, "SerializedSCT" is an opaque byte string that contains the
   serialized TLS 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).

   Likewise, SCTs can be embedded in a TLS extension.  See below for
   details.

   TLS clients MUST implement all three mechanisms.  Servers MUST
   implement at least one of the three mechanisms.  Note that existing
   TLS servers can generally use the certificate extension mechanism
   without modification.

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







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<span class="h4"><a class="selflink" id="section-3.3.1" href="#section-3.3.1">3.3.1</a>.  TLS Extension</span>

   The SCT can be sent during the TLS handshake using a TLS extension
   with type "signed_certificate_timestamp".

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

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

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

<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>.  Merkle Tree</span>

   The hashing algorithm for the Merkle Tree Hash is SHA-256.

   Structure of the Merkle Tree input:

       enum { timestamped_entry(0), (255) }
         MerkleLeafType;

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

       struct {
           Version version;
           MerkleLeafType leaf_type;
           select (leaf_type) {
               case timestamped_entry: TimestampedEntry;
           }
       } MerkleTreeLeaf;

   Here, "version" is the version of the protocol to which the
   MerkleTreeLeaf corresponds.  This version is v1.





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   "leaf_type" is the type of the leaf input.  Currently, only
   "timestamped_entry" (corresponding to an SCT) is defined.  Future
   revisions of this protocol version may add new MerkleLeafType types.
   <a href="#section-4">Section 4</a> explains how clients should handle unknown leaf types.

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

   "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.

<span class="h3"><a class="selflink" id="section-3.5" href="#section-3.5">3.5</a>.  Signed Tree Head</span>

   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 <a href="#section-4.3">Section 4.3</a>).  The
   signature for that data is structured as follows:

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

   "version" is the version of the protocol to which the
   TreeHeadSignature conforms.  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.





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<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Log Client Messages</span>

   Messages are sent as HTTPS GET or POST requests.  Parameters for
   POSTs and all responses are encoded as JavaScript Object Notation
   (JSON) objects [<a href="./rfc4627" title="&quot;The application/json Media Type for JavaScript Object Notation (JSON)&quot;">RFC4627</a>].  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" [<a href="#ref-HTML401" title="&quot;HTML 4.01 Specification&quot;">HTML401</a>].  Binary data is base64 encoded [<a href="./rfc4648" title="&quot;The Base16, Base32, and Base64 Data Encodings&quot;">RFC4648</a>] 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 &lt;log server&gt; prefix can 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.

   Any errors will be returned as HTTP 4xx or 5xx responses, with human-
   readable error messages.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Add Chain to Log</span>

   POST https://&lt;log server&gt;/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.  Since log clients who request an
         SCT for inclusion in TLS handshakes are not required to verify
         it, we do not assume they know the ID of the log.

      timestamp:  The SCT timestamp, in decimal.






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      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.

   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.  (Note:
   Log clients don't need to be able to verify this structure; only TLS
   clients do.  If we were to serve the structure as a binary blob, then
   we could completely change it without requiring an upgrade to v1
   clients.)

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Add PreCertChain to Log</span>

   POST https://&lt;log server&gt;/ct/v1/add-pre-chain

   Inputs:

      chain:  An array of base64-encoded Precertificates.  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 are the same as in <a href="#section-4.1">Section 4.1</a>.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Retrieve Latest Signed Tree Head</span>

   GET https://&lt;log server&gt;/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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<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  Retrieve Merkle Consistency Proof between Two Signed Tree Heads</span>

   GET https://&lt;log server&gt;/ct/v1/get-sth-consistency

   Inputs:

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

      second:  The tree_size of the second tree, in decimal.

   Both tree sizes must be from existing v1 STHs (Signed Tree Heads).

   Outputs:

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

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

<span class="h3"><a class="selflink" id="section-4.5" href="#section-4.5">4.5</a>.  Retrieve Merkle Audit Proof from Log by Leaf Hash</span>

   GET https://&lt;log server&gt;/ct/v1/get-proof-by-hash

   Inputs:

      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 <a href="#section-3.4">Section 3.4</a>.  The
   "tree_size" must designate an existing v1 STH.

   Outputs:

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

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











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<span class="h3"><a class="selflink" id="section-4.6" href="#section-4.6">4.6</a>.  Retrieve Entries from Log</span>

   GET https://&lt;log server&gt;/ct/v1/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 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 PrecertChainEntry,
            this is the whole "PrecertChainEntry".

   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.  However, a compliant v1
   client MUST NOT construe an unrecognized MerkleLeafType or
   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 &lt;= x &lt;
   "tree_size" as returned by "get-sth" in <a href="#section-4.3">Section 4.3</a>.

   Logs MAY honor requests where 0 &lt;= "start" &lt; "tree_size" and "end" &gt;=
   "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".








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<span class="h3"><a class="selflink" id="section-4.7" href="#section-4.7">4.7</a>.  Retrieve Accepted Root Certificates</span>

   GET https://&lt;log server&gt;/ct/v1/get-roots

   No inputs.

   Outputs:

      certificates:  An array of base64-encoded root certificates that
         are acceptable to the log.

<span class="h3"><a class="selflink" id="section-4.8" href="#section-4.8">4.8</a>.  Retrieve Entry+Merkle Audit Proof from Log</span>

   GET https://&lt;log server&gt;/ct/v1/get-entry-and-proof

   Inputs:

      leaf_index:  The index of the desired entry.

      tree_size:  The tree_size of the tree for which the proof is
         desired.

   The tree size must designate an existing STH.

   Outputs:

      leaf_input:  The base64-encoded MerkleTreeLeaf structure.

      extra_data:  The base64-encoded unsigned data, same as in
         <a href="#section-4.6">Section 4.6</a>.

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

   This API is probably only useful for debugging.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Clients</span>

   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 should gossip with each other, exchanging STHs at least;
   this is all that is required to ensure that they all have a
   consistent view.  The exact mechanism for gossip will be described in
   a separate document, but it is expected there will be a variety.



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<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Submitters</span>

   Submitters submit certificates or Precertificates to the log as
   described above.  They may go on to use the returned SCT to construct
   a certificate or use it directly in a TLS handshake.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  TLS Client</span>

   TLS clients are not directly clients of the log, but they receive
   SCTs alongside or in server certificates.  In addition to normal
   validation of the certificate and its chain, they 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.  Note that this document does not describe how
   clients obtain the logs' public keys.

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

<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  Monitor</span>

   Monitors watch logs and check that they behave correctly.  They also
   watch for certificates of interest.

   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 (<a href="#section-4.3">Section 4.3</a>).

   2.  Verify the STH signature.

   3.  Fetch all the entries in the tree corresponding to the STH
       (<a href="#section-4.6">Section 4.6</a>).

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

   5.  Fetch the current STH (<a href="#section-4.3">Section 4.3</a>).  Repeat until the STH
       changes.

   6.  Verify the STH signature.

   7.  Fetch all the new entries in the tree corresponding to the STH
       (<a href="#section-4.6">Section 4.6</a>).  If they remain unavailable for an extended
       period, then this should be viewed as misbehavior on the part of
       the log.





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   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:

       2.  Fetch a consistency proof for the new STH with the previous
           STH (<a href="#section-4.4">Section 4.4</a>).

       3.  Verify the consistency proof.

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

   9.  Go to Step 5.

<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>.  Auditor</span>

   Auditors take partial information about a log as input and verify
   that this information is consistent with other partial information
   they have.  An auditor might be an integral component of a TLS
   client; it might be a standalone service; or it might be a secondary
   function of a monitor.

   Any pair of STHs from the same log can be verified by requesting a
   consistency proof (<a href="#section-4.4">Section 4.4</a>).

   A certificate accompanied by an SCT can be verified against any STH
   dated after the SCT timestamp + the Maximum Merge Delay by requesting
   a Merkle audit proof (<a href="#section-4.5">Section 4.5</a>).

   Auditors can fetch STHs from time to time of their own accord, of
   course (<a href="#section-4.3">Section 4.3</a>).

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  IANA Considerations</span>

   IANA has allocated an <a href="./rfc5246">RFC 5246</a> ExtensionType value (18) for the SCT
   TLS extension.  The extension name is "signed_certificate_timestamp".

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Security Considerations</span>

   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 the certificate has been published in a log.  From this,
   the client knows that the subject of the certificate has had some



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   time to notice the misissue and take some action, such as asking a CA
   to revoke a misissued certificate.  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.

   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.

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  Misissued Certificates</span>

   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.

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  Detection of Misissue</span>

   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.

<span class="h3"><a class="selflink" id="section-7.3" href="#section-7.3">7.3</a>.  Misbehaving Logs</span>

   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
   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 is detected by global
   gossiping, i.e., everyone auditing logs comparing their versions of
   the latest Signed Tree Heads.  As soon as two conflicting Signed Tree



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   Heads for the same log are detected, this is cryptographic proof of
   that log's misbehavior.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Efficiency Considerations</span>

   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.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Future Changes</span>

   This section lists things we might address in a Standards Track
   version of this document.

   o  Rather than forcing a log operator to create a new log in order to
      change the log signing key, we may allow some key roll mechanism.

   o  We may add hash and signing algorithm agility.

   o  We may describe some gossip protocols.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Acknowledgements</span>

   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, Rob Stradling, and Carl Wallace for their
   valuable contributions.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  References</span>

<span class="h3"><a class="selflink" id="section-11.1" href="#section-11.1">11.1</a>.  Normative Reference</span>

   [<a id="ref-RFC2119">RFC2119</a>]        Bradner, S., "Key words for use in RFCs to Indicate
                    Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.









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<span class="h3"><a class="selflink" id="section-11.2" href="#section-11.2">11.2</a>.  Informative References</span>

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

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

   [<a id="ref-FIPS.180-4">FIPS.180-4</a>]     National Institute of Standards and Technology,
                    "Secure Hash Standard", FIPS PUB 180-4, March 2012,
                    &lt;<a href="http://csrc.nist.gov/publications/fips/fips180-4/fips-180-4.pdf">http://csrc.nist.gov/publications/fips/fips180-4/</a>
                    <a href="http://csrc.nist.gov/publications/fips/fips180-4/fips-180-4.pdf">fips-180-4.pdf</a>&gt;.

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

   [<a id="ref-RFC2560">RFC2560</a>]        Myers, M., Ankney, R., Malpani, A., Galperin, S.,
                    and C. Adams, "X.509 Internet Public Key
                    Infrastructure Online Certificate Status Protocol -
                    OCSP", <a href="./rfc2560">RFC 2560</a>, June 1999.

   [<a id="ref-RFC3447">RFC3447</a>]        Jonsson, J. and B. Kaliski, "Public-Key Cryptography
                    Standards (PKCS) #1: RSA Cryptography Specifications
                    Version 2.1", <a href="./rfc3447">RFC 3447</a>, February 2003.

   [<a id="ref-RFC4627">RFC4627</a>]        Crockford, D., "The application/json Media Type for
                    JavaScript Object Notation (JSON)", <a href="./rfc4627">RFC 4627</a>,
                    July 2006.

   [<a id="ref-RFC4648">RFC4648</a>]        Josefsson, S., "The Base16, Base32, and Base64 Data
                    Encodings", <a href="./rfc4648">RFC 4648</a>, October 2006.

   [<a id="ref-RFC5246">RFC5246</a>]        Dierks, T. and E. Rescorla, "The Transport Layer
                    Security (TLS) Protocol Version 1.2", <a href="./rfc5246">RFC 5246</a>,
                    August 2008.

   [<a id="ref-RFC5280">RFC5280</a>]        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", <a href="./rfc5280">RFC 5280</a>, May 2008.




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   [<a id="ref-RFC5905">RFC5905</a>]        Mills, D., Martin, J., Burbank, J., and W. Kasch,
                    "Network Time Protocol Version 4: Protocol and
                    Algorithms Specification", <a href="./rfc5905">RFC 5905</a>, June 2010.

   [<a id="ref-RFC6066">RFC6066</a>]        Eastlake, D., "Transport Layer Security (TLS)
                    Extensions: Extension Definitions", <a href="./rfc6066">RFC 6066</a>,
                    January 2011.

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

























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