rfc6962-bis.xml

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      'http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml'>
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<?rfc symrefs="yes"?>
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<?rfc compact="yes"?>
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<?rfc sortrefs="yes" ?>

<rfc ipr="trust200902" category="std" submissionType="IETF" docName="draft-ietf-trans-rfc6962-bis-12">

  <front>
    <title>Certificate Transparency</title>
    <author initials="B" surname="Laurie" fullname="Ben Laurie">
      <organization abbrev="Google">
        Google UK Ltd.
      </organization>
      <address>
        <email>benl@google.com</email>
      </address>
    </author>
    <author initials="A" surname="Langley" fullname="Adam Langley">
      <organization abbrev="Google">
        Google Inc.
      </organization>
      <address>
        <email>agl@google.com</email>
      </address>
    </author>
    <author initials="E" surname="Kasper" fullname="Emilia Kasper">
      <organization abbrev="Google">
        Google Switzerland GmbH
      </organization>
      <address>
        <email>ekasper@google.com</email>
      </address>
    </author>
    <author initials="E" surname="Messeri" fullname="Eran Messeri">
      <organization abbrev="Google">
        Google UK Ltd.
      </organization>
      <address>
        <email>eranm@google.com</email>
      </address>
    </author>
    <author initials="R" surname="Stradling" fullname="Rob Stradling">
      <organization abbrev="Comodo">
        Comodo CA, Ltd.
      </organization>
      <address>
        <email>rob.stradling@comodo.com</email>
      </address>
    </author>
    <date />
    <workgroup>Public Notary Transparency Working Group</workgroup>

    <abstract>
      <t>
        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.
      </t>
      <t>
        Logs are network services that implement the protocol operations for
submissions and queries that are defined in this document.
      </t>
    </abstract>
  </front>
  <middle>
    <section title="Introduction">
      <t>
        Certificate transparency aims to mitigate the problem of misissued
certificates by providing append-only logs of issued certificates. The logs
do not need to be trusted because they are publicly auditable. Anyone may verify
the correctness of each log and monitor when new certificates are added to it.
The logs do not themselves prevent misissue, but
they ensure that interested parties (particularly those named in certificates)
can detect such misissuance. Note that this is a general mechanism, but in this
document, we only describe its use for public TLS server certificates issued by
public certification authorities (CAs).
      </t>
      <t>
        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 are accompanied by signed timestamps.
      </t>
      <t>
        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.
      </t>
      <t>
        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.
      </t>
      <t>
        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.
      </t>
      <section title="Requirements Language">
        <t>
          The key words &quot;MUST&quot;, &quot;MUST NOT&quot;, &quot;REQUIRED&quot;, &quot;SHALL&quot;, &quot;SHALL NOT&quot;, &quot;SHOULD&quot;, &quot;SHOULD NOT&quot;, &quot;RECOMMENDED&quot;, &quot;MAY&quot;, and &quot;OPTIONAL&quot; in this document are to be interpreted as described in <xref target="RFC2119">RFC 2119</xref>.
        </t>
      </section>
      <section title="Data Structures">
        <t>
          Data structures are defined according to the conventions laid out in
Section 4 of <xref target="RFC5246"/>.
        </t>
      </section>
    </section>
    <section title="Cryptographic Components">
      <section title="Merkle Hash Trees" anchor="mht">
        <t>
          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 <xref target="hash_algorithms"/>. The hashing algorithm in use is referred to as HASH throughout this document and the size of its output in bytes as HASH_SIZE. The input to the Merkle Tree Hash is a list
of data entries; these entries will be hashed to form the leaves of the Merkle
Hash Tree. The output is a single HASH_SIZE Merkle Tree Hash. Given an ordered
list of n inputs, D[n] = {d(0), d(1), ..., d(n-1)}, the Merkle Tree Hash (MTH)
is thus defined as follows:
        </t>
        <t>
          The hash of an empty list is the hash of an empty string:
          <vspace blankLines='1' />
          MTH({}) = HASH().
          <vspace blankLines='1' />
          The hash of a list with one entry (also known as a leaf hash) is:
          <vspace blankLines='1' />
          MTH({d(0)}) = HASH(0x00 || d(0)).
          <vspace blankLines='1' />
          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
          <vspace blankLines='1' />
          MTH(D[n]) = HASH(0x01 || MTH(D[0:k]) || MTH(D[k:n])),
          <vspace blankLines='1' />
          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.)
        </t>
        <t>
          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 <xref target='CrosbyWallach'>history tree</xref>
proposal, except our definition handles non-full trees differently.)
        </t>
      <section title="Merkle Inclusion Proofs" anchor='merkle_inclusion_proof'>
        <t>
          A Merkle inclusion proof for a leaf in a Merkle Hash Tree is the shortest
list of additional nodes in the Merkle Tree required to compute the Merkle Tree
Hash for that tree. Each node in the tree is either a leaf node or is computed
from the two nodes immediately below it (i.e., towards the leaves). At each
step up the tree (towards the root), a node from the inclusion proof is combined
with the node computed so far. In other words, the inclusion proof consists of the
list of missing nodes required to compute the nodes leading from a leaf to the
root of the tree. If the root computed from the inclusion proof matches the true
root, then the inclusion proof proves that the leaf exists in the tree.
        </t>
        <t> 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 &lt;= m &lt; n, is defined as follows:
        </t>
        <t>
          The proof for the single leaf in a tree with a one-element input list D[1] = {d(0)} is empty:
          <vspace blankLines='1' />
          PATH(0, {d(0)}) = {}
          <vspace blankLines='1' />
          For n &gt; 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 &gt; m elements is then defined recursively as
          <vspace blankLines='1' />
          PATH(m, D[n]) = PATH(m, D[0:k]) : MTH(D[k:n]) for m &lt; k; and
          <vspace blankLines='1' />
          PATH(m, D[n]) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for m &gt;= k,
          <vspace blankLines='1' />
          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.
        </t>
      </section>

      <section title="Merkle Consistency Proofs" anchor='consistency'>
        <t>
          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.
        </t>
        <t>
          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:
          <vspace blankLines='1' />
          PROOF(m, D[n]) = SUBPROOF(m, D[n], true)
          <vspace blankLines='1' />
          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):
          <vspace blankLines='1' />
          SUBPROOF(m, D[m], true) = {}
          <vspace blankLines='1' />
           The subproof for m = n is the Merkle Tree Hash committing inputs D[0:m]; otherwise:
          <vspace blankLines='1' />
          SUBPROOF(m, D[m], false) = {MTH(D[m])}
          <vspace blankLines='1' />
          For m &lt; n, let k be the largest power of two smaller than n. The subproof is then defined recursively.
          <vspace blankLines='1' />
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]:
          <vspace blankLines='1' />
          SUBPROOF(m, D[n], b) = SUBPROOF(m, D[0:k], b) : MTH(D[k:n])
          <vspace blankLines='1' />
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].
          <vspace blankLines='1' />
          SUBPROOF(m, D[n], b) = SUBPROOF(m - k, D[k:n], false) : MTH(D[0:k])
          <vspace blankLines='1' />
          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.
        </t>
        <t>
          The number of nodes in the resulting proof is bounded above by ceil(log2(n)) + 1.
        </t>
      </section>

        <section title="Example">
          <figure>
            <preamble>
              The binary Merkle Tree with 7 leaves:
            </preamble>
            <artwork>
            hash
           /    \
          /      \
         /        \
        /          \
       /            \
      k              l
     / \            / \
    /   \          /   \
   /     \        /     \
  g       h      i      j
 / \     / \    / \     |
 a b     c d    e f     d6
 | |     | |    | |
d0 d1   d2 d3  d4 d5</artwork>
          </figure>
          <t>
            The inclusion proof for d0 is [b, h, l].
          </t>
          <t>
            The inclusion proof for d3 is [c, g, l].
          </t>
          <t>
            The inclusion proof for d4 is [f, j, k].
          </t>
          <t>
            The inclusion proof for d6 is [i, k].
          </t>
          <figure>
            <preamble>
              The same tree, built incrementally in four steps:
            </preamble>
            <artwork>
    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</artwork>
          </figure>
          <t>
            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.
          </t>
          <t>
            The consistency proof between hash1 and hash is PROOF(4, D[7]) =
[l]. hash can be verified using hash1=k and l.
          </t>
          <t>
            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.
          </t>
        </section>
        <section title="Signatures" anchor="signatures">
          <t>
            Various data structures are signed. A log MUST use one of the signature algorithms defined in the <xref target="signature_algorithms"/> section.
          </t>
        </section>
      </section>
    </section>
    <section title="Submitters">
      <t>
        Submitters submit certificates or precertificates to logs for public auditing, as described below. In order to enable attribution of each logged certificate or precertificate to its issuer, each submission MUST be accompanied by all additional certificates required to verify the chain up to an accepted root certificate. The root certificate itself MAY be omitted from the submission.
      </t>
      <t>
        If a log accepts a submission, it will return a Signed Certificate Timestamp (SCT) (see <xref target="sct"/>). The submitter SHOULD validate the returned SCT as described in <xref target="tls_clients"/> if they understand its format and they intend to use it directly in a TLS handshake or to construct a certificate.
      </t>
      <section title="Certificates">
        <t>
          Anyone can <xref target="add-chain">submit a certificate</xref> to a log. Since certificates may not be accepted by TLS clients unless logged, it is expected that certificate owners or their CAs will usually submit them.
        </t>
      </section>
      <section title="Precertificates" anchor="Precertificates">
        <t>
          Alternatively, (root as well as intermediate) CAs may preannounce a certificate prior to issuance by <xref target="add-pre-chain">submitting a precertificate</xref> that the log can use to create an entry that will be valid against the issued certificate. The CA MAY incorporate the returned SCT in the issued certificate.
        </t>
        <t>
          A precertificate is a CMS <xref target="RFC5652"/> <spanx style="verb">signed-data</spanx> object that conforms to the following requirements:
          <list style="symbols">
            <t>
              It MUST be DER encoded.
            </t>
            <t>
              <spanx style="verb">SignedData.encapContentInfo.eContentType</spanx> MUST be the OID &lt;TBD&gt;.
            </t>
            <t>
              <spanx style="verb">SignedData.encapContentInfo.eContent</spanx> MUST contain a TBSCertificate <xref target="RFC5280"/>, which MAY redact certain domain name labels that will be present in the issued certificate (see <xref target="redacting_subdomains"/>) and MUST NOT contain any SCTs, but which will be otherwise identical to the TBSCertificate in the issued certificate.
            </t>
            <t>
              <spanx style="verb">SignedData.signerInfos</spanx> MUST contain a signature from the same (root or intermediate) CA that will ultimately issue the certificate. This signature indicates the CA's intent to issue the certificate. This intent is considered binding (i.e. misissuance of the precertificate is considered equivalent to misissuance of the certificate). (Note that, because of the structure of CMS, the signature on the CMS object will not be a valid X.509v3 signature and so cannot be used to construct a certificate from the precertificate).
            </t>
            <t>
              <spanx style="verb">SignedData.certificates</spanx> SHOULD be omitted.
            </t>
          </list>
        </t>
      </section>
    </section>
    <section title="Private Domain Name Labels">
      <t>
        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.
      </t>
      <section title="Wildcard Certificates">
        <t>
          A certificate containing a <xref target="RFC6125">DNS-ID</xref> of <spanx style="verb">*.example.com</spanx> could be used to secure the domain <spanx style="verb">topsecret.example.com</spanx>, without revealing the string <spanx style="verb">topsecret</spanx> publicly.
        </t>
        <t>
          Since TLS clients only match the wildcard character to the complete leftmost label of the DNS domain name (see Section 6.4.3 of <xref target="RFC6125"/>), this approach would not work for a DNS-ID such as <spanx style="verb">top.secret.example.com</spanx>. Also, wildcard certificates are prohibited in some cases, such as <xref target="EVSSLGuidelines">Extended Validation Certificates</xref>.
        </t>
      </section>
      <section title="Redacting Domain Name Labels in Precertificates" anchor="redacting_subdomains">
        <t>
          When creating a precertificate, the CA MAY substitute one or more labels in each DNS-ID with a corresponding number of <spanx style="verb">?</spanx> labels. Every label to the left of a <spanx style="verb">?</spanx> label MUST also be redacted. For example, if a certificate contains a DNS-ID of <spanx style="verb">top.secret.example.com</spanx>, then the corresponding precertificate could contain <spanx style="verb">?.?.example.com</spanx> instead, but not <spanx style="verb">top.?.example.com</spanx> instead.
        </t>
        <t>
          Wildcard <spanx style="verb">*</spanx> labels MUST NOT be redacted. However, if the complete leftmost label of a DNS-ID is <spanx style="verb">*</spanx>, 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 <spanx style="verb">*.top.secret.example.com</spanx>, then the corresponding precertificate could contain <spanx style="verb">*.?.?.example.com</spanx> instead, but not <spanx style="verb">?.?.?.example.com</spanx> instead.
        </t>
        <t>
          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 <spanx style="verb">*</spanx> labels in the 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.
        </t>
        <t>
          When a precertificate contains that extension and contains a <xref target="RFC6125">CN-ID</xref>, 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.
        </t>
      </section>
      <section title="Using a Name-Constrained Intermediate CA" anchor="name_constrained">
        <t>
          An intermediate CA certificate or intermediate CA precertificate that contains the critical or non-critical <xref target="RFC5280">Name Constraints</xref> 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:
          <list style="symbols">
            <t>
              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.
            </t>
            <t>
              permittedSubtrees MUST specify one or more dNSNames.
            </t>
            <t>
              excludedSubtrees MUST specify the entire IPv4 and IPv6 address ranges.
            </t>
          </list>
        </t>
        <figure>
          <preamble>
            Below is an example Name Constraints extension that meets these conditions:
          </preamble>
          <artwork>
    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
              }
            }
          }
        }
      }</artwork>
        </figure>
      </section>
    </section>
    <section title="Log Format and Operation">
      <t>
        A log is a single, append-only Merkle Tree of submitted certificate and precertificate entries.
      </t>
      <t>
        When it receives a valid submission, the log MUST return an SCT that corresponds to the submitted certificate or precertificate. If the log has previously seen this valid submission, it 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 of type <spanx style="verb">precert_sct</spanx> would not be appropriate; instead, a fresh SCT of type <spanx style="verb">x509_sct</spanx> should be generated).
      </t>
      <t>
        An SCT is the log's promise to incorporate the submitted entry in its Merkle Tree no later than a fixed amount of time, known as the Maximum Merge Delay (MMD), after the issuance of the SCT. Periodically, the log MUST append all its new entries to its Merkle Tree and sign the root of the tree.
      </t>
      <t>
        Log operators MUST NOT impose any conditions on retrieving or sharing data from the log.
      </t>
      <section title="Accepting Submissions">
        <t>
          Logs MUST verify that each 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 and precertificates that are fully valid according to <xref target="RFC5280">RFC 5280</xref> 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 RFC 5280 verification rules in order to accommodate quirks of CA certificate-issuing software. However, logs MUST reject submissions without a valid signature chain to an accepted root certificate. Logs MUST also reject precertificates that do not conform to the requirements in <xref target="Precertificates"/>.
        </t>
        <t>
          Logs SHOULD limit the length of chain they will accept. The maximum chain length is specified in the log's metadata.
        </t>
        <t>
          The log SHALL allow retrieval of its list of accepted root certificates (see <xref target="get-roots"/>). This list might usefully be the union of root certificates trusted by major browser vendors.
        </t>
      </section>
      <section title="Log Entries" anchor="log_entries">
        <t>
          If a submission is accepted and an SCT issued, the accepting log MUST store the entire chain used for verification. This chain MUST include the certificate or precertificate itself, the zero or more intermediate CA certificates provided by the submitter, and the root certificate used to verify the chain (even if it was omitted from the submission). The log MUST present this chain for auditing upon request (see <xref target="get-entries"/>). This chain is required to prevent a CA from avoiding blame by logging a partial or empty chain.
        </t>
        <figure>
          <preamble>
            Each certificate entry in a log MUST include a <spanx style="verb">X509ChainEntry</spanx> structure, and each precertificate entry MUST include a <spanx style="verb">PrecertChainEntryV2</spanx> structure:
          </preamble>
          <artwork>
    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;

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

    struct {
        CMSPrecert pre_certificate;
        ASN.1Cert precertificate_chain&lt;1..2^24-1&gt;;
    } PrecertChainEntryV2;</artwork>
        </figure>
        <t>
          <spanx style="verb">leaf_certificate</spanx> is a submitted certificate that has been accepted by the log.
        </t>
        <t>
          <spanx style="verb">certificate_chain</spanx> is a vector of 0 or more additional certificates required to verify <spanx style="verb">leaf_certificate</spanx>. The first certificate MUST certify <spanx style="verb">leaf_certificate</spanx>. Each following certificate MUST directly certify the one preceding it. The final certificate MUST be a root certificate accepted by the log. If <spanx style="verb">leaf_certificate</spanx> is a root certificate, then this vector is empty.
        </t>
        <t>
          <spanx style="verb">pre_certificate</spanx> is a submitted precertificate that has been accepted by the log.
        </t>
        <t>
          <spanx style="verb">precertificate_chain</spanx> is a vector of 1 or more additional certificates required to verify <spanx style="verb">pre_certificate</spanx>. The first certificate MUST certify <spanx style="verb">pre_certificate</spanx>. Each following certificate MUST directly certify the one preceding it. The final certificate MUST be a root certificate accepted by the log.
        </t>
      </section>
      <section title="Log ID" anchor="log_id">
        <figure>
          <preamble>
            Each log's operator allocates an OID for the purpose of uniquely identifying that log. This OID is specified in the log's metadata. Various data structures include the DER encoding of this OID, excluding the ASN.1 tag and length bytes, in an opaque vector:
          </preamble>
          <artwork>
    opaque LogID&lt;2..127&gt;;</artwork>
        </figure>
        <t>
          Note that the ASN.1 length and the opaque vector length are identical in size (1 byte) and value, so the DER encoding of the OID can be reproduced simply by prepending an OBJECT IDENTIFIER tag (0x06) to the opaque vector length and contents.
        </t>
      </section>
      <section title="The TransItem Structure">
        <figure>
          <preamble>
            Various data structures produced by logs are encapsulated in the <spanx style="verb">TransItem</spanx> structure to ensure that the type and version of each one is identified in a common fashion:
          </preamble>
          <artwork>
    enum {
        v1(0), v2(1), (255)
    } Version;

    enum {
        x509_entry(0), precert_entry(1), x509_sct(2), precert_sct(3),
        tree_head(4), signed_tree_head(5), consistency_proof(6),
        inclusion_proof(7), (65535)
    } TransType;

    enum {
        reserved(65535)
    } ItemExtensionType;

    struct {
        ItemExtensionType item_extension_type;
        opaque item_extension_data&lt;0..2^16-1&gt;;
    } ItemExtension;

    struct {
        TransType type;
        select (type) {
            case x509_entry: TimestampedCertificateEntryDataV2;
            case precert_entry: TimestampedCertificateEntryDataV2;
            case x509_sct: SignedCertificateTimestampDataV2;
            case precert_sct: SignedCertificateTimestampDataV2;
            case tree_head: TreeHeadDataV2;
            case signed_tree_head: SignedTreeHeadDataV2;
            case consistency_proof: ConsistencyProofDataV2;
            case inclusion_proof: InclusionProofDataV2;
        } data;
        ItemExtension item_extensions&lt;0..2^16-1&gt;;
    } TransItemV2;

    struct {
        Version version;
        select (version) {
            case v1: TransItemV1;
            case v2: TransItemV2;
        }
    } TransItem;</artwork>
        </figure>
        <t>
          <spanx style="verb">version</spanx> is the earliest version of this protocol to which the encapsulated data structure conforms. This document is v2. Note that <xref target="RFC6962">v1</xref> did not define <spanx style="verb">TransItem</spanx>, but this document specifies a mechanism (see <xref target="TransItemV1"/>) for v2 implementations to encapsulate existing v1 objects in the <spanx style="verb">TransItem</spanx> structure. Note also that, since each <spanx style="verb">TransItem</spanx> object is individually versioned, future revisions to this protocol could conceivably update some encapsulated data structures without having to update all of them.
        </t>
        <t>
          <spanx style="verb">type</spanx> is the type of the encapsulated data structure. (Note that <spanx style="verb">TransType</spanx> combines the v1 type enumerations <spanx style="verb">LogEntryType</spanx>, <spanx style="verb">SignatureType</spanx> and <spanx style="verb">MerkleLeafType</spanx>). Future revisions of this protocol may add new <spanx style="verb">TransType</spanx> values.
        </t>
        <t>
          <spanx style="verb">data</spanx> is the encapsulated data structure. The various structures named with the <spanx style="verb">DataV2</spanx> suffix are defined in later sections of this document.
        </t>
        <t>
          <spanx style="verb">item_extension_type</spanx> identifies a single extension from the IANA registry in <xref target="item_extension_types"/>.
        </t>
        <t>
          The interpretation of the <spanx style="verb">item_extension_data</spanx> field is determined solely by the value of the <spanx style="verb">item_extension_type</spanx> field. Each document that registers a new <spanx style="verb">item_extension_type</spanx> must describe how to interpret the corresponding <spanx style="verb">item_extension_data</spanx>.
        </t>
        <t>
          <spanx style="verb">item_extensions</spanx> is a vector of 0 or more item extensions. This vector MUST NOT include more than one extension with the same <spanx style="verb">item_extension_type</spanx>. The extensions in the vector MUST be ordered by the value of the <spanx style="verb">item_extension_type</spanx> field, smallest value first.
        </t>
      </section>
      <section title="Merkle Tree Leaves" anchor="tree_leaves">
        <figure>
          <preamble>
            The leaves of a log's Merkle Tree correspond to the log's entries (see <xref target="log_entries"/>). Each leaf is the <xref target="mht">leaf hash</xref> of a <spanx style="verb">TransItem</spanx> structure of type <spanx style="verb">x509_entry</spanx> or <spanx style="verb">precert_entry</spanx>, which in this version (v2) encapsulates a <spanx style="verb">TimestampedCertificateEntryDataV2</spanx> structure. Note that leaf hashes are calculated as HASH(0x00 || TransItem), where the hashing algorithm is specified in the log's metadata.
          </preamble>
          <artwork>
    opaque TBSCertificate&lt;1..2^24-1&gt;;

    struct {
        uint64 timestamp;
        opaque issuer_key_hash[HASH_SIZE];
        TBSCertificate tbs_certificate;
        SctExtension sct_extensions&lt;0..2^16-1&gt;;
    } TimestampedCertificateEntryDataV2;</artwork>
        </figure>
        <t>
          <spanx style="verb">timestamp</spanx> is the <xref target="RFC5905">NTP Time</xref> at which the certificate or precertificate was accepted by the log, measured in milliseconds since the epoch (January 1, 1970, 00:00), ignoring leap seconds.
        </t>
        <t>
          <spanx style="verb">issuer_key_hash</spanx> is the HASH of the public key of the CA that issued the certificate or precertificate, calculated over the DER encoding of the key represented as <xref target="RFC5280">SubjectPublicKeyInfo</xref>. This is needed to bind the CA to the certificate or precertificate, making it impossible for the corresponding SCT to be valid for any other certificate or precertificate whose TBSCertificate matches <spanx style="verb">tbs_certificate</spanx>.
        </t>
        <t>
          <spanx style="verb">tbs_certificate</spanx> is the DER encoded TBSCertificate from either the <spanx style="verb">leaf_certificate</spanx> (in the case of an <spanx style="verb">X509ChainEntry</spanx>) or the <spanx style="verb">pre_certificate</spanx> (in the case of a <spanx style="verb">PrecertChainEntryV2</spanx>). (Note that a precertificate's TBSCertificate can be reconstructed from the issued certificate's TBSCertificate by redacting the domain name labels indicated by the redacted labels extension, and deleting the SCT list extension and redacted labels extension).
        </t>
        <t>
          <spanx style="verb">sct_extensions</spanx> matches the SCT extensions of the corresponding SCT.
        </t>
      </section>
      <section title="Signed Certificate Timestamp (SCT)" anchor="sct">
        <figure>
          <preamble>
            An SCT is a <spanx style="verb">TransItem</spanx> structure of type <spanx style="verb">x509_sct</spanx> or <spanx style="verb">precert_sct</spanx>, which in this version (v2) encapsulates a <spanx style="verb">SignedCertificateTimestampDataV2</spanx> structure:
          </preamble>
          <artwork>
    enum {
        reserved(65535)
    } SctExtensionType;

    struct {
        SctExtensionType sct_extension_type;
        opaque sct_extension_data&lt;0..2^16-1&gt;;
    } SctExtension;

    struct {
        LogID log_id;
        uint64 timestamp;
        SctExtension sct_extensions&lt;0..2^16-1&gt;;
        digitally-signed struct {
            TransItem timestamped_entry;
        } signature;
    } SignedCertificateTimestampDataV2;</artwork>
        </figure>
        <t>
          <spanx style="verb">log_id</spanx> is this log's unique ID, encoded in an opaque vector as described in <xref target="log_id"/>.
        </t>
        <t>
          <spanx style="verb">timestamp</spanx> is equal to the timestamp from the <spanx style="verb">TimestampedCertificateEntryDataV2</spanx> structure encapsulated in the <spanx style="verb">timestamped_entry</spanx>.
        </t>
        <t>
          <spanx style="verb">sct_extension_type</spanx> identifies a single extension from the IANA registry in <xref target="sct_extension_types"/>. At the time of writing, no extensions are specified.
        </t>
        <t>
          The interpretation of the <spanx style="verb">sct_extension_data</spanx> field is determined solely by the value of the <spanx style="verb">sct_extension_type</spanx> field. Each document that registers a new <spanx style="verb">sct_extension_type</spanx> must describe how to interpret the corresponding <spanx style="verb">sct_extension_data</spanx>.
        </t>
        <t>
          <spanx style="verb">sct_extensions</spanx> is a vector of 0 or more SCT extensions. This vector MUST NOT include more than one extension with the same <spanx style="verb">sct_extension_type</spanx>. The extensions in the vector MUST be ordered by the value of the <spanx style="verb">sct_extension_type</spanx> field, smallest value first. If an implementation sees an extension that it does not understand, it SHOULD ignore that extension. Furthermore, an implementation MAY choose to ignore any extension(s) that it does understand.
        </t>
        <t>
          The encoding of the digitally-signed element is defined in <xref target='RFC5246'/>.
        </t>
        <t>
          <spanx style="verb">timestamped_entry</spanx> is a <spanx style="verb">TransItem</spanx> structure that MUST be of type <spanx style="verb">x509_entry</spanx> or <spanx style="verb">precert_entry</spanx> (see <xref target="tree_leaves"/>) and MUST have an empty <spanx style="verb">item_extensions</spanx> vector.
        </t>
      </section>
      <section title="Merkle Tree Head" anchor="tree_head">
        <figure>
          <preamble>
            The log stores information about its Merkle Tree in a <spanx style="verb">TransItem</spanx> structure of type <spanx style="verb">tree_head</spanx>, which in this version (v2) encapsulates a <spanx style="verb">TreeHeadDataV2</spanx> structure:
          </preamble>
          <artwork>
    opaque NodeHash[HASH_SIZE];

    struct {
        uint64 timestamp;
        uint64 tree_size;
        NodeHash root_hash;
        SthExtension sth_extensions&lt;0..2^16-1&gt;;
    } TreeHeadDataV2;</artwork>
        </figure>
        <t>
          <spanx style="verb">timestamp</spanx> is the current <xref target="RFC5905">NTP Time</xref>, measured in milliseconds since the epoch (January 1, 1970, 00:00), ignoring leap seconds.
        </t>
        <t>
          <spanx style="verb">tree_size</spanx> is the number of entries currently in the log's Merkle Tree.
        </t>
        <t>
          <spanx style="verb">root_hash</spanx> is the root of the Merkle Hash Tree.
        </t>
        <t>
          <spanx style="verb">sth_extensions</spanx> matches the STH extensions of the corresponding STH.
        </t>
      </section>
      <section title="Signed Tree Head (STH)" anchor="STH">
        <t>
          Periodically each log SHOULD sign its current tree head information (see <xref target="tree_head"/>) to produce an STH. When a client requests a log's latest STH (see <xref target="get-sth"/>), the log MUST return an STH that is no older than the log's MMD. However, STHs could be used to mark individual clients (by producing a new one for each query), so logs MUST NOT produce them more frequently than is declared in their metadata. In general, there is no need to produce a new STH unless there are new entries in the log; however, in the unlikely event that it receives no new submissions during an MMD period, the log SHALL sign the same Merkle Tree Hash with a fresh timestamp.
        </t>
        <figure>
          <preamble>
            An STH is a <spanx style="verb">TransItem</spanx> structure of type <spanx style="verb">signed_tree_head</spanx>, which in this version (v2) encapsulates a <spanx style="verb">SignedTreeHeadDataV2</spanx> structure:
          </preamble>
          <artwork>
    enum {
        reserved(65535)
    } SthExtensionType;

    struct {
        SthExtensionType sth_extension_type;
        opaque sth_extension_data&lt;0..2^16-1&gt;;
    } SthExtension;

    struct {
        LogID log_id;
        uint64 timestamp;
        uint64 tree_size;
        NodeHash root_hash;
        SthExtension sth_extensions&lt;0..2^16-1&gt;;
        digitally-signed struct {
            TransItem merkle_tree_head;
        } signature;
    } SignedTreeHeadDataV2;</artwork>
        </figure>
        <t>
          <spanx style="verb">log_id</spanx> is this log's unique ID, encoded in an opaque vector as described in <xref target="log_id"/>.
        </t>
        <t>
          <spanx style="verb">timestamp</spanx> is equal to the timestamp from the <spanx style="verb">TreeHeadDataV2</spanx> structure encapsulated in <spanx style="verb">merkle_tree_head</spanx>. This 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.
        </t>
        <t>
          <spanx style="verb">tree_size</spanx> is equal to the tree size from the <spanx style="verb">TreeHeadDataV2</spanx> structure encapsulated in <spanx style="verb">merkle_tree_head</spanx>.
        </t>
        <t>
          <spanx style="verb">root_hash</spanx> is equal to the root hash from the <spanx style="verb">TreeHeadDataV2</spanx> structure encapsulated in <spanx style="verb">merkle_tree_head</spanx>.
        </t>
        <t>
          <spanx style="verb">sth_extension_type</spanx> identifies a single extension from the IANA registry in <xref target="sth_extension_types"/>. At the time of writing, no extensions are specified.
        </t>
        <t>
          The interpretation of the <spanx style="verb">sth_extension_data</spanx> field is determined solely by the value of the <spanx style="verb">sth_extension_type</spanx> field. Each document that registers a new <spanx style="verb">sth_extension_type</spanx> must describe how to interpret the corresponding <spanx style="verb">sth_extension_data</spanx>.
        </t>
        <t>
          <spanx style="verb">sth_extensions</spanx> is a vector of 0 or more STH extensions. This vector MUST NOT include more than one extension with the same <spanx style="verb">sth_extension_type</spanx>. The extensions in the vector MUST be ordered by the value of the <spanx style="verb">sth_extension_type</spanx> field, smallest value first. If an implementation sees an extension that it does not understand, it SHOULD ignore that extension. Furthermore, an implementation MAY choose to ignore any extension(s) that it does understand.
        </t>
        <t>
          <spanx style="verb">merkle_tree_head</spanx> is a <spanx style="verb">TransItem</spanx> structure that MUST be of type <spanx style="verb">tree_head</spanx> (see <xref target="tree_head"/>) and MUST have an empty <spanx style="verb">item_extensions</spanx> vector.
        </t>
      </section>
      <section title="Merkle Consistency Proofs">
        <figure>
          <preamble>
            To prepare a Merkle Consistency Proof for distribution to clients, the log produces a <spanx style="verb">TransItem</spanx> structure of type <spanx style="verb">consistency_proof</spanx>, which in this version (v2) encapsulates a <spanx style="verb">ConsistencyProofDataV2</spanx> structure:
          </preamble>
          <artwork>
    struct {
        LogID log_id;
        uint64 tree_size_1;
        uint64 tree_size_2;
        NodeHash consistency_path&lt;1..2^8-1&gt;;
    } ConsistencyProofDataV2;</artwork>
        </figure>
        <t>
          <spanx style="verb">log_id</spanx> is this log's unique ID, encoded in an opaque vector as described in <xref target="log_id"/>.
        </t>
        <t>
          <spanx style="verb">tree_size_1</spanx> is the size of the older tree.
        </t>
        <t>
          <spanx style="verb">tree_size_2</spanx> is the size of the newer tree.
        </t>
        <t>
          <spanx style="verb">consistency_path</spanx> is a vector of Merkle Tree nodes proving the consistency of two STHs.
        </t>
      </section>
      <section title="Merkle Inclusion Proofs">
        <figure>
          <preamble>
            To prepare a Merkle Inclusion Proof for distribution to clients, the log produces a <spanx style="verb">TransItem</spanx> structure of type <spanx style="verb">inclusion_proof</spanx>, which in this version (v2) encapsulates an <spanx style="verb">InclusionProofDataV2</spanx> structure:
          </preamble>
          <artwork>
    struct {
        LogID log_id;
        uint64 tree_size;
        uint64 leaf_index;
        NodeHash inclusion_path&lt;1..2^8-1&gt;;
    } InclusionProofDataV2;</artwork>
        </figure>
        <t>
          <spanx style="verb">log_id</spanx> is this log's unique ID, encoded in an opaque vector as described in <xref target="log_id"/>.
        </t>
        <t>
          <spanx style="verb">tree_size</spanx> is the size of the tree on which this inclusion proof is based.
        </t>
        <t>
          <spanx style="verb">leaf_index</spanx> is the 0-based index of the log entry corresponding to this inclusion proof.
        </t>
        <t>
          <spanx style="verb">inclusion_path</spanx> is a vector of Merkle Tree nodes proving the inclusion of the chosen certificate or precertificate.
        </t>
      </section>
    </section>
    <section title="Log Client Messages" anchor="client_messages">
      <t>
        Messages are sent as HTTPS GET or POST requests. Parameters for POSTs
and all responses are encoded as <xref target="RFC4627">JavaScript Object
Notation (JSON)
objects</xref>. Parameters for GETs are encoded as order-independent key/value
URL parameters, using the "application/x-www-form-urlencoded" format described
in the <xref target="HTML401">"HTML 4.01 Specification"</xref>. Binary data is
<xref target="RFC4648">base64 encoded</xref> as specified in the individual
messages.
      </t>
      <t>
        Note that JSON objects and URL parameters may contain fields not specified here. These extra fields should be ignored.
      </t>
      <t>
        The &lt;log server&gt; prefix, which is part of the log's metadata, MAY include a path as well as a server name and a port.
      </t>
      <t>
        In practice, log servers may include multiple front-end machines. Since it is impractical to keep these machines in perfect sync, errors may occur that are caused by skew between the machines. Where such errors are possible, the front-end will return additional information (as specified below) making it possible for clients to make progress, if progress is possible. Front-ends MUST only serve data that is free of gaps (that is, for example, no front-end will respond with an STH unless it is also able to prove consistency from all log entries logged within that STH).
      </t>
      <t>
        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).
      </t>
      <t>
        If the log is unable to process a client's request, it MUST return an HTTP response code of 4xx/5xx (see <xref target="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:
        <list style="hanging">
          <t hangText="error_message:">
                A human-readable string describing the error which prevented the log from processing the request.
          </t>
          <t>
                In the case of a malformed request, the string SHOULD provide sufficient detail for the error to be rectified.
          </t>
          <t hangText="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.
            <list style="hanging">
              <t hangText="not compliant">
                The request is not compliant with this RFC.
              </t>
            </list>
          </t>
        </list>
      </t>
      <figure>
        <preamble>
          e.g. In response to a request of <spanx style="verb">/ct/v2/get-entries?start=100&amp;end=99</spanx>, the log would return a <spanx style="verb">400 Bad Request</spanx> response code with a body similar to the following:
        </preamble>
        <artwork>
    {
        "error_message": "'start' cannot be greater than 'end'",
        "error_code": "not compliant",
    }</artwork>
      </figure>
      <t>
        Clients SHOULD treat <spanx style="verb">500 Internal Server Error</spanx> and <spanx style="verb">503 Service Unavailable</spanx> responses as transient failures and MAY retry the same request without modification at a later date.  Note that as per <xref target="RFC2616"/>, in the case of a 503 response the log MAY include a <spanx style="verb">Retry-After:</spanx> header in order to request a minimum time for the client to wait before retrying the request.
      </t>
      <section title="Add Chain to Log" anchor="add-chain">
        <t>
          POST https://&lt;log server&gt;/ct/v2/add-chain
        </t>
        <t>
          <list style="hanging">
            <t hangText="Inputs:">
              <list style="hanging">
                <t hangText="chain:">
                  An array of base64 encoded certificates. The first element is the certificate for which the submitter desires an SCT; the second chains to the first and so on to the last, which is either an accepted root certificate or a certificate that chains to an accepted root certificate.
                </t>
              </list>
            </t>
            <t hangText="Outputs:">
              <list style="hanging">
                <t hangText="sct:">
                  A base64 encoded <spanx style="verb">TransItem</spanx> of type <spanx style="verb">x509_sct</spanx>, signed by this log, that corresponds to the submitted certificate.
                </t>
              </list>
            </t>
            <t hangText="Error codes:">
              <list style="hanging">
                <t hangText="unknown root">
                  The root of the chain is not one accepted by the log.
                </t>
                <t hangText="bad chain">
                  The alleged chain is not actually a chain of certificates.
                </t>
                <t hangText="bad certificate">
                  One or more certificates in the chain are not valid (e.g. not properly encoded).
                </t>
              </list>
            </t>
          </list>
        </t>
        <t>
          If the version of <spanx style="verb">sct</spanx> is not v2, then a v2 client may be unable to verify the signature. It MUST NOT construe this as an error. This is to avoid forcing an upgrade of compliant v2 clients that do not use the returned SCTs.
        </t>
        <t>
          If a log detects bad encoding in a chain that otherwise verifies correctly 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 <xref target="monitor">monitors</xref> can then detect these encoding errors, which may be accepted by some TLS clients.
        </t>
        <t>
          Note that not all certificate handling software is capable of detecting all encoding errors (e.g. some software will accept BER instead of DER encodings in certificates, or incorrect character encodings, even though these are technically incorrect) .
        </t>
      </section>

      <section title="Add PreCertChain to Log" anchor="add-pre-chain">
        <t>
          POST https://&lt;log server&gt;/ct/v2/add-pre-chain
        </t>
        <t>
          <list style="hanging">
            <t hangText="Inputs:">
              <list style="hanging">
                <t hangText="precertificate:">
                  The base64 encoded precertificate.
                </t>
                <t hangText="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 an accepted root certificate or a certificate that chains to an accepted root certificate.
                </t>
              </list>
            </t>
            <t hangText="Outputs:">
              <list style="hanging">
                <t hangText="sct:">
                  A base64 encoded <spanx style="verb">TransItem</spanx> of type <spanx style="verb">precert_sct</spanx>, signed by this log, that corresponds to the submitted precertificate.
                </t>
              </list>
            </t>
          </list>
        </t>
        <t>
          Errors are the same as in <xref target="add-chain"/>.
        </t>
      </section>

      <section title="Retrieve Latest Signed Tree Head" anchor="get-sth">
        <t>
          GET https://&lt;log server&gt;/ct/v2/get-sth
        </t>
        <t>
          No inputs.
        </t>
        <t>
          <list style="hanging">
            <t hangText="Outputs:">
              <list style="hanging">
                <t hangText="sth:">
                  A base64 encoded <spanx style="verb">TransItem</spanx> of type <spanx style="verb">signed_tree_head</spanx>, signed by this log, that is no older than the log's MMD.
                </t>
              </list>
            </t>
          </list>
        </t>
      </section>

      <section title="Retrieve Merkle Consistency Proof between Two Signed Tree Heads" anchor="get-sth-consistency">
        <t>
          GET https://&lt;log server&gt;/ct/v2/get-sth-consistency
        </t>
        <t>
          <list style="hanging">
            <t hangText="Inputs:">
              <list style="hanging">
                <t hangText="first:">
                  The tree_size of the older tree, in decimal.
                </t>
                <t hangText="second:">
                  The tree_size of the newer tree, in decimal (optional).
                </t>
              </list>
            </t>
            <t>
              Both tree sizes must be from existing v2 STHs. However, because of skew, the receiving front-end may not know one or both of the existing STHs. If both are known, then only the <spanx style="verb">consistency</spanx> 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.
            </t>
            <t hangText="Outputs:">
              <list style="hanging">
                <t hangText="consistency:">
                  A base64 encoded <spanx style="verb">TransItem</spanx> of type <spanx style="verb">consistency_proof</spanx>, whose <spanx style="verb">tree_size_1</spanx> MUST match the <spanx style="verb">first</spanx> input. If the <spanx style="verb">sth</spanx> output is omitted, then <spanx style="verb">tree_size_2</spanx> MUST match the <spanx style="verb">second</spanx> input.
                </t>
                <t hangText="sth:">
                  A base64 encoded <spanx style="verb">TransItem</spanx> of type <spanx style="verb">signed_tree_head</spanx>, signed by this log.
                </t>
              </list>
            </t>
            <t>
              Note that no signature is required for the <spanx style="verb">consistency</spanx> output as it is used to verify the consistency between two STHs, which are signed.
            </t>
            <t hangText="Error codes:">
              <list style="hanging">
                <t hangText="first unknown">
                  <spanx style="verb">first</spanx> is before the latest known STH but is not from an existing STH.
                </t>
                <t hangText="second unknown">
                  <spanx style="verb">second</spanx> is before the latest known STH but is not from an existing STH.
                </t>
              </list>
            </t>
          </list>
        </t>
        <t>
          See <xref target="verify_consistency"/> for an outline of how to use the <spanx style="verb">consistency</spanx> output.
        </t>
      </section>

      <section title="Retrieve Merkle Inclusion Proof from Log by Leaf Hash" anchor="get-proof-by-hash">
        <t>
          GET https://&lt;log server&gt;/ct/v2/get-proof-by-hash
        </t>
        <t>
          <list style="hanging">
            <t hangText="Inputs:">
              <list style="hanging">
                <t hangText="hash:">
                  A base64 encoded v1 leaf hash.
                </t>
                <t hangText="tree_size:">
                  The tree_size of the tree on which to base the proof, in decimal.
                </t>
              </list>
            </t>
            <t>
              The <spanx style="verb">hash</spanx> must be calculated as defined in <xref target="tree_leaves"/>. The <spanx style="verb">tree_size</spanx> must designate an existing v2 STH. Because of skew, the front-end may not know the requested STH. In that case, it will return the latest STH it knows, along with an inclusion proof to that STH. If the front-end knows the requested STH then only <spanx style="verb">inclusion</spanx> is returned.
            </t>
            <t hangText="Outputs:">
              <list style="hanging">
                <t hangText="inclusion:">
                  A base64 encoded <spanx style="verb">TransItem</spanx> of type <spanx style="verb">inclusion_proof</spanx> whose <spanx style="verb">inclusion_path</spanx> array of Merkle Tree nodes proves the inclusion of the chosen certificate in the selected STH.
                </t>
                <t hangText="sth:">
                  A base64 encoded <spanx style="verb">TransItem</spanx> of type <spanx style="verb">signed_tree_head</spanx>, signed by this log.
                </t>
              </list>
            </t>
            <t>
              Note that no signature is required for the <spanx style="verb">inclusion</spanx> output as it is used to verify inclusion in the selected STH, which is signed.
            </t>
            <t hangText="Error codes:">
              <list style="hanging">
                <t hangText="hash unknown">
                  <spanx style="verb">hash</spanx> is not the hash of a known leaf (may be caused by skew or by a known certificate not yet merged).
                </t>
                <t hangText="tree_size unknown">
                  <spanx style="verb">hash</spanx> is before the latest known STH but is not from an existing STH.
                </t>
              </list>
            </t>
          </list>
        </t>
        <t>See <xref target="verify_inclusion"/> for an outline of how to use the <spanx style="verb">inclusion</spanx> output.</t>
      </section>

      <section title="Retrieve Merkle Inclusion Proof, Signed Tree Head and Consistency Proof by Leaf Hash" anchor="get-all-by-hash">
        <t>
          GET https://&lt;log server&gt;/ct/v2/get-all-by-hash
        </t>
        <t>
          <list style="hanging">
            <t hangText="Inputs:">
              <list style="hanging">
                <t hangText="hash:">
                  A base64 encoded v1 leaf hash.
                </t>
                <t hangText="tree_size:">
                  The tree_size of the tree on which to base the proofs, in decimal.
                </t>
              </list>
            </t>
            <t>
              The <spanx style="verb">hash</spanx> must be calculated as defined in <xref target="tree_leaves"/>. The <spanx style="verb">tree_size</spanx> must designate an existing v2 STH.
            </t>
            <t>
              Because of skew, the front-end may not know the requested STH or the requested hash, which leads to a number of cases.
            </t>
            <t>
              <list style="hanging">
                <t hangText="latest STH &lt; requested STH">
                  Return latest STH.
                </t>
                <t hangText="latest STH &gt; requested STH">
                  Return latest STH and a consistency proof between it and the requested STH (see <xref target="get-sth-consistency"/>).
                </t>
                <t hangText="index of requested hash &lt; latest STH">
                  Return <spanx style="verb">inclusion</spanx>.
                </t>
              </list>
            </t>
            <t>
              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.
            </t>
            <t hangText="Outputs:">
              <list style="hanging">
                <t hangText="inclusion:">
                  A base64 encoded <spanx style="verb">TransItem</spanx> of type <spanx style="verb">inclusion_proof</spanx> whose <spanx style="verb">inclusion_path</spanx> array of Merkle Tree nodes proves the inclusion of the chosen certificate in the selected STH.
                </t>
                <t hangText="sth:">
                  A base64 encoded <spanx style="verb">TransItem</spanx> of type <spanx style="verb">signed_tree_head</spanx>, signed by this log.
                </t>
                <t hangText="consistency:">
                  A base64 encoded <spanx style="verb">TransItem</spanx> of type <spanx style="verb">consistency_proof</spanx> that proves the consistency of the requested STH and the returned STH.
                </t>
              </list>
            </t>
            <t>
              Note that no signature is required for the <spanx style="verb">inclusion</spanx> or <spanx style="verb">consistency</spanx> outputs as they are used to verify inclusion in and consistency of STHs, which are signed.
            </t>
          </list>
        </t>
        <t>
          Errors are the same as in <xref target="get-proof-by-hash"/>.
        </t>
        <t>
          See <xref target="verify_inclusion"/> for an outline of how to use the <spanx style="verb">inclusion</spanx> output, and see <xref target="verify_consistency"/> for an outline of how to use the <spanx style="verb">consistency</spanx> output.
        </t>
      </section>

      <section title="Retrieve Entries and STH from Log" anchor="get-entries">
        <t>
          GET https://&lt;log server&gt;/ct/v2/get-entries
        </t>
        <t>
          <list style="hanging">
            <t hangText="Inputs:">
              <list style="hanging">
                <t hangText="start:">
                  0-based index of first entry to retrieve, in decimal.
                </t>
                <t hangText="end:">
                  0-based index of last entry to retrieve, in decimal.
                </t>
              </list>
            </t>
            <t hangText="Outputs:">
              <list style="hanging">
                <t hangText="entries:">
                  An array of objects, each consisting of
                  <list style="hanging">
                    <t hangText="leaf_input:">
                      The base64 encoded <spanx style="verb">TransItem</spanx> structure of type <spanx style="verb">x509_entry</spanx> or <spanx style="verb">precert_entry</spanx> (see <xref target="tree_leaves"/>).
                    </t>
                    <t hangText="log_entry:">
                      The base64 encoded log entry (see <xref target="log_entries"/>). In the case of an <spanx style="verb">x509_entry</spanx> entry, this is the whole <spanx style="verb">X509ChainEntry</spanx>; and in the case of a <spanx style="verb">precert_entry</spanx>, this is the whole <spanx style="verb">PrecertChainEntryV2</spanx>.
                    </t>
                    <t hangText="sct:">
                      A base64 encoded <spanx style="verb">TransItem</spanx> of type <spanx style="verb">x509_sct</spanx> or <spanx style="verb">precert_sct</spanx> corresponding to this log entry. Note that more than one SCT may have been returned for the same entry - only one of those is returned in this field. It may not be possible to retrieve others.
                    </t>
                  </list>
                </t>
                <t hangText="sth:">
                  A base64 encoded <spanx style="verb">TransItem</spanx> of type <spanx style="verb">signed_tree_head</spanx>, signed by this log.
                </t>
              </list>
            </t>
          </list>
        </t>
        <t>
          Note that this message is not signed -- the <spanx style="verb">entries</spanx> 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.
        </t>
        <t>
          The <spanx style="verb">start</spanx> and <spanx style="verb">end</spanx> parameters SHOULD be within the range 0 &lt;= x &lt; <spanx style="verb">tree_size</spanx> as returned by <spanx style="verb">get-sth</spanx> in <xref target="get-sth"/>.
        </t>
        <t>
          The <spanx style="verb">start</spanx> parameter MUST be less than or equal to the <spanx style="verb">end</spanx> parameter.
        </t>
        <t>
          Log servers MUST honor requests where 0 &lt;= <spanx style="verb">start</spanx> &lt; <spanx style="verb">tree_size</spanx> and <spanx style="verb">end</spanx> &gt;= <spanx style="verb">tree_size</spanx> by returning a partial response covering only the valid entries in the specified range. <spanx style="verb">end</spanx> &gt;= <spanx style="verb">tree_size</spanx> could be caused by skew. Note that the following restriction may also apply:
        </t>
        <t>
          Logs MAY restrict the number of entries that can be retrieved per
<spanx style="verb">get-entries</spanx> 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 <spanx style="verb">start</spanx>.
        </t>
        <t>
          Because of skew, it is possible the log server will not have any entries between <spanx style="verb">start</spanx> and <spanx style="verb">end</spanx>. In this case it MUST return an empty <spanx style="verb">entries</spanx> array.
        </t>
        <t>
          In any case, the log server MUST return the latest STH it knows about.
        </t>
        <t>See <xref target="verify_hash" /> for an outline of how to use a complete list of <spanx style="verb">leaf_input</spanx> entries to verify the <spanx style="verb">root_hash</spanx>.</t>
      </section>

      <section title="Retrieve Accepted Root Certificates" anchor="get-roots">
        <t>
          GET https://&lt;log server&gt;/ct/v2/get-roots
        </t>
        <t>
          No inputs.
        </t>
        <t>
          <list style="hanging">
            <t hangText="Outputs:">
              <list style="hanging">
                <t hangText="certificates:">
                  An array of base64 encoded root certificates that are acceptable to the log.
                </t>
                <t hangText="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.
                </t>
              </list>
            </t>
          </list>
        </t>
      </section>
    </section>
    <section title="TLS Servers" anchor="tls_servers">
      <t>
        TLS servers MUST use at least one of the three mechanisms listed below to present one or more SCTs or inclusion proofs from one or more logs to each TLS client during TLS handshakes, where each SCT or inclusion proof corresponds to the server certificate or to a name-constrained intermediate the server certificate chains to. Three mechanisms are provided because they have different tradeoffs.
        <list style="symbols">
          <t>
            A TLS extension (Section 7.4.1.4 of <xref target='RFC5246'/>) with type <spanx style="verb">transparency_info</spanx> (see <xref target="tls_transinfo_extension"/>). This mechanism allows TLS servers to participate in CT without the cooperation of CAs, unlike the other two mechanisms. It also allows SCTs and inclusion proofs to be updated on the fly.
          </t>
          <t>
            An <xref target='RFC6960'>Online Certificate Status Protocol (OCSP)</xref> response extension (see <xref target="ocsp_transinfo_extension"/>), where the OCSP response is provided in the <spanx style="verb">certificate_status</spanx> TLS extension (Section 8 of <xref target='RFC6066'/>), also known as OCSP stapling. This mechanism is already widely (but not universally) implemented. It also allows SCTs and inclusion proofs to be updated on the fly.
          </t>
          <t>
            An X509v3 certificate extension (see <xref target="cert_transinfo_extension"/>). This mechanism allows the use of unmodified TLS servers, but the SCTs and inclusion proofs cannot be updated on the fly. Since the logs from where the SCTs and inclusion proofs originated won't necessarily be accepted by TLS clients for the full lifetime of the certificate, there is a risk that TLS clients will subsequently consider the certificate to be non-compliant and in need of re-issuance.
          </t>
        </list>
      </t>
      <t>
        TLS servers SHOULD send SCTs or inclusion proofs 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).
      </t>
      <figure>
        <preamble>
          Multiple SCTs, inclusion proofs, and indeed <spanx style="verb">TransItem</spanx> structures of any type, are combined into a list as follows:
        </preamble>
        <artwork>
    opaque SerializedTransItem&lt;1..2^16-1&gt;;

    struct {
        SerializedTransItem trans_item_list&lt;1..2^16-1&gt;;
    } TransItemList;</artwork>
      </figure>
      <t>
        Here, <spanx style="verb">SerializedTransItem</spanx> is an opaque byte string that contains the serialized <spanx style="verb">TransItem</spanx> structure. This encoding ensures that TLS clients can decode each <spanx style="verb">TransItem</spanx> individually (so, for example, if there is a version upgrade, out-of-date clients can still parse old <spanx style="verb">TransItem</spanx> structures while skipping over new <spanx style="verb">TransItem</spanx> structures whose versions they don't understand).
      </t>
      <t>
        TODO: We need to define at least one ItemExtensionType for associating SCT and inclusion proof TransItems with the relevant certificate.
      </t>
      <section title="TLS Extension" anchor="tls_transinfo_extension">
        <t>
          If a TLS client includes the <spanx style="verb">transparency_info</spanx> extension type in the ClientHello, the TLS server MAY include the <spanx style="verb">transparency_info</spanx> extension in the ServerHello with <spanx style="verb">extension_data</spanx> set to a <spanx style="verb">TransItemList</spanx>. The TLS server is not expected to process or include this extension when a TLS session is resumed, since session resumption uses the original session information.
        </t>
      </section>
    </section>
    <section title="Certification Authorities">
      <section title="Transparency Information X.509v3 Extension" anchor="x509v3_transinfo_extension">
        <figure>
          <preamble>
            One or more <spanx style="verb">TransItem</spanx> structures can be embedded in the Transparency Information X.509v3 extension, which has OID &lt;TBD&gt; and SHOULD be non-critical. This extension can be included in OCSP responses and certificates. Since RFC5280 requires the <spanx style="verb">extnValue</spanx> field (an OCTET STRING) of each X.509v3 extension to include the DER encoding of an ASN.1 value, we cannot embed a <spanx style="verb">TransItemList</spanx> directly. Instead, we have to wrap it inside an additional OCTET STRING, which we then put into the <spanx style="verb">extnValue</spanx> field:
          </preamble>
          <artwork>
    TransparencyInformationSyntax ::= OCTET STRING</artwork>
        </figure>
        <t>
          <spanx style="verb">TransparencyInformationSyntax</spanx> contains a <spanx style="verb">TransItemList</spanx>.
        </t>
        <section title="OCSP Response Extension" anchor="ocsp_transinfo_extension">
          <t>
            A certification authority may include a Transparency Information X.509v3 extension in the <spanx style="verb">singleExtensions</spanx> of a <spanx style="verb">SingleResponse</spanx> in an OCSP response. The included SCTs or inclusion proofs MUST be for the certificate identified by the <spanx style="verb">certID</spanx> of that <spanx style="verb">SingleResponse</spanx>, or for a precertificate that corresponds to that certificate, or for a name-constrained intermediate to which that certificate chains.
          </t>
        </section>
        <section title="Certificate Extension" anchor="cert_transinfo_extension">
          <t>
            A certification authority may include a Transparency Information X.509v3 extension in a certificate. Any included SCTs or inclusion proofs MUST be either for a precertificate that corresponds to this certificate, or for a name-constrained intermediate to which this certificate chains.
          </t>
        </section>
      </section>
    </section>
    <section title="Clients">
      <t>
        There are various different functions clients of logs might perform. We
describe here some typical clients and how they should function. Any
inconsistency may be used as evidence that a log has not behaved correctly, and
the signatures on the data structures prevent the log from denying that
misbehavior.
      </t>
      <t>
        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, <xref target="Chromium.Policy"/>).
      </t>
      <t>
        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.
      </t>
      <section title="Metadata" anchor="metadata">
        <t>
          In order to communicate with and verify a log, clients need metadata about the log.
        </t>
        <t><list style="hanging">
          <t hangText="Base URL:">
            The URL to substitute for &lt;log server&gt; in <xref target="client_messages"/>.
          </t>
          <t hangText="Hash Algorithm">
            The hash algorithm used for the Merkle Tree (see <xref target="hash_algorithms"/>).
          </t>
          <t hangText="Signing Algorithm">
            The signing algorithm used (see <xref target="signatures"/>).
          </t>
          <t hangText="Public Key">
            The public key used to verify signatures generated by the log. A log MUST NOT use the same keypair as any other log.
          </t>
          <t hangText="Log ID">
            The OID that uniquely identifies the log.
          </t>
          <t hangText="Maximum Merge Delay">
            The MMD the log has committed to.
          </t>
          <t hangText="Version">
            The version of the protocol supported by the log (currently 1 or 2).
          </t>
          <t hangText="Maximum Chain Length">
            The longest chain submission the log is willing to accept, if the log chose to limit it.
          </t>
          <t hangText="STH Frequency Count">
            The maximum number of STHs the log may produce in any period equal to the <spanx style="verb">Maximum Merge Delay</spanx> (see <xref target="STH"/>).
          </t>
          <t hangText="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.
          </t>
        </list></t>
        <t>
          <xref target="JSON.Metadata"/> is an example of a metadata format which includes the above elements.
        </t>
      </section>
      <section title="TLS Client" anchor="tls_clients">
        <t>
          TLS clients receive SCTs alongside or in certificates, either for the server certificate itself or for a name-constrained intermediate the server certificate chains to. TLS clients MUST implement all of the three mechanisms by which TLS servers may present SCTs (see <xref target="tls_servers"/>). TLS clients that support the <spanx style="verb">transparency_info</spanx> TLS extension SHOULD include it in ClientHello messages, with <spanx style="verb">extension_data</spanx> set to &lt;TBD&gt;.
        </t>
        <t>
          TODO: What should the TLS client communicate in the extension_data? Version(s) of CT that it supports? Certain types of TransItem that it can handle? Whether or not it wants to gossip?
        </t>
        <t>
          In addition to normal validation of the certificate and its chain, TLS clients SHOULD validate each supplied 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. TLS clients MUST NOT consider SCTs whose timestamp is in the future as valid.
        </t>
        <t>
          TLS clients SHOULD also validate each supplied inclusion proof (see <xref target="verify_inclusion"/>), in order to audit the log. If no inclusion proof was supplied by the TLS server, the TLS client MAY request one directly from the corresponding log using <spanx style="verb">get-proof-by-hash</spanx> (<xref target="get-proof-by-hash"/>) or <spanx style="verb">get-all-by-hash</spanx> (<xref target="get-all-by-hash"/>), and then validate it.
        </t>
        <t>
          To be considered compliant, a certificate MUST be accompanied by at least one valid SCT or at least one valid inclusion proof. A certificate not accompanied by any valid SCTs or any valid inclusion proofs MUST NOT be considered compliant by TLS clients. However, specifying the TLS clients' behavior once compliance or non-compliance has been determined (for example, whether a certificate should be rejected due to non-compliance) is outside the scope of this document.
        </t>
        <t>
          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 (<xref target="get-sth"/>), then verify it by requesting a consistency proof (<xref target="get-sth-consistency"/>). Note that if the TLS client uses <spanx style="verb">get-all-by-hash</spanx>, then it will already have the new STH.
        </t>
      </section>
      <section title="Monitor" anchor="monitor">
        <t>
          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.
        </t>
        <t>
          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:
          <list style="numbers">
            <t>
              Fetch the current STH (<xref target="get-sth"/>).
            </t>
            <t>
              Verify the STH signature.
            </t>
            <t>
              Fetch all the entries in the tree corresponding to the STH (<xref target="get-entries"/>).
            </t>
            <t>
              Confirm that the tree made from the fetched entries produces the same hash as that in the STH.
            </t>
            <t anchor="monitor_loop">
              Fetch the current STH (<xref target="get-sth"/>). Repeat until the STH changes.
            </t>
            <t>
              Verify the STH signature.
            </t>
            <t>
              Fetch all the new entries in the tree corresponding to the STH
(<xref target="get-entries"/>). If they remain unavailable for an extended period, then this should be viewed as misbehavior on the part of the log.
            </t>
            <t>
              Either:
              <list style="numbers">
                <t>
                  Verify that the updated list of all entries generates a tree with the same hash as the new STH.
                </t>
              </list>
              Or, if it is not keeping all log entries:
              <list style="numbers">
                <t>
                  Fetch a consistency proof for the new STH with the previous STH (<xref target="get-sth-consistency"/>).
                </t>
                <t>
                  Verify the consistency proof.
                </t>
                <t>
                  Verify that the new entries generate the corresponding elements in the consistency proof.
                </t>
              </list>
            </t>
            <t>
              Go to Step 5.
            </t>
          </list>
        </t>
      </section>
      <section title="Auditing">
        <t>
          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.
        </t>
        <t>
          A <xref target="monitor">monitor</xref> 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.
        </t>
        <t>
          A <xref target="tls_clients">TLS client</xref> can audit by verifying an SCT against any STH dated after the SCT timestamp + the Maximum Merge Delay by requesting a Merkle inclusion proof (<xref target="get-proof-by-hash"/>). 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 certificate or is an appropriate name-constrained intermediate [see <xref target="name_constrained"/>]).
        </t>
        <t>
          The following algorithm outlines may be useful for clients that wish to perform various audit operations.
        </t>
        <section title="Verifying an inclusion proof" anchor="verify_inclusion">
          <t>
            When a client has received a <spanx style="verb">TransItem</spanx> of type <spanx style="verb">inclusion_proof</spanx> and wishes to verify inclusion of an input <spanx style="verb">hash</spanx> for an STH with a given <spanx style="verb">tree_size</spanx> and <spanx style="verb">root_hash</spanx>, the following algorithm may be used to prove the <spanx style="verb">hash</spanx> was included in the <spanx style="verb">root_hash</spanx>:
            <list style="numbers">
              <t>Set <spanx style="verb">fn</spanx> to <spanx style="verb">leaf_index</spanx> and <spanx style="verb">sn</spanx> to <spanx style="verb">tree_size - 1</spanx>.</t>
              <t>Set <spanx style="verb">r</spanx> to <spanx style="verb">hash</spanx>.</t>
              <t>
                For each value <spanx style="verb">p</spanx> in the <spanx style="verb">inclusion_path</spanx> array:
                <vspace blankLines="1" />
                If <spanx style="verb">LSB(fn)</spanx> is set, or if <spanx style="verb">fn</spanx> is equal to <spanx style="verb">sn</spanx>, then:
                <list style="numbers">
                  <t>
                    Set <spanx style="verb">r</spanx> to <spanx style="verb">HASH(0x01 || p || r)</spanx>
                  </t>
                  <t>If <spanx style="verb">LSB(fn)</spanx> is not set, then right-shift both <spanx style="verb">fn</spanx> and <spanx style="verb">sn</spanx> equally until either <spanx style="verb">LSB(fn)</spanx> is set or <spanx style="verb">fn</spanx> is <spanx style="verb">0</spanx>.</t>
                </list>
                Otherwise:
                <list style="empty">
                  <t>Set <spanx style="verb">r</spanx> to <spanx style="verb">HASH(0x01 || r || p)</spanx></t>
                </list>
                <vspace blankLines="1" />
                Finally, right-shift both <spanx style="verb">fn</spanx> and <spanx style="verb">sn</spanx> one time.
              </t>
              <t>Compare <spanx style="verb">r</spanx> against the <spanx style="verb">root_hash</spanx>. If they are equal, then the log has proven the inclusion of <spanx style="verb">hash</spanx>.</t>
            </list>
          </t>
        </section>
        <section title="Verifying consistency between two STHs" anchor="verify_consistency">
          <t>
            When a client has an STH <spanx style="verb">first_hash</spanx> for tree size <spanx style="verb">first</spanx>, an STH <spanx style="verb">second_hash</spanx> for tree size <spanx style="verb">second</spanx> where <spanx style="verb">0 &lt; first &lt; second</spanx>, and has received a <spanx style="verb">TransItem</spanx> of type <spanx style="verb">consistency_proof</spanx> that they wish to use to verify both hashes, the following algorithm may be used:
            <list style="numbers">
              <t>If <spanx style="verb">first</spanx> is an exact power of 2, then prepend <spanx style="verb">first_hash</spanx> to the <spanx style="verb">consistency_path</spanx> array.</t>
              <t>Set <spanx style="verb">fn</spanx> to <spanx style="verb">first - 1</spanx> and <spanx style="verb">sn</spanx> to <spanx style="verb">second - 1</spanx>.</t>
              <t>If <spanx style="verb">LSB(fn)</spanx> is set, then right-shift both <spanx style="verb">fn</spanx> and <spanx style="verb">sn</spanx> equally until <spanx style="verb">LSB(fn)</spanx> is not set.</t>
              <t>Set both <spanx style="verb">fr</spanx> and <spanx style="verb">sr</spanx> to the first value in the <spanx style="verb">consistency_path</spanx> array.</t>
              <t>
                For each subsequent value <spanx style="verb">c</spanx> in the <spanx style="verb">consistency_path</spanx> array:
                <vspace blankLines="1" />
                If <spanx style="verb">LSB(fn)</spanx> is set, or if <spanx style="verb">fn</spanx> is equal to <spanx style="verb">sn</spanx>, then:
                <list style="numbers">
                  <t>
                    Set <spanx style="verb">fr</spanx> to <spanx style="verb">HASH(0x01 || c || fr)</spanx>
                    <vspace blankLines="0" />
                    Set <spanx style="verb">sr</spanx> to <spanx style="verb">HASH(0x01 || c || sr)</spanx>
                  </t>
                  <t>If <spanx style="verb">LSB(fn)</spanx> is not set, then right-shift both <spanx style="verb">fn</spanx> and <spanx style="verb">sn</spanx> equally until either <spanx style="verb">LSB(fn)</spanx> is set or <spanx style="verb">fn</spanx> is <spanx style="verb">0</spanx>.</t>
                </list>
                Otherwise:
                <list style="empty">
                  <t>Set <spanx style="verb">sr</spanx> to <spanx style="verb">HASH(0x01 || sr || c)</spanx></t>
                </list>
                <vspace blankLines="1" />
                Finally, right-shift both <spanx style="verb">fn</spanx> and <spanx style="verb">sn</spanx> one time.
              </t>
              <t>After completing iterating through the <spanx style="verb">consistency_path</spanx> array as described above, verify that the <spanx style="verb">fr</spanx> calculated is equal to the <spanx style="verb">first_hash</spanx> supplied and that the <spanx style="verb">sr</spanx> calculated is equal to the <spanx style="verb">second_hash</spanx> supplied.</t>
            </list>
          </t>
        </section>
        <section title="Verifying root hash given entries" anchor="verify_hash">
          <t>
            When a client has a complete list of leaf input <spanx style="verb">entries</spanx> from <spanx style="verb">0</spanx> up to <spanx style="verb">tree_size - 1</spanx> and wishes to verify this list against an STH <spanx style="verb">root_hash</spanx> returned by the log for the same <spanx style="verb">tree_size</spanx>, the following algorithm may be used:
            <list style="numbers">
              <t>Set <spanx style="verb">stack</spanx> to an empty stack.</t>
              <t>
                 For each <spanx style="verb">i</spanx> from <spanx style="verb">0</spanx> up to <spanx style="verb">tree_size - 1</spanx>:
                 <list style="numbers">
                   <t>Push <spanx style="verb">HASH(0x00 || entries[i])</spanx> to <spanx style="verb">stack</spanx>.</t>
                   <t>Set <spanx style="verb">merge_count</spanx> to the lowest value (<spanx style="verb">0</spanx> included) such that <spanx style="verb">LSB(i >> merge_count)</spanx> is not set.  In other words, set <spanx style="verb">merge_count</spanx> to the number of consecutive <spanx style="verb">1</spanx>s found starting at the least significant bit of <spanx style="verb">i</spanx>.</t>
                   <t>
                     Repeat <spanx style="verb">merge_count</spanx> times:
                     <list style="numbers">
                       <t>Pop <spanx style="verb">right</spanx> from <spanx style="verb">stack</spanx>.</t>
                       <t>Pop <spanx style="verb">left</spanx> from <spanx style="verb">stack</spanx>.</t>
                       <t>Push <spanx style="verb">HASH(0x01 || left || right)</spanx> to <spanx style="verb">stack</spanx>.</t>
                     </list>
                   </t>
                 </list>
               </t>
               <t>If there is more than one element in the <spanx style="verb">stack</spanx>, repeat the same merge procedure (Step 2.3 above) until only a single element remains.</t>
               <t>The remaining element in <spanx style="verb">stack</spanx> is the Merkle Tree hash for the given <spanx style="verb">tree_size</spanx> and should be compared by equality against the supplied <spanx style="verb">root_hash</spanx>.</t>
            </list>
          </t>
        </section>
      </section>
    </section>

    <section title="Algorithm Agility">
      <t>
        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 <xref target="metadata"/> 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.
      </t>
    </section>

    <section title="IANA Considerations">
      <section title="TLS Extension Type">
        <t>
          IANA is asked to allocate an RFC 5246 ExtensionType value for the <spanx style="verb">transparency_info</spanx> TLS extension. IANA should update this extension type to point at this document.
        </t>
      </section>
      <section title="Hash Algorithms" anchor="hash_algorithms">
        <t>
          IANA is asked to establish a registry of hash values, initially consisting of:
        </t>
        <texttable>
          <ttcol>Index</ttcol><ttcol>Hash</ttcol>
          <c>0</c><c><xref target="FIPS.180-4">SHA-256</xref></c>
        </texttable>
      </section>
      <section title="TransItem Extensions" anchor="item_extension_types">
        <t>
          IANA is asked to establish a registry of TransItem extensions, initially consisting of:
        </t>
        <texttable>
          <ttcol>Type</ttcol><ttcol>Extension</ttcol>
          <c>65535</c><c>reserved</c>
        </texttable>
        <t>
          TBD: policy for adding to the registry
        </t>
      </section>
      <section title="Signature Algorithms" anchor="signature_algorithms">
        <t>
          IANA is asked to establish a registry of signature algorithm values, initially consisting of:
        </t>
        <texttable>
          <ttcol>Index</ttcol><ttcol>Signature Algorithm</ttcol>
          <c>0</c><c><xref target="RFC6979">deterministic ECDSA</xref> using the NIST P-256 curve (Section D.1.2.3 of the <xref target="DSS">Digital Signature Standard </xref>) and HMAC-SHA256</c>
          <c>1</c><c>RSA signatures (RSASSA-PKCS1-v1_5 with SHA-256, Section 8.2 of <xref target="RFC3447"/>) using a key of at least 2048 bits.</c>
        </texttable>
      </section>
      <section title="SCT Extensions" anchor="sct_extension_types">
        <t>
          IANA is asked to establish a registry of SCT extensions, initially consisting of:
        </t>
        <texttable>
          <ttcol>Type</ttcol><ttcol>Extension</ttcol>
          <c>65535</c><c>reserved</c>
        </texttable>
        <t>
          TBD: policy for adding to the registry
        </t>
      </section>
      <section title="STH Extensions" anchor="sth_extension_types">
        <t>
          IANA is asked to establish a registry of STH extensions, initially consisting of:
        </t>
        <texttable>
          <ttcol>Type</ttcol><ttcol>Extension</ttcol>
          <c>65535</c><c>reserved</c>
        </texttable>
        <t>
          TBD: policy for adding to the registry
        </t>
      </section>
    </section>

    <section title="Security Considerations">
      <t>
        With CAs, logs, and servers performing the actions described here, TLS clients can use logs and signed timestamps to reduce the likelihood that they will accept misissued certificates.  If a server presents a valid signed timestamp for a certificate, then the client knows that a log has committed to publishing the certificate.  From this, the client knows that monitors acting for the subject of the certificate have had some time to notice the misissue and take some action, such as asking a CA to revoke a misissued certificate, or that the log has misbehaved, which will be discovered when the SCT is audited.  A signed timestamp is not a guarantee that the certificate is not misissued, since appropriate monitors might not have checked the logs or the CA might have refused to revoke the certificate.
      </t>
      <t>
        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.
      </t>
      <section title="Misissued Certificates">
        <t>
          Misissued certificates that have not been publicly logged, and thus do not have a valid SCT, are not considered compliant (so TLS clients may decide, for example, to reject them). 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.
        </t>
      </section>
      <section title="Detection of Misissue">
        <t>
          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.
        </t>
      </section>
      <section title="Redaction of Public Domain Name Labels">
        <t>
          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. <spanx style="verb">?.com</spanx> and
<spanx style="verb">?.co.uk</spanx> 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.
        </t>
      </section>
      <section title="Misbehaving Logs">
        <t>
          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.
        </t>
        <t>
          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.
        </t>
        <t>
          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).
        </t>
      </section>
      <section title="Multiple SCTs">
        <t>
          TLS servers may wish to offer multiple SCTs, each from a different log.
          <list style="symbols">
            <t>
              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.
            </t>
            <t>
              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.
            </t>
            <t>
              TLS clients may have policies related to the above risks requiring servers to present multiple SCTs. For example <xref target="Chromium.Log.Policy">Chromium</xref> currently requires multiple SCTs to be presented with EV certificates in order for the EV indicator to be shown.
            </t>
          </list>
        </t>
      </section>
    </section>
    <section title="Acknowledgements">
      <t>
        The authors would like to thank Erwann Abelea, Robin Alden, Al Cutter,
Francis Dupont, Adam Eijdenberg, Stephen Farrell, Daniel Kahn Gillmor, Brad Hill, Jeff Hodges, Paul Hoffman, Jeffrey Hutzelman, Stephen Kent, SM, Alexey Melnikov, Linus Nordberg, Chris Palmer, Trevor Perrin, Pierre Phaneuf, Melinda Shore, Ryan Sleevi, Carl Wallace and Paul Wouters for their valuable contributions.
      </t>
    </section>
  </middle>

  <back>
    <references title="Normative References">

      <?rfc include="reference.RFC.2119" ?>

      <?rfc include="reference.RFC.2616"?>

      <?rfc include="reference.RFC.3447"?>

      <?rfc include="reference.RFC.4627"?>

      <?rfc include="reference.RFC.4648"?>

      <?rfc include="reference.RFC.5246"?>

      <?rfc include="reference.RFC.5280"?>

      <?rfc include="reference.RFC.5652"?>

      <?rfc include="reference.RFC.5905"?>

      <?rfc include="reference.RFC.6066"?>

      <?rfc include="reference.RFC.6125"?>

      <?rfc include="reference.RFC.6960"?>

      <?rfc include="reference.RFC.6979"?>

      <reference anchor="DSS" target="http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf">
        <front>
          <title>Digital Signature Standard (DSS)</title>
          <author>
            <organization abbrev="NIST">National Institute of Standards and
Technology</organization>
          </author>
          <date month="June" year="2009"/>
        </front>
        <seriesInfo name="FIPS" value="186-3"/>
        <format type="PDF" target="http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf"/>
      </reference>

      <reference anchor="FIPS.180-4"
target="http://csrc.nist.gov/publications/fips/fips180-4/fips-180-4.pdf">
        <front>
          <title>Secure Hash Standard</title>
          <author>
            <organization>National Institute of Standards and Technology</organization>
          </author>
          <date month="March" year="2012"/>
        </front>
        <seriesInfo name="FIPS" value="PUB 180-4"/>
      </reference>

      <reference anchor="HTML401" target="http://www.w3.org/TR/1999/REC-html401-19991224">
        <front>
          <title>HTML 4.01 Specification</title>
          <author initials="D." surname="Raggett" fullname="David Raggett">
            <organization/>
          </author>
          <author initials="A." surname="Le Hors" fullname="Arnaud Le Hors">
            <organization/>
          </author>
          <author initials="I." surname="Jacobs" fullname="Ian Jacobs">
            <organization/>
          </author>
          <date month="December" day="24" year="1999"/>
        </front>
        <seriesInfo name="World Wide Web Consortium Recommendation" value="REC-html401-19991224"/>
        <format type="HTML" target="http://www.w3.org/TR/1999/REC-html401-19991224"/>
      </reference>

    </references>

<references title="Informative References">

    <?rfc include="reference.RFC.6962"?>

      <reference anchor="CrosbyWallach"
target="http://static.usenix.org/event/sec09/tech/full_papers/crosby.pdf">
        <front>
          <title>Efficient Data Structures for Tamper-Evident Logging</title>
          <author initials='S.' surname='Crosby' fullname='Scott A. Crosby'>
            <organization/>
          </author>
          <author initials='D.' surname='Wallach' fullname='Dan S. Wallach'>
            <organization/>
          </author>
          <date month="August" year="2009"/>
        </front>
<seriesInfo name="Proceedings of the 18th USENIX Security Symposium,"
value="Montreal"/>
        <format type="PDF" target="http://static.usenix.org/event/sec09/tech/full_papers/crosby.pdf"/>
      </reference>

      <reference anchor="EVSSLGuidelines" target="https://cabforum.org/wp-content/uploads/EV_Certificate_Guidelines.pdf">
        <front>
          <title>Guidelines For The Issuance And Management Of Extended Validation Certificates</title>
          <author>
            <organization>CA/Browser Forum</organization>
          </author>
          <date year="2007"/>
        </front>
        <format type="PDF" target="https://cabforum.org/wp-content/uploads/EV_Certificate_Guidelines.pdf"/>
      </reference>

      <reference anchor="Chromium.Policy" target="http://www.chromium.org/Home/chromium-security/certificate-transparency">
        <front>
          <title>Chromium Certificate Transparency</title>
          <author>
            <organization>The Chromium Projects</organization>
          </author>
          <date year="2014"/>
        </front>
      </reference>

      <reference anchor="JSON.Metadata" target="http://www.certificate-transparency.org/known-logs/log_list_schema.json">
        <front>
          <title>Chromium Log Metadata JSON Schema</title>
          <author>
            <organization>The Chromium Projects</organization>
          </author>
          <date year="2014"/>
        </front>
      </reference>

      <reference anchor="Chromium.Log.Policy" target="http://www.chromium.org/Home/chromium-security/certificate-transparency/log-policy">
        <front>
          <title>Chromium Certificate Transparency Log Policy</title>
          <author>
            <organization>The Chromium Projects</organization>
          </author>
          <date year="2014"/>
        </front>
      </reference>

    </references>

    <section title="TransItemV1" anchor="TransItemV1">
      <figure>
        <preamble>
          TODO: Finish writing this section. Or should it be in a separate document?
        </preamble>
        <artwork>
    struct {
        TransType type;
        select (type) {
            case x509_sct: SignedCertificateTimestampV1;
            case precert_sct: SignedCertificateTimestampV1;
            case signed_tree_head: SignedTreeHeadDataV1;
            case consistency_proof: ConsistencyProofDataV1;
            case inclusion_proof: InclusionProofDataV1;
        } data;
    } TransItemV1;

    opaque SHA256Hash[32];

    struct {
        Version version = v1;
        SHA256Hash log_id;
        uint64 timestamp;
        SctExtensions extensions;
        digitally-signed struct {
            Version version = v1;
            uint8 signature_type = 0;  /* "certificate_timestamp" */
            uint64 timestamp;
            TransType type;  /* x509_entry(0) or precert_entry(1) */
            select (type) {
                case x509_entry: ASN.1Cert;
                case precert_entry: PreCert;
            } signed_entry;
            SctExtensions extensions;
        } signature;
    } SignedCertificateTimestampV1;

    struct {
        SHA256Hash log_id;
        uint64 timestamp;
        uint64 tree_size;
        SHA256Hash sha256_root_hash;
        digitally-signed struct {
            Version version = v1;
            uint8 signature_type = 1;  /* "tree_hash" */
            uint64 timestamp;
            uint64 tree_size;
            SHA256Hash sha256_root_hash;
        } signature;
    } SignedTreeHeadDataV1;

    struct {
        SHA256Hash log_id;
        uint64 tree_size_1;
        uint64 tree_size_2;
        SHA256Hash consistency_path&lt;1..2^8-1&gt;;
    } ConsistencyProofDataV1;

    struct {
        SHA256Hash log_id;
        uint64 tree_size;
        uint64 leaf_index;
        SHA256Hash inclusion_path&lt;1..2^8-1&gt;;
    } InclusionProofDataV1;</artwork>
      </figure>
    </section>
  </back>
</rfc>