draft-laurie-pki-sunlight-03.txt

Network Working Group                                          B. Laurie
Internet-Draft                                                A. Langley
Expires: June 3, 2013                                          E. Kasper
                                                       November 30, 2012


                        Certificate Transparency
                      draft-laurie-pki-sunlight-03

Abstract

   The aim of Certificate Transparency is to have every public end-
   entity and intermediate TLS certificate issued by a known Certificate
   Authority recorded in one or more certificate logs.  In order to
   detect mis-issuance of certificates, all logs are publicly auditable.
   In particular, domain owners or their agents will be able to monitor
   logs for certificates issued on their own domain.

   To protect clients from unlogged mis-issued certificates, logs sign
   all recorded certificates, and clients can choose not to trust
   certificates that are not accompanied by an appropriate log
   signature.  For privacy and performance reasons log signatures are
   embedded in the TLS handshake via the TLS authorization extension
   [RFC5878], in a stapled [RFC6066] OCSP extension [RFC2560], or in the
   certificate itself via an X.509v3 certificate extension [RFC5280].

   To ensure a globally consistent view of the log, logs also provide a
   global signature over the entire log.  Any inconsistency of logs can
   be detected through cross-checks on the global signature.
   Consistency between any pair of global signatures, corresponding to
   snapshots of the log at different times, can be efficiently shown.

   Logs are only expected to certify that they have seen a certificate,
   and thus we do not specify any revocation mechanism for log
   signatures in this document.  Logs are append-only, and log
   signatures will be valid indefinitely.

Status of this Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months



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   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 3, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.































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Table of Contents

   1.  Informal introduction  . . . . . . . . . . . . . . . . . . . .  4
   2.  Cryptographic components . . . . . . . . . . . . . . . . . . .  5
     2.1.  Merkle Hash Trees  . . . . . . . . . . . . . . . . . . . .  5
       2.1.1.  Merkle audit paths . . . . . . . . . . . . . . . . . .  5
       2.1.2.  Merkle consistency proofs  . . . . . . . . . . . . . .  6
       2.1.3.  Example  . . . . . . . . . . . . . . . . . . . . . . .  7
       2.1.4.  Signatures . . . . . . . . . . . . . . . . . . . . . .  8
   3.  Log Format . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Log Entries  . . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Including the Signed Certificate Timestamp in the TLS
           Handshake  . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.3.  Merkle Tree  . . . . . . . . . . . . . . . . . . . . . . . 13
     3.4.  Tree Head Signature  . . . . . . . . . . . . . . . . . . . 14
   4.  Client Messages  . . . . . . . . . . . . . . . . . . . . . . . 16
     4.1.  Add Chain to Log . . . . . . . . . . . . . . . . . . . . . 16
     4.2.  Add PreCertChain to Log  . . . . . . . . . . . . . . . . . 17
     4.3.  Retrieve Latest Signed Tree Head . . . . . . . . . . . . . 17
     4.4.  Retrieve Merkle Consistency Proof between two Signed
           Tree Heads . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.5.  Retrieve Merkle Audit Proof from Log by Leaf Hash  . . . . 18
     4.6.  Retrieve Entries from Log  . . . . . . . . . . . . . . . . 18
     4.7.  Retrieve Entry+Merkle Audit Proof from Log . . . . . . . . 19
   5.  Clients  . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     5.1.  Monitor  . . . . . . . . . . . . . . . . . . . . . . . . . 20
     5.2.  Auditor  . . . . . . . . . . . . . . . . . . . . . . . . . 21
   6.  Security and Privacy Considerations  . . . . . . . . . . . . . 22
     6.1.  Misissued Certificates . . . . . . . . . . . . . . . . . . 22
     6.2.  Detection of Misissue  . . . . . . . . . . . . . . . . . . 22
     6.3.  Misbehaving logs . . . . . . . . . . . . . . . . . . . . . 22
   7.  Efficiency Considerations  . . . . . . . . . . . . . . . . . . 23
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25

















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1.  Informal introduction

   Certificate Transparency aims to solve the problem of mis-issued
   certificates by providing a publicly auditable, append-only,
   untrusted log of all issued certificates.  The logs are publicly
   auditable so that it is possible for anyone to verify the correct
   operation of the log, and to monitor when new certificates added to
   it.  The logs do not themselves prevent mis-issue, but they ensure
   that interested parties (particularly those named in certificates)
   can detect such mis-issuance.  Note that this is a general mechanism,
   but in this document we only decsribe its use for public TLS
   certificates issued by public CAs.

   The log consists of certificate chains, which can be submitted by
   anyone.  It is expected that most public CAs will contribute all
   their newly-issued certificates to the log; it is also expected that
   certificate holders will also contribute their own certificate
   chains.  In order to avoid the log being spammed into uselessness, it
   is required that the chain is rooted in a known CA certificate.  When
   a chain is submitted to the log, a signed timestamp is returned,
   which can later be used to prove to clients that the chain has been
   submitted.  Clients can thus require that all certificates they see
   have been logged.

   Those who are concerned about mis-issue can monitor the log, asking
   it 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 mis-issuance has happened, is beyond the scope of
   this document, but broadly speaking they can invoke existing business
   mechanisms for dealing with mis-issued certificates.  Of course,
   anyone who wants can monitor the log, and if they believe a
   certificate is incorrectly issued, take action as they see fit.

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

   The append-only property of a log is technically achieved using
   Merkle Trees, which can be used to show that any particular version
   of the log is a superset of any particular previous version.
   Likewise, Merkle Trees avoid the need to trust the log: if the log
   attempts to show different things to different people, this can be
   efficiently detected by comparing tree roots and consistency proofs.



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2.  Cryptographic components

2.1.  Merkle Hash Trees

   Logs use a binary Merkle hash tree for efficient auditing.  The
   hashing algorithm is SHA-256.  The input to the Merkle tree hash is a
   list of data entries; these entries will be hashed to form the leaves
   of the Merkle hash tree.  The output is a single 32-byte root hash.
   Given an ordered list of n inputs, D[n] = {d(0), d(1), ..., d(n-1)},
   the Merkle Tree Hash (MTH) is thus defined as follows:

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

   MTH({}) = SHA-256().

   The hash of a list with one entry is:

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

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

   MTH(D[n]) = SHA-256(1 || MTH(D[0:k]) || MTH(D[k:n])),

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

   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.
   [This Merkle tree is essentially the same as the history tree [1]
   proposal, except our definition omits dummy leaves.]

2.1.1.  Merkle audit paths

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




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   Given an ordered list of n inputs to the tree, D[n] = {d(0), ...,
   d(n-1)}, the Merkle audit path PATH(m, D[n]) for the (m+1)th input
   d(m), 0 <= m < n, is defined as follows:

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

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

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

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

   PATH(m, D[n]) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for m >= k,

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

2.1.2.  Merkle consistency proofs

   Merkle consistency proofs prove the append-only property of the tree.
   A Merkle consistency proof for a Merkle Tree Hash MTH(D[n]) and a
   previously advertised hash MTH(D[0:m]) of the first m leaves, m <= n,
   is the list of nodes in the Merkle tree required to verify that the
   first m inputs D[0:m] are equal in both trees.  Thus, a consistency
   proof must contain a set of intermediate nodes (i.e., commitments to
   inputs) sufficient to verify MTH(D[n]), such that (a subset of) the
   same nodes can be used to verify MTH(D[0:m]).  We define an algorithm
   that outputs the (unique) minimal consistency proof.

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

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

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

   The subproof for m = n is the root hash committing inputs D[0:m]
   otherwise:

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




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

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

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

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

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

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

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

2.1.3.  Example

   The binary Merkle tree with 7 leaves:

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

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

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

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

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



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   The same tree, built incrementally in four steps:

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

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

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

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

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

2.1.4.  Signatures

   Various data structures are signed.  A log can use either elliptic
   curve signatures using the NIST P-256 curve
   (http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf
   section D.1.2.3) or RSA signatures using a key of at least 2048 bits.






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3.  Log Format

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

   After accepting a certificate submission, the log MUST immediately
   return a Signed Certificate Timestamp (SCT).  The SCT is the log's
   promise to incorporate the certificate in the Merkle Tree within a
   fixed amount of time known as the Maximum Merge Delay (MMD).  If the
   log has previously seen the certificate, it MAY return the same SCT
   as it returned before.  Servers MUST present an SCT from one or more
   logs to the client together with the certificate.  Clients MUST
   reject certificates that do not have a valid SCT for the end-entity
   certificate.

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

3.1.  Log Entries

   Anyone can submit a certificate to the log.  In order to attribute
   each logged certificate to its issuer, the log shall publish a list
   of acceptable root certificates (this list should be the union of
   root certificates trusted by major browser vendors).  Each submitted
   certificate MUST be accompanied by all additional certificates
   required to verify the certificate chain up to an accepted root
   certificate.  The self-signed root certificate itself MAY be omitted
   from this list.

   Alternatively, (root as well as intermediate) Certificate Authorities
   may submit a certificate to the log prior to issuance.  To do so, a
   Certificate Authority constructs a Precertificate by adding a special
   critical poison extension (OID 1.3.6.1.4.1.11129.2.4.3, ASN.1 NULL
   data) to the leaf TBSCertificate, and signing the resulting
   TBSCertificate [RFC5280] with a special-purpose (Extended Key Usage:
   Certificate Transparency, OID 1.3.6.1.4.1.11129.2.4.4,
   basicConstraints=critical,CA:FALSE) Precertificate Signing
   Certificate.  The Precertificate Signing Certificate MUST be
   certified by the CA certificate.  As above, the Precertificate
   submission MUST be accompanied by the Precertificate Signing
   Certificate and all additional certificates required to verify the



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   chain up to an accepted root certificate.  The signature on the
   TBSCertificate indicates the Certificate Authority's intent to issue
   a certificate.  This intent is considered binding (i.e., misissuance
   of the Precertificate is considered equal to misissuance of the final
   certificate).  The log verifies the Precertificate signature chain,
   and issues a Signed Certificate Timestamp on the corresponding
   TBSCertificate.

   The log MUST verify that the submitted leaf certificate or
   Precertificate has a valid signature chain leading back to a trusted
   root CA certificate, using the chain of intermediate CA certificates
   provided by the submitter.  In case of Precertificates, the log MUST
   also verify that the Precertificate Signing Certificate has the
   correct Extended Key Usage extension.  The log MAY accept
   certificates that have expired, are not yet valid, have been revoked
   or are otherwise not fully valid according to X.509 verification
   rules.  However, the log MUST refuse to publish certificates without
   a valid chain to a known root CA.  If a certificate is accepted and
   an SCT issued, the log MUST store the chain used for verification
   including the certificate or Precertificate itself, and MUST present
   this chain for auditing upon request.

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

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

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

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

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

       struct {
           ASN.1Cert tbs_certificate;
           ASN.1Cert precertificate_chain<1..2^24-1>;
       } PrecertChainEntry;

   Logs MAY limit the length of chain they will accept.



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

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

   "certificate_chain" is a chain of additional certificates required to
   verify the leaf certificate.  The first certificate MUST certify the
   leaf certificate.  Each following certificate MUST directly certify
   the one preceding it.  The self-signed root certificate MAY be
   omitted from the chain.

   "tbs_certificate" is the TBSCertificate component of the
   Precertificate (i.e., the original TBSCertificate, without the
   Precertificate signature and the SCT extension).

   "precertificate_chain" is a chain of certificates required to verify
   the Precertificate submission.  The first certificate MUST be the
   original Precertificate, with its unsigned part matching the
   "tbs_certificate".  The second certificate MUST be a valid
   Precertificate Signing Certificate, and MUST certify the first
   certificate.  Each following certificate MUST directly certify the
   one preceding it.  The self-signed root certificate MAY be omitted
   from the chain.

   Structure of the Signed Certificate Timestamp:

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

       enum { v1(0), 255 }
         Version;

         struct {
             opaque key_id[32];
         } LogID;

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

   "key_id" is the SHA-256 hash of the log's public key [TODO: define
   how to calculate this].









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

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

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

   "timestamp" is the current UTC time since epoch (January 1, 1970,
   00:00), in milliseconds.

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

   "signed_entry" is the "leaf_certificate" (in case of an
   X509ChainEntry), or "tbs_certificate" (in case of a
   PrecertChainEntry).

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

3.2.  Including the Signed Certificate Timestamp in the TLS Handshake

   The SCT data from at least one log must be included in the TLS
   handshake, either by using an Authorization Extension [RFC5878] with
   type 182, or by using OCSP Stapling (section 8 of [RFC6066]), where
   the response includes an OCSP extension with OID
   1.3.6.1.4.1.11129.2.4.5 (see [RFC2560]) and body:

       SignedCertificateTimestampList ::= OCTET STRING

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



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

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

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

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

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

   SCTs embedded in the TLS Authorization Extension are each encoded as
   an individual AuthorizationDataEntry [RFC5878].

3.3.  Merkle Tree

   A certificate log MUST periodically append all new log entries to the
   log Merkle Tree.  The log MUST sign these entries by constructing a
   binary Merkle Tree with log entries as consecutive inputs to the
   tree, signing the corresponding Merkle Tree Hash, and publishing each
   update to the tree in a Signed Merkle Tree Update.  The hashing
   algorithm for the Merkle Tree Hash is SHA-256.

   Structure of the Merkle Tree input:














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       enum { timestamped_entry(0), 255 }
         MerkleLeafType;

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

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

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

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

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

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

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

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

3.4.  Tree Head Signature

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






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

   "version" is the version of the protocol the TreeHeadSignature
   conforms to.  This version is v1.

   "timestamp" is the current time.  The timestamp MUST be at least as
   recent as the most recent SCT timestamp in the tree.  Each subsequent
   timestamp MUST be more recent than the timestamp of the previous
   update.

   "tree_size" equals the number of entries in the new tree.

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

   The log MUST produce a Tree Head Signature at least as often as the
   Maximum Merge Delay.  In the unlikely event that it receives no new
   submissions during an MMD period, the log SHALL sign the same Merkle
   Tree Hash with a fresh timestamp.



























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4.  Client Messages

   Messages are sent as HTTPS GET or POST requests.  Parameters for
   POSTs and all responses are encoded as JSON objects.  Parameters for
   GETs are encoded as URL parameters.  Binary data is base64 encoded as
   specified in the individual messages.

   The <log server> prefix can include a path as well as a server name
   and a port.  It must map one-to-one to a known public key (how this
   mapping is distributed is out of scope for this document).

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

4.1.  Add Chain to Log

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

   Inputs

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

   Outputs

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

   id The log ID, base64 encoded.  Since clients who request an SCT for
      inclusion in the TLS handshake are not required to verify it, we
      do not assume they know the ID of the log.

   timestamp  The SCT timestamp, in decimal.

   extensions  [TBD]

   signature  The SCT signature, base64 encoded.

   If the "sct_version" is not v1, then a v1 client may be unable to
   verify the signature.  It MUST NOT construe this as an error.  [Note:
   log clients don't need to be able to verify this structure, only TLS
   clients do - if we were to serve the structure binary, then we could
   completely change it without requiring an upgrade to v1 clients].





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4.2.  Add PreCertChain to Log

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

   Inputs

   chain  An array of base64 encoded precertificates.  The first element
      is the leaf certificate, the second chains to the first and so on
      to the last, which is either the root certificate or a certificate
      that chains to a known root certificate.

   Outputs are the same as Section 4.1.

4.3.  Retrieve Latest Signed Tree Head

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

   No inputs.

   Outputs

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

   timestamp  The timestamp, in decimal.

   sha256_root_hash  The root hash of the tree, in base64.

   tree_head_signature  A TreeHeadSignature for the above data.

4.4.  Retrieve Merkle Consistency Proof between two Signed Tree Heads

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

   Inputs

   first  The tree_size of the first tree, in decimal.

   second  The tree_size of the second tree, in decimal.

   Both tree sizes must be from published v1 STHs.

   Outputs

   consistency  An array of Merkle tree nodes, base64 encoded.

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




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4.5.  Retrieve Merkle Audit Proof from Log by Leaf Hash

   GET https://<log server>/ct/v1/get-proof-by-hash

   Inputs

   hash  A base64 encoded v1 leaf hash.

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

   The "hash" must be calculated as defined in Section 3.3.  The
   "tree_size" must designate a published v1 STH.

   Outputs

   timestamp  The tree's timestamp, in decimal.

   leaf_index  The index of the leaf corresponding to the "hash"
      parameter.

   audit_path  An array of base64 encoded Merkle tree nodes proving the
      inclusion of the chosen certificate.

4.6.  Retrieve Entries from Log

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

   Inputs

   start  Index of first entry to retrieve, in decimal.

   end  Index of last entry to retrieve, in decimal.

   Outputs

   entries  An array of objects, each consisting of

      leaf_input  The base64-encoded MerkleTreeLeaf structure.

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

   Note that this message is not signed - the retrieved data can be
   verified by constructing the root hash corresponding to a retrieved
   STH.  All leaves MUST be v1.  However, a compliant v1 client MUST NOT



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   construe an unrecognized MerkleLeafType or LogEntryType value as an
   error.  This means it may be unable to parse some entries, but note
   that each client can inspect the entries it does recognize, as well
   as verify the integrity of the data by treating unrecognized leaves
   as opaque input to the tree.

4.7.  Retrieve Entry+Merkle Audit Proof from Log

   GET https://<log server>/ct/v1/get-entry-and-proof

   Inputs

   leaf_index  The index of the desired entry.

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

   The tree size must designate a published STH.

   Outputs

   entries  An array of objects, each consisting of

      leaf_input  The base64-encoded MerkleTreeLeaf structure.

      auxiliary_data  The base64-encoded unsigned data, same as in
         Section 4.6.

   timestamp  The tree's timestamp, in decimal.

   audit_path  An array of base64 encoded Merkle tree nodes proving the
      inclusion of the chosen certificate.

   This API is probably only useful for debugging.


















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5.  Clients

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

   All clients should gossip with each other, exchanging STHs at least:
   this is all that is required to ensure that they all have a
   consistent view.  The exact mechanism for gossip is TBD, but it is
   expected there will be a variety.

5.1.  Monitor

   Monitors watch the log and check that it behaves correctly.  They
   also watch for certificates of interest.

   A monitor needs to, at least, inspect every new entry in the log.  It
   may also want to keep a copy of the entire log.  In order to do this,
   it should follow these steps:

   1.  Fetch the current STH using Section 4.3.

   2.  Verify the STH signature.

   3.  Fetch all the entries in the tree corresponding to the STH using
       Section 4.6.

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

   5.  Fetch the current STH using Section 4.3.  Repeat until STH
       changes.

   6.  Verify the STH signature.

   7.  Fetch all the new entries in the tree corresponding to the STH
       using Section 4.6.  If they remain unavailable for an extended
       period, then this should be viewed as misbehaviour on the part of
       the log.

   8.  Either:

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

       Or, if it is not keeping all log entries:



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       2.  Fetch a consistency proof for the new STH with the previous
           STH using Section 4.4.

       3.  Verify the consistency proof.

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

   9.  Go to Step 5.

5.2.  Auditor

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

   Any pair of STHs from the same log can be verified by requesting a
   consistency proof using Section 4.4.

   A certificate accompanied by an SCT can be verified against any STH
   dated after the SCT timestamp + the Maximum Merge Delay by requesting
   a Merkle Audit Proof using Section 4.5.

   Auditors can fetch STHs from time to time of their own accord, of
   course, using Section 4.3.
























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6.  Security and Privacy Considerations

6.1.  Misissued Certificates

   Misissued certificates that have not been publicly logged, and thus
   do not have a valid SCT, will be rejected by clients.  Misissued
   certificates that do have an SCT from a log will appear in the 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.

6.2.  Detection of Misissue

   The log does not itself detect misissued certificate, it relies
   instead on interested parties, such as domain owners, to monitor it
   and take corrective action when a misissue is detected.

6.3.  Misbehaving logs

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

   Violation of the MMD contract is detected by clients requesting a
   Merkle audit proof for each observed SCT.  These checks can be
   asynchronous, and need only be done once per each certificate.  In
   order to protect the clients' privacy, these checks need not reveal
   the exact certificate to the log.  Clients can instead request the
   proof from a trusted auditor (since anyone can compute the audit
   proofs from the log), or request Merkle proofs for a batch of
   certificates around the SCT timestamp.

   Violation of the append-only property is detected by global
   gossiping, i.e., everyone auditing the log comparing their versions
   of the latest signed tree head.  As soon as two conflicting signed
   tree heads are detected, this is cryptographic proof of the log's
   misbehaviour.











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7.  Efficiency Considerations

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

   Auditing the log for integrity does not require third parties to
   maintain a copy of the entire log.  The Signed Tree Head can be
   updated as new entries become available, without recomputing the
   entire tree.  Third party auditors need only fetch the Merkle
   consistency proof against an existing STH to efficiently verify the
   append-only property of an update to the Merkle Tree, without
   auditing the entire tree.







































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8.  References

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

   [RFC2560]  Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
              Adams, "X.509 Internet Public Key Infrastructure Online
              Certificate Status Protocol - OCSP", RFC 2560, June 1999.

   [RFC5878]  Brown, M. and R. Housley, "The Transport Layer Security
              (TLS) Authorization Extensions", RFC 5280, May 2010.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

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

   [1]  <http://tamperevident.cs.rice.edu/Logging.html/>






























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Authors' Addresses

   Ben Laurie

   Email: benl@google.com


   Adam Langley

   Email: agl@google.com


   Emilia Kasper

   Email: ekasper@google.com




































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