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Versions: 00 01 02 03 04 05 06 07 09 10 11 12 RFC 6962

Network Working Group                                          B. Laurie
Internet-Draft                                                A. Langley
Expires: March 16, 2013                                        E. Kasper
                                                      September 12, 2012


                        Certificate Transparency
                      draft-laurie-pki-sunlight-00

Abstract

   The aim of Certificate Transparency is to have every public end-
   entity 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 will be able to monitor logs for
   certificates issued on their own domain.

   In order 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], or in the certificate itself via an X.509v3 certificate
   extension [RFC5280].

   In order 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.

   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 will be 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
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference



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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 16, 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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1.  Cryptographic components

1.1.  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[0: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[0:n] is then defined
   recursively as

   MTH(D[0: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 current definition omits dummy leaves._

1.1.1.  Merkle audit paths

   A Merkle audit path for a leaf in a Merkle hash tree is the list of
   all additional nodes in the Merkle tree required to compute the
   Merkle Tree Hash for that tree.  If the root computed from the audit
   path matches the true root, then the audit path is proof that the
   leaf exists in the tree.

   Given an ordered list of n inputs to the tree, D[0:n] = {d(0), ...,
   d(n-1)}, the Merkle audit path PATH(m, D[0: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[0:1] = {d(0)} is empty:



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   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 &gt m elements is
   then defined recursively as

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

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

   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.

1.1.2.  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[0: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[0: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[0:n] = {d(0), ...,
   d(n-1)}, the Merkle consistency proof PROOF(m, D[0:n]) for a previous
   root hash MTH(D[0:m]), 0 < m < n, is defined as PROOF(m, D[0:n]) =
   SUBPROOF(m, D[0: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[0:m], true) = {}

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

   SUBPROOF(m, D[0:m], false) = {MTH(D[0:m])}

   For m &lt 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]:




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   SUBPROOF(m, D[0: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[0: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.

1.1.3.  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].

   The same tree, built incrementally in four steps:







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

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

   The consistency proof between hash0 and hash is PROOF(3, D[0:7]) =
   [c, d, g, l]. c, g are used to verify hash0, and d, l are
   additionally used to verify hash.

   The consistency proof between hash1 and hash is PROOF(4, D[0:7]) =
   [l]. hash can be verified, using hash1=k and l.

   The consistency proof between hash2 and hash is PROOF(6, D[0:7]) =
   [i, j, k]. k, i are used to verify hash1, and j is additionally used
   to verify hash.















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2.  Log Format

   Certificate owners will be expected to submit certificates to
   certificate logs for public auditing.  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).  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 Signed Certificate Timestamp.

   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.

2.1.  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 signing the leaf
   TBSCertificate [RFC5280] with a special-purpose (Extended Key Usage:
   Certificate Transparency) 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 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 SCT can then be directly embedded in the final



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   certificate, by appending it to the TBSCertificate as an X.509v3
   certificate extension.  Upon receiving the certificate, clients can
   reconstruct the original TBSCertificate to verify the SCT signature.

   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 X509 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 (or
   in case of Precertificates, the entire Precertificate chain,
   including the signed Precertificate), 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;

   [benl: Should we consider a small number for the certificate_chain?
   Or perhaps note that a log can impose a limit?]

   "leaf_certificate" is the end-entity certificate submitted for



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   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;


       struct {
           uint64 timestamp;
           digitally-signed struct {
               SignatureType signature_type = certificate_timestamp;
               uint64 timestamp;
               LogEntryType entry_type;
               ASN.1Cert certificate;
           };
       } SignedCertificateTimestamp;

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

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

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






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2.2.  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:

       struct {
           uint64 timestamp;
           LogEntryType entry_type;
           ASN.1Cert certificate;
       } MerkleTreeLeaf;

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

   Structure of the Signed Merkle Tree Update:

       struct {
           uint64 old_tree_size;
           uint64 timestamp;
           MerkleTreeLeaf new_leaves<0..2^64-1>;
           digitally-signed struct {
               SignatureType signature_type = tree_hash;
               uint64 timestamp;
               uint64 tree_size;
               opaque sha256_root_hash[32];
           } TreeHeadSignature;
       } SignedMerkleTreeUpdate;

   "old_tree_size" is the size of the tree prior to this update.

   "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.

   "new_leaves" is the list of leaves added to the tree in this update,
   ordered by leaf index.  This order can be fixed arbitrarily amongst
   new entries.

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



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   The log MUST produce a Signed Merkle Tree Update 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.

2.3.  Audit Proofs

   It is possible to audit the entire log by computing the current
   "sha256_root_hash" value from consecutive Signed Merkle Tree Updates,
   and verifying the Tree Head Signature.  We rely on cross-checks of
   the Signed Tree Head between auditors to verify that their views of
   the log are consistent.

   Additionally, logs provide Merkle audit proofs for efficient partial
   checks.  (In fact, anyone can compute audit proofs from the full
   log.)  Merkle audit proofs allow auditors to efficiently verify that
   a certificate for which an SCT has been issued indeed appears in the
   log, without inspecting the entire log.

   Structure of the Merkle audit proof:

       struct {
           opaque sha256_hash[32];
       } MerkleNode;


       struct {
           uint64 tree_size;
           uint64 timestamp;
           uint64 leaf_index;
           MerkleNode audit_path<0..2^16-1>;
           TreeHeadSignature tree_head_signature;
       } MerkleAuditProof;

   "tree_size" is the generation of the tree that this proof is for.

   "timestamp" is the corresponding timestamp.

   "leaf_index" is the index of the audited node in the Merkle tree,
   from 0 to "tree_size - 1".

   "audit_path" is a list of additional nodes in the Merkle tree
   required for reconstructing the root hash corresponding to the
   "tree_size".  Nodes must be listed from leaf to root level, i.e., in
   the order they are used in the Merkle Tree Hash computation, as
   defined in Section 1.1.1. _Notice that the left-right ordering is
   determined by the position of the leaf._ The leaf node under audit as
   well as the root node shall be omitted from the path.



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   "tree_head_signature" is the TreeHeadSignature for generation
   "tree_size".

   A valid audit proof for a Merkle Tree Leaf MUST satisfy the
   following:

   o  The "tree_size" MUST be at least 1;

   o  The "leaf_index" MUST NOT exceed "tree_size - 1";

   o  The "tree_signature" MUST be a valid signature on the
      corresponding "timestamp", "tree_size", and the root hash
      reconstructed from the Merkle Tree Leaf, "leaf_index" and
      "audit_path".





































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3.  Security and Privacy Considerations

3.1.  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 update 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 notice is the MMD.

3.2.  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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4.  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 root hash
   can be updated incrementally as new entries become available, without
   recomputing the entire tree.  Third party auditors need only store a
   logarithmic number of intermediate nodes in the Merkle Tree.

   Additionally, the Merkle consistency proofs defined in Section 1.1.2
   can be used to efficiently prove the append-only property of an
   incremental update to the Merkle Tree, without auditing the entire
   tree.




































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5.  References

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

   [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.

   [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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