GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc6962

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

                                                             June 2013
                      Certificate Transparency

Abstract

 This document describes an experimental protocol for publicly logging
 the existence of Transport Layer Security (TLS) certificates as they
 are issued or observed, in a manner that allows anyone to audit
 certificate authority (CA) activity and notice the issuance of
 suspect certificates as well as to audit the certificate logs
 themselves.  The intent is that eventually clients would refuse to
 honor certificates that do not appear in a log, effectively forcing
 CAs to add all issued certificates to the logs.
 Logs are network services that implement the protocol operations for
 submissions and queries that are defined in this document.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  This document is a product of the Internet Engineering
 Task Force (IETF).  It represents the consensus of the IETF
 community.  It has received public review and has been approved for
 publication by the Internet Engineering Steering Group (IESG).  Not
 all documents approved by the IESG are a candidate for any level of
 Internet Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6962.

Laurie, et al. Experimental [Page 1] RFC 6962 Certificate Transparency June 2013

Copyright Notice

 Copyright (c) 2013 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Informal Introduction ...........................................3
    1.1. Requirements Language ......................................4
    1.2. Data Structures ............................................4
 2. Cryptographic Components ........................................4
    2.1. Merkle Hash Trees ..........................................4
         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 and Operation ........................................9
    3.1. Log Entries ................................................9
    3.2. Structure of the Signed Certificate Timestamp .............12
    3.3. Including the Signed Certificate Timestamp in the
         TLS Handshake .............................................13
         3.3.1. TLS Extension ......................................15
    3.4. Merkle Tree ...............................................15
    3.5. Signed Tree Head ..........................................16
 4. Log Client Messages ............................................17
    4.1. Add Chain to Log ..........................................17
    4.2. Add PreCertChain to Log ...................................18
    4.3. Retrieve Latest Signed Tree Head ..........................18
    4.4. Retrieve Merkle Consistency Proof between Two
         Signed Tree Heads .........................................19
    4.5. Retrieve Merkle Audit Proof from Log by Leaf Hash .........19
    4.6. Retrieve Entries from Log .................................20
    4.7. Retrieve Accepted Root Certificates .......................21
    4.8. Retrieve Entry+Merkle Audit Proof from Log ................21
 5. Clients ........................................................21
    5.1. Submitters ................................................22
    5.2. TLS Client ................................................22
    5.3. Monitor ...................................................22

Laurie, et al. Experimental [Page 2] RFC 6962 Certificate Transparency June 2013

    5.4. Auditor ...................................................23
 6. IANA Considerations ............................................23
 7. Security Considerations ........................................23
    7.1. Misissued Certificates ....................................24
    7.2. Detection of Misissue .....................................24
    7.3. Misbehaving Logs ..........................................24
 8. Efficiency Considerations ......................................25
 9. Future Changes .................................................25
 10. Acknowledgements ..............................................25
 11. References ....................................................25
    11.1. Normative Reference ......................................25
    11.2. Informative References ...................................26

1. Informal Introduction

 Certificate transparency aims to mitigate the problem of misissued
 certificates by providing publicly auditable, append-only, untrusted
 logs of all issued certificates.  The logs are publicly auditable so
 that it is possible for anyone to verify the correctness of each log
 and to monitor when new certificates are added to it.  The logs do
 not themselves prevent misissue, but they ensure that interested
 parties (particularly those named in certificates) can detect such
 misissuance.  Note that this is a general mechanism, but in this
 document, we only describe its use for public TLS server certificates
 issued by public certificate authorities (CAs).
 Each log consists of certificate chains, which can be submitted by
 anyone.  It is expected that public CAs will contribute all their
 newly issued certificates to one or more logs; it is also expected
 that certificate holders will contribute their own certificate
 chains.  In order to avoid logs being spammed into uselessness, it is
 required that each chain is rooted in a known CA certificate.  When a
 chain is submitted to a log, a signed timestamp is returned, which
 can later be used to provide evidence to clients that the chain has
 been submitted.  TLS clients can thus require that all certificates
 they see have been logged.
 Those who are concerned about misissue can monitor the logs, asking
 them regularly for all new entries, and can thus check whether
 domains they are responsible for have had certificates issued that
 they did not expect.  What they do with this information,
 particularly when they find that a misissuance has happened, is
 beyond the scope of this document, but broadly speaking, they can
 invoke existing business mechanisms for dealing with misissued
 certificates.  Of course, anyone who wants can monitor the logs and,
 if they believe a certificate is incorrectly issued, take action as
 they see fit.

Laurie, et al. Experimental [Page 3] RFC 6962 Certificate Transparency June 2013

 Similarly, those who have seen signed timestamps from a particular
 log can later demand a proof of inclusion from that log.  If the log
 is unable to provide this (or, indeed, if the corresponding
 certificate is absent from monitors' copies of that log), that is
 evidence of the incorrect operation of the log.  The checking
 operation is asynchronous to allow TLS connections to proceed without
 delay, despite network connectivity issues and the vagaries of
 firewalls.
 The append-only property of each log is technically achieved using
 Merkle Trees, which can be used to show that any particular version
 of the log is a superset of any particular previous version.
 Likewise, Merkle Trees avoid the need to blindly trust logs: if a log
 attempts to show different things to different people, this can be
 efficiently detected by comparing tree roots and consistency proofs.
 Similarly, other misbehaviors of any log (e.g., issuing signed
 timestamps for certificates they then don't log) can be efficiently
 detected and proved to the world at large.

1.1. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. Data Structures

 Data structures are defined according to the conventions laid out in
 Section 4 of [RFC5246].

2. Cryptographic Components

2.1. Merkle Hash Trees

 Logs use a binary Merkle Hash Tree for efficient auditing.  The
 hashing algorithm is SHA-256 [FIPS.180-4] (note that this is fixed
 for this experiment, but it is anticipated that each log would be
 able to specify a hash algorithm).  The input to the Merkle Tree Hash
 is a list of data entries; these entries will be hashed to form the
 leaves of the Merkle Hash Tree.  The output is a single 32-byte
 Merkle Tree Hash.  Given an ordered list of n inputs, D[n] = {d(0),
 d(1), ..., d(n-1)}, the Merkle Tree Hash (MTH) is thus defined as
 follows:
 The hash of an empty list is the hash of an empty string:
 MTH({}) = SHA-256().

Laurie, et al. Experimental [Page 4] RFC 6962 Certificate Transparency June 2013

 The hash of a list with one entry (also known as a leaf hash) is:
 MTH({d(0)}) = SHA-256(0x00 || d(0)).
 For n > 1, let k be the largest power of two smaller than n (i.e.,
 k < n <= 2k).  The Merkle Tree Hash of an n-element list D[n] is then
 defined recursively as
 MTH(D[n]) = SHA-256(0x01 || MTH(D[0:k]) || MTH(D[k:n])),
 where || is concatenation and D[k1:k2] denotes the list {d(k1),
 d(k1+1),..., d(k2-1)} of length (k2 - k1).  (Note that the hash
 calculations for leaves and nodes differ.  This domain separation is
 required to give second preimage resistance.)
 Note that we do not require the length of the input list to be a
 power of two.  The resulting Merkle Tree may thus not be balanced;
 however, its shape is uniquely determined by the number of leaves.
 (Note: This Merkle Tree is essentially the same as the history tree
 [CrosbyWallach] proposal, except our definition handles non-full
 trees differently.)

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.
 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)}) = {}

Laurie, et al. Experimental [Page 5] RFC 6962 Certificate Transparency June 2013

 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
 Merkle Tree Hash MTH(D[0:m]), 0 < m < n, is defined as:
 PROOF(m, D[n]) = SUBPROOF(m, D[n], true)
 The subproof for m = n is empty if m is the value for which PROOF was
 originally requested (meaning that the subtree Merkle Tree Hash
 MTH(D[0:m]) is known):
 SUBPROOF(m, D[m], true) = {}
 The subproof for m = n is the Merkle Tree Hash committing inputs
 D[0:m]; otherwise:
 SUBPROOF(m, D[m], false) = {MTH(D[m])}
 For m < n, let k be the largest power of two smaller than n.  The
 subproof is then defined recursively.
 If m <= k, the right subtree entries D[k:n] only exist in the current
 tree.  We prove that the left subtree entries D[0:k] are consistent
 and add a commitment to D[k:n]:
 SUBPROOF(m, D[n], b) = SUBPROOF(m, D[0:k], b) : MTH(D[k:n])

Laurie, et al. Experimental [Page 6] RFC 6962 Certificate Transparency June 2013

 If m > k, the left subtree entries D[0:k] are identical in both
 trees.  We prove that the right subtree entries D[k:n] are consistent
 and add a commitment to D[0:k].
 SUBPROOF(m, D[n], b) = SUBPROOF(m - k, D[k:n], false) : MTH(D[0:k])
 Here, : is a concatenation of lists, and D[k1:k2] denotes the length
 (k2 - k1) list {d(k1), d(k1+1),..., d(k2-1)} as before.
 The number of nodes in the resulting proof is bounded above by
 ceil(log2(n)) + 1.

2.1.3. Example

 The binary Merkle Tree with 7 leaves:
             hash
            /    \
           /      \
          /        \
         /          \
        /            \
       k              l
      / \            / \
     /   \          /   \
    /     \        /     \
   g       h      i      j
  / \     / \    / \     |
  a b     c d    e f     d6
  | |     | |    | |
 d0 d1   d2 d3  d4 d5
 The 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].

Laurie, et al. Experimental [Page 7] RFC 6962 Certificate Transparency June 2013

 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 MUST use either elliptic
 curve signatures using the NIST P-256 curve (Section D.1.2.3 of the
 Digital Signature Standard [DSS]) or RSA signatures (RSASSA-PKCS1-
 V1_5 with SHA-256, Section 8.2 of [RFC3447]) using a key of at least
 2048 bits.

Laurie, et al. Experimental [Page 8] RFC 6962 Certificate Transparency June 2013

3. Log Format and Operation

 Anyone can submit certificates to certificate logs for public
 auditing; however, since certificates will not be accepted by TLS
 clients unless logged, it is expected that certificate owners or
 their CAs will usually submit them.  A log is a single, ever-growing,
 append-only Merkle Tree of such certificates.
 When a valid certificate is submitted to a log, the log MUST
 immediately return a Signed Certificate Timestamp (SCT).  The SCT is
 the log's promise to incorporate the certificate in the Merkle Tree
 within a fixed amount of time known as the Maximum Merge Delay (MMD).
 If the log has previously seen the certificate, it MAY return the
 same SCT as it returned before.  TLS servers MUST present an SCT from
 one or more logs to the TLS client together with the certificate.
 TLS clients MUST reject certificates that do not have a valid SCT for
 the end-entity certificate.
 Periodically, each log appends all its new entries to the Merkle Tree
 and signs the root of the tree.  Auditors can thus verify that each
 certificate for which an SCT has been issued indeed appears in the
 log.  The log MUST incorporate a certificate in its Merkle Tree
 within the Maximum Merge Delay period after the issuance of the SCT.
 Log operators MUST NOT impose any conditions on retrieving or sharing
 data from the log.

3.1. Log Entries

 Anyone can submit a certificate to any log.  In order to enable
 attribution of each logged certificate to its issuer, the log SHALL
 publish a list of acceptable root certificates (this list might
 usefully be the union of root certificates trusted by major browser
 vendors).  Each submitted certificate MUST be accompanied by all
 additional certificates required to verify the certificate chain up
 to an accepted root certificate.  The root certificate itself MAY be
 omitted from the chain submitted to the log server.
 Alternatively, (root as well as intermediate) certificate authorities
 may submit a certificate to logs prior to issuance.  To do so, the CA
 submits a Precertificate that the log can use to create an entry that
 will be valid against the issued certificate.  The Precertificate is
 constructed from the certificate to be issued by adding a special
 critical poison extension (OID 1.3.6.1.4.1.11129.2.4.3, whose
 extnValue OCTET STRING contains ASN.1 NULL data (0x05 0x00)) to the
 end-entity TBSCertificate (this extension is to ensure that the
 Precertificate cannot be validated by a standard X.509v3 client) and
 signing the resulting TBSCertificate [RFC5280] with either

Laurie, et al. Experimental [Page 9] RFC 6962 Certificate Transparency June 2013

 o  a special-purpose (CA:true, Extended Key Usage: Certificate
    Transparency, OID 1.3.6.1.4.1.11129.2.4.4) Precertificate Signing
    Certificate.  The Precertificate Signing Certificate MUST be
    directly certified by the (root or intermediate) CA certificate
    that will ultimately sign the end-entity TBSCertificate yielding
    the end-entity certificate (note that the log may relax standard
    validation rules to allow this, so long as the issued certificate
    will be valid),
 o  or, the CA certificate that will sign the final certificate.
 As above, the Precertificate submission MUST be accompanied by the
 Precertificate Signing Certificate, if used, and all additional
 certificates required to verify the chain up to an accepted root
 certificate.  The signature on the TBSCertificate indicates the
 certificate authority's intent to issue a certificate.  This intent
 is considered binding (i.e., misissuance of the Precertificate is
 considered equal to misissuance of the final certificate).  Each log
 verifies the Precertificate signature chain and issues a Signed
 Certificate Timestamp on the corresponding TBSCertificate.
 Logs MUST verify that the submitted end-entity certificate or
 Precertificate has a valid signature chain leading back to a trusted
 root CA certificate, using the chain of intermediate CA certificates
 provided by the submitter.  Logs MAY accept certificates that have
 expired, are not yet valid, have been revoked, or are otherwise not
 fully valid according to X.509 verification rules in order to
 accommodate quirks of CA certificate-issuing software.  However, logs
 MUST refuse to publish certificates without a valid chain to a known
 root CA.  If a certificate is accepted and an SCT issued, the
 accepting log MUST store the entire chain used for verification,
 including the certificate or Precertificate itself and including the
 root certificate used to verify the chain (even if it was omitted
 from the submission), and MUST present this chain for auditing upon
 request.  This chain is required to prevent a CA from avoiding blame
 by logging a partial or empty chain.  (Note: This effectively
 excludes self-signed and DANE-based certificates until some mechanism
 to control spam for those certificates is found.  The authors welcome
 suggestions.)

Laurie, et al. Experimental [Page 10] RFC 6962 Certificate Transparency June 2013

 Each certificate entry in a log MUST include the following
 components:
     enum { x509_entry(0), precert_entry(1), (65535) } LogEntryType;
     struct {
         LogEntryType entry_type;
         select (entry_type) {
             case x509_entry: X509ChainEntry;
             case precert_entry: PrecertChainEntry;
         } entry;
     } LogEntry;
     opaque ASN.1Cert<1..2^24-1>;
     struct {
         ASN.1Cert leaf_certificate;
         ASN.1Cert certificate_chain<0..2^24-1>;
     } X509ChainEntry;
     struct {
         ASN.1Cert pre_certificate;
         ASN.1Cert precertificate_chain<0..2^24-1>;
     } PrecertChainEntry;
 Logs MAY limit the length of chain they will accept.
 "entry_type" is the type of this entry.  Future revisions of this
 protocol version may add new LogEntryType values.  Section 4 explains
 how clients should handle unknown entry types.
 "leaf_certificate" is the end-entity certificate submitted for
 auditing.
 "certificate_chain" is a chain of additional certificates required to
 verify the end-entity certificate.  The first certificate MUST
 certify the end-entity certificate.  Each following certificate MUST
 directly certify the one preceding it.  The final certificate MUST be
 a root certificate accepted by the log.
 "pre_certificate" is the Precertificate submitted for auditing.
 "precertificate_chain" is a chain of additional certificates required
 to verify the Precertificate submission.  The first certificate MAY
 be a valid Precertificate Signing Certificate and MUST certify the
 first certificate.  Each following certificate MUST directly certify
 the one preceding it.  The final certificate MUST be a root
 certificate accepted by the log.

Laurie, et al. Experimental [Page 11] RFC 6962 Certificate Transparency June 2013

3.2. 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 TBSCertificate<1..2^24-1>;
       struct {
         opaque issuer_key_hash[32];
         TBSCertificate tbs_certificate;
       } PreCert;
       opaque CtExtensions<0..2^16-1>;
 "key_id" is the SHA-256 hash of the log's public key, calculated over
 the DER encoding of the key represented as SubjectPublicKeyInfo.
 "issuer_key_hash" is the SHA-256 hash of the certificate issuer's
 public key, calculated over the DER encoding of the key represented
 as SubjectPublicKeyInfo.  This is needed to bind the issuer to the
 final certificate.
 "tbs_certificate" is the DER-encoded TBSCertificate (see [RFC5280])
 component of the Precertificate -- that is, without the signature and
 the poison extension.  If the Precertificate is not signed with the
 CA certificate that will issue the final certificate, then the
 TBSCertificate also has its issuer changed to that of the CA that
 will issue the final certificate.  Note that it is also possible to
 reconstruct this TBSCertificate from the final certificate by
 extracting the TBSCertificate from it and deleting the SCT extension.
 Also note that since the TBSCertificate contains an
 AlgorithmIdentifier that must match both the Precertificate signature
 algorithm and final certificate signature algorithm, they must be
 signed with the same algorithm and parameters.  If the Precertificate
 is issued using a Precertificate Signing Certificate and an Authority
 Key Identifier extension is present in the TBSCertificate, the
 corresponding extension must also be present in the Precertificate
 Signing Certificate -- in this case, the TBSCertificate also has its
 Authority Key Identifier changed to match the final issuer.

Laurie, et al. Experimental [Page 12] RFC 6962 Certificate Transparency June 2013

     struct {
         Version sct_version;
         LogID id;
         uint64 timestamp;
         CtExtensions extensions;
         digitally-signed struct {
             Version sct_version;
             SignatureType signature_type = certificate_timestamp;
             uint64 timestamp;
             LogEntryType entry_type;
             select(entry_type) {
                 case x509_entry: ASN.1Cert;
                 case precert_entry: PreCert;
             } signed_entry;
            CtExtensions extensions;
         };
     } SignedCertificateTimestamp;
 The encoding of the digitally-signed element is defined in [RFC5246].
 "sct_version" is the version of the protocol to which the SCT
 conforms.  This version is v1.
 "timestamp" is the current NTP Time [RFC5905], measured since the
 epoch (January 1, 1970, 00:00), ignoring leap seconds, in
 milliseconds.
 "entry_type" may be implicit from the context in which the SCT is
 presented.
 "signed_entry" is the "leaf_certificate" (in the case of an
 X509ChainEntry) or is the PreCert (in the case of a
 PrecertChainEntry), as described above.
 "extensions" are future extensions to this protocol version (v1).
 Currently, no extensions are specified.

3.3. Including the Signed Certificate Timestamp in the TLS Handshake

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

Laurie, et al. Experimental [Page 13] RFC 6962 Certificate Transparency June 2013

 Request" TLS extension; see [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.
 Similarly, a certificate authority MAY submit a Precertificate to
 more than one log, and all obtained SCTs can be directly embedded in
 the final certificate, by encoding the SignedCertificateTimestampList
 structure as an ASN.1 OCTET STRING and inserting the resulting data
 in the TBSCertificate as an X.509v3 certificate extension (OID
 1.3.6.1.4.1.11129.2.4.2).  Upon receiving the certificate, clients
 can reconstruct the original TBSCertificate to verify the SCT
 signature.
 The contents of the ASN.1 OCTET STRING embedded in an OCSP extension
 or X509v3 certificate extension are as follows:
      opaque SerializedSCT<1..2^16-1>;
      struct {
          SerializedSCT sct_list <1..2^16-1>;
      } SignedCertificateTimestampList;
 Here, "SerializedSCT" is an opaque byte string that contains the
 serialized TLS structure.  This encoding ensures that TLS clients can
 decode each SCT individually (i.e., if there is a version upgrade,
 out-of-date clients can still parse old SCTs while skipping over new
 SCTs whose versions they don't understand).
 Likewise, SCTs can be embedded in a TLS extension.  See below for
 details.
 TLS clients MUST implement all three mechanisms.  Servers MUST
 implement at least one of the three mechanisms.  Note that existing
 TLS servers can generally use the certificate extension mechanism
 without modification.
 TLS servers should send SCTs from multiple logs in case one or more
 logs are not acceptable to the client (for example, if a log has been
 struck off for misbehavior or has had a key compromise).

Laurie, et al. Experimental [Page 14] RFC 6962 Certificate Transparency June 2013

3.3.1. TLS Extension

 The SCT can be sent during the TLS handshake using a TLS extension
 with type "signed_certificate_timestamp".
 Clients that support the extension SHOULD send a ClientHello
 extension with the appropriate type and empty "extension_data".
 Servers MUST only send SCTs to clients who have indicated support for
 the extension in the ClientHello, in which case the SCTs are sent by
 setting the "extension_data" to a "SignedCertificateTimestampList".
 Session resumption uses the original session information: clients
 SHOULD include the extension type in the ClientHello, but if the
 session is resumed, the server is not expected to process it or
 include the extension in the ServerHello.

3.4. Merkle Tree

 The hashing algorithm for the Merkle Tree Hash is SHA-256.
 Structure of the Merkle Tree input:
     enum { timestamped_entry(0), (255) }
       MerkleLeafType;
     struct {
         uint64 timestamp;
         LogEntryType entry_type;
         select(entry_type) {
             case x509_entry: ASN.1Cert;
             case precert_entry: PreCert;
         } signed_entry;
         CtExtensions extensions;
     } TimestampedEntry;
     struct {
         Version version;
         MerkleLeafType leaf_type;
         select (leaf_type) {
             case timestamped_entry: TimestampedEntry;
         }
     } MerkleTreeLeaf;
 Here, "version" is the version of the protocol to which the
 MerkleTreeLeaf corresponds.  This version is v1.

Laurie, et al. Experimental [Page 15] RFC 6962 Certificate Transparency June 2013

 "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 leaf hashes of the
 corresponding "MerkleTreeLeaf" structures.

3.5. Signed Tree Head

 Every time a log appends new entries to the tree, the log SHOULD sign
 the corresponding tree hash and tree information (see the
 corresponding Signed Tree Head client message in Section 4.3).  The
 signature for that data is structured as follows:
     digitally-signed struct {
         Version version;
         SignatureType signature_type = tree_hash;
         uint64 timestamp;
         uint64 tree_size;
         opaque sha256_root_hash[32];
     } TreeHeadSignature;
 "version" is the version of the protocol to which the
 TreeHeadSignature conforms.  This version is v1.
 "timestamp" is the current time.  The timestamp MUST be at least as
 recent as the most recent SCT timestamp in the tree.  Each subsequent
 timestamp MUST be more recent than the timestamp of the previous
 update.
 "tree_size" equals the number of entries in the new tree.
 "sha256_root_hash" is the root of the Merkle Hash Tree.
 Each log MUST produce on demand a Signed Tree Head that is no older
 than the Maximum Merge Delay.  In the unlikely event that it receives
 no new submissions during an MMD period, the log SHALL sign the same
 Merkle Tree Hash with a fresh timestamp.

Laurie, et al. Experimental [Page 16] RFC 6962 Certificate Transparency June 2013

4. Log Client Messages

 Messages are sent as HTTPS GET or POST requests.  Parameters for
 POSTs and all responses are encoded as JavaScript Object Notation
 (JSON) objects [RFC4627].  Parameters for GETs are encoded as order-
 independent key/value URL parameters, using the "application/
 x-www-form-urlencoded" format described in the "HTML 4.01
 Specification" [HTML401].  Binary data is base64 encoded [RFC4648] as
 specified in the individual messages.
 Note that JSON objects and URL parameters may contain fields not
 specified here.  These extra fields should be ignored.
 The <log server> prefix can include a path as well as a server name
 and a port.
 In general, where needed, the "version" is v1 and the "id" is the log
 id for the log server queried.
 Any errors will be returned as HTTP 4xx or 5xx responses, with human-
 readable error messages.

4.1. Add Chain to Log

 POST https://<log server>/ct/v1/add-chain
 Inputs:
    chain:  An array of base64-encoded certificates.  The first
       element is the end-entity certificate; the second chains to the
       first and so on to the last, which is either the root
       certificate or a certificate that chains to a known root
       certificate.
 Outputs:
    sct_version:  The version of the SignedCertificateTimestamp
       structure, in decimal.  A compliant v1 implementation MUST NOT
       expect this to be 0 (i.e., v1).
    id:  The log ID, base64 encoded.  Since log clients who request an
       SCT for inclusion in TLS handshakes are not required to verify
       it, we do not assume they know the ID of the log.
    timestamp:  The SCT timestamp, in decimal.

Laurie, et al. Experimental [Page 17] RFC 6962 Certificate Transparency June 2013

    extensions:  An opaque type for future expansion.  It is likely
       that not all participants will need to understand data in this
       field.  Logs should set this to the empty string.  Clients
       should decode the base64-encoded data and include it in the
       SCT.
    signature:  The SCT signature, base64 encoded.
 If the "sct_version" is not v1, then a v1 client may be unable to
 verify the signature.  It MUST NOT construe this as an error.  (Note:
 Log clients don't need to be able to verify this structure; only TLS
 clients do.  If we were to serve the structure as a binary blob, then
 we could completely change it without requiring an upgrade to v1
 clients.)

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 end-entity certificate; the second chains to the
       first and so on to the last, which is either the root
       certificate or a certificate that chains to a known root
       certificate.
 Outputs are the same as in Section 4.1.

4.3. Retrieve Latest Signed Tree Head

 GET https://<log server>/ct/v1/get-sth
 No inputs.
 Outputs:
    tree_size:  The size of the tree, in entries, in decimal.
    timestamp:  The timestamp, in decimal.
    sha256_root_hash:  The Merkle Tree Hash of the tree, in base64.
    tree_head_signature:  A TreeHeadSignature for the above data.

Laurie, et al. Experimental [Page 18] RFC 6962 Certificate Transparency June 2013

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 existing v1 STHs (Signed Tree Heads).
 Outputs:
    consistency:  An array of Merkle Tree nodes, base64 encoded.
 Note that no signature is required on this data, as it is used to
 verify an STH, which is signed.

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 on which to base the proof,
       in decimal.
 The "hash" must be calculated as defined in Section 3.4.  The
 "tree_size" must designate an existing v1 STH.
 Outputs:
    leaf_index:  The 0-based index of the end entity corresponding to
       the "hash" parameter.
    audit_path:  An array of base64-encoded Merkle Tree nodes proving
       the inclusion of the chosen certificate.

Laurie, et al. Experimental [Page 19] RFC 6962 Certificate Transparency June 2013

4.6. Retrieve Entries from Log

 GET https://<log server>/ct/v1/get-entries
 Inputs:
    start:  0-based index of first entry to retrieve, in decimal.
    end:  0-based index of last entry to retrieve, in decimal.
 Outputs:
    entries:  An array of objects, each consisting of
       leaf_input:  The base64-encoded MerkleTreeLeaf structure.
       extra_data:  The base64-encoded unsigned data pertaining to the
          log entry.  In the case of an X509ChainEntry, this is the
          "certificate_chain".  In the case of a PrecertChainEntry,
          this is the whole "PrecertChainEntry".
 Note that this message is not signed -- the retrieved data can be
 verified by constructing the Merkle Tree Hash corresponding to a
 retrieved STH.  All leaves MUST be v1.  However, a compliant v1
 client MUST NOT construe an unrecognized MerkleLeafType or
 LogEntryType value as an error.  This means it may be unable to parse
 some entries, but note that each client can inspect the entries it
 does recognize as well as verify the integrity of the data by
 treating unrecognized leaves as opaque input to the tree.
 The "start" and "end" parameters SHOULD be within the range 0 <= x <
 "tree_size" as returned by "get-sth" in Section 4.3.
 Logs MAY honor requests where 0 <= "start" < "tree_size" and "end" >=
 "tree_size" by returning a partial response covering only the valid
 entries in the specified range.  Note that the following restriction
 may also apply:
 Logs MAY restrict the number of entries that can be retrieved per
 "get-entries" request.  If a client requests more than the permitted
 number of entries, the log SHALL return the maximum number of entries
 permissible.  These entries SHALL be sequential beginning with the
 entry specified by "start".

Laurie, et al. Experimental [Page 20] RFC 6962 Certificate Transparency June 2013

4.7. Retrieve Accepted Root Certificates

 GET https://<log server>/ct/v1/get-roots
 No inputs.
 Outputs:
    certificates:  An array of base64-encoded root certificates that
       are acceptable to the log.

4.8. 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 an existing STH.
 Outputs:
    leaf_input:  The base64-encoded MerkleTreeLeaf structure.
    extra_data:  The base64-encoded unsigned data, same as in
       Section 4.6.
    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.

5. Clients

 There are various different functions clients of logs might perform.
 We describe here some typical clients and how they could function.
 Any inconsistency may be used as evidence that a log has not behaved
 correctly, and the signatures on the data structures prevent the log
 from denying that misbehavior.
 All clients should gossip with each other, exchanging STHs at least;
 this is all that is required to ensure that they all have a
 consistent view.  The exact mechanism for gossip will be described in
 a separate document, but it is expected there will be a variety.

Laurie, et al. Experimental [Page 21] RFC 6962 Certificate Transparency June 2013

5.1. Submitters

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

5.2. TLS Client

 TLS clients are not directly clients of the log, but they receive
 SCTs alongside or in server certificates.  In addition to normal
 validation of the certificate and its chain, they should validate the
 SCT by computing the signature input from the SCT data as well as the
 certificate and verifying the signature, using the corresponding
 log's public key.  Note that this document does not describe how
 clients obtain the logs' public keys.
 TLS clients MUST reject SCTs whose timestamp is in the future.

5.3. Monitor

 Monitors watch logs and check that they behave correctly.  They also
 watch for certificates of interest.
 A monitor needs to, at least, inspect every new entry in each log it
 watches.  It may also want to keep copies of entire logs.  In order
 to do this, it should follow these steps for each log:
 1.  Fetch the current STH (Section 4.3).
 2.  Verify the STH signature.
 3.  Fetch all the entries in the tree corresponding to the STH
     (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 (Section 4.3).  Repeat until the STH
     changes.
 6.  Verify the STH signature.
 7.  Fetch all the new entries in the tree corresponding to the STH
     (Section 4.6).  If they remain unavailable for an extended
     period, then this should be viewed as misbehavior on the part of
     the log.

Laurie, et al. Experimental [Page 22] RFC 6962 Certificate Transparency June 2013

 8.  Either:
     1.  Verify that the updated list of all entries generates a tree
         with the same hash as the new STH.
     Or, if it is not keeping all log entries:
     2.  Fetch a consistency proof for the new STH with the previous
         STH (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.

5.4. 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 (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 (Section 4.5).
 Auditors can fetch STHs from time to time of their own accord, of
 course (Section 4.3).

6. IANA Considerations

 IANA has allocated an RFC 5246 ExtensionType value (18) for the SCT
 TLS extension.  The extension name is "signed_certificate_timestamp".

7. Security Considerations

 With CAs, logs, and servers performing the actions described here,
 TLS clients can use logs and signed timestamps to reduce the
 likelihood that they will accept misissued certificates.  If a server
 presents a valid signed timestamp for a certificate, then the client
 knows that the certificate has been published in a log.  From this,
 the client knows that the subject of the certificate has had some

Laurie, et al. Experimental [Page 23] RFC 6962 Certificate Transparency June 2013

 time to notice the misissue and take some action, such as asking a CA
 to revoke a misissued certificate.  A signed timestamp is not a
 guarantee that the certificate is not misissued, since the subject of
 the certificate might not have checked the logs or the CA might have
 refused to revoke the certificate.
 In addition, if TLS clients will not accept unlogged certificates,
 then site owners will have a greater incentive to submit certificates
 to logs, possibly with the assistance of their CA, increasing the
 overall transparency of the system.

7.1. Misissued Certificates

 Misissued certificates that have not been publicly logged, and thus
 do not have a valid SCT, will be rejected by TLS clients.  Misissued
 certificates that do have an SCT from a log will appear in that
 public log within the Maximum Merge Delay, assuming the log is
 operating correctly.  Thus, the maximum period of time during which a
 misissued certificate can be used without being available for audit
 is the MMD.

7.2. Detection of Misissue

 The logs do not themselves detect misissued certificates; they rely
 instead on interested parties, such as domain owners, to monitor them
 and take corrective action when a misissue is detected.

7.3. 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 log clients requesting a
 Merkle audit proof for each observed SCT.  These checks can be
 asynchronous and need only be done once per each certificate.  In
 order to protect the clients' privacy, these checks need not reveal
 the exact certificate to the log.  Clients can instead request the
 proof from a trusted auditor (since anyone can compute the audit
 proofs from the log) or request Merkle proofs for a batch of
 certificates around the SCT timestamp.
 Violation of the append-only property is detected by global
 gossiping, i.e., everyone auditing logs comparing their versions of
 the latest Signed Tree Heads.  As soon as two conflicting Signed Tree

Laurie, et al. Experimental [Page 24] RFC 6962 Certificate Transparency June 2013

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

8. Efficiency Considerations

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

9. Future Changes

 This section lists things we might address in a Standards Track
 version of this document.
 o  Rather than forcing a log operator to create a new log in order to
    change the log signing key, we may allow some key roll mechanism.
 o  We may add hash and signing algorithm agility.
 o  We may describe some gossip protocols.

10. Acknowledgements

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

11. References

11.1. Normative Reference

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

Laurie, et al. Experimental [Page 25] RFC 6962 Certificate Transparency June 2013

11.2. Informative References

 [CrosbyWallach]  Crosby, S. and D. Wallach, "Efficient Data
                  Structures for Tamper-Evident Logging", Proceedings
                  of the 18th USENIX Security Symposium, Montreal,
                  August 2009, <http://static.usenix.org/event/sec09/
                  tech/full_papers/crosby.pdf>.
 [DSS]            National Institute of Standards and Technology,
                  "Digital Signature Standard (DSS)", FIPS 186-3,
                  June 2009, <http://csrc.nist.gov/publications/fips/
                  fips186-3/fips_186-3.pdf>.
 [FIPS.180-4]     National Institute of Standards and Technology,
                  "Secure Hash Standard", FIPS PUB 180-4, March 2012,
                  <http://csrc.nist.gov/publications/fips/fips180-4/
                  fips-180-4.pdf>.
 [HTML401]        Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
                  Specification", World Wide Web Consortium
                  Recommendation REC-html401-19991224, December 1999,
                  <http://www.w3.org/TR/1999/REC-html401-19991224>.
 [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.
 [RFC3447]        Jonsson, J. and B. Kaliski, "Public-Key Cryptography
                  Standards (PKCS) #1: RSA Cryptography Specifications
                  Version 2.1", RFC 3447, February 2003.
 [RFC4627]        Crockford, D., "The application/json Media Type for
                  JavaScript Object Notation (JSON)", RFC 4627,
                  July 2006.
 [RFC4648]        Josefsson, S., "The Base16, Base32, and Base64 Data
                  Encodings", RFC 4648, October 2006.
 [RFC5246]        Dierks, T. and E. Rescorla, "The Transport Layer
                  Security (TLS) Protocol Version 1.2", RFC 5246,
                  August 2008.
 [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.

Laurie, et al. Experimental [Page 26] RFC 6962 Certificate Transparency June 2013

 [RFC5905]        Mills, D., Martin, J., Burbank, J., and W. Kasch,
                  "Network Time Protocol Version 4: Protocol and
                  Algorithms Specification", RFC 5905, June 2010.
 [RFC6066]        Eastlake, D., "Transport Layer Security (TLS)
                  Extensions: Extension Definitions", RFC 6066,
                  January 2011.

Authors' Addresses

 Ben Laurie
 Google UK Ltd.
 EMail: benl@google.com
 Adam Langley
 Google Inc.
 EMail: agl@google.com
 Emilia Kasper
 Google Switzerland GmbH
 EMail: ekasper@google.com

Laurie, et al. Experimental [Page 27]

/data/webs/external/dokuwiki/data/pages/rfc/rfc6962.txt · Last modified: 2013/06/06 20:56 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki