Public Notary Transparency Working Group B. Laurie
Internet-Draft A. Langley
Intended status: Standards Track E. Kasper
Expires: April 21, 2016 E. Messeri
Google
R. Stradling
Comodo
October 19, 2015
Certificate Transparency
draft-ietf-trans-rfc6962-bis-10
Abstract
This document describes a protocol for publicly logging the existence
of Transport Layer Security (TLS) certificates as they are issued or
observed, in a manner that allows anyone to audit certification
authority (CA) activity and notice the issuance of suspect
certificates as well as to audit the certificate logs themselves.
The intent is that eventually clients would refuse to honor
certificates that do not appear in a log, effectively forcing CAs to
add all issued certificates to the logs.
Logs are network services that implement the protocol operations for
submissions and queries that are defined in this document.
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
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 21, 2016.
Laurie, et al. Expires April 21, 2016 [Page 1]
Internet-Draft Certificate Transparency October 2015
Copyright Notice
Copyright (c) 2015 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. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
1.2. Data Structures . . . . . . . . . . . . . . . . . . . . . 5
2. Cryptographic Components . . . . . . . . . . . . . . . . . . 5
2.1. Merkle Hash Trees . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Merkle Inclusion Proofs . . . . . . . . . . . . . . . 6
2.1.2. Merkle Consistency Proofs . . . . . . . . . . . . . . 6
2.1.3. Example . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.4. Signatures . . . . . . . . . . . . . . . . . . . . . 9
3. Submitters . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Certificates . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Precertificates . . . . . . . . . . . . . . . . . . . . . 10
4. Private Domain Name Labels . . . . . . . . . . . . . . . . . 11
4.1. Wildcard Certificates . . . . . . . . . . . . . . . . . . 11
4.2. Redacting Domain Name Labels in Precertificates . . . . . 11
4.3. Using a Name-Constrained Intermediate CA . . . . . . . . 12
5. Log Format and Operation . . . . . . . . . . . . . . . . . . 13
5.1. Accepting Submissions . . . . . . . . . . . . . . . . . . 14
5.2. Log Entries . . . . . . . . . . . . . . . . . . . . . . . 14
5.3. Structure of the Signed Certificate Timestamp . . . . . . 16
5.4. Merkle Tree . . . . . . . . . . . . . . . . . . . . . . . 18
5.5. Signed Tree Head (STH) . . . . . . . . . . . . . . . . . 19
5.5.1. Structure of the STH . . . . . . . . . . . . . . . . 19
6. Log Client Messages . . . . . . . . . . . . . . . . . . . . . 20
6.1. Add Chain to Log . . . . . . . . . . . . . . . . . . . . 22
6.2. Add PreCertChain to Log . . . . . . . . . . . . . . . . . 23
6.3. Retrieve Latest Signed Tree Head . . . . . . . . . . . . 23
6.4. Retrieve Merkle Consistency Proof between Two Signed Tree
Heads . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.5. Retrieve Merkle Inclusion Proof from Log by Leaf Hash . . 24
6.6. Retrieve Merkle Inclusion Proof, Signed Tree Head and
Laurie, et al. Expires April 21, 2016 [Page 2]
Internet-Draft Certificate Transparency October 2015
Consistency Proof by Leaf Hash . . . . . . . . . . . . . 25
6.7. Retrieve Entries and STH from Log . . . . . . . . . . . . 26
6.8. Retrieve Accepted Root Certificates . . . . . . . . . . . 28
7. TLS Servers . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.1. TLS Extension . . . . . . . . . . . . . . . . . . . . . . 29
8. Certification Authorities . . . . . . . . . . . . . . . . . . 29
8.1. X.509v3 Extension . . . . . . . . . . . . . . . . . . . . 29
8.1.1. OCSP Response Extension . . . . . . . . . . . . . . . 29
8.1.2. Certificate Extension . . . . . . . . . . . . . . . . 30
9. Clients . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.1. Metadata . . . . . . . . . . . . . . . . . . . . . . . . 31
9.2. TLS Client . . . . . . . . . . . . . . . . . . . . . . . 31
9.3. Monitor . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.4. Auditing . . . . . . . . . . . . . . . . . . . . . . . . 33
9.4.1. Verifying an inclusion proof . . . . . . . . . . . . 33
9.4.2. Verifying consistency between two STHs . . . . . . . 34
9.4.3. Verifying root hash given entries . . . . . . . . . . 35
10. Algorithm Agility . . . . . . . . . . . . . . . . . . . . . . 36
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
11.1. TLS Extension Type . . . . . . . . . . . . . . . . . . . 36
11.2. Hash Algorithms . . . . . . . . . . . . . . . . . . . . 36
11.3. SCT Extensions . . . . . . . . . . . . . . . . . . . . . 36
11.4. STH Extensions . . . . . . . . . . . . . . . . . . . . . 36
12. Security Considerations . . . . . . . . . . . . . . . . . . . 37
12.1. Misissued Certificates . . . . . . . . . . . . . . . . . 37
12.2. Detection of Misissue . . . . . . . . . . . . . . . . . 37
12.3. Redaction of Public Domain Name Labels . . . . . . . . . 38
12.4. Misbehaving Logs . . . . . . . . . . . . . . . . . . . . 38
12.5. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . 38
13. Efficiency Considerations . . . . . . . . . . . . . . . . . . 39
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 39
15.1. Normative References . . . . . . . . . . . . . . . . . . 39
15.2. Informative References . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
1. 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 certification authorities (CAs).
Laurie, et al. Expires April 21, 2016 [Page 3]
Internet-Draft Certificate Transparency October 2015
Each log consists of certificate chains, which can be submitted by
anyone. It is expected that public CAs will contribute all their
newly issued certificates to one or more logs, however certificate
holders can also contribute their own certificate chains, as can
third parties. In order to avoid logs being rendered useless by
submitting large numbers of spurious certificates, it is required
that each chain is rooted in a CA certificate accepted by the log.
When a chain is submitted to a log, a signed timestamp is returned,
which can later be used to provide evidence to TLS clients that the
chain has been submitted. TLS clients can thus require that all
certificates they accept as valid are accompanied by signed
timestamps.
Those who are concerned about misissue can monitor the logs, asking
them regularly for all new entries, and can thus check whether
domains they are responsible for have had certificates issued that
they did not expect. What they do with this information,
particularly when they find that a misissuance has happened, is
beyond the scope of this document, but broadly speaking, they can
invoke existing business mechanisms for dealing with misissued
certificates, such as working with the CA to get the certificate
revoked, or with maintainers of trust anchor lists to get the CA
removed. Of course, anyone who wants can monitor the logs and, if
they believe a certificate is incorrectly issued, take action as they
see fit.
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 instance
of the log is a superset of any particular previous instance.
Likewise, Merkle Trees avoid the need to blindly trust logs: if a log
attempts to show different things to different people, this can be
efficiently detected by comparing tree roots and consistency proofs.
Similarly, other misbehaviors of any log (e.g., issuing signed
timestamps for certificates they then don't log) can be efficiently
detected and proved to the world at large.
Laurie, et al. Expires April 21, 2016 [Page 4]
Internet-Draft Certificate Transparency October 2015
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 used by each log is expected to be specified as
part of the metadata relating to that log. We have established a
registry of acceptable algorithms, see Section 11.2. The hashing
algorithm in use is referred to as HASH throughout this document and
the size of its output in bytes as HASH_SIZE. The input to the
Merkle Tree Hash is a list of data entries; these entries will be
hashed to form the leaves of the Merkle Hash Tree. The output is a
single HASH_SIZE Merkle Tree Hash. Given an ordered list of n
inputs, D[n] = {d(0), d(1), ..., d(n-1)}, the Merkle Tree Hash (MTH)
is thus defined as follows:
The hash of an empty list is the hash of an empty string:
MTH({}) = HASH().
The hash of a list with one entry (also known as a leaf hash) is:
MTH({d(0)}) = HASH(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]) = HASH(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;
Laurie, et al. Expires April 21, 2016 [Page 5]
Internet-Draft Certificate Transparency October 2015
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 Inclusion Proofs
A Merkle inclusion proof for a leaf in a Merkle Hash Tree is the
shortest list of additional nodes in the Merkle Tree required to
compute the Merkle Tree Hash for that tree. Each node in the tree is
either a leaf node or is computed from the two nodes immediately
below it (i.e., towards the leaves). At each step up the tree
(towards the root), a node from the inclusion proof is combined with
the node computed so far. In other words, the inclusion proof
consists of the list of missing nodes required to compute the nodes
leading from a leaf to the root of the tree. If the root computed
from the inclusion proof matches the true root, then the inclusion
proof proves that the leaf exists in the tree.
Given an ordered list of n inputs to the tree, D[n] = {d(0), ...,
d(n-1)}, the Merkle inclusion proof PATH(m, D[n]) for the (m+1)th
input d(m), 0 <= m < n, is defined as follows:
The proof 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
proof 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
Laurie, et al. Expires April 21, 2016 [Page 6]
Internet-Draft Certificate Transparency October 2015
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])
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
Laurie, et al. Expires April 21, 2016 [Page 7]
Internet-Draft Certificate Transparency October 2015
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 inclusion proof for d0 is [b, h, l].
The inclusion proof for d3 is [c, g, l].
The inclusion proof for d4 is [f, j, k].
The inclusion proof for d6 is [i, k].
Laurie, et al. Expires April 21, 2016 [Page 8]
Internet-Draft Certificate Transparency October 2015
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
deterministic ECDSA [RFC6979] using the NIST P-256 curve
(Section D.1.2.3 of the Digital Signature Standard [DSS]) and HMAC-
SHA256 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. Expires April 21, 2016 [Page 9]
Internet-Draft Certificate Transparency October 2015
3. Submitters
Submitters submit certificates or precertificates to logs for public
auditing, as described below. In order to enable attribution of each
logged certificate or precertificate to its issuer, each submission
MUST be accompanied by all additional certificates required to verify
the chain up to an accepted root certificate. The root certificate
itself MAY be omitted from the submission.
If a log accepts a submission, it will return a Signed Certificate
Timestamp (SCT). The submitter SHOULD validate the returned SCT as
described in Section 9.2 if they understand its format and they
intend to use it directly in a TLS handshake or to construct a
certificate.
3.1. Certificates
Anyone can submit a certificate (Section 6.1) to a log. Since
certificates may not be accepted by TLS clients unless logged, it is
expected that certificate owners or their CAs will usually submit
them.
3.2. Precertificates
Alternatively, (root as well as intermediate) CAs may preannounce a
certificate prior to issuance by submitting a precertificate
(Section 6.2) that the log can use to create an entry that will be
valid against the issued certificate. The CA MAY incorporate the
returned SCT in the issued certificate.
A precertificate is a CMS [RFC5652] "signed-data" object that
conforms to the following requirements:
o It MUST be DER encoded.
o "SignedData.encapContentInfo.eContentType" MUST be the OID <TBD>.
o "SignedData.encapContentInfo.eContent" MUST contain a
TBSCertificate [RFC5280], which MAY redact certain domain name
labels that will be present in the issued certificate (see
Section 4.2) and MUST NOT contain any SCTs, but which will be
otherwise identical to the TBSCertificate in the issued
certificate.
o "SignedData.signerInfos" MUST contain a signature from the same
(root or intermediate) CA that will ultimately issue the
certificate. This signature indicates the CA's intent to issue
the certificate. This intent is considered binding (i.e.
Laurie, et al. Expires April 21, 2016 [Page 10]
Internet-Draft Certificate Transparency October 2015
misissuance of the precertificate is considered equivalent to
misissuance of the certificate). (Note that, because of the
structure of CMS, the signature on the CMS object will not be a
valid X.509v3 signature and so cannot be used to construct a
certificate from the precertificate).
o "SignedData.certificates" SHOULD be omitted.
4. Private Domain Name Labels
Some regard some DNS domain name labels within their registered
domain space as private and security sensitive. Even though these
domains are often only accessible within the domain owner's private
network, it's common for them to be secured using publicly trusted
TLS server certificates. We define a mechanism to allow these
private labels to not appear in public logs.
4.1. Wildcard Certificates
A certificate containing a DNS-ID [RFC6125] of "*.example.com" could
be used to secure the domain "topsecret.example.com", without
revealing the string "topsecret" publicly.
Since TLS clients only match the wildcard character to the complete
leftmost label of the DNS domain name (see Section 6.4.3 of
[RFC6125]), this approach would not work for a DNS-ID such as
"top.secret.example.com". Also, wildcard certificates are prohibited
in some cases, such as Extended Validation Certificates
[EVSSLGuidelines].
4.2. Redacting Domain Name Labels in Precertificates
When creating a precertificate, the CA MAY substitute one or more
labels in each DNS-ID with a corresponding number of "?" labels.
Every label to the left of a "?" label MUST also be redacted. For
example, if a certificate contains a DNS-ID of
"top.secret.example.com", then the corresponding precertificate could
contain "?.?.example.com" instead, but not "top.?.example.com"
instead.
Wildcard "*" labels MUST NOT be redacted. However, if the complete
leftmost label of a DNS-ID is "*", it is considered redacted for the
purposes of determining if the label to the right may be redacted.
For example, if a certificate contains a DNS-ID of
"*.top.secret.example.com", then the corresponding precertificate
could contain "*.?.?.example.com" instead, but not
"?.?.?.example.com" instead.
Laurie, et al. Expires April 21, 2016 [Page 11]
Internet-Draft Certificate Transparency October 2015
When a precertificate contains one or more redacted labels, a non-
critical extension (OID 1.3.6.1.4.1.11129.2.4.6, whose extnValue
OCTET STRING contains an ASN.1 SEQUENCE OF INTEGERs) MUST be added to
the corresponding certificate: the first INTEGER indicates the total
number of redacted labels and wildcard "*" labels in the
precertificate's first DNS-ID; the second INTEGER does the same for
the precertificate's second DNS-ID; etc. There MUST NOT be more
INTEGERs than there are DNS-IDs. If there are fewer INTEGERs than
there are DNS-IDs, the shortfall is made up by implicitly repeating
the last INTEGER. Each INTEGER MUST have a value of zero or more.
The purpose of this extension is to enable TLS clients to accurately
reconstruct the TBSCertificate component of the precertificate from
the certificate without having to perform any guesswork.
When a precertificate contains that extension and contains a CN-ID
[RFC6125], the CN-ID MUST match the first DNS-ID and have the same
labels redacted. TLS clients will use the first entry in the
SEQUENCE OF INTEGERs to reconstruct both the first DNS-ID and the CN-
ID.
4.3. Using a Name-Constrained Intermediate CA
An intermediate CA certificate or intermediate CA precertificate that
contains the critical or non-critical Name Constraints [RFC5280]
extension MAY be logged in place of end-entity certificates issued by
that intermediate CA, as long as all of the following conditions are
met:
o there MUST be a non-critical extension (OID
1.3.6.1.4.1.11129.2.4.7, whose extnValue OCTET STRING contains
ASN.1 NULL data (0x05 0x00)). This extension is an explicit
indication that it is acceptable to not log certificates issued by
this intermediate CA.
o permittedSubtrees MUST specify one or more dNSNames.
o excludedSubtrees MUST specify the entire IPv4 and IPv6 address
ranges.
Laurie, et al. Expires April 21, 2016 [Page 12]
Internet-Draft Certificate Transparency October 2015
Below is an example Name Constraints extension that meets these
conditions:
SEQUENCE {
OBJECT IDENTIFIER '2 5 29 30'
OCTET STRING, encapsulates {
SEQUENCE {
[0] {
SEQUENCE {
[2] 'example.com'
}
}
[1] {
SEQUENCE {
[7] 00 00 00 00 00 00 00 00
}
SEQUENCE {
[7]
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
}
}
}
}
}
5. Log Format and Operation
A log is a single, append-only Merkle Tree of submitted certificate
and precertificate entries.
When it receives a valid submission, the log MUST return an SCT that
corresponds to the submitted certificate or precertificate. If the
log has previously seen this valid submission, it MAY return the same
SCT as it returned before (note that if a certificate was previously
logged as a precertificate, then the precertificate's SCT would not
be appropriate, instead a fresh SCT of type x509_entry should be
generated).
An SCT is the log's promise to incorporate the submitted entry in its
Merkle Tree no later than a fixed amount of time, known as the
Maximum Merge Delay (MMD), after the issuance of the SCT.
Periodically, the log MUST append all its new entries to its Merkle
Tree and sign the root of the tree. This provides auditable evidence
that the log kept all its promises.
Log operators MUST NOT impose any conditions on retrieving or sharing
data from the log.
Laurie, et al. Expires April 21, 2016 [Page 13]
Internet-Draft Certificate Transparency October 2015
5.1. Accepting Submissions
Logs MUST verify that each submitted certificate or precertificate
has a valid signature chain to an accepted root certificate, using
the chain of intermediate CA certificates provided by the submitter.
Logs MUST accept certificates and precertificates that are fully
valid according to RFC 5280 [RFC5280] verification rules and are
submitted with such a chain. Logs MAY accept certificates and
precertificates that have expired, are not yet valid, have been
revoked, or are otherwise not fully valid according to RFC 5280
verification rules in order to accommodate quirks of CA certificate-
issuing software. However, logs MUST reject submissions without a
valid signature chain to an accepted root certificate. Logs MUST
also reject precertificates that do not conform to the requirements
in Section 3.2.
Logs SHOULD limit the length of chain they will accept. The maximum
chain length is specified in the log's metadata.
The log SHALL allow retrieval of its list of accepted root
certificates (see Section 6.8). This list might usefully be the
union of root certificates trusted by major browser vendors.
5.2. Log Entries
If a submission is accepted and an SCT issued, the accepting log MUST
store the entire chain used for verification. This chain MUST
include the certificate or precertificate itself, the zero or more
intermediate CA certificates provided by the submitter, and the root
certificate used to verify the chain (even if it was omitted from the
submission). The log MUST present this chain for auditing upon
request. This chain is required to prevent a CA from avoiding blame
by logging a partial or empty chain.
Laurie, et al. Expires April 21, 2016 [Page 14]
Internet-Draft Certificate Transparency October 2015
Each certificate entry in a log MUST include a "X509ChainEntry"
structure, and each precertificate entry MUST include a
"PrecertChainEntryV2" structure:
enum {
x509_entry(0), precert_entry_V2(2), (65535)
} LogEntryType;
opaque ASN.1Cert<1..2^24-1>;
struct {
ASN.1Cert leaf_certificate;
ASN.1Cert certificate_chain<0..2^24-1>;
} X509ChainEntry;
opaque CMSPrecert<1..2^24-1>;
struct {
CMSPrecert pre_certificate;
ASN.1Cert precertificate_chain<0..2^24-1>;
} PrecertChainEntryV2;
"entry_type" is the type of this entry. Future revisions of this
protocol may add new LogEntryType values. Section 6 explains how
clients should handle unknown entry types.
"leaf_certificate" is a submitted certificate that has been accepted
by the log.
"certificate_chain" is a vector of 0 or more additional certificates
required to verify "leaf_certificate". The first certificate MUST
certify "leaf_certificate". Each following certificate MUST directly
certify the one preceding it. The final certificate MUST be a root
certificate accepted by the log. If "leaf_certificate" is a root
certificate, then this vector is empty.
"pre_certificate" is a submitted precertificate that has been
accepted by the log.
"precertificate_chain" is a vector of 1 or more additional
certificates required to verify "pre_certificate". The first
certificate MUST certify "pre_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. Expires April 21, 2016 [Page 15]
Internet-Draft Certificate Transparency October 2015
5.3. Structure of the Signed Certificate Timestamp
enum {
certificate_timestamp(0), tree_hash(1), (255)
} SignatureType;
enum {
v2(1), (255)
} Version;
struct {
opaque key_id[HASH_SIZE];
} LogID;
opaque TBSCertificate<1..2^24-1>;
struct {
opaque issuer_key_hash[HASH_SIZE];
TBSCertificate tbs_certificate;
} CertInfo;
enum {
reserved(65535)
} SctExtensionType;
struct {
SctExtensionType sct_extension_type;
opaque sct_extension_data<0..2^16-1>;
} SctExtension;
SctExtension SctExtensions<0..2^16-1>;
"key_id" is the HASH of the log's public key, calculated over the DER
encoding of the key represented as SubjectPublicKeyInfo.
"issuer_key_hash" is the 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, making it impossible for the SCT to be valid for any
other certificate.
"tbs_certificate" is the DER-encoded TBSCertificate component of the
precertificate. Note that it is also possible to reconstruct this
TBSCertificate from the issued certificate by extracting the
TBSCertificate from it, redacting the domain name labels indicated by
the redacted labels extension, and deleting the SCT list extension
and redacted labels extension.
Laurie, et al. Expires April 21, 2016 [Page 16]
Internet-Draft Certificate Transparency October 2015
"sct_extension_type" identifies a single extension from the IANA
registry in Section 11.3.
The interpretation of the "sct_extension_data" field is determined
solely by the value of the "sct_extension_type" field. Each document
that registers a new "sct_extension_type" must describe how to
interpret the corresponding "sct_extension_data".
The "SctExtensions" type is a vector of 0 or more extensions. This
vector MUST NOT include more than one extension with the same
"sct_extension_type". The extensions in the vector MUST be ordered
by the value of the "sct_extension_type" field, smallest value first.
struct {
Version sct_version;
LogID id;
uint64 timestamp;
SctExtensions extensions;
digitally-signed struct {
Version sct_version;
SignatureType signature_type = certificate_timestamp;
uint64 timestamp;
LogEntryType entry_type;
select(entry_type) {
case x509_entry: CertInfo;
case precert_entry_V2: CertInfo;
} signed_entry;
SctExtensions 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 v2. Note that SignedCertificateTimestamp
v1 [RFC6962] had a different definition of "signed_entry".
"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" includes the TBSCertificate from either the
"leaf_certificate" (in the case of an X509ChainEntry) or the
"pre_certificate" (in the case of a PrecertChainEntryV2).
Laurie, et al. Expires April 21, 2016 [Page 17]
Internet-Draft Certificate Transparency October 2015
"extensions" are future extensions to SignedCertificateTimestamp v2.
Currently, no extensions are specified. If an implementation sees an
extension that it does not understand, it SHOULD ignore that
extension. Furthermore, an implementation MAY choose to ignore any
extension(s) that it does understand.
5.4. Merkle Tree
The hashing algorithm for the Merkle Tree Hash is specified in the
log's metadata.
Structure of the Merkle Tree input:
enum {
v1(0), v2(1), (255)
} LeafVersion;
struct {
uint64 timestamp;
LogEntryType entry_type;
select(entry_type) {
case x509_entry: CertInfo;
case precert_entry_V2: CertInfo;
} signed_entry;
SctExtensions extensions;
} TimestampedEntry;
struct {
LeafVersion version;
TimestampedEntry timestamped_entry;
} MerkleTreeLeaf;
Here, "version" is the version of the MerkleTreeLeaf structure. This
version is v2. Note that MerkleTreeLeaf v1 [RFC6962] had another
layer of indirection which is removed in v2.
"timestamp" is the timestamp of the corresponding SCT issued for this
certificate.
"entry_type" is the type of entry stored in "signed_entry". New
"LogEntryType" values may be added to "signed_entry" without
increasing the "MerkleTreeLeaf" version. Section 6 explains how
clients should handle unknown entry types.
"signed_entry" is the "signed_entry" of the corresponding SCT.
"extensions" are the "extensions" of the corresponding SCT.
Laurie, et al. Expires April 21, 2016 [Page 18]
Internet-Draft Certificate Transparency October 2015
The leaves of the Merkle Tree are the leaf hashes of the
corresponding "MerkleTreeLeaf" structures. Note that leaf hashes
(Section 2.1) are calculated as HASH(0x00 || MerkleTreeLeaf).
5.5. Signed Tree Head (STH)
Periodically the log SHOULD sign the corresponding tree hash and tree
information (see the corresponding Signed Tree Head client message in
Section 6.3).
Each log MUST produce on demand a Signed Tree Head that is no older
than the Maximum Merge Delay. However, Signed Tree Heads could be
used to mark individual clients (by producing a new one for each
query), so logs MUST NOT produce them more frequently than is
declared in their metadata. In general, there is no need to produce
a new Signed Tree Head unless there are new entries in the log,
however, in the unlikely event that it receives no new submissions
during an MMD period, the log SHALL sign the same Merkle Tree Hash
with a fresh timestamp.
5.5.1. Structure of the STH
enum {
v2(1), (255)
} TreeHeadVersion;
enum {
reserved(65535)
} SthExtensionType;
struct {
SthExtensionType sth_extension_type;
opaque sth_extension_data<0..2^16-1>;
} SthExtension;
SthExtension SthExtensions<0..2^16-1>;
"sth_extension_type" identifies a single extension from the IANA
registry in Section 11.4.
The interpretation of the "sth_extension_data" field is determined
solely by the value of the "sth_extension_type" field. Each document
that registers a new "sth_extension_type" must describe how to
interpret the corresponding "sth_extension_data".
The "SthExtensions" type is a vector of 0 or more extensions. This
vector MUST NOT include more than one extension with the same
Laurie, et al. Expires April 21, 2016 [Page 19]
Internet-Draft Certificate Transparency October 2015
"sth_extension_type". The extensions in the vector MUST be ordered
by the value of the "sth_extension_type" field, smallest value first.
struct {
TreeHeadVersion version;
LogID id;
uint64 timestamp;
uint64 tree_size;
opaque root_hash[HASH_SIZE];
SthExtensions extensions;
digitally-signed struct {
TreeHeadVersion version;
SignatureType signature_type = tree_hash;
LogID id;
uint64 timestamp;
uint64 tree_size;
opaque root_hash[HASH_SIZE];
SthExtensions extensions;
};
} SignedTreeHead;
"version" is the version of the SignedTreeHead structure. This
version is v2. Note that TreeHeadSignature v1 [RFC6962] only
included the inner "digitally-signed struct" and did not include the
"id" or "extensions" fields.
"timestamp" is the current NTP Time [RFC5905], measured since the
epoch (January 1, 1970, 00:00), ignoring leap seconds, in
milliseconds. 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.
"root_hash" is the root of the Merkle Hash Tree.
"extensions" are future extensions to SignedTreeHead v2. Currently,
no extensions are specified. If an implementation sees an extension
that it does not understand, it SHOULD ignore that extension.
Furthermore, an implementation MAY choose to ignore any extension(s)
that it does understand.
6. 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-
Laurie, et al. Expires April 21, 2016 [Page 20]
Internet-Draft Certificate Transparency October 2015
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 MAY 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.
In practice, log servers may include multiple front-end machines.
Since it is impractical to keep these machines in perfect sync,
errors may occur that are caused by skew between the machines. Where
such errors are possible, the front-end will return additional
information (as specified below) making it possible for clients to
make progress, if progress is possible. Front-ends MUST only serve
data that is free of gaps (that is, for example, no front-end will
respond with an STH unless it is also able to prove consistency from
all log entries logged within that STH).
For example, when a consistency proof between two STHs is requested,
the front-end reached may not yet be aware of one or both STHs. In
the case where it is unaware of both, it will return the latest STH
it is aware of. Where it is aware of the first but not the second,
it will return the latest STH it is aware of and a consistency proof
from the first STH to the returned STH. The case where it knows the
second but not the first should not arise (see the "no gaps"
requirement above).
If the log is unable to process a client's request, it MUST return an
HTTP response code of 4xx/5xx (see [RFC2616]), and, in place of the
responses outlined in the subsections below, the body SHOULD be a
JSON structure containing at least the following field:
error_message: A human-readable string describing the error which
prevented the log from processing the request.
In the case of a malformed request, the string SHOULD provide
sufficient detail for the error to be rectified.
error_code: An error code readable by the client. Some codes are
generic and are detailed here. Others are detailed in the
individual requests. Error codes are fixed text strings.
not compliant The request is not compliant with this RFC.
Laurie, et al. Expires April 21, 2016 [Page 21]
Internet-Draft Certificate Transparency October 2015
e.g. In response to a request of "/ct/v2/get-
entries?start=100&end=99", the log would return a "400 Bad Request"
response code with a body similar to the following:
{
"error_message": "'start' cannot be greater than 'end'",
"error_code": "not compliant",
}
Clients SHOULD treat "500 Internal Server Error" and "503 Service
Unavailable" responses as transient failures and MAY retry the same
request without modification at a later date. Note that as per
[RFC2616], in the case of a 503 response the log MAY include a
"Retry-After:" header in order to request a minimum time for the
client to wait before retrying the request.
6.1. Add Chain to Log
POST https://<log server>/ct/v2/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: The base64 encoded "SignedCertificateTimestamp" for the
submitted certificate.
Error codes:
unknown root The root of the chain is not one accepted by the
log.
bad chain The alleged chain is not actually a chain of
certificates.
bad certificate One or more certificates in the chain are not
valid (e.g. not properly encoded).
If the version of "sct" is not v2, then a v2 client may be unable to
verify the signature. It MUST NOT construe this as an error. This
is to avoid forcing an upgrade of compliant v2 clients that do not
use the returned SCTs.
Laurie, et al. Expires April 21, 2016 [Page 22]
Internet-Draft Certificate Transparency October 2015
If a log detects bad encoding in a chain that otherwise verifies
correctly then the log MAY still log the certificate but SHOULD NOT
return an SCT. It should instead return the "bad certificate" error.
Logging the certificate is useful, because monitors (Section 9.3) can
then detect these encoding errors, which may be accepted by some TLS
clients.
Note that not all certificate handling software is capable of
detecting all encoding errors (e.g. some software will accept BER
instead of DER encodings in certificates, or incorrect character
encodings, even though these are technically incorrect) .
6.2. Add PreCertChain to Log
POST https://<log server>/ct/v2/add-pre-chain
Inputs:
precertificate: The base64 encoded precertificate.
chain: An array of base64 encoded CA certificates. The first
element is the signer of the precertificate; the second chains
to the first and so on to the last, which is either the root
certificate or a certificate that chains to an accepted root
certificate.
Outputs and errors are the same as in Section 6.1.
6.3. Retrieve Latest Signed Tree Head
GET https://<log server>/ct/v2/get-sth
No inputs.
Outputs:
sth: A base64 encoded SignedTreeHead.
6.4. Retrieve Merkle Consistency Proof between Two Signed Tree Heads
GET https://<log server>/ct/v2/get-sth-consistency
Inputs:
first: The tree_size of the older tree, in decimal.
second: The tree_size of the newer tree, in decimal (optional).
Laurie, et al. Expires April 21, 2016 [Page 23]
Internet-Draft Certificate Transparency October 2015
Both tree sizes must be from existing v2 STHs (Signed Tree Heads).
However, because of skew, the receiving front-end may not know one
or both of the existing STHs. If both are known, then only the
"consistency" output is returned. If the first is known but the
second is not (or has been omitted), then the latest known STH is
returned, along with a consistency proof between the first STH and
the latest. If neither are known, then the latest known STH is
returned without a consistency proof.
Outputs:
consistency: An array of base64 encoded Merkle Tree nodes.
sth: A base64 encoded SignedTreeHead.
Note that no signature is required for the "consistency" output as
it is used to verify "sth", which is signed.
Error codes:
first unknown "first" is before the latest known STH but is not
from an existing STH.
second unknown "second" is before the latest known STH but is not
from an existing STH.
See Section 9.4.2 for an outline of how to use the "consistency"
array.
6.5. Retrieve Merkle Inclusion Proof from Log by Leaf Hash
GET https://<log server>/ct/v2/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 5.4. The
"tree_size" must designate an existing v2 STH. Because of skew,
the front-end may not know the requested STH. In that case, it
will return the latest STH it knows, along with an inclusion proof
to that STH. If the front-end knows the requested STH then only
"leaf_index" and "audit_path" are returned.
Outputs:
Laurie, et al. Expires April 21, 2016 [Page 24]
Internet-Draft Certificate Transparency October 2015
leaf_index: The 0-based index of the entry corresponding to the
"hash" parameter.
audit_path: An array of base64 encoded Merkle Tree nodes proving
the inclusion of the chosen certificate.
sth: A base64 encoded SignedTreeHead.
Note that no signature is required for the "leaf_index" or
"audit_path" outputs as they are used to verify inclusion in
"sth", which is signed.
Error codes:
hash unknown "hash" is not the hash of a known leaf (may be
caused by skew or by a known certificate not yet merged).
tree_size unknown "hash" is before the latest known STH but is
not from an existing STH.
See Section 9.4.1 for an outline of how to use the "audit_path"
array.
6.6. Retrieve Merkle Inclusion Proof, Signed Tree Head and Consistency
Proof by Leaf Hash
GET https://<log server>/ct/v2/get-all-by-hash
Inputs:
hash: A base64 encoded v1 leaf hash.
tree_size: The tree_size of the tree on which to base the proofs,
in decimal.
The "hash" must be calculated as defined in Section 5.4. The
"tree_size" must designate an existing v2 STH.
Because of skew, the front-end may not know the requested STH or
the requested hash, which leads to a number of cases.
latest STH < requested STH Return latest STH.
latest STH > requested STH Return latest STH and a consistency
proof between it and the requested STH (see Section 6.4).
Laurie, et al. Expires April 21, 2016 [Page 25]
Internet-Draft Certificate Transparency October 2015
index of requested hash < latest STH Return "leaf_index" and
"audit_path".
Note that more than one case can be true, in which case the
returned data is their concatenation. It is also possible for
none to be true, in which case the front-end MUST return an empty
response.
Outputs:
leaf_index: The 0-based index of the entry corresponding to the
"hash" parameter.
audit_path: An array of base64 encoded Merkle Tree nodes proving
the inclusion of the chosen certificate.
sth: A base64 encoded SignedTreeHead.
consistency: An array of base64 encoded Merkle Tree nodes proving
the consistency of the requested STH and the returned STH.
Note that no signature is required for the "leaf_index",
"audit_path" or "consistency" outputs as they are used to verify
inclusion in and consistency of "sth", which is signed.
Errors are the same as in Section 6.5.
See Section 9.4.1 for an outline of how to use the "audit_path" array
and see Section 9.4.2 for an outline of how to use the "consistency"
array.
6.7. Retrieve Entries and STH from Log
GET https://<log server>/ct/v2/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.
Laurie, et al. Expires April 21, 2016 [Page 26]
Internet-Draft Certificate Transparency October 2015
extra_data: The base64 encoded unsigned data pertaining to the
log entry. In the case of an X509ChainEntry, this is the
whole "X509ChainEntry". In the case of a
PrecertChainEntryV2, this is the whole
"PrecertChainEntryV2".
sct: A base64 encoded "SignedCertificateTimestamp" for this
entry. Note that more than one SCT may have been returned
for the same entry - only one of those is returned in this
field. It may not be possible to retrieve others.
sth: A base64 encoded SignedTreeHead.
Note that this message is not signed -- the "entries" data can be
verified by constructing the Merkle Tree Hash corresponding to a
retrieved STH. All leaves MUST be v1 or v2. However, a compliant v1
client MUST NOT construe an unrecognized LogEntryType value as an
error. This means it may be unable to parse some entries, but note
that each client can inspect the entries it does recognize as well as
verify the integrity of the data by treating unrecognized leaves as
opaque input to the tree.
The "start" and "end" parameters SHOULD be within the range 0 <= x <
"tree_size" as returned by "get-sth" in Section 6.3.
The "start" parameter MUST be less than or equal to the "end"
parameter.
Log servers MUST 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. "end" >= "tree_size" could
be caused by skew. 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".
Because of skew, it is possible the log server will not have any
entries between "start" and "end". In this case it MUST return an
empty "entries" array.
In any case, the log server MUST return the latest STH it knows
about.
Laurie, et al. Expires April 21, 2016 [Page 27]
Internet-Draft Certificate Transparency October 2015
See Section 9.4.3 for an outline of how to use a complete list of
"leaf_input" entries to verify the "root_hash".
6.8. 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.
max_chain: If the server has chosen to limit the length of chains
it accepts, this is the maximum number of certificates in the
chain, in decimal. If there is no limit, this is omitted.
7. TLS Servers
TLS servers MUST use at least one of the three mechanisms listed
below to present one or more SCTs from one or more logs to each TLS
client during TLS handshakes, where each SCT corresponds to the
server certificate or to a name-constrained intermediate the server
certificate chains to. Three mechanisms are provided because they
have different tradeoffs.
o A TLS extension (Section 7.4.1.4 of [RFC5246]) with type
"signed_certificate_timestamp" (see Section 7.1). This mechanism
allows TLS servers to participate in CT without the cooperation of
CAs, unlike the other two mechanisms. It also allows SCTs to be
updated on the fly.
o An Online Certificate Status Protocol (OCSP) [RFC6960] response
extension (see Section 8.1.1), where the OCSP response is provided
in the "certificate_status" TLS extension (Section 8 of
[RFC6066]), also known as OCSP stapling. This mechanism is
already widely (but not universally) implemented. It also allows
SCTs to be updated on the fly.
o An X509v3 certificate extension (see Section 8.1.2). This
mechanism allows the use of unmodified TLS servers, but the SCTs
cannot be updated on the fly. Since the logs that signed the SCTs
won't necessarily be accepted by TLS clients for the full lifetime
of the certificate, there is a risk that TLS clients will
subsequently consider the certificate to be non-compliant and in
need of re-issuance.
Laurie, et al. Expires April 21, 2016 [Page 28]
Internet-Draft Certificate Transparency October 2015
TLS servers SHOULD send SCTs from multiple logs in case one or more
logs are not acceptable to the TLS client (for example, if a log has
been struck off for misbehavior, has had a key compromise or is not
known to the TLS client).
Multiple SCTs are combined into an SCT list 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 SCT 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).
7.1. TLS Extension
If a TLS client includes the "signed_certificate_timestamp" extension
type in the ClientHello, the TLS server MAY include the
"signed_certificate_timestamp" extension in the ServerHello with
"extension_data" set to a "SignedCertificateTimestampList". The TLS
server is not expected to process or include this extension when a
TLS session is resumed, since session resumption uses the original
session information.
8. Certification Authorities
8.1. X.509v3 Extension
One or more SCTs can be embedded in an X.509v3 extension that is
included in a certificate or an OCSP response. Since RFC5280
requires the "extnValue" field (an OCTET STRING) of each X.509v3
extension to include the DER encoding of an ASN.1 value, we cannot
embed a "SignedCertificateTimestampList" directly. Instead, we have
to wrap it inside an additional OCTET STRING (see below), which we
then put into the "extnValue" field.
8.1.1. OCSP Response Extension
Laurie, et al. Expires April 21, 2016 [Page 29]
Internet-Draft Certificate Transparency October 2015
A certification authority may embed one or more SCTs in OCSP
responses pertaining to the end-entity certificate, by including a
non-critical "singleExtensions" extension with OID
1.3.6.1.4.1.11129.2.4.5 whose "extnValue" contains:
CertificateSCTList ::= OCTET STRING
"CertificateSCTList" contains a "SignedCertificateTimestampList"
whose SCTs all have the "x509_entry" "LogEntryType".
8.1.2. Certificate Extension
A certification authority that has submitted a precertificate to one
or more logs may embed the obtained SCTs in the "TBSCertificate" that
will be signed to produce the certificate, by including a non-
critical X.509v3 extension with OID 1.3.6.1.4.1.11129.2.4.2 whose
"extnValue" contains:
PrecertificateSCTList ::= OCTET STRING
"PrecertificateSCTList" contains a "SignedCertificateTimestampList"
whose SCTs all have the "precert_entry_V2" "LogEntryType".
Upon receiving the certificate, clients can reconstruct the original
"TBSCertificate" to verify the SCT signatures.
9. Clients
There are various different functions clients of logs might perform.
We describe here some typical clients and how they should function.
Any inconsistency may be used as evidence that a log has not behaved
correctly, and the signatures on the data structures prevent the log
from denying that misbehavior.
All clients need various metadata in order to communicate with logs
and verify their responses. This metadata is described below, but
note that this document does not describe how the metadata is
obtained, which is implementation dependent (see, for example,
[Chromium.Policy]).
Clients should somehow exchange STHs they see, or make them available
for scrutiny, in order to ensure that they all have a consistent
view. The exact mechanisms will be in separate documents, but it is
expected there will be a variety.
Laurie, et al. Expires April 21, 2016 [Page 30]
Internet-Draft Certificate Transparency October 2015
9.1. Metadata
In order to communicate with and verify a log, clients need metadata
about the log.
Base URL: The URL to substitute for <log server> in Section 6.
Hash Algorithm The hash algorithm used for the Merkle Tree (see
Section 11.2).
Signing Algorithm The signing algorithm used (see Section 2.1.4).
Public Key The public key used for signing.
Maximum Merge Delay The MMD the log has committed to.
Version The version of the protocol supported by the log (currently
1 or 2).
Maximum Chain Length The longest chain submission the log is willing
to accept, if the log chose to limit it.
STH Frequency Count The maximum number of STHs the log may produce
in any period equal to the "Maximum Merge Delay" (see
Section 5.5).
Final STH If a log has been closed down (i.e. no longer accepts new
entries), existing entries may still be valid. In this case, the
client should know the final valid STH in the log to ensure no new
entries can be added without detection.
[JSON.Metadata] is an example of a metadata format which includes the
above elements.
9.2. TLS Client
TLS clients receive SCTs alongside or in certificates, either for the
server certificate itself or for a name-constrained intermediate the
server certificate chains to. TLS clients MUST implement all of the
three mechanisms by which TLS servers may present SCTs (see
Section 7). TLS clients that support the
"signed_certificate_timestamp" TLS extension SHOULD include it, with
empty "extension_data", in ClientHello messages.
In addition to normal validation of the certificate and its chain,
TLS clients SHOULD validate each SCT by computing the signature input
from the SCT data as well as the certificate and verifying the
Laurie, et al. Expires April 21, 2016 [Page 31]
Internet-Draft Certificate Transparency October 2015
signature, using the corresponding log's public key. TLS clients
MUST reject SCTs whose timestamp is in the future.
By validating SCTs, TLS clients can thus determine whether
certificates are compliant. A certificate not accompanied by a valid
SCT MUST NOT be considered compliant by TLS clients. However,
specifying the TLS clients' behavior once compliance or non-
compliance has been determined (for example, whether a certificate
should be rejected due to the lack of valid SCTs) is outside the
scope of this document.
A TLS client MAY audit the corresponding log by requesting, and
verifying, a Merkle audit proof for said certificate. If the TLS
client holds an STH that predates the SCT, it MAY, in the process of
auditing, request a new STH from the log (Section 6.3), then verify
it by requesting a consistency proof (Section 6.4).
9.3. Monitor
Monitors watch logs and check that they behave correctly. Monitors
may additionally watch for certificates of interest. For example, a
monitor may be configured to report on all certificates that apply to
a specific domain name when fetching new entries for consistency
validation.
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 6.3).
2. Verify the STH signature.
3. Fetch all the entries in the tree corresponding to the STH
(Section 6.7).
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 6.3). Repeat until the STH
changes.
6. Verify the STH signature.
7. Fetch all the new entries in the tree corresponding to the STH
(Section 6.7). If they remain unavailable for an extended
period, then this should be viewed as misbehavior on the part of
the log.
Laurie, et al. Expires April 21, 2016 [Page 32]
Internet-Draft Certificate Transparency October 2015
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:
1. Fetch a consistency proof for the new STH with the previous
STH (Section 6.4).
2. Verify the consistency proof.
3. Verify that the new entries generate the corresponding
elements in the consistency proof.
9. Go to Step 5.
9.4. Auditing
Auditing is taking partial information about a log as input and
verifying that this information is consistent with other partial
information held. All clients described above may perform auditing
as an additional function. The action taken by the client if audit
fails is not specified, but note that in general if audit fails, the
client is in possession of signed proof of the log's misbehavior.
A monitor (Section 9.3) can audit by verifying the consistency of
STHs it receives, ensure that each entry can be fetched and that the
STH is indeed the result of making a tree from all fetched entries.
A TLS client (Section 9.2) can audit by verifying an SCT against any
STH dated after the SCT timestamp + the Maximum Merge Delay by
requesting a Merkle inclusion proof (Section 6.5). It can also
verify that the SCT corresponds to the certificate it arrived with
(i.e. the log entry is that certificate, is a precertificate for that
certificate or is an appropriate name-constrained intermediate [see
Section 4.3]).
The following algorithm outlines may be useful for clients that wish
to perform various audit operations.
9.4.1. Verifying an inclusion proof
When a client has received an "audit_path" and "leaf_index" and
wishes to verify inclusion of an input "hash" for an STH with a given
"tree_size" and "root_hash", the following algorithm may be used to
prove the "hash" was included in the "root_hash":
Laurie, et al. Expires April 21, 2016 [Page 33]
Internet-Draft Certificate Transparency October 2015
1. Set "fn" to "leaf_index" and "sn" to "tree_size - 1".
2. Set "r" to "hash".
3. For each value "p" in the "audit_path" array:
If "LSB(fn)" is set, or if "fn" is equal to "sn", then:
1. Set "r" to "HASH(0x01 || p || r)"
2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn"
equally until either "LSB(fn)" is set or "fn" is "0".
Otherwise:
Set "r" to "HASH(0x01 || r || p)"
Finally, right-shift both "fn" and "sn" one time.
4. Compare "r" against the "root_hash". If they are equal, then the
log has proven the inclusion of "hash".
9.4.2. Verifying consistency between two STHs
When a client has an STH "first_hash" for tree size "first", an STH
"second_hash" for tree size "second" where "0 < first < second", and
has received a "consistency" array that they wish to use to verify
both hashes, the following algorithm may be used:
1. If "first" is an exact power of 2, then prepend "first_hash" to
the "consistency" array.
2. Set "fn" to "first - 1" and "sn" to "second - 1".
3. If "LSB(fn)" is set, then right-shift both "fn" and "sn" equally
until "LSB(fn)" is not set.
4. Set both "fr" and "sr" to the first value in the "consistency"
array.
5. For each subsequent value "c" in the "consistency" array:
If "LSB(fn)" is set, or if "fn" is equal to "sn", then:
1. Set "fr" to "HASH(0x01 || c || fr)"
Set "sr" to "HASH(0x01 || c || sr)"
Laurie, et al. Expires April 21, 2016 [Page 34]
Internet-Draft Certificate Transparency October 2015
2. If "LSB(fn)" is not set, then right-shift both "fn" and "sn"
equally until either "LSB(fn)" is set or "fn" is "0".
Otherwise:
Set "sr" to "HASH(0x01 || sr || c)"
Finally, right-shift both "fn" and "sn" one time.
6. After completing iterating through the "consistency" array as
described above, verify that the "fr" calculated is equal to the
"first_hash" supplied and that the "sr" calculated is equal to
the "second_hash" supplied.
9.4.3. Verifying root hash given entries
When a client has a complete list of leaf input "entries" from "0" up
to "tree_size - 1" and wishes to verify this list against an STH
"root_hash" returned by the log for the same "tree_size", the
following algorithm may be used:
1. Set "stack" to an empty stack.
2. For each "i" from "0" up to "tree_size - 1":
1. Push "HASH(0x00 || entries[i])" to "stack".
2. Set "merge_count" to the lowest value ("0" included) such
that "LSB(i >> merge_count)" is not set. In other words, set
"merge_count" to the number of consecutive "1"s found
starting at the least significant bit of "i".
3. Repeat "merge_count" times:
1. Pop "right" from "stack".
2. Pop "left" from "stack".
3. Push "HASH(0x01 || left || right)" to "stack".
3. If there is more than one element in the "stack", repeat the same
merge procedure (Step 2.3 above) until only a single element
remains.
4. The remaining element in "stack" is the Merkle Tree hash for the
given "tree_size" and should be compared by equality against the
supplied "root_hash".
Laurie, et al. Expires April 21, 2016 [Page 35]
Internet-Draft Certificate Transparency October 2015
10. Algorithm Agility
It is not possible for a log to change any of its algorithms part way
through its lifetime. If it should become necessary to deprecate an
algorithm used by a live log, then the log should be frozen as
specified in Section 9.1 and a new log should be started. If
necessary, the new log can contain existing entries from the frozen
log, which monitors can verify are an exact match.
11. IANA Considerations
11.1. TLS Extension Type
IANA has allocated an RFC 5246 ExtensionType value (18) for the SCT
TLS extension. The extension name is "signed_certificate_timestamp".
IANA should update this extension type to point at this document.
11.2. Hash Algorithms
IANA is asked to establish a registry of hash values, initially
consisting of:
+-------+----------------------+
| Index | Hash |
+-------+----------------------+
| 0 | SHA-256 [FIPS.180-4] |
+-------+----------------------+
11.3. SCT Extensions
IANA is asked to establish a registry of SCT extensions, initially
consisting of:
+-------+-----------+
| Type | Extension |
+-------+-----------+
| 65535 | reserved |
+-------+-----------+
TBD: policy for adding to the registry
11.4. STH Extensions
IANA is asked to establish a registry of STH extensions, initially
consisting of:
Laurie, et al. Expires April 21, 2016 [Page 36]
Internet-Draft Certificate Transparency October 2015
+-------+-----------+
| Type | Extension |
+-------+-----------+
| 65535 | reserved |
+-------+-----------+
TBD: policy for adding to the registry
12. 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 a log has committed to publishing the certificate. From
this, the client knows that the subject of the certificate has had
some time to notice the misissue and take some action, such as asking
a CA to revoke a misissued certificate, or that the log has
misbehaved, which will be discovered when the SCT is audited. A
signed timestamp is not a guarantee that the certificate is not
misissued, since 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.
12.1. Misissued Certificates
Misissued certificates that have not been publicly logged, and thus
do not have a valid SCT, are not considered compliant (so TLS clients
may decide, for example, to reject them). Misissued certificates
that do have an SCT from a log will appear in that public log within
the Maximum Merge Delay, assuming the log is operating correctly.
Thus, the maximum period of time during which a misissued certificate
can be used without being available for audit is the MMD.
12.2. 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.
Laurie, et al. Expires April 21, 2016 [Page 37]
Internet-Draft Certificate Transparency October 2015
12.3. Redaction of Public Domain Name Labels
CAs SHOULD NOT redact domain name labels in precertificates such that
the entirety of the domain space below the unredacted part of the
domain name is not owned or controlled by a single entity (e.g.
"?.com" and "?.co.uk" would both be problematic). Logs MUST NOT
reject any precertificate that is overly redacted but which is
otherwise considered compliant. It is expected that monitors will
treat overly redacted precertificates as potentially misissued. TLS
clients MAY reject a certificate whose corresponding precertificate
would be overly redacted, perhaps using the same mechanism for
determining whether a wildcard in a domain name of a certificate is
too broad.
12.4. 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 can be detected by clients
comparing their instances of the Signed Tree Heads. As soon as two
conflicting Signed Tree Heads for the same log are detected, this is
cryptographic proof of that log's misbehavior. There are various
ways this could be done, for example via gossip (see
http://trac.tools.ietf.org/id/draft-linus-trans-gossip-00.txt) or
peer-to-peer communications or by sending STHs to monitors (who could
then directly check against their own copy of the relevant log).
12.5. Multiple SCTs
TLS servers may wish to offer multiple SCTs, each from a different
log.
Laurie, et al. Expires April 21, 2016 [Page 38]
Internet-Draft Certificate Transparency October 2015
o If a CA and a log collude, it is possible to temporarily hide
misissuance from clients. Including SCTs from different logs
makes it more difficult to mount this attack.
o If a log misbehaves, a consequence may be that clients cease to
trust it. Since the time an SCT may be in use can be considerable
(several years is common in current practice when the SCT is
embedded in a certificate), servers may wish to reduce the
probability of their certificates being rejected as a result by
including SCTs from different logs.
o TLS clients may have policies related to the above risks requiring
servers to present multiple SCTs. For example Chromium
[Chromium.Log.Policy] currently requires multiple SCTs to be
presented with EV certificates in order for the EV indicator to be
shown.
13. 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.
14. Acknowledgements
The authors would like to thank Erwann Abelea, Robin Alden, Al
Cutter, Francis Dupont, Adam Eijdenberg, Stephen Farrell, Daniel Kahn
Gillmor, Brad Hill, Jeff Hodges, Paul Hoffman, Jeffrey Hutzelman,
Stephen Kent, SM, Alexey Melnikov, Linus Nordberg, Chris Palmer,
Trevor Perrin, Pierre Phaneuf, Melinda Shore, Ryan Sleevi, Carl
Wallace and Paul Wouters for their valuable contributions.
15. References
15.1. Normative References
[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>.
Laurie, et al. Expires April 21, 2016 [Page 39]
Internet-Draft Certificate Transparency October 2015
[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>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, DOI 10.17487/
RFC2616, June 1999,
<http://www.rfc-editor.org/info/rfc2616>.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
2003, <http://www.rfc-editor.org/info/rfc3447>.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, DOI
10.17487/RFC4627, July 2006,
<http://www.rfc-editor.org/info/rfc4627>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[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, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
Laurie, et al. Expires April 21, 2016 [Page 40]
Internet-Draft Certificate Transparency October 2015
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<http://www.rfc-editor.org/info/rfc5652>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<http://www.rfc-editor.org/info/rfc5905>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, DOI
10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<http://www.rfc-editor.org/info/rfc6960>.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <http://www.rfc-editor.org/info/rfc6979>.
15.2. Informative References
[Chromium.Log.Policy]
The Chromium Projects, "Chromium Certificate Transparency
Log Policy", 2014, <http://www.chromium.org/Home/chromium-
security/certificate-transparency/log-policy>.
[Chromium.Policy]
The Chromium Projects, "Chromium Certificate
Transparency", 2014, <http://www.chromium.org/Home/
chromium-security/certificate-transparency>.
Laurie, et al. Expires April 21, 2016 [Page 41]
Internet-Draft Certificate Transparency October 2015
[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>.
[EVSSLGuidelines]
CA/Browser Forum, "Guidelines For The Issuance And
Management Of Extended Validation Certificates", 2007,
<https://cabforum.org/wp-content/uploads/
EV_Certificate_Guidelines.pdf>.
[JSON.Metadata]
The Chromium Projects, "Chromium Log Metadata JSON
Schema", 2014, <http://www.certificate-transparency.org/
known-logs/log_list_schema.json>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<http://www.rfc-editor.org/info/rfc6962>.
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
Eran Messeri
Google UK Ltd.
EMail: eranm@google.com
Laurie, et al. Expires April 21, 2016 [Page 42]
Internet-Draft Certificate Transparency October 2015
Rob Stradling
Comodo CA, Ltd.
EMail: rob.stradling@comodo.com
Laurie, et al. Expires April 21, 2016 [Page 43]
