Internet Engineering Task Force (IETF) B. Laurie
Request for Comments: 6962 A. Langley
Category: Experimental E. Kasper
ISSN: 2070-1721 Google
May 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.
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 . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
1.2. Data Structures . . . . . . . . . . . . . . . . . . . . . 5
2. Cryptographic Components . . . . . . . . . . . . . . . . . . . 5
2.1. Merkle Hash Trees . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Merkle Audit Paths . . . . . . . . . . . . . . . . . . 6
2.1.2. Merkle Consistency Proofs . . . . . . . . . . . . . . 6
2.1.3. Example . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.4. Signatures . . . . . . . . . . . . . . . . . . . . . . 9
3. Log Format and Operation . . . . . . . . . . . . . . . . . . . 10
3.1. Log Entries . . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Structure of the Signed Certificate Timestamp . . . . . . 13
3.3. Including the Signed Certificate Timestamp in the TLS
Handshake . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3.1. TLS Extension . . . . . . . . . . . . . . . . . . . . 15
3.4. Merkle Tree . . . . . . . . . . . . . . . . . . . . . . . 16
3.5. Signed Tree Head . . . . . . . . . . . . . . . . . . . . . 17
4. Log Client Messages . . . . . . . . . . . . . . . . . . . . . 17
4.1. Add Chain to Log . . . . . . . . . . . . . . . . . . . . . 18
4.2. Add PreCertChain to Log . . . . . . . . . . . . . . . . . 19
4.3. Retrieve Latest Signed Tree Head . . . . . . . . . . . . . 19
4.4. Retrieve Merkle Consistency Proof between Two Signed
Tree Heads . . . . . . . . . . . . . . . . . . . . . . . . 19
4.5. Retrieve Merkle Audit Proof from Log by Leaf Hash . . . . 20
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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.1. Submitters . . . . . . . . . . . . . . . . . . . . . . . . 22
5.2. TLS Client . . . . . . . . . . . . . . . . . . . . . . . . 22
5.3. Monitor . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.4. Auditor . . . . . . . . . . . . . . . . . . . . . . . . . 23
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
7.1. Misissued Certificates . . . . . . . . . . . . . . . . . . 24
7.2. Detection of Misissue . . . . . . . . . . . . . . . . . . 24
7.3. Misbehaving Logs . . . . . . . . . . . . . . . . . . . . . 25
8. Efficiency Considerations . . . . . . . . . . . . . . . . . . 25
9. Future Changes . . . . . . . . . . . . . . . . . . . . . . . . 25
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
11.1. Normative References . . . . . . . . . . . . . . . . . . . 26
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.
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 Hash Tree for efficient auditing. The
hashing algorithm is SHA-256 [FIPS.180-2.2002] [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. 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().
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 Tree may thus not be balanced;
however, its shape is uniquely determined by the number of leaves.
(Note: This Merkle tree 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 Hash Tree is the shortest
list of additional nodes in the Merkle tree 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)}) = {}
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 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])
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 Tree with 7 leaves:
hash
/ \
/ \
/ \
/ \
/ \
k l
/ \ / \
/ \ / \
/ \ / \
g h i j
/ \ / \ / \ |
a b c d e f d6
| | | | | |
d0 d1 d2 d3 d4 d5
The audit path for d0 is [b, h, l].
The audit path for d3 is [c, g, l].
The audit path for d4 is [f, j, k].
The audit path for d6 is [i, k].
The same tree, built incrementally in four steps:
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.
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
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.)
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.
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 pre-certificate 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.
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 (Section 8 of (also known as the "Certificate Status
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 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).
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.
"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.
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.
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 binary, 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. 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.
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 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 Tree nodes proving
the inclusion of the chosen certificate.
4.6. Retrieve Entries from Log
GET https://<log server>/ct/v1/get-entries
Inputs:
start: 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".
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 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.
5.1. Submitters
Submitters submit certificates or precertificates 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.
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 CTS 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
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. Signed Tree Heads. As soon as two conflicting signed tree
heads Signed Tree
Heads for the same log are detected, this is cryptographic proof of
that log's misbehavior.
8. Efficiency Considerations
The Merkle tree 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 References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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.
[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.
[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>.
[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>.
[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>.
[FIPS.180-2.2002] National Institute of Standards and Technology,
"Secure Hash Standard", FIPS PUB 180-2,
August 2002, <http://csrc.nist.gov/publications/
fips/fips180-2/fips180-2.pdf>.
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