Certificate Transparency
draft-ietf-trans-rfc6962-bis-06
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9162.
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|
|---|---|---|---|
| Authors | Ben Laurie , Adam Langley , Emilia Kasper , Eran Messeri , Rob Stradling | ||
| Last updated | 2015-03-09 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Additional resources | Mailing list discussion | ||
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| IESG | IESG state | Became RFC 9162 (Experimental) | |
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draft-ietf-trans-rfc6962-bis-06
Public Notary Transparency Working Group B. Laurie
Internet-Draft A. Langley
Intended status: Standards Track E. Kasper
Expires: September 10, 2015 E. Messeri
Google
R. Stradling
Comodo
March 9, 2015
Certificate Transparency
draft-ietf-trans-rfc6962-bis-06
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 September 10, 2015.
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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 . . . . . . . . . . . . . . . . . . 4
1.2. Data Structures . . . . . . . . . . . . . . . . . . . . . 4
2. Cryptographic Components . . . . . . . . . . . . . . . . . . 4
2.1. Merkle Hash Trees . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Merkle Inclusion Proofs . . . . . . . . . . . . . . . 5
2.1.2. Merkle Consistency Proofs . . . . . . . . . . . . . . 6
2.1.3. Example . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.4. Signatures . . . . . . . . . . . . . . . . . . . . . 9
3. Log Format and Operation . . . . . . . . . . . . . . . . . . 9
3.1. Log Entries . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Private Domain Name Labels . . . . . . . . . . . . . . . 12
3.2.1. Wildcard Certificates . . . . . . . . . . . . . . . . 12
3.2.2. Redacting Domain Name Labels in Precertificates . . . 12
3.2.3. Using a Name-Constrained Intermediate CA . . . . . . 13
3.3. Structure of the Signed Certificate Timestamp . . . . . . 14
3.4. Including the Signed Certificate Timestamp in the TLS
Handshake . . . . . . . . . . . . . . . . . . . . . . . . 15
3.4.1. TLS Extension . . . . . . . . . . . . . . . . . . . . 17
3.5. Merkle Tree . . . . . . . . . . . . . . . . . . . . . . . 17
3.6. Signed Tree Head . . . . . . . . . . . . . . . . . . . . 18
4. Log Client Messages . . . . . . . . . . . . . . . . . . . . . 19
4.1. Add Chain to Log . . . . . . . . . . . . . . . . . . . . 21
4.2. Add PreCertChain to Log . . . . . . . . . . . . . . . . . 22
4.3. Retrieve Latest Signed Tree Head . . . . . . . . . . . . 22
4.4. Retrieve Merkle Consistency Proof between Two Signed Tree
Heads . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.5. Retrieve Merkle Inclusion Proof from Log by Leaf Hash . . 23
4.6. Retrieve Merkle Inclusion Proof, Signed Tree Head and
Consistency Proof by Leaf Hash . . . . . . . . . . . . . 24
4.7. Retrieve Entries from Log . . . . . . . . . . . . . . . . 25
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4.8. Retrieve Accepted Root Certificates . . . . . . . . . . . 26
5. Clients . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.1. Metadata . . . . . . . . . . . . . . . . . . . . . . . . 27
5.2. Submitters . . . . . . . . . . . . . . . . . . . . . . . 28
5.3. TLS Client . . . . . . . . . . . . . . . . . . . . . . . 28
5.4. Monitor . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.5. Auditing . . . . . . . . . . . . . . . . . . . . . . . . 29
6. Algorithm Agility . . . . . . . . . . . . . . . . . . . . . . 30
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
7.1. TLS Extension Type . . . . . . . . . . . . . . . . . . . 30
7.2. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 30
8. Security Considerations . . . . . . . . . . . . . . . . . . . 30
8.1. Misissued Certificates . . . . . . . . . . . . . . . . . 31
8.2. Detection of Misissue . . . . . . . . . . . . . . . . . . 31
8.3. Redaction of Public Domain Name Labels . . . . . . . . . 31
8.4. Misbehaving Logs . . . . . . . . . . . . . . . . . . . . 31
8.5. Multiple SCTs . . . . . . . . . . . . . . . . . . . . . . 32
9. Efficiency Considerations . . . . . . . . . . . . . . . . . . 32
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 33
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
11.1. Normative References . . . . . . . . . . . . . . . . . . 33
11.2. Informative References . . . . . . . . . . . . . . . . . 34
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).
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 have been logged.
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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.
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
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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 7.2. The hashing
algorithm in use is referred to as HASH throughout this document.
The input to the Merkle Tree Hash is a list of data entries; these
entries will be hashed to form the leaves of the Merkle Hash Tree.
The output is a single 32-byte Merkle Tree Hash. Given an ordered
list of n inputs, D[n] = {d(0), d(1), ..., d(n-1)}, the Merkle Tree
Hash (MTH) is thus defined as follows:
The hash of an empty list is the hash of an empty string:
MTH({}) = 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;
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
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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
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) = {}
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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 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].
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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].
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.
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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 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 (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). TLS servers MUST present an SCT from one or more logs to
the TLS client together with the certificate. A certificate not
accompanied by an SCT (either for the end-entity certificate or for a
name-constrained intermediate the end-entity certificate chains to)
MUST NOT be considered compliant by TLS clients.
Periodically, each log appends all its new entries to the Merkle Tree
and signs the root of the tree. The log MUST incorporate a
certificate in its Merkle Tree within the Maximum Merge Delay period
after the issuance of the SCT. When encountering an SCT, an Auditor
can verify that the certificate was added to the Merkle Tree within
that timeframe.
Log operators MUST NOT impose any conditions on retrieving or sharing
data from the log.
3.1. Log Entries
In order to enable attribution of each logged certificate to its
issuer, 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. The log SHALL
allow retrieval of a list of accepted root certificates (this list
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might usefully be the union of root certificates trusted by major
browser vendors).
Alternatively, (root as well as intermediate) certification
authorities may preannounce a certificate to logs prior to issuance
in order to incorporate the SCT in the issued certificate. To do
this, the CA submits a precertificate that the log can use to create
an entry that will be valid against the issued certificate. A
precertificate is a CMS [RFC5652] "signed-data" object that contains
a TBSCertificate [RFC5280] in its
"SignedData.encapContentInfo.eContent" field, identified by the OID
<TBD> in the "SignedData.encapContentInfo.eContentType" field. This
TBSCertificate MAY redact certain domain name labels that will be
present in the issued certificate (see Section 3.2.2) and MUST NOT
contain any SCTs, but it will be otherwise identical to the
TBSCertificate in the issued certificate. "SignedData.signerInfos"
MUST contain a signature from the same (root or intermediate) CA that
will ultimately issue the certificate. This signature indicates the
certification authority's intent to issue the certificate. This
intent is considered binding (i.e., misissuance of the precertificate
is considered equivalent to misissuance of the certificate). As
above, the precertificate submission MUST be accompanied by all the
additional certificates required to verify the chain up to an
accepted root certificate. This does not involve using the
"SignedData.certificates" field, so that field SHOULD be omitted.
Logs MUST verify that the 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 that are fully valid according to X.509
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 X.509 verification rules in order to accommodate quirks
of CA certificate-issuing software. However, logs MUST reject
certificates without a valid signature chain to an accepted root
certificate. 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 limit the submission of spurious certificates is found. The
authors welcome suggestions.)
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Each certificate or precertificate entry in a log MUST include the
following components:
enum { x509_entry(0), precert_entry_V2(3), (65535) } LogEntryType;
struct {
LogEntryType entry_type;
select (entry_type) {
case x509_entry: X509ChainEntry;
case precert_entry_V2: PrecertChainEntryV2;
} entry;
} LogEntry;
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;
Logs SHOULD limit the length of chain they will accept.
"entry_type" is the type of this entry. Future revisions of this
protocol 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
either be, or be issued by, 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 MUST
certify the precertificate. Each following certificate MUST directly
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certify the one preceding it. The final certificate MUST be a root
certificate accepted by the log.
3.2. 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.
3.2.1. 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].
3.2.2. 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.
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
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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.
3.2.3. 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.
Below is an example Name Constraints extension that meets these
conditions:
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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
}
}
}
}
}
3.3. Structure of the Signed Certificate Timestamp
enum { certificate_timestamp(0), tree_hash(1), (255) }
SignatureType;
enum { v1(0), v2(1), (255) }
Version;
struct {
opaque key_id[32];
} LogID;
opaque TBSCertificate<1..2^24-1>;
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.
"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.
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struct {
Version sct_version;
LogID id;
uint64 timestamp;
CtExtensions extensions;
digitally-signed struct {
Version sct_version;
SignatureType signature_type = certificate_timestamp;
uint64 timestamp;
LogEntryType entry_type;
select(entry_type) {
case x509_entry: ASN.1Cert;
case precert_entry_V2: TBSCertificate;
} 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 v2.
"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 TBSCertificate (in the case of a
PrecertChainEntryV2), as described above.
"extensions" are future extensions to SignedCertificateTimestamp v2.
Currently, no extensions are specified.
3.4. Including the Signed Certificate Timestamp in the TLS Handshake
The SCT data corresponding to at least one certificate in the chain
from at least one log must be included in the TLS handshake, either
by using an X509v3 certificate extension as described below, by using
a TLS extension (Section 7.4.1.4 of [RFC5246]) with type
"signed_certificate_timestamp", or by using Online Certificate Status
Protocol (OCSP) Stapling (also known as the "Certificate Status
Request" TLS extension; see [RFC6066]), where the OCSP response
includes a non-critical extension with OID 1.3.6.1.4.1.11129.2.4.5
(see [RFC2560]) and body:
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SignedCertificateTimestampList ::= OCTET STRING
in the singleExtensions component of the SingleResponse pertaining to
the end-entity certificate.
At least one SCT MUST be included. Server operators MAY include more
than one SCT.
Similarly, a certification authority MAY submit a precertificate to
more than one log, and all obtained SCTs can be directly embedded in
the issued certificate, by encoding the
SignedCertificateTimestampList structure as an ASN.1 OCTET STRING and
inserting the resulting data in the TBSCertificate as a non-critical
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 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).
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, has had a key compromise or is not known
to the client).
The three mechanisms are provided because they have different
tradeoffs. Embedding the SCTs in the certificate allows the use of
unmodified TLS servers, but, because they cannot be changed without
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re-issuing the certificate, increases the risk that the certificate
will be refused if the SCTs become invalid. OCSP Stapling is already
widely (but not universally) implemented, and provides a mechanism by
which TLS servers that already support it can serve SCTs that are
generated on the fly. Finally, the TLS extension permits 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.
3.4.1. 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 in this TLS extension 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.
3.5. Merkle Tree
The hashing algorithm for the Merkle Tree Hash is specified in the
log's metadata.
Structure of the Merkle Tree input:
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enum { v1(0), v2(1), (255) }
LeafVersion;
struct {
uint64 timestamp;
LogEntryType entry_type;
select(entry_type) {
case x509_entry: ASN.1Cert;
case precert_entry_V2: TBSCertificate;
} signed_entry;
CtExtensions 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 4 explains how
clients should handle unknown entry types.
"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.
3.6. 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:
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enum { v1(0), (255) } TreeHeadVersion;
digitally-signed struct {
TreeHeadVersion version;
SignatureType signature_type = tree_hash;
uint64 timestamp;
uint64 tree_size;
opaque sha256_root_hash[32];
} TreeHeadSignature;
"version" is the version of the TreeHeadSignature structure. 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 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,
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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.
e.g. In response to a request of "/ct/v1/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
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"Retry-After:" header in order to request a minimum time for the
client to wait before retrying the request.
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.
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.
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 "sct_version" is not v1, then a v1 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 v1 clients that do not
use the returned SCTs.
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If a log detects bad encoding in a chain that otherwise verifies
correctly (e.g. some software will accept BER instead of DER
encodings in certificates, or incorrect character encodings, even
though these are technically incorrect) 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 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.
4.2. Add PreCertChain to Log
POST https://<log server>/ct/v1/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 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.
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4.4. 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).
Both tree sizes must be from existing v1 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 Merkle Tree nodes, base64 encoded.
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.
Note that no signature is required on this data, as it is used to
verify an 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.
4.5. Retrieve Merkle Inclusion Proof from Log by Leaf Hash
GET https://<log server>/ct/v2/get-proof-by-hash
Inputs:
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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 3.5. The
"tree_size" must designate an existing v1 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:
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.
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.
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.
4.6. 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.
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The "hash" must be calculated as defined in Section 3.5. The
"tree_size" must designate an existing v1 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 4.4).
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.
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.
consistency: An array of base64-encoded Merkle Tree nodes proving
the consistency of the requested STH and the returned STH.
Errors are the same as in Section 4.5.
4.7. Retrieve Entries from Log
GET https://<log server>/ct/v1/get-entries
Inputs:
start: 0-based index of first entry to retrieve, in decimal.
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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 PrecertChainEntryV2,
this is the whole "PrecertChainEntryV2".
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 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 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".
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.
4.8. Retrieve Accepted Root Certificates
GET https://<log server>/ct/v1/get-roots
No inputs.
Outputs:
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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.
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 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.
5.1. 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 4.
Hash Algorithm The hash algorithm used for the Merkle Tree (see
Section 7.2).
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.
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.
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[JSON.Metadata] is an example of a metadata format which includes the
above elements.
5.2. Submitters
Submitters submit certificates or precertificates to the log as
described above. When a Submitter intends to use the returned SCT
directly in a TLS handshake or to construct a certificate, they
SHOULD validate the SCT as described in Section 5.3 if they
understand its format.
5.3. TLS Client
TLS clients receive SCTs alongside or in certificates, either for the
server certificate itself or for intermediate CA precertificates. In
addition to normal validation of the certificate and its chain, TLS
clients 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.
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 4.3), then verify
it by requesting a consistency proof (Section 4.4).
TLS clients MUST reject SCTs whose timestamp is in the future.
5.4. 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 4.3).
2. Verify the STH signature.
3. Fetch all the entries in the tree corresponding to the STH
(Section 4.7).
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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 4.7). 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:
1. Fetch a consistency proof for the new STH with the previous
STH (Section 4.4).
2. Verify the consistency proof.
3. Verify that the new entries generate the corresponding
elements in the consistency proof.
9. Go to Step 5.
5.5. 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 5.4) 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 5.3) 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 4.5). 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
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certificate or is an appropriate name-constrained intermediate [see
Section 3.2.3]).
6. 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 5.1 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.
7. IANA Considerations
7.1. 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.
7.2. Hash Algorithms
IANA is asked to establish a registry of hash values, initially
consisting of:
+-------+----------------------+
| Index | Hash |
+-------+----------------------+
| 0 | SHA-256 [FIPS.180-4] |
+-------+----------------------+
8. 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.
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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.
8.1. 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.
8.2. 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.
8.3. 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.
8.4. 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
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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).
8.5. Multiple SCTs
TLS servers may wish to offer multiple SCTs, each from a different
log.
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.
9. 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.
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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 and Carl Wallace for their valuable
contributions.
11. References
11.1. 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>.
[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, March 1997.
[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.
[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, 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.
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[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.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[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.
[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, March 2011.
11.2. 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>.
[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>.
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[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, June 2013.
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
Rob Stradling
Comodo CA, Ltd.
EMail: rob.stradling@comodo.com
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