Certificate Transparency
draft-laurie-pki-sunlight-03
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 6962.
|
|
|---|---|---|---|
| Authors | Ben Laurie , Adam Langley , Emilia Kasper | ||
| Last updated | 2012-11-29 | ||
| RFC stream | (None) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
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by Francis Dupont
Almost ready
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||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | Became RFC 6962 (Experimental) | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-laurie-pki-sunlight-03
Network Working Group B. Laurie
Internet-Draft A. Langley
Expires: June 2, 2013 E. Kasper
November 29, 2012
Certificate Transparency
draft-laurie-pki-sunlight-03
Abstract
The aim of Certificate Transparency is to have every public end-
entity and intermediate TLS certificate issued by a known Certificate
Authority recorded in one or more certificate logs. In order to
detect mis-issuance of certificates, all logs are publicly auditable.
In particular, domain owners or their agents will be able to monitor
logs for certificates issued on their own domain.
To protect clients from unlogged mis-issued certificates, logs sign
all recorded certificates, and clients can choose not to trust
certificates that are not accompanied by an appropriate log
signature. For privacy and performance reasons log signatures are
embedded in the TLS handshake via the TLS authorization extension
[RFC5878], in a stapled [RFC6066] OCSP extension [RFC2560], or in the
certificate itself via an X.509v3 certificate extension [RFC5280].
To ensure a globally consistent view of the log, logs also provide a
global signature over the entire log. Any inconsistency of logs can
be detected through cross-checks on the global signature.
Consistency between any pair of global signatures, corresponding to
snapshots of the log at different times, can be efficiently shown.
Logs are only expected to certify that they have seen a certificate,
and thus we do not specify any revocation mechanism for log
signatures in this document. Logs are append-only, and log
signatures will be valid indefinitely.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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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 June 2, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Informal introduction . . . . . . . . . . . . . . . . . . . . 4
2. Cryptographic components . . . . . . . . . . . . . . . . . . . 5
2.1. Merkle Hash Trees . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Merkle audit paths . . . . . . . . . . . . . . . . . . 5
2.1.2. Merkle consistency proofs . . . . . . . . . . . . . . 6
2.1.3. Example . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.4. Signatures . . . . . . . . . . . . . . . . . . . . . . 8
3. Log Format . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Log Entries . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Including the Signed Certificate Timestamp in the TLS
Handshake . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3. Merkle Tree . . . . . . . . . . . . . . . . . . . . . . . 13
3.4. Tree Head Signature . . . . . . . . . . . . . . . . . . . 14
4. Client Messages . . . . . . . . . . . . . . . . . . . . . . . 16
4.1. Add Chain to Log . . . . . . . . . . . . . . . . . . . . . 16
4.2. Add PreCertChain to Log . . . . . . . . . . . . . . . . . 16
4.3. Retrieve Latest Signed Tree Head . . . . . . . . . . . . . 17
4.4. Retrieve Merkle Consistency Proof between two Signed
Tree Heads . . . . . . . . . . . . . . . . . . . . . . . . 17
4.5. Retrieve Merkle Audit Proof from Log by Leaf Hash . . . . 17
4.6. Retrieve Entries from Log . . . . . . . . . . . . . . . . 18
4.7. Retrieve Entry+Merkle Audit Proof from Log . . . . . . . . 19
5. Clients . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.1. Monitor . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.2. Auditor . . . . . . . . . . . . . . . . . . . . . . . . . 21
6. Security and Privacy Considerations . . . . . . . . . . . . . 22
6.1. Misissued Certificates . . . . . . . . . . . . . . . . . . 22
6.2. Detection of Misissue . . . . . . . . . . . . . . . . . . 22
6.3. Misbehaving logs . . . . . . . . . . . . . . . . . . . . . 22
7. Efficiency Considerations . . . . . . . . . . . . . . . . . . 23
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Informal introduction
Certificate Transparency aims to solve the problem of mis-issued
certificates by providing a publicly auditable, append-only,
untrusted log of all issued certificates. The logs are publicly
auditable so that it is possible for anyone to verify the correct
operation of the log, and to monitor when new certificates added to
it. The logs do not themselves prevent mis-issue, but they ensure
that interested parties (particularly those named in certificates)
can detect such mis-issuance. Note that this is a general mechanism,
but in this document we only decsribe its use for public TLS
certificates issued by public CAs.
The log consists of certificate chains, which can be submitted by
anyone. It is expected that most public CAs will contribute all
their newly-issued certificates to the log; it is also expected that
certificate holders will also contribute their own certificate
chains. In order to avoid the log being spammed into uselessness, it
is required that the chain is rooted in a known CA certificate. When
a chain is submitted to the log, a signed timestamp is returned,
which can later be used to prove to clients that the chain has been
submitted. Clients can thus require that all certificates they see
have been logged.
Those who are concerned about mis-issue can monitor the log, asking
it 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 mis-issuance has happened, is beyond the scope of
this document, but broadly speaking they can invoke existing business
mechanisms for dealing with mis-issued certificates. Of course,
anyone who wants can monitor the log, and if they believe a
certificate is incorrectly issued, take action as they see fit.
Similarly, those who have seen signed timestamps from the log can
later demand a proof of inclusion from the log. If the log is unable
to provide this (or, indeed, if the corresponding certificate is
absent from monitors' copies of the log), that is evidence of the
incorrect operation of the log. This 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 a 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 trust the log: if the log
attempts to show different things to different people, this can be
efficiently detected by comparing tree roots and consistency proofs.
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2. Cryptographic components
2.1. Merkle Hash Trees
Logs use a binary Merkle hash tree for efficient auditing. The
hashing algorithm is SHA-256. The input to the Merkle tree hash is a
list of data entries; these entries will be hashed to form the leaves
of the Merkle hash tree. The output is a single 32-byte root hash.
Given an ordered list of n inputs, D[n] = {d(0), d(1), ..., d(n-1)},
the Merkle Tree Hash (MTH) is thus defined as follows:
The hash of an empty list is the hash of an empty string:
MTH({}) = SHA-256().
The hash of a list with one entry is:
MTH({d(0)}) = SHA-256(0 || d(0)).
For n > 1, let k be the largest power of two smaller than n. The
Merkle Tree Hash of an n-element list D[n] is then defined
recursively as
MTH(D[n]) = SHA-256(1 || MTH(D[0:k]) || MTH(D[k:n])),
where || is concatenation and D[k1:k2] denotes the length (k2 - k1)
list {d(k1), d(k1+1),..., d(k2-1)}.
Note that we do not require the length of the input list to be a
power of two. The resulting Merkle tree may thus not be balanced,
however, its shape is uniquely determined by the number of leaves.
[This Merkle tree is essentially the same as the history tree [1]
proposal, except our definition omits dummy leaves.]
2.1.1. Merkle audit paths
A Merkle audit path for a leaf in a Merkle hash tree is the shortest
list of additional nodes in the Merkle tree required to compute the
Merkle Tree Hash for that tree. Each node in the tree is either a
leaf node, or is computed from the two nodes immediately below it
(i.e. towards the leaves). At each step up the tree (towards the
root), a node from the audit path is combined with the node computed
so far. In other words, the audit path consists of the list of
missing nodes required to compute the nodes leading from a leaf to
the root of the tree. If the root computed from the audit path
matches the true root, then the audit path is proof that the leaf
exists in the tree.
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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 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
root 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 root hash MTH(D[0:m])
is known):
SUBPROOF(m, D[m], true) = {}
The subproof for m = n is the root hash committing inputs D[0:m]
otherwise:
SUBPROOF(m, D[m], false) = {MTH(D[m])}
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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 concatenation of lists and D[k1:k2] denotes the length (k2
- k1) list {d(k1), d(k1+1),..., d(k2-1)} as before.
The number of nodes in the resulting proof is bounded above by
ceil(log2(n)) + 1.
2.1.3. Example
The binary Merkle tree with 7 leaves:
hash
/ \
/ \
/ \
/ \
/ \
k l
/ \ / \
/ \ / \
/ \ / \
g h i j
/ \ / \ / \ |
a b c d e f d6
| | | | | |
d0 d1 d2 d3 d4 d5
The audit path for d0 is [b, h, l].
The audit path for d3 is [c, g, l].
The audit path for d4 is [f, j, k].
The audit path for d6 is [i, k].
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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 can use either elliptic
curve signatures using the NIST P-256 curve
(http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf
section D.1.2.3) or RSA signatures using a key of at least 2048 bits.
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3. Log Format
Anyone can submit certificates to certificate logs for public
auditing, however, since certificates will not be accepted by 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.
After accepting a certificate submission, the log MUST immediately
return a Signed Certificate Timestamp (SCT). The SCT is the log's
promise to incorporate the certificate in the Merkle Tree within a
fixed amount of time known as the Maximum Merge Delay (MMD). If the
log has previously seen the certificate, it MAY return the same SCT
as it returned before. Servers MUST present an SCT from one or more
logs to the client together with the certificate. Clients MUST
reject certificates that do not have a valid SCT for the end-entity
certificate.
Periodically, the log appends all new entries to the Merkle Tree, and
signs the root of the tree. Clients and auditors can thus verify
that each certificate for which an SCT has been issued indeed appears
in the log. The log MUST incorporate a certificate in its Merkle
Tree within the Maximum Merge Delay period after the issuance of the
SCT.
3.1. Log Entries
Anyone can submit a certificate to the log. In order to attribute
each logged certificate to its issuer, the log shall publish a list
of acceptable root certificates (this list should be the union of
root certificates trusted by major browser vendors). Each submitted
certificate MUST be accompanied by all additional certificates
required to verify the certificate chain up to an accepted root
certificate. The self-signed root certificate itself MAY be omitted
from this list.
Alternatively, (root as well as intermediate) Certificate Authorities
may submit a certificate to the log prior to issuance. To do so, a
Certificate Authority constructs a Precertificate by adding a special
critical poison extension (OID 1.3.6.1.4.1.11129.2.4.3, ASN.1 NULL
data) to the leaf TBSCertificate, and signing the resulting
TBSCertificate [RFC5280] with a special-purpose (Extended Key Usage:
Certificate Transparency, OID 1.3.6.1.4.1.11129.2.4.4,
basicConstraints=critical,CA:FALSE) Precertificate Signing
Certificate. The Precertificate Signing Certificate MUST be
certified by the CA certificate. As above, the Precertificate
submission MUST be accompanied by the Precertificate Signing
Certificate and all additional certificates required to verify the
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chain up to an accepted root certificate. The signature on the
TBSCertificate indicates the Certificate Authority's intent to issue
a certificate. This intent is considered binding (i.e., misissuance
of the Precertificate is considered equal to misissuance of the final
certificate). The log verifies the Precertificate signature chain,
and issues a Signed Certificate Timestamp on the corresponding
TBSCertificate.
The log MUST verify that the submitted leaf certificate or
Precertificate has a valid signature chain leading back to a trusted
root CA certificate, using the chain of intermediate CA certificates
provided by the submitter. In case of Precertificates, the log MUST
also verify that the Precertificate Signing Certificate has the
correct Extended Key Usage extension. The log MAY accept
certificates that have expired, are not yet valid, have been revoked
or are otherwise not fully valid according to X.509 verification
rules. However, the log MUST refuse to publish certificates without
a valid chain to a known root CA. If a certificate is accepted and
an SCT issued, the log MUST store the chain used for verification
including the certificate or Precertificate itself, and MUST present
this chain for auditing upon request.
Each certificate entry in the log MUST include the following
components:
enum { x509_entry(0), precert_entry(1), (65535) } LogEntryType;
struct {
LogEntryType entry_type;
select (entry_type) {
case x509_entry: X509ChainEntry;
case precert_entry: PrecertChainEntry;
} entry;
} LogEntry;
opaque ASN.1Cert<1..2^24-1>;
struct {
ASN.1Cert leaf_certificate;
ASN.1Cert certificate_chain<0..2^24-1>;
} X509ChainEntry;
struct {
ASN.1Cert tbs_certificate;
ASN.1Cert precertificate_chain<1..2^24-1>;
} PrecertChainEntry;
Logs MAY limit the length of chain they will accept.
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"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 leaf certificate. The first certificate MUST certify the
leaf certificate. Each following certificate MUST directly certify
the one preceding it. The self-signed root certificate MAY be
omitted from the chain.
"tbs_certificate" is the TBSCertificate component of the
Precertificate (i.e., the original TBSCertificate, without the
Precertificate signature and the SCT extension).
"precertificate_chain" is a chain of certificates required to verify
the Precertificate submission. The first certificate MUST be the
original Precertificate, with its unsigned part matching the
"tbs_certificate". The second certificate MUST be a valid
Precertificate Signing Certificate, and MUST certify the first
certificate. Each following certificate MUST directly certify the
one preceding it. The self-signed root certificate MAY be omitted
from the chain.
Structure of the Signed Certificate Timestamp:
enum { certificate_timestamp(0), tree_hash(1), 255 }
SignatureType;
enum { v1(0), 255 }
Version;
struct {
opaque key_id[32];
} LogID;
opaque CtExtensions<0..2^16-1>;
"key_id" is the SHA-256 hash of the log's public key [TODO: define
how to calculate this].
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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: ASN.1Cert;
} signed_entry;
CtExtensions extensions;
};
} SignedCertificateTimestamp;
The encoding of the digitally-signed element is defined in [RFC5246].
"sct_version" is the version of the protocol the SCT conforms to.
This version is v1.
"timestamp" is the current UTC time since epoch (January 1, 1970,
00:00), in milliseconds.
"entry_type" is assumed to be implicit from the context in which the
SCT is presented.
"signed_entry" is the "leaf_certificate" (in case of an
X509ChainEntry), or "tbs_certificate" (in case of a
PrecertChainEntry).
"extensions" are future extensions to this protocol version (v1).
Currently, no extensions are specified.
3.2. Including the Signed Certificate Timestamp in the TLS Handshake
The SCT data from at least one log must be included in the TLS
handshake, either by using an Authorization Extension [RFC5878] with
type 182, or by using OCSP Stapling (section 8 of [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.
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Similarly, the Certificate Authority MAY submit the 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 bytestring that contains the
serialized TLS structure. This encoding ensures that 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 version they don't understand).
SCTs embedded in the TLS Authorization Extension are each encoded as
an individual AuthorizationDataEntry [RFC5878].
3.3. Merkle Tree
A certificate log MUST periodically append all new log entries to the
log Merkle Tree. The log MUST sign these entries by constructing a
binary Merkle Tree with log entries as consecutive inputs to the
tree, signing the corresponding Merkle Tree Hash, and publishing each
update to the tree in a Signed Merkle Tree Update. The hashing
algorithm for the Merkle Tree Hash is SHA-256.
Structure of the Merkle Tree input:
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enum { timestamped_entry(0), 255 }
MerkleLeafType;
struct {
uint64 timestamp;
LogEntryType entry_type;
select(entry_type) {
case x509_entry: ASN.1Cert;
case precert_entry: ASN.1Cert;
} 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 the MerkleTreeLeaf
corresponds to. 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 hashes of the corresponding
"MerkleTreeLeaf" structures.
3.4. Tree Head Signature
Every time the log appends new entries to the tree, the log MUST sign
the corresponding tree hash and tree information (see also the
corresponding Signed Tree Head client message in Section 4.3). The
signature input is structured as follows:
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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 the TreeHeadSignature
conforms to. 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.
The log MUST produce a Tree Head Signature at least as often as the
Maximum Merge Delay. In the unlikely event that it receives no new
submissions during an MMD period, the log SHALL sign the same Merkle
Tree Hash with a fresh timestamp.
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4. Client Messages
Messages are sent as HTTPS GET or POST requests. Parameters for
POSTs and all responses are encoded as JSON objects. Parameters for
GETs are encoded as URL parameters. Binary data is base64 encoded as
specified in the individual messages.
The <log server> prefix can include a path as well as a server name
and a port. It must map one-to-one to a known public key (how this
mapping is distributed is out of scope for this document).
In general, where needed, the "version" is v1 and the "id" is the log
id for the log server queried.
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 leaf 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 clients who request an SCT for
inclusion in the TLS handshake are not required to verify it, we
do not assume they know the ID of the log.
timestamp The SCT timestamp, in decimal.
extensions [TBD]
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.
4.2. Add PreCertChain to Log
POST https://<log server>/ct/v1/add-pre-chain
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Inputs
chain An array of base64 encoded precertificates. The first element
is the leaf 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 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 root 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 published v1 STHs.
Outputs
consistency An array of Merkle tree nodes, base64 encoded.
Note that no signature is required on this data, as it is used to
verify an STH, which is signed.
4.5. Retrieve Merkle Audit Proof from Log by Leaf Hash
GET https://<log server>/ct/v1/get-proof-by-hash
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Inputs
hash A base64 encoded v1 leaf hash.
tree_size The tree_size of the tree to base the proof on, in
decimal.
The "hash" must be calculated as defined in Section 3.3. The
"tree_size" must designate a published v1 STH.
Outputs
timestamp The tree's timestamp, in decimal.
leaf_index The index of the leaf corresponding to the "hash"
parameter.
audit_path An array of base64 encoded Merkle 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 Index of first entry to retrieve, in decimal.
end 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 "precertificate_chain".
Note that this message is not signed - the retrieved data can be
verified by constructing the root 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
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as opaque input to the tree.
4.7. 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 a published STH.
Outputs
entries An array of objects, each consisting of
leaf_input The base64-encoded MerkleTreeLeaf structure.
auxiliary_data The base64-encoded unsigned data, same as in
Section 4.6.
timestamp The tree's timestamp, in decimal.
audit_path An array of base64 encoded Merkle tree nodes proving the
inclusion of the chosen certificate.
This API is probably only useful for debugging.
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5. Clients
There are various different functions clients of the log 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 misbehaviour.
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 is TBD, but it is
expected there will be a variety.
5.1. Monitor
Monitors watch the log and check that it behaves correctly. They
also watch for certificates of interest.
A monitor needs to, at least, inspect every new entry in the log. It
may also want to keep a copy of the entire log. In order to do this,
it should follow these steps:
1. Fetch the current STH using Section 4.3.
2. Verify the STH signature.
3. Fetch all the entries in the tree corresponding to the STH using
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 using Section 4.3. Repeat until STH
changes.
6. Verify the STH signature.
7. Fetch all the new entries in the tree corresponding to the STH
using Section 4.6. If they remain unavailable for an extended
period, then this should be viewed as misbehaviour 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:
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2. Fetch a consistency proof for the new STH with the previous
STH using 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.
5.2. 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 using 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 using Section 4.5.
Auditors can fetch STHs from time to time of their own accord, of
course, using Section 4.3.
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6. Security and Privacy Considerations
6.1. Misissued Certificates
Misissued certificates that have not been publicly logged, and thus
do not have a valid SCT, will be rejected by clients. Misissued
certificates that do have an SCT from a log will appear in the public
log 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.
6.2. Detection of Misissue
The log does not itself detect misissued certificate, it relies
instead on interested parties, such as domain owners, to monitor it
and take corrective action when a misissue is detected.
6.3. 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 clients requesting a
Merkle audit proof for each observed SCT. These checks can be
asynchronous, and need only be done once per each certificate. In
order to protect the clients' privacy, these checks need not reveal
the exact certificate to the log. Clients can instead request the
proof from a trusted auditor (since anyone can compute the audit
proofs from the log), or request Merkle proofs for a batch of
certificates around the SCT timestamp.
Violation of the append-only property is detected by global
gossiping, i.e., everyone auditing the log comparing their versions
of the latest signed tree head. As soon as two conflicting signed
tree heads are detected, this is cryptographic proof of the log's
misbehaviour.
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7. Efficiency Considerations
The Merkle tree design serves the purpose of keeping communication
overhead low.
Auditing the log for integrity does not require third parties to
maintain a copy of the entire log. The Signed Tree Head can be
updated as new entries become available, without recomputing the
entire tree. Third party auditors need only fetch the Merkle
consistency proof against an existing STH to efficiently verify the
append-only property of an update to the Merkle Tree, without
auditing the entire tree.
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8. References
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[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.
[RFC5878] Brown, M. and R. Housley, "The Transport Layer Security
(TLS) Authorization Extensions", RFC 5280, May 2010.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[1] <http://tamperevident.cs.rice.edu/Logging.html/>
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Authors' Addresses
Ben Laurie
Email: benl@google.com
Adam Langley
Email: agl@google.com
Emilia Kasper
Email: ekasper@google.com
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