Certificate Transparency
draft-laurie-pki-sunlight-00
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 | 2012-09-12 | ||
| RFC stream | (None) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-07)
by Francis Dupont
Almost ready
|
||
| 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-00
Network Working Group B. Laurie
Internet-Draft A. Langley
Expires: March 16, 2013 E. Kasper
September 12, 2012
Certificate Transparency
draft-laurie-pki-sunlight-00
Abstract
The aim of Certificate Transparency is to have every public end-
entity 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 will be able to monitor logs for
certificates issued on their own domain.
In order 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], or in the certificate itself via an X.509v3 certificate
extension [RFC5280].
In order 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.
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 will be 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
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 16, 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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1. Cryptographic components
1.1. 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[0: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[0:n] is then defined
recursively as
MTH(D[0: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 current definition omits dummy leaves._
1.1.1. Merkle audit paths
A Merkle audit path for a leaf in a Merkle hash tree is the list of
all additional nodes in the Merkle tree required to compute the
Merkle Tree Hash for that tree. If the root computed from the audit
path matches the true root, then the audit path is proof that the
leaf exists in the tree.
Given an ordered list of n inputs to the tree, D[0:n] = {d(0), ...,
d(n-1)}, the Merkle audit path PATH(m, D[0: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[0:1] = {d(0)} is empty:
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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[0:n]) = PATH(m, D[0:k]) : MTH(D[k:n]) for m < k; and
PATH(m, D[0:n]) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for k <= m < n,
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.
1.1.2. 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[0: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[0: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[0:n] = {d(0), ...,
d(n-1)}, the Merkle consistency proof PROOF(m, D[0:n]) for a previous
root hash MTH(D[0:m]), 0 < m < n, is defined as PROOF(m, D[0:n]) =
SUBPROOF(m, D[0: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[0:m], true) = {}
The subproof for m = n is the root hash committing inputs D[0:m]
otherwise:
SUBPROOF(m, D[0:m], false) = {MTH(D[0: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]:
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SUBPROOF(m, D[0: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[0: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.
1.1.3. 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].
The same tree, built incrementally in four steps:
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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[0:7]) =
[c, d, g, l]. c, g are used to verify hash0, and d, l are
additionally used to verify hash.
The consistency proof between hash1 and hash is PROOF(4, D[0:7]) =
[l]. hash can be verified, using hash1=k and l.
The consistency proof between hash2 and hash is PROOF(6, D[0:7]) =
[i, j, k]. k, i are used to verify hash1, and j is additionally used
to verify hash.
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2. Log Format
Certificate owners will be expected to submit certificates to
certificate logs for public auditing. 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). 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 Signed Certificate Timestamp.
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.
2.1. 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 signing the leaf
TBSCertificate [RFC5280] with a special-purpose (Extended Key Usage:
Certificate Transparency) 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 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 SCT can then be directly embedded in the final
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certificate, by appending it to the TBSCertificate as an X.509v3
certificate extension. Upon receiving the certificate, clients can
reconstruct the original TBSCertificate to verify the SCT signature.
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 X509 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 (or
in case of Precertificates, the entire Precertificate chain,
including the signed Precertificate), 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;
[benl: Should we consider a small number for the certificate_chain?
Or perhaps note that a log can impose a limit?]
"leaf_certificate" is the end-entity certificate submitted for
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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;
struct {
uint64 timestamp;
digitally-signed struct {
SignatureType signature_type = certificate_timestamp;
uint64 timestamp;
LogEntryType entry_type;
ASN.1Cert certificate;
};
} SignedCertificateTimestamp;
The encoding of the digitally-signed element is defined in [RFC5246].
"timestamp" is the current UTC time since epoch (January 1, 1970,
00:00), in milliseconds.
"signed_certificate" is the "leaf_certificate" (in case of an
X509ChainEntry), or "tbs_certificate" (in case of a
PrecertChainEntry).
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2.2. 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:
struct {
uint64 timestamp;
LogEntryType entry_type;
ASN.1Cert certificate;
} MerkleTreeLeaf;
Here "timestamp" is the timestamp of the corresponding SCT issued for
this certificate.
Structure of the Signed Merkle Tree Update:
struct {
uint64 old_tree_size;
uint64 timestamp;
MerkleTreeLeaf new_leaves<0..2^64-1>;
digitally-signed struct {
SignatureType signature_type = tree_hash;
uint64 timestamp;
uint64 tree_size;
opaque sha256_root_hash[32];
} TreeHeadSignature;
} SignedMerkleTreeUpdate;
"old_tree_size" is the size of the tree prior to this update.
"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.
"new_leaves" is the list of leaves added to the tree in this update,
ordered by leaf index. This order can be fixed arbitrarily amongst
new entries.
"sha256_root_hash" is the root of the Merkle Hash Tree.
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The log MUST produce a Signed Merkle Tree Update 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.
2.3. Audit Proofs
It is possible to audit the entire log by computing the current
"sha256_root_hash" value from consecutive Signed Merkle Tree Updates,
and verifying the Tree Head Signature. We rely on cross-checks of
the Signed Tree Head between auditors to verify that their views of
the log are consistent.
Additionally, logs provide Merkle audit proofs for efficient partial
checks. (In fact, anyone can compute audit proofs from the full
log.) Merkle audit proofs allow auditors to efficiently verify that
a certificate for which an SCT has been issued indeed appears in the
log, without inspecting the entire log.
Structure of the Merkle audit proof:
struct {
opaque sha256_hash[32];
} MerkleNode;
struct {
uint64 tree_size;
uint64 timestamp;
uint64 leaf_index;
MerkleNode audit_path<0..2^16-1>;
TreeHeadSignature tree_head_signature;
} MerkleAuditProof;
"tree_size" is the generation of the tree that this proof is for.
"timestamp" is the corresponding timestamp.
"leaf_index" is the index of the audited node in the Merkle tree,
from 0 to "tree_size - 1".
"audit_path" is a list of additional nodes in the Merkle tree
required for reconstructing the root hash corresponding to the
"tree_size". Nodes must be listed from leaf to root level, i.e., in
the order they are used in the Merkle Tree Hash computation, as
defined in Section 1.1.1. _Notice that the left-right ordering is
determined by the position of the leaf._ The leaf node under audit as
well as the root node shall be omitted from the path.
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"tree_head_signature" is the TreeHeadSignature for generation
"tree_size".
A valid audit proof for a Merkle Tree Leaf MUST satisfy the
following:
o The "tree_size" MUST be at least 1;
o The "leaf_index" MUST NOT exceed "tree_size - 1";
o The "tree_signature" MUST be a valid signature on the
corresponding "timestamp", "tree_size", and the root hash
reconstructed from the Merkle Tree Leaf, "leaf_index" and
"audit_path".
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3. Security and Privacy Considerations
3.1. 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 update 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 notice is the MMD.
3.2. 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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4. 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 root hash
can be updated incrementally as new entries become available, without
recomputing the entire tree. Third party auditors need only store a
logarithmic number of intermediate nodes in the Merkle Tree.
Additionally, the Merkle consistency proofs defined in Section 1.1.2
can be used to efficiently prove the append-only property of an
incremental update to the Merkle Tree, without auditing the entire
tree.
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5. References
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[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.
[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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