Introduction
The goal is to make it impossible (or at
least very difficult) for a Certificate Authority to issue a certificate for a
domain without it being visible to the owner of that domain. A secondary goal
is to protect users as much as possible from mis-issued certificates.
It is also intended that the solution
should be backwards compatible with existing browsers and other clients.
This is achieved by creating a number of
cryptographically assured, publicly auditable, append-only logs of
certificates. Every certificate will be accompanied by a signature from one or
more logs asserting that the certificate has been included in those logs.
Browsers, auditors and monitors
will collaborate to ensure that the log is
honest. Domain owners and other interested parties will monitor the log for
mis-issued certificates.
The logs will not deal with revocation:
that will be accomplished by existing mechanisms.
The Log
Each log is an append-only log of all
certificates submitted to it. It is designed so that monitors can efficiently
ensure that any certificate logged is promptly visible to them and can be
checked for legitimacy (for example, by knowing which certificates the domain
owner has got from CAs). It is
also possible for auditors to efficiently
check whatever partial information they have about the log is consistent with
the append-only nature of the log.
In other words, monitors see the whole of
the log, and watch over it on behalf of domain owners and other interested
parties. Auditors gather partial information and then verify that all that
partial information is consistent with the current state of the log.
Inconsistencies indicate dishonesty on the part of the log. For example, an
auditor built into a browser would verify that the certificate for each website
the browser visited actually appears in the log.
In general, “consistent with the current
state of the log” means that the current log provably contains every
certificate ever signed by the log.
If the log ever attempts to claim that it
has logged a certificate which is not actually in the log, then this will
become apparent to auditors and monitors.
Thus, domain owners are assured that only
their own legitimate certificates are in circulation, and can take action when
certificates are mis-issued. This ultimately protects users as well as domain
owners by effectively preventing masquerading as websites that are monitoring
the logs.
Detailed Operation
Anyone can submit a certificate and its
validation chain to the log. The log will immediately return a signed data
structure known as a Signed Certificate Timestamp (SCT) containing
. The time the certificate was submitted.
. A signature over the certificate and
timestamp.
The SCT is served along with the
certificate each time a TLS session is initiated, either through a TLS
extension or through incorporation into the certificate itself. Clients will
decline to connect to servers that do not include a SCT from a trusted log.
Clients will also later check that the certificate has been correctly
incorporated into the log (see below).
The log promises to incorporate the
certificate and chain within a certain amount of time1. Failure to do so is
considered a breach of contract by the log. This time is known as the Maximum
Merge Delay (MMD)2. We anticipate the MMD being measured in hours. Clearly, the
MMD is the longest possible time a rogue certificate can be used without
detection.
The log itself consists of an ever-growing
Merkle tree of all certificates ever issued. As we show in the detailed
protocol document[ref] it is possible for anyone with a complete copy of the
tree to efficiently show that any two versions of the tree are consistent: that
is, the later version includes
everything in the earlier version, in the
same order, and all new entries come after all entries from the earlier
version3. The size of this proof is logarithmically proportional to the number
of entries.
As frequently as possible, but at least as
often as the MMD, the log will produce a new version of the tree and will sign
the following data, known as a Signed Tree Head (STH)
. The root hash of the Merkle tree.
. The number of entries in the tree.
. A timestamp.
In the unlikely event there are no new
entries by the time MMD has expired, the log will reissue the STH with a new
timestamp.
Monitors will be able to fetch this new
version and a copy of all new certificates in the tree.
Since they will have the previous version,
they can check for themselves that the two trees are consistent, simply by
constructing the consistency proof themselves. Any discrepancy will be a breach
of contract by the log. Furthermore, this discrepancy will be provable - the
monitor will be able to show the two signed versions of the tree, and that they
are not consistent. Because they are both signed by the log, the inconsistent
version can only have been produced by the log misbehaving.
It is also possible to show the log has
failed to honour the MMD by showing an SCT and an STH whose timestamps differ
by more than the MMD, and that the corresponding certificate is not present in
the tree4. The log must always produce an STH that is more recent than the MMD
on request; failure to do so is an indication of misbehaviour.
Auditors will also be able to request from
either the log or a monitor (or anyone with a copy of the log) a proof that any
particular SCT is consistent with any particular STH, so long as the STH was
issued after the MMD had passed since the SCT’s timestamp5. This proof consists
of a Merkle
path from the leaf hash corresponding to
the SCT up to the root STH.
However, this will, of course, reveal the
particular certificate that was queried and so we must also provide a privacy
preserving mechanism for verifying SCTs. We provide two mechanisms.
The first makes the various proofs
available via DNS. To get a sense of how this works, say you wanted to see the
proof that a certificate with SCT hash 89abcdef is in the current tree (which
contains, say, 1300000 entries). To find the location of the SCT in the tree,
you would request a
TXT record containing its index from
. 89abcdef.hash.<domain>
Say the returned index is 1234567. The
Merkle path you want then contains the hash of certificate 1234568, the hash of
1234565 + 1234566, and so on. To get these values, you would request a TXT
record containing the hash from
. 1234567.0.1300000.tree.<domain>
. 617283.1.1300000.tree.<domain>
(617283 = 1234566 / 2).
and so on. In general, you request
<entry number>.<level>.<tree size>.tree.<domain> to get
the hash at that position in the particular tree you are interested in6.
This method does not hide which certificate
is being checked, but it does hide who is checking it from the log: most
clients are configured to use their ISP’s nameservers (or some other caching
resolver), so all the log will see is that some client of a particular ISP is
interested in a particular
certificate. The ISP will, of course, know
which client, but they also already have access to the IP addresses that client
visited.
The second method is for the auditor to
request a range of certificates around the one of interest and check all of
them, including the one they want to check. In order to do this, the log must
also make available a range of timestamps for each chunk of log. That is, for
every, say, 256
certificates the log will say what the
lowest and highest timestamp in the chunk is. Note that there may be overlap
between chunks. The client can thus choose a set of chunks that should include
the certificate it has, since it knows the timestamp from the SCT, and fetch
all the corresponding
hashes and their proofs. All the log learns
is that the client wants to verify one of the certificates it fetched.
Finally, since the proofs can be generated
by anyone with a copy of the log, clients can also choose to either keep their
own copy, or verify via some trusted third party that keeps a copy (note that
because of the signatures on the STH and SCT this third party only need be
trusted to preserve privacy, not to be honest about the log contents - the
client still verifies everything itself, but because it trusts the third party
to preserve its privacy it no longer needs to request “dummy” entries to hide
the sites it has visited).
Gossip
All the above allows any particular client
to check that their view of the world is consistent with their past views, but
the final piece in the puzzle is to show that everyone’s views are consistent
with each other.
This can be achieved by exchanging STHs and
SCTs between different clients through gossip protocols. These can then be
checked for consistency using the methods described above. Clients wishing to
preserve privacy can verify their own SCTs against STHs fetched from the log or
its mirrors and only gossip the latest STH they have seen.
Security Considerations
A misissued certificate can be used without
detection for at most the MMD. Once the MMD has passed since the SCT was
issued, either the certificate appears in a public log, or the log issuing the
SCT is no longer trusted, since it has failed in its duty to include the
certificate in the log within
the MMD. In the first case, the misissue
can be detected and the certificate revoked. In the second, the log signing key
is revoked. In both case the certificate/SCT pair will no longer be accepted by
clients.
The log contains the certificate plus the
intermediates chaining it to a trusted root, but the client only verifies that
the end certificate appears in the log. The client only needs to validate the
end certificate because that is sufficient to check for revocation, which is
all the client needs to know.
The log, however, needs to record the entire
chain so that when a certificate is misissued it is possible to correctly
assign blame. The reason the log does not sign the chain is that many CAs issue
certificates that may be presented with multiple chains. Permitting validation
of all possible
chains would bloat the log and complicate
protocols for no particular gain.
Sizes
(Assuming SHA-256 as the hash function.)
The number of currently valid public
certificates is estimated to be around 1.5M in 2012.
Data transmitted as part of a TLS handshake:
8 bytes timestamp + signature (<100 bytes for ECDSA, 256+ bytes for RSA)
Signed tree head (STH): 8 bytes timestamp +
32 bytes root hash + 8 bytes tree size + signature
Merkle proof: log2(size of tree) x 32 bytes
+ STH. For a tree with 1M certificates: 640 bytes + 8 bytes timestamp +
signature.
Caching: assume for example a client that
caches an intermediate node hash of every subtree of 256 certificates. Cache
size for a tree with 1M certificates: 32 MB/256 + current signature ~ 128 kB. Merkle
proof size: 8 x 32 = 256 bytes. A client will only need to request the rest of
the proof +
signature if there is a mismatch at the
cached node. It is also possible for many clients to cache the hash of every
certificate it ever verifies so each certificate only needs to be validated
once. For example, 1,000 certificates/day would be 12 MB/year.
Size of entire tree (end certificates
only): assume certificates are, on average, 1 kB long, then the tree for 1.5M
certificates is 1.5 GB. There is sufficient information in this data to
reconstruct the STH, which would mean performing around 3M hashes. A MacBook
Pro can do about 1,000,000
SHA-256 hashes a second, so this would take
3 seconds. Merkle trees are easily parallelised, so time can be reduced almost
arbitrarily.
Certificate Transparency v2.1a
Ben Laurie
(benl@google.com)
Emilia Kasper
(ekasper@google.com)
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