Certificate TransparencyGoogle UK Ltd.benl@google.comGoogle Inc.agl@google.comGoogle Switzerland GmbHekasper@google.comGoogle UK Ltd.eranm@google.comComodo CA, Ltd.rob.stradling@comodo.com
Security
TRANS (Public Notary Transparency)This document describes a protocol for publicly logging the existence of
Transport Layer Security (TLS) server 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.Certificate transparency aims to mitigate the problem of misissued certificates
by providing append-only logs of issued certificates. The logs do not need to be
trusted because they are publicly auditable. Anyone may verify the correctness
of each log and 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 that could be used for transparently logging
any form of binary data, subject to some kind of inclusion criteria. In this
document, we only describe its use for public TLS server certificates (i.e.,
where the inclusion criteria is a valid certificate issued by a public
certification authority (CA)).Each log contains 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 the submission of large numbers of spurious certificates, it is
required that each chain ends with a trust anchor that is accepted by the log.
When a chain is accepted by 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 are accompanied by signed timestamps.Those who are concerned about misissuance can monitor the logs, asking them
regularly for all new entries, and can thus check whether domains for which they
are responsible 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. However, 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 clients to proceed without delay, despite
possible issues such as network connectivity and the vagaries of firewalls.The append-only property of each log is 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.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 .Data structures are defined according to the conventions laid out in Section 4
of .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 (see ). We have established a registry of acceptable
algorithms, see . The hashing algorithm in use is referred to
as HASH throughout this document and the size of its output in bytes as
HASH_SIZE. 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 HASH_SIZE 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:The hash of a list with one entry (also known as a leaf hash) is: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 aswhere || 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 proposal, except our
definition handles non-full trees differently.)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 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: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 aswhere : is concatenation of lists and D[k1:k2] denotes the length (k2 - k1)
list {d(k1), d(k1+1),…, d(k2-1)} as before.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:In SUBPROOF, the boolean value represents whether the subtree created from
D[0:m] is a complete subtree of the Merkle Tree created from D[n], and,
consequently, whether the subtree Merkle Tree Hash MTH(D[0:m]) is known. The
initial call to SUBPROOF sets this to be true, and SUBPROOF is then defined as
follows:The subproof for m = n is empty if m is the value for which PROOF was originally
requested (meaning that the subtree created from D[0:m] is a complete subtree
of the Merkle Tree created from the original D[n] for which PROOF was
requested, and the subtree Merkle Tree Hash MTH(D[0:m]) is known):Otherwise, the subproof for m = n is the Merkle Tree Hash committing inputs
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]: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].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.The binary Merkle Tree with 7 leaves:The inclusion proof for d0 is [b, h, l].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: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.Various data structures are signed. A log MUST use one of the signature
algorithms defined in .Submitters submit certificates or preannouncements of certificates prior to
issuance (precertificates) to logs for public auditing, as described below. In
order to enable attribution of each logged certificate or precertificate to its
issuer, each submission MUST be accompanied by all additional certificates
required to verify the chain up to an accepted trust anchor. The trust anchor (a
root or intermediate CA certificate) MAY be omitted from the submission.If a log accepts a submission, it will return a Signed Certificate Timestamp
(SCT) (see ). The submitter SHOULD validate the returned SCT as described
in if they understand its format and they intend to use it
directly in a TLS handshake or to construct a certificate. If the submitter does
not need the SCT (for example, the certificate is being submitted simply to make
it available in the log), it MAY validate the SCT.Any entity can submit a certificate () to a log. Since it is
anticipated that TLS clients will reject certificates that are not logged, it is
expected that certificate issuers and subjects will be strongly motivated to
submit them.CAs may preannounce a certificate prior to issuance by submitting a
precertificate () that the log can use to create an entry that
will be valid against the issued certificate. The CA MAY incorporate the
returned SCT in the issued certificate. One example of where the returned SCT is
not incorporated in the issued certificate is when a CA sends the precertificate
to multiple logs, but only incorporates the SCTs that are returned first.A precertificate is a CMS signed-data object that conforms to the
following requirements:It MUST be DER encoded.SignedData.encapContentInfo.eContentType MUST be the OID 1.3.101.78.SignedData.encapContentInfo.eContent MUST contain a TBSCertificate
that will be identical to the TBSCertificate in the issued certificate, except
that the Transparency Information () extension
MUST be omitted.SignedData.signerInfos MUST contain a signature from the same (root or
intermediate) CA that will ultimately issue the certificate. This signature
indicates the CA'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). (Note that, because of the structure of CMS,
the signature on the CMS object will not be a valid X.509v3 signature and so
cannot be used to construct a certificate from the precertificate).SignedData.certificates SHOULD be omitted.Some regard certain 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.A certificate containing a DNS-ID 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 ), a different
approach is needed when any label other than the leftmost label in a DNS-ID is
considered private (e.g., top.secret.example.com). Also, wildcard certificates
are prohibited in some cases, such as Extended Validation Certificates
.An intermediate CA certificate or intermediate CA precertificate that contains
the Name Constraints 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:there MUST be a non-critical extension (OID 1.3.101.76, 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.there MUST be a Name Constraints extension, in which: permittedSubtrees MUST specify one or more dNSNames.excludedSubtrees MUST specify the entire IPv4 and IPv6 address ranges.Below is an example Name Constraints extension that meets these conditions:A log is a single, append-only Merkle Tree of submitted certificate and
precertificate entries.When it receives a valid submission, the log MUST return an SCT that corresponds
to the submitted certificate or precertificate. If the log has previously seen
this valid submission, it SHOULD return the same SCT as it returned before (to
reduce the ability to track clients as described in
). If different SCTs are produced for the same
submission, multiple log entries will have to be created, one for each SCT (as
the timestamp is a part of the leaf structure). Note that if a certificate was
previously logged as a precertificate, then the precertificate's SCT of type
precert_sct_v2 would not be appropriate; instead, a fresh SCT of type
x509_sct_v2 should be generated.An SCT is the log's promise to incorporate the submitted entry in its Merkle
Tree no later than a fixed amount of time, known as the Maximum Merge Delay
(MMD), after the issuance of the SCT. Periodically, the log MUST append all its
new entries to its Merkle Tree and sign the root of the tree.Log operators MUST NOT impose any conditions on retrieving or sharing data from
the log.Logs MUST verify that each submitted certificate or precertificate has a valid
signature chain to an accepted trust anchor, using the chain of intermediate CA
certificates provided by the submitter. Logs MUST accept certificates and
precertificates that are fully valid according to RFC 5280
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 RFC 5280 verification
rules in order to accommodate quirks of CA certificate-issuing software.
However, logs MUST reject submissions without a valid signature chain to an
accepted trust anchor. Logs MUST also reject precertificates that do not conform
to the requirements in .Logs SHOULD limit the length of chain they will accept. The maximum chain length
is specified in the log's metadata.The log SHALL allow retrieval of its list of accepted trust anchors (see
), each of which is a root or intermediate CA certificate. This
list might usefully be the union of root certificates trusted by major browser
vendors.If a submission is accepted and an SCT issued, the accepting log MUST store the
entire chain used for verification. This chain MUST include the certificate or
precertificate itself, the zero or more intermediate CA certificates provided by
the submitter, and the trust anchor used to verify the chain (even if it was
omitted from the submission). The log MUST present this chain for auditing upon
request (see ). This chain is required to prevent a CA from
avoiding blame by logging a partial or empty chain.Each certificate entry in a log MUST include a X509ChainEntry structure, and
each precertificate entry MUST include a PrecertChainEntryV2 structure:leaf_certificate is a submitted certificate that has been accepted by the log.certificate_chain is a vector of 0 or more additional certificates required to
verify leaf_certificate. The first certificate MUST certify
leaf_certificate. Each following certificate MUST directly certify the one
preceding it. The final certificate MUST be a trust anchor accepted by the log.
If leaf_certificate is an accepted trust anchor, then this vector is empty.pre_certificate is a submitted precertificate that has been accepted by the
log.precertificate_chain is a vector of 1 or more additional certificates required
to verify pre_certificate. The first certificate MUST certify
pre_certificate. Each following certificate MUST directly certify the one
preceding it. The final certificate MUST be a trust anchor accepted by the log.Each log is identified by an OID, which is specified in the log's metadata and
which MUST NOT be used to identify any other log. A log's operator MUST either
allocate the OID themselves or request an OID from one of the two Log ID
Registries (see and ). Various data
structures include the DER encoding of this OID, excluding the ASN.1 tag and
length bytes, in an opaque vector:Note that the ASN.1 length and the opaque vector length are identical in size (1
byte) and value, so the DER encoding of the OID can be reproduced simply by
prepending an OBJECT IDENTIFIER tag (0x06) to the opaque vector length and
contents.OIDs used to identify logs are limited such that the DER encoding of their value
is less than or equal to 127 octets.Various data structures are encapsulated in the TransItem structure to ensure
that the type and version of each one is identified in a common fashion:versioned_type is the type of the encapsulated data structure and the earliest
version of this protocol to which it conforms. This document is v2.data is the encapsulated data structure. The various structures named with the
DataV2 suffix are defined in later sections of this document.Note that VersionedTransType combines the v1 type enumerations
Version, LogEntryType, SignatureType and MerkleLeafType. Note also that
v1 did not define TransItem, but this document provides guidelines (see
) on how v2 implementations can co-exist with v1
implementations.Future versions of this protocol may reuse VersionedTransType values defined
in this document as long as the corresponding data structures are not modified,
and may add new VersionedTransType values for new or modified data structures.The leaves of a log's Merkle Tree correspond to the log's entries (see
). Each leaf is the leaf hash () of a TransItem
structure of type x509_entry_v2 or precert_entry_v2, which encapsulates a
TimestampedCertificateEntryDataV2 structure. Note that leaf hashes are
calculated as HASH(0x00 || TransItem), where the hashing algorithm is specified
in the log's metadata.timestamp is the NTP Time at which the certificate or precertificate
was accepted by the log, measured in milliseconds since the epoch (January 1,
1970, 00:00 UTC), ignoring leap seconds. Note that the leaves of a log's Merkle
Tree are not required to be in strict chronological order.issuer_key_hash is the HASH of the public key of the CA that issued the
certificate or precertificate, calculated over the DER encoding of the key
represented as SubjectPublicKeyInfo . This is needed to bind the CA to
the certificate or precertificate, making it impossible for the corresponding
SCT to be valid for any other certificate or precertificate whose TBSCertificate
matches tbs_certificate. The length of the issuer_key_hash MUST match
HASH_SIZE.tbs_certificate is the DER encoded TBSCertificate from either the
leaf_certificate (in the case of an X509ChainEntry) or the pre_certificate
(in the case of a PrecertChainEntryV2). (Note that a precertificate's
TBSCertificate can be reconstructed from the corresponding certificate as
described in ).sct_extensions matches the SCT extensions of the corresponding SCT.An SCT is a TransItem structure of type x509_sct_v2 or precert_sct_v2,
which encapsulates a SignedCertificateTimestampDataV2 structure:log_id is this log's unique ID, encoded in an opaque vector as described in
.timestamp is equal to the timestamp from the
TimestampedCertificateEntryDataV2 structure encapsulated in the
timestamped_entry.sct_extension_type identifies a single extension from the IANA registry in
. At the time of writing, no extensions are specified.The interpretation of the sct_extension_data field is determined solely by the
value of the sct_extension_type field. Each document that registers a new
sct_extension_type must describe how to interpret the corresponding
sct_extension_data.sct_extensions is a vector of 0 or more SCT extensions. This vector MUST NOT
include more than one extension with the same sct_extension_type. The
extensions in the vector MUST be ordered by the value of the
sct_extension_type field, smallest value first. If an implementation sees an
extension that it does not understand, it SHOULD ignore that extension.
Furthermore, an implementation MAY choose to ignore any extension(s) that it
does understand.The encoding of the digitally-signed element is defined in .timestamped_entry is a TransItem structure that MUST be of type
x509_entry_v2 or precert_entry_v2 (see ).The log stores information about its Merkle Tree in a TreeHeadDataV2:The length of NodeHash MUST match HASH_SIZE of the log.sth_extension_type identifies a single extension from the IANA registry in
. At the time of writing, no extensions are specified.The interpretation of the sth_extension_data field is determined solely by the
value of the sth_extension_type field. Each document that registers a new
sth_extension_type must describe how to interpret the corresponding
sth_extension_data.timestamp is the current NTP Time , measured in milliseconds since
the epoch (January 1, 1970, 00:00 UTC), ignoring leap seconds.tree_size is the number of entries currently in the log's Merkle Tree.root_hash is the root of the Merkle Hash Tree.sth_extensions is a vector of 0 or more STH extensions. This vector MUST NOT
include more than one extension with the same sth_extension_type. The
extensions in the vector MUST be ordered by the value of the
sth_extension_type field, smallest value first. If an implementation sees an
extension that it does not understand, it SHOULD ignore that extension.
Furthermore, an implementation MAY choose to ignore any extension(s) that it
does understand.Periodically each log SHOULD sign its current tree head information (see
) to produce an STH. When a client requests a log's latest STH (see
), the log MUST return an STH that is no older than the log's MMD.
However, STHs could be used to mark individual clients (by producing a new one
for each query), so logs MUST NOT produce them more frequently than is declared
in their metadata. In general, there is no need to produce a new STH unless
there are new entries in the log; however, 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.An STH is a TransItem structure of type signed_tree_head_v2, which
encapsulates a SignedTreeHeadDataV2 structure:log_id is this log's unique ID, encoded in an opaque vector as described in
.The timestamp in tree_head 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_head contains the latest tree head information (see ).signature is a signature over the encoded tree_head field.To prepare a Merkle Consistency Proof for distribution to clients, the log
produces a TransItem structure of type consistency_proof_v2, which
encapsulates a ConsistencyProofDataV2 structure:log_id is this log's unique ID, encoded in an opaque vector as described in
.tree_size_1 is the size of the older tree.tree_size_2 is the size of the newer tree.consistency_path is a vector of Merkle Tree nodes proving the consistency of
two STHs.To prepare a Merkle Inclusion Proof for distribution to clients, the log
produces a TransItem structure of type inclusion_proof_v2, which
encapsulates an InclusionProofDataV2 structure:log_id is this log's unique ID, encoded in an opaque vector as described in
.tree_size is the size of the tree on which this inclusion proof is based.leaf_index is the 0-based index of the log entry corresponding to this
inclusion proof.inclusion_path is a vector of Merkle Tree nodes proving the inclusion of the
chosen certificate or precertificate.Log operators may decide to shut down a log for various reasons, such as
deprecation of the signature algorithm. If there are entries in the log for
certificates that have not yet expired, simply making TLS clients stop
recognizing that log will have the effect of invalidating SCTs from that log.
To avoid that, the following actions are suggested:Make it known to clients and monitors that the log will be frozen.Stop accepting new submissions (the error code "shutdown" should be returned
for such requests).Once MMD from the last accepted submission has passed and all pending
submissions are incorporated, issue a final STH and publish it as a part of
the log's metadata. Having an STH with a timestamp that is after the MMD has
passed from the last SCT issuance allows clients to audit this log regularly
without special handling for the final STH. At this point the log's private
key is no longer needed and can be destroyed.Keep the log running until the certificates in all of its entries have expired
or exist in other logs (this can be determined by scanning other logs or
connecting to domains mentioned in the certificates and inspecting the SCTs
served).Messages are sent as HTTPS GET or POST requests. Parameters for POSTs and all
responses are encoded as JavaScript Object Notation (JSON) objects .
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" . Binary data is base64 encoded 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, which is part of the log's metadata, MAY include a
path as well as a server name and a port.In practice, log servers may include multiple front-end machines. Since it is
impractical to keep these machines in perfect sync, 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 ), and, in place of the responses
outlined in the subsections below, the body SHOULD be a JSON structure
containing at least the following field:
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.
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.Error CodeMeaningnot compliantThe request is not compliant with this RFC.e.g., In response to a request of /ct/v2/get-entries?start=100&end=99, the log
would return a 400 Bad Request response code with a body similar to the
following: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 , in the case of a 503
response the log MAY include a Retry-After: header in order to request a
minimum time for the client to wait before retrying the request.POST https://<log server>/ct/v2/add-chain
An array of base64 encoded certificates. The first element is the
certificate for which the submitter desires an SCT; the second certifies the
first and so on to the last, which either is, or is certified by, an
accepted trust anchor.
A base64 encoded TransItem of type x509_sct_v2, signed by this log, that
corresponds to the submitted certificate.Error codes:Error CodeMeaningunknown anchorThe last certificate in the chain both is not, and is not certified by, an accepted trust anchor.bad chainThe alleged chain is not actually a chain of certificates.bad certificateOne or more certificates in the chain are not valid (e.g., not properly encoded).shutdownThe log has ceased operation and is not accepting new submissions.If the version of sct is not v2, then a v2 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 v2 clients that do not use the returned SCTs.If a log detects bad encoding in a chain that otherwise verifies correctly then
the log MUST either log the certificate or return the "bad certificate" error.
If the certificate is logged, an SCT MUST be issued. Logging the certificate is
useful, because monitors () can then detect these encoding errors,
which may be accepted by some TLS clients.POST https://<log server>/ct/v2/add-pre-chain
The base64 encoded precertificate.
An array of base64 encoded CA certificates. The first element is the signer
of the precertificate; the second certifies the first and so on to the last,
which either is, or is certified by, an accepted trust anchor.
A base64 encoded TransItem of type precert_sct_v2, signed by this log,
that corresponds to the submitted precertificate.Errors are the same as in .GET https://<log server>/ct/v2/get-sthNo inputs.
A base64 encoded TransItem of type signed_tree_head_v2, signed by this
log, that is no older than the log's MMD.GET https://<log server>/ct/v2/get-sth-consistency
The tree_size of the older tree, in decimal.
The tree_size of the newer tree, in decimal (optional).Both tree sizes must be from existing v2 STHs. 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.
A base64 encoded TransItem of type consistency_proof_v2, whose
tree_size_1 MUST match the first input. If the sth output is omitted,
then tree_size_2 MUST match the second input.
A base64 encoded TransItem of type signed_tree_head_v2, signed by this
log.Note that no signature is required for the consistency output as it is used
to verify the consistency between two STHs, which are signed.Error codes:Error CodeMeaningfirst unknownfirst is before the latest known STH but is not from an existing STH.second unknownsecond is before the latest known STH but is not from an existing STH.See for an outline of how to use the consistency
output.GET https://<log server>/ct/v2/get-proof-by-hash
A base64 encoded v2 leaf hash.
The tree_size of the tree on which to base the proof, in decimal.The hash must be calculated as defined in . The tree_size
must designate an existing v2 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 inclusion is returned.
A base64 encoded TransItem of type inclusion_proof_v2 whose
inclusion_path array of Merkle Tree nodes proves the inclusion of the
chosen certificate in the selected STH.
A base64 encoded TransItem of type signed_tree_head_v2, signed by this
log.Note that no signature is required for the inclusion output as it is used to
verify inclusion in the selected STH, which is signed.Error codes:Error CodeMeaninghash unknownhash is not the hash of a known leaf (may be caused by skew or by a known certificate not yet merged).tree_size unknownhash is before the latest known STH but is not from an existing STH.See for an outline of how to use the inclusion output.GET https://<log server>/ct/v2/get-all-by-hash
A base64 encoded v2 leaf hash.
The tree_size of the tree on which to base the proofs, in decimal.The hash must be calculated as defined in . The tree_size
must designate an existing v2 STH.Because of skew, the front-end may not know the requested STH or the requested
hash, which leads to a number of cases.
Return latest STH.
Return latest STH and a consistency proof between it and the requested STH
(see ).
Return inclusion.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.
A base64 encoded TransItem of type inclusion_proof_v2 whose
inclusion_path array of Merkle Tree nodes proves the inclusion of the
chosen certificate in the returned STH.
A base64 encoded TransItem of type signed_tree_head_v2, signed by this
log.
A base64 encoded TransItem of type consistency_proof_v2 that proves the
consistency of the requested STH and the returned STH.Note that no signature is required for the inclusion or consistency
outputs as they are used to verify inclusion in and consistency of STHs, which
are signed.Errors are the same as in .See for an outline of how to use the inclusion output,
and see for an outline of how to use the consistency
output.GET https://<log server>/ct/v2/get-entries
0-based index of first entry to retrieve, in decimal.
0-based index of last entry to retrieve, in decimal.
An array of objects, each consisting of
The base64 encoded TransItem structure of type x509_entry_v2 or
precert_entry_v2 (see ).
The base64 encoded log entry (see ). In the case of an
x509_entry_v2 entry, this is the whole X509ChainEntry; and in the case
of a precert_entry_v2, this is the whole PrecertChainEntryV2.
The base64 encoded TransItem of type x509_sct_v2 or precert_sct_v2
corresponding to this log entry.
A base64 encoded TransItem of type signed_tree_head_v2, signed by this
log.Note that this message is not signed -- the entries data can be verified by
constructing the Merkle Tree Hash corresponding to a retrieved STH. All leaves
MUST be v2. However, a compliant v2 client MUST NOT construe an unrecognized
TransItem type 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 .The start parameter MUST be less than or equal to the end parameter.Log servers MUST 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. end >= tree_size could be caused by skew. 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.In any case, the log server MUST return the latest STH it knows about.See for an outline of how to use a complete list of leaf_input
entries to verify the root_hash.GET https://<log server>/ct/v2/get-anchorsNo inputs.
An array of base64 encoded trust anchors that are acceptable to the log.
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.Logs MAY implement these messages. They are not required for correct operation
of logs or their clients, but may be convenient in some circumstances.GET https://<log server>/ct/v2/get-entry-for-sct
A base64 encoded TransItem of type x509_sct_v2 or precert_sct_v2
signed by this log.
0-based index of the log entry corresponding to the supplied SCT.Error codes:Error CodeMeaningbad signaturesct is not signed by this log.not foundsct does not correspond to an entry that is currently available.Note that any SCT signed by a log MUST have a corresponding entry in the log,
but it may not be retrievable until the MMD has passed since the SCT was issued.GET https://<log server>/ct/v2/get-entry-for-tbscertificate
A base64 encoded HASH of a TBSCertificate for which the log has previously
issued an SCT. (Note that a precertificate's TBSCertificate is reconstructed
from the corresponding certificate as described in
).
An array of 0-based indices of log entries corresponding to the supplied
HASH.Error codes:Error CodeMeaningbad hashhash is not the right size or format.not foundsct does not correspond to an entry that is currently available.Note that it is possible for a certificate to be logged more than once. If that
is the case, the log MAY return more than one entry index. If the certificate is
present in the log, then the log MUST return at least one entry index.TLS servers MUST use at least one of the three mechanisms listed below to
present one or more SCTs from one or more logs to each TLS client during full
TLS handshakes, where each SCT corresponds to the server certificate or to a
name-constrained intermediate the server certificate chains to. TLS servers
SHOULD also present corresponding inclusion proofs and STHs (see
).Three mechanisms are provided because they have different tradeoffs.A TLS extension (Section 7.4.1.4 of ) with type transparency_info
(see ). This mechanism allows TLS servers to
participate in CT without the cooperation of CAs, unlike the other two
mechanisms. It also allows SCTs and inclusion proofs to be updated on the fly.An Online Certificate Status Protocol (OCSP) response extension (see
), where the OCSP response is provided in the
CertificateStatus message, provided that the TLS client included the
status_request extension in the (extended) ClientHello (Section 8 of
). This mechanism, popularly known as OCSP stapling, is already
widely (but not universally) implemented. It also allows SCTs and inclusion
proofs to be updated on the fly.An X509v3 certificate extension (see ). This
mechanism allows the use of unmodified TLS servers, but the SCTs and inclusion
proofs cannot be updated on the fly. Since the logs from which the SCTs and
inclusion proofs originated won't necessarily be accepted by TLS clients for
the full lifetime of the certificate, there is a risk that TLS clients will
subsequently consider the certificate to be non-compliant and in need of
re-issuance.Additionally, a TLS server which supports presenting SCTs using an OCSP response
MAY provide it when the TLS client included the status_request_v2 extension
() in the (extended) ClientHello, but only in addition to at least
one of the three mechanisms listed above.TLS servers SHOULD send SCTs from multiple logs in case one or more logs are not
acceptable to the TLS client (for example, if a log has been struck off for
misbehavior, has had a key compromise, or is not known to the TLS client). For
example: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.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 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.TLS clients may have policies related to the above risks requiring servers to
present multiple SCTs. For example, at the time of writing, Chromium
requires multiple SCTs to be presented with EV
certificates in order for the EV indicator to be shown.To select the logs from which to obtain SCTs, a TLS server can, for example,
examine the set of logs popular TLS clients accept and recognize.Multiple SCTs, inclusion proofs, and indeed TransItem structures of any type,
are combined into a list as follows:Here, SerializedTransItem is an opaque byte string that contains the
serialized TransItem structure. This encoding ensures that TLS clients can
decode each TransItem individually (so, for example, if there is a version
upgrade, out-of-date clients can still parse old TransItem structures while
skipping over new TransItem structures whose versions they don't understand).When constructing a TransItemList structure, a TLS server SHOULD construct and
include TransItem structures of type x509_sct_with_proof_v2 (for an SCT of
type x509_sct_v2) or precert_sct_with_proof_v2 (for an SCT of type
precert_sct_v2), both of which encapsulate a SCTWithProofDataV2 structure:sct is the encapsulated data structure from an SCT that corresponds to the
server certificate or to a name-constrained intermediate the server certificate
chains to.sth is the encapsulated data structure from an STH that was signed by the same
log as sct.inclusion_proof is the encapsulated data structure from an inclusion proof
that corresponds to sct and can be used to compute the root in sth.Presenting inclusion proofs and STHs in the TLS handshake helps to protect the
client's privacy (see ) and reduces load on log
servers. However, if a TLS server is unable to obtain an inclusion proof and STH
that correspond to an SCT, then it MUST include TransItem structures of type
x509_sct_v2 or precert_sct_v2 in the TransItemList.Provided that a TLS client includes the transparency_info extension type in
the ClientHello, the TLS server SHOULD include the transparency_info extension
in the ServerHello with extension_data set to a TransItemList. The TLS
server SHOULD ignore any extension_data sent by the TLS client. Additionally,
the TLS server MUST NOT process or include this extension when a TLS session is
resumed, since session resumption uses the original session information.When a TLS server includes the transparency_info extension in the ServerHello,
it SHOULD NOT include any TransItem structures of type
x509_sct_with_proof_v2, x509_sct_v2, precert_sct_with_proof_v2 or
precert_sct_v2 in the TransItemList if all of the following conditions are
met:The TLS client includes the transparency_info extension type in the
ClientHello.The TLS client includes the cached_info () extension type in the
ClientHello, with a CachedObject of type ct_compliant (see
) and at least one CachedObject of type cert.The TLS server sends a modified Certificate message (as described in section
4.1 of ).TLS servers SHOULD ignore the hash_value fields of each CachedObject of type
ct_compliant sent by TLS clients.The Transparency Information X.509v3 extension, which has OID 1.3.101.75 and
SHOULD be non-critical, contains one or more TransItem structures in a
TransItemList. This extension MAY be included in OCSP responses (see
) and certificates (see
). Since RFC5280 requires the extnValue field (an
OCTET STRING) of each X.509v3 extension to include the DER encoding of an ASN.1
value, a TransItemList MUST NOT be included directly. Instead, it MUST be
wrapped inside an additional OCTET STRING, which is then put into the
extnValue field:TransparencyInformationSyntax contains a TransItemList.A certification authority MAY include a Transparency Information X.509v3
extension in the singleExtensions of a SingleResponse in an OCSP response.
The included SCTs or inclusion proofs MUST be for the certificate identified by
the certID of that SingleResponse, or for a precertificate that corresponds
to that certificate, or for a name-constrained intermediate to which that
certificate chains.A certification authority MAY include a Transparency Information X.509v3
extension in a certificate. Any included SCTs or inclusion proofs MUST be either
for a precertificate that corresponds to this certificate, or for a
name-constrained intermediate to which this certificate chains.A certification authority MAY include the transparency_info
() TLS extension identifier in the TLS Feature
certificate extension in root, intermediate and end-entity
certificates. When a certificate chain includes such a certificate, this
indicates that CT compliance is required.There are various different functions clients of logs might perform. We describe
here some typical clients and how they should 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, ).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.In order to communicate with and verify a log, clients need metadata about the
log.
The URL to substitute for <log server> in .
The hash algorithm used for the Merkle Tree (see ).
The signing algorithm used (see ).
The public key used to verify signatures generated by the log. A log MUST NOT
use the same keypair as any other log.
The OID that uniquely identifies the log.
The MMD the log has committed to.
The version of the protocol supported by the log (currently 1 or 2).
The longest chain submission the log is willing to accept, if the log chose to
limit it.
The maximum number of STHs the log may produce in any period equal to the
Maximum Merge Delay (see ).
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.
The final STH should be provided in the form of a TransItem of type
signed_tree_head_v2. is an example of a metadata format which includes the above
elements.TLS clients receive SCTs alongside or in certificates. TLS clients MUST
implement all of the three mechanisms by which TLS servers may present SCTs (see
). TLS clients MAY also accept SCTs via the status_request_v2
extension (). TLS clients that support the transparency_info TLS
extension SHOULD include it in ClientHello messages, with empty
extension_data. TLS clients may also receive inclusion proofs in addition to
SCTs, which should be checked once the SCTs are validated.To reconstruct the TBSCertificate component of a precertificate from a
certificate, TLS clients should remove the Transparency Information extension
described in .If the SCT checked is for a Precertificate (where the type of the TransItem
is precert_sct_v2), then the client SHOULD also remove embedded v1 SCTs,
identified by OID 1.3.6.1.4.1.11129.2.4.2 (See Section 3.3. of ), in
the process of reconstructing the TBSCertificate. That is to allow embedded v1
and v2 SCTs to co-exist in a certificate (See ).In addition to normal validation of the server certificate and its chain, TLS
clients SHOULD validate each received SCT for which they have the corresponding
log's metadata. To validate an SCT, a TLS client computes the signature input
from the SCT data and the corresponding certificate, and then verifies the
signature using the corresponding log's public key. TLS clients MUST NOT
consider valid any SCT whose timestamp is in the future.Before considering any SCT to be invalid, the TLS client MUST attempt to
validate it against the server certificate and against each of the zero or more
suitable name-constrained intermediates () in the chain.
These certificates may be evaluated in the order they appear in the chain, or,
indeed, in any order.After validating a received SCT, a TLS client MAY request a corresponding
inclusion proof (if one is not already available) and then verify it. An
inclusion proof can be requested directly from a log using get-proof-by-hash
() or get-all-by-hash (), but note
that this will disclose to the log which TLS server the client has been
communicating with.Alternatively, if the TLS client has received an inclusion proof (and an STH)
alongside the SCT, it can proceed to verifying the inclusion proof to the
provided STH. The client then has to verify consistency between the provided STH
and an STH it knows about, which is less sensitive from a privacy perspective.TLS clients SHOULD also verify each received inclusion proof (see
) for which they have the corresponding log's metadata, to
audit the log and gain confidence that the certificate is logged.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 (), then verify it by
requesting a consistency proof (). Note that if the TLS
client uses get-all-by-hash, then it will already have the new STH.To be considered compliant, a certificate MUST be accompanied by at least one
valid SCT. A certificate not accompanied by any valid SCTs MUST NOT be
considered compliant by TLS clients.A TLS client MUST NOT evaluate compliance if it did not send both the
transparency_info and status_request TLS extensions in the ClientHello.If any certificate in a chain includes the transparency_info
() TLS extension identifier in the TLS Feature
certificate extension, then CT compliance (using any of the mechanisms
from ) is required.If a TLS client uses the cached_info TLS extension () to indicate 1
or more cached certificates, all of which it already considers to be CT
compliant, the TLS client MAY also include a CachedObject of type
ct_compliant in the cached_info extension. The hash_value field MUST be 1
byte long with the value 0.If a TLS server presents a certificate chain that is non-compliant, and the use
of a compliant certificate is mandated by an explicit security policy,
application protocol specification, the TLS Feature extension or any other
means, the TLS client MUST refuse the connection.Monitors watch logs to check that they behave correctly, for certificates of
interest, or both. 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:Fetch the current STH ().Verify the STH signature.Fetch all the entries in the tree corresponding to the STH
().Confirm that the tree made from the fetched entries produces the same hash as
that in the STH.Fetch the current STH (). Repeat until the STH changes.Verify the STH signature.Fetch all the new entries in the tree corresponding to the STH
(). If they remain unavailable for an extended period, then
this should be viewed as misbehavior on the part of the log.Either: 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: Fetch a consistency proof for the new STH with the previous STH
().Verify the consistency proof.Verify that the new entries generate the corresponding elements in the
consistency proof.Go to Step 5.Auditing ensures that the current published state of a log is reachable from
previously published states that are known to be good, and that the promises
made by the log in the form of SCTs have been kept. Audits are performed by
monitors or TLS clients.In particular, there are four log behaviour properties that should be checked:The Maximum Merge Delay (MMD).The STH Frequency Count.The append-only property.The consistency of the log view presented to all query sources.A benign, conformant log publishes a series of STHs over time, each derived from
the previous STH and the submitted entries incorporated into the log since
publication of the previous STH. This can be proven through auditing of STHs.
SCTs returned to TLS clients can be audited by verifying against the
accompanying certificate, and using Merkle Inclusion Proofs, against the log's
Merkle tree.The action taken by the auditor if an audit fails is not specified, but note
that in general if audit fails, the auditor is in possession of signed proof of
the log's misbehavior.A monitor () 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 () can audit by verifying an SCT against any STH
dated after the SCT timestamp + the Maximum Merge Delay by requesting a Merkle
inclusion proof (). 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 certificate or is an appropriate
name-constrained intermediate ().Checking of the consistency of the log view presented to all entities is more
difficult to perform because it requires a way to share log responses among a
set of CT-aware entities, and is discussed in .The following algorithm outlines may be useful for clients that wish to perform
various audit operations.When a client has received a TransItem of type inclusion_proof_v2 and wishes
to verify inclusion of an input hash for an STH with a given tree_size and
root_hash, the following algorithm may be used to prove the hash was
included in the root_hash:Compare leaf_index against tree_size. If leaf_index is greater than or
equal to tree_size fail the proof verification.Set fn to leaf_index and sn to tree_size - 1.Set r to hash.For each value p in the inclusion_path array:
If LSB(fn) is set, or if fn is equal to sn, then: Set r to HASH(0x01 || p || r)If LSB(fn) is not set, then right-shift both fn and sn equally
until either LSB(fn) is set or fn is 0.
Otherwise: Set r to HASH(0x01 || r || p)
Finally, right-shift both fn and sn one time.Compare sn to 0. Compare r against the root_hash. If sn is equal to
0, and r and the root_hash are equal, then the log has proven the
inclusion of hash. Otherwise, fail the proof verification.When a client has an STH first_hash for tree size first, an STH
second_hash for tree size second where 0 < first < second, and has
received a TransItem of type consistency_proof_v2 that they wish to use to
verify both hashes, the following algorithm may be used:If first is an exact power of 2, then prepend first_hash to the
consistency_path array.Set fn to first - 1 and sn to second - 1.If LSB(fn) is set, then right-shift both fn and sn equally until
LSB(fn) is not set.Set both fr and sr to the first value in the consistency_path array.For each subsequent value c in the consistency_path array:
If sn is 0, stop the iteration and fail the proof verification.
If LSB(fn) is set, or if fn is equal to sn, then: Set fr to HASH(0x01 || c || fr)
Set sr to HASH(0x01 || c || sr)If LSB(fn) is not set, then right-shift both fn and sn equally
until either LSB(fn) is set or fn is 0.
Otherwise: Set sr to HASH(0x01 || sr || c)
Finally, right-shift both fn and sn one time.After completing iterating through the consistency_path array as described
above, verify that the fr calculated is equal to the first_hash supplied,
that the sr calculated is equal to the second_hash supplied and that sn
is 0.When a client has a complete list of leaf input entries from 0 up to
tree_size - 1 and wishes to verify this list against an STH root_hash
returned by the log for the same tree_size, the following algorithm may be
used:Set stack to an empty stack.For each i from 0 up to tree_size - 1: Push HASH(0x00 || entries[i]) to stack.Set merge_count to the lowest value (0 included) such that LSB(i >>
merge_count) is not set. In other words, set merge_count to the number
of consecutive 1s found starting at the least significant bit of i.Repeat merge_count times: Pop right from stack.Pop left from stack.Push HASH(0x01 || left || right) to stack.If there is more than one element in the stack, repeat the same merge
procedure (Step 2.3 above) until only a single element remains.The remaining element in stack is the Merkle Tree hash for the given
tree_size and should be compared by equality against the supplied
root_hash.It is not possible for a log to change any of its algorithms part way through
its lifetime:
SCT signatures must remain valid so signature algorithms can only be added,
not removed.
A log would have to support the old and new hash algorithms to allow
backwards-compatibility with clients that are not aware of a hash algorithm
change.Allowing multiple signature or hash algorithms for a log would require that all
data structures support it and would significantly complicate client
implementation, which is why it is not supported by this document.If it should become necessary to deprecate an algorithm used by a live log, then
the log should be frozen as specified in and a new log should be
started. Certificates in the frozen log that have not yet expired and require
new SCTs SHOULD be submitted to the new log and the SCTs from that log used
instead.IANA is asked to allocate an RFC 5246 ExtensionType value for the
transparency_info TLS extension. IANA should update this extension type to
point at this document.IANA is asked to add an entry for ct_compliant(TBD) to the "TLS
CachedInformationType Values" registry that was defined in .IANA is asked to establish a registry of hash algorithm values, initially
consisting of:IndexHash0SHA-256 255reservedIANA is asked to establish a registry of signature algorithm values, initially
consisting of:IndexSignature Algorithm0deterministic ECDSA using the NIST P-256 curve (Section D.1.2.3 of the Digital Signature Standard ) and HMAC-SHA256.1RSA signatures (RSASSA-PKCS1-v1_5 with SHA-256, Section 8.2 of ) using a key of at least 2048 bits.IANA is asked to establish a registry of SCT extensions, initially consisting
of:TypeExtension65535reservedTBD: policy for adding to the registryIANA is asked to establish a registry of STH extensions, initially consisting
of:TypeExtension65535reservedTBD: policy for adding to the registryThis document uses object identifiers (OIDs) to identify Log IDs (see
), the precertificate CMS eContentType (see ),
and X.509v3 extensions in certificates (see and
) and OCSP responses (see
). The OIDs are defined in an arc that was selected
due to its short encoding.All OIDs in the range from 1.3.101.8192 to 1.3.101.16383 have been reserved.
This is a limited resource of 8,192 OIDs, each of which has an encoded length of
4 octets.IANA is requested to establish a registry that will allocate Log IDs from this
range.TBD: policy for adding to the registry. Perhaps "Expert Review"?The 1.3.101.80 arc has been delegated. This is an unlimited resource, but only
the 128 OIDs from 1.3.101.80.0 to 1.3.101.80.127 have an encoded length of only
4 octets.IANA is requested to establish a registry that will allocate Log IDs from this
arc.TBD: policy for adding to the registry. Perhaps "Expert Review"?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 monitors acting for the subject of
the certificate have 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 appropriate
monitors might not have checked the logs or the CA might have refused to revoke
the certificate.In addition, if TLS clients will not accept unlogged certificates, then site
owners will have a greater incentive to submit certificates to logs, possibly
with the assistance of their CA, increasing the overall transparency of the
system. provides a more detailed threat analysis of the
Certificate Transparency architecture.Misissued certificates that have not been publicly logged, and thus do not have
a valid SCT, are not considered compliant. 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.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.A log can misbehave in several ways. Examples include failing to incorporate a
certificate with an SCT in the Merkle Tree within the MMD, presenting different,
conflicting views of the Merkle Tree at different times and/or to different
parties and issuing STHs too frequently. Such misbehavior is detectable and the
provides more details on how this can be done.Violation of the MMD contract is detected by log clients requesting a Merkle
inclusion 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. Instead, clients can request the proof from a trusted auditor (since
anyone can compute the proofs from the log) or communicate with the log via
proxies.Violation of the append-only property or the STH issuance rate limit can be
detected by clients comparing their instances of the Signed Tree Heads. There
are various ways this could be done, for example via gossip (see
) or peer-to-peer communications or by sending STHs to
monitors (who could then directly check against their own copy of the relevant
log). A proof of misbehavior in such cases would be a series of STHs that were
issued too closely together, proving violation of the STH issuance rate limit,
or an STH with a root hash that does not match the one calculated from a copy of
the log, proving violation of the append-only property.Logs are required to use deterministic signatures for the following reasons:Using non-deterministic ECDSA with a predictable source of randomness means
that each signature can potentially expose the secret material of the signing
key.Clients that gossip STHs or report back SCTs can be tracked or traced if a log
was to produce multiple STHs or SCTs with the same timestamp and data but
different signatures.By offering multiple SCTs, each from a different log, TLS servers reduce the
effectiveness of an attack where a CA and a log collude (see ).The authors would like to thank Erwann Abelea, Robin Alden, Andrew Ayer, Al
Cutter, David Drysdale, Francis Dupont, Adam Eijdenberg, Stephen Farrell, Daniel
Kahn Gillmor, Paul Hadfield, Brad Hill, Jeff Hodges, Paul Hoffman, Jeffrey
Hutzelman, Kat Joyce, Stephen Kent, SM, Alexey Melnikov, Linus Nordberg, Chris
Palmer, Trevor Perrin, Pierre Phaneuf, Melinda Shore, Ryan Sleevi, Martin Smith,
Carl Wallace and Paul Wouters for their valuable contributions.A big thank you to Symantec for kindly donating the OIDs from the 1.3.101 arc
that are used in this document.Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Hypertext Transfer Protocol -- HTTP/1.1HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as "HTTP/1.1", and is an update to RFC 2068. [STANDARDS-TRACK]Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1This memo represents a republication of PKCS #1 v2.1 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document is taken directly from the PKCS #1 v2.1 document, with certain corrections made during the publication process. This memo provides information for the Internet community.The application/json Media Type for JavaScript Object Notation (JSON)JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This memo provides information for the Internet community.The Base16, Base32, and Base64 Data EncodingsThis document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK]The Transport Layer Security (TLS) Protocol Version 1.2This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK]Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) ProfileThis memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK]Cryptographic Message Syntax (CMS)This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK]Network Time Protocol Version 4: Protocol and Algorithms SpecificationThe Network Time Protocol (NTP) is widely used to synchronize computer clocks in the Internet. This document describes NTP version 4 (NTPv4), which is backwards compatible with NTP version 3 (NTPv3), described in RFC 1305, as well as previous versions of the protocol. NTPv4 includes a modified protocol header to accommodate the Internet Protocol version 6 address family. NTPv4 includes fundamental improvements in the mitigation and discipline algorithms that extend the potential accuracy to the tens of microseconds with modern workstations and fast LANs. It includes a dynamic server discovery scheme, so that in many cases, specific server configuration is not required. It corrects certain errors in the NTPv3 design and implementation and includes an optional extension mechanism. [STANDARDS-TRACK]Transport Layer Security (TLS) Extensions: Extension DefinitionsThis document provides specifications for existing TLS extensions. It is a companion document for RFC 5246, "The Transport Layer Security (TLS) Protocol Version 1.2". The extensions specified are server_name, max_fragment_length, client_certificate_url, trusted_ca_keys, truncated_hmac, and status_request. [STANDARDS-TRACK]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)Many application technologies enable secure communication between two entities by means of Internet Public Key Infrastructure Using X.509 (PKIX) certificates in the context of Transport Layer Security (TLS). This document specifies procedures for representing and verifying the identity of application services in such interactions. [STANDARDS-TRACK]X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSPThis document specifies a protocol useful in determining the current status of a digital certificate without requiring Certificate Revocation Lists (CRLs). Additional mechanisms addressing PKIX operational requirements are specified in separate documents. This document obsoletes RFCs 2560 and 6277. It also updates RFC 5912.The Transport Layer Security (TLS) Multiple Certificate Status Request ExtensionThis document defines the Transport Layer Security (TLS) Certificate Status Version 2 Extension to allow clients to specify and support several certificate status methods. (The use of the Certificate Status extension is commonly referred to as "OCSP stapling".) Also defined is a new method based on the Online Certificate Status Protocol (OCSP) that servers can use to provide status information about not only the server's own certificate but also the status of intermediate certificates in the chain.Deterministic Usage of the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA)This document defines a deterministic digital signature generation procedure. Such signatures are compatible with standard Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA) digital signatures and can be processed with unmodified verifiers, which need not be aware of the procedure described therein. Deterministic signatures retain the cryptographic security features associated with digital signatures but can be more easily implemented in various environments, since they do not need access to a source of high-quality randomness.X.509v3 Transport Layer Security (TLS) Feature ExtensionThe purpose of the TLS feature extension is to prevent downgrade attacks that are not otherwise prevented by the TLS protocol. In particular, the TLS feature extension may be used to mandate support for revocation checking features in the TLS protocol such as Online Certificate Status Protocol (OCSP) stapling. Informing clients that an OCSP status response will always be stapled permits an immediate failure in the case that the response is not stapled. This in turn prevents a denial-of-service attack that might otherwise be possible.Transport Layer Security (TLS) Cached Information ExtensionTransport Layer Security (TLS) handshakes often include fairly static information, such as the server certificate and a list of trusted certification authorities (CAs). This information can be of considerable size, particularly if the server certificate is bundled with a complete certificate chain (i.e., the certificates of intermediate CAs up to the root CA).This document defines an extension that allows a TLS client to inform a server of cached information, thereby enabling the server to omit already available information.Digital Signature Standard (DSS)National Institute of Standards and TechnologySecure Hash StandardNational Institute of Standards and TechnologyHTML 4.01 SpecificationCertificate TransparencyThis document describes an experimental protocol for publicly logging the existence of Transport Layer Security (TLS) certificates as they are issued or observed, in a manner that allows anyone to audit certificate authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs.Logs are network services that implement the protocol operations for submissions and queries that are defined in this document.Gossiping in CTThe logs in Certificate Transparency are untrusted in the sense that the users of the system don't have to trust that they behave correctly since the behavior of a log can be verified to be correct. This document tries to solve the problem with logs presenting a "split view" of their operations. It describes three gossiping mechanisms for Certificate Transparency: SCT Feedback, STH Pollination and Trusted Auditor Relationship.Efficient Data Structures for Tamper-Evident LoggingGuidelines For The Issuance And Management Of Extended Validation CertificatesCA/Browser ForumChromium Certificate TransparencyThe Chromium ProjectsChromium Log Metadata JSON SchemaThe Chromium ProjectsChromium Certificate Transparency Log PolicyThe Chromium ProjectsAttack and Threat Model for Certificate TransparencyThis document describes an attack model and discusses threats for the Web PKI context in which security mechanisms to detect mis-issuance of web site certificates are being developed. The model provides an analysis of detection and remediation mechanisms for both syntactic and semantic mis-issuance. The model introduces an outline of attacks to organize the discussion. The model also describes the roles played by the elements of the Certificate Transparency (CT) system, to establish a context for the model.Certificate Transparency logs have to be either v1 (conforming to ) or
v2 (conforming to this document), as the data structures are incompatible and so
a v2 log could not issue a valid v1 SCT.CT clients, however, can support v1 and v2 SCTs, for the same certificate,
simultaneously, as v1 SCTs are delivered in different TLS, X.509 and OCSP
extensions than v2 SCTs.v1 and v2 SCTs for X.509 certificates can be validated independently. For
precertificates, v2 SCTs should be embedded in the TBSCertificate before
submission of the TBSCertificate (inside a v1 precertificate, as described in
Section 3.1. of ) to a v1 log so that TLS clients conforming to
but not this document are oblivious to the embedded v2 SCTs. An issuer
can follow these steps to produce an X.509 certificate with embedded v1 and v2
SCTs:Create a CMS precertificate as described in and submit it
to v2 logs.Embed the obtained v2 SCTs in the TBSCertificate, as described in
.Use that TBSCertificate to create a v1 precertificate, as described in
Section 3.1. of and submit it to v1 logs.Embed the v1 SCTs in the TBSCertificate, as described in Section 3.3. of
.Sign that TBSCertificate (which now contains v1 and v2 SCTs) to issue the
final X.509 certificate.