| TRANS (Public Notary Transparency) | B. Laurie |
| Internet-Draft | A. Langley |
| Obsoletes: 6962 (if approved) | E. Kasper |
| Intended status: Standards Track | E. Messeri |
| Expires: May 3, 2018 | |
| R. Stradling | |
| Comodo | |
| October 30, 2017 |
Certificate Transparency Version 2.0
draft-ietf-trans-rfc6962-bis-27
This document describes version 2.0 of the Certificate Transparency (CT) 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.
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 https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 3, 2018.
Copyright (c) 2017 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 (https://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.
Certificate Transparency aims to mitigate the problem of misissued certificates by providing append-only logs of issued certificates. The logs do not themselves prevent misissuance, 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 efficiently prove that any particular instance of the log is a superset of any particular previous instance and to efficiently detect various misbehaviors of the log (e.g., issuing a signed timestamp for a certificate that is not subsequently logged).
It is necessary to treat each log as a trusted third party, because the log auditing mechanisms described in this document can be circumvented by a misbehaving log that shows different, inconsistent views of itself to different clients. Whilst it is anticipated that additional mechanisms could be developed to address these shortcomings and thereby avoid the need to blindly trust logs, such mechanisms are outside the scope of this document.
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 [RFC2119].
Data structures are defined and encoded according to the conventions laid out in Section 3 of [I-D.ietf-tls-tls13].
This document revises and obsoletes the experimental CT 1.0 [RFC6962] protocol, drawing on insights gained from CT 1.0 deployments and on feedback from the community. The major changes are:
The log uses a binary Merkle Hash Tree for efficient auditing. The hash algorithm used is one of the log's parameters (see Section 4.1). We have established a registry of acceptable hash algorithms (see Section 10.3). Throughout this document, the hash algorithm in use is referred to as HASH 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:
MTH({}) = HASH().
The hash of a list with one entry (also known as a leaf hash) is:
MTH({d[0]}) = HASH(0x00 || d[0]).
For n > 1, let k be the largest power of two smaller than n (i.e., k < n <= 2k). The Merkle Tree Hash of an n-element list D_n is then defined recursively as
MTH(D_n) = HASH(0x01 || MTH(D[0:k]) || MTH(D[k:n])),
Where || is concatenation and D[k1:k2] = D'_(k2-k1) denotes the list {d'[0] = d[k1], d'[1] = d[k1+1], …, d'[k2-k1-1] = d[k2-1]} of length (k2 - k1). (Note that the hash calculations for leaves and nodes differ; this domain separation is required to give second preimage resistance).
Note that we do not require the length of the input list to be a power of two. The resulting Merkle Tree may thus not be balanced; however, its shape is uniquely determined by the number of leaves. (Note: This Merkle Tree is essentially the same as the history tree [CrosbyWallach] proposal, except our definition handles non-full trees differently).
When a client has a complete list of n input entries from 0 up to tree_size - 1 and wishes to verify this list against a tree head root_hash returned by the log for the same tree_size, the following algorithm may be used:
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[1], …, d[n-1]}, the Merkle inclusion proof PATH(m, D_n) for the (m+1)th input d[m], 0 <= m < n, is defined as follows:
The proof for the single leaf in a tree with a one-element input list D[1] = {d[0]} is empty:
PATH(0, {d[0]}) = {}
For n > 1, let k be the largest power of two smaller than n. The proof for the (m+1)th element d[m] in a list of n > m elements is then defined recursively as
PATH(m, D_n) = PATH(m, D[0:k]) : MTH(D[k:n]) for m < k; and PATH(m, D_n) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for m >= k,
The : operator and D[k1:k2] are defined the same as in Section 2.1.1.
When a client has received an inclusion proof (e.g., in a TransItem of type inclusion_proof_v2) and wishes to verify inclusion of an input hash for a given tree_size and root_hash, the following algorithm may be used to prove the hash was included in the root_hash:
Otherwise:
Finally, right-shift both
fn and sn one time.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[1], …, d[n-1]}, the Merkle consistency proof PROOF(m, D_n) for a previous Merkle Tree Hash MTH(D[0:m]), 0 < m < n, is defined as:
PROOF(m, D_n) = SUBPROOF(m, D_n, true)
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):
SUBPROOF(m, D[m], true) = {}
Otherwise, the subproof for m = n is the Merkle Tree Hash committing inputs D[0:m]:
SUBPROOF(m, D[m], false) = {MTH(D[m])}
For m < n, let k be the largest power of two smaller than n. The subproof is then defined recursively.
If m <= k, the right subtree entries D[k:n] only exist in the current tree. We prove that the left subtree entries D[0:k] are consistent and add a commitment to D[k:n]:
SUBPROOF(m, D_n, b) = SUBPROOF(m, D[0:k], b) : MTH(D[k:n])
If m > k, the left subtree entries D[0:k] are identical in both trees. We prove that the right subtree entries D[k:n] are consistent and add a commitment to D[0:k].
SUBPROOF(m, D_n, b) = SUBPROOF(m - k, D[k:n], false) : MTH(D[0:k])
The number of nodes in the resulting proof is bounded above by ceil(log2(n)) + 1.
The : operator and D[k1:k2] are defined the same as in Section 2.1.1.
When a client has a tree head first_hash for tree size first, a tree head second_hash for tree size second where 0 < first < second, and has received a consistency proof between the two (e.g., in a TransItem of type consistency_proof_v2), the following algorithm may be used to verify the consistency proof:
Otherwise:
Finally, right-shift both
fn and sn one time.The binary Merkle Tree with 7 leaves:
hash
/ \
/ \
/ \
/ \
/ \
k l
/ \ / \
/ \ / \
/ \ / \
g h i j
/ \ / \ / \ |
a b c d e f d6
| | | | | |
d0 d1 d2 d3 d4 d5
The inclusion proof for d0 is [b, h, l].
The inclusion proof for d3 is [c, g, l].
The inclusion proof for d4 is [f, j, k].
The inclusion proof for d6 is [i, k].
The same tree, built incrementally in four steps:
hash0 hash1=k
/ \ / \
/ \ / \
/ \ / \
g c g h
/ \ | / \ / \
a b d2 a b c d
| | | | | |
d0 d1 d0 d1 d2 d3
hash2 hash
/ \ / \
/ \ / \
/ \ / \
/ \ / \
/ \ / \
k i k l
/ \ / \ / \ / \
/ \ e f / \ / \
/ \ | | / \ / \
g h d4 d5 g h i j
/ \ / \ / \ / \ / \ |
a b c d a b c d e f d6
| | | | | | | | | |
d0 d1 d2 d3 d0 d1 d2 d3 d4 d5
The consistency proof between hash0 and hash is PROOF(3, D[7]) = [c, d, g, l]. c, g are used to verify hash0, and d, l are additionally used to show hash is consistent with hash0.
The consistency proof between hash1 and hash is PROOF(4, D[7]) = [l]. hash can be verified using hash1=k and l.
The consistency proof between hash2 and hash is PROOF(6, D[7]) = [i, j, k]. k, i are used to verify hash2, and j is additionally used to show hash is consistent with hash2.
Various data structures Section 1.2 are signed. A log MUST use one of the signature algorithms defined in Section 10.4.
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 (Section 5.7). 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 Section 4.8). The submitter SHOULD validate the returned SCT as described in Section 8.1 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 (Section 5.1) 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 (Section 5.1) 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 [RFC5652] signed-data object that conforms to the following profile:
SignerInfo.signedAttrs is included in the message digest calculation process (see Section 5.4 of [RFC5652]), which ensures that the SignerInfo.signature value will not be a valid X.509v3 signature that could be used in conjunction with the TBSCertificate (from SignedData.encapContentInfo.eContent) to construct a valid certificate.
A log is a single, append-only Merkle Tree of submitted certificate and precertificate entries.
When it receives and accepts 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 Section 11.4). 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 append to its Merkle Tree an entry for the accepted submission. Upon producing an SCT, the log MUST fulfil this promise by performing the following actions within a fixed amount of time known as the Maximum Merge Delay (MMD), which is one of the log's parameters (see Section 4.1):
The log may append multiple entries before signing the root of the tree.
Log operators SHOULD NOT impose any conditions on retrieving or sharing data from the log.
A log is defined by a collection of parameters, which are used by clients to communicate with the log and to verify log artifacts.
[JSON.Metadata] is an example of a metadata format which includes the above elements.
To avoid being overloaded by invalid submissions, the log MUST NOT accept any submission until it has verified that the certificate or precertificate was submitted with a valid signature chain to an accepted trust anchor. The log MUST NOT use other sources of intermediate CA certificates to attempt certification path construction; instead, it MUST only use the intermediate CA certificates provided in the submission, in the order provided.
Logs SHOULD accept certificates and precertificates that are fully valid according to RFC 5280 [RFC5280] verification rules and are submitted with such a chain. (A log may decide, for example, to temporarily reject valid submissions to protect itself against denial-of-service attacks).
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 Section 3.2.
Logs SHOULD limit the length of chain they will accept. The maximum chain length is one of the log's parameters (see Section 4.1).
The log SHALL allow retrieval of its list of accepted trust anchors (see Section 5.7), 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 Section 5.6). This prevents the CA from avoiding blame by logging a partial or empty chain. Each log entry is a TransItem structure of type x509_entry_v2 or precert_entry_v2. However, a log may store its entries in any format. If a log does not store this TransItem in full, it must store the timestamp and sct_extensions of the corresponding TimestampedCertificateEntryDataV2 structure. The TransItem can be reconstructed from these fields and the entire chain that the log used to verify the submission.
Each log is identified by an OID, which is one of the log's parameters (see Section 4.1) 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 the Log ID Registry (see Section 10.7.1). Various data structures include the DER encoding of this OID, excluding the ASN.1 tag and length bytes, in an opaque vector:
opaque LogID<2..127>;
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:
enum {
reserved(0),
x509_entry_v2(1), precert_entry_v2(2),
x509_sct_v2(3), precert_sct_v2(4),
signed_tree_head_v2(5), consistency_proof_v2(6),
inclusion_proof_v2(7),
(65535)
} VersionedTransType;
struct {
VersionedTransType versioned_type;
select (versioned_type) {
case x509_entry_v2: TimestampedCertificateEntryDataV2;
case precert_entry_v2: TimestampedCertificateEntryDataV2;
case x509_sct_v2: SignedCertificateTimestampDataV2;
case precert_sct_v2: SignedCertificateTimestampDataV2;
case signed_tree_head_v2: SignedTreeHeadDataV2;
case consistency_proof_v2: ConsistencyProofDataV2;
case inclusion_proof_v2: InclusionProofDataV2;
} data;
} TransItem;
versioned_type is a value from the IANA registry in Section 10.5 that identifies 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 [RFC6962] type enumerations Version, LogEntryType, SignatureType and MerkleLeafType. Note also that v1 did not define TransItem, but this document provides guidelines (see Appendix A) 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.
enum {
reserved(65535)
} ExtensionType;
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
The Extension structure provides a generic extensibility for log artifacts, including Signed Certificate Timestamps (Section 4.8) and Signed Tree Heads (Section 4.10). The interpretation of the extension_data field is determined solely by the value of the extension_type field.
This document does not define any extensions, but it does establish a registry for future ExtensionType values (see Section 10.6). Each document that registers a new ExtensionType must specify the context in which it may be used (e.g., SCT, STH, or both) and describe how to interpret the corresponding extension_data.
The leaves of a log's Merkle Tree correspond to the log's entries (see Section 4.3). Each leaf is the leaf hash (Section 2.1) 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 hash algorithm is one of the log's parameters.
opaque TBSCertificate<1..2^24-1>;
struct {
uint64 timestamp;
opaque issuer_key_hash<32..2^8-1>;
TBSCertificate tbs_certificate;
Extension sct_extensions<0..2^16-1>;
} TimestampedCertificateEntryDataV2;
timestamp is the NTP Time [RFC5905] 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 [RFC5280]. 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 the submission. (Note that a precertificate's TBSCertificate can be reconstructed from the corresponding certificate as described in Section 8.1.2).
sct_extensions matches the SCT extensions of the corresponding SCT.
The type of the TransItem corresponds to the value of the type parameter supplied in the Section 5.1 call.
An SCT is a TransItem structure of type x509_sct_v2 or precert_sct_v2, which encapsulates a SignedCertificateTimestampDataV2 structure:
struct {
LogID log_id;
uint64 timestamp;
Extension sct_extensions<0..2^16-1>;
opaque signature<0..2^16-1>;
} SignedCertificateTimestampDataV2;
log_id is this log's unique ID, encoded in an opaque vector as described in Section 4.4.
timestamp is equal to the timestamp from the corresponding TimestampedCertificateEntryDataV2 structure.
sct_extensions is a vector of 0 or more SCT extensions. This vector MUST NOT include more than one extension with the same extension_type. The extensions in the vector MUST be ordered by the value of the 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.
signature is computed over a TransItem structure of type x509_entry_v2 or precert_entry_v2 (see Section 4.7) using the signature algorithm declared in the log's parameters (see Section 4.1).
The log stores information about its Merkle Tree in a TreeHeadDataV2:
opaque NodeHash<32..2^8-1>;
struct {
uint64 timestamp;
uint64 tree_size;
NodeHash root_hash;
Extension sth_extensions<0..2^16-1>;
} TreeHeadDataV2;
The length of NodeHash MUST match HASH_SIZE of the log.
timestamp is the current NTP Time [RFC5905], 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 extension_type. The extensions in the vector MUST be ordered by the value of the 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 Section 4.9) to produce an STH. When a client requests a log's latest STH (see Section 5.2), the log MUST return an STH that is no older than the log's MMD. However, since STHs could be used to mark individual clients (by producing a new STH for each query), a log MUST NOT produce STHs more frequently than its parameters declare (see Section 4.1). In general, there is no need to produce a new STH unless there are new entries in the log; however, in the event that a log does not accept any submissions during an MMD period, the log MUST 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:
struct {
LogID log_id;
TreeHeadDataV2 tree_head;
opaque signature<0..2^16-1>;
} SignedTreeHeadDataV2;
log_id is this log's unique ID, encoded in an opaque vector as described in Section 4.4.
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 Section 4.9).
signature is computed over the tree_head field using the signature algorithm declared in the log's parameters (see Section 4.1).
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:
struct {
LogID log_id;
uint64 tree_size_1;
uint64 tree_size_2;
NodeHash consistency_path<1..2^16-1>;
} ConsistencyProofDataV2;
log_id is this log's unique ID, encoded in an opaque vector as described in Section 4.4.
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:
struct {
LogID log_id;
uint64 tree_size;
uint64 leaf_index;
NodeHash inclusion_path<1..2^16-1>;
} InclusionProofDataV2;
log_id is this log's unique ID, encoded in an opaque vector as described in Section 4.4.
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:
Messages are sent as HTTPS GET or POST requests. Parameters for POSTs and all responses are encoded as JavaScript Object Notation (JSON) objects [RFC7159]. Parameters for GETs are encoded as order-independent key/value URL parameters, using the "application/x-www-form-urlencoded" format described in the "HTML 4.01 Specification" [HTML401]. Binary data is base64 encoded [RFC4648] as specified in the individual messages.
Clients are configured with a base URL for a log and construct URLs for requests by appending suffixes to this base URL. This structure places some degree of restriction on how log operators can deploy these services, as noted in [RFC7320]. However, operational experience with version 1 of this protocol has not indicated that these restrictions are a problem in practice.
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 one of the log's parameters, 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 [RFC7231]), and, in place of the responses outlined in the subsections below, the body SHOULD be a JSON structure containing at least the following field:
| Error Code | Meaning |
|---|---|
| not compliant | The 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:
{
"error_message": "'start' cannot be greater than 'end'",
"error_code": "not compliant",
}
Clients SHOULD treat 500 Internal Server Error and 503 Service Unavailable responses as transient failures and MAY retry the same request without modification at a later date. Note that as per [RFC7231], 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/submit-entry
If the submitted entry is immediately appended to (or already exists in) this log's tree, then the log SHOULD also output:
Error codes:
| Error Code | Meaning |
|---|---|
| bad submission | submission is neither a valid certificate nor a valid precertificate. |
| bad type | type is neither 1 nor 2. |
| bad chain | The first element of chain is not the certifier of the submission, or the second element does not certify the first, etc. |
| bad certificate | One or more certificates in the chain are not valid (e.g., not properly encoded). |
| unknown anchor | The last element of chain (or, if chain is an empty array, the submission) both is not, and is not certified by, an accepted trust anchor. |
| shutdown | The log is no longer accepting 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 (Section 8.2) can then detect these encoding errors, which may be accepted by some TLS clients.
If submission is an accepted trust anchor whose certifier is neither an accepted trust anchor nor the first element of chain, then the log MUST return the "unknown anchor" error. A log cannot generate an SCT for a submission if it does not have access to the issuer's public key.
If the returned sct is intended to be provided to TLS clients, then sth and inclusion (if returned) SHOULD also be provided to TLS clients (e.g., if type was 2 (for precert_sct_v2) then all three TransItems could be embedded in the certificate).
GET https://<log server>/ct/v2/get-sth
No inputs.
GET https://<log server>/ct/v2/get-sth-consistency
Error codes:
| Error Code | Meaning |
|---|---|
| first unknown | first is before the latest known STH but is not from an existing STH. |
| second unknown | second is before the latest known STH but is not from an existing STH. |
See Section 2.1.4.2 for an outline of how to use the consistency output.
GET https://<log server>/ct/v2/get-proof-by-hash
Error codes:
| Error Code | Meaning |
|---|---|
| hash unknown | hash is not the hash of a known leaf (may be caused by skew or by a known certificate not yet merged). |
| tree_size unknown | hash is before the latest known STH but is not from an existing STH. |
See Section 2.1.3.2 for an outline of how to use the inclusion output.
GET https://<log server>/ct/v2/get-all-by-hash
Because of skew, the front-end may not know the requested STH or the requested hash, which leads to a number of cases:
| Case | Response |
|---|---|
| latest STH < requested STH | Return latest STH |
| latest STH > requested STH | Return latest STH and a consistency proof between it and the requested STH (see Section 5.3) |
| index of requested hash < latest STH | Return inclusion |
Note that more than one case can be true, in which case the returned data is their union. It is also possible for none to be true, in which case the front-end MUST return an empty response.
Errors are the same as in Section 5.4.
See Section 2.1.3.2 for an outline of how to use the inclusion output, and see Section 2.1.4.2 for an outline of how to use the consistency output.
GET https://<log server>/ct/v2/get-entries
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 Section 5.2.
The start parameter MUST be less than or equal to the end parameter.
Each submitted_entry output parameter MUST include the trust anchor that the log used to verify the submission, even if that trust anchor was not provided to submit-entry (see Section 5.1). If the submission does not certify itself, then the first element of chain MUST be present and MUST certify the submission.
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 Section 2.1.2 for an outline of how to use a complete list of log_entry entries to verify the root_hash.
GET https://<log server>/ct/v2/get-anchors
No inputs.
CT-using 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. They SHOULD also present corresponding inclusion proofs and STHs.
Three mechanisms are provided because they have different tradeoffs.
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 ([RFC6961]) in the (extended) ClientHello, but only in addition to at least one of the three mechanisms listed above.
CT-using TLS servers SHOULD send SCTs from multiple logs, because:
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:
opaque SerializedTransItem<1..2^16-1>;
struct {
SerializedTransItem trans_item_list<1..2^16-1>;
} TransItemList;
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).
In each TransItemList that is sent to a client during a TLS handshake, the TLS server MUST include a TransItem structure of type x509_sct_v2 or precert_sct_v2 (except as described in Section 6.5).
Presenting inclusion proofs and STHs in the TLS handshake helps to protect the client's privacy (see Section 8.1.4) and reduces load on log servers. Therefore, if the TLS server can obtain them, it SHOULD also include TransItems of type inclusion_proof_v2 and signed_tree_head_v2 in the TransItemList.
Provided that a TLS client includes the transparency_info extension type in the ClientHello and the TLS server supports the transparency_info extension:
TLS servers 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_v2 or precert_sct_v2 in the TransItemList if all of the following conditions are met:
If the hash_value of any CachedObject of type ct_compliant sent by a TLS client is not 1 byte long with the value 0, the CT-using TLS server MUST abort the handshake.
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 Section 7.1.1) and certificates (see Section 7.1.2). 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 ::= OCTET STRING
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. All included SCTs and inclusion proofs MUST be for the certificate identified by the certID of that SingleResponse, or for a precertificate that corresponds to that certificate.
A certification authority MAY include a Transparency Information X.509v3 extension in a certificate. All included SCTs and inclusion proofs MUST be for a precertificate that corresponds to this certificate.
A certification authority SHOULD NOT issue any certificate that identifies the transparency_info TLS extension in a TLS feature extension [RFC7633], because TLS servers are not required to support the transparency_info TLS extension in order to participate in CT (see Section 6).
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 parameters in order to communicate with logs and verify their responses. These parameters are described in Section 4.1, but note that this document does not describe how the parameters are obtained, which is implementation-dependent (see, for example, [Chromium.Policy]).
TLS clients receive SCTs and inclusion proofs alongside or in certificates. CT-using TLS clients MUST implement all of the three mechanisms by which TLS servers may present SCTs (see Section 6) and MAY also accept SCTs via the status_request_v2 extension ([RFC6961]).
TLS clients that support the transparency_info TLS extension SHOULD include it in ClientHello messages, with empty extension_data. If a TLS server includes the transparency_info TLS extension when resuming a TLS session, the TLS client MUST abort the handshake.
Validation of an SCT for a certificate (where the type of the TransItem is x509_sct_v2) uses the unmodified TBSCertificate component of the certificate.
Before an SCT for a precertificate (where the type of the TransItem is precert_sct_v2) can be validated, the TBSCertificate component of the precertificate needs to be reconstructed from the TBSCertificate component of the certificate as follows:
In addition to normal validation of the server certificate and its chain, CT-using TLS clients MUST validate each received SCT for which they have the corresponding log's parameters. To validate an SCT, a TLS client computes the signature input by constructing a TransItem of type x509_entry_v2 or precert_entry_v2, depending on the SCT's TransItem type. The TimestampedCertificateEntryDataV2 structure is constructed in the following manner:
The SCT's signature is then verified using the public key of the corresponding log, which is identified by the log_id. The required signature algorithm is one of the log's parameters.
When a TLS client has validated a received SCT but does not yet possess a corresponding inclusion proof, the TLS client MAY request the inclusion proof directly from a log using get-proof-by-hash (Section 5.4) or get-all-by-hash (Section 5.5).
Note that fetching inclusion proofs directly from a log will disclose to the log which TLS server the client has been communicating with. This may be regarded as a significant privacy concern, and so it is preferable for the TLS server to send the inclusion proofs (see Section 6.3).
When a TLS client has received, or fetched, an inclusion proof (and an STH), it SHOULD proceed to verifying the inclusion proof to the provided STH. The TLS client SHOULD also verify consistency between the provided STH and an STH it knows about.
If the TLS client holds an STH that predates the SCT, it MAY, in the process of auditing, request a new STH from the log (Section 5.2), then verify it by requesting a consistency proof (Section 5.3). Note that if the TLS client uses get-all-by-hash, then it will already have the new STH.
It is up to a client's local policy to specify the quantity and form of evidence (SCTs, inclusion proofs or a combination) needed to achieve compliance and how to handle non-compliance.
A TLS client can only evaluate compliance if it has given the TLS server the opportunity to send SCTs and inclusion proofs by any of the three mechanisms that are mandatory to implement for CT-using TLS clients (see Section 8.1.1). Therefore, a TLS client MUST NOT evaluate compliance if it did not include both the transparency_info and status_request TLS extensions in the ClientHello.
If a TLS client uses the cached_info TLS extension ([RFC7924]) 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. Its hash_value field MUST have the value 0 and be 1 byte long (the minimum length permitted by [RFC7924]).
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 MUST at least inspect every new entry in every log it watches, and it MAY also choose to keep copies of entire logs.
To inspect all of the existing entries, the monitor SHOULD follow these steps once for each log:
To inspect new entries, the monitor SHOULD follow these steps repeatedly for each log:
Or, if it is not keeping all log entries:
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 behavior properties that should be checked:
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 (Section 8.2) can audit by verifying the consistency of STHs it receives, ensure that each entry can be fetched and that the STH is indeed the result of making a tree from all fetched entries.
A TLS client (Section 8.1) can audit by verifying an SCT against any STH dated after the SCT timestamp + the Maximum Merge Delay by requesting a Merkle inclusion proof (Section 5.4). It can also verify that the SCT corresponds to the server certificate it arrived with (i.e., the log entry is that certificate, or is a precertificate corresponding to that certificate).
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-using entities, and is discussed in Section 11.3.
It is not possible for a log to change any of its algorithms part way through its lifetime:
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 MUST be frozen as specified in Section 4.13 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.
The assignment policy criteria mentioned in this section refer to the policies outlined in [RFC5226].
IANA is asked to add an entry for transparency_info(TBD) to the "TLS ExtensionType Values" registry defined in [I-D.ietf-tls-tls13], citing this document as the "Reference" and setting the "Recommended" value to "Yes".
IANA is asked to add an entry for ct_compliant(TBD) to the "TLS CachedInformationType Values" registry defined in [RFC7924], citing this document as the "Reference".
IANA is asked to establish a registry of hash algorithm values, named "CT Hash Algorithms", that initially consists of:
| Value | Hash Algorithm | OID | Reference / Assignment Policy |
|---|---|---|---|
| 0x00 | SHA-256 | 2.16.840.1.101.3.4.2.1 | [RFC6234] |
| 0x01 - 0xDF | Unassigned | Specification Required and Expert Review | |
| 0xE0 - 0xEF | Reserved | Experimental Use | |
| 0xF0 - 0xFF | Reserved | Private Use |
The appointed Expert should ensure that the proposed algorithm has a public specification and is suitable for use as a cryptographic hash algorithm with no known preimage or collision attacks. These attacks can damage the integrity of the log.
IANA is asked to establish a registry of signature algorithm values, named "CT Signature Algorithms", that initially consists of:
| SignatureScheme Value | Signature Algorithm | Reference / Assignment Policy |
|---|---|---|
| ecdsa_secp256r1_sha256(0x0403) | ECDSA (NIST P-256) with SHA-256 | [FIPS186-4] |
| ecdsa_secp256r1_sha256(0x0403) | Deterministic ECDSA (NIST P-256) with HMAC-SHA256 | [RFC6979] |
| ed25519(0x0807) | Ed25519 (PureEdDSA with the edwards25519 curve) | [RFC8032] |
| private_use(0xFE00..0xFFFF) | Reserved | Private Use |
The appointed Expert should ensure that the proposed algorithm has a public specification, has a value assigned to it in the TLS SignatureScheme Registry (that IANA is asked to establish in [I-D.ietf-tls-tls13]) and is suitable for use as a cryptographic signature algorithm.
IANA is asked to establish a registry of VersionedTransType values, named "CT VersionedTransTypes", that initially consists of:
| Value | Type and Version | Reference / Assignment Policy |
|---|---|---|
| 0x0000 | Reserved | [RFC6962] (*) |
| 0x0001 | x509_entry_v2 | RFCXXXX |
| 0x0002 | precert_entry_v2 | RFCXXXX |
| 0x0003 | x509_sct_v2 | RFCXXXX |
| 0x0004 | precert_sct_v2 | RFCXXXX |
| 0x0005 | signed_tree_head_v2 | RFCXXXX |
| 0x0006 | consistency_proof_v2 | RFCXXXX |
| 0x0007 | inclusion_proof_v2 | RFCXXXX |
| 0x0008 - 0xDFFF | Unassigned | Specification Required and Expert Review |
| 0xE000 - 0xEFFF | Reserved | Experimental Use |
| 0xF000 - 0xFFFF | Reserved | Private Use |
(*) The 0x0000 value is reserved so that v1 SCTs are distinguishable from v2 SCTs and other TransItem structures.
[RFC Editor: please update 'RFCXXXX' to refer to this document, once its RFC number is known.]
The appointed Expert should review the public specification to ensure that it is detailed enough to ensure implementation interoperability.
IANA is asked to establish a registry of ExtensionType values, named "CT Log Artifact Extensions", that initially consists of:
| ExtensionType | Status | Use | Reference / Assignment Policy |
|---|---|---|---|
| 0x0000 - 0xDFFF | Unassigned | n/a | Specification Required and Expert Review |
| 0xE000 - 0xEFFF | Reserved | n/a | Experimental Use |
| 0xF000 - 0xFFFF | Reserved | n/a | Private Use |
The "Use" column should contain one or both of the following values:
The appointed Expert should review the public specification to ensure that it is detailed enough to ensure implementation interoperability. The Expert should also verify that the extension is appropriate to the contexts in which it is specified to be used (SCT, STH, or both).
This document uses object identifiers (OIDs) to identify Log IDs (see Section 4.4), the precertificate CMS eContentType (see Section 3.2), and X.509v3 extensions in certificates (see Section 7.1.2) and OCSP responses (see Section 7.1.1). The OIDs are defined in an arc that was selected due to its short encoding.
IANA is asked to establish a registry of Log IDs, named "CT Log ID Registry", that initially consists of:
| Value | Log | Reference / Assignment Policy |
|---|---|---|
| 1.3.101.8192 - 1.3.101.16383 | Unassigned | Parameters Required and First Come First Served |
| 1.3.101.80.0 - 1.3.101.80.* | Unassigned | Parameters Required and First Come First Served |
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.
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.
Each application for the allocation of a Log ID should be accompanied by all of the required parameters (except for the Log ID) listed in Section 4.1.
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 misissuance and take some action, such as asking a CA to revoke a misissued certificate. A signed timestamp does not guarantee this though, 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.
[I-D.ietf-trans-threat-analysis] 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. Since a log is allowed to serve an STH of any age up to the MMD, the maximum period of time during which a misissued certificate can be used without being available for audit is twice 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; issuing STHs too frequently; mutating the signature of a logged certificate; and failing to present a chain containing the certifier of a logged certificate. Such misbehavior is detectable and [I-D.ietf-trans-threat-analysis] provides more details on how this can be done.
Violation of the MMD contract is detected by log clients requesting a Merkle inclusion proof (Section 5.4) for each observed SCT. These checks can be asynchronous and need only be done once per certificate. However, note that there may be privacy concerns (see Section 8.1.4).
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 [I-D.ietf-trans-gossip]) or peer-to-peer communications or by sending STHs to monitors (who could then directly check against their own copy of the relevant log). 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.
Clients that gossip STHs or report back SCTs can be tracked or traced if a log produces multiple STHs or SCTs with the same timestamp and data but different signatures. Logs SHOULD mitigate this risk by either:
By requiring TLS servers to offer multiple SCTs, each from a different log, TLS clients reduce the effectiveness of an attack where a CA and a log collude (see Section 6.1).
The authors would like to thank Erwann Abelea, Robin Alden, Andrew Ayer, Richard Barnes, 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, Eric Rescorla, 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.
Certificate Transparency logs have to be either v1 (conforming to [RFC6962]) 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 [RFC6962]) to a v1 log so that TLS clients conforming to [RFC6962] 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: