1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

1.2. Data Structures

Data structures are defined and encoded according to the conventions laid out in Section 3 of [RFC8446].¶

This document uses object identifiers (OIDs) to identify Log IDs (see Section 4.4), the precertificate Cryptographic Message Syntax (CMS) eContentType (see Section 3.2), X.509v3 extensions in certificates (see Section 7.1.2), and Online Certificate Status Protocol (OCSP) responses (see Section 7.1.1). The OIDs are defined in an arc that was selected due to its short encoding.¶

1.3. Major Differences from CT 1.0

This document revises and obsoletes the CT 1.0 protocol [RFC6962], drawing on insights gained from CT 1.0 deployments and on feedback from the community. The major changes are:¶

• Hash and signature algorithm agility: Permitted algorithms are now specified in IANA registries.¶
• Precertificate format: Precertificates are now CMS objects rather than X.509 certificates, which avoids violating the certificate serial number uniqueness requirement in Section 4.1.2.2 of [RFC5280].¶
• Removal of precertificate signing certificates and the precertificate poison extension: The change of precertificate format means that these are no longer needed.¶
• Logs IDs: Each log is now identified by an OID rather than by the hash of its public key. OID allocations are available from an IANA registry.¶
• TransItem structure: This new data structure is used to encapsulate most types of CT data. A TransItemList, consisting of one or more TransItem structures, can be used anywhere that SignedCertificateTimestampList was used in [RFC6962].¶
• Merkle Tree leaves: The MerkleTreeLeaf structure has been replaced by the TransItem structure, which eases extensibility and simplifies the leaf structure by removing one layer of abstraction.¶
• Unified leaf format: The structure for both certificate and precertificate entries now includes only the TBSCertificate (whereas certificate entries in [RFC6962] included the entire certificate).¶
• Log artifact extensions: These are now typed and managed by an IANA registry, and they can now appear not only in Signed Certificate Timestamps (SCTs) but also in Signed Tree Heads (STHs).¶
• API outputs: Complete TransItem structures are returned rather than the constituent parts of each structure.¶
• get-all-by-hash: This is a new client API for obtaining an inclusion proof and the corresponding consistency proof at the same time.¶
• submit-entry: This is a new client API, replacing add-chain and add-pre-chain.¶
• Presenting SCTs with proofs: TLS servers may present SCTs together with the corresponding inclusion proofs, using any of the mechanisms that [RFC6962] defined for presenting SCTs only. (Presenting SCTs only is still supported).¶
• CT TLS extension: The signed_certificate_timestamp TLS extension has been replaced by the transparency_info TLS extension.¶
• Verification algorithms: Detailed algorithms for verifying inclusion proofs, for verifying consistency between two STHs, and for verifying a root hash given a complete list of the relevant leaf input entries have been added.¶
• Extensive clarifications and editorial work.¶

2.1. Merkle Trees

A full description of the Merkle Tree is beyond the scope of this document. Briefly, it is a binary tree where each non-leaf node is a hash of its children. For CT, the number of children is at most two. Additional information can be found in the Introduction and Reference sections of [RFC8391].¶

MTH({}) = HASH().

MTH({d[0]}) = HASH(0x00 || d[0]).

MTH(D_n) = HASH(0x01 || MTH(D[0:k]) || MTH(D[k:n])),

PATH(0, {d[0]}) = {}

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,

PROOF(m, D_n) = SUBPROOF(m, D_n, true)

SUBPROOF(m, D_m, true) = {}

SUBPROOF(m, D_m, false) = {MTH(D_m)}

SUBPROOF(m, D_n, b) = SUBPROOF(m, D[0:k], b) : MTH(D[k:n])

SUBPROOF(m, D_n, b) = SUBPROOF(m - k, D[k:n], false) : MTH(D[0:k])

            hash
           /    \
          /      \
         /        \
        /          \
       /            \
      k              l
     / \            / \
    /   \          /   \
   /     \        /     \
  g       h      i      j
 / \     / \    / \     |
 a b     c d    e f     d6
 | |     | |    | |
d0 d1   d2 d3  d4 d5

    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

2.2. Signatures

When signing data structures, a log MUST use one of the signature algorithms from the IANA "Signature Algorithms" registry, described in Section 10.2.2.¶

3.1. Certificates

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.¶

3.2. Precertificates

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:¶

• It MUST be DER encoded, as described in [X690].¶
• SignedData.version MUST be v3(3).¶
• SignedData.digestAlgorithms MUST be the same as the SignerInfo.digestAlgorithm OID value (see below).¶

• SignedData.encapContentInfo:¶

  • eContentType MUST be the OID 1.3.101.78.¶
  • eContent MUST contain a TBSCertificate [RFC5280] that will be identical to the TBSCertificate in the issued certificate, except that the Transparency Information (Section 7.1) extension MUST be omitted.¶
• SignedData.certificates MUST be omitted.¶
• SignedData.crls MUST be omitted.¶

• SignedData.signerInfos MUST contain one SignerInfo:¶

  • version MUST be v3(3).¶
  • sid MUST use the subjectKeyIdentifier option.¶
  • digestAlgorithm MUST be one of the hash algorithm OIDs listed in the IANA "Hash Algorithms" registry, described in Section 10.2.1.¶

  • signedAttrs MUST be present and MUST contain two attributes:¶

    • a content-type attribute whose value is the same as SignedData.encapContentInfo.eContentType and¶
    • a message-digest attribute whose value is the message digest of SignedData.encapContentInfo.eContent.¶
  • signatureAlgorithm MUST be the same OID as TBSCertificate.signature.¶
  • signature MUST be from the same (root or intermediate) CA that intends to issue the corresponding certificate (see Section 3.2.1).¶
  • unsignedAttrs MUST be omitted.¶

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.¶

4.1. Log Parameters

A log is defined by a collection of immutable parameters, which are used by clients to communicate with the log and to verify log artifacts. Except for the Final STH, each of these parameters MUST be established before the log operator begins to operate the log.¶

Base URL:

The prefix used to construct URLs [RFC3986] for client messages (see Section 5). The base URL MUST be an "https" URL, MAY contain a port, and MAY contain a path with any number of path segments but MUST NOT contain a query string, fragment, or trailing "/". Example: https://ct.example.org/blue.¶

Hash Algorithm:

The hash algorithm used for the Merkle Tree (see Section 10.2.1).¶

Signature Algorithm:

The signature algorithm used (see Section 2.2).¶

Public Key:

The public key used to verify signatures generated by the log. A log MUST NOT use the same keypair as any other log.¶

Log ID:

The OID that uniquely identifies the log.¶

Maximum Merge Delay:

The MMD the log has committed to. This document deliberately does not specify any limits on the value to allow for experimentation.¶

Version:

The version of the protocol supported by the log (currently 1 or 2).¶

Maximum Chain Length:

The longest certificate chain submission the log is willing to accept, if the log imposes any limit.¶

STH Frequency Count:

The maximum number of STHs the log may produce in any period equal to the Maximum Merge Delay (see Section 4.10).¶

Final STH:

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. This value MUST be provided in the form of a TransItem of type signed_tree_head_v2. If a log is still accepting entries, this value should not be provided.¶

[JSON.Metadata] is an example of a metadata format that includes the above elements.¶

4.2. Evaluating Submissions

A log determines whether to accept or reject a submission by evaluating it against the minimum acceptance criteria (see Section 4.2.1) and against the log's discretionary acceptance criteria (see Section 4.2.2).¶

If the acceptance criteria are met, the log SHOULD accept the submission. (A log may decide, for example, to temporarily reject acceptable submissions to protect itself against denial-of-service attacks.)¶

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.¶

4.3. Log Entries

If a submission is accepted and an SCT is 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 provide this chain for auditing upon request (see Section 5.6) so that the CA cannot avoid 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.¶

4.4. Log ID

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 10.2.5). One way to get an OID arc, from which OIDs can be allocated, is to request a Private Enterprise Number from IANA by completing the registration form. The only advantage of the registry is that the DER encoding can be small. (Recall that OID allocations do not require a central registration, although logs will most likely want to make themselves known to potential clients through out-of-band means.) 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 full DER encoding (including the tag and length) of the OID can be reproduced simply by prepending an OBJECT IDENTIFIER tag (0x06) to the opaque vector length and contents.¶

The OID used to identify a log is limited such that the DER encoding of its value, excluding the tag and length, MUST be no longer than 127 octets.¶

4.5. TransItem Structure

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 {
        x509_entry_v2(0x0100), precert_entry_v2(0x0101),
        x509_sct_v2(0x0102), precert_sct_v2(0x0103),
        signed_tree_head_v2(0x0104), consistency_proof_v2(0x0105),
        inclusion_proof_v2(0x0106),

        /* Reserved Code Points */
        reserved_rfc6962(0x0000..0x00FF),
        reserved_experimentaluse(0xE000..0xEFFF),
        reserved_privateuse(0xF000..0xFFFF),
        (0xFFFF)
    } 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 10.2.3 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 type enumerations Version, LogEntryType, SignatureType, and MerkleLeafType [RFC6962]. Note also that v1 did not define TransItem, but this document provides guidelines (see Appendix A) on how v2 implementations can coexist 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.¶

4.6. Log Artifact Extensions

    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 SCTs (Section 4.8) and STHs (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 10.2.4). 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.¶

4.7. Merkle Tree Leaves

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 date and time at which the certificate or precertificate was accepted by the log, in the form of a 64-bit unsigned number of milliseconds elapsed since the Unix Epoch (1 January 1970 00:00:00 UTC -- see [UNIXTIME]), ignoring leap seconds, in network byte order. 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 is byte-for-byte identical to 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.¶

4.8. Signed Certificate Timestamp (SCT)

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<1..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. All SCT extensions are similar to noncritical X.509v3 extensions (i.e., the mustUnderstand field is not set), and a recipient SHOULD ignore any extension it does not understand. 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).¶

4.9. Merkle Tree Head

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 date and time, using the format defined in Section 4.7.¶

tree_size is the number of entries currently in the log's Merkle Tree.¶

root_hash is the root of the Merkle 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.¶

4.10. Signed Tree Head (STH)

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<1..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).¶

4.11. Merkle Consistency Proofs

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<0..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, as described in Section 2.1.4.¶

4.12. Merkle Inclusion Proofs

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<0..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, as described in Section 2.1.3.¶

4.13. Shutting Down a Log

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. In order to avoid that, the following actions SHOULD be taken:¶

• Make it known to clients and monitors that the log will be frozen. This is not part of the API, so it will have to be done via a relevant out-of-band mechanism.¶
• 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 one of the log's parameters. 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).¶

5.1. Submit Entry to Log

POST <Base URL>/ct/v2/submit-entry¶

Inputs:

submission:

The base64-encoded certificate or precertificate.¶

type:

The VersionedTransType integer value that indicates the type of the submission: 1 for x509_entry_v2 or 2 for precert_entry_v2.¶

chain:

An array of zero or more JSON strings, each of which is a base64-encoded CA certificate. The first element is the certifier of the submission, the second certifies the first, etc. The last element of chain (or, if chain is an empty array, the submission) is certified by an accepted trust anchor.¶

Outputs:

sct:

A base64-encoded TransItem of type x509_sct_v2 or precert_sct_v2, signed by this log, that corresponds to the submission.¶

If the submitted entry is immediately appended to (or already exists in) this log's tree, then the log SHOULD also output:¶

sth:

A base64-encoded TransItem of type signed_tree_head_v2 signed by this log.¶

inclusion:

A base64-encoded TransItem of type inclusion_proof_v2 whose inclusion_path array of Merkle Tree nodes proves the inclusion of the submission in the returned sth.¶

Error codes:¶

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 "badCertificate" 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 "unknownAnchor" error. A log is not able to 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. For example, if type was 2 (indicating precert_sct_v2), then all three TransItems could be embedded in the certificate.¶

5.2. Retrieve Latest STH

GET <Base URL>/ct/v2/get-sth¶

No inputs.¶

Outputs:

sth:

A base64-encoded TransItem of type signed_tree_head_v2 signed by this log that is no older than the log's MMD.¶

5.3. Retrieve Merkle Consistency Proof between Two STHs

GET <Base URL>/ct/v2/get-sth-consistency¶

Inputs:

first:

The tree_size of the older tree, in decimal.¶

second:

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.¶

Outputs:

consistency:

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. If first and second are equal and correspond to a known STH, the returned consistency proof MUST be empty (a consistency_path array with zero elements).¶

sth:

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 signed STHs.¶

Error codes:¶

See Section 2.1.4.2 for an outline of how to use the consistency output.¶

5.4. Retrieve Merkle Inclusion Proof from Log by Leaf Hash

GET <Base URL>/ct/v2/get-proof-by-hash¶

Inputs:

hash:

A base64-encoded v2 leaf hash.¶

tree_size:

The tree_size of the tree on which to base the proof, in decimal.¶

The hash must be calculated as defined in Section 4.7. A v2 STH must exist for the tree_size. Because of skew, the front end may not know the requested tree head. 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 tree head, then only inclusion is returned.¶

Outputs:

inclusion:

A base64-encoded TransItem of type inclusion_proof_v2 whose inclusion_path array of Merkle Tree nodes proves the inclusion of the certificate (as specified by the hash parameter) in the selected STH.¶

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:¶

See Section 2.1.3.2 for an outline of how to use the inclusion output.¶

5.5. Retrieve Merkle Inclusion Proof, STH, and Consistency Proof by Leaf Hash

GET <Base URL>/ct/v2/get-all-by-hash¶

Inputs:

hash:

A base64-encoded v2 leaf hash.¶

tree_size:

The tree_size of the tree on which to base the proofs, in decimal.¶

The hash must be calculated as defined in Section 4.7. A v2 STH must exist for the tree_size.¶

Because of skew, the front end may not know the requested tree head or the requested hash, which leads to a number of cases:¶

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.¶

Outputs:

inclusion:

A base64-encoded TransItem of type inclusion_proof_v2 whose inclusion_path array of Merkle Tree nodes proves the inclusion of the certificate (as specified by the hash parameter) in the selected STH.¶

sth:

A base64-encoded TransItem of type signed_tree_head_v2, signed by this log.¶

consistency:

A base64-encoded TransItem of type consistency_proof_v2 that proves the consistency of the requested tree head 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 signed STHs.¶

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.¶

5.6. Retrieve Entries and STH from Log

GET <Base URL>/ct/v2/get-entries¶

Inputs:

start:

0-based index of first entry to retrieve, in decimal.¶

end:

0-based index of last entry to retrieve, in decimal.¶

Outputs:

entries:

An array of objects, each consisting of:¶

log_entry:

The base64-encoded TransItem structure of type x509_entry_v2 or precert_entry_v2 (see Section 4.3).¶

submitted_entry:

JSON object equivalent to inputs that were submitted to submit-entry, with the addition of the trust anchor to the chain field if the submission did not include it.¶

sct:

The base64-encoded TransItem of type x509_sct_v2 or precert_sct_v2, corresponding to this log entry.¶

sth:

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 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. Note that a limit on the number of entries is not immutable, and therefore the restriction may be changed or lifted at any time and is not listed with the other Log Parameters in Section 4.1.¶

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.¶

Error codes:¶

5.7. Retrieve Accepted Trust Anchors

GET <Base URL>/ct/v2/get-anchors¶

No inputs.¶

Outputs:

certificates:

An array of JSON strings, each of which is a base64-encoded CA certificate that is acceptable to the log.¶

max_chain_length:

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.¶

This data is not signed, and the protocol depends on the security guarantees of TLS to ensure correctness.¶

6.1. TLS Client Authentication

This specification includes no description of how a TLS server can use CT for TLS client certificates. While this may be useful, it is not documented here for the following reasons:¶

• The greater security exposure is for clients to end up interacting with an illegitimate server.¶
• In general, TLS client certificates are not expected to be submitted to CT logs, particularly those intended for general public use.¶

A future version could include such information.¶

6.2. Multiple SCTs

CT-using TLS servers SHOULD send SCTs from multiple logs because:¶

• The set of logs trusted by TLS clients is neither unified nor static; each client vendor may maintain an independent list of trusted logs, and, over time, new logs may become trusted and current logs may become distrusted. Note that client discovery, trust, and distrust of logs are expected to be handled out of band and are out of scope of this document.¶
• If a CA and a log collude, it is possible to temporarily hide misissuance from clients. When a TLS client requires SCTs from multiple logs to be provided, it is more difficult to mount this attack.¶
• If a log misbehaves or suffers a key compromise, 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), including SCTs from multiple logs reduces the probability of the certificate being rejected by TLS clients.¶
• TLS clients may have policies related to the above risks requiring TLS servers to present multiple SCTs. For example, at the time of writing, Chromium [Chromium.Log.Policy] requires multiple SCTs to be presented with Extended Validation (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.¶

6.3. TransItemList Structure

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).¶

6.4. Presenting SCTs, Inclusions Proofs, and STHs

In each TransItemList that is sent during a TLS handshake, the TLS server MUST include a TransItem structure of type x509_sct_v2 or precert_sct_v2.¶

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.¶

6.5. transparency_info TLS Extension

Provided that a TLS client includes the transparency_info extension type in the ClientHello and the TLS server supports the transparency_info extension:¶

• The TLS server MUST verify that the received extension_data is empty.¶
• The TLS server MUST construct a TransItemList of relevant TransItems (see Section 6.4), which SHOULD omit any TransItems that are already embedded in the server certificate or the stapled OCSP response (see Section 7.1). If the constructed TransItemList is not empty, then the TLS server MUST include the transparency_info extension with the extension_data set to this TransItemList. If the list is empty, then the server SHOULD omit the extension_data element but MAY send it with an empty array.¶

TLS servers MUST only include this extension in the following messages:¶

• the ServerHello message (for TLS 1.2 or earlier)¶
• the Certificate or CertificateRequest message (for TLS 1.3)¶

TLS servers MUST NOT process or include this extension when a TLS session is resumed, since session resumption uses the original session information.¶

7.1. Transparency Information X.509v3 Extension

The Transparency Information X.509v3 extension, which has OID 1.3.101.75 and SHOULD be noncritical, 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.¶

7.2. TLS Feature X.509v3 Extension

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).¶

8.1. TLS Client

8.2. Monitor

Monitors watch logs to check for correct behavior, for certificates of interest, or for 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:¶

1. Fetch the current STH (Section 5.2).¶
2. Verify the STH signature.¶
3. Fetch all the entries in the tree corresponding to the STH (Section 5.6).¶
4. If applicable, check each entry to see if it's a certificate of interest.¶
5. Confirm that the tree made from the fetched entries produces the same hash as that in the STH.¶

To inspect new entries, the monitor SHOULD follow these steps repeatedly for each log:¶

1. Fetch the current STH (Section 5.2). Repeat until the STH changes. To allow for experimentation, this document does not specify the polling frequency.¶
2. Verify the STH signature.¶
3. Fetch all the new entries in the tree corresponding to the STH (Section 5.6). If they remain unavailable for an extended period, then this should be viewed as misbehavior on the part of the log.¶
4. If applicable, check each entry to see if it's a certificate of interest.¶

5. Either:¶

   1. Verify that the updated list of all entries generates a tree with the same hash as the new STH.¶

   Or, if it is not keeping all log entries:¶

   1. Fetch a consistency proof for the new STH with the previous STH (Section 5.3).¶
   2. Verify the consistency proof.¶
   3. Verify that the new entries generate the corresponding elements in the consistency proof.¶
6. Repeat from Step 1.¶

8.3. Auditing

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 properties of log behavior 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 an 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, ensuring 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.¶

10.1. Additions to Existing Registries

This subsection defines additions to existing registries.¶

10.2. New CT-Related Registries

IANA has added a new protocol registry, "Public Notary Transparency", to the list that appears at <https://www.iana.org/assignments/>¶

The rest of this section defines the subregistries that have been created within the new "Public Notary Transparency" registry.¶

10.3. OID Assignment

IANA has assigned an object identifier from the "SMI Security for PKIX Module Identifier" registry to identify the ASN.1 module in Appendix B of this document.¶

11.1. Misissued Certificates

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.¶

11.2. Detection of Misissue

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.¶

11.3. Misbehaving Logs

A log can misbehave in several ways. Examples include the following: 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.¶

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 multiple clients comparing their instances of the STHs. This technique, known as "gossip", is an active area of research and not defined here. Proof of misbehavior in such cases would be either 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 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:¶

• using deterministic signature schemes or¶
• producing no more than one SCT for each distinct submission and no more than one STH for each distinct tree_size. Each of these SCTs and STHs can be stored by the log and served to other clients that submit the same certificate or request the same STH.¶

11.4. Multiple SCTs

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 6.2).¶

11.5. Leakage of DNS Information

Malicious monitors can use logs to learn about the existence of domain names that might not otherwise be easy to discover. Some subdomain labels may reveal information about the service and software for which the subdomain is used, which in turn might facilitate targeted attacks.¶