Google UK Ltd.

benl@google.com

Google Inc.

agl@google.com

Google Switzerland GmbH

ekasper@google.com

Google UK Ltd.

eranm@google.com

Sectigo Ltd.

rob@sectigo.com

Security TRANS (Public Notary Transparency) 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. This document obsoletes RFC 6962. It also specifies a new TLS extension that is used to send various CT log artifacts. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. [RFC Editor: please update 'RFCXXXX' to refer to this document, once its RFC number is known, through the document.]

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)). A typical definition of "public" can be found in . 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. A log may also limit the length of the chain it is willing to accept; such chains must also end with an acceptable trust anchor. 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. While mechanisms are being 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", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

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

This document revises and obsoletes the CT 1.0 protocol, 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 . Removed 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 managed by 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 . 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 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 SCTs but also in STHs. API outputs: complete TransItem structures are returned, rather than the constituent parts of each structure. get-all-by-hash: new client API for obtaining an inclusion proof and the corresponding consistency proof at the same time. submit-entry: 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 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: added 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. Extensive clarifications and editorial work.

A full description of Merkle Hash 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 section of .

The log uses a binary Merkle Hash Tree for efficient auditing. The hash algorithm used is one of the log's parameters (see ). This document establishes a registry of acceptable hash algorithms (see ). 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:

The hash of a list with one entry (also known as a leaf hash) is:

For n > 1, let k be the largest power of two smaller than n (i.e., k < n <= 2k). The Merkle Tree Hash of an n-element list D_n is then defined recursively as

where: || denotes concatenation : denotes concatenation of lists 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 proposal, except our definition handles non-full trees differently).

When a client has a complete list of 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: Set stack to an empty stack. For each i from 0 up to tree_size - 1: Push HASH(0x00 || entries[i]) to stack. Set merge_count to the lowest value (0 included) such that LSB(i >> merge_count) is not set, where LSB means the least significant bit. In other words, set merge_count to the number of consecutive 1s found starting at the least significant bit of i. Repeat merge_count times: Pop right from stack. Pop left from stack. Push HASH(0x01 || left || right) to stack. If there is more than one element in the stack, repeat the same merge procedure (the sub-items of Step 2.3 above) until only a single element remains. The remaining element in stack is the Merkle Tree hash for the given tree_size and should be compared by equality against the supplied root_hash.

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:

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

= k, ]]>

The : operator and D[k1:k2] are defined the same as in .

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: Compare leaf_index from the inclusion_proof_v2 structure against tree_size. If leaf_index is greater than or equal to tree_size then fail the proof verification. Set fn to leaf_index and sn to tree_size - 1. Set r to hash. For each value p in the inclusion_path array: If sn is 0, stop the iteration and fail the proof verification. If LSB(fn) is set, or if fn is equal to sn, then: Set r to HASH(0x01 || p || r) If LSB(fn) is not set, then right-shift both fn and sn equally until either LSB(fn) is set or fn is 0. Otherwise: Set r to HASH(0x01 || r || p) Finally, right-shift both fn and sn one time. Compare sn to 0. Compare r against the root_hash. If sn is equal to 0, and r and the root_hash are equal, then the log has proven the inclusion of hash. Otherwise, fail the proof verification.

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:

In SUBPROOF, the boolean value represents whether the subtree created from D[0:m] is a complete subtree of the Merkle Tree created from D_n, and, consequently, whether the subtree Merkle Tree Hash MTH(D[0:m]) is known. The initial call to SUBPROOF sets this to be true, and SUBPROOF is then defined as follows: The subproof for m = n is empty if m is the value for which PROOF was originally requested (meaning that the subtree created from D[0:m] is a complete subtree of the Merkle Tree created from the original D_n for which PROOF was requested, and the subtree Merkle Tree Hash MTH(D[0:m]) is known):

Otherwise, the subproof for m = n is the Merkle Tree Hash committing inputs D[0:m]:

For m < n, let k be the largest power of two smaller than n. The subproof is then defined recursively, using the appropriate step below: If m <= k, the right subtree entries D[k:n] only exist in the current tree. We prove that the left subtree entries D[0:k] are consistent and add a commitment to D[k:n]:

If m > k, the left subtree entries D[0:k] are identical in both trees. We prove that the right subtree entries D[k:n] are consistent and add a commitment to D[0:k].

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 .

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: If consistency_path is an empty array, stop and fail the proof verification. If first is an exact power of 2, then prepend first_hash to the consistency_path array. Set fn to first - 1 and sn to second - 1. If LSB(fn) is set, then right-shift both fn and sn equally until LSB(fn) is not set. Set both fr and sr to the first value in the consistency_path array. For each subsequent value c in the consistency_path array: If sn is 0, stop the iteration and fail the proof verification. If LSB(fn) is set, or if fn is equal to sn, then: Set fr to HASH(0x01 || c || fr) Set sr to HASH(0x01 || c || sr) If LSB(fn) is not set, then right-shift both fn and sn equally until either LSB(fn) is set or fn is 0. Otherwise: Set sr to HASH(0x01 || sr || c) Finally, right-shift both fn and sn one time. After completing iterating through the consistency_path array as described above, verify that the fr calculated is equal to the first_hash supplied, that the sr calculated is equal to the second_hash supplied and that sn is 0.

The binary Merkle Tree with 7 leaves:

The inclusion proof for d0 is [b, h, l]. The inclusion proof for d3 is [c, g, l]. The inclusion proof for d4 is [f, j, k]. The inclusion proof for d6 is [i, k]. The same tree, built incrementally in four steps:

The consistency proof between hash0 and hash is PROOF(3, D[7]) = [c, d, g, l]. c, g are used to verify hash0, and d, l are additionally used to show hash is consistent with hash0. The consistency proof between hash1 and hash is PROOF(4, D[7]) = [l]. hash can be verified using hash1=k and l. The consistency proof between hash2 and hash is PROOF(6, D[7]) = [i, j, k]. k, i are used to verify hash2, and j is additionally used to show hash is consistent with hash2.

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

Submitters submit certificates or preannouncements of certificates prior to issuance (precertificates) to logs for public auditing, as described below. In order to enable attribution of each logged certificate or precertificate to its issuer, each submission MUST be accompanied by all additional certificates required to verify the chain up to an accepted trust anchor (). The trust anchor (a root or intermediate CA certificate) MAY be omitted from the submission. If a log accepts a submission, it will return a Signed Certificate Timestamp (SCT) (see ). The submitter SHOULD validate the returned SCT as described in if they understand its format and they intend to use it directly in a TLS handshake or to construct a certificate. If the submitter does not need the SCT (for example, the certificate is being submitted simply to make it available in the log), it MAY validate the SCT.

Any entity can submit a certificate () to a log. Since it is anticipated that TLS clients will reject certificates that are not logged, it is expected that certificate issuers and subjects will be strongly motivated to submit them.

CAs may preannounce a certificate prior to issuance by submitting a precertificate () that the log can use to create an entry that will be valid against the issued certificate. The CA MAY incorporate the returned SCT in the issued certificate. One example of where the returned SCT is not incorporated in the issued certificate is when a CA sends the precertificate to multiple logs, but only incorporates the SCTs that are returned first. A precertificate is a CMS signed-data object that conforms to the following profile: It MUST be DER encoded as described in . 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 that will be identical to the TBSCertificate in the issued certificate, except that the Transparency Information () 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 CT Hash Algorithms Registry, described in . signedAttrs MUST be present and MUST contain two attributes: A content-type attribute whose value is the same as SignedData.encapContentInfo.eContentType. 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 ). unsignedAttrs MUST be omitted. SignerInfo.signedAttrs is included in the message digest calculation process (see Section 5.4 of ), 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.

Under normal circumstances, there will be a short delay between precertificate submission and issuance of the corresponding certificate. Longer delays are to be expected occasionally (e.g., due to log server downtime), and in some cases the CA might not actually issue the corresponding certificate. Nevertheless, a precertificate's signature indicates the CA's binding intent to issue the corresponding certificate, which means that: Misissuance of a precertificate is considered equivalent to misissuance of the corresponding certificate. The CA should expect to be held to account, even if the corresponding certificate has not actually been issued. Upon observing a precertificate, a client can reasonably presume that the corresponding certificate has been issued. A client may wish to obtain status information (e.g., by using the Online Certificate Status Protocol or by checking a Certificate Revocation List ) about a certificate that is presumed to exist, especially if there is evidence or suspicion that the corresponding precertificate was misissued. TLS clients may have policies that require CAs to be able to revoke, and to provide certificate status services for, each certificate that is presumed to exist based on the existence of a corresponding precertificate.

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, as discussed in . If different SCTs are produced for the same submission, multiple log entries will have to be created, one for each SCT (as the timestamp is a part of the leaf structure). Note that if a certificate was previously logged as a precertificate, then the precertificate's SCT of type precert_sct_v2 would not be appropriate; instead, a fresh SCT of type x509_sct_v2 should be generated. An SCT is the log's promise to 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 ): Allocate a tree index to the entry representing the accepted submission. Calculate the root of the tree. Sign the root of the tree (see ). 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 immutable parameters, which are used by clients to communicate with the log and to verify log artifacts. Except for the Final Signed Tree Head (STH), each of these parameters MUST be established before the log operator begins to operate the log. The prefix used to construct URLs () for client messages (see ). The base URL MUST be an "https" URL, MAY contain a port, 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 The hash algorithm used for the Merkle Tree (see ). The signature algorithm used (see ). The public key used to verify signatures generated by the log. A log MUST NOT use the same keypair as any other log. The OID that uniquely identifies the log. The MMD the log has committed to. This document deliberately does not specify any limits on the value, to allow for experimentation. The version of the protocol supported by the log (currently 1 or 2). The longest certificate chain submission the log is willing to accept, if the log imposes any limit. The maximum number of STHs the log may produce in any period equal to the Maximum Merge Delay (see ). If a log has been closed down (i.e., no longer accepts new entries), existing entries may still be valid. In this case, the client should know the final valid STH in the log to ensure no new entries can be added without detection. 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. is an example of a metadata format which includes the above elements.

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

To ensure that logged certificates and precertificates are attributable to an accepted trust anchor, and to set clear expectations for what monitors would find in the log, and to avoid being overloaded by invalid submissions, the log MUST reject a submission if any of the following conditions are not met: The submission, type and chain inputs MUST be set as described in . The log MUST NOT accommodate misordered CA certificates or use any other source of intermediate CA certificates to attempt certification path construction. Each of the zero or more intermediate CA certificates in the chain MUST have one or both of the following features: The Basic Constraints extension with the cA boolean asserted. The Key Usage extension with the keyCertSign bit asserted. Each certificate in the chain MUST fall within the limits imposed by the zero or more Basic Constraints pathLenConstraint values found higher up the chain. Precertificate submissions MUST conform to all of the requirements in .

If the minimum acceptance criteria are met but the submission is not fully valid according to verification rules (e.g., the certificate or precertificate has expired, is not yet valid, has been revoked, exhibits ASN.1 DER encoding errors but the log can still parse it, etc), then the acceptability of the submission is left to the log's discretion. It is useful for logs to accept such submissions in order to accommodate quirks of CA certificate-issuing software and to facilitate monitoring of CA compliance with applicable policies and technical standards. However, it is impractical for this document to enumerate, and for logs to consider, all of the ways that a submission might fail to comply with . Logs SHOULD limit the length of chain they will accept. The maximum chain length is one of the log's parameters (see ).

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 provide this chain for auditing upon request (see ) 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.

Each log is identified by an OID, which is one of the log's parameters (see ) 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 ). 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:

; ]]>

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.

Various data structures are encapsulated in the TransItem structure to ensure that the type and version of each one is identified in a common fashion:

versioned_type is a value from the IANA registry in 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. Note also that v1 did not define TransItem, but this document provides guidelines (see ) on how v2 implementations can co-exist with v1 implementations. Future versions of this protocol may reuse VersionedTransType values defined in this document as long as the corresponding data structures are not modified, and may add new VersionedTransType values for new or modified data structures.

; } Extension; ]]>

The Extension structure provides a generic extensibility for log artifacts, including Signed Certificate Timestamps () and Signed Tree Heads (). 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 ). 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 ). Each leaf is the leaf hash () of a TransItem structure of type x509_entry_v2 or precert_entry_v2, which encapsulates a TimestampedCertificateEntryDataV2 structure. Note that leaf hashes are calculated as HASH(0x00 || TransItem), where the hash algorithm is one of the log's parameters.

; 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 ), 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 . 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 ). 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 call.

An SCT is a TransItem structure of type x509_sct_v2 or precert_sct_v2, which encapsulates a SignedCertificateTimestampDataV2 structure:

; opaque signature<1..2^16-1>; } SignedCertificateTimestampDataV2; ]]>

log_id is this log's unique ID, encoded in an opaque vector as described in . 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 non-critical 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 ) using the signature algorithm declared in the log's parameters (see ).

The log stores information about its Merkle Tree in a TreeHeadDataV2:

; 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 {tree_leaves}. 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 ) to produce an STH. When a client requests a log's latest STH (see ), the log MUST return an STH that is no older than the log's MMD. However, 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 ). 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:

; } SignedTreeHeadDataV2; ]]>

log_id is this log's unique ID, encoded in an opaque vector as described in . The timestamp in tree_head MUST be at least as recent as the most recent SCT timestamp in the tree. Each subsequent timestamp MUST be more recent than the timestamp of the previous update. tree_head contains the latest tree head information (see ). signature is computed over the tree_head field using the signature algorithm declared in the log's parameters (see ).

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:

; } ConsistencyProofDataV2; ]]>

log_id is this log's unique ID, encoded in an opaque vector as described in . tree_size_1 is the size of the older tree. tree_size_2 is the size of the newer tree. consistency_path is a vector of Merkle Tree nodes proving the consistency of two STHs as described in {consistency}.

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:

; } InclusionProofDataV2; ]]>

log_id is this log's unique ID, encoded in an opaque vector as described in . tree_size is the size of the tree on which this inclusion proof is based. leaf_index is the 0-based index of the log entry corresponding to this inclusion proof. inclusion_path is a vector of Merkle Tree nodes proving the inclusion of the chosen certificate or precertificate as described in {merkle_inclusion_proof}.

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).

Messages are sent as HTTPS GET or POST requests. Parameters for POSTs and all responses are encoded as JavaScript Object Notation (JSON) objects . Parameters for GETs are encoded as order-independent key/value URL parameters, using the "application/x-www-form-urlencoded" format described in the "HTML 4.01 Specification" . Binary data is base64 encoded according to section 4 of as specified in the individual messages. Clients are configured with a log's base URL, which is one of the log's parameters. Clients 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 . 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, to allow for experimentation. Any fields that are not understood SHOULD be ignored. In practice, log servers may include multiple front-end machines. Since it is impractical to keep these machines in perfect sync, errors may occur that are caused by skew between the machines. Where such errors are possible, the front-end will return additional information (as specified below) making it possible for clients to make progress, if progress is possible. Front-ends MUST only serve data that is free of gaps (that is, for example, no front-end will respond with an STH unless it is also able to prove consistency from all log entries logged within that STH). For example, when a consistency proof between two STHs is requested, the front-end reached may not yet be aware of one or both STHs. In the case where it is unaware of both, it will return the latest STH it is aware of. Where it is aware of the first but not the second, it will return the latest STH it is aware of and a consistency proof from the first STH to the returned STH. The case where it knows the second but not the first should not arise (see the "no gaps" requirement above). If the log is unable to process a client's request, it MUST return an HTTP response code of 4xx/5xx (see ), and, in place of the responses outlined in the subsections below, the body SHOULD be a JSON Problem Details Object (see Section 3), containing: A URN reference identifying the problem. To facilitate automated response to errors, this document defines a set of standard tokens for use in the type field, within the URN namespace of: "urn:ietf:params:trans:error:". A human-readable string describing the error that prevented the log from processing the request, ideally with sufficient detail to enable the error to be rectified. e.g., In response to a request of <Base URL>/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:

Most error types are specific to the type of request and are defined in the respective subsections below. The one exception is the "malformed" error type, which indicates that the log server could not parse the client's request because it did not comply with this document: type detail malformed The request could not be parsed. Clients SHOULD treat 500 Internal Server Error and 503 Service Unavailable responses as transient failures and MAY retry the same request without modification at a later date. Note that as per , in the case of a 503 response the log MAY include a Retry-After: header in order to request a minimum time for the client to wait before retrying the request. In the absence of this header, this document does not specify a minimum. Clients SHOULD treat any 4xx error as a problem with the request and not attempt to resubmit without some modification to the request. The full status code MAY provide additional details. This document deliberately does not provide more specific guidance on the use of HTTP status codes.

POST <Base URL>/ct/v2/submit-entry The base64 encoded certificate or precertificate. The VersionedTransType integer value that indicates the type of the submission: 1 for x509_entry_v2, or 2 for precert_entry_v2. 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. 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: A base64 encoded TransItem of type signed_tree_head_v2, signed by this log. 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: type detail badSubmission submission is neither a valid certificate nor a valid precertificate. badType type is neither 1 nor 2. badChain The first element of chain is not the certifier of the submission, or the second element does not certify the first, etc. badCertificate One or more certificates in the chain are not valid (e.g., not properly encoded). unknownAnchor 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 () 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 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.

GET <Base URL>/ct/v2/get-sth No inputs. A base64 encoded TransItem of type signed_tree_head_v2, signed by this log, that is no older than the log's MMD.

GET <Base URL>/ct/v2/get-sth-consistency The tree_size of the older tree, in decimal. The tree_size of the newer tree, in decimal (optional). Both tree sizes must be from existing v2 STHs. However, because of skew, the receiving front-end may not know one or both of the existing STHs. If both are known, then only the consistency output is returned. If the first is known but the second is not (or has been omitted), then the latest known STH is returned, along with a consistency proof between the first STH and the latest. If neither are known, then the latest known STH is returned without a consistency proof. A base64 encoded TransItem of type consistency_proof_v2, whose tree_size_1 MUST match the first input. If the sth output is omitted, then tree_size_2 MUST match the second input. 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). A base64 encoded TransItem of type signed_tree_head_v2, signed by this log. Note that no signature is required for the consistency output as it is used to verify the consistency between two STHs, which are signed. Error codes: type detail firstUnknown first is before the latest known STH but is not from an existing STH. secondUnknown second is before the latest known STH but is not from an existing STH. secondBeforeFirst second is smaller than first. See for an outline of how to use the consistency output.

GET <Base URL>/ct/v2/get-proof-by-hash A base64 encoded v2 leaf hash. The tree_size of the tree on which to base the proof, in decimal. The hash must be calculated as defined in . 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. 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. 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: type detail hashUnknown hash is not the hash of a known leaf (may be caused by skew or by a known certificate not yet merged). treeSizeUnknown hash is before the latest known STH but is not from an existing STH. See for an outline of how to use the inclusion output.

GET <Base URL>/ct/v2/get-all-by-hash A base64 encoded v2 leaf hash. The tree_size of the tree on which to base the proofs, in decimal. The hash must be calculated as defined in . 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: Case Response latest STH < requested tree head Return latest STH latest STH > requested tree head Return latest STH and a consistency proof between it and the requested tree head (see ) 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. 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. A base64 encoded TransItem of type signed_tree_head_v2, signed by this log. A base64 encoded TransItem of type consistency_proof_v2 that proves the consistency of the requested 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 STHs, which are signed. Errors are the same as in . See for an outline of how to use the inclusion output, and see for an outline of how to use the consistency output.

GET <Base URL>/ct/v2/get-entries 0-based index of first entry to retrieve, in decimal. 0-based index of last entry to retrieve, in decimal. An array of objects, each consisting of The base64 encoded TransItem structure of type x509_entry_v2 or precert_entry_v2 (see ). 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. The base64 encoded TransItem of type x509_sct_v2 or precert_sct_v2 corresponding to this log entry. A base64 encoded TransItem of type signed_tree_head_v2, signed by this log. Note that this message is not signed -- the entries data can be verified by constructing the Merkle Tree Hash corresponding to a retrieved STH. All leaves MUST be v2. However, a compliant v2 client MUST NOT construe an unrecognized TransItem type as an error. This means it may be unable to parse some entries, but note that each client can inspect the entries it does recognize as well as verify the integrity of the data by treating unrecognized leaves as opaque input to the tree. The start and end parameters SHOULD be within the range 0 <= x < tree_size as returned by get-sth in . The start parameter MUST be less than or equal to the end parameter. 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 ). 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 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 . Because of skew, it is possible the log server will not have any entries between start and end. In this case it MUST return an empty entries array. In any case, the log server MUST return the latest STH it knows about. See for an outline of how to use a complete list of log_entry entries to verify the root_hash. Error codes: type detail startUnknown start is greater than the number of entries in the Merkle tree. endBeforeStart start cannot be greater than end.

GET <Base URL>/ct/v2/get-anchors No inputs. 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.

CT-using TLS servers MUST use at least one of the mechanisms described 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. (Of course, a server can only send a TLS extension if the client has specified it first.) Servers SHOULD also present corresponding inclusion proofs and STHs. A server can provide SCTs using a TLS 1.3 extension (Section 4.2 of ) with type transparency_info (see ). This mechanism allows TLS servers to participate in CT without the cooperation of CAs, unlike the other two mechanisms. It also allows SCTs and inclusion proofs to be updated on the fly. The server may also use an Online Certificate Status Protocol (OCSP) response extension (see ), providing the OCSP response as part of the TLS handshake. Providing a response during a TLS handshake is popularly known as "OCSP stapling." For TLS 1.3, the information is encoded as an extension in the status_request extension data; see Section 4.4.2.1 of . For TLS 1.2 (), the information is encoded as an extension in the CertificateStatus message; see Section 8 of . Using stapling also allows SCTs and inclusion proofs to be updated on the fly. CT information can also be encoded as an extension in the X.509v3 certificate (see ). This mechanism allows the use of unmodified TLS servers, but the SCTs and inclusion proofs cannot be updated on the fly. Since the logs from which the SCTs and inclusion proofs originated won't necessarily be accepted by TLS clients for the full lifetime of the certificate, there is a risk that TLS clients may subsequently consider the certificate to be non-compliant and in need of re-issuance or the use of one of the other two methods for delivering CT information.

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.

CT-using TLS servers SHOULD send SCTs from multiple logs, because: One or more logs may not have become acceptable to all CT-using TLS clients. Note that client discovery, trust, and distrust of logs is expected to be handled out-of-band and is 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 requires multiple SCTs to be presented with EV certificates in order for the EV indicator to be shown. To select the logs from which to obtain SCTs, a TLS server can, for example, examine the set of logs popular TLS clients accept and recognize.

Multiple SCTs, inclusion proofs, and indeed TransItem structures of any type, are combined into a list as follows:

; 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 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 ) 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: The TLS server MUST verify that the received extension_data is empty. The TLS server MUST construct a TransItemList of relevant TransItems (see ), which SHOULD omit any TransItems that are already embedded in the server certificate or the stapled OCSP response (see ). 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.

The Transparency Information X.509v3 extension, which has OID 1.3.101.75 and SHOULD be non-critical, contains one or more TransItem structures in a TransItemList. This extension MAY be included in OCSP responses (see ) and certificates (see ). Since RFC5280 requires the extnValue field (an OCTET STRING) of each X.509v3 extension to include the DER encoding of an ASN.1 value, a TransItemList MUST NOT be included directly. Instead, it MUST be wrapped inside an additional OCTET STRING, which is then put into the extnValue field:

TransparencyInformationSyntax contains a TransItemList.

A certification authority MAY include a Transparency Information X.509v3 extension in the singleExtensions of a SingleResponse in an OCSP response. 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 , because TLS servers are not required to support the transparency_info TLS extension in order to participate in CT (see ).

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 , but note that this document does not describe how the parameters are obtained, which is implementation-dependent (see, for example, ).

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 ). TLS clients that support the transparency_info TLS extension (see ) 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: Remove the Transparency Information extension (see ). Remove embedded v1 SCTs, identified by OID 1.3.6.1.4.1.11129.2.4.2 (see section 3.3 of ). This allows embedded v1 and v2 SCTs to co-exist in a certificate (see ).

In order to make use of a received SCT, the TLS client MUST first validate it as follows: Compute 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: timestamp is copied from the SCT. tbs_certificate is the reconstructed TBSCertificate portion of the server certificate, as described in . issuer_key_hash is computed as described in . sct_extensions is copied from the SCT. Verify the SCT's signature against the computed signature input 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. If the TLS client does not have the corresponding log's parameters, it cannot attempt to validate the SCT. When evaluating compliance (see ), the TLS client will consider only those SCTs that it was able to validate. Note that SCT validation is not a substitute for the normal validation of the server certificate and its chain.

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 () or get-all-by-hash (). 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 ).

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 (), then verify it by requesting a consistency proof (). Note that if the TLS client uses get-all-by-hash, then it will already have the new STH.

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 ). 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.

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: Fetch the current STH (). Verify the STH signature. Fetch all the entries in the tree corresponding to the STH (). If applicable, check each entry to see if it's a certificate of interest. 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: Fetch the current STH (). Repeat until the STH changes. This document does not specify the polling frequency, to allow for experimentation. Verify the STH signature. Fetch all the new entries in the tree corresponding to the STH (). If they remain unavailable for an extended period, then this should be viewed as misbehavior on the part of the log. If applicable, check each entry to see if it's a certificate of interest. Either: Verify that the updated list of all entries generates a tree with the same hash as the new STH. Or, if it is not keeping all log entries: Fetch a consistency proof for the new STH with the previous STH (). Verify the consistency proof. Verify that the new entries generate the corresponding elements in the consistency proof. Repeat from step 1.

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: The Maximum Merge Delay (MMD). The STH Frequency Count. The append-only property. The consistency of the log view presented to all query sources. A benign, conformant log publishes a series of STHs over time, each derived from the previous STH and the submitted entries incorporated into the log since publication of the previous STH. This can be proven through auditing of STHs. SCTs returned to TLS clients can be audited by verifying against the accompanying certificate, and using Merkle Inclusion Proofs, against the log's Merkle tree. The action taken by the auditor if an audit fails is not specified, but note that in general if audit fails, the auditor is in possession of signed proof of the log's misbehavior. A monitor () can audit by verifying the consistency of STHs it receives, ensure that each entry can be fetched and that the STH is indeed the result of making a tree from all fetched entries. A TLS client () can audit by verifying an SCT against any STH dated after the SCT timestamp + the Maximum Merge Delay by requesting a Merkle inclusion proof (). It can also verify that the SCT corresponds to the 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 .

It is not possible for a log to change any of its algorithms part way through its lifetime: SCT signatures must remain valid so signature algorithms can only be added, not removed. A log would have to support the old and new hash algorithms to allow backwards-compatibility with clients that are not aware of a hash algorithm change. Allowing multiple signature or hash algorithms for a log would require that all data structures support it and would significantly complicate client implementation, which is why it is not supported by this document. If it should become necessary to deprecate an algorithm used by a live log, then the log MUST be frozen as specified in and a new log SHOULD be started. Certificates in the frozen log that have not yet expired and require new SCTs SHOULD be submitted to the new log and the SCTs from that log used instead.

The assignment policy criteria mentioned in this section refer to the policies outlined in .

IANA is asked to add an entry for transparency_info(TBD) to the "TLS ExtensionType Values" registry defined in , setting the "Recommended" value to "Y", setting the "TLS 1.3" value to "CH, CR, CT", and 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 0x01 - 0xDF Unassigned   Specification Required 0xE0 - 0xEF Reserved   Experimental Use 0xF0 - 0xFF Reserved   Private Use

The appointed Expert(s) 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" The following notes should be added: This is a subset of the TLS SignatureScheme Registry, limited to those algorithms that are appropriate for CT. A major advantage of this is leveraging the expertise of the TLS working group and its designated experts. The value 0x0403 appears twice. While this may be confusing, it is okay because the verification process is the same for both algorithms, and the choice of which to use when generating a signature is purely internal to the log server. The registry should initially consist of: SignatureScheme Value Signature Algorithm Reference / Assignment Policy 0x0000 - 0x0402 Unassigned Expert Review ecdsa_secp256r1_sha256(0x0403) ECDSA (NIST P-256) with SHA-256 ecdsa_secp256r1_sha256(0x0403) Deterministic ECDSA (NIST P-256) with HMAC-SHA256 0x0404 - 0x0806 Unassigned Expert Review ed25519(0x0807) Ed25519 (PureEdDSA with the edwards25519 curve) 0x0808 - 0xFDFF Unassigned Expert Review 0xFE00 - 0xFEFF Reserved Experimental Use 0xFF00 - 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 ) 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 * 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 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. ### Specification Required guidance 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 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: "SCT", for extensions specified for use in Signed Certificate Timestamps. "STH", for extensions specified for use in Signed Tree Heads.

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 ), the precertificate CMS eContentType (see ), and X.509v3 extensions in certificates (see ) and OCSP responses (see ). 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: Log ID Log Base URL Log Operator Reference / Assignment Policy 1.3.101.8192 - 1.3.101.16383 Unassigned Unassigned First Come First Served 1.3.101.80.0 - 1.3.101.80.* Unassigned Unassigned First Come First Served All OIDs in the range from 1.3.101.8192 to 1.3.101.16383 have been set aside for Log IDs. 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 also been set assigned for LogIDs. 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 MUST be accompanied by: the Log's Base URL (see ). the Log Operator's contact details. IANA is asked to reject any request to update a Log ID or Log Base URL in this registry, because these fields are immutable (see ). IANA is asked to accept requests from log operators to update their contact details in this registry. Since log operators can choose to not use this registry (see ), it is not expected to be a global directory of all logs.

IANA is requested to add a new entry in the "IETF URN Sub-namespace for Registered Protocol Parameter Identifiers" registry, following the template in : Registry name: trans:error Specification: RFCXXXX Repository: https://www.iana.org/assignments/trans Index value: No transformation needed.

IANA is requested to create a new registry for errors. Requirements for this registry are Specification Required. This registry should have the following three fields: Field Name Type Reference identifier string RFCXXXX meaning string RFCXXXX reference string RFCXXXX The initial values are as follows, taken from the text above: Identifier Meaning Reference malformed The request could not be parsed. RFCXXXX badSubmission submission is neither a valid certificate nor a valid precertificate RFCXXXX badType type is neither 1 nor 2 RFCXXXX badChain The first element of chain is not the certifier of the submission, or the second element does not certify the first, etc. RFCXXXX badCertificate One or more certificates in the chain are not valid (e.g., not properly encoded) RFCXXXX unknownAnchor 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 RFCXXXX shutdown The log is no longer accepting submissions RFCXXXX firstUnknown first is before the latest known STH but is not from an existing STH. RFCXXXX secondUnknown second is before the latest known STH but is not from an existing STH. RFCXXXX secondBeforeFirst second is smaller than first. RFCXXXX hashUnknown hash is not the hash of a known leaf (may be caused by skew or by a known certificate not yet merged). RFCXXXX treeSizeUnknown hash is before the latest known STH but is not from an existing STH. RFCXXXX startUnknown start is greater than the number of entries in the Merkle tree. RFCXXXX endBeforeStart start cannot be greater than end. RFCXXXX

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.

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. Violation of the MMD contract is detected by log clients requesting a Merkle inclusion proof () for each observed SCT. These checks can be asynchronous and need only be done once per certificate. However, note that there may be privacy concerns (see ). Violation of the append-only property or the STH issuance rate limit can be detected by multiple clients comparing their instances of the Signed Tree Heads. This technique, known as "gossip," is an active area of research and not defined here. 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 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.

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 ).

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.

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, Rich Salz, 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.

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] This document provides specifications for existing TLS extensions. It is a companion document for RFC 5246, "The Transport Layer Security (TLS) Protocol Version 1.2". The extensions specified are server_name, max_fragment_length, client_certificate_url, trusted_ca_keys, truncated_hmac, and status_request. [STANDARDS-TRACK] Federal Information Processing Standard, FIPS This document specifies a protocol useful in determining the current status of a digital certificate without requiring Certificate Revocation Lists (CRLs). Additional mechanisms addressing PKIX operational requirements are specified in separate documents. This document obsoletes RFCs 2560 and 6277. It also updates RFC 5912. This document defines a deterministic digital signature generation procedure. Such signatures are compatible with standard Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA) digital signatures and can be processed with unmodified verifiers, which need not be aware of the procedure described therein. Deterministic signatures retain the cryptographic security features associated with digital signatures but can be more easily implemented in various environments, since they do not need access to a source of high-quality randomness. The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation. The purpose of the TLS feature extension is to prevent downgrade attacks that are not otherwise prevented by the TLS protocol. In particular, the TLS feature extension may be used to mandate support for revocation checking features in the TLS protocol such as Online Certificate Status Protocol (OCSP) stapling. Informing clients that an OCSP status response will always be stapled permits an immediate failure in the case that the response is not stapled. This in turn prevents a denial-of-service attack that might otherwise be possible. This document defines a "problem detail" as a way to carry machine- readable details of errors in a HTTP response to avoid the need to define new error response formats for HTTP APIs. This document describes elliptic curve signature scheme Edwards-curve Digital Signature Algorithm (EdDSA). The algorithm is instantiated with recommended parameters for the edwards25519 and edwards448 curves. An example implementation and test vectors are provided. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data.This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. This note describes the eXtended Merkle Signature Scheme (XMSS), a hash-based digital signature system that is based on existing descriptions in scientific literature. This note specifies Winternitz One-Time Signature Plus (WOTS+), a one-time signature scheme; XMSS, a single-tree scheme; and XMSS^MT, a multi-tree variant of XMSS. Both XMSS and XMSS^MT use WOTS+ as a main building block. XMSS provides cryptographic digital signatures without relying on the conjectured hardness of mathematical problems. Instead, it is proven that it only relies on the properties of cryptographic hash functions. XMSS provides strong security guarantees and is even secure when the collision resistance of the underlying hash function is broken. It is suitable for compact implementations, is relatively simple to implement, and naturally resists side-channel attacks. Unlike most other signature systems, hash-based signatures can so far withstand known attacks using quantum computers. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery.This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. NIST IEEE ITU-T This document describes a new sub-delegation for the 'ietf' URN namespace for registered protocol items. The 'ietf' URN namespace is defined in RFC 2648 as a root for persistent URIs that refer to IETF- defined resources. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes an experimental protocol for publicly logging the existence of Transport Layer Security (TLS) certificates as they are issued or observed, in a manner that allows anyone to audit certificate authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs.Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. Section 1.1.1 of RFC 3986 defines URI syntax as "a federated and extensible naming system wherein each scheme's specification may further restrict the syntax and semantics of identifiers using that scheme." In other words, the structure of a URI is defined by its scheme. While it is common for schemes to further delegate their substructure to the URI's owner, publishing independent standards that mandate particular forms of URI substructure is inappropriate, because that essentially usurps ownership. This document further describes this problematic practice and provides some acceptable alternatives for use in standards. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.This is the third edition of this document; it obsoletes RFC 5226. The Chromium Projects The Chromium Projects The Chromium Projects CA/Browser Forum

Certificate Transparency logs have to be either v1 (conforming to ) or v2 (conforming to this document), as the data structures are incompatible and so a v2 log could not issue a valid v1 SCT. CT clients, however, can support v1 and v2 SCTs, for the same certificate, simultaneously, as v1 SCTs are delivered in different TLS, X.509 and OCSP extensions than v2 SCTs. v1 and v2 SCTs for X.509 certificates can be validated independently. For precertificates, v2 SCTs should be embedded in the TBSCertificate before submission of the TBSCertificate (inside a v1 precertificate, as described in Section 3.1. of ) to a v1 log so that TLS clients conforming to but not this document are oblivious to the embedded v2 SCTs. An issuer can follow these steps to produce an X.509 certificate with embedded v1 and v2 SCTs: Create a CMS precertificate as described in and submit it to v2 logs. Embed the obtained v2 SCTs in the TBSCertificate, as described in . Use that TBSCertificate to create a v1 precertificate, as described in Section 3.1. of and submit it to v1 logs. Embed the v1 SCTs in the TBSCertificate, as described in Section 3.3 of . Sign that TBSCertificate (which now contains v1 and v2 SCTs) to issue the final X.509 certificate.

The following ASN.1 module may be useful to implementors.