Public Notary Transparency Working Group B. Laurie
Internet-Draft A. Langley
Intended status: Standards Track E. Kasper
Expires: January 28, 2017 E. Messeri
Google
R. Stradling
Comodo
July 27, 2016

Certificate Transparency
draft-ietf-trans-rfc6962-bis-18

Abstract

This document describes a 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 certification authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs.

Logs are network services that implement the protocol operations for submissions and queries that are defined in this document.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on January 28, 2017.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.


Table of Contents

1. Introduction

Certificate transparency aims to mitigate the problem of misissued certificates by providing append-only logs of issued certificates. The logs do not need to be trusted because they are publicly auditable. Anyone may verify the correctness of each log and monitor when new certificates are added to it. The logs do not themselves prevent misissue, but they ensure that interested parties (particularly those named in certificates) can detect such misissuance. Note that this is a general mechanism; but in this document, we only describe its use for public TLS server certificates issued by public certification authorities (CAs).

Each log contains certificate chains, which can be submitted by anyone. It is expected that public CAs will contribute all their newly issued certificates to one or more logs; however certificate holders can also contribute their own certificate chains, as can third parties. In order to avoid logs being rendered useless by the submission of large numbers of spurious certificates, it is required that each chain ends with a trust anchor that is accepted by the log. When a chain is accepted by a log, a signed timestamp is returned, which can later be used to provide evidence to TLS clients that the chain has been submitted. TLS clients can thus require that all certificates they accept as valid are accompanied by signed timestamps.

Those who are concerned about misissuance can monitor the logs, asking them regularly for all new entries, and can thus check whether domains for which they are responsible have had certificates issued that they did not expect. What they do with this information, particularly when they find that a misissuance has happened, is beyond the scope of this document; but, broadly speaking, they can invoke existing business mechanisms for dealing with misissued certificates, such as working with the CA to get the certificate revoked, or with maintainers of trust anchor lists to get the CA removed. Of course, anyone who wants can monitor the logs and, if they believe a certificate is incorrectly issued, take action as they see fit.

Similarly, those who have seen signed timestamps from a particular log can later demand a proof of inclusion from that log. If the log is unable to provide this (or, indeed, if the corresponding certificate is absent from monitors' copies of that log), that is evidence of the incorrect operation of the log. The checking operation is asynchronous to allow clients to proceed without delay, despite possible issues such as network connectivity and the vagaries of firewalls.

The append-only property of each log is achieved using Merkle Trees, which can be used to show that any particular instance of the log is a superset of any particular previous instance. Likewise, Merkle Trees avoid the need to blindly trust logs: if a log attempts to show different things to different people, this can be efficiently detected by comparing tree roots and consistency proofs. Similarly, other misbehaviors of any log (e.g., issuing signed timestamps for certificates they then don't log) can be efficiently detected and proved to the world at large.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. Data Structures

Data structures are defined according to the conventions laid out in Section 4 of [RFC5246].

2. Cryptographic Components

2.1. Merkle Hash Trees

Logs use a binary Merkle Hash Tree for efficient auditing. The hashing algorithm used by each log is expected to be specified as part of the metadata relating to that log (see Section 10.1). We have established a registry of acceptable algorithms, see Section 12.2. The hashing algorithm in use is referred to as HASH throughout this document and the size of its output in bytes as HASH_SIZE. The input to the Merkle Tree Hash is a list of data entries; these entries will be hashed to form the leaves of the Merkle Hash Tree. The output is a single HASH_SIZE Merkle Tree Hash. Given an ordered list of n inputs, D[n] = {d(0), d(1), ..., d(n-1)}, the Merkle Tree Hash (MTH) is thus defined as follows:

The hash of an empty list is the hash of an empty string:

MTH({}) = HASH().

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

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

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

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

where || is concatenation and D[k1:k2] denotes the list {d(k1), d(k1+1),..., d(k2-1)} of length (k2 - k1). (Note that the hash calculations for leaves and nodes differ. This domain separation is required to give second preimage resistance.)

Note that we do not require the length of the input list to be a power of two. The resulting Merkle Tree may thus not be balanced; however, its shape is uniquely determined by the number of leaves. (Note: This Merkle Tree is essentially the same as the history tree [CrosbyWallach] proposal, except our definition handles non-full trees differently.)

2.1.1. Merkle Inclusion Proofs

A Merkle inclusion proof for a leaf in a Merkle Hash Tree is the shortest list of additional nodes in the Merkle Tree required to compute the Merkle Tree Hash for that tree. Each node in the tree is either a leaf node or is computed from the two nodes immediately below it (i.e., towards the leaves). At each step up the tree (towards the root), a node from the inclusion proof is combined with the node computed so far. In other words, the inclusion proof consists of the list of missing nodes required to compute the nodes leading from a leaf to the root of the tree. If the root computed from the inclusion proof matches the true root, then the inclusion proof proves that the leaf exists in the tree.

Given an ordered list of n inputs to the tree, D[n] = {d(0), ..., d(n-1)}, the Merkle inclusion proof PATH(m, D[n]) for the (m+1)th input d(m), 0 <= m < n, is defined as follows:

The proof for the single leaf in a tree with a one-element input list D[1] = {d(0)} is empty:

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

For n > 1, let k be the largest power of two smaller than n. The proof for the (m+1)th element d(m) in a list of n > m elements is then defined recursively as

PATH(m, D[n]) = PATH(m, D[0:k]) : MTH(D[k:n]) for m < k; and

PATH(m, D[n]) = PATH(m - k, D[k:n]) : MTH(D[0:k]) for m >= k,

where : is concatenation of lists and D[k1:k2] denotes the length (k2 - k1) list {d(k1), d(k1+1),..., d(k2-1)} as before.

2.1.2. Merkle Consistency Proofs

Merkle consistency proofs prove the append-only property of the tree. A Merkle consistency proof for a Merkle Tree Hash MTH(D[n]) and a previously advertised hash MTH(D[0:m]) of the first m leaves, m <= n, is the list of nodes in the Merkle Tree required to verify that the first m inputs D[0:m] are equal in both trees. Thus, a consistency proof must contain a set of intermediate nodes (i.e., commitments to inputs) sufficient to verify MTH(D[n]), such that (a subset of) the same nodes can be used to verify MTH(D[0:m]). We define an algorithm that outputs the (unique) minimal consistency proof.

Given an ordered list of n inputs to the tree, D[n] = {d(0), ..., d(n-1)}, the Merkle consistency proof PROOF(m, D[n]) for a previous Merkle Tree Hash MTH(D[0:m]), 0 < m < n, is defined as:

PROOF(m, D[n]) = SUBPROOF(m, D[n], true)

In SUBPROOF, the boolean value represents whether the subtree created from D[0:m] is a complete subtree of the Merkle Tree created from D[n], and, consequently, whether the subtree Merkle Tree Hash MTH(D[0:m]) is known. The initial call to SUBPROOF sets this to be true, and SUBPROOF is then defined as follows:

The subproof for m = n is empty if m is the value for which PROOF was originally requested (meaning that the subtree created from D[0:m] is a complete subtree of the Merkle Tree created from the original D[n] for which PROOF was requested, and the subtree Merkle Tree Hash MTH(D[0:m]) is known):

SUBPROOF(m, D[m], true) = {}

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

SUBPROOF(m, D[m], false) = {MTH(D[m])}

For m < n, let k be the largest power of two smaller than n. The subproof is then defined recursively.

If m <= k, the right subtree entries D[k:n] only exist in the current tree. We prove that the left subtree entries D[0:k] are consistent and add a commitment to D[k:n]:

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

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

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

Here, : is a concatenation of lists, and D[k1:k2] denotes the length (k2 - k1) list {d(k1), d(k1+1),..., d(k2-1)} as before.

The number of nodes in the resulting proof is bounded above by ceil(log2(n)) + 1.

2.1.3. Example

The binary Merkle Tree with 7 leaves:

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

The inclusion proof for d0 is [b, h, l].

The inclusion proof for d3 is [c, g, l].

The inclusion proof for d4 is [f, j, k].

The inclusion proof for d6 is [i, k].

The same tree, built incrementally in four steps:

    hash0          hash1=k
    / \              /  \
   /   \            /    \
  /     \          /      \
  g      c         g       h
 / \     |        / \     / \
 a b     d2       a b     c d
 | |              | |     | |
d0 d1            d0 d1   d2 d3

          hash2                    hash
          /  \                    /    \
         /    \                  /      \
        /      \                /        \
       /        \              /          \
      /          \            /            \
     k            i          k              l
    / \          / \        / \            / \
   /   \         e f       /   \          /   \
  /     \        | |      /     \        /     \
 g       h      d4 d5    g       h      i      j
/ \     / \             / \     / \    / \     |
a b     c d             a b     c d    e f     d6
| |     | |             | |     | |    | |
d0 d1   d2 d3           d0 d1   d2 d3  d4 d5

The consistency proof between hash0 and hash is PROOF(3, D[7]) = [c, d, g, l]. c, g are used to verify hash0, and d, l are additionally used to show hash is consistent with hash0.

The consistency proof between hash1 and hash is PROOF(4, D[7]) = [l]. hash can be verified using hash1=k and l.

The consistency proof between hash2 and hash is PROOF(6, D[7]) = [i, j, k]. k, i are used to verify hash2, and j is additionally used to show hash is consistent with hash2.

2.1.4. Signatures

Various data structures are signed. A log MUST use one of the signature algorithms defined in the Section 12.3.

3. Submitters

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 Section 5.6). The submitter SHOULD validate the returned SCT as described in Section 10.2 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.

3.1. Certificates

Any entity can submit a certificate [add-chain] 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 [add-pre-chain] 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. Examples of situations where the returned SCT is not incorporated into the issued certificate would be when a CA sends the precertificate to multiple logs, but only incorporates the SCTs that are returned first, or the CA is using domain name redaction [domain_name_redaction] and intends to use another mechanism to publish SCTs (such as an OCSP response [ocsp_transinfo_extension] or the TLS extension [tls_transinfo_extension]).

A precertificate is a CMS [RFC5652] signed-data object that conforms to the following requirements:

4. Private Domain Name Labels

Some regard certain DNS domain name labels within their registered domain space as private and security sensitive. Even though these domains are often only accessible within the domain owner's private network, it's common for them to be secured using publicly trusted TLS server certificates. We define a mechanism (see Section 4.2) to allow these private labels to not appear in public logs, while still retaining most of the security benefits that accrue from using Certificate Transparency mechanisms.

4.1. Wildcard Certificates

A certificate containing a DNS-ID [RFC6125] of *.example.com could be used to secure the domain topsecret.example.com, without revealing the string topsecret publicly.

Since TLS clients only match the wildcard character to the complete leftmost label of the DNS domain name (see Section 6.4.3 of [RFC6125]), a different approach is needed when any label other than the leftmost label in a DNS-ID is considered private (e.g. top.secret.example.com). Also, wildcard certificates are prohibited in some cases, such as Extended Validation Certificates [EVSSLGuidelines].

4.2. Redaction of Domain Name Labels

4.2.1. Redacting Labels in Precertificates

When creating a precertificate, the CA MAY omit the subjectAltName extension, even if it intends to include the extension in the final certificate. If omitting the subjectAltName extension, the CA MUST include a redactedSubjectAltName [redacted_san_extension] extension that contains, in a redacted form, the same entries that will be included in the certificate's subjectAltName extension.

Wildcard * labels MUST NOT be redacted, but one or more non-wildcard labels in each DNS-ID [RFC6125] can each be replaced with a redacted label as follows:

  REDACT(label) = prefix || BASE32(index || _label_hash)
    _label_hash = LABELHASH(keyid_len || keyid || label_len || label)

label is the case-sensitive label to be redacted.

prefix is the "?" character (ASCII value 63).

index is the 1 byte index of a hash function in Section 12.2. The value 255 is reserved.

keyid_len is the 1 byte length of the keyid.

keyid is the keyIdentifier from the Subject Key Identifier extension (section 4.2.1.2 of [RFC5280]), excluding the ASN.1 OCTET STRING tag and length bytes.

label_len is the 1 byte length of the label.

|| denotes concatenation.

BASE32 is the Base 32 Encoding function (section 6 of [RFC4648]). Pad characters MUST NOT be appended to the encoded data.

LABELHASH is the hash function identified by index.

4.2.2. redactedSubjectAltName Certificate Extension

The redactedSubjectAltName extension is a non-critical extension (OID 1.3.101.77) that is identical in structure to the subjectAltName extension, except that DNS-IDs MAY contain redacted labels (see Section 4.2.1).

When used, the redactedSubjectAltName extension MUST be present in both the precertificate and the corresponding certificate.

This extension informs TLS clients of the DNS-ID labels that were redacted and the degree of redaction, while minimizing the complexity of TBSCertificate reconstruction (as described in Section 10.2.2). Hashing the redacted labels allows the legitimate domain owner to identify whether or not each redacted label correlates to a label they know of.

Only DNS-ID labels can be redacted using this mechanism. However, CAs can use Name Constraints (Section 4.3) to allow DNS domain name labels in other subjectAltName entries to not appear in logs.

4.3. Using a Name-Constrained Intermediate CA

An intermediate CA certificate or intermediate CA precertificate that contains the Name Constraints [RFC5280] extension MAY be logged in place of end-entity certificates issued by that intermediate CA, as long as all of the following conditions are met:

Below is an example Name Constraints extension that meets these conditions:

    SEQUENCE {
      OBJECT IDENTIFIER '2 5 29 30'
      OCTET STRING, encapsulates {
        SEQUENCE {
          [0] {
            SEQUENCE {
              [2] 'example.com'
              }
            }
          [1] {
            SEQUENCE {
              [7] 00 00 00 00 00 00 00 00
              }
            SEQUENCE {
              [7]
                00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
                00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
              }
            }
          }
        }
      }

5. Log Format and Operation

A log is a single, append-only Merkle Tree of submitted certificate and precertificate entries.

When it receives a valid submission, the log MUST return an SCT that corresponds to the submitted certificate or precertificate. If the log has previously seen this valid submission, it SHOULD return the same SCT as it returned before (to reduce the ability to track clients as described in Section 13.5). If different SCTs are produced for the same submission, multiple log entries will have to be created, one for each SCT (as the timestamp is a part of the leaf structure). Note that if a certificate was previously logged as a precertificate, then the precertificate's SCT of type precert_sct_v2 would not be appropriate; instead, a fresh SCT of type x509_sct_v2 should be generated.

An SCT is the log's promise to incorporate the submitted entry in its Merkle Tree no later than a fixed amount of time, known as the Maximum Merge Delay (MMD), after the issuance of the SCT. Periodically, the log MUST append all its new entries to its Merkle Tree and sign the root of the tree.

Log operators MUST NOT impose any conditions on retrieving or sharing data from the log.

5.1. Accepting Submissions

Logs MUST verify that each submitted certificate or precertificate has a valid signature chain to an accepted trust anchor, using the chain of intermediate CA certificates provided by the submitter. Logs MUST accept certificates and precertificates that are fully valid according to RFC 5280 [RFC5280] verification rules and are submitted with such a chain. Logs MAY accept certificates and precertificates that have expired, are not yet valid, have been revoked, or are otherwise not fully valid according to RFC 5280 verification rules in order to accommodate quirks of CA certificate-issuing software. However, logs MUST reject submissions without a valid signature chain to an accepted trust anchor. Logs MUST also reject precertificates that do not conform to the requirements in Section 3.2.

Logs SHOULD limit the length of chain they will accept. The maximum chain length is specified in the log's metadata.

The log SHALL allow retrieval of its list of accepted trust anchors (see Section 6.8), 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.

5.2. Log Entries

If a submission is accepted and an SCT issued, the accepting log MUST store the entire chain used for verification. This chain MUST include the certificate or precertificate itself, the zero or more intermediate CA certificates provided by the submitter, and the trust anchor used to verify the chain (even if it was omitted from the submission). The log MUST present this chain for auditing upon request (see Section 6.7). This chain is required to prevent a CA from avoiding blame by logging a partial or empty chain.

Each certificate entry in a log MUST include a X509ChainEntry structure, and each precertificate entry MUST include a PrecertChainEntryV2 structure:

    opaque ASN.1Cert<1..2^24-1>;

    struct {
        ASN.1Cert leaf_certificate;
        ASN.1Cert certificate_chain<0..2^24-1>;
    } X509ChainEntry;

    opaque CMSPrecert<1..2^24-1>;

    struct {
        CMSPrecert pre_certificate;
        ASN.1Cert precertificate_chain<1..2^24-1>;
    } PrecertChainEntryV2;

leaf_certificate is a submitted certificate that has been accepted by the log.

certificate_chain is a vector of 0 or more additional certificates required to verify leaf_certificate. The first certificate MUST certify leaf_certificate. Each following certificate MUST directly certify the one preceding it. The final certificate MUST be a trust anchor accepted by the log. If leaf_certificate is an accepted trust anchor, then this vector is empty.

pre_certificate is a submitted precertificate that has been accepted by the log.

precertificate_chain is a vector of 1 or more additional certificates required to verify pre_certificate. The first certificate MUST certify pre_certificate. Each following certificate MUST directly certify the one preceding it. The final certificate MUST be a trust anchor accepted by the log.

5.3. Log ID

Each log is identified by an OID, which is specified in the log's metadata and which MUST NOT be used to identify any other log. A log's operator MUST either allocate the OID themselves or request an OID from one of the two Log ID Registries (see Section 12.6.1 and Section 12.6.2). Various data structures include the DER encoding of this OID, excluding the ASN.1 tag and length bytes, in an opaque vector:

    opaque LogID<2..127>;

Note that the ASN.1 length and the opaque vector length are identical in size (1 byte) and value, so the DER encoding of the OID can be reproduced simply by prepending an OBJECT IDENTIFIER tag (0x06) to the opaque vector length and contents.

5.4. 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 {
        reserved(0),
        x509_entry_v2(1), precert_entry_v2(2),
        x509_sct_v2(3), precert_sct_v2(4),
        tree_head_v2(5), signed_tree_head_v2(6),
        consistency_proof_v2(7), inclusion_proof_v2(8),
        x509_sct_with_proof_v2(9), precert_sct_with_proof_v2(10),
        (65535)
    } VersionedTransType;

    struct {
        VersionedTransType versioned_type;
        select (versioned_type) {
            case x509_entry_v2: TimestampedCertificateEntryDataV2;
            case precert_entry_v2: TimestampedCertificateEntryDataV2;
            case x509_sct_v2: SignedCertificateTimestampDataV2;
            case precert_sct_v2: SignedCertificateTimestampDataV2;
            case tree_head_v2: TreeHeadDataV2;
            case signed_tree_head_v2: SignedTreeHeadDataV2;
            case consistency_proof_v2: ConsistencyProofDataV2;
            case inclusion_proof_v2: InclusionProofDataV2;
            case x509_sct_with_proof_v2: SCTWithProofDataV2;
            case precert_sct_with_proof_v2: SCTWithProofDataV2;
        } data;
    } TransItem;
          

versioned_type is the type of the encapsulated data structure and the earliest version of this protocol to which it conforms. This document is v2.

data is the encapsulated data structure. The various structures named with the DataV2 suffix are defined in later sections of this document.

Note that VersionedTransType combines the v1 [RFC6962] type enumerations Version, LogEntryType, SignatureType and MerkleLeafType. Note also that v1 did not define TransItem, but this document provides guidelines (see Appendix A) on how v2 implementations can co-exist with v1 implementations.

Future versions of this protocol may reuse VersionedTransType values defined in this document as long as the corresponding data structures are not modified, and may add new VersionedTransType values for new or modified data structures.

5.5. Merkle Tree Leaves

The leaves of a log's Merkle Tree correspond to the log's entries (see Section 5.2). Each leaf is the leaf hash [mht] of a TransItem structure of type x509_entry_v2 or precert_entry_v2, which encapsulates a TimestampedCertificateEntryDataV2 structure. Note that leaf hashes are calculated as HASH(0x00 || TransItem), where the hashing algorithm is specified in the log's metadata.

    opaque TBSCertificate<1..2^24-1>;

    struct {
        uint64 timestamp;
        opaque issuer_key_hash[HASH_SIZE];
        TBSCertificate tbs_certificate;
        SctExtension sct_extensions<0..2^16-1>;
    } TimestampedCertificateEntryDataV2;

timestamp is the NTP Time [RFC5905] at which the certificate or precertificate was accepted by the log, measured in milliseconds since the epoch (January 1, 1970, 00:00), ignoring leap seconds. Note that the leaves of a log's Merkle Tree are not required to be in strict chronological order.

issuer_key_hash is the HASH of the public key of the CA that issued the certificate or precertificate, calculated over the DER encoding of the key represented as SubjectPublicKeyInfo [RFC5280]. This is needed to bind the CA to the certificate or precertificate, making it impossible for the corresponding SCT to be valid for any other certificate or precertificate whose TBSCertificate matches tbs_certificate.

tbs_certificate is the DER encoded TBSCertificate from either the leaf_certificate (in the case of an X509ChainEntry) or the pre_certificate (in the case of a PrecertChainEntryV2). (Note that a precertificate's TBSCertificate can be reconstructed from the corresponding certificate as described in Section 10.2.2).

sct_extensions matches the SCT extensions of the corresponding SCT.

5.6. Signed Certificate Timestamp (SCT)

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

    enum {
        reserved(65535)
    } SctExtensionType;

    struct {
        SctExtensionType sct_extension_type;
        opaque sct_extension_data<0..2^16-1>;
    } SctExtension;

    struct {
        LogID log_id;
        uint64 timestamp;
        SctExtension sct_extensions<0..2^16-1>;
        digitally-signed struct {
            TransItem timestamped_entry;
        } signature;
    } SignedCertificateTimestampDataV2;

log_id is this log's unique ID, encoded in an opaque vector as described in Section 5.3.

timestamp is equal to the timestamp from the TimestampedCertificateEntryDataV2 structure encapsulated in the timestamped_entry.

sct_extension_type identifies a single extension from the IANA registry in Section 12.4. At the time of writing, no extensions are specified.

The interpretation of the sct_extension_data field is determined solely by the value of the sct_extension_type field. Each document that registers a new sct_extension_type must describe how to interpret the corresponding sct_extension_data.

sct_extensions is a vector of 0 or more SCT extensions. This vector MUST NOT include more than one extension with the same sct_extension_type. The extensions in the vector MUST be ordered by the value of the sct_extension_type field, smallest value first. If an implementation sees an extension that it does not understand, it SHOULD ignore that extension. Furthermore, an implementation MAY choose to ignore any extension(s) that it does understand.

The encoding of the digitally-signed element is defined in [RFC5246].

timestamped_entry is a TransItem structure that MUST be of type x509_entry_v2 or precert_entry_v2 (see Section 5.5).

5.7. Merkle Tree Head

The log stores information about its Merkle Tree in a TransItem structure of type tree_head_v2, which encapsulates a TreeHeadDataV2 structure:

    opaque NodeHash[HASH_SIZE];

    struct {
        uint64 timestamp;
        uint64 tree_size;
        NodeHash root_hash;
        SthExtension sth_extensions<0..2^16-1>;
    } TreeHeadDataV2;

timestamp is the current NTP Time [RFC5905], measured in milliseconds since the epoch (January 1, 1970, 00:00), ignoring leap seconds.

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

root_hash is the root of the Merkle Hash Tree.

sth_extensions matches the STH extensions of the corresponding STH.

5.8. Signed Tree Head (STH)

Periodically each log SHOULD sign its current tree head information (see Section 5.7) to produce an STH. When a client requests a log's latest STH (see Section 6.3), the log MUST return an STH that is no older than the log's MMD. However, STHs could be used to mark individual clients (by producing a new one for each query), so logs MUST NOT produce them more frequently than is declared in their metadata. In general, there is no need to produce a new STH unless there are new entries in the log; however, in the unlikely event that it receives no new submissions during an MMD period, the log SHALL sign the same Merkle Tree Hash with a fresh timestamp.

An STH is a TransItem structure of type signed_tree_head_v2, which encapsulates a SignedTreeHeadDataV2 structure:

    enum {
        reserved(65535)
    } SthExtensionType;

    struct {
        SthExtensionType sth_extension_type;
        opaque sth_extension_data<0..2^16-1>;
    } SthExtension;

    struct {
        LogID log_id;
        uint64 timestamp;
        uint64 tree_size;
        NodeHash root_hash;
        SthExtension sth_extensions<0..2^16-1>;
        digitally-signed struct {
            TransItem merkle_tree_head;
        } signature;
    } SignedTreeHeadDataV2;

log_id is this log's unique ID, encoded in an opaque vector as described in Section 5.3.

timestamp is equal to the timestamp from the TreeHeadDataV2 structure encapsulated in merkle_tree_head. This timestamp 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_size is equal to the tree size from the TreeHeadDataV2 structure encapsulated in merkle_tree_head.

root_hash is equal to the root hash from the TreeHeadDataV2 structure encapsulated in merkle_tree_head.

sth_extension_type identifies a single extension from the IANA registry in Section 12.5. At the time of writing, no extensions are specified.

The interpretation of the sth_extension_data field is determined solely by the value of the sth_extension_type field. Each document that registers a new sth_extension_type must describe how to interpret the corresponding sth_extension_data.

sth_extensions is a vector of 0 or more STH extensions. This vector MUST NOT include more than one extension with the same sth_extension_type. The extensions in the vector MUST be ordered by the value of the sth_extension_type field, smallest value first. If an implementation sees an extension that it does not understand, it SHOULD ignore that extension. Furthermore, an implementation MAY choose to ignore any extension(s) that it does understand.

merkle_tree_head is a TransItem structure that MUST be of type tree_head_v2 (see Section 5.7).

5.9. 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<1..2^8-1>;
    } ConsistencyProofDataV2;

log_id is this log's unique ID, encoded in an opaque vector as described in Section 5.3.

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.

5.10. 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<1..2^8-1>;
    } InclusionProofDataV2;

log_id is this log's unique ID, encoded in an opaque vector as described in Section 5.3.

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.

5.11. 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. To avoid that, the following actions are suggested:

6. Log Client Messages

Messages are sent as HTTPS GET or POST requests. Parameters for POSTs and all responses are encoded as JavaScript Object Notation (JSON) objects [RFC4627]. Parameters for GETs are encoded as order-independent key/value URL parameters, using the "application/x-www-form-urlencoded" format described in the "HTML 4.01 Specification" [HTML401]. Binary data is base64 encoded [RFC4648] as specified in the individual messages.

Note that JSON objects and URL parameters may contain fields not specified here. These extra fields should be ignored.

The <log server> prefix, which is part of the log's metadata, MAY include a path as well as a server name and a port.

In practice, log servers may include multiple front-end machines. Since it is impractical to keep these machines in perfect sync, errors may occur that are caused by skew between the machines. Where such errors are possible, the front-end will return additional information (as specified below) making it possible for clients to make progress, if progress is possible. Front-ends MUST only serve data that is free of gaps (that is, for example, no front-end will respond with an STH unless it is also able to prove consistency from all log entries logged within that STH).

For example, when a consistency proof between two STHs is requested, the front-end reached may not yet be aware of one or both STHs. In the case where it is unaware of both, it will return the latest STH it is aware of. Where it is aware of the first but not the second, it will return the latest STH it is aware of and a consistency proof from the first STH to the returned STH. The case where it knows the second but not the first should not arise (see the "no gaps" requirement above).

If the log is unable to process a client's request, it MUST return an HTTP response code of 4xx/5xx (see [RFC2616]), and, in place of the responses outlined in the subsections below, the body SHOULD be a JSON structure containing at least the following field:

error_message:
A human-readable string describing the error which prevented the log from processing the request.
In the case of a malformed request, the string SHOULD provide sufficient detail for the error to be rectified.
error_code:
An error code readable by the client. Some codes are generic and are detailed here. Others are detailed in the individual requests. Error codes are fixed text strings.
not compliant
The request is not compliant with this RFC.

e.g. In response to a request of /ct/v2/get-entries?start=100&end=99, the log would return a 400 Bad Request response code with a body similar to the following:

    {
        "error_message": "'start' cannot be greater than 'end'",
        "error_code": "not compliant",
    }

Clients SHOULD treat 500 Internal Server Error and 503 Service Unavailable responses as transient failures and MAY retry the same request without modification at a later date. Note that as per [RFC2616], 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.

6.1. Add Chain to Log

POST https://<log server>/ct/v2/add-chain

Inputs:
chain:
An array of base64 encoded certificates. The first element is the certificate for which the submitter desires an SCT; the second certifies the first and so on to the last, which either is, or is certified by, an accepted trust anchor.

Outputs:
sct:
A base64 encoded TransItem of type x509_sct_v2, signed by this log, that corresponds to the submitted certificate.

Error codes:
unknown anchor
The last certificate in the chain both is not, and is not certified by, an accepted trust anchor.
bad chain
The alleged chain is not actually a chain of certificates.
bad certificate
One or more certificates in the chain are not valid (e.g. not properly encoded).
shutdown
The log has ceased operation and is not accepting new submissions.

If the version of sct is not v2, then a v2 client may be unable to verify the signature. It MUST NOT construe this as an error. This is to avoid forcing an upgrade of compliant v2 clients that do not use the returned SCTs.

If a log detects bad encoding in a chain that otherwise verifies correctly then the log MUST either log the certificate or return the "bad certificate" error. If the certificate is logged, an SCT MUST be issued. Logging the certificate is useful, because monitors [monitor] can then detect these encoding errors, which may be accepted by some TLS clients.

6.2. Add PreCertChain to Log

POST https://<log server>/ct/v2/add-pre-chain

Inputs:
precertificate:
The base64 encoded precertificate.
chain:
An array of base64 encoded CA certificates. The first element is the signer of the precertificate; the second certifies the first and so on to the last, which either is, or is certified by, an accepted trust anchor.

Outputs:
sct:
A base64 encoded TransItem of type precert_sct_v2, signed by this log, that corresponds to the submitted precertificate.

Errors are the same as in Section 6.1.

6.3. Retrieve Latest Signed Tree Head

GET https://<log server>/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.

6.4. Retrieve Merkle Consistency Proof between Two Signed Tree Heads

GET https://<log server>/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.
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 STHs, which are signed.
Error codes:
first unknown
first is before the latest known STH but is not from an existing STH.
second unknown
second is before the latest known STH but is not from an existing STH.

See Section 10.4.2 for an outline of how to use the consistency output.

6.5. Retrieve Merkle Inclusion Proof from Log by Leaf Hash

GET https://<log server>/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 5.5. The tree_size must designate an existing v2 STH. Because of skew, the front-end may not know the requested STH. In that case, it will return the latest STH it knows, along with an inclusion proof to that STH. If the front-end knows the requested STH then only inclusion is returned.
Outputs:
inclusion:
A base64 encoded TransItem of type inclusion_proof_v2 whose inclusion_path array of Merkle Tree nodes proves the inclusion of the chosen certificate in the selected STH.
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:
hash unknown
hash is not the hash of a known leaf (may be caused by skew or by a known certificate not yet merged).
tree_size unknown
hash is before the latest known STH but is not from an existing STH.

See Section 10.4.1 for an outline of how to use the inclusion output.

6.6. Retrieve Merkle Inclusion Proof, Signed Tree Head and Consistency Proof by Leaf Hash

GET https://<log server>/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 5.5. The tree_size must designate an existing v2 STH.
Because of skew, the front-end may not know the requested STH or the requested hash, which leads to a number of cases.
latest STH < requested STH
Return latest STH.
latest STH > requested STH
Return latest STH and a consistency proof between it and the requested STH (see Section 6.4).
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 concatenation. 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 chosen certificate in the returned 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 STH and the returned STH.

Note that no signature is required for the inclusion or consistency outputs as they are used to verify inclusion in and consistency of STHs, which are signed.

Errors are the same as in Section 6.5.

See Section 10.4.1 for an outline of how to use the inclusion output, and see Section 10.4.2 for an outline of how to use the consistency output.

6.7. Retrieve Entries and STH from Log

GET https://<log server>/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
leaf_input:
The base64 encoded TransItem structure of type x509_entry_v2 or precert_entry_v2 (see Section 5.5).
log_entry:
The base64 encoded log entry (see Section 5.2). In the case of an x509_entry_v2 entry, this is the whole X509ChainEntry; and in the case of a precert_entry_v2, this is the whole PrecertChainEntryV2.
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 6.3.

The start parameter MUST be less than or equal to the end parameter.

Log servers MUST honor requests where 0 <= start < tree_size and end >= tree_size by returning a partial response covering only the valid entries in the specified range. end >= tree_size could be caused by skew. Note that the following restriction may also apply:

Logs MAY restrict the number of entries that can be retrieved per get-entries request. If a client requests more than the permitted number of entries, the log SHALL return the maximum number of entries permissible. These entries SHALL be sequential beginning with the entry specified by start.

Because of skew, it is possible the log server will not have any entries between start and end. In this case it MUST return an empty entries array.

In any case, the log server MUST return the latest STH it knows about.

See Section 10.4.3 for an outline of how to use a complete list of leaf_input entries to verify the root_hash.

6.8. Retrieve Accepted Trust Anchors

GET https://<log server>/ct/v2/get-anchors

No inputs.

Outputs:
certificates:
An array of base64 encoded trust anchors that are acceptable to the log.
max_chain:
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.

7. Optional Client Messages

Logs MAY implement these messages. They are not required for correct operation of logs or their clients, but may be convenient in some circumstances. Note that mirrors can implement these even if the log they are mirroring does not.

7.1. Get Entry Number for SCT

GET https://<log server>/ct/v2/get-entry-for-sct

Inputs:
sct:
A base64 encoded TransItem of type x509_sct_v2 or precert_sct_v2 signed by this log.

Outputs:
entry:
0-based index of the log entry corresponding to the supplied SCT.

Error codes:
bad signature
sct is not signed by this log.
not found
sct does not correspond to an entry that is currently available.

Note that any SCT signed by a log MUST have a corresponding entry in the log, but it may not be retrievable until the MMD has passed since the SCT was issued.

7.2. Get Entry Numbers for Certificate

GET https://<log server>/ct/v2/get-entry-for-certificate

Inputs:
hash:
A base64 encoded HASH of a TBSCertificate. Note that if the certificate has redacted labels then the TBSCertificate must be constructed as described above [domain_name_redaction].

Outputs:
entries:
An array of 0-based indices of log entries corresponding to the supplied HASH.

Error codes:
bad hash
hash is not the right size or format.
not found
sct does not correspond to an entry that is currently available.

Note that it is possible for a certificate to be logged more than once. If that is the case, the log MAY return more than one entry index. If the certificate is present in the log, then the log MUST return at least one entry index.

8. TLS Servers

TLS servers MUST use at least one of the three mechanisms listed below to present one or more SCTs from one or more logs to each TLS client during full TLS handshakes, where each SCT corresponds to the server certificate or to a name-constrained intermediate the server certificate chains to. TLS servers SHOULD also present corresponding inclusion proofs and STHs (see Section 8.3).

Three mechanisms are provided because they have different tradeoffs.

Additionally, a TLS server which supports presenting SCTs using an OCSP response MAY provide it when the TLS client included the status_request_v2 extension ([RFC6961]) in the (extended) ClientHello, but only in addition to at least one of the three mechanisms listed above.

8.1. Multiple SCTs

TLS servers SHOULD send SCTs from multiple logs in case one or more logs are not acceptable to the TLS client (for example, if a log has been struck off for misbehavior, has had a key compromise, or is not known to the TLS client). For example:

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.

8.2. 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).

8.3. Presenting SCTs, inclusion proofs and STHs

When constructing a TransItemList structure, a TLS server SHOULD construct and include TransItem structures of type x509_sct_with_proof_v2 (for an SCT of type x509_sct_v2) or precert_sct_with_proof_v2 (for an SCT of type precert_sct_v2), both of which encapsulate a SCTWithProofDataV2 structure:

    struct {
        SignedCertificateTimestampDataV2 sct;
        SignedTreeHeadDataV2 sth;
        InclusionProofDataV2 inclusion_proof;
    } SCTWithProofDataV2;

sct is the encapsulated data structure from an SCT that corresponds to the server certificate or to a name-constrained intermediate the server certificate chains to.

sth is the encapsulated data structure from an STH that was signed by the same log as sct.

inclusion_proof is the encapsulated data structure from an inclusion proof that corresponds to sct and can be used to compute the root in sth.

8.4. Presenting SCTs only

Presenting inclusion proofs and STHs in the TLS handshake helps to protect the client's privacy (see Section 10.2.5) and reduces load on log servers. However, if a TLS server is unable to obtain an inclusion proof and STH that correspond to an SCT, then it MUST include TransItem structures of type x509_sct_v2 or precert_sct_v2 in the TransItemList.

8.5. transparency_info TLS Extension

Provided that a TLS client includes the transparency_info extension type in the ClientHello, the TLS server SHOULD include the transparency_info extension in the ServerHello with extension_data set to a TransItemList. The TLS server SHOULD ignore any extension_data sent by the TLS client. Additionally, the TLS server MUST NOT process or include this extension when a TLS session is resumed, since session resumption uses the original session information.

9. Certification Authorities

9.1. Transparency Information X.509v3 Extension

The Transparency Information X.509v3 extension, which has OID 1.3.101.75 and SHOULD be non-critical, contains one or more TransItem structures in a TransItemList. This extension MAY be included in OCSP responses (see Section 9.1.1) and certificates (see Section 9.1.2). 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.

9.1.1. OCSP Response Extension

A certification authority MAY include a Transparency Information X.509v3 extension in the singleExtensions of a SingleResponse in an OCSP response. The included SCTs or inclusion proofs MUST be for the certificate identified by the certID of that SingleResponse, or for a precertificate that corresponds to that certificate, or for a name-constrained intermediate to which that certificate chains.

9.1.2. Certificate Extension

A certification authority MAY include a Transparency Information X.509v3 extension in a certificate. Any included SCTs or inclusion proofs MUST be either for a precertificate that corresponds to this certificate, or for a name-constrained intermediate to which this certificate chains.

9.2. TLS Feature Extension

A certification authority MAY include the transparency_info [tls_transinfo_extension] TLS extension identifier in the TLS Feature [RFC7633] certificate extension in root, intermediate and end-entity certificates. When a certificate chain includes such a certificate, this indicates that CT compliance is required.

10. Clients

There are various different functions clients of logs might perform. We describe here some typical clients and how they should function. Any inconsistency may be used as evidence that a log has not behaved correctly, and the signatures on the data structures prevent the log from denying that misbehavior.

All clients need various metadata in order to communicate with logs and verify their responses. This metadata is described below, but note that this document does not describe how the metadata is obtained, which is implementation dependent (see, for example, [Chromium.Policy]).

Clients should somehow exchange STHs they see, or make them available for scrutiny, in order to ensure that they all have a consistent view. The exact mechanisms will be in separate documents, but it is expected there will be a variety.

10.1. Metadata

In order to communicate with and verify a log, clients need metadata about the log.

Base URL:
The URL to substitute for <log server> in Section 6.
Hash Algorithm
The hash algorithm used for the Merkle Tree (see Section 12.2).
Signing Algorithm
The signing algorithm used (see Section 2.1.4).
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.
Version
The version of the protocol supported by the log (currently 1 or 2).
Maximum Chain Length
The longest chain submission the log is willing to accept, if the log chose to limit it.
STH Frequency Count
The maximum number of STHs the log may produce in any period equal to the Maximum Merge Delay (see Section 5.8).
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. The final STH should be provided in the form of a TransItem of type signed_tree_head_v2.

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

10.2. TLS Client

10.2.1. Receiving SCTs

TLS clients receive SCTs alongside or in certificates. TLS clients MUST implement all of the three mechanisms by which TLS servers may present SCTs (see Section 8). TLS clients MAY also accept SCTs via the status_request_v2 extension ([RFC6961]). TLS clients that support the transparency_info TLS extension SHOULD include it in ClientHello messages, with empty extension_data. TLS clients may also receive inclusion proofs in addition to SCTs, which should be checked once the SCTs are validated.

10.2.2. Reconstructing the TBSCertificate

To reconstruct the TBSCertificate component of a precertificate from a certificate, TLS clients should:

If the SCT checked is for a Precertificate (where the type of the TransItem is precert_sct_v2), then the client SHOULD also remove embedded v1 SCTs, identified by OID 1.3.6.1.4.1.11129.2.4.2 (See Section 3.3. of [RFC6962]), in the process of reconstructing the TBSCertificate. That is to allow embedded v1 and v2 SCTs to co-exist in a certificate (See Appendix A).

10.2.3. Verifying the redactedSubjectAltName extension

If the redactedSubjectAltName extension is present, TLS clients MUST check that the subjectAltName extension is present, that the subjectAltName extension contains the same number of entries as the redactedSubjectAltName extension, and that each entry in the subjectAltName extension has a matching entry at the same position in the redactedSubjectAltName extension. Two entries are matching if either:

If any of these checks fail, the certificate MUST NOT be considered compliant.

10.2.4. Validating SCTs

In addition to normal validation of the server certificate and its chain, TLS clients SHOULD validate each received SCT for which they have the corresponding log's metadata. To validate an SCT, a TLS client computes the signature input from the SCT data and the corresponding certificate, and then verifies the signature using the corresponding log's public key. TLS clients MUST NOT consider valid any SCT whose timestamp is in the future.

Before considering any SCT to be invalid, the TLS client MUST attempt to validate it against the server certificate and against each of the zero or more suitable name-constrained intermediates [name_constrained] in the chain. These certificates may be evaluated in the order they appear in the chain, or, indeed, in any order.

10.2.5. Validating inclusion proofs

After validating a received SCT, a TLS client MAY request a corresponding inclusion proof (if one is not already available) and then verify it. An inclusion proof can be requested directly from a log using get-proof-by-hash (Section 6.5) or get-all-by-hash (Section 6.6), but note that this will disclose to the log which TLS server the client has been communicating with.

Alternatively, if the TLS client has received an inclusion proof (and an STH) alongside the SCT, it can proceed to verifying the inclusion proof to the provided STH. The client then has to verify consistency between the provided STH and an STH it knows about, which is less sensitive from a privacy perspective.

TLS clients SHOULD also verify each received inclusion proof (see Section 10.4.1) for which they have the corresponding log's metadata, to audit the log and gain confidence that the certificate is logged.

If the TLS client holds an STH that predates the SCT, it MAY, in the process of auditing, request a new STH from the log (Section 6.3), then verify it by requesting a consistency proof (Section 6.4). Note that if the TLS client uses get-all-by-hash, then it will already have the new STH.

10.2.6. Evaluating compliance

To be considered compliant, a certificate MUST be accompanied by at least one valid SCT. A certificate not accompanied by any valid SCTs MUST NOT be considered compliant by TLS clients.

10.2.7. TLS Feature Extension

If any certificate in a chain includes the transparency_info [tls_transinfo_extension] TLS extension identifier in the TLS Feature [RFC7633] certificate extension, then CT compliance (using any of the mechanisms from Section 8) is required.

10.2.8. Handling of Non-compliance

If a TLS server presents a certificate chain that is non-compliant, and the use of a compliant certificate is mandated by an explicit security policy, application protocol specification, the TLS Feature extension or any other means, the TLS client MUST refuse the connection.

10.3. Monitor

Monitors watch logs to check that they behave correctly, for certificates of interest, or both. For example, a monitor may be configured to report on all certificates that apply to a specific domain name when fetching new entries for consistency validation.

A monitor needs to, at least, inspect every new entry in each log it watches. It may also want to keep copies of entire logs. In order to do this, it should follow these steps for each log:

  1. Fetch the current STH (Section 6.3).
  2. Verify the STH signature.
  3. Fetch all the entries in the tree corresponding to the STH (Section 6.7).
  4. Confirm that the tree made from the fetched entries produces the same hash as that in the STH.
  5. Fetch the current STH (Section 6.3). Repeat until the STH changes.
  6. Verify the STH signature.
  7. Fetch all the new entries in the tree corresponding to the STH (Section 6.7). If they remain unavailable for an extended period, then this should be viewed as misbehavior on the part of the log.
  8. 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 6.4).
    2. Verify the consistency proof.
    3. Verify that the new entries generate the corresponding elements in the consistency proof.

  9. Go to Step 5.

10.4. 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 log behaviour properties that should be checked:

A benign, conformant log publishes a series of STHs over time, each derived from the previous STH and the submitted entries incorporated into the log since publication of the previous STH. This can be proven through auditing of STHs. SCTs returned to TLS clients can be audited by verifying against the accompanying certificate, and using Merkle Inclusion Proofs, against the log's Merkle tree.

The action taken by the auditor if an audit fails is not specified, but note that in general if audit fails, the auditor is in possession of signed proof of the log's misbehavior.

A monitor [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 [tls_clients] 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 6.5). It can also verify that the SCT corresponds to the certificate it arrived with (i.e. the log entry is that certificate, is a precertificate for that certificate or is an appropriate name-constrained intermediate [see Section 4.3]).

Checking of the consistency of the log view presented to all entities is more difficult to perform because it requires a way to share log responses among a set of CT-aware entities, and is discussed in Section 13.4.

The following algorithm outlines may be useful for clients that wish to perform various audit operations.

10.4.1. Verifying an inclusion proof

When a client has received a TransItem of type inclusion_proof_v2 and wishes to verify inclusion of an input hash for an STH with a given tree_size and root_hash, the following algorithm may be used to prove the hash was included in the root_hash:

  1. Compare leaf_index against tree_size. If leaf_index is greater than or equal to tree_size fail the proof verification.
  2. Set fn to leaf_index and sn to tree_size - 1.
  3. Set r to hash.
  4. For each value p in the inclusion_path array:

    If LSB(fn) is set, or if fn is equal to sn, then:
    1. Set r to HASH(0x01 || p || r)
    2. 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:



    Finally, right-shift both fn and sn one time.
  5. 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.

10.4.2. Verifying consistency between two STHs

When a client has an STH first_hash for tree size first, an STH second_hash for tree size second where 0 < first < second, and has received a TransItem of type consistency_proof_v2 that they wish to use to verify both hashes, the following algorithm may be used:

  1. If first is an exact power of 2, then prepend first_hash to the consistency_path array.
  2. Set fn to first - 1 and sn to second - 1.
  3. If LSB(fn) is set, then right-shift both fn and sn equally until LSB(fn) is not set.
  4. Set both fr and sr to the first value in the consistency_path array.
  5. 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:
    1. Set fr to HASH(0x01 || c || fr)
      Set sr to HASH(0x01 || c || sr)
    2. 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:



    Finally, right-shift both fn and sn one time.
  6. 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.

10.4.3. Verifying root hash given entries

When a client has a complete list of leaf input entries from 0 up to tree_size - 1 and wishes to verify this list against an STH root_hash returned by the log for the same tree_size, the following algorithm may be used:

  1. Set stack to an empty stack.
  2. For each i from 0 up to tree_size - 1:
    1. Push HASH(0x00 || entries[i]) to stack.
    2. Set merge_count to the lowest value (0 included) such that LSB(i >> merge_count) is not set. In other words, set merge_count to the number of consecutive 1s found starting at the least significant bit of i.
    3. Repeat merge_count times:
      1. Pop right from stack.
      2. Pop left from stack.
      3. Push HASH(0x01 || left || right) to stack.

  3. If there is more than one element in the stack, repeat the same merge procedure (Step 2.3 above) until only a single element remains.
  4. 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.

11. Algorithm Agility

It is not possible for a log to change any of its algorithms part way through its lifetime:

Signature algorithm:
SCT signatures must remain valid so signature algorithms can only be added, not removed.
Hash algorithm:
A log would have to support the old and new hash algorithms to allow backwards-compatibility with clients that are not aware of a hash algorithm change.

Allowing multiple signature or hash algorithms for a log would require that all data structures support it and would significantly complicate client implementation, which is why it is not supported by this document.

If it should become necessary to deprecate an algorithm used by a live log, then the log should be frozen as specified in Section 10.1 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.

12. IANA Considerations

12.1. TLS Extension Type

IANA is asked to allocate an RFC 5246 ExtensionType value for the transparency_info TLS extension. IANA should update this extension type to point at this document.

12.2. Hash Algorithms

IANA is asked to establish a registry of hash values, initially consisting of:

Index Hash
0 SHA-256 [FIPS.180-4]

12.3. Signature Algorithms

IANA is asked to establish a registry of signature algorithm values, initially consisting of:

Index Signature Algorithm
0 deterministic ECDSA [RFC6979] using the NIST P-256 curve (Section D.1.2.3 of the Digital Signature Standard [DSS]) and HMAC-SHA256
1 RSA signatures (RSASSA-PKCS1-v1_5 with SHA-256, Section 8.2 of [RFC3447]) using a key of at least 2048 bits.

12.4. SCT Extensions

IANA is asked to establish a registry of SCT extensions, initially consisting of:

Type Extension
65535 reserved

TBD: policy for adding to the registry

12.5. STH Extensions

IANA is asked to establish a registry of STH extensions, initially consisting of:

Type Extension
65535 reserved

TBD: policy for adding to the registry

12.6. Object Identifiers

This document uses object identifiers (OIDs) to identify Log IDs (see Section 5.3), the precertificate CMS eContentType (see Section 3.2), and X.509v3 extensions in certificates (see Section 4.2.2, Section 4.3 and Section 9.1.2) and OCSP responses (see Section 9.1.1). The OIDs are defined in an arc that was selected due to its short encoding.

12.6.1. Log ID Registry 1

All OIDs in the range from 1.3.101.8192 to 1.3.101.16383 have been reserved. This is a limited resource of 8,192 OIDs, each of which has an encoded length of 4 octets.

IANA is requested to establish a registry that will allocate Log IDs from this range.

TBD: policy for adding to the registry. Perhaps "Expert Review"?

12.6.2. Log ID Registry 2

The 1.3.101.80 arc has been delegated. This is an unlimited resource, but only the 128 OIDs from 1.3.101.80.0 to 1.3.101.80.127 have an encoded length of only 4 octets.

IANA is requested to establish a registry that will allocate Log IDs from this arc.

TBD: policy for adding to the registry. Perhaps "Expert Review"?

13. Security Considerations

With CAs, logs, and servers performing the actions described here, TLS clients can use logs and signed timestamps to reduce the likelihood that they will accept misissued certificates. If a server presents a valid signed timestamp for a certificate, then the client knows that a log has committed to publishing the certificate. From this, the client knows that monitors acting for the subject of the certificate have had some time to notice the misissue and take some action, such as asking a CA to revoke a misissued certificate, or that the log has misbehaved, which will be discovered when the SCT is audited. A signed timestamp is not a guarantee that the certificate is not misissued, since appropriate monitors might not have checked the logs or the CA might have refused to revoke the certificate.

In addition, if TLS clients will not accept unlogged certificates, then site owners will have a greater incentive to submit certificates to logs, possibly with the assistance of their CA, increasing the overall transparency of the system.

[I-D.ietf-trans-threat-analysis] provides a more detailed threat analysis of the Certificate Transparency architecture.

13.1. 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. Thus, the maximum period of time during which a misissued certificate can be used without being available for audit is the MMD.

13.2. 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.

13.3. Avoiding Overly Redacting Domain Name Labels

Redaction of domain name labels carries the same risks as the use of wildcards (See Section 7.2 of [RFC6125], for example). If the entirety of the domain space below the unredacted part of a domain name is not registered by a single domain owner (e.g. REDACT(label).com, REDACT(label).co.uk and other public suffixes [Public.Suffix.List]), then the domain name may be considered by clients to be overly redacted.

CAs should take care to avoid overly redacting domain names in precertificates. It is expected that monitors will treat precertificates that contain overly redacted domain names as potentially misissued. TLS clients MAY consider a certificate to be non-compliant if the reconstructed TBSCertificate [reconstructing_tbscertificate] contains any overly redacted domain names.

13.4. Misbehaving Logs

A log can misbehave in several ways. Examples include failing to incorporate a certificate with an SCT in the Merkle Tree within the MMD, presenting different, conflicting views of the Merkle Tree at different times and/or to different parties and issuing STHs too frequently. Such misbehavior is detectable and the [I-D.ietf-trans-threat-analysis] provides more details on how this can be done.

Violation of the MMD contract is detected by log clients requesting a Merkle inclusion proof (Section 6.5) for each observed SCT. These checks can be asynchronous and need only be done once per each certificate. In order to protect the clients' privacy, these checks need not reveal the exact certificate to the log. Instead, clients can request the proof from a trusted auditor (since anyone can compute the proofs from the log) or communicate with the log via proxies.

Violation of the append-only property or the STH issuance rate limit can be detected by clients comparing their instances of the Signed Tree Heads. There are various ways this could be done, for example via gossip (see [I-D.ietf-trans-gossip]) or peer-to-peer communications or by sending STHs to monitors (who could then directly check against their own copy of the relevant log). A proof of misbehavior in such cases would be a series of STHs that were issued too closely together, proving violation of the STH issuance rate limit, or an STH with a root hash that does not match the one calculated from a copy of the log, proving violation of the append-only property.

13.5. Deterministic Signatures

Logs are required to use deterministic signatures for the following reasons:

13.6. Multiple SCTs

By offering multiple SCTs, each from a different log, TLS servers reduce the effectiveness of an attack where a CA and a log collude (see Section 8.1).

14. Privacy Considerations

14.1. Ensuring Effective Redaction

Although the domain name redaction mechanism (Section 4.2) removes the need for private labels to appear in logs, it does not guarantee that this will never happen. Anyone who encounters a certificate could choose to submit it to one or more logs, thereby rendering the redaction futile. Therefore, domain owners are advised to take the following steps to minimize the likelihood that their private labels will become known outside their closed communities:

CAs are advised to carefully consider each request to redact a label. When a CA believes that redacting a particular label would be futile, we advise rejecting the redaction request. TLS clients may have policies that forbid redaction, so redaction should only be used when it's absolutely necessary and likely to be effective.

15. Acknowledgements

The authors would like to thank Erwann Abelea, Robin Alden, Andrew Ayer, Al Cutter, Francis Dupont, Adam Eijdenberg, Stephen Farrell, Daniel Kahn Gillmor, Paul Hadfield, Brad Hill, Jeff Hodges, Paul Hoffman, Jeffrey Hutzelman, Kat Joyce, Stephen Kent, SM, Alexey Melnikov, Linus Nordberg, Chris Palmer, Trevor Perrin, Pierre Phaneuf, Melinda Shore, Ryan Sleevi, Martin Smith, Carl Wallace and Paul Wouters for their valuable contributions.

A big thank you to Symantec for kindly donating the OIDs from the 1.3.101 arc that are used in this document.

16. References

16.1. Normative References

[DSS] National Institute of Standards and Technology, "Digital Signature Standard (DSS)", FIPS 186-3, June 2009.
[FIPS.180-4] National Institute of Standards and Technology, "Secure Hash Standard", FIPS PUB 180-4, March 2012.
[HTML401] Raggett, D., Le Hors, A. and I. Jacobs, "HTML 4.01 Specification", World Wide Web Consortium Recommendation REC-html401-19991224, December 1999.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, DOI 10.17487/RFC2616, June 1999.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 2003.
[RFC4627] Crockford, D., "The application/json Media Type for JavaScript Object Notation (JSON)", RFC 4627, DOI 10.17487/RFC4627, July 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009.
[RFC5905] Mills, D., Martin, J., Burbank, J. and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., Galperin, S. and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 6960, DOI 10.17487/RFC6960, June 2013.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS) Multiple Certificate Status Request Extension", RFC 6961, DOI 10.17487/RFC6961, June 2013.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2013.
[RFC7633] Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS) Feature Extension", RFC 7633, DOI 10.17487/RFC7633, October 2015.

16.2. Informative References

[Chromium.Log.Policy] The Chromium Projects, "Chromium Certificate Transparency Log Policy", 2014.
[Chromium.Policy] The Chromium Projects, "Chromium Certificate Transparency", 2014.
[CrosbyWallach] Crosby, S. and D. Wallach, "Efficient Data Structures for Tamper-Evident Logging", Proceedings of the 18th USENIX Security Symposium, Montreal, August 2009.
[EVSSLGuidelines] CA/Browser Forum, "Guidelines For The Issuance And Management Of Extended Validation Certificates", 2007.
[I-D.ietf-trans-gossip] Nordberg, L., Gillmor, D. and T. Ritter, "Gossiping in CT", Internet-Draft draft-ietf-trans-gossip-03, July 2016.
[I-D.ietf-trans-threat-analysis] Kent, S., "Attack Model and Threat for Certificate Transparency", Internet-Draft draft-ietf-trans-threat-analysis-06, May 2016.
[JSON.Metadata] The Chromium Projects, "Chromium Log Metadata JSON Schema", 2014.
[Public.Suffix.List] Mozilla Foundation, "Public Suffix List", 2016.
[RFC6962] Laurie, B., Langley, A. and E. Kasper, "Certificate Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013.

Appendix A. Supporting v1 and v2 simultaneously

Certificate Transparency logs have to be either v1 (conforming to [RFC6962]) or v2 (conforming to this document), as the data structures are incompatible and so a v2 log could not issue a valid v1 SCT.

CT clients, however, can support v1 and v2 SCTs, for the same certificate, simultaneously, as v1 SCTs are delivered in different TLS, X.509 and OCSP extensions than v2 SCTs.

v1 and v2 SCTs for X.509 certificates can be validated independently. For precertificates, v2 SCTs should be embedded in the TBSCertificate before submission of the TBSCertificate (inside a v1 precertificate, as described in Section 3.1. of [RFC6962]) to a v1 log so that TLS clients conforming to [RFC6962] but not this document are oblivious to the embedded v2 SCTs. An issuer can follow these steps to produce an X.509 certificate with embedded v1 and v2 SCTs:

Authors' Addresses

Ben Laurie Google UK Ltd. EMail: benl@google.com
Adam Langley Google Inc. EMail: agl@google.com
Emilia Kasper Google Switzerland GmbH EMail: ekasper@google.com
Eran Messeri Google UK Ltd. EMail: eranm@google.com
Rob Stradling Comodo CA, Ltd. EMail: rob.stradling@comodo.com