TLS Trust Anchor Identifiers

2.1. Terminology and Roles

This document discusses three roles:¶

Authenticating party:

The party authenticating itself in the protocol. In TLS, this is the side sending the Certificate and CertificateVerify message.¶

Relying party:

The party whom the authenticating party presents its identity to. In TLS, this is the side that validates a Certificate and CertificateVerify message.¶

Certification authority (CA):

The service issuing certificates to the authenticating party.¶

Additionally, there are several terms used throughout this document to describe this proposal:¶

Trust anchor:

A pre-distributed X.509 name and public key that relying parties use to determine whether a certification path is trusted. See Section 6.1.1 of [RFC5280]. Trust anchors are sometimes configured as self-signed certificates.¶

Certification path:

An ordered list of X.509 certificates starting with the target certificate. Each certificate is issued by the next certificate, except the last, which is issued by a trust anchor.¶

3.1. Relying Party Configuration

Relying parties are configured with one or more supported trust anchors. Each trust anchor that participates in this protocol must have an associated trust anchor identifier.¶

When trust anchors are represented as X.509 certificates, the X.509 trust anchor identifier extension MAY be used to carry this identifier. The trust anchor identifier extension has an extnID of id-trustAnchorIdentifier and an extnValue containing a DER-encoded TrustAnchorIdentifier structure, defined below. The TrustAnchorIdentifier is the trust anchor identifier's ASN.1 representation, described in Section 3. This extension MUST be non-critical.¶

id-trustAnchorIdentifier OBJECT IDENTIFIER ::= { TBD }

TrustAnchorIdentifier ::= RELATIVE-OID

Relying parties MAY instead or additionally configure trust anchor identifiers via some application-specific out-of-band information.¶

Relying parties MAY support trust anchors without associated trust anchor identifiers, but such trust anchors will not participate in this protocol. Those trust anchors MAY participate in other trust anchor negotiation protocols, such as the certificate_authorities extension.¶

3.2. Authenticating Party Configuration

Authenticating parties are configured with one or more candidate certification paths to present in TLS, in some preference order. This preference order is used when multiple candidate paths are usable for a connection. For example, the authenticating party may prefer candidates that minimize message size or have more performant private keys.¶

Each candidate path which participates in this protocol must be configured with the trust anchor identifier for its corresponding trust anchor. It is RECOMMENDED, though not required, that this information come from the CA. Section 6 defines a RECOMMENDED format for this information, along with an optional ACME [RFC8555] extension for CAs to send it.¶

Authenticating parties MAY have candidate certification paths without associated trust anchor identifiers, but such paths will not participate in this protocol. Those paths MAY participate in other trust anchor negotiation protocols, such as the certificate_authorities extension.¶

4.1. Overview

The trust_anchors extension is defined using the structures below:¶

enum { trust_anchors(TBD), (2^16-1) } ExtensionType;

opaque TrustAnchorIdentifier<1..2^8-1>;

TrustAnchorIdentifier TrustAnchorIdentifierList<0..2^16-1>;

When the extension is sent in the ClientHello or CertificateRequest messages, the extension_data is a TrustAnchorIdentifierList and indicates that the sender supports the specified trust anchors. The list is unordered, and MAY be empty. Each TrustAnchorIdentifier uses the binary representation, as described in Section 3.¶

When the extension is sent in EncryptedExtensions, the extension_data is a TrustAnchorIdentifierList containing the list of trust anchors that server has available, in the server's preference order, and MUST NOT be empty.¶

When the extension is sent in Certificate, the extension_data MUST be empty and indicates that the sender sent the certificate because the certificate matched a trust anchor identifier sent by the peer. When used in this form, the extension may only be sent in the first CertificateEntry. It MUST NOT be sent in subsequent ones.¶

4.2. Certificate Selection

A trust_anchors extension in the ClientHello or CertificateRequest is processed similarly to the certificate_authorities extension. The relying party indicates some set of supported trust anchors in the ClientHello or CertificateRequest trust_anchors extension. The authenticating party then selects a certificate from its candidate certification paths (see Section 3.2), as described in Section 4.4.2.2 of [RFC8446] and Section 4.4.2.3 of [RFC8446]. This process is extended as follows:¶

If the ClientHello or CertificateRequest contains a trust_anchors extension, the authenticating party SHOULD send a certification path whose trust anchor identifier appears in the relying party's trust_anchors extension.¶

If the ClientHello or CertificateRequest contains both trust_anchors and certificate_authorities, certification paths that satisfy either extension's criteria may be used. This additionally applies to future extensions which play a similar role.¶

If no certification paths satisfy either extension, the authenticating party MAY return a handshake_failure alert, or choose among fallback certification paths without considering trust_anchors or certification_authorities. See Section 4.3 for additional guidance on selecting a fallback when the ClientHello contains trust_anchors.¶

Sending a fallback allows the authenticating party to retain support for relying parties that do not implement any form of trust anchor negotiation. In this case, the authenticating party must find a sufficiently ubiquitous trust anchor, if one exists. However, only those relying parties need to be considered in this ubiquity determination. Updated relying parties may continue to evolve without restricting fallback certificate selection.¶

If the authenticating party sends a certification path that matches the relying party's trust_anchors extension, as described in Section 4.2, the authenticating party MUST send an empty trust_anchors extension in the first CertificateEntry of the Certificate message. In this case, the certificate_list flexibility described in Section 4.4.2 of [RFC8446] no longer applies. The certificate_list MUST contain a complete certification path, issued by the matching trust anchor, correctly ordered and with no extraneous certificates. That is, each certificate MUST certify the one immediately preceding it, and the trust anchor MUST certify the final certificate. The authenticating party MUST NOT send the trust_anchors extension in the Certificate message in other situations.¶

If a relying party receives this extension in the Certificate message, it MAY choose to disable path building [RFC4158] and validate the peer's certificate list as pre-built certification path. Doing so avoids the unpredictable behavior of path-building, and helps ensure CAs and authenticating parties do not inadvertently provision incorrect paths.¶

4.3. Retry Mechanism

When the relying party is a client, it may choose not to send its full trust anchor identifier list due to fingerprinting risks (see Section 8), or because the list is too large. The client MAY send a subset of supported trust anchors, or an empty list. This subset may be determined by, possibly outdated, prior knowledge about the server, such as Section 5 or past connections.¶

To accommodate this, when receiving a ClientHello with trust_anchors, the server collects all candidate certification paths which:¶

• Have a trust anchor identifier, and¶

• Satisfy the conditions in Section 4.4.2.2 of [RFC8446], with the exception of certification_authorities, and any future extensions that play a similar role¶

If this collection is non-empty, the server sends a trust_anchors extension in EncryptedExtensions, containing the corresponding trust anchor identifiers in preference order.¶

When a client sends a subset or empty list in trust_anchors, it SHOULD implement the following retry mechanism:¶

If the client receives either a connection error or an untrusted certificate, the client looks in server's EncryptedExtensions for a trust anchor identifier that it trusts. If there are multiple, it selects an option based on the server's preference order and its local preferences. It then makes a new connection to the same endpoint, sending only the selected trust anchor identifier in the ClientHello trust_anchors extension. If the EncryptedExtensions had no trust_anchor extension, or no match was found, the client returns the error to the application.¶

Clients SHOULD retry at most once per connection attempt.¶

[[TODO: Retrying in a new connection is expensive and cannot be done from within the TLS stack in most implementations. Consider handshake modifications to instead retry within the same connection. https://github.com/davidben/tls-trust-expressions/issues/53 ]]¶

This mechanism allows the connection to recover from a certificate selection failure, e.g. due to the client not revealing its full preference list, at additional latency cost. Section 5 describes an optimization which can avoid this cost.¶

This mechanism also allows servers to safely send fallback certificates that may not be as ubiquitously acceptable. Without some form of trust anchor negotiation, servers are limited to selecting certification paths that are ubiquitously trusted in all supported clients. This often means sending extra cross-certificates to target the lowest common denominator at a bandwidth cost. If the ClientHello contains trust_anchors, the server MAY opportunistically send a less ubiquitous, more bandwidth-efficient path based on local heuristics, with the expectation that the client will retry when the heuristics fail.¶

5.1. Syntax

The tls-trust-anchors parameter contains an ordered list of one or more trust anchor identifiers, in server preference order.¶

The presentation value of the SvcParamValue is a non-empty comma-separated list (Appendix A.1 of [RFC9460]). Each element of the list is a trust anchor identifier in the ASCII representation defined in Section 3. Any other value is a syntax error. To enable simpler parsing, this SvcParam MUST NOT contain escape sequences.¶

The wire format of the SvcParamValue is determined by prefixing each trust anchor identifier with its length as a single octet, then concatenating each of these length-value pairs to form the SvcParamValue. These pairs MUST exactly fill the SvcParamValue; otherwise, the SvcParamValue is malformed.¶

For example, if a TLS server has three available certification paths issued by 32473.1, 32473.2.1, and 32473.2.2, respectively, the DNS record in presentation syntax may be:¶

example.net.  7200  IN SVCB 3 server.example.net. (
    tls-trust-anchors=32473.1,32473.2.1,32473.2.2 )

The wire format of the SvcParamValue would be the 17 octets below. In the example, the octets comprising each trust anchor identifier are placed on separate lines for clarity¶

0x04, 0x81, 0xfd, 0x59, 0x01,
0x05, 0x81, 0xfd, 0x59, 0x02, 0x01,
0x05, 0x81, 0xfd, 0x59, 0x02, 0x02,

5.2. Configuring Services

Services SHOULD include the trust anchor identifier for each of their available certification paths, in preference order, in the tls-trust-anchors of their HTTPS or SVCB endpoints. As TLS configuration is updated, services SHOULD update the DNS record to match. The mechanism for this is out of scope for this document, but services are RECOMMENDED to automate this process.¶

Services MAY have certification paths without trust anchor identifiers, but those paths will not participate in this mechanism.¶

5.3. Client Behavior

When connecting to a service endpoint whose HTTPS or SVCB record contains the tls-trust-anchors parameter, the client first computes the intersection between its configured trust anchors and the server's provided list. If this intersection is non-empty, the client MAY use it to determine the trust_anchors extension in the ClientHello (see Section 4.3).¶

If doing so, the client MAY send a subset of this intersection to meet size constraints, but SHOULD offer multiple options. This reduces the chance of a reconnection if, for example, the first option in the intersection uses a signature algorithm that the client doesn't support, or if the TLS server and DNS configuration are out of sync.¶

Although this service parameter is intended to reduce trust anchor mismatches, mismatches may still occur in some scenarios. Clients and servers MUST continue to implement the provisions described in Section 4.3, even when using this service parameter.¶

6.1. Media Type

A certification path with its associated CertificatePropertyList may be represented in a PEM [RFC7468] structure in a file of type "application/pem-certificate-chain-with-properties". Files of this type MUST use the strict encoding and MUST NOT include explanatory text. The ABNF [RFC5234] for this format is as follows, where "stricttextualmsg" and "eol" are as defined in Section 3 of [RFC7468]:¶

certchainwithproperties = stricttextualmsg eol stricttextualmsg
                          *(eol stricttextualmsg)

The first element MUST be the encoded CertificatePropertyList. The second element MUST be an end-entity certificate. Each following certificate MUST directly certify the one preceding it. The certificate representing the trust anchor MUST be omitted from the path.¶

CertificatePropertyLists are encoded using the "CERTIFICATE PROPERTIES" label. The encoded data is a serialized CertificatePropertyList, defined in Section 6.¶

Certificates are encoded as in Section 5.1 of [RFC7468], except DER [X690] MUST be used.¶

The following is an example file with a certification path containing an end-entity certificate and an intermediate certificate.¶

-----BEGIN CERTIFICATE PROPERTIES-----
TODO fill in an example
-----END CERTIFICATE PROPERTIES-----
-----BEGIN CERTIFICATE-----
TODO fill in an example
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
TODO fill in an example
-----END CERTIFICATE-----

The IANA registration for this media type is described in Section 10.2.¶

6.2. ACME Extension

The format defined in Section 6.1 can be used with ACME's alternate format mechanism (see Section 7.4.2 of [RFC8555]) as follows. When downloading certificates, a supporting client SHOULD include "application/pem-certificate-chain-with-properties" in its HTTP Accept header (Section 12.5.1 of [RFC9110]). When a supporting server sees such a header, it MAY then respond with that format to include a CertificatePropertyList with the certification path. This CertificatePropertyList MAY include a trust_anchor_identifier property for use with this protocol, or other properties defined in another document.¶

When used with ACME's alternate certificate chain mechanism (see Section 7.4.2 of [RFC8555]), this protocol removes the need for heuristics in determining which path to serve to which relying party.¶

The authenticating party MAY combine the resulting certification paths with those from other ACME orders, or other sources, for a complete set of candidate paths to serve.¶

7.1. Making Use of Newly-Trusted CAs

When one relying party trusts a new CA, other relying parties, such as older ones, may not yet trust it. Trust anchor negotiation allows an authenticating party to negotiate a certificate from the newer CA with relying parties that do trust it, while continuing to negotiate another certificate with relying parties that do not. This allows PKI transitions to progress smoothly. Connections can make use of, for example, a new CA's stronger signature algorithms, stronger validation practices, better automation, or more efficient certificate sizes, without interruptions to other connections.¶

Without negotiation, the authenticating party is limited to its relying parties' intersection and must wait for every supported relying party to be updated before the transition even begins. This wait could often take many years. In some cases, such as with IoT devices, relying parties may never receive updates.¶

In some contexts, other fields can provide a partial signal. For example, post-quantum-capable relying parties may be detected with the signature_algorithms and signature_algorithms_cert extensions. However, this relies on all post-quantum CAs being added at roughly the same time and that they are sufficiently interchangeable to be negotiated with these extensions. Trust anchor negotiation directly addresses this problem and allows for both gradual and possibly heterogeneous deployment of post-quantum CAs across relying parties.¶

7.2. Removing Untrustworthy CAs

When CAs are determined to be untrustworthy, relying parties must remove them to mitigate the risk to user security. Over time, this shrinks their intersection with older relying parties. Without negotiation, the result is authenticating parties have fewer and fewer CA choices available. Even determining the intersecting CAs can be difficult. Often, the only option is to try the new certificate and monitor errors. For authenticating parties that serve many diverse relying parties, this is a disruptive and risky process.¶

Trust anchor negotiation removes this constraint. If an authenticating party's CA is distrusted, it can use a new CA in addition to the existing one. The addition does not risk outages for older relying parties and may be chosen from a wider set of CAs, as it only needs to be compatible with the relying parties that distrusted the other CA.¶

Over time, the authenticating party can monitor which certificates it serves, and re-evaluate which CA or CAs to use. For example, it may find the new CA was sufficient, or that older relying parties have since all been updated. However, user security depends on the relying party's trust anchors, not the authenticating party's choice of CA, so this can occur asynchronously, based on serving needs and costs, rather than delay the response to a security incident.¶

7.3. Key Rotation

Despite the severity of root CA private key compromise and the benefits of routinely rotating cryptographic key material, such rotation in PKIs is often very rare. In 2023, the oldest root in [CHROME-ROOTS] and [MOZILLA-ROOTS] was 25 years old, dating to 1998.¶

Key rotation in PKIs used in TLS is challenging, as it combines the challenges described in both Section 7.1 and Section 7.2. Without trust anchor negotiation, authenticating parties cannot switch to the new root as long as any supported older relying party requires the old root. That, in turn, means relying parties cannot distrust the old root, leaving them vulnerable.¶

Trust anchor negotiation offers a smooth transition for CA key rotation. The CA can provide certification paths for the old and new roots. The authenticating party can then serve both paths without impacting older relying parties. New relying parties can then distrust the old root.¶

7.4. Other Root Transitions

The mechanisms in this document can aid PKI transitions beyond key rotation. For example, a CA operator may generate a postquantum root CA and issue from the classical and postquantum roots concurrently. The authenticating party will then, transparently and with no configuration change, serve both. As in Section 7.3, newer relying parties can then remove the classical roots, while older relying parties continue to function.¶

This same procedure may also be used to transition between newer, more size-efficient signature algorithms, as they are developed.¶

7.5. Intermediate Elision

In my PKIs, root CAs issue shorter-lived intermediate certificates which, in turn, issue end-entity certificates. This comes at a bandwidth cost: the TLS handshake includes an extra certificate, which includes a public key, signature, and X.509 metadata. Post-quantum signature algorithms will dramatically increase this cost. ML-DSA-65 [FIPS204], for example, has a total public key and signature size of 5,261 bytes.¶

[I-D.ietf-tls-cert-abridge] predistributes a specific set of intermediate CA certificates to relying parties so that these certificates can be omitted from TLS connections, as a compression scheme. Negotiating intermediate CAs as short-lived trust anchors also achieves this effect, but is usable by more relying parties than the specific intermediate set accommodates.¶

In this model, a CA operator provides authenticating parties with two certification paths: a longer path ending at a long-lived root and shorter path the other ending at a short-lived root. Relying parties trust both the long-lived root and the most recent short-lived root. The authenticating party sends the shorter path when possible, falling back to the longer path when the relying party’s short-lived root is stale.¶

This achieves the same effect with a simpler and more flexible, general-purpose mechanism.¶

7.6. Conflicting Relying Party Requirements

An authenticating party may need to support relying parties with different, potentially conflicting requirements. For example, in contexts where online revocation checks are expensive, unreliable, or privacy-sensitive, user security is best served by short-lived certificates. In other contexts, long-lived certificates may be more appropriate for, e.g., systems that are offline for long periods of time or have unreliable clocks.¶

Trust anchor negotiation allows these conflicts to be resolved by different trust anchors where necessary. This avoids the need to compromise on user security or service availability.¶

7.7. Backup Certificates

An authenticating party may obtain certificate paths from multiple CAs for redundancy. If one CA is compromised and removed from newer relying parties, the TLS server software will be able to gracefully serve a backup certificate path, avoiding the immediate breakage that would otherwise be caused by this removal.¶

7.8. Public Key Pinning

To reduce security risk from misissued certificates, relying parties sometimes employ public key pinning [RFC7469]. Pinning effectively reduces a relying party's trust anchor list to a subset of the original set.¶

As other relying parties in the PKI evolve, the pinning relying party limits the authenticating party to satisfy both the pinning constraint and newer constraints in the PKI. This can lead to conflicts if, for example, the pinned CA is distrusted by a newer relying party. The authenticating party is then forced to either break the pinning relying party, or break the newer ones.¶

Trust anchor negotiation reduces this conflict, provided the pinning relying party negotiates with its reduced trust anchor list. The authenticating party can then use a certificate from the pinned CA with the pinning relying party, and another CA with other relying parties.¶

8.1. Relying Parties

The trust_anchors extension is analogous to the certificate_authorities extension (Section 4.2.4 of [RFC8446]), but more size-efficient. Like certificate_authorities, trust_anchors reveals some information about the relying party's trust anchors. However, unlike certificate_authorities, trust_anchors allows a relying party to only reveal a trust anchor in response to the authenticating party's list, which reduces the fingerprinting exposure. This section provides guidance for a relying party to configure this mechanism, based on its privacy goals.¶

When using this extension, a relying party's trust anchors may be divided into three categories:¶

1. Trust anchors whose identifiers the relying party never sends, but still trusts. These are trust anchors that do not participate in this mechanism.¶

2. Trust anchors whose identifiers the relying party sends conditionally, i.e. only if the server offers them. For example, the relying party may indicate support for a trust anchor if its identifier is listed in the server's HTTPS/SVCB record or trust anchor list in EncryptedExtensions.¶

3. Trust anchors whose identifiers the relying party sends unconditionally, i.e. independently of the authenticating party's behavior.¶

Each of these categories carries a different fingerprinting exposure:¶

Trust anchors that do not participate are not revealed by this extension. However, they have some fingerprinting exposure due to being trusted. Given a certification path, an authenticating party can probe whether the relying party trusts the trust anchor by seeing if the relying party accepts it.¶

Trust anchor identifiers sent in response to the authenticating party can only be observed actively. That is, the authenticating party could vary its list and observe how the client responds, in order to probe for the client's trust anchor list. This is similar to the exposure of trust anchors not participating in this extension, except that the trust anchor can be probed by only knowing the trust anchor identifier.¶

Trust anchor identifiers sent unconditionally can be observed passively. This mode is analogous to the certificate_authorities extension. Relying parties SHOULD NOT unconditionally advertise trust anchor lists that are unique to an individual user. Rather, unconditionally-advertised lists SHOULD be empty or computed only from the trust anchors common to the relying party's anonymity set (Section 3.3 of [RFC6973]).¶

Relying parties SHOULD determine which trust anchors participate in this mechanism, and whether to advertise them unconditionally or conditionally, based on their privacy goals. PKIs that reliably use the DNS service parameter (Section 5) can rely on conditional advertisement for stronger privacy properties without a round-trip penalty.¶

Additionally, a relying party that computes the trust_anchors extension based on prior state may allow observers to correlate across connections. Relying parties SHOULD NOT maintain such state across connections that are intended to be uncorrelated. As above, implementing the DNS service parameter can avoid a round-trip penalty without such state.¶

8.2. Authenticating Parties

If the authenticating party is a server, the mechanisms in Section 5 and Section 4.3 enumerate the trust anchors for the server's available certification paths. This mechanism assumes they are not sensitive. Servers SHOULD NOT use this mechanism to negotiate certification paths with sensitive trust anchors.¶

In servers that host multiple services, this protocol only enumerates certification paths for the requested service. If, for example, a server uses the server_name extension to select services, the addition to EncryptedExtensions in Section 4.3 is expected to be filtered by server_name. Likewise, the DNS parameter in Section 5 only contains information for the corresponding service. In both cases, co-located services are not revealed.¶

The above does not apply if the authenticating party is a client. This protocol does not enumerate the available certification paths for a client.¶

9.1. Incorrect Selection Metadata

If the authenticating party has provisioned certification paths with incorrect trust anchor identifiers, it may negotiate inaccurately and send an untrusted path to the relying party when another candidate would have been trusted. This will not result in the untrusted path becoming trusted, but the connection will fail.¶

9.2. Trust Anchor Negotiation

Both the trust_anchors and certificate_authorities (Section 4.2.4 of [RFC8446]) extensions implement trust anchor negotiation, so security considerations are largely unchanged from certificate_authorities. This section discusses security considerations for trust anchor negotiation in general.¶

10.1. TLS ExtensionType Updates

IANA is requested to create the following entry in the TLS ExtensionType Values registry, defined by [RFC8446]:¶

10.2. Media Type Updates

IANA is requested to create the following entry in the "Media Types" registry, defined in [RFC6838]:¶

Type name:

application¶

Subtype name:

pem-certificate-chain-with-properties¶

Required parameters:

None¶

Optional parameters:

None¶

Encoding considerations:

7bit¶

Security considerations:

Carries a cryptographic certificate and its associated certificate chain and additional properties. This media type carries no active content.¶

Interoperability considerations:

None¶

Published specification:

[this-RFC, Section 6.1]¶

Applications that use this media type:

ACME clients and servers, HTTP servers, other applications that need to be configured with a certificate chain¶

Additional information:

Deprecated alias names for this type:

n/a¶

Magic number(s):

n/a¶

File extension(s):

.pem¶

Macintosh file type code(s):

n/a¶

Person & email address to contact for further information:

See Authors' Addresses section.¶

Intended usage:

COMMON¶

Restrictions on usage:

n/a¶

Author:

See Authors' Addresses section.¶

Change controller:

IETF¶

10.3. CertificatePropertyType Registry

[[TODO: Establish a CertificatePropertyType registry.]]¶

11.1. Normative References

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.

[RFC4158]

Cooper, M., Dzambasow, Y., Hesse, P., Joseph, S., and R. Nicholas, "Internet X.509 Public Key Infrastructure: Certification Path Building", RFC 4158, DOI 10.17487/RFC4158, September 2005, <https://www.rfc-editor.org/rfc/rfc4158>.

[RFC5234]

Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <https://www.rfc-editor.org/rfc/rfc5234>.

[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, <https://www.rfc-editor.org/rfc/rfc5280>.

[RFC6838]

Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013, <https://www.rfc-editor.org/rfc/rfc6838>.

[RFC6973]

Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., Morris, J., Hansen, M., and R. Smith, "Privacy Considerations for Internet Protocols", RFC 6973, DOI 10.17487/RFC6973, July 2013, <https://www.rfc-editor.org/rfc/rfc6973>.

[RFC7468]

Josefsson, S. and S. Leonard, "Textual Encodings of PKIX, PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468, April 2015, <https://www.rfc-editor.org/rfc/rfc7468>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

[RFC8446]

Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/rfc/rfc8446>.

[RFC8555]

Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, <https://www.rfc-editor.org/rfc/rfc8555>.

[RFC9110]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, <https://www.rfc-editor.org/rfc/rfc9110>.

[RFC9371]

Baber, A. and P. Hoffman, "Registration Procedures for Private Enterprise Numbers (PENs)", RFC 9371, DOI 10.17487/RFC9371, March 2023, <https://www.rfc-editor.org/rfc/rfc9371>.

[RFC9460]

Schwartz, B., Bishop, M., and E. Nygren, "Service Binding and Parameter Specification via the DNS (SVCB and HTTPS Resource Records)", RFC 9460, DOI 10.17487/RFC9460, November 2023, <https://www.rfc-editor.org/rfc/rfc9460>.

[X680]

ITU-T, "Information technology - Abstract Syntax Notation One (ASN.1): Specification of basic notation", ISO/IEC 8824-1:2021, 2021.

[X690]

ITU-T, "Information technology - ASN.1 encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ISO/IEC 8825-1:2021, 2021.

11.2. Informative References

[CHROME-ROOTS]

Chromium, "Chrome Root Store", 30 August 2023, <https://chromium.googlesource.com/chromium/src/+/main/net/data/ssl/chrome_root_store>.

[FIPS204]

National Institute of Standards and Technology (NIST), "Module-Lattice-based Digital Signature Standard", FIPS PUB 204, August 2023, <https://csrc.nist.gov/projects/post-quantum-cryptography>.

[I-D.ietf-tls-cert-abridge]

Jackson, D., "Abridged Compression for WebPKI Certificates", Work in Progress, Internet-Draft, draft-ietf-tls-cert-abridge-02, 16 September 2024, <https://datatracker.ietf.org/doc/html/draft-ietf-tls-cert-abridge-02>.

[MOZILLA-ROOTS]

Mozilla, "Mozilla Included CA Certificate List", 30 August 2023, <https://wiki.mozilla.org/CA/Included_Certificates>.

[RFC6962]

Laurie, B., Langley, A., and E. Kasper, "Certificate Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, <https://www.rfc-editor.org/rfc/rfc6962>.

[RFC7469]

Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April 2015, <https://www.rfc-editor.org/rfc/rfc7469>.