TLS Trust Anchor Identifiers

2.1. Terminology and Roles

This document discusses three roles:¶

Subscriber:

The party whose identity is being validated in the protocol. In TLS, this is the side sending the Certificate and CertificateVerify message.¶

Relying party:

The party authenticating the subscriber. In TLS, this is the side that validates a Certificate and CertificateVerify message.¶

Certification authority (CA):

The service issuing certificates to the subscriber.¶

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

Trust anchor:

A pre-distributed public key or certificate that relying parties use to determine whether a certification path is trusted.¶

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 which participates in this protocol must have an associated trust anchor identifier.¶

When trust anchors are self-signed 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 a relative object identifier, determined as 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. Subscriber Configuration

Subscribers 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 subscriber may prefer candidates that minimize message size or have more performant private keys.¶

Each candidate path which participates in this protocol must be provisioned with the trust anchor identifier for its corresponding trust anchor.¶

To carry this information, this document defines the trust_anchor_identifier property for a CertificatePropertyList Section 5 of [I-D.davidben-tls-trust-expr]. This data field of the property contains the binary representation of the trust anchor identifier of the certification path’s trust anchor, as described in Section 3.¶

enum {
    trust_anchor_identifier(TBD), (2^16-1)
} CertificatePropertyType;

Subscribers 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.¶

[[TODO: Move the definition of CertificatePropertyList from the trust expressions draft to here, if this ends up being the canonical one. When doing so, also import media type definition.]]¶

4.1. Overview

The trust_anchors extension is sent in ClientHello, CertificateRequest, or Certificate messages, 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.¶

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 subscriber 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 subscriber 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 subscriber 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 subscriber to retain support for relying parties that do not implement any form of trust anchor negotiation. In this case, the subscriber 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 subscriber sends a certification path that matches the relying party’strust_anchors extension, as described in Section 4.2, the subscriber 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 subscriber 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 subscribers 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.]]¶

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 text 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,

## 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.¶

5.2. 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, even when using this service parameter.¶

8.1. Relying Parties

The negotiation mechanism described in this document is analogous to the certificate_authorities extension (Section 4.2.4 of [RFC8446]), but more size-efficient. Like that extension, it presumes the requested trust anchor list is not sensitive.¶

The privacy implications of this are determined by how a relying party uses this extension. Trust anchors supported by a relying party may be divided into three categories:¶

1. Trust anchors whose identifiers the relying party sends unconditionally, i.e. not in response to the server’s HTTPS/SVCB record, trust anchor list in EncryptedExtensions, etc.¶

2. Trust anchors whose identifiers the relying party sends 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 that the relying party accepts but will never send a corresponding trust anchor identifier in trust_anchors. These are trust anchors which do not participate in this mechanism.¶

Each of these categories carries a different fingerprinting exposure:¶

Trust anchor identifiers sent unconditionally can be observed passively. Thus, relying parties SHOULD NOT unconditionally advertise trust anchor lists which are unique to an individual user. Rather, such lists SHOULD be either empty or computed only from the trust anchors common to the relying party's anonymity set (Section 3.3 of [RFC6973]).¶

Trust anchor identifiers sent in response to the subscriber can only be observed actively. That is, the subscriber 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 baseline exposure for trust anchors that do not participate in negotiation. A subscriber in possession of a certificate can send it and determine if the relying party accepts or rejects it. Unlike this baseline exposure, trust anchors that participate in this protocol can be probed by only knowing the trust anchor identifier.¶

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. Subscribers

If the subscriber 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 sensitive trust anchors.¶

This does not apply if the subscriber is a client.¶

9.1. Relying Party Policies and Agility

The negotiation mechanism described in this document facilitates which certification path is served to relying parties, but has no impact on the relying party's trust preferences themselves.¶

As a result, this enables a more flexible and agile PKI, which can better mitigate security risks to users. Section 7 discusses some scenarios which benefit from the multi-certificate deployment this document enables. In general, robust trust anchor negotiation helps subscribers navigate differences in relying party requirements. This means security improvements for one set of relying parties can be deployed without needing to risk incompatibility or breakage for others.¶

For example, agility reduces pressures on relying parties to sacrifice user security for compatibility. Suppose a subscriber currently uses some CA, but a relying party deems trusting that CA to pose an unacceptable security risk to its users. In a single-certificate deployment, those subscribers may be unwilling to adopt a CA trusted by the relying party, as switching CAs risks compatibility problems elsewhere. The relying party then faces compatibility pressure and adds this CA, sacrificing user security. However, in a multi-certificate deployment, the subscriber can use its existing CA in addition to another CA trusted by relying party B. This allows the ecosystem to improve interoperability, while still meeting relying party B's user-security goals.¶

9.2. Incorrect Selection Metadata

If the subscriber has incorrect information about trust anchor identifiers, it may send an untrusted certification path. This will not result in that path being trusted, but does present the possibility of a degradation of service. However, this information is provisioned by the CA, which is already expected to provide accurate information.¶

9.3. Serving Multiple Certificates

In a multi-certificate deployment, subscribers have a collection of certificates available to satisfy relying parties with potentially different security policies. It is possible that some of these policies will only be satisfied by certificates from CAs that have been distrusted by other relying parties, such as if the relying party is out of date but still important for the subscriber to support. If a certificate asserts the correct information about the subscriber (TLS key, DNS names, etc.), the subscriber can safely present it, even if the CA is otherwise untrustworthy. In particular, doing so does not allow the CA to decrypt or intercept the connection.¶

However, the subscriber presenting a certificate is not an endorsement of the CA. The subscriber's role is to present a certificate which will convince the relying party of the correct subscriber information. Subscribers do not vet CAs for trustworthiness, only the correctness of their specific, configured certificates and the CA's ability to meet the subscriber's requirements, such as availability, compatibility, and performance. It is the responsibility of the relying party, and its corresponding root program, to determine the set of trusted CAs. Trusting a CA means trusting all certificates issued by that CA, so it is not enough to observe subscribers serving correct certificates. An untrustworthy CA may sign one correct certificate, but also sign incorrect certificates, possibly in the future, that can attack the relying party. Root program management is a complex, security-critical process, the full considerations of which are outside the scope of this document.¶

10.1. TLS ExtensionType Updates

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

10.2. CertificatePropertyType Registry

[[TODO: Establish / add to the CertificatePropertyType registry]]¶