TLS Trust Expressions

2.1. Time

Times in this document are represented by POSIX timestamps, which are integers containing a number of seconds since the Epoch, defined in Section 4.16 of [POSIX]. That is, the number of seconds after 1970-01-01 00:00:00 UTC, excluding leap seconds. A UTC time is converted to a POSIX timestamp as described in [POSIX].¶

Durations of time are integers, representing a number of seconds not including leap seconds. They can be added to POSIX timestamps to produce other POSIX timestamps.¶

The current time is a POSIX timestamp determined by converting the current UTC time to seconds since the Epoch as per Section 4.16 of [POSIX]. One POSIX timestamp is said to be before (respectively, after) another POSIX timestamp if it is less than (respectively, greater than) the other value.¶

2.2. Terminology and Roles

This document discusses four 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 that authenticates the subscriber. In TLS, this is the side that validates a Certificate and CertificateVerify message.¶

Certification authority (CA):

The service that issues certificates to the subscriber.¶

Root program:

An entity that manages a set of common trusted CAs for a set of relying parties.¶

While the first three roles are common to most X.509 deployments, this document discusses the role of a root program as distinct from the relying party. Trust decisions are often common between related relying parties, such as multiple clients running the same application. The root program is the entity making those common decisions, such as the software vendor for the application or operating system.¶

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.¶

Trust store:

A collection of trust anchors managed by the root program.¶

Certification path:

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

Trust store manifest:

A structure describing a series of versioned trust stores.¶

Trust expression:

A compact description of a relying party's trust store contents, referencing named, versioned trust stores.¶

CertificatePropertyList:

A structure associated with a certification path, containing additional information from the CA, for use by the subscriber when presenting the certification path.¶

TrustStoreInclusionList:

A property in the CertificatePropertyList which describes the corresponding trust anchor's inclusion in trust stores. This is used by the subscriber when evaluating trust expressions.¶

4.1. Trust Store Identifiers

To simplify allocation, trust store identifiers use object identifiers (OIDs) [X680] based on Private Enterprise Numbers (PENs) [RFC9371]. An organization may define a trust store by allocating some OID under its PEN. For example, an organization with PEN 32473 might allocate a trust store identified by the OID 1.3.6.1.4.1.32473.1.¶

For compactness, this document uses relative object identifiers (see Section 33 of [X680]), relative to the OID prefix 1.3.6.1.4.1. In the text form, these are written in dotted decimal notation, such as 32473.1. In a binary protocol, such as TLS, this is encoded as the contents octets of the relative object identifier's DER encoding, as described in Section 8.20 of [X690]. Note this omits the tag and length portion of the encoding. For example, the binary encoding of 32473.1 would be the four-octet sequence {0x81, 0xfd, 0x59, 0x01}.¶

4.2. Trust Store Entry Expiration

The max_age and max_lifetime fields define an expiration time for trust store entries in versions other than the latest version. For a trust store entry in version V, the expiration time is the sum of:¶

• The timestamp of the subsequent version, i.e. version V+1¶

• The max_age of the manifest¶

• The max_lifetime of the trust store entry¶

Expiration times for entries in the latest version are not defined. They are determined once the root store appends a new version.¶

Trust store entries are not removed from their containing version after they expire. Rather, the expiration time is the point at which all unexpired certificates have incorporated information about subsequent trust store versions, per Section 5.2. This ensures the negotiation procedures in Section 6.3 and Section 6.5 interoperate.¶

4.3. Labels

Trust store entries reference labels, which are 24-bit integers that identify subsets of a trust store version. These integers are defined relative to the trust store manifest and are not globally unique. Root programs allocate labels when defining versions of a trust store.¶

Labels are used in trust expressions (Section 6.1) to exclude trust anchor entries from the expression. A label may identify an individual trust anchor if it is the only one with that label, or multiple trust anchors if they share the label.¶

The root program's label allocation scheme determines which sets may be efficiently represented. In particular, when trust anchors are removed in a later version of a trust store, trust expressions must express the removed set with labels as described in Section 6.5. Root programs SHOULD allocate individual labels for each trust anchor, and shared labels for trust anchors that will likely be removed together. For example, the root program may allocate shared labels for trust anchors with the same operator, or trust anchors that use some signature algorithm if it expects some relying parties to exclude by algorithm.¶

When allocating labels, root programs MAY repurpose a previously allocated label value after all previous entries referencing it have expired (Section 4.2).¶

5.1. Trust Store Inclusion

The trust_stores property specifies which trust stores contain the certification path's trust anchor. Its data field contains a TrustStoreInclusionList, defined below:¶

enum {
    previous_version(0),
    latest_version_at_issuance(1)
} TrustStoreStatus;

struct {
    opaque id<1..2^8-1>;
    uint24 version;
} TrustStore;

struct {
    TrustStore trust_store;
    TrustStoreStatus status;
    uint24 labels<1..2^16-1>;
} TrustStoreInclusion;

TrustStoreInclusion TrustStoreInclusionList<1..2^16-1>;

Each TrustStoreInclusion describes a trust store version which contains this certification path's trust anchor. trust_store specifies a version of the trust store, and labels specifies the trust anchor's labels within that trust store.¶

If status is latest_version_at_issuance, trust_store was the latest version at the time the certificate was issued. In this case, the trust expression evaluation algorithm (Section 6.3) predicts this information additionally applies to all versions greater than trust_store's version, up to the expiration of the certification path.¶

A TrustStoreInclusionList MUST satisfy the following invariants:¶

• Each TrustStoreInclusion has a unique trust_store value.¶

• The TrustStoreInclusion structures are sorted, first by length of trust_store's id, then by trust_store's id lexicographically, then by trust_store's version.¶

• If status is latest_version_at_issuance in some TrustStoreInclusion, no trust_store with the same id but higher version appears in the list.¶

5.2. Computing Trust Store Inclusions

Each CA regularly fetches trust store manifests from root programs in which it participates, caching the most recently fetched manifest from each root program. When issuing a certification path to a subscriber, it runs the following procedure on each root program's trust store manifest, to compute a list of TrustStoreInclusions:¶

1. If the cached manifest was fetched more than max_age seconds ago, fetch and cache a new copy.¶

2. For each trust store version in the cached manifest's versions:¶

   1. Look for a trust store entry whose trust_anchor field, when indexed into trust_anchors, matches the certification path's trust anchor.¶

   2. If found, output a TrustStoreInclusion structure:¶

      • Set trust_store to the manifest's id, encoded in binary form as described in Section 4.1, and the version's version number.¶

      • Set status to latest_version_at_issuance if the trust store version is currently the latest version. Otherwise, set it to previous_version.¶

      • Set labels to the trust store entry's labels.¶

If the above procedure outputs a TrustStoreInclusion with status of latest_version_at_issuance, the certification path's lifetime MUST NOT exceed the corresponding trust store entry's max_lifetime field. This ensures the procedures in Section 6.3 and Section 6.5 interoperate correctly.¶

The CA combines the outputs of this procedure for each known manifest to form the final TrustStoreInclusionList. To aid the CA in collecting manifests, repositories such as [CCADB] can publish a directory of trust store manifests from participating root programs.¶

5.3. Media Type

A certification path with its associated CertificationPropertyList 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 5.¶

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 12.2.¶

6.1. Trust Expressions

When the trust_expressions extension is sent in ClientHello and CertificateRequest, the extension_data field is a TrustExpressionList, defined below:¶

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

struct {
    TrustStore trust_store;
    uint24 excluded_labels<0..2^16-1>;
} TrustExpression;

TrustExpression TrustExpressionList<1..2^16-1>;

A TrustExpressionList is an unordered set of TrustExpression objects. When a relying party sends a TrustExpressionList, it indicates support for all trust anchors specified by any TrustExpression contained in the list. A TrustExpression specifies a list of trust anchors in two parts:¶

First, trust_store specifies a trust store by ID and version (see Section 4). Any trust anchors contained in the specified trust store are included.¶

Second, excluded_labels specifies a set of labels, each of which identify one or more trust anchors in a trust store manifest. Any trust anchors identified by any label in excluded_labels are excluded. Section 6.5 discusses this set in more detail.¶

excluded_labels MUST be sorted in ascending order and contain no duplicate values to be valid. If invalid, receivers MUST terminate the connection with an illegal_parameter alert.¶

Together, a TrustExpression indicates that the relying party accepts all trust anchors in the referenced trust store version, except for any that were excluded via excluded_labels.¶

6.2. Subscriber Acknowledgement

When the trust_expressions extension is sent in a Certificate message, the extension MUST be in the first CertificateEntry and the extension_data field MUST be empty. This extension indicates the subscriber selected a certification path using trust expressions.¶

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, 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.¶

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.¶

6.3. Evaluating Trust Expressions

Given a certification path and a TrustExpressionList from the relying party, the subscriber runs the following procedure to determine if the relying party is claiming support for the path's trust anchor. If so, the TrustExpressionList is said to match the certification path:¶

1. If the certification path does not have an associated trust_store certificate property (Section 5.1), containing a TrustStoreInclusionList, terminate this procedure and return false. The certification path does not match the TrustExpressionList.¶

2. If the certification path has expired, terminate this procedure and return false. Expired certification paths are expected to be rejected by the relying party and may also have stale TrustStoreInclusionLists.¶

3. Otherwise, for each TrustExpression in the relying party's TrustExpressionList:¶

   1. Find the TrustExpression's corresponding TrustStoreInclusion in the TrustStoreInclusionList, if any. The corresponding TrustStoreInclusion is the one where both of the following are true:¶

      • The id fields of two structures' trust_store fields are equal.¶

      • Either the version fields of the two structures' trust_store fields are equal, or the TrustStoreInclusion's status is latest_version_at_issuance and the version field of TrustExpression's trust_store is greater than that of the TrustStoreInclusion's trust_store.¶

      The invariants in Section 5.1 imply that at most one TrustStoreInclusion will satisfy this. Implementations may evaluate this efficiently by performing a binary search over the TrustStoreInclusionList.¶

   2. If there was no corresponding TrustStoreInclusion, continue to the next TrustExpression.¶

   3. Otherwise, check for common values between the TrustStoreInclusion's labels field and the TrustExpression's excluded_labels field. If there are no common values, terminate this procedure and return true. The certification path matches the TrustExpressionList. Otherwise, continue to the next TrustExpression.¶

4. If no match was found in the TrustExpressionList, return false. The certification path does not match the TrustExpressionList.¶

When a TrustStoreInclusion's status is latest_version_at_issuance, the above procedure predicts that trust anchors in the latest version will continue to be present in later versions. This allows relying parties using later trust store versions to continue to interoperate with certification paths that predate the later version. If this prediction is incorrect and the trust anchor has been removed from the later version, Section 6.5 requires that the TrustExpression include appropriate excluded_labels values to mitigate this.¶

6.4. Subscriber Behavior

Subscribers using this negotiation scheme are configured with a list of candidate certification paths, with corresponding CertificatePropertyList (Section 5) structures. As a server (respectively, client) responding to a ClientHello (respectively, CertificateRequest), the subscriber selects the best candidate path, incorporating the matching algorithm described in Section 6.3, and other criteria defined in TLS.¶

If the subscriber selected a certification path which matched via trust expressions, it MUST include an empty trust_expressions extension in the first CertificateEntry.¶

This document does not prescribe a precise selection procedure and configuration, but the following is an example implementation strategy:¶

The TLS implementation allows the application to configure a list of candidate credentials, each with certification path, private key, and optional CertificatePropertyList, in preference order. This order may reflect a preference to, e.g., minimize message size, or more performant private keys. When selecting a credential, the TLS implementation iterates over each candidate in preference order and checks:¶

1. Either the matching algorithm described in Section 6.3 returns true, or some other TLS extension, such as the certificate_authorities extension, indicates the relying party supports the trust anchor.¶

2. The certification path is suitable for use based on other TLS criteria. For example, the TLS signature_algorithms (Section 4.2.3 of [RFC8446]) extension constrains the types of keys which may be used.¶

3. The certification path satisfies any other application-specific constraints which may apply. For example, TLS servers often select certificates based on the server_name (Section 3 of [RFC6066]) extension.¶

The TLS implementation selects the first credential which satisfies all of its selection criteria. If no credentials match, the TLS implementation may return a handshake_failure alert, or consider a configured fallback set of credentials, which skip the first check above. These fallback credentials may be a separate credential list, or appended to the same list with an option to control the matching criteria.¶

Fallback credentials allow the subscriber to retain support for relying parties that do not implement trust expressions. As they are be selected without trust anchor negotiation, the subscriber must find a sufficiently ubiquitous trust anchor, if one exists. However, unlike existing PKIs that do not use trust expressions, only relying parties without trust expressions need to be considered in this ubiquity determination. Updated relying parties may continue to evolve without restricting fallback credential selection.¶

6.5. Constructing Trust Expressions

Relying parties send the trust_expressions extension to advertise a set of accepted trust anchors. Section 10 discusses which trust anchors to advertise. Each trust expression to be sent advertises a subset of some trust store version. To communicate a set of trust anchors that span multiple trust stores, relying parties MAY send multiple trust expressions, each referencing a different trust store.¶

For each referenced trust store version, the following procedure constructs a trust expression. This procedure is expected to be run by the root program, as part of defining new trust store versions and provisioning supported trust anchors in relying parties.¶

[[TODO: This is written as a procedure, but we expect root programs to fill in the expected exclusions when they define a new trust store version, and then trim the compatibility exclusions as they expire. Also the root programs know their label allocation scheme and are the ones deciding on removals, so they're best situated to pick a set of excluded_labels. Perhaps this should get moved to the manifest?]]¶

1. Let include_entries and exclude_entries be two empty sets of trust anchor entries.¶

2. For each trust store entry in the trust store version:¶

   1. If the trust store entry's trust_anchor references a trust anchor that is in the desired subset, add it to include_entries.¶

   2. Otherwise, add it to exclude_entries.¶

3. For all trust store entries in trust store versions before the specified version:¶

   1. If the current time is before the entry's expiration time (Section 4.2) and if the entry's trust_anchor references a trust anchor that is not in the desired subset, add the entry to exclude_entries.¶

4. Compute a set of labels, excluded_labels such that:¶

   1. No label appears in any entry in include_entries.¶

   2. For each entry in exclude_entries, there is some label in common between excluded_labels and the labels in the entry.¶

   How to compute this set depends on the root program's label allocation scheme (Section 4.3). If the root program allocates a label for each trust anchor, this set is always computable, though more efficient sets may be possible depending on the allocation scheme.¶

5. Output a TrustExpression such that trust_store is the referenced trust store version, and excluded_labels is the computed excluded_labels value.¶

This procedure uses excluded_labels for two kinds of exclusions:¶

First, if the trust store version includes undesired trust anchors, the trust expression should exclude them. This may occur if, for example, the trust store is used by all versions of the relying party's software, but some trust anchors are gated by software version.¶

Second, trust expressions exclude unexpired entries from previous versions. This is because the matching criteria described in Section 6.3 predictively applies TrustStoreInclusion values with status of latest_version_at_issuance to all future versions of a trust store. This allows relying parties to interoperate with subscribers with stale information. Unexpired entries are those for which such an unexpired certification path may still exist. Where this prediction is incorrect, trust expressions MUST mitigate this by excluding the past entries.¶

7.1. ACME Extension

This section extends ACME [RFC8555] to be able to issue certificate paths, each with an associated CertificatePropertyList by defining a new media type in Section 5.3.¶

When an ACME server processes an order object, it MAY issue multiple certification paths, each with an associated TrustStoreInclusionList, computed as in Section 5.2. These paths MAY share an end-entity certificate but end at different trust anchors, or they MAY be completely distinct. The ACME server encodes each certification path using the application/pem-certificate-chain-with-properties format, defined in Section 5.3). Note this format is required to form a complete certification path. The CA MUST return a result that may be verified by relying parties without path building [RFC4158].¶

The ACME server provides additional results to the client with the link relation header fields described in Section 7.4.2 of [RFC8555]. When fetching certificates, the ACME client includes application/pem-certificate-chain-with-properties in its Accept header to indicate it supports the new format. Unlike the original mechanism described in [RFC8555], these certification paths do not require heuristics to be used. Instead, the ACME client uses the associated TrustStoreInclusionLists to select a path as described in Section 6.4.¶

When the ACME client wishes to renew any of the certification paths issued in this order, it repeats this process to renew each path concurrently. Thus this extension is suitable when the CA is willing to issue and renew all paths together. It may not be appropriate if the paths have significantly different processing times or lifetimes. Future enhancements to ACME may be defined to address these cases, e.g. by allowing the ACME client to make independent orders.¶

9.1. Key Rotation

In most X.509 deployments, a compromise of any root CA's private key compromises the entire PKI. Yet key rotation in PKIs is rare. In 2023, the oldest root in [CHROME-ROOTS] and [MOZILLA-ROOTS] was 25 years old, dating to 1998. Key rotation is challenging in a single-certificate deployment model. As long as any older relying party requires the old root, subscribers cannot switch to the new root, which in turn means relying parties cannot distrust the old root, leaving them vulnerable.¶

A multi-certificate deployment model avoids these transition problems. Key rotation may proceed as follows:¶

1. The CA operator generates a new root CA with a separate key, but continues operating the old root CA.¶

2. Root programs begin trusting the new root CA alongside the old one. They update their root store manifests.¶

3. When subscribers request certificates, the CA issues certificates from both roots and provisions the subscriber with both certificates.¶

4. Relying parties send trust expressions that evaluate to either the old or new root, and are served the appropriate option.¶

5. Once subscribers have been provisioned with new certificates, root programs can safely distrust the old root in new relying parties. The CA operator continues to operate the old root CA for as long as it wishes to serve subscribers that, in turn, wish to serve older relying parties.¶

This process requires no configuration changes to the subscriber, given an automated, multi-certificate-aware certificate issuance process. The subscriber does not need to know why it received two certificates, only how to select between them for each relying party.¶

9.2. Adding CAs

In the single-certificate model, subscribers cannot use TLS certificates issued from a new root CA until all supported relying parties have been updated to trust the new root CA. This can take years or more. Some relying parties, such as IoT devices, may never receive trust store updates at all.¶

As a result, it is very difficult for subscribers that serve a wide variety of relying parties to use a newly-trusted root CA. When trust stores diverge too far, subscribers often must partition their services into multiple TLS endpoints (i.e. different DNS names) and direct different relying parties to different endpoints. Subscribers sometimes resort to TLS fingerprinting, to detect particular relying parties. But, as this repurposes other TLS fields for unintended purposes, this is unreliable and usually requires writing custom service-specific logic.¶

In a multi-certificate deployment model, subscribers can begin serving certificates from new root CAs without interrupting relying parties that depend on existing ones.¶

In some contexts, it may be possible to use other fields to select the new CA. For example, post-quantum-capable clients may be detected with the signature_algorithms and signature_algorithms_cert extensions. However, this assumes all post-quantum CAs are added at the same time. A multi-certificate model avoids this problem and allows for a more gradual deployment of post-quantum CAs.¶

9.3. Removing CAs

Subscribers in a single-certificate model are limited to CAs in the intersection of their supported relying parties. As newer relying parties remove untrusted CAs over time,the intersection with older relying parties shrinks. Moreover, the subscriber may not even know which CAs are in the intersection. Often, the only option is to try the new certificate and monitor errors. For subscribers that serve many diverse relying parties, this is a disruptive and risky process.¶

The multi-certificate model removes this constraint. If a subscriber's CA is distrusted, it can continue to use that CA, in addition to a newer one. This removes the risk that some older relying party required that CA and was incompatible with the new one. The mechanisms in this document will select an appropriate certificate for each relying party.¶

9.4. 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 use the mechanism in Section 7.1 to issue from the classical and postquantum roots concurrently. The subscriber will then, transparently and with no configuration change, serve both. As in Section 9.1, 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.¶

[[TODO: There's one missing piece, which is that some servers may attempt to parse the signature algorithms out of the certificate chain. See https://github.com/davidben/tls-trust-expressions/issues/9 ]]¶

9.5. Intermediate Elision

Today, root CAs typically issue shorter-lived intermediate certificates which, in turn, issue end-entity certificates. The long-lived root key is less exposed to attack, while the short-lived intermediate key can be more easily replaced without changes to relying parties.¶

This operational improvement comes at a bandwidth cost: the TLS handshake includes an extra certificate, which includes a public key, signature, and X.509 metadata. An average X.509 name in the Chrome Root Store [CHROME-ROOTS] or Mozilla CA Certificate Program [MOZILLA-ROOTS] is around 100 bytes alone. Post-quantum signature algorithms will dramatically shift this tradeoff. Dilithium3 [Dilithium], for example, has a total public key and signature size of 5,245 bytes.¶

[I-D.ietf-tls-cert-abridge] proposes to predistribute known intermediate certificates to relying parties, as a compression scheme. A multi-certificate deployment model provides another way to achieve this effect. To relying parties, a predistributed intermediate certificate is functionally equivalent to a root certificate. PKIs use intermediate certificates because changing root certificates requires updating relying parties, but predistributed intermediates already presume updated relying parties.¶

A CA operator could provide subscribers with two certification paths: a longer path ending at a long-lived trust anchor and shorter path the other ending at a short-lived, revocable root. Relying parties would be configured to trust both the long-lived root and the most recent short-lived root. The negotiation mechanism in Section 6 would then send the shorter path to up-to-date relying parties, and the longer path to older relying parties.¶

This achieves the same effect with a more general-purpose multi-certificate mechanism. It is also more flexible, as the two paths need not be related. For example, root CA public keys are not distributed in each TLS connection, so a post-quantum signature algorithm that optimizes for signature size may be preferable. In this model, both the long-lived and short-lived roots may use such an algorithm. In a compression-based model, the same intermediate must optimize both its compressed and uncompressed size, so such an algorithm may not be suitable.¶

9.6. Conflicting Relying Party Requirements

A subscriber may need to support relying parties with different 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.¶

A single-certificate deployment model forces subscribers to find a single certificate that meets all requirements. User security then suffers in all contexts, as the PKI may not quite meet anyone's needs. In a multi-certificate deployment model, different contexts may use different trust anchors. A subscriber that supports multiple contexts would provision certificates for each, with certificate negotiation logic directing the right one to each relying party.¶

9.7. Backup Certificates

A subscriber may obtain certificate paths from multiple CAs for redundancy in the face of future CA compromises. If one CA is compromised and removed from newer relying parties, the TLS server software will transparently serve the other one.¶

To support this, TLS serving software SHOULD permit users to configure multiple ACME endpoints and select from the union of the certificate paths returned by each ACME server.¶

11.1. Relying Party Policies

PKI-based TLS authentication depends on the relying party's certificate policies. If the relying party trusts an untrustworthy CA, that CA can intercept TLS connections made by that relying party by issuing certificates associating the target name with the wrong TLS key.¶

This attack vector is available with or without trust anchor negotiation. The negotiation mechanism described in this document allows certificate selection to reflect a relying party's certificate policies. It does not determine the certificate policies themselves. Relying parties remain responsible for trusting only trustworthy CAs, and untrustworthy CAs remain a security risk when trusted.¶

11.2. Agility

As with other TLS parameters, negotiation reduces a conflict between availability and security, which allows PKIs to better mitigate security risks to users. When relying parties in an existing TLS ecosystem improve their certificate policies, trust anchor negotiation helps subscribers navigate differences between those relying parties and existing relying parties. Each set of requirements may be satisfied without compatibility risk to the other. Section 9 discusses such scenarios in more detail.¶

Negotiation also 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.¶

11.3. Incorrect Selection Metadata

If either the subscriber's TrustStoreInclusionList or the relying party's TrustExpressionList are incorrect, the matching algorithm described in Section 6.4 may incorrectly identify an untrusted certification path. This mechanism will not result in that path being trusted, but does present the possibility of a denial of service. These structures are provisioned by the CA and root program, respectively, who are already expected to provide accurate information.¶

11.4. Serving Multiple Certificates

Negotiation reduces compatibility pressures against subscribers choosing to serve certificates from a less common CA, as the subscriber can serve it in addition to other CAs that, together, satisfy all supported relying parties. The CA may even have been distrusted by some relying parties, e.g. if it is needed for older, unupdated relying parties that are still important for the subscriber to support. As discussed in Section 9 and Section 11.2, this deployment model avoids compatibility risks during PKI transitions which mitigate security risks to users.¶

Serving such certificates does not enable the CA to decrypt or intercept the connection. If a certificate asserts the correct information about the subscriber, notably the correct public key, the subscriber can safely present it, whether or not the issuing CA is otherwise trustworthy. Issuing a certificate for the subscriber's public key does not grant the CA access to the corresponding private key. Conversely, if the attacker already has access to the subscriber's private key, they do not need to be in control of a CA to intercept a connection.¶

While the choice of CA is a security-critical decision in a PKI, it is the relying party's choice of trusted CAs that determines interceptibility, not the subscriber's choice of certificates to present. If the relying party trusts an attacker-controlled CA, the attacker can intercept the connection independent of whether the subscriber uses that CA or another CA. In both cases, the attacker would need to present a different certificate, with a different public key. Conversely, if the relying party does not trust the attacker's CA, the subscriber's correct but untrusted attacker-issued certificate will not enable the attacker to substitute in a different public key. The ability to intercept a connection via the PKI is determined solely by relying party trust decisions.¶

Relying parties thus SHOULD NOT interpret the subscriber's choice of CA list as 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.¶

11.5. Targeting TLS Interception

Trust Expressions, like certificate_authorities, enables a TLS server to differentiate between clients that do and do not trust some CA. If this CA mis-issued a certificate, this could be used by a network attacker to only enable TLS interception with clients that trust the CA. The network attacker may wish to do this reduce the odds of detection. Clients which do not trust the CA will not accept the mis-issued certificate, which may be user-visible.¶

However, the attacker could also use any existing mechanism for differentiating clients. In TLS parameter negotiation, the client offers all its available TLS features, including cipher suites and other extensions, in the TLS ClientHello. This means any variation in any client TLS policies, related or unrelated to trust anchors, may be used as an implementation fingerprint to differentiate clients. While fingerprinting's heuristic nature makes it not viable for a broad, diverse set of TLS servers, it is viable for a network attacker's single interception service. Trust Expressions only impacts detection where this differentiation was not already possible.¶

If the attacker targets any clients that enforce Certificate Transparency [RFC6962], the mis-issued certificates will need to be publicly logged. In this case, detection is more robust, and client differentiation, with or without Trust Expressions, has no significant impact.¶

12.1. TLS ExtensionType Updates

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

12.2. 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 5.3]¶

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¶

12.3. CertificatePropertyType Registry

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

13.1. Normative References

[POSIX]

"IEEE Standard for Information Technology--Portable Operating System Interface (POSIX(TM)) Base Specifications, Issue 7", IEEE, DOI 10.1109/ieeestd.2018.8277153, ISBN ["9781504445429"], January 2018, <https://doi.org/10.1109/ieeestd.2018.8277153>.

[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>.

[RFC4648]

Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, <https://www.rfc-editor.org/rfc/rfc4648>.

[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>.

[RFC8259]

Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017, <https://www.rfc-editor.org/rfc/rfc8259>.

[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>.

[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>.

[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.

13.2. Informative References

[CCADB]

Mozilla, Microsoft, Google, Apple, and Cisco, "Common CA Database", 9 October 2023, <https://www.ccadb.org/>.

[CHROME-ROOTS]

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

[Dilithium]

Bai, S., Ducas, L., Kiltz, E., Lepoint, T., Lyubashevsky, V., Schwabe, P., Seiler, G., and D. Stehlé, "CRYSTALS-Dilithium Algorithm Specifications and Supporting Documentation", 8 February 2021, <https://pq-crystals.org/dilithium/data/dilithium-specification-round3-20210208.pdf>.

[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>.

[RFC6066]

Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011, <https://www.rfc-editor.org/rfc/rfc6066>.

[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>.

[RFC8610]

Birkholz, H., Vigano, C., and C. Bormann, "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.