]> Qwixl

luke.chapman@qwixl.com

Security Independent Submission MLS agent end-to-end encryption data minimisation A2A Agent-to-agent protocols increasingly carry messages between autonomous software agents acting on behalf of distinct principals, including across organisational boundaries. Existing protocols in this space secure the transport hop and authenticate the calling party, but do not provide message-level confidentiality, do not provide non-repudiable evidence of what a counterparty asserted, and do not carry machine-enforceable constraints on how a recipient may use the data conveyed. This document specifies a profile that addresses those three gaps. It defines a Governed Object: a signed, purpose-bound, expiring message envelope identified by a decentralised identifier. It specifies how such objects are exchanged inside end-to-end encrypted sessions established using the Messaging Layer Security (MLS) protocol , and how the MLS credential is cryptographically bound to the sending agent's identity. It defines the encapsulation of these constructs as an extension to an agent-to-agent transport, using the Agent2Agent (A2A) protocol as the reference binding, and specifies mandatory receiver-side processing rules including replay rejection and purpose enforcement.

Introduction

Problem Statement Deployments of agent-to-agent messaging are moving from single-organisation orchestration towards interactions between agents that represent different principals: a person and a business, two businesses, or a citizen and a public body. Three properties that are routinely required in such interactions are not provided by the agent-to-agent protocols currently in use.

Confidentiality against intermediaries.

Current practice secures the transport hop, typically with TLS, and authenticates the caller, typically with OAuth 2.0, OpenID Connect, or mutual TLS. Any intermediary that terminates TLS -- a gateway, a broker, a hosting provider, or the platform operating either agent -- can read the full message content. Where the two principals are mutually independent and the message content is confidential, commercially sensitive, or personal data, transport security is insufficient.

Non-repudiable assertion.

A recipient frequently needs durable evidence of what the counterparty's agent asserted, and needs it to remain verifiable after the session has ended, after key rotation, and independently of any log held by an intermediary. Transport-level authentication does not produce such evidence, because it authenticates a connection rather than a statement.

Machine-enforceable use constraints.

When an agent discloses data to a counterparty, the disclosing principal frequently intends the disclosure to be limited: to a stated purpose, and for a bounded period. Today that intent, if expressed at all, is expressed in prose outside the protocol and is therefore not available to the receiving implementation at the moment of processing.

These are not hypothetical requirements. In several jurisdictions the second and third are closely related to statutory obligations; see .

Scope This document specifies:

• a signed message envelope, the Governed Object, carrying a semantic type, a payload, a declared purpose, and an expiry ();
• an agent identity scheme based on Ed25519 decentralised identifiers, and a mandatory cryptographic binding between that identity and the agent's MLS credential ();
• establishment of MLS sessions between agents, including publication and retrieval of MLS KeyPackages ();
• encapsulation of MLS wire messages and Governed Objects as an extension to an agent-to-agent transport, with A2A as the reference binding ();
• mandatory receiver-side processing rules, including signature verification, expiry rejection, replay rejection, and purpose enforcement ().

Non-Goals This document does not:

• define a new agent-to-agent transport, discovery mechanism, or capability description format; it layers on an existing one;
• define agent tool invocation or local resource access, for which the Model Context Protocol and comparable mechanisms are used;
• provide cryptographic enforcement of purpose limitation. The purpose field is signed, tamper-evident, and mandatory to enforce at the receiver, but a non-conforming receiver that has decrypted a message can disregard it. This is a deliberate limitation, discussed in ;
• specify authorisation policy, business semantics, or user interface behaviour;
• specify how a decentralised identifier is bound to a legal or natural identity. That binding, where required, is provided by credential presentation layered above this specification and is out of scope.

Relationship to Existing Work MLS provides the group key establishment and message protection used here. This document does not modify MLS. It specifies a ciphersuite constraint, a credential-binding requirement, and a transport encapsulation. A2A provides agent discovery, capability description, and message transport. A2A version 1.0 supports declared protocol extensions, and this document is specified as such an extension; states the version this binding targets. Nothing here requires changes to A2A itself. The mechanism is transport-agnostic in principle. A2A is used as the reference binding because it is widely deployed and because its extension mechanism makes the binding expressible without protocol changes.

Conventions and Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Agent:

A software process that sends and receives messages on behalf of a principal.

Principal:

The natural person, legal person, or organisation on whose behalf an agent acts.

Agent Identity:

A did:key identifier encoding an Ed25519 public key, used to identify an agent and to verify its signatures.

Governed Object:

The signed message envelope specified in .

Purpose:

A short string in a Governed Object declaring the use for which the payload is disclosed.

Relying Receiver:

An implementation that accepts a Governed Object and acts upon its payload.

Architecture Overview Two agents, each holding an Ed25519 key pair, establish an MLS group containing exactly two members (a pair session) or more than two members (a group session). Application messages within that group carry Governed Objects serialised as JSON . The MLS wire bytes are carried inside the agent-to-agent transport as opaque data. The transport therefore learns the identity of the endpoints and the size and timing of messages, but not the content.

Confidential agent messaging | | (2) MLS Welcome + ratchet tree | |---------------------------------------------------->| | | | (3) MLS application message | | = AEAD( JSON( GovernedObject ) ) | |<--------------------------------------------------->| | | +--+-----------------------------------------------------+--+ | agent-to-agent transport (e.g. A2A over HTTPS) | | sees: endpoints, size, timing. not: content | +----------------------------------------------------------+ ]]>

Agent Identity

Identifier Construction An Agent Identity is a did:key identifier encoding an Ed25519 public key. The identifier is the string did:key: followed by the multibase base58btc encoding (prefix z) of the multicodec ed25519-pub header (0xed 0x01) concatenated with the 32-byte public key. Verifiers MUST derive the public key from the identifier itself. Conforming implementations MUST NOT require network resolution of the identifier in order to verify a signature. This property is required so that a Governed Object remains verifiable offline and after the issuer's service has ceased to operate. Implementations MAY support additional DID methods. Implementations MUST support did:key.

MLS Ciphersuite Implementations MUST support MLS ciphersuite MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519 (value 0x0001 in the MLS Ciphersuites registry established by ). This ciphersuite is mandatory because its signature scheme is Ed25519, matching the Agent Identity key type and enabling the binding specified in . Implementations MAY negotiate other ciphersuites, subject to that section.

Credential Binding An agent's MLS credential MUST be a basic credential whose identity is the UTF-8 encoding of the agent's Agent Identity. Furthermore, the MLS signature_key in the agent's LeafNode MUST be the same Ed25519 public key that is encoded in that Agent Identity. This requirement is the load-bearing element of this section. Without it, an attacker who can publish a KeyPackage may assert an arbitrary DID string in the credential, and a receiver has no cryptographic basis for concluding that messages from that MLS member originate from the holder of the corresponding DID. With it, MLS membership and Governed Object authorship are bound to a single key. On receiving a KeyPackage or LeafNode, an implementation MUST verify that the credential identity parses as a did:key Ed25519 identifier and that the public key so derived is octet-for-octet equal to the LeafNode signature_key. If the check fails, the implementation MUST reject the KeyPackage and MUST NOT add the member to a group. Where a ciphersuite whose signature scheme is not Ed25519 is negotiated, the binding of cannot be expressed by key equality. In that case implementations MUST instead require the credential identity to be accompanied by a proof of possession: a signature, made with the Agent Identity key, over the MLS signature_key. Definition of that proof's encoding is deferred to a future revision of this document; until it is specified, implementations that require the binding MUST use ciphersuite 0x0001.

The Governed Object

Structure A Governed Object is a JSON object with the following members. All listed members are mandatory unless stated otherwise.

Member	Type	Description
version	number	Format version. MUST be 1 for this specification.
id	string	Unique object identifier. MUST be unique per issuer.
issuerDid	string	Agent Identity of the signer.
issuedAt	string	Issuance time, RFC 3339 date-time in UTC.
semantic	object	Type descriptor; see below.
payload	object	Application data.
governance	object	Use constraints; see .
signatureAlgorithm	string	MUST be "ed25519".
signature	string	Base64 Ed25519 signature.

The semantic member has a mandatory schema member containing an absolute URI identifying the payload type, and OPTIONAL version and embeddingHint members. The id member MUST be unique per issuer and SHOULD be a UUID or other value with negligible collision probability. Receivers rely on this for replay rejection ().

Governance The governance member carries:

Member	Type	Description
purpose	string	Mandatory. Declared use for the payload.
ttlSeconds	number	OPTIONAL. Lifetime in seconds from issuedAt.
expiresAt	string	OPTIONAL. Absolute expiry, RFC 3339 date-time.

If both ttlSeconds and expiresAt are present, the earlier resulting instant is the effective expiry. If neither is present, the object does not expire, and receivers SHOULD apply a locally configured maximum lifetime. Purpose strings are lowercase, colon-delimited, and hierarchical, for example comms:message or coordination:proposal. Purpose values are application domain vocabulary; this document does not define a registry, but see for a discussion of whether one is warranted.

Canonical Serialisation The signature is computed over a canonical serialisation of the object's content, excluding signatureAlgorithm and signature. The canonical form is produced by serialising the JSON object consisting of exactly the members version, id, issuerDid, issuedAt, semantic, payload, and governance, such that:

1. object members are ordered lexicographically by their UTF-16 code unit sequence;
2. members whose value is undefined or absent are omitted;
3. array element order is preserved;
4. no insignificant whitespace is emitted;
5. strings and numbers are serialised as specified by .

Implementations SHOULD use JSON Canonicalization Scheme where available; it satisfies the requirements above for the value space used by this specification. Nested objects within semantic, payload, and governance are canonicalised recursively by the same rules. Payload authors SHOULD avoid floating-point numbers, because canonical serialisation of floating-point values is a known source of cross-language interoperability failure. Where numeric precision matters, payloads SHOULD encode values as strings or as integers in a stated minor unit.

Signing The signature is Ed25519(privateKey, UTF8(canonicalForm)) as specified in , encoded per Section 4 (base64 with padding).

Verification A verifier MUST:

1. confirm version equals 1;
2. confirm signatureAlgorithm equals "ed25519";
3. derive the Ed25519 public key from issuerDid per ;
4. recompute the canonical form per ;
5. verify the signature.

Verification failure MUST cause the object to be rejected.

Session Establishment

KeyPackage Publication An agent that accepts incoming sessions publishes MLS KeyPackages. Where the A2A binding of is used, the agent SHOULD expose them at the path /mls/key-package relative to its A2A service endpoint, returning a JSON object with members did (the Agent Identity) and wire (the base64-encoded MLS KeyPackage). Publishing endpoints MUST rate-limit KeyPackage retrieval. KeyPackages are single-use in MLS; unbounded retrieval permits exhaustion of an agent's pre-published supply. Retrievers MUST perform the credential binding check of on the retrieved KeyPackage before use.

Establishing a Pair Session The initiating agent creates an MLS group containing itself, then commits an Add proposal for the responding agent using the retrieved KeyPackage, and delivers the resulting Welcome message and ratchet tree to the responder using the handshake encapsulation of . Where the responder's identity was learned from an out-of-band invitation that asserted a specific Agent Identity, the initiator MUST verify that the retrieved KeyPackage's credential identity equals the asserted identity, and MUST abort establishment on mismatch. Absent this check, an attacker who controls the endpoint named in an invitation can substitute their own identity.

Group Sessions Sessions with more than two members follow MLS group semantics unchanged. Any member with commit rights, as determined by local policy, MAY add or remove members. This document places no additional requirements on group membership policy, but implementations MUST apply the credential binding check of to every joining member.

Session Persistence Implementations that persist MLS group state across process restarts MUST protect that state at rest with confidentiality and integrity equivalent to the protection afforded to long-term private keys. Group state contains the secrets necessary to decrypt past and future epochs.

Encapsulation This section specifies the A2A binding.

Protocol Version This binding is specified against A2A version 1.0. Two properties of that version are relied upon: a message part carries a mediaType member of its own, and a message may enumerate the extensions it depends upon. A2A version 0.3 provided neither. Implementations MAY interoperate with peers speaking version 0.3 by applying , and the reference implementation does so, but a conforming implementation of this document transmits version 1.0. Throughout this section, part and message members are named as they appear in the JSON serialisation on the wire. An implementation generated from the A2A protocol buffer schema will present the part content as a language-level tagged union; that representation is an artefact of code generation and is not observable by a peer.

Extension Declaration An agent supporting the Governed Object profile defined in this document MUST declare the extension URI https://atom.qwixl.dev/a2a/data-object/v1 in the capabilities.extensions member of its Agent Card, with required set to false. A message carrying one or more Governed Object parts SHOULD list that URI in the message's extensions member. A message that carries only MLS wire or MLS handshake parts MUST NOT stamp the Governed Object URI in extensions (the GO claim stays narrow; MLS MAY later ship under its own extension URI). A receiver MUST NOT require the extensions member to be present, since a peer speaking A2A version 0.3 has no such member to populate, and MUST NOT rely upon it to identify a part: the media type is authoritative. The A2A extension specification for the Governed Object profile alone (without MLS) is published under spec/extensions/data-object-v1/ in the reference implementation repository. This Internet-Draft remains the broader provenance document covering Governed Objects together with MLS session mechanics. Editor's note: this URI reflects the deployed reference implementation. Should this document be adopted by a standards body, the URI is expected to be reassigned under that body's namespace. Implementations SHOULD treat the URI as an opaque identifier.

Media Type Placement Each part specified below is identified by one of the media types in . A sender MUST set the media type in the part's mediaType member, and SHOULD also set a mediaType member within the part's data object with the identical value. A receiver MUST determine the media type from the part's mediaType member when that member is present and non-empty, and MUST fall back to the mediaType member within the data object otherwise. Where both are present and they disagree, the receiver MUST reject the part. The duplication exists because version 0.3 of A2A had no mediaType member on a part, obliging deployments written against A2A version 0.3 to carry the media type inside the payload. A receiver that reads only the inner member therefore continues to interoperate. A sender MAY omit the inner member once no such receiver remains reachable; this document retains it as a SHOULD rather than a MUST for that reason.

Governed Object Part A Governed Object MAY be carried unencrypted in an A2A message part whose data member is a JSON object with members mediaType, set to application/vnd.atom.data-object+json;version=1 per , and object, set to the Governed Object. Unencrypted carriage provides authenticity and governance metadata but not confidentiality. Implementations MUST NOT use unencrypted carriage for personal data or otherwise confidential payloads where an MLS session can be established. Implementations SHOULD default to encrypted carriage.

MLS Wire Part MLS wire messages are carried in an A2A message part whose data member is a JSON object with members mediaType, set to application/vnd.atom.mls-wire+cbor;version=1 per , and wire, set to the base64 encoding of the MLSMessage as serialised per . " }, "mediaType": "application/vnd.atom.mls-wire+cbor;version=1" } ]]> The decrypted application message plaintext is the UTF-8 encoding of a JSON object with a member object containing a Governed Object. Implementations MAY define additional members; receivers MUST ignore members they do not recognise.

Handshake Part MLS Welcome messages and the accompanying ratchet tree are carried in an A2A message part whose data member is a JSON object with member mediaType set to application/vnd.atom.mls-handshake+json;version=1 per , and members conveying the base64-encoded Welcome and ratchet tree.

Agent Card An agent's endpoints are declared in the supportedInterfaces member of its Agent Card, an ordered list in which the first entry is the sender's preferred interface and each entry states the A2A protocol version that interface speaks. A2A version 0.3 instead declared a single endpoint in a top-level url member; an implementation reading a card MUST NOT depend on that member. An agent MAY declare more than one interface at the same URL in order to serve more than one protocol version, and the reference implementation does so. An Agent Card MAY carry one or more JWS signatures in its signatures member. Where an agent's identity is a did:key as specified in , the signature's kid header parameter SHOULD be that identifier, because the verifying key is then recoverable from the identifier itself and card verification requires no key distribution mechanism and no further network retrieval. A relying party that verifies a card signature MUST treat the identifier in kid as the identity that produced the card, and MUST NOT infer from a valid signature that the card's contents are endorsed by any other identity named within it. Transport security establishes control of the origin serving a card; it establishes nothing about the agent the card describes. An unsigned card therefore permits any party controlling an origin to publish a card asserting any agent identity, and a relying party that acts on such an assertion -- for instance, to associate a domain with an agent -- SHOULD require a signature whose kid matches the asserted identity.

Version Compatibility An implementation MAY accept and originate A2A version 0.3 messages in addition to version 1.0. Where it does:

• The version of a received message is determined by the A2A-Version header field. Its absence denotes version 0.3.
• The version to use towards a given peer is determined by that peer's Agent Card, per .
• A part received from a version 0.3 peer carries the media type only within the data object, which accommodates.

Deployment order is not symmetric, and this has operational consequence. A receiver that accepts both versions is compatible with senders of either, but a sender that has begun transmitting version 1.0 is not compatible with a receiver that accepts only version 0.3. In a network upgraded incrementally, every receiver MUST therefore be capable of the newer version before any sender originates it.

Transport Authentication The transport carrying these parts MUST authenticate the calling party using a mechanism defined by the underlying protocol, such as those enumerated in . An unauthenticated transport endpoint permits an unbounded set of parties to consume the receiver's resources, to enqueue messages, and to attempt session establishment. End-to-end encryption does not mitigate this: MLS protects content, not availability. Implementations MUST NOT expose an unauthenticated message-submission endpoint in production.

Receiver Processing Rules

Mandatory Ordered Checks On receiving a Governed Object, whether from a decrypted MLS application message or from unencrypted carriage, a Relying Receiver MUST perform the following checks, and MUST perform them in this order:

1. Structural validation against . Reject on failure.
2. Signature verification per . Reject on failure.
3. Where the object arrived inside an MLS session, confirm that issuerDid equals the Agent Identity in the credential of the MLS member that sent the message. Reject on mismatch.
4. Expiry evaluation per . Reject if expired.
5. Replay rejection per . Reject if already seen.
6. Purpose enforcement per . Reject if not permitted.

Only after all six checks succeed may the receiver act upon payload. Check 3 is essential and is easily omitted. Without it, any member of a group can emit an object bearing another member's issuerDid; the signature check alone does not detect this, because the signature is valid -- it is simply not made by the party that sent it. Implementations MUST NOT treat a valid signature as evidence of who transmitted the message.

Replay Rejection A Relying Receiver MUST reject a Governed Object whose id it has previously accepted from the same issuerDid. Receivers MUST retain accepted (issuerDid, id) pairs for at least the maximum object lifetime they will accept. Where a receiver applies a maximum accepted lifetime, retention for that period is sufficient, because an older object is rejected by check 4 regardless. Absent this requirement, a passive observer of unencrypted carriage, or any member of an MLS group, can re-present a previously valid object. Where objects convey instructions with side effects -- a payment authorisation, a booking confirmation, a consent grant -- replay is directly exploitable.

Purpose Enforcement A Relying Receiver MUST maintain, per processing context, a set of permitted purpose values, and MUST reject an object whose governance.purpose is not in that set. The set MUST be determined by the receiver's own configuration for the context in which the object is being processed. It MUST NOT be derived from the object itself, nor from any value under the sender's control. Receivers MUST NOT use a payload for a purpose other than the one declared, and MUST NOT retain it beyond its expiry, save where an independent legal obligation requires retention.

Rejection Behaviour A receiver rejecting an object SHOULD emit a diagnostic distinguishable by cause, for operator use. A receiver SHOULD NOT return to the sender information that distinguishes between rejection causes beyond what is necessary for interoperability, because fine-grained rejection reasons assist an attacker in probing receiver configuration -- in particular in enumerating the permitted purpose set.

Regulatory Alignment This section is informative. The constructs specified here correspond to obligations that exist independently of this document in several jurisdictions. In the European Union, Article 5(1)(b) requires that personal data be collected for specified purposes and not further processed incompatibly with them; Article 5(1)(c) requires data minimisation; and Article 5(1)(e) requires that data be kept no longer than necessary. A Governed Object carries the specified purpose and the retention limit as signed metadata available to the receiving implementation at the moment of processing, rather than as prose in an agreement negotiated out of band. The mandatory receiver-side enforcement of makes those constraints operative in code. This does not constitute compliance, and this document does not offer legal advice. It observes only that the mechanism places the relevant facts where an implementation can act on them, which is a precondition for compliance being demonstrable rather than merely asserted.

Security Considerations

What This Specification Provides Within an MLS session, message content is confidential from the transport and from any intermediary, with the forward secrecy and post-compromise security properties of . Governed Objects are authenticated and integrity protected by Ed25519 signatures that remain verifiable independently of the session and after it ends, providing durable evidence of authorship. The credential binding of ties MLS membership to the same key, so that transmission and authorship cannot be separated.

What It Does Not Provide Purpose limitation is not cryptographically enforced. A receiver that has decrypted a message is in possession of the plaintext and can disregard the declared purpose and expiry. The mechanism provides a signed, non-repudiable record of the constraint under which disclosure was made, which converts an undetectable breach into an attributable one. It does not prevent the breach. Constructions that would provide stronger enforcement, such as purpose-bound decryption keys or confidential computing attestation, are out of scope. A DID is not an identity. It denotes control of a key. Binding a DID to a legal or natural person requires a credential presentation layer above this specification. Compromise of an agent's Ed25519 private key compromises both its MLS membership and its ability to sign Governed Objects, precisely because unifies them. Implementations MUST protect this key accordingly, and SHOULD use non-exportable key storage where the platform provides it. The unification is a deliberate trade: it eliminates a class of impersonation attack at the cost of concentrating key-compromise impact. MLS provides no protection against traffic analysis, and this specification adds none. Endpoints, message sizes, and timing are visible to the transport.

Availability An unauthenticated message-submission endpoint is a denial-of-service vector regardless of encryption; see . KeyPackage exhaustion is a related vector; see . Implementations that queue messages for offline recipients MUST bound queue size and MUST apply the checks of on dequeue rather than only on enqueue. Deferring validation to dequeue without bounding the queue permits an attacker to cause unbounded storage consumption with unvalidated input.

IANA Considerations

Media Types This document uses three media types, currently registered in neither the standards nor the vendor tree. The deployed reference implementation uses vendor-tree names. If this document advances, the author requests registration of the following in the standards tree, with the vendor-tree names retained as deprecated aliases:

Proposed	Deployed	Purpose
application/governed-object+json	application/vnd.atom.data-object+json	Governed Object
application/mls-wire+cbor	application/vnd.atom.mls-wire+cbor	MLS wire message
application/mls-handshake+json	application/vnd.atom.mls-handshake+json	Welcome and ratchet tree

Complete registration templates will be supplied in a subsequent revision.

Purpose Value Registry This document does not establish a registry of purpose values. Purpose vocabulary is application domain specific, and premature centralisation would impede the domain experimentation that ought to inform any eventual registry. The author solicits input on whether a registry of a small number of cross-domain purposes -- for instance a value denoting an authorisation to effect a payment -- would be of sufficient value to warrant the coordination cost, given that such values are the ones for which cross-implementation misinterpretation is most consequential.

Implementation Status This section is to be removed before publication as an RFC, per . An open source implementation exists under the Apache License 2.0 at https://github.com/Qwixl/Atom, and is deployed in a small public network. Implemented and interoperability-tested between two independent processes over HTTP:

• Governed Object construction, canonical serialisation, Ed25519 signing, and verification, including tamper and expiry rejection.
• did:key Ed25519 identity derivation without network resolution.
• MLS pair and group sessions using ciphersuite 0x0001 via the ts-mls library, including KeyPackage publication and retrieval, Welcome and ratchet tree delivery, application message encryption and decryption, and group state persistence.
• The A2A binding of at protocol version 1.0, including extension declaration in the Agent Card and per message, all three part encodings, and media type placement in both positions per .
• Version compatibility with A2A version 0.3 per , in both directions, tested against the published version 0.3 implementation rather than against a substitute: a version 1.0 sender delivering to a version 0.3 receiver, and a version 0.3 sender delivering to a version 1.0 receiver.
• Agent Card signing and verification per , with kid set to the agent's did:key and the verifying key recovered from that identifier without network retrieval. The relying check described there -- requiring the signing identity to match an asserted identity before associating a domain with an agent -- is applied.
• Purpose enforcement by receiver-configured allowlist, and expiry rejection.
• Out-of-band invitation with identity assertion, and abort on identity mismatch.
• Transport authentication on the message-submission endpoint (): Agent Cards declare an HTTP Bearer scheme (atomDidBearer), and /a2a/jsonrpc requires an Atom DID Bearer token (Authorization: Bearer atom.<payload>.<sig>) signed by the caller's did:key, with audience bound to the peer's public base URL.

Previously deferred and now implemented in the reference libraries:

• The credential binding key-equality check of (@qwixl/protocol + @qwixl/mls-session KeyPackage generation and admission).
• Replay rejection () via ReplayGuard in @qwixl/protocol, wired through the agent backend receive paths.
• Check 3 of , confirming issuerDid against the sending MLS member's LeafNode credential (decrypt returns senderDid; verifyDataObject({ expectedMlsSenderDid })).
• Validation on dequeue for messages queued to offline recipients (): dequeueAsleepMessages applies when the agent wakes; the queue remains size-bounded at enqueue.

The author notes that several of these items were identified by the exercise of writing this specification, having not been apparent from the working implementation. This is offered as evidence for the general proposition that specification and implementation are not redundant activities. A further instance arose in preparing this document. The reference implementation's own documentation described a part using the tagged-union form produced by its code generator, rather than the JSON actually transmitted. The discrepancy was invisible to that implementation, which both produced and consumed the same internal representation, and would have been discovered only by a second implementation in another language attempting to interoperate from the description. The requirement in that members be named as they appear on the wire, and the encapsulation vectors of , exist to prevent a recurrence.

Changes from draft-chapman-a2a-mls-00

• Clarified Extension Declaration: messages carrying only MLS wire or MLS handshake parts MUST NOT stamp the Governed Object extension URI; GO-carrying messages SHOULD list it. The Agent Card declaration for the GO URI uses required false.
• Noted the separately published GO-only A2A extension profile under spec/extensions/data-object-v1/ in the reference implementation repository. This document remains the broader GO+MLS provenance draft.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Messaging applications are increasingly making use of end-to-end security mechanisms to ensure that messages are only accessible to the communicating endpoints, and not to any servers involved in delivering messages. Establishing keys to provide such protections is challenging for group chat settings, in which more than two clients need to agree on a key but may not be online at the same time. In this document, we specify a key establishment protocol that provides efficient asynchronous group key establishment with forward secrecy (FS) and post-compromise security (PCS) for groups in size ranging from two to thousands. This document describes elliptic curve signature scheme Edwards-curve Digital Signature Algorithm (EdDSA). The algorithm is instantiated with recommended parameters for the edwards25519 and edwards448 curves. An example implementation and test vectors are provided. JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] Informative References This document describes a simple process that allows authors of Internet-Drafts to record the status of known implementations by including an Implementation Status section. This will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. This process is not mandatory. Authors of Internet-Drafts are encouraged to consider using the process for their documents, and working groups are invited to think about applying the process to all of their protocol specifications. This document obsoletes RFC 6982, advancing it to a Best Current Practice. Cryptographic operations like hashing and signing need the data to be expressed in an invariant format so that the operations are reliably repeatable. One way to address this is to create a canonical representation of the data. Canonicalization also permits data to be exchanged in its original form on the "wire" while cryptographic operations performed on the canonicalized counterpart of the data in the producer and consumer endpoints generate consistent results. This document describes the JSON Canonicalization Scheme (JCS). This specification defines how to create a canonical representation of JSON data by building on the strict serialization methods for JSON primitives defined by ECMAScript, constraining JSON data to the Internet JSON (I-JSON) subset, and by using deterministic property sorting. A2A Project (Linux Foundation) W3C Credentials Community Group n.d. Anthropic n.d. European Union

Test Vectors Machine-readable test vectors accompany this document, comprising: signed Governed Objects with known Ed25519 key pairs and their canonical serialisations; objects with mutated payloads that MUST fail verification; expired objects; objects whose declared purpose is outside a stated permitted set; a replayed object pair; and a KeyPackage whose credential identity does not match its signature key, which MUST be rejected per . A second group of vectors covers the encapsulation of as serialised JSON, independently of any Governed Object contained within: parts carrying the media type in the part member, in the data member, and in both; a part whose two media type members disagree, which MUST be rejected per ; and a part bearing no media type in either position. These vectors exist because the encapsulation is the layer at which two implementations in different languages first fail to interoperate, and it is the layer least well served by reading either implementation's source. Vectors are published at https://github.com/Qwixl/Atom under spec/vectors/. An implementation claiming conformance to this document SHOULD produce the specified outcome for every vector. The vectors were derived from this document's text rather than generated from the reference implementation, so that the two are capable of disagreeing. On first execution they did: the reference implementation treated governance.expiresAt as authoritative whenever present and never compared it to the interval implied by ttlSeconds, so a sender could extend a short-lived object's lifetime indefinitely by supplying an additional distant absolute expiry. Because the governance block is covered by the signature, the resulting object was valid under every other check. This is the origin of the requirement in that the earlier instant govern, and it is offered as a concrete instance of why vectors written from specification text are worth the effort of producing.

Acknowledgements This work builds directly on the MLS protocol and on the A2A protocol . The author thanks the authors and contributors of both.