Proposal: Cryptographic Agent Identity extension (CTEF v0.3.1-aligned)

## Abstract

Proposal for an A2A extension carrying cryptographic agent identity claims aligned with the CTEF v0.3.1 substrate (live at [`/.well-known/cte-test-vectors.json`](https://agentgraph.co/.well-known/cte-test-vectors.json), commit `agentgraph-co/agentgraph@69ad94d`). Claims declare a `claim_type` discriminator over the closed set `{identity, transport, authority, continuity}` (reserved: `envelope`), with per-layer composition rules and structural-before-semantic error codes (`INVALID_CLAIM_SCOPE`, `INVALID_COMPOSITION`).

## Motivation

A2A's existing `AgentCardSignature` (Section 4.4.7) signs the AgentCard as a document. Production deployments also need protocol-level handles for the agent's *subject identity* — composable across the lifecycle, not just at card publication. Specifically: who the agent is (DID/key binding), what the upstream authority ceiling is (delegation chain), whether the identity has been continuous since a prior interaction (rotation-attestation chain).

The four-layer split was developed across multiple implementations on [#1672](https://github.com/a2aproject/A2A/issues/1672) over the last week, and the wire format is now byte-match-validated across three independent canonicalizers.

## Technical approach

The extension uses A2A's existing `AgentExtension` mechanism (Section 4.4.4) with `params` carrying the per-claim payload. No proto schema changes proposed. Reference URI: `https://a2a-protocol.org/extensions/cryptographic-agent-identity/v0.3.1` (experimental prefix until graduation).

Wire-format substrate: CTEF v0.3.1 at `agentgraph-co/agentgraph@69ad94d`. Composition rules and negative-path test vectors (`scope_violation_vector`, `composition_failure_vector`) are published at the cited endpoint.

Reference implementations and alignments as of 2026-04-24:

- **AgentGraph** — CTEF v0.3.1 normative substrate (commit `69ad94d`)
- **APS / Agent Passport System** — JCS canonicalizer + bilateral-delegation and rotation-attestation fixture sets
- **AgentID** — JCS canonicalizer byte-matching APS fixtures; `claim_type` live on `/verify` as of 2026-04-24
- **Nobulex** (`@nobulex/crypto`) — TS canonicalizer byte-matching APS + AgentGraph fixtures
- **HiveTrust** — concurs with the four-layer split and "history-stability under rotation" framing on #1672; concrete continuity-layer mapping in progress

## Out of scope

- Identity issuance — extension consumes existing DID methods without prescribing one
- Action-boundary verification — separate concern, see [PIC Standard (aaif/project-proposals#16)](https://github.com/aaif/project-proposals/issues/16)
- Payment receipts — `evidence_basis.evidence_type.payment_execution` reserved separately in CTEF v0.3.1

## Sponsorship request

Per [`extension-and-binding-governance.md`](https://github.com/a2aproject/A2A/blob/main/docs/topics/extension-and-binding-governance.md) Proposal Phase, requesting maintainer sponsorship to create the corresponding `experimental-ext-*` repository per governance. The detailed spec, conformance suite, and reference implementations land in that repo once sponsored. Happy to address scope, interface, or governance questions ahead of sponsorship.

## References

- [#1672](https://github.com/a2aproject/A2A/issues/1672) — design discussion
- [#1734](https://github.com/a2aproject/A2A/discussions/1734) — CTEF v0.3.1 RFC
- [CTEF v0.3.1 normative](https://agentgraph.co/.well-known/cte-test-vectors.json) — frozen 2026-04-24, `agentgraph-co/agentgraph@69ad94d`
- [APS rotation-attestation fixtures (SHA table + canonical bytes)](https://aeoess.com/fixtures/rotation-attestation/test-vectors.json) — five fixtures: happy-path, cross-signed, migration-attested, happy-path-compound, negative-no-attestation

The issue proposes a new A2A extension for cryptographic agent identity. Review the existing AgentExtension mechanism (Section 4.4.4 of the A2A spec) and the CTEF v0.3.1 substrate at the provided URI. Examine the linked discussion #1672 and the test vectors to understand the four-layer claim structure. The work involves protocol design and alignment with multiple implementations, not a specific code change.

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