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