Agent Identity and Discovery (AID)

2.1. Requirements Language

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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

2.2. Terms

AID Client

Software that performs AID discovery for a domain.¶

Provider

The entity that controls a domain and publishes the AID TXT record.¶

_agent subdomain

The DNS name `_agent.<domain>` where the canonical AID TXT record is published.¶

A-label

The Punycode representation of an Internationalized Domain Name as described in [RFC5890].¶

PKA

Public Key for Agent: an optional Ed25519 endpoint-proof key in the AID record.¶

JWK `x`

The unpadded base64url public-key member of an Ed25519 OKP JWK.¶

JWK thumbprint

An RFC 7638 hash of a canonical JWK representation. AID v2 uses this value as the HTTP Message Signature `keyid`.¶

Trust source

The source from which the selected AID record was obtained: `dns` or `well-known-tls`.¶

3.1. Syntax and Parsing

The record MUST be a single semicolon-delimited string of `key=value` pairs. Clients SHOULD trim leading and trailing whitespace from keys and values. Clients MUST ignore unknown keys unless the key has a known legacy meaning that this specification explicitly rejects for `aid2`.¶

If a DNS server splits the TXT record into multiple 255-octet character strings, the client MUST concatenate them in order before parsing. Providers SHOULD keep the total payload below 255 octets when practical.¶

Clients MUST recognize single-letter lowercase aliases. A record MUST NOT include both a full key and its alias. Key comparisons are case-insensitive. Providers SHOULD emit the short-key form for compact DNS deployment.¶

3.2. Defined Keys

AID v2 records MUST NOT use `kid` or `i` for endpoint proof. A `v=aid2` record containing `kid` or `i` is invalid. The expected key identifier is derived from `k` as described in Appendix D.¶

3.3. Examples

_agent.example.com. 300 IN TXT "v=aid2;u=https://api.example.com/mcp;p=mcp;a=pat;s=Example AI Tools"

_agent.example.com. 300 IN TXT "v=aid2;p=mcp;u=https://api.example.com/mcp;k=JrQLj5P_89iXES9-vFgrIy29clF9CC_oPPsw3c5D0bs;a=oauth2_code;s=Secure AI Gateway"

_agent.example.com. 300 IN TXT "v=aid2;p=websocket;u=wss://agent.example.com/session;a=oauth2_code;s=Streaming Agent"

_agent.grafana.com. 300 IN TXT "v=aid2;u=docker:grafana/mcp:latest;p=local;a=pat;s=Run Grafana agent locally"

_agent.example.com. 300 IN TXT "v=aid1;p=mcp;u=https://api.example.com/mcp;k=<legacy-v1-multibase-key>;i=g1"
_agent.example.com. 300 IN TXT "v=aid2;p=mcp;u=https://api.example.com/mcp;k=<same-key-as-v2-base64url>"

Clients that support both versions partition records by version before ambiguity checks. A same-key migration from legacy `aid1` to current `aid2` is not a key rotation when the decoded Ed25519 public key bytes are the same.¶

4.1. Discovery Algorithm

When an AID client is given a domain, it MUST perform the following steps:¶

1. Normalize the domain. If the domain contains non-ASCII labels, convert them to A-label form as described in [RFC5890].¶
2. Query TXT records for the canonical base name `_agent.<exact-host-user-entered>`. Clients MUST NOT walk up to parent domains.¶
3. Parse returned TXT answers as semicolon-delimited `key=value` records. Key comparisons are case-insensitive.¶
4. Validate records using versioned rules. For `aid2`, require `v`, `u`, and `p`; reject a record containing both a full key and alias; reject `kid` or `i`; and reject malformed `k` values.¶
5. Partition valid records by AID major version.¶
6. Select the highest supported valid version allowed by local policy. Clients that support `aid2` SHOULD prefer `aid2` over `aid1`.¶
7. Within the selected version, if exactly one valid record exists, use it. If more than one valid record exists, fail with ambiguity. Clients MUST NOT choose among multiple valid same-version records by DNS answer order.¶
8. Process optional metadata. If `docs` is present, the client MAY display it. If `dep` is in the future, the client SHOULD warn. If `dep` is in the past, the client SHOULD fail gracefully.¶
9. If `k` is present, perform PKA endpoint proof using Appendix D.¶
10. Return the discovered endpoint, protocol, metadata, PKA state, and trust source.¶

Malformed answers do not matter when there is exactly one valid record in the selected version. Returning clients that previously selected `aid2` SHOULD treat an `aid1`-only result as a version downgrade.¶

If no DNS record is found or DNS lookup fails, the client MAY attempt the `.well-known` fallback on the same exact host as described in Appendix E. If both DNS and fallback fail, discovery fails.¶

4.2. Standard Client Error Codes

4.3. Exact-Host Semantics and Delegation

Discovery is exact-host by default. If the application asks for `app.team.example.com`, the base query is `_agent.app.team.example.com`.¶

Clients MUST NOT implicitly retry parent hosts such as `_agent.team.example.com` or `_agent.example.com`.¶

If an operator wants child hosts to inherit a shared record, that inheritance MUST be expressed in DNS for the exact queried name. A common pattern is a CNAME from the child host's `_agent` label to a shared `_agent` record.¶

_agent.app.team.example.com. 300 IN CNAME _agent.shared.team.example.com.
_agent.shared.team.example.com. 300 IN TXT "v=aid2;p=mcp;u=https://gateway.team.example.com/mcp;k=<current-key>"

4.4. Protocol-Specific Names

The canonical v2 location is the base record `_agent.<domain>`. Providers MAY additionally publish protocol-specific names such as `_agent._mcp.<domain>` or `_agent._a2a.<domain>` for legacy clients, diagnostics, or explicitly configured base-failure probing.¶

When an application explicitly requests a protocol, v2 clients still query the canonical base name `_agent.<exact-host-user-entered>` first and filter that record for the requested protocol. Protocol-prefixed `_agent._<proto>.<exact-host-user-entered>` probing is legacy, diagnostic, or base-failure-only behavior where supported and explicitly configured.¶

Clients MUST NOT perform a compatibility lookup at `_agent.<proto>.<domain>`. Protocol-specific names, when used, always use the underscore form `_agent._<proto>.<domain>`.¶

5.1. What PKA Proves

PKA proves exactly this: the endpoint reached at the discovered URI controls the Ed25519 private key corresponding to the public key currently published in the domain's selected AID record.¶

PKA does not prove that a user authorized a specific action, that an OAuth token is valid, that a workload identity belongs to a trust domain, that an internal policy engine approved a request, or that a key change is cryptographically continuous with a previous key.¶

5.2. Threat Model

AID addresses DNS spoofing and cache poisoning with optional DNSSEC validation, endpoint impersonation with PKA endpoint proof using Ed25519 HTTP Message Signatures, downgrade detection with remembered state, command injection risks for local execution, and cross-origin redirect abuse for remote endpoints.¶

Returning clients can detect PKA removal, key replacement, and `aid2` to `aid1` version downgrade when they retain previous security state. If the key changes, the key changed; AID v2 core does not define DNS-level cryptographic continuity across key changes.¶

Compromised authoritative DNS servers, active attackers after TLS validation fails, authorization, delegation, user consent, reputation, workload federation, and managed cryptographic rotation in the core DNS record are outside the direct scope of AID v2.¶

5.3. Enterprise Policy Modes

Clients that expose enterprise controls SHOULD provide simple policy knobs.¶

• PKA policy: `if-present | require`¶
• DNSSEC policy: `off | prefer | require`¶
• Well-known policy: `auto | disable`¶
• Downgrade policy: `off | warn | fail`¶

Policy semantics are as follows:¶

• `pka=require`: discovery fails if the selected record has no `k`.¶
• `dnssec=require`: discovery fails when DNSSEC validation is unavailable or unsuccessful for the selected DNS answer.¶
• `well-known=disable`: clients do not use `/.well-known/agent` fallback.¶
• `downgrade=warn|fail`: applies to PKA removal, key replacement, and `aid2` to `aid1` downgrade when previous state exists.¶

If discovery succeeds only through `.well-known`, that result cannot satisfy a policy that requires DNSSEC validation.¶

7.1. No Multi-Key RRset in Core

AID v2 core rejects the following shape as a core discovery result:¶

_agent.acme.com. 300 IN TXT "v=aid2;p=mcp;u=https://agent.acme.com/mcp;k=<old-key>"
_agent.acme.com. 300 IN TXT "v=aid2;p=mcp;u=https://agent.acme.com/mcp;k=<new-key>"

This changes the model from one selected endpoint record with an optional proof key into one endpoint descriptor with an active key set where the HTTP response `keyid` selects a key. That larger model requires rules for normalized non-key field equivalence, duplicate handling, partial DNS propagation, key-set pinning, downgrade policy, SDK return types, and conformance fixtures. It also does not solve lost-key or compromised-key recovery.¶

7.2. Future Rotation Profile

If AID later needs managed rotation for pinned clients or provenance profiles, it should be a separate HTTP key-directory profile, likely using JWKS or Web Bot Auth style overlap.¶

{
  "keys": [
    {
      "kty": "OKP",
      "crv": "Ed25519",
      "kid": "<jwk-thumbprint>",
      "x": "<publicKey>",
      "use": "sig",
      "nbf": 1712793600,
      "exp": 1715385600
    }
  ]
}

If such a directory chains to DNS `k`, that chaining is the defining property of an AID-anchored key directory. If it does not chain to DNS `k`, it is a normal external key directory outside AID core.¶

D.1. Key Decoding

For v2, `k` MUST be unpadded base64url. Decoding MUST produce exactly 32 octets. Legacy `z...` multibase keys MUST NOT be accepted in `v=aid2`.¶

The corresponding JWK is:¶

{ "kty": "OKP", "crv": "Ed25519", "x": "<k>" }

D.2. Derived Key Identifier

The expected HTTP Message Signature `keyid` is the RFC 7638 JWK thumbprint using SHA-256 over this exact UTF-8 JSON serialization, with no extra spaces:¶

{"crv":"Ed25519","kty":"OKP","x":"<k>"}

The SHA-256 digest is encoded as unpadded base64url. Implementations MUST NOT hash the raw public key bytes directly for `keyid`.¶

D.3. Request and Response Shape

The validated RFC 9421 Structured Fields shape is:¶

Accept-Signature: aid-pka=("@method";req "@target-uri";req "@authority";req "@status");created;expires;keyid="<jwk-thumbprint>";alg="ed25519";nonce="<client-challenge>";tag="aid-pka-v2"
Signature-Input: aid-pka=("@method";req "@target-uri";req "@authority";req "@status");created=<unix>;expires=<unix>;keyid="<jwk-thumbprint>";alg="ed25519";nonce="<client-challenge>";tag="aid-pka-v2"
Signature: aid-pka=:<base64-signature>:
Cache-Control: no-store

The signature MUST NOT cover HTTP `Date`.¶

The client challenge MUST contain at least 32 bytes of entropy and SHOULD be transported as unpadded base64url in the RFC 9421 `nonce` signature parameter. The verifier MUST compare the received `nonce` exactly to the challenge it sent.¶

Servers are not required to store nonce state in v2 core because the verifier supplies the one-shot nonce and the signed response is not cacheable.¶

`created` and `expires` are mandatory. `expires` MUST be greater than `created`. `expires - created` MUST NOT exceed 300 seconds and SHOULD be 60 seconds or less. Verifiers MAY allow a small clock-skew tolerance when evaluating `created` and `expires`.¶

Signers MUST emit `alg="ed25519"` lowercase. Verifiers MUST compare the semantic algorithm value case-insensitively and MUST reconstruct `@signature-params` from the received Structured Field value.¶

D.4. Covered Components

The v2 PKA response signature covers:¶

• `"@method";req`¶
• `"@target-uri";req`¶
• `"@authority";req`¶
• `"@status"`¶

`@method`, `@target-uri`, and `@authority` are request-derived components and therefore use `;req`. `@status` is response-derived and does not use `;req`.¶

`@status` signs the status actually returned. PKA does not require status `200`. A signed `401` can still prove endpoint authenticity before an OAuth or auth.md handoff continues.¶

D.5. URI, Authority, and Redirects

Clients MUST NOT follow redirects during PKA verification. The request context is the discovered endpoint URI after fragment removal. Query strings are preserved.¶

`@authority` uses the externally visible request authority: lowercase hostname, omitted default port, and retained non-default port.¶

Servers behind reverse proxies must sign the externally visible request context, not internal hop-local scheme, host, or port values.¶

D.6. Verifier Summary

A verifier accepts a v2 PKA response only when:¶

1. the selected AID record contains valid v2 `k`;¶
2. the response contains a valid `Signature-Input` and `Signature`;¶
3. the covered components and `tag="aid-pka-v2"` match this profile;¶
4. `keyid` equals the RFC 7638 thumbprint derived from DNS `k`;¶
5. `alg` has semantic value `ed25519`;¶
6. `nonce` exactly equals the verifier-generated challenge;¶
7. `created` and `expires` pass freshness checks;¶
8. the response includes `Cache-Control: no-store`;¶
9. Ed25519 verification succeeds over the reconstructed RFC 9421 signature base.¶

F.1. Web Bot Auth

AID v2 follows Web Bot Auth where the layers match: Ed25519, RFC 9421 HTTP Message Signatures, RFC 7638 JWK thumbprints, `created`, `expires`, `nonce`, and `tag`.¶

AID does not become Web Bot Auth. Web Bot Auth signs automated client requests to origins. AID PKA proves endpoint control for a DNS-discovered agent endpoint.¶

Operators may reuse key material across AID and Web Bot Auth if their threat model allows it, but AID does not recommend reuse. Reuse shares blast radius between endpoint proof and request-signing.¶

F.2. auth.md and OAuth

AID can support auth.md without adding auth.md-specific fields to core:¶

1. AID resolves a domain to `u` and `p`.¶
2. If `k` is present, PKA verifies the endpoint.¶
3. The client follows the endpoint, protocol, OAuth, or auth.md layer where present.¶

AID does not specify ID-JAG `aud`, provider trust lists, registration payloads, scopes, credential types, revocation, or auth.md metadata registries.¶

F.3. SPIFFE and WIMSE

AID may be used by future SPIFFE or WIMSE profiles as a public first-contact anchor. AID v2 core does not define SPIFFE trust-domain mapping, SPIFFE bundle federation, WIMSE hop re-binding, OAuth client registration, or workload authorization.¶

F.4. Pkarr

AID v2 remains compatible with the Pkarr-inspired idea of compact Ed25519 key material, but it does not adopt Pkarr's identity model.¶

Pkarr makes the public key the address and signs DNS packets under that key. AID remains DNS-authority-rooted: the DNS owner publishes the current endpoint and current endpoint-proof key inside a TXT payload.¶

Reviewer-facing language should say "DNS-current endpoint/key" and should not imply Pkarr-style self-certifying names, key-addressed identity, or cryptographic continuity across key changes.¶

G.1. Provider Migration

1. Inventory current `aid1` records.¶
2. For PKA records, decode the v1 multibase/base58btc `k` to the 32-byte Ed25519 public key.¶
3. Encode those bytes as unpadded base64url. This is the v2 `k`.¶
4. Publish `v=aid2` with the same endpoint, protocol, and v2 `k`, without `i`.¶
5. Keep `aid1` during the compatibility window.¶
6. Remove `aid1` after old-client support is no longer needed.¶

G.2. Client Migration

1. Support both `aid1` and `aid2` parsing.¶
2. Partition records by version before ambiguity checks.¶
3. Prefer `aid2` when both versions are valid and policy allows it.¶
4. Keep legacy PKA only for selected `aid1` records.¶
5. Store previous security state using derived JWK thumbprints so same-key `aid1` to `aid2` migration does not look like key replacement.¶