HTTP Signature-Key Header

2.1. Label Consistency

If a label appears in Signature or Signature-Input, and the verifier attempts to verify it, the corresponding member MUST exist in Signature-Key. If Signature-Key contains members for labels not being verified, verifiers MAY ignore them.¶

2.2. Multiple Signatures

The dictionary format supports multiple signatures per message. Each signature has its own dictionary member keyed by its unique label:¶

Signature-Input: sig1=(... "signature-key"), sig2=(... "signature-key")
Signature: sig1=:...:, sig2=:...:
Signature-Key: sig1=jwt;jwt="eyJ...", sig2=jwks_uri;id="https://example.com";well-known="meta";kid="k1"

Most deployments SHOULD use a single signature. When multiple signatures are required, the complete Signature-Key header (containing all keys) MUST be populated before any signature is created, and each signature MUST cover signature-key. This ensures all signatures protect the integrity of all key material. See Signature-Key Integrity in Security Considerations. Alternative key distribution mechanisms outside this specification may be used for scenarios requiring independent signature addition.¶

2.3. Header Web Key (hwk)

The hwk scheme provides a self-contained public key inline in the header, enabling pseudonymous verification without key discovery.¶

Parameters by key type:¶

OKP (Octet Key Pair):¶

• kty (REQUIRED) - "OKP"¶

• crv (REQUIRED) - Curve name (e.g., "Ed25519")¶

• x (REQUIRED) - Public key value¶

Signature-Key: sig=hwk;kty="OKP";crv="Ed25519";x="JrQLj5P..."

EC (Elliptic Curve):¶

• kty (REQUIRED) - "EC"¶

• crv (REQUIRED) - Curve name (e.g., "P-256", "P-384")¶

• x (REQUIRED) - X coordinate¶

• y (REQUIRED) - Y coordinate¶

Signature-Key: sig=hwk;kty="EC";crv="P-256";x="f83OJ3D...";y="x_FEzRu..."

RSA:¶

• kty (REQUIRED) - "RSA"¶

• n (REQUIRED) - Modulus¶

• e (REQUIRED) - Exponent¶

Signature-Key: sig=hwk;kty="RSA";n="0vx7agoebGcQ...";e="AQAB"

Constraints:¶

• The alg parameter MUST NOT be present (algorithm is specified in Signature-Input)¶

• The kid parameter SHOULD NOT be used¶

Use cases:¶

• Privacy-preserving agents that avoid identity disclosure¶

• Experimental or temporary access without registration¶

• Rate limiting and reputation building on a per-key basis¶

2.4. JWKS URI Discovery (jwks_uri)

The jwks_uri scheme identifies the signer and enables key discovery via a metadata document containing a jwks_uri property.¶

Parameters:¶

• id (REQUIRED) - Signer identifier (HTTPS URL)¶

• well-known (REQUIRED) - Metadata document name under /.well-known/¶

• kid (REQUIRED) - Key identifier¶

Note: The well-known parameter may be shortened to wk once the semantics are stable.¶

Discovery procedure:¶

1. Fetch {id}/.well-known/{well-known}¶

2. Parse as JSON metadata¶

3. Extract jwks_uri property¶

4. Fetch JWKS from jwks_uri¶

5. Find key with matching kid¶

Example:¶

Signature-Key: sig=jwks_uri;id="https://agent.example";well-known="aauth-agent";kid="key-1"

Use cases:¶

• Identified services with stable HTTPS identity¶

• Search engine crawlers and monitoring services¶

• Services requiring explicit entity identification¶

2.5. X.509 Certificates (x509)

The x509 scheme provides certificate-based verification using PKI trust chains.¶

Parameters:¶

• x5u (REQUIRED) - URL to X.509 certificate chain (PEM format, [RFC7517] Section 4.6)¶

• x5t (REQUIRED) - Certificate thumbprint: BASE64URL(SHA256(DER(leaf_cert)))¶

Verification procedure:¶

1. Check cache for certificate with matching x5t¶

2. If not cached or expired, fetch PEM from x5u¶

3. Validate certificate chain to trusted root CA¶

4. Check certificate validity and revocation status¶

5. Verify x5t matches leaf certificate¶

6. Extract public key from end-entity certificate¶

7. Verify signature using extracted key¶

8. Cache certificate indexed by x5t¶

Example:¶

Signature-Key: sig=x509;x5u="https://agent.example/.well-known/cert.pem";x5t="bWcoon4QTVn8Q6xiY0ekMD6L8bNLMkuDV2KtvsFc1nM"

Use cases:¶

• Enterprise environments with PKI infrastructure¶

• Integration with existing certificate management systems¶

• Scenarios requiring certificate revocation checking¶

• Regulated industries requiring certificate-based authentication¶

2.6. JWT Confirmation Key (jwt)

The jwt scheme embeds a public key inside a signed JWT using the cnf (confirmation) claim [RFC7800], enabling delegation and horizontal scale.¶

Parameters:¶

• jwt (REQUIRED) - Compact-serialized JWT¶

JWT requirements:¶

• MUST contain cnf.jwk claim with embedded JWK¶

• SHOULD contain standard claims: iss, sub, exp, iat¶

Note: The mechanism by which the JWT is obtained is out of scope of this specification.¶

Verification procedure:¶

1. Validate the JWT: verify signature using issuer's public key and verify claims per policy (iss, exp, etc.)¶

2. Extract JWK from cnf.jwk¶

3. Verify HTTP Message Signature using extracted key¶

Example:¶

Signature-Key: sig=jwt;jwt="eyJhbGciOiJFUzI1NiI..."

JWT payload example:¶

{
  "iss": "https://issuer.example",
  "sub": "instance-123",
  "exp": 1732210000,
  "cnf": {
    "jwk": {
      "kty": "OKP",
      "crv": "Ed25519",
      "x": "JrQLj5P_89iXES9-vFgrIy29clF9CC_oPPsw3c5D0bs"
    }
  }
}

Use cases:¶

• Distributed services with ephemeral instance keys¶

• Delegation scenarios where instances act on behalf of an authority¶

• Short-lived credentials for horizontal scaling¶

3.1. Key Validation

Verifiers MUST validate all cryptographic material before use:¶

• hwk: Validate JWK structure and key parameters¶

• jwks_uri: Verify HTTPS transport and validate fetched JWKS¶

• x509: Validate complete certificate chain, check revocation status¶

• jwt: Verify JWT signature and validate embedded JWK¶

3.2. Caching and Performance

Verifiers MAY cache keys to improve performance but MUST implement appropriate cache expiration:¶

• jwks_uri: Respect cache-control headers, implement reasonable TTLs¶

• x509: Cache by x5t, invalidate on certificate expiry¶

• jwt: Cache embedded keys until JWT expiration¶

Verifiers SHOULD implement cache limits to prevent resource exhaustion attacks.¶

3.3. Scheme-Specific Risks

hwk: No identity verification - suitable only for scenarios where pseudonymous access is acceptable.¶

jwks_uri: Relies on HTTPS security - vulnerable to DNS/CA compromise. Verifiers should implement certificate pinning where appropriate.¶

x509: Requires robust certificate validation including revocation checking. Verifiers MUST NOT skip certificate chain validation.¶

jwt: Delegation trust depends on JWT issuer verification. Verifiers MUST validate JWT signatures and claims before trusting embedded keys.¶

3.4. Algorithm Selection

The alg parameter in Signature-Input (RFC 9421) determines the signature algorithm. Verifiers MUST:¶

• Validate algorithm against policy (reject weak algorithms)¶

• Ensure key type matches algorithm requirements¶

• Reject algorithm/key mismatches¶

3.5. Signature-Key Integrity

The Signature-Key header SHOULD be included as a covered component in Signature-Input:¶

Signature-Input: sig=("@method" "@authority" "@path" "signature-key"); created=1732210000

If signature-key is not covered, an attacker can modify the header without invalidating the signature. Attacks include:¶

Scheme substitution: An attacker extracts the public key from an hwk scheme and republishes it via jwks_uri under their own identity, causing verifiers to attribute the request to the attacker.¶

Identity substitution: An attacker modifies the id parameter in a jwks_uri scheme to point to their own metadata endpoint that returns the same public key, impersonating a different signer.¶

Verifiers SHOULD reject requests where signature-key is not a covered component.¶

4.1. Pseudonymity vs. Identity

The hwk scheme enables pseudonymous operation where the signer's identity is not disclosed. Verifiers should be aware that:¶

• hwk provides no identity linkage across requests (unless keys are reused)¶

• Key reuse enables tracking but may be necessary for reputation/rate-limiting¶

• Verifiers should not log or retain hwk keys beyond operational necessity¶

The jwks_uri, x509, and jwt schemes all reveal signer identity. Protocols using these schemes should inform signers that their identity will be disclosed to verifiers.¶

4.2. Key Discovery Tracking

The jwks_uri and x509 schemes require verifiers to fetch resources from signer-controlled URLs. This creates potential tracking vectors:¶

• Signers can observe when and from where keys are fetched¶

• Verifiers should cache keys to minimize fetches¶

• Verifiers may wish to use shared caching infrastructure to reduce fingerprinting¶

4.3. JWT Contents

JWTs in the jwt scheme may contain additional claims beyond cnf. Verifiers should:¶

• Only process claims necessary for verification¶

• Not log or retain unnecessary JWT claims¶

• Be aware that JWT contents are visible to network observers unless using TLS¶

5.1. HTTP Field Name Registration

This document registers the Signature-Key header field in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" defined in [RFC9110].¶

Header field name: Signature-Key¶

Applicable protocol: http¶

Status: standard¶

Author/Change controller: IETF¶

Specification document(s): [this document]¶

5.2. Signature-Key Scheme Registry

This document establishes the "HTTP Signature-Key Scheme" registry. This registry allows for the definition of additional key distribution schemes beyond those defined in this document.¶