HTTP Signature-Key Header

2.1. Label Consistency

If a label appears in Signature or Signature-Input, and the verifier attempts to verify it using Signature-Key, 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";dwk="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. The parameter names and values correspond directly to the JWK parameters defined in [RFC7517].¶

Parameters by key type:¶

OKP (Octet Key Pair):¶

• kty (REQUIRED, String) - "OKP"¶

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

• x (REQUIRED, String) - Public key value¶

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

EC (Elliptic Curve):¶

• kty (REQUIRED, String) - "EC"¶

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

• x (REQUIRED, String) - X coordinate¶

• y (REQUIRED, String) - Y coordinate¶

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

RSA:¶

• kty (REQUIRED, String) - "RSA"¶

• n (REQUIRED, String) - Modulus¶

• e (REQUIRED, String) - 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¶

Design Note: The hwk parameters use structured field strings rather than byte sequences. JWK key values are base64url-encoded per [RFC7517], while structured field byte sequences use base64 encoding per [RFC8941]. Using strings allows implementations to pass JWK values directly without converting between base64url and base64, avoiding a potential source of encoding bugs.¶

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, String) - Signer identifier (HTTPS URL)¶

• dwk (REQUIRED, String) - Dot well-known metadata document name under /.well-known/¶

• kid (REQUIRED, String) - Key identifier¶

Discovery procedure:¶

1. Fetch {id}/.well-known/{dwk}¶

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";dwk="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, String) - URL to X.509 certificate chain (PEM format, [RFC7517] Section 4.6)¶

• x5t (REQUIRED, Byte Sequence) - Certificate thumbprint: SHA-256 hash of DER-encoded end-entity certificate¶

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 end-entity 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, String) - Compact-serialized JWT¶

JWT requirements:¶

• MUST contain iss claim (HTTPS URL of the issuer)¶

• MUST contain dwk claim (dot well-known metadata document name) — the verifier constructs {iss}/.well-known/{dwk} to discover the issuer's jwks_uri¶

• MUST contain cnf.jwk claim with embedded JWK¶

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

• Verifiers SHOULD verify the JWT typ header parameter has an expected value per deployment policy¶

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

Verification procedure:¶

1. Verify the JWT typ header parameter has an expected value per policy. Reject if unexpected.¶

2. Extract iss and dwk claims from the JWT payload¶

3. Fetch {iss}/.well-known/{dwk}, parse as JSON metadata, extract jwks_uri¶

4. Fetch JWKS from jwks_uri, find key matching kid in JWT header¶

5. Verify JWT signature using the discovered key¶

6. Validate JWT claims per policy (iss, exp, etc.)¶

7. Extract JWK from cnf.jwk¶

8. Verify HTTP Message Signature using extracted key¶

Example:¶

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

JWT payload example:¶

{
  "iss": "https://issuer.example",
  "dwk": "oauth-authorization-server",
  "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.¶

A.1. draft-hardt-httpbis-signature-key-02

• Changed x5t parameter to byte sequence per reviewer feedback¶
• Added structured field types to all parameters¶
• Added design note explaining string vs byte sequence choice for hwk¶

A.2. draft-hardt-httpbis-signature-key-01

• Initial public draft with four schemes: hwk, jwks_uri, x509, jwt¶

B.1. Why a Separate Header?

An alternative design would extend Signature-Input with additional parameters to carry key material. This was considered and rejected for several reasons:¶

1. Parameter complexity: Each scheme has a different set of parameters (e.g., hwk needs kty, crv, x, y; jwks_uri needs id, dwk, kid; jwt needs a full JWT string). Overloading Signature-Input with all possible key parameters across all schemes would make the Signature-Input grammar unwieldy and harder to parse.¶

2. Separation of concerns: Signature-Input describes what is signed and how (covered components, algorithm, timestamps). Signature-Key describes who signed it and where to find the key. These are distinct concerns, and separating them into distinct headers makes each easier to understand and process independently.¶

3. Extensibility: A separate header with a scheme registry allows new key distribution mechanisms to be added without modifying the Signature-Input grammar. New schemes can define arbitrary parameters without coordination with RFC 9421.¶

4. Multiple signatures: With a dictionary structure keyed by label, each signature can use a different scheme. This is natural in a separate header but would create complex nesting if embedded in Signature-Input.¶

B.2. Why Schemes Instead of Just a Key and Key ID?

A simpler design would define Signature-Key as carrying only a public key (or key reference) and a key identifier, without the scheme abstraction. This was considered insufficient because:¶

1. Trust model varies: A bare key tells the verifier nothing about the trust model. Is this a pseudonymous key to be evaluated on its own merits (hwk)? A key bound to a discoverable identity (jwks_uri)? A delegated key from an authority (jwt)? A certificate-backed key (x509)? The scheme token tells the verifier which verification procedure to follow and what trust properties the key carries.¶

2. Verification procedure differs: Each scheme has a fundamentally different verification path. hwk requires no external fetches. jwks_uri requires metadata discovery. x509 requires certificate chain validation. jwt requires JWT signature verification before the HTTP signature can be verified. A key-and-ID-only design would push scheme detection to heuristics or out-of-band agreement.¶

3. Security properties differ: Without an explicit scheme, a verifier cannot distinguish between a self-asserted key and a CA-certified key. The scheme makes the trust model explicit, allowing verifiers to enforce policy (e.g., "only accept jwt or x509 schemes").¶

4. Interoperability: Explicit schemes create clear interoperability targets. Two implementations that support the jwt scheme know exactly what to expect from each other. Without schemes, the same key material could be interpreted differently by different implementations.¶