Foundations

Error Model

AAuth adopts the Signature-Error response header from the Signature-Key draft. Every verification failure returns a 401 with a machine-readable error code — the response body is not authoritative.

Anatomy

HTTP/1.1 401 Unauthorized
Signature-Error: error=invalid_input,
    required_input=("@method" "@authority" "@path"
    "signature-key" "content-digest")
Content-Type: application/problem+json

{
  "type": "urn:ietf:params:sig-error:invalid_input",
  "title": "Missing required covered components",
  "status": 401
}

The header is the authoritative source. Problem Details (RFC 9457) in the body is a convenience for operators — machine clients MUST read the header.

Error codes AAuth uses

error	Status	When it fires
`invalid_request`	400 / 401	Required signature header (Signature, Signature-Input, or Signature-Key) is missing or malformed.
`invalid_input`	401	Signature-Input does not cover all required components. Response SHOULD include `required_input` listing the components the server requires.
`invalid_signature`	401	Signature does not verify, or the created timestamp is outside the validity window.
`unsupported_algorithm`	401	The signing algorithm in the key/signature is not on the server's accept list. Response MUST include `supported_algorithms` listing the algorithms the server accepts.
`invalid_key`	401	The public key in Signature-Key could not be parsed, is malformed, or doesn't meet trust requirements.
`unknown_key`	401	For sig=jwks_uri — the referenced kid was not found at the published jwks_uri. Server SHOULD re-fetch the JWKS once before returning this to handle key rotation gracefully.
`invalid_jwt`	401	For sig=jwt or sig=jkt-jwt — the JWT is malformed or its signature failed verification.
`expired_jwt`	401	For sig=jwt or sig=jkt-jwt — the JWT exp claim is in the past.

Authentication vs Authorization

401 + Signature-Error

The signature or its keying material is wrong. The client can (and often should) retry with corrected parameters.

403 Forbidden

Signature verified and identity is known, but policy denies the request. The response MUST NOT include Signature-Error or Accept-Signature— this isn't a signing problem.