IETF · February 2024
RFC 9421
A. Backman, J. Richer, M. Sporny
This is the cryptographic primitive that lets an agent prove 'this request came from my key' without a bearer secret in the Authorization header.
At a glance
Actors
01
HTTP requests need application-level integrity that survives some intermediaries, without ad-hoc HMAC headers. RFC 9421 is the cryptographic primitive that lets an agent prove 'this request came from my key' without a bearer secret in Authorization.
AAuth profiles this. WIMSE also has an HTTP signature method alongside WPT. DPoP is a different proof (a JWT in a header), not 9421.
02
Signer and verifier share a key-discovery story (Signature-Key, WIT, static config). Intermediaries that add unsigned headers are fine; intermediaries that change covered components break the signature. Do not cover hop-by-hop headers you cannot control.
03
Signer covers chosen components (@method, @authority, @path, selected headers, content-digest, created, …) in Signature-Input and produces Signature. Verifier reconstitutes the signature base and checks against a known key. Require coverage of method, authority, path, and the key-identifying header (signature-key or equivalent).
04
Flow 1
Choose components (minimum method, authority, path, key header).
Build Signature-Input with created and a nonce or keyid as required by the profile.
Sign the signature base; send Signature plus key discovery (Signature-Key or WIT).
Verifier rebuilds the base, fetches the key, checks alg, created window, and signature.
05
Authentication and integrity of the HTTP message. Authorization is left to the application (AAuth tokens, OAuth, local policy).
06
Key distribution companion.
Profiles 9421 as the only API-call PoP.
Alternative application-layer PoP using a JWT in Authorization.
Different PoP; do not assume a DPoP proof satisfies a 9421 verifier.
07
If you do not cover the path, an attacker rewrites /read to /admin. If you do not cover signature-key, they swap identity.
Replay windows depend on created and verifier policy. Pair with jti-like uniqueness if the profile provides it.
08
RFC 9421, February 2024, A. Backman, J. Richer, M. Sporny. Stable. https://www.rfc-editor.org/rfc/rfc9421.html
AAuth profiles RFC 9421 rather than inventing a new signature scheme. WIMSE also has an HTTP signature method alongside WPT. Require coverage of method, authority, path, and the key-identifying header.