RFC 9421 vs draft-cavage: what changed?

RFC 9421 ("HTTP Message Signatures") replaced the expired draft-cavage-http-signatures with a standard that uses structured fields, introduces @-prefixed derived components, and clarifies the Signature-Input header semantics. The changes make signatures more interoperable, easier to debug, and future-proof for new HTTP features like digest fields and content negotiation.

If you implemented draft-cavage, you must migrate to RFC 9421 to stay compliant with modern HTTP stacks and avoid security warnings in browsers and proxies.

From strings to structured fields

The draft-cavage spec treated HTTP headers as raw strings. RFC 9421 moves to structured fields (RFC 8941), so every component is a typed item (string, token, byte sequence, or integer). This eliminates ambiguity in header parsing and lets you sign complex values like timestamps or lists without string munging.

Example: in draft-cavage you signed content-length: 123 as the literal string "content-length: 123". In RFC 9421 you sign the structured item content-length=123, preserving the integer type.

Derived components: the @-prefix revolution

RFC 9421 adds derived components, prefixed with @. These let you sign parts of the message that aren't headers, like the request target, the method, or the entire message body digest.

Common derived components: - @method: the HTTP method (GET, POST, etc.) - @target-uri: the full request URI - @authority: the Host header value - @request-response: ties a request to its response (for bidirectional signing) - @query-param: signs a specific query parameter by name

These components are canonicalised per RFC 8941, so you don't have to worry about whitespace or case folding.

Signature-Input: what you actually signed

In draft-cavage, the Signature header contained the signature bytes, but the covered components were buried in the key metadata. RFC 9421 splits this into two headers:

• Signature-Input: a structured field listing every component you signed, with parameters like keyid, alg, and created. This header is human-readable and machine-verifiable.
• Signature: the raw signature bytes, base64-encoded.

This separation makes it trivial to debug which parts of the message were signed, and it lets you attach multiple signatures to the same message (for example, a short-lived Ed25519 signature and a long-term RSA signature).

Why migrate now

1. Security warnings: Browsers and proxies flag draft-cavage signatures as non-standard. RFC 9421 is the IETF-approved way to sign HTTP messages.
2. Digest fields: RFC 9421 integrates with RFC 9530 (Digest Fields), so you can sign the message body without reading it into memory. This is critical for large payloads like AI training corpora.
3. Interoperability: Structured fields are the future of HTTP. Tools like OpenAPI and service meshes already expect them.
4. Future-proofing: Derived components let you sign new HTTP features (for example, @path for the request path) without breaking changes.

Do it with AlgoVoi

AlgoVoi's free, open (Apache-2.0) algovoi-rfc9421-verifier (v0.4.4) is a reference implementation of RFC 9421. It verifies both the Signature-Input and Signature headers, checks the covered components, and validates the signature against the key material.

Install: - Python: pip install algovoi-rfc9421-verifier - TypeScript: npm install @algovoi/rfc9421-verifier

Or try the hosted verifier at verify.algovoi.co.uk/rfc9421.

Next step

Start with the docs: docs.algovoi.co.uk/quickstart.