Capability	Support
Native `Request` and `Response`	Yes, using Fetch-observable values
Final request/response descriptors	Yes, with ordered field occurrences and optional trailers
Related request components (`req`)	Yes, on response descriptors
Ordinary field components	Yes
Structured Field dictionary member (`key`)	Yes, strict RFC 8941 parsing
Derived components	`@method`, `@target-uri`, `@authority`, `@scheme`, `@request-target`, `@path`, `@query`, `@status`
Multiple signatures	Yes
Async signing and verification	Yes
Synchronous signing	Yes
Explicit verification policy	Required
WebCrypto providers	Ed25519 and RSA-PSS with SHA-512

Signing

import { appendSignature, createSignature } from "http-message-sig";

const fields = await createSignature(request, {
  label: "sig1",
  components: ["@method", "@authority", "@path", "content-digest"],
  parameters: { created: 1_700_000_000, alg: signer.algorithm, keyid: "key-1" },
  signer,
});

const headers = appendSignature(request.headers, fields);

Signers consume UTF-8 signature-base bytes and expose readonly algorithm. appendSignature accepts Headers only, returns a clone, and atomically parses and merges both signature dictionaries.

Use component("example-dict", { key: "member" }) for parameterized components. componentIdentity returns the exact serialized component identifier.

Verification

import { verifySignature } from "http-message-sig";

const verified = await verifySignature(request, {
  policy: {
    algorithms: ["ed25519"],
    requiredComponents: ["@method", "@authority", "@path"],
    requiredParameters: ["created", "keyid"],
    maxAge: 300,
    clockSkew: 5,
  },
  resolveVerifier(untrustedCandidate, context) {
    return lookupVerifier(untrustedCandidate.parameters.keyid, context);
  },
});

The resolver candidate is untrusted until cryptographic verification succeeds. The optional policy validate callback receives authenticated data and runs only after successful cryptographic verification.

Limitations

sf, bs, tr, @query-param, and unknown derived components are rejected with UnsupportedFeature.
RFC 9651 dates and display strings are rejected in signature fields. Signature metadata is limited to RFC 8941 bare items.
Fetch does not expose raw field occurrences or trailers. Use descriptors when those values matter.
Response has no related request. Use a response descriptor for req.
Cryptographic algorithms and key discovery are supplied by the caller.
The former directory helpers and draft-era signatureHeaders, verify, and RequestLike APIs were removed. Directory behavior belongs in the consuming package and must migrate to createSignature plus appendSignature.

Errors

Library validation failures use SignatureError, stable SignatureErrorCode values, and isSignatureError.

Security

This software has not been audited. Applications must define policy appropriate to their protocol, including algorithm, component, parameter, and freshness requirements.