Activity-Relay verifies signed inbound ActivityPub requests and signs outbound
deliveries with the relay actor identity. It also signs remote actor and object
GET requests for authorized-fetch and Mastodon secure-mode interoperability.
The established Fediverse Signature/Digest profile remains supported. The
3.0 omitted/default outbound policy is destination-aware dual; explicit
legacy remains available for fixed compatibility. Legacy authorized-fetch
requests sign (request-target), Host, and Date; legacy delivery requests
additionally sign Digest and Content-Type. Redirected fetches are re-signed
for the new target and authority, and the signed authority matches the request
transmitted on the wire.
Activity-Relay 3.0 also implements RFC 9421 HTTP Message Signatures and RFC 9530
Content-Digest. Inbound requests containing Signature-Input are committed to
the modern verifier and do not downgrade to the legacy parser after modern
verification failure. Modern verification enforces bounded creation time,
request authority and covered-component policy, RSA signature validity, body
digest integrity, actor/key binding, and Redis-backed nonce replay prevention.
Outbound OUTBOUND_SIGNATURE_PROFILE accepts fixed legacy, fixed rfc9421,
or destination-aware dual. dual selects one concrete wire profile per
operation. It may make one bounded legacy fallback for an idempotent GET after
recognized compatibility evidence, but a delivery POST is never re-sent under
another signature grammar. Capability state is bounded, origin- and
scope-specific, and contains no raw request bodies, signatures, or key material.
The 3.0 stable default is dual. This is a compatibility policy rather than a
wire format: unknown fetches can probe RFC 9421 with one bounded legacy fallback,
while unknown delivery POSTs begin with legacy and never change signature
grammar across retries. Fetch and delivery capability evidence remain separate.
- Mastodon security specification: https://docs.joinmastodon.org/spec/security/
- RFC 9421, HTTP Message Signatures: https://www.rfc-editor.org/rfc/rfc9421.html
- RFC 9530, Digest Fields: https://www.rfc-editor.org/rfc/rfc9530.html
Do not log private keys, complete signatures, request bodies, credentials, nonces, or unbounded remote response bodies. Delivery diagnostics may contain public ActivityPub identifiers, destination inbox URLs, and remote response text up to the documented bound; operators should therefore protect and retain logs as operationally sensitive data. Public status and metric labels must remain bounded and must not expose private relay topology or operator configuration.