# description
A digitally signed authentication credential transmitted in the HTTP Authorization header of PN solicited callbacks to CN `/on_*` endpoints. Extends the standard Signature by chaining the PN's signature to the CN's original request signature, allowing the CN to verify that the callback is a genuine response to a request it sent.

The PN produces this by signing a four-line canonical signing string with its Ed25519 private key:

    (created): {unix_timestamp}
    (expires): {unix_timestamp}
    digest: BLAKE2b-512={base64_callback_body_hash}
    request-signature: {cn_raw_base64_signature}

The fourth line (`request-signature`) contains the raw Base64 Ed25519 signature value extracted verbatim from the `signature="..."` field of the CN's original `Authorization` header. This binds the callback cryptographically to the triggering request.

The `headers` attribute MUST be `"(created) (expires) digest request-signature"`. The `keyId` identifies the PN signer. The algorithm MUST be `ed25519`. The body digest covers the callback body using BLAKE2b-512.

Used for: PN→CN solicited callbacks only. For PN-initiated notifications (no preceding CN request), use the standard Signature schema instead. For synchronous response signing, see components/headers/AckSignatureHeader. See NFH-004 Authentication and Trust §6 for the full callback signing procedure.