Signs Base64-encoded data with the private key associated with kid and returns the produced JWS inside a signing-metadata envelope. Supported algorithms: ML-DSA, Falcon, RSA-PSS (PS256), ECDSA (ES256/384/512), and COMPOSITE hybrid keys.

Serialization selection (PRD §60). The optional serialization field expresses the client's chosen JWS representation and is orthogonal to the backend-authoritative RFC 3161 time-stamping policy (§49). Two honest compliance tiers exist: RFC 7515 JWS (COMPACT = §7.1 three-segment compact, or JSON = §7.2 JWS JSON Serialization) and JAdES B-T (JWS JSON Serialization only, carrying an RFC 3161 qualified timestamp sigTst in the JWS Unprotected Header per ETSI TS 119 182-1 — a COMPACT token structurally cannot carry it).

Behavior truth table (serialization × effective stamping → output form):

An explicit COMPACT is never silently upgraded and a mandated stamp is never dropped — the compact+REQUIRED row is the only rejection. The output column names the logical serialization tier; see "Output to parse" for the exact wire shape.

Output to parse. On the HTTP wire the produced JWS is ALWAYS carried in the typed jwsToken (RFC 7515 §7.2 JWS JSON Serialization) envelope. A COMPACT / unstamped result is a single-signature object with no sigTst (losslessly convertible to the 3-segment RFC 7515 §7.1 compact form, as the SDK renders it); a JAdES B-T result carries the RFC 3161 sigTst in the unprotected header (signatures[].header.etsiU[].sigTst). The surrounding metadata envelope (keyRequested, materialVersion, algorithmUsed, warnings) is ALWAYS present regardless of serialization.