Signing

Sign produces a detached, ASCII-armored OpenPGP signature over the bytes you give it, with the heavy lifting done on the card.

func (c *Card) Sign(data []byte, pin string, pub *packet.PublicKey) ([]byte, error)

• data — the bytes to sign, covered verbatim as a binary document (signature type 0x00).
• pin — the user PIN (PW1). Authorizes this one operation.
• pub — the signing key's public half, for the signature-packet metadata.

How the packet is built

The card can sign a digest, but it can't build an OpenPGP signature packet — that's this library's job. The steps, all in buildSignaturePacket:

1. Hashed subpackets are assembled: signature creation time (type 2), issuer key ID (type 16), and issuer fingerprint (type 33). These bind the signature to when and by which key it was made, and are covered by the hash.
2. The hash suffix is laid out per RFC 4880 §5.2.4: version 0x04, signature type 0x00, the public-key algorithm, the hash algorithm (SHA-256), then the two-octet length of the hashed subpackets followed by the subpackets themselves.
3. The hash is computed over data || hash-suffix || v4-trailer, where the trailer is 0x04 0xFF plus the 4-byte big-endian length of the suffix.
4. The card signs the resulting digest via its crypto.Signer.
5. The raw signature is MPI-encoded for the key's algorithm.
6. The body is wrapped in a new-format packet (tag 2) and ASCII-armored.

Per-algorithm signature encoding

The card returns a raw signature; OpenPGP wants algorithm-specific MPIs.

The ECDSA DER parser (parseASN1Signature) bounds-checks every field length against the buffer: a truncated or malformed signature yields a typed error, never an out-of-range panic.

Choosing what to sign

Because the signature covers data verbatim, you decide what bytes are covered. That keeps the library out of your framing:

• git commit/tag signing — git hands the object to the signing program on stdin and expects the armored signature on stdout. Pass that object to Sign.
• multipart/signed mail (PGP/MIME) — canonicalize the MIME part you want to protect (CRLF line endings, the right Content-Type), pass it to Sign, and place the armored result in the application/pgp-signature part.
• Plain detached signatures — pass the file contents; ship the .asc alongside.