OpenPGP as mail carries it: Digital signatures checked and Public keys read. No I/O, no clock and no keys of its own. The caller hands in the message's bytes, the Public keys it keeps, the time, and whether the owner has verified a key.

verify checks a PGP/MIME message (multipart/signed; protocol="application/pgp-signature", RFC 3156 §5) over the signed part's canonical bytes, or a cleartext-signed text (RFC 9580 §7), and gives an Outcome (verified, valid but not verified, bad, or can't be checked) with what the signature covers (Coverage). MD5, SHA-1 and RIPEMD-160 signatures are refused as no longer safe.
keys_in_message reads the keys a message carries: attached application/pgp-keys parts and the Autocrypt: header (Autocrypt Level 1). Nothing is fetched or sent.
PublicKey, KeyRing, Fingerprint, KeyId: transferable public keys (armoured or binary) with their self-signatures, expiry and revocations checked.
parse_autocrypt: one Autocrypt: header value.
secret::read, secret::holds_secret: what a transferable secret key file is (its fingerprint, addresses, public half, whether every secret part has a passphrase, whether it can sign), without keeping or using the secret.

The packets and the cryptography are rPGP's (pgp, MIT OR Apache-2.0). The armour, the Cleartext Signature Framework and the little MIME needed to find the signed bytes exactly as they came are read here, from RFC 2045/2046, RFC 3156 and RFC 9580, so that every size is capped by this crate.

Everything from outside is hostile: messages past MAX_MESSAGE are not read, signatures, keys, packets, user IDs, subkeys, MIME parts, nesting and the number of signature checks per call are capped, work is linear in the input, and a panic inside rPGP is caught and reported as an unreadable signature.

Moved from MailCraft's mailcraft-openpgp.