Function decrypt 

pub fn decrypt(
    message: &[u8],
    keys: &Keys,
    unlock: &Unlock<'_>,
) -> Result<Opened, String>

Decrypts an OpenPGP message (armored or not) with your keys, checking the signatures inside. unlock gives the passphrase of a key of yours here, the session keys kept, and your security key when you asked to use it.