In addition / supplement to what @heavenlyhash says: Gnupg cannot do a diffie hellman key exchange to set up the session/symmetric key: one party unilaterally

In addition / supplement to what @heavenlyhash says: Gnupg cannot do a diffie hellman key exchange to set up the session/symmetric key: one party unilaterally generates a random symmetric key (eg: aes key), encrypts the session key with the recipient's public (asymmetric, eg: RSA) key, the message with the symmetric key and sends both cipher texts as the message. A passive attacker can store this, and will only need to crack/gain access to the recipient's private key, to recover the symmetric key, and then the message. Active/online protocols can use a key exchange: today that typically involves ephemeral (elliptic curve) diffie-hellman. The exchange is authenticated with the asymmetric key - but the session keys are independent of these "permanent" keys. So the eg: RSA key is used to make sure you're playing "guess the number I'm thinking of (the random symmetric session key)" with the intended recipient, and not an active attacker - but when the session key is thrown away, there's no way to recover it for either of the participants or a passive attacker. See also: https://lwn.net/Articles/572926/ https://lwn.net/Articles/572926/