We must assume that an attacker has copies of message ciphertext sent over an insecure network such as the Internet. These messages remain vulnerable until the corresponding private key is deleted. Messages therefore MAY be stored temporarily encrypted with a short-lifetime key, but are unreadable once it has been deleted. Clients MUST allow messages to be stored encrypted under a long- term storage key. A mail client MAY implement its own secure storage facilities, or use those provided by other software.
Key generation is typically a time-consuming operation.
A group of keys can share a common public modulus with no negative security implications other than that the key then presents a fatter target for pre-computation attacks. Multiple forward secret Elgamal keys MAY therefore use the same prime modulus with minimal security reduction. Key distribution can be eased by submitting new keys to key servers, where they will be available for other users to retrieve. Expired public encryption keys MAY be deleted by users and keyservers to save space. If an OpenPGP client has more than one valid encryption key available for a given message recipient, the key nearest its expiration date MUST be used.