physical security

Physical security for pgp is also necessary if you store your pass phrase in memory. As far as modification, detection is good enough, but you'd better make sure your program to detect modifications is not itself compromised! (Does anybody detect an imminent arms race here?) Eric Hollander is correct. Ideally, your keys and your encryption mechanism should be kept secure. At some point in the future, a small card which contains all of this will be standard equipment, as well as a port to plug it into. Eric