Cheswick, William R. and Steven M. Bellovin.
Firewalls and Internet Security: Repelling the Wily
Hacker. Reading, Mass. : Addison-Wesley, 1995.
ISBN 0-201-63357-4
Garfinkel, Simson. PGP Pretty Good
Privacy O'Reilly & Associates, Inc., 1995.
ISBN 1-56592-098-8