1. In encryption, the word symmetric means those cases where the same key both encrypts and decrypts. This has been historically the "normal" encryption, but new public-key cryptography is changing things. Analogy: In your house, the same keys are used to lock and unlock your door. Examples: Some symmetric encryption ciphers are: DES The forerunner to most of today's popular symmetric ciphers. RC2, RC4, and RC5 Popular ciphers by RSA used in today's browsers for secure connections to websites. IDEA A cipher made popular by the fact that it was used in PGP. Blowfish A well-regarded cipher with free source code, no license required, unpatented, and royalty-free. As such, it is an extremely popular symmetric encryption algorithm. http://www.counterpane.com/blowfish.html TwoFish A new cipher with many of the same restrictions as Blowfish (i.e. none). It is even more efficient, and destined to become very popular. http://www.counterpane.com/twofish.html From Hacking-Lexicon