Developed by Xuejia Lai and James Massey in 1992. A block cipher, IDEA operates on 64-bit blocks with a 128-bit key and is considered to be very secure. IDEA is used by Pretty Good Privacy (PGP), a very secure public key encryption application for MS-DOS, UNIX, and VAX/VMS. Originally written by Philip Zimmermann, PGP was later improved by Hal Finney, Branko Lankester, and Peter Gutmann. Farlex, Inc. The Free Dictionary: International Data Encryption Algorithm. [Online, 2004.] Farlex, Inc. Website. http://computing-dictionary.thefreedictionary .com/International%20Data%20Encryption%20Algorithm.