From The Jargon File (version 4.4.7, 29 Dec 2003):
ice
n.
[coined by Usenetter Tom Maddox, popularized by William Gibson's cyberpunk
SF novels: a contrived acronym for ‘Intrusion Countermeasure Electronics’]
Security software (in Gibson's novels, software that responds to intrusion
by attempting to immobilize or even literally kill the intruder). Hence,
icebreaker: a program designed for cracking security on a system.
Neither term is in serious use yet as of late 2003, but many hackers find
the metaphor attractive, and each may develop a denotation in the future.
In the meantime, the speculative usage could be confused with ‘ICE’, an
acronym for “in-circuit emulator”.
In ironic reference to the speculative usage, however, some hackers and
computer scientists formed ICE (International Cryptographic Experiment) in
1994. ICE is a consortium to promote uniform international access to strong
cryptography.