Ad·ver·sary

Dictionary:
Ad?ver?sar?y (ad-v&(r)-"ser-E)
???1. An opponent; an enemy.
???2. The Devil; Satan. Often used with the.
[Middle English adversarie, from Latin adversrius, enemy, from adversus, against.]
Thesaurus:
One that opposes another in a battle, contest, controversy, or debate: antagonist, opponent, opposer, opposition, oppositionist, resister.
Wikipedia:
In cryptography, an adversary is a malicious entity whose aim is to prevent the users of the cryptosystem from achieving their goal (primarily privacy, integrity and availability of data). An adversary's efforts might take the form of attempting to discover secret data, corrupting some of the data in the system, spoofing the identity of a message sender or receiver, or forcing system downtime.