In cryptography, an adversary (rarely opponent, enemy) 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.
Eavesdropper Eve, malicious attacker Mallory, opponent Oscar, and intruder Trudy are all adversarial characters widely used in both types of texts.
The notion of security of a cryptosystem is meaningful only with respect to particular attacks (usually presumed to be carried out by particular sorts of adversaries).
In actual security practice, the attacks assigned to such adversaries are often seen, so such notional analysis is not merely theoretical.