Cryptography is concerned with the construction of schemes that withstand any abuse. Such schemes are constructed so as to maintain a desired functionality, even under malicious attempts aimed at making them deviate from their prescribed functionality.

The design of cryptographic schemes is a very difficult task. One cannot rely on intuitions regarding the typical state of the environment in which the system operates. For sure, the adversary attacking the system will try to manipulate the environment into untypical states. Nor can one be content with countermeasures designed to withstand specific attacks because the adversary (which acts after the design of the system is completed) will try to attack the schemes in ways that are typically different from the ones envisioned by the designer. The validity of the foregoing assertions seems self–evident; still, some people hope that in practice, ignoring these tautologies will not result in actual damage. Experience shows that these hopes rarely come true; cryptographic schemes based on make–believe are broken, typically sooner than later.

In view of these assertions, we believe that it makes little sense to make assumptions regarding the specific strategy that the adversary may use. The only assumptions that can be justified refer to the computational abilities of the adversary.