Information theoretic security
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A cryptosystem is information-theoretically secure or perfectly secure if a ciphertext produced using it provides no information about the plaintext to anyone other than the key holders.
This means that an information-theoretically secure cryptosystem is secure against attacks by an adversary with infinite or unbounded computing power. An example (simple and unusable in nearly all actual practice) is the one-time pad. Indeed the proof by Shannon that the one time pad is secure in this sense may be interpreted to mean that no system can be more secure, and further that any system which is so secure will be equivalent to the one time pad.
If E is a information-theoretically secure encryption function, for any fixed message m there must exist for each ciphertext c at least one key such that <math>c=E_{k}(m)<math>.