Perfect secrecy
Information-theoretic security is a cryptosystem whose security derives purely from information theory. In other words, it cannot be broken even if the adversary had unlimited computing power. The adversary simply does not have enough information to break the encryption and so the cryptosystems are considered cryptanalytically-unbreakable.
An encryption protocol with information-theoretic security does not depend for its effectiveness on unproven assumptions about computational hardness, and such an algorithm is not vulnerable to future developments in computer power such as quantum computing. An example of an information-theoretically secure cryptosystem is the one-time pad. The concept of information-theoretically secure communication was introduced in 1949 by American mathematician Claude Shannon, the inventor of information theory, who used it to prove that the one-time pad system was secure. Information-theoretically secure cryptosystems have been used for the most sensitive governmental communications, such as diplomatic cables and high-level military communications, because of the great efforts enemy governments expend toward breaking them.
Perfect security is an interesting special case. It means that for an encryption algorithm, if there is ciphertext produced that uses it, no information about the plaintext is provided without knowledge of the key. If E is a perfectly secure encryption function, for any fixed message m, there must be, for each ciphertext c, at least one key k such that . It has been proved that any cipher with the perfect secrecy property must use keys with effectively the same requirements as one-time pad keys.
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