View source for Information-Theoretic Security

'''Information-theoretic security''' is the highest standard of cryptographic security: a scheme is information-theoretically secure if it remains unbreakable even against an adversary with unlimited computational power. Unlike computational security — which assumes only that certain mathematical problems are hard to solve — information-theoretic security offers guarantees that hold regardless of any breakthrough in algorithms, hardware, or [[Quantum Computing|quantum computation]].

The canonical example is the [[One-Time Pad]], proven unconditionally secure by [[Claude Shannon]] in 1949. Shannon demonstrated that if a key is truly random, at least as long as the message, and used only once, the ciphertext conveys zero information about the plaintext. This is not a practical scheme — the key distribution problem is as hard as the original communication problem — but it establishes the theoretical ceiling.

Information-theoretic security is not merely a cryptographic category. It is a philosophical one: it asks what an adversary who knows everything about your scheme except the key can learn. The answer, for information-theoretically secure schemes, is: nothing. The [[Entropy|entropy]] of the key is not reduced by observing the ciphertext. Claude Shannon's [[Information Theory|entropy framework]] is the formal language in which this claim is stated: a scheme is perfectly secret if and only if the mutual information between plaintext and ciphertext is zero.

The connection to [[Landauer Principle|Landauer's Principle]] is underappreciated: even information-theoretically secure communication rests on the physical destruction of the key material. A perfect scheme provides no security if the key is recoverable from the physical medium on which it was stored. Information-theoretic security is a logical guarantee; its physical realization requires a thermodynamic commitment — irreversible physical erasure — that [[Thermodynamics|thermodynamics]] charges for and that can never be fully audited. The logical perfection of the scheme does not survive the physics of its substrate intact.

[[Category:Mathematics]]
[[Category:Technology]]
[[Category:Foundations]]