Editing Key size (section)

==Significance==
[[Key (cryptography)|Keys]] are used to control the operation of a cipher so that only the correct key can convert encrypted text ([[ciphertext]]) to [[plaintext]]. All commonly-used ciphers are based on publicly known [[algorithm]]s or are [[open-source model|open source]] and so it is only the difficulty of obtaining the key that determines security of the system, provided that there is no analytic attack (i.e. a "structural weakness" in the algorithms or protocols used), and assuming that the key is not otherwise available (such as via theft, extortion, or compromise of computer systems). The widely accepted notion that the security of the system should depend on the key alone has been explicitly formulated by [[Auguste Kerckhoffs]] (in the 1880s) and [[Claude Shannon]] (in the 1940s); the statements are known as [[Kerckhoffs' principle]] and Shannon's Maxim respectively.

A key should, therefore, be large enough that a brute-force attack (possible against any encryption algorithm) is infeasible &ndash; i.e. would take too long and/or would take too much memory to execute. [[Claude Shannon|Shannon's]] work on [[information theory]] showed that to achieve so-called '[[perfect secrecy]]', the key length must be at least as large as the message and only used once (this algorithm is called the [[one-time pad]]). In light of this, and the practical difficulty of managing such long keys, modern cryptographic practice has discarded the notion of perfect secrecy as a requirement for encryption, and instead focuses on [[computational security]], under which the computational requirements of breaking an encrypted text must be infeasible for an attacker.