Key Size - Significance

Significance

Keys are used to control the operation of a cipher so that only the correct key can convert encrypted text (ciphertext) to plaintext. Many ciphers are based on publicly known algorithms or are 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 – i.e., would take too long to execute. Shannon's work on information theory showed that to achieve so called perfect secrecy, it is necessary for the key length to be at least as large as the message to be transmitted 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.

The preferred numbers commonly used as key sizes (in bits) are powers of two, potentially multiplied with a small odd integer.