Key size

In cryptography, key size or key length is the size measured in bits of the key used in a cryptographic algorithm (such as a cipher). An algorithm's key length is distinct from its original cryptographic security, which is a logarithmic measure of the fastest known computational attack on the algorithm, also measured in bits. The security of an algorithm cannot exceed its key length (since any algorithm can be cracked by brute force), but it can be smaller. For example, Triple DES has a key size of 168 bits but provides at most 112 bits of security, since an attack of complexity 2¹¹² is known. This property of Triple DES is not a weakness provided 112 bits of security is sufficient for an application. Most symmetric-key algorithms in common use are designed to have security equal to their key length. No asymmetric-key algorithms with this property are known; elliptic curve cryptography comes the closest with an effective security of roughly half its key length.

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 actually 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.