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 2
112 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.