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=== Known attacks ===
For cryptographers, a [[cryptanalysis|cryptographic]] "break" is anything faster than a [[brute-force attack]]{{px2}}{{mdash}}{{spcs|2|hair}}i.e., performing one trial decryption for each possible key in sequence {{xref|(see {{slink|Cryptanalysis|Computational resources required}})}}. A break can thus include results that are infeasible with current technology. Despite being impractical, theoretical breaks can sometimes provide insight into vulnerability patterns. The largest successful publicly known brute-force attack against a widely implemented block-cipher encryption algorithm was against a 64-bit [[RC5]] key by [[distributed.net]] in 2006.<ref name=ZD20060430>{{cite web |url=
The key space increases by a factor of 2 for each additional bit of key length, and if every possible value of the key is equiprobable; this translates into a doubling of the average brute-force key search time with every additional bit of key length. This implies that the effort of a brute-force search increases exponentially with key length. Key length in itself does not imply security against attacks, since there are ciphers with very long keys that have been found to be vulnerable.
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