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Symmetric key ciphers can be distinguished into two types, depending on whether they work on blocks of symbols of fixed size (''[[block cipher]]s''), or on a continuous stream of symbols (''[[stream cipher]]s'').
==Key size and vulnerability==
In a pure mathematical attack (ie, lacking any other information to help break a cypher), three factors above all, count:
* Mathematical advances, that allow new attacks or weaknesses to be discovered and exploited.
* Computational power available, ie the computer power which can be brought to bear on the problem.
* [[Key size]], ie the size of key used to encrypt a message. As the key size increases, so does the complexity of [[brute search]] to the point where it becomes infeasible to crack encryption directly.
Since the desired effect is computational difficulty, in theory one would choose an algorithm and desired difficulty level, thus decide the key length accordingly.
An example of this process can be found at [[http://www.keylength.com keylength.com]] which uses multiple reports to suggest that a symmetric cypher with 128 bits, an assymetric cypher with 3072 bit keys, and an elliptic curve cypher with 512 bits, all have similar difficulty at present.
== See also ==
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