Editing Cipher (section)

==Key size and vulnerability==
In a pure mathematical attack, (i.e., lacking any other information to help break a cipher) two factors above all count:

* Computational power available, i.e., the computing power which can be brought to bear on the problem. It is important to note that average performance/capacity of a single computer is not the only factor to consider. An adversary can use multiple computers at once, for instance, to increase the speed of [[exhaustive search]] for a key (i.e., "brute force" attack) substantially.
* [[Key size]], i.e., the size of key used to encrypt a message. As the key size increases, so does the complexity of [[exhaustive search]] to the point where it becomes impractical 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.

[[Claude Shannon]] proved, using information theory considerations, that any theoretically unbreakable cipher must have keys which are at least as long as the plaintext, and used only once: [[one-time pad]].<ref name="Shannon One-Time Pad Secrecy">{{cite web |url=http://netlab.cs.ucla.edu/wiki/files/shannon1949.pdf#page=27 |title=Communication Theory of Secrecy Systems |access-date=February 3, 2019 |archive-date=June 5, 2007 |archive-url=https://web.archive.org/web/20070605092733/http://netlab.cs.ucla.edu/wiki/files/shannon1949.pdf#page=27 |url-status=dead }}</ref>