Editing Unicity distance (section)

=== Unicity distance of substitution cipher ===
For a simple [[substitution cipher]], the number of possible keys is {{math|26! {{=}} 4.0329 × 10<sup>26</sup> {{=}} 2<sup>88.4</sup>}}, the number of ways in which the alphabet can be permuted. Assuming all keys are equally likely, {{math|''H''(''k'') {{=}} log<sub>2</sub>(26!) {{=}} 88.4}} bits. For English text {{math|''D'' {{=}} 3.2}}, thus {{math|''U'' {{=}} 88.4/3.2 {{=}} 28}}.

So given 28 characters of ciphertext it should be theoretically possible to work out an English plaintext and hence the key.