In cryptography, unicity distance is the length of an original ciphertext needed to break the cipher by reducing the number of possible spurious keys to zero in a brute force attack.
[1] Claude Shannon defined the unicity distance in his 1949 paper "Communication Theory of Secrecy Systems".
[2] Consider an attack on the ciphertext string "WNAIW" encrypted using a Vigenère cipher with a five letter key.
In the example above we see only upper case English characters, so if we assume that the plaintext has this form, then there are 26 possible letters for each position in the string.
Since M/N gets arbitrarily small as the length L of the message increases, there is eventually some L that is large enough to make the number of spurious keys equal to zero.
The unicity distance can equivalently be defined as the minimum amount of ciphertext required to permit a computationally unlimited adversary to recover the unique encryption key.
For a one time pad of unlimited size, given the unbounded entropy of the key space, we have
Unicity distance is a useful theoretical measure, but it does not say much about the security of a block cipher when attacked by an adversary with real-world (limited) resources.
One way to do this is to deploy data compression techniques prior to encryption, for example by removing redundant vowels while retaining readability.