Unicity-distance of Playfair Given the classical scheme Playfair, say, for 5*5 matrices, I am interested to determine its unicity-distance. How should one proceed to properly do that? I should appreciate it, if I could obtain a very terse recipe, listing the kind of computations such that I could effectively compute that value myself. (I am attempting to determine the unicity-distance of a suggested variant of Playfair.)
The unicity distance is defined to be the minimum number of ciphertext characters needed to have a unique significant decryption.
It answers the question 'if we try all the keys, how much ciphertext would we need to be sure our solution was the true solution?'.
The answer depends on the redundancy of the language in which the plaintext was written.

To find the unicity distance of a given cipher algorithm you calculate the ratio of the number of bits required to express the key divided by the redundacy of the language in question. Assuming the plaintext was written in English and that the Playfair Cipher is used to encipher it, we have that the unicity distance equals to 27 characters.

For a slightly more detailed discussion consider practicalcryptography.com