Unicity distance The length of text required to decode enciphered text uniquely when keys are randomly generated is known as the unicity distance.Denoting byH(K) the entropy of the key and by D the redundancy of the text, the unicity distance is defined as HðKÞ=D:

There are 26! possible substitution ciphers for a 26-letter alphabet, while the redundancy of English expressed using log10 is 0.7. What length of text is sufficient to decode any substitution cipher uniquely? What does this say about the security of such ciphers?