Number of different substitution alphabet ciphers possible with given conditions?
Iterate through the alphabet (using a smaller alphabet is always a good way to start). So you take letter A and you can combine it with letter B..Z. Now if you take B, then you can combine it with letter A and C..Z. But the combination (A,B) is equivalent to (B,A). So you only have C..Z to consider. Thus you would get (n - 1) + (n - 2) + ... pairs, leaving you with 26 * (25 / 2) = 325 combinations.