How is knowing the inverse key useful? To use the encoding-decoding routines on transposition ciphers such as columnar, swagman, redefense, etc, you must enter the key. But sometimes the "key" that is given as a solution to a Cryptogram cipher is not the key as defined in the "ACA and You" reference book. Instead you are sometimes given the inverse of the key. That is, you are given the "decoding key" instead of the "encoding key". These keys are inverses of each other. Given one, it's easy to calculate the other; this program does the calculation for you.