Use a 27-letter alphabet consisting of the standard 26 letters and a blank space, which well denote as ''. We encode the letters in the usual way with A = 0, ... , Z = 25, and = 26. Then we encode digraphs (though for a 27-letter alphabet). We are able to intercept some ciphertext encrypted with C aP + b(Mod 729). From frequency analysis, we can determine that the ciphertexts corresponding to the two most frequent digraphs in English (namely S and T) are NG and KX, respectively.

1. Decrypt this piece of the ciphertext: TCUGARXKOK.