Monoalphabetic encipherment of N-grams of ASCII plaintext with N > 1 is attractive for two reasons:
Lester Hill  described a simple and elegant way to encipher N-grams of ASCII plaintext. Each character will be identified by its ordinal position in the ASCII character alphabet, integers in . We suppose the length n of plaintext x = (x0, x1,…, xn−1) is a multiple of N; various modifications are possible when n ≠ kN and will be mentioned later. x is divided into N-grams whose components are integers in :
The Hill encipherment of ASCII plaintext x denoted by
is defined by
and A = (ai,j) is an N × N matrix with entries in and which is invertible. ...