Monoalphabetic encipherment of *N*-grams of ASCII plaintext with *N* > 1 is attractive for two reasons:

- The probability distribution of
*N*-grams with*N*≈ 4 is much flatter than for 1-grams, making it harder to recognize letter fragments; and - There is a very large number 128
^{N}of*N*-grams with*N*≥ 4.

Lester Hill [1929] 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* = (*x*_{0}, *x*_{1},…, *x*_{n−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

where

and *A* = (*a*_{i,j}) is an *N* × *N* matrix with entries in and which is invertible. ...

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