1. L. S. Hill, “Concerning certain linear transformation apparatus of cryptography,” American Math. Monthly, 38 (1931), 135–154.
2. D. Kahn, The Codebreakers, the Story of Secret Writing, Macmillan, 1967.
3. K. H. Rosen, Elementary Number Theory and Its Applications, 3rd ed., Addison-Wesley, 1993.