Sabine Baring-Gould,
Once a Week
(1863)
“ How any man could have solved the mystery of this cypher is astonishing. Now let us recommend a far simpler system, and one which is very difficult of detection. It consists of a combination of numbers and letters. Both parties must be agreed on an arrangement such as that in the second line below, for on it all depends. Now in turning a sentence such as “The army must retire” into cypher, you count the letters which make the sentence, and find that t is the first, h the second, f the third, a the fourth, r the fifth, and so on. Then look at the table. t is the first letter; 4 answers to 1 ”
