From the Riddles of Ancient Egypt to Cryptography in the Renaissance — 3500 Years in the Making - Codes: The Guide to Secrecy from Ancient to Modern Times - page 53

conceived of using a key or key words in their polyalphabetic ciphers. The first
in recorded history to do so was Belaso in 1553. His idea was to use a keyphrase ,
which he called a countersign , repeated as often as needed, to select the cipher
alphabets. (We may think of this as the modern invention of the notion of a
password .) Here is how his countersign works. First, we are going to be using
Trithemius's table (page 51).
BELASO'S KEYPHRASE POLYALPHABETIC CIPHER
To employ Belaso's idea, we do three things to encipher .

(a) Put the plaintext letters in a row.

(b) Above each plaintext letter place the keyphrase letters, repeated as often
as necessary, to cover all the plaintext.

(c) Replace each letter of the plaintext with the letter at the intersection of the
row labelled by the keyphrase letter and column labelled by the plaintext
letter in Trithemius's table.

We illustrate these rules with the following.

Example 1.1 We will suppose the keyphrase, used by Belaso, is OPTARE ME-
LIORA , and the plaintext is countersign is key . Then one places the keyphrase
over the plaintext, repeated until the plaintext runs out as illustrated below.

o p t a r e m e l i o r a o p t

c o u n t e r s i g n i s k e y

Q D O N L I D X T P B A S Y T R

For example, the letter o labels the row that intersects the column headed by
the letter c at the ciphertext letter Q , and so on.

To decipher using the Trithemius square, we do three things.

(a) Put the ciphertext letters in a row.

(b) Put the keyword letters above the ciphertext letters, repeating them as
required, to cover all ciphertext.

(c) Locate the column labelled by each keyword letter, and find the row in
which the ciphertext letter sits below it. Then the label of that row is the
plaintext.

Applying these rules to Example 1.1, we get the following.

Example 1.2
o p t a r e m e l i o r a o p t
Q D O N L I D X T P B A S Y T R
c o u n t e r s i g n i s k e y
For instance, since the letter o sits over the ciphertext letter Q , the row of
which is labelled by c , then this is the first letter of plaintext, and so on.