The Playfair
Cipher is a
simple transposition cipher offering a relatively
weak method of
encryption. It was used by many countries during wartime and extensively by
Britain in
World War 1, and
Germany during
World War 2. It is based around a 5x5
matrix, a copy of which is held by both communicating parties, into which 25 of the 26 letters of the
alphabet (normally either j and i are represented by the same
letter or x is ignored) are placed in a
random fashion i.e.
T J A M U
E V G S Q
P Z B W D
I N O F Y
C H L R K
The
plaintext message is then created, stripped of any spaces or
punctuation, any required
substitutions take place (i.e. I for J, eliminating X etc.) and it is then grouped into pairs of letters. These letter pairs then undergo the following
encryption techniques
1) If a double letter occurs insert a 'dummy' letter between them
2) If a letter pair are at the corners of a rectangle,substitute the letters at the opposing corners (eg TO -> AI)
3) If a letter pair are in the same row, substitute the letters one to the right (eg MA -> UM)
4) If the letter pair are in the same column, substitute the letters one below (eg EC -> PT)
To decrypt a message divide the ciphertext into letter pairs and follow the following decryption techniques
1) If the letter pair are in the same column then substitute the letters one position above
2) If the letter pair are in the same row then substitute the letters one position to the left.
If there are any dummy letters it should now become obvious
The main weakness in this method is the fixed encryption/decryption table.