I am studying in CSE and in my recent exam paper, A question was asked as:

Construct a playfair cipher for Plaintext: semester5 and key:technology.

Generally as we are taught yet, there is no example of playfair matrix substitution for numeric value.

So, what to do with number 5? Is there any trick for this kind of encryption?

  • $\begingroup$ If the matrix doesn't have 5, then you simply cannot process that character with that matrix. Perhaps you were expected to think of the use of a larger matrix than the one shown in the lessons. $\endgroup$ – Mok-Kong Shen Mar 26 '14 at 20:13

use letters to represent digits

"a captured German revealed under interrogation that Enigma operators had been instructed to encode numbers by spelling them out ... Alan Turing reviewed decrypted messages and determined that the number "eins" ("one") was the most common string in the plaintext." -- Wikipedia: known-plaintext_attack

"The Enigma machine didn't encode spaces (nor punctuation characters), so either leave out the spaces in your message or replace them with X's. ... If you wish to encode numbers in your message, you must spell them out." -- Madlab: Enigma

"The German Army, Air Force and Police used the Double Playfair system as a medium-grade cipher in WWII, ... numbers were spelled out. As the German numbers 1 (eins) to twelve (zwölf) contain all but eight of the letters in the Double Playfair squares, pro forma traffic was relatively easy to break" -- Wikipedia: Playfair cipher

"each digit was represented by 2 letters" -- "Enigma Uhr"

use some other cipher that can handle letters and digits directly

"... we proposed a 6 X 6 Playfair cipher ... 6 X 6 Playfair cipher supports all 26 alphabets (A-Z) and 10 digits (0- 9)" -- "6 X 6 Playfair Cipher using LFSR"

"This extended play fair algorithm is based on the use of a 6 X 6 matrix of letters ... and digits..." -- "An Extension to Traditional Playfair Cryptographic Method"

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