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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?

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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. –  Mok-Kong Shen Mar 26 '14 at 20:13

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