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Showing posts from July, 2013

Simple symmetric / Shared key encryption tutorial

I was looking for a way to explain cryptography to my 8 year old son this evening and we devised a simple share key encryption / decryption method.  The key is symmetrical because both sides have the same key  and the same key is used for both encrypting and decrypting. We took a simple ceasar cipher and extended it slightly by creating a stronger key.  Something like this: Plain Text = HELLO WORLD Key = [2, -5, 3, 7] to encrypt we take the positional value for each letter and apply the replacement for the next key bit in turn.  For example: Start at keybit = 1 [2] H = 8, H + 2 = 10, 10 = J keybit = 2 [-5] E = 5, E - 5 = 0 (26), 26 = Z keybit = 3 [3] L = 12, 12 + 3 = 15, 15 = O keybit = 4 [7] L = 12, 12 + 7 = 19, 19 = S Now start at key bit =1  O = 15, 15 + 2 = 17, 17 = Q and so on. So long as the same key is used for decryption the thing works a treat. So I explained that simple key can be made and shared between friends then private messag