Simple one time pad

1 pointdublinclontarf4 comments

I've been reading quite a bit about encryption and what not and came across the one time pad. And am wondering if this is an accurate example or have I missed something.

Message:loveyou

key of random characters:spgbwoz

encrypted message = message + key modulo 26

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you have it right M=loveyou K=spgbwoz

C_i =letter(order(M_i)+order(K_i) mod 26)

where order() and letter() backward and forward lookup in this table: 0 - a 1 - b 2 - c ... 24 - y 25 - z

The Decryption function is similar to the encryption function but you subtract the key from the cyphertext to get back the message.

Claude Shannon proved that the one-time pad is unconditionally secure using the concept of entropy H(). Very simple argument which you could try to lookup.

Yes, as long as the random characters are really random. This seems trivial but is actually quite difficult to get right.

Also you technically leak a little information by not padding your plaintext to a larger block size, but its not really an issue in this particular example.

Yes, and it can also be tricky to communicate the key.

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