One-Time Pad

January 9, 2015

In a previous exercise Ben Simon showed us how to use a trigraph for secret communication. For illustration, he used a one-time pad based on a simple random number generator, but that is not sufficient for proper cryptographic secrecy. In today’s exercise we build a secure one-time pad.

We need two things. One is a cryptographically-secure source of random bits. Solutions include counting the clicks of a geiger counter or reading the background radiation, but being a software guy rather than a hardware guy I suggest the Blum-Blum-Shub cryptographically-secure random number generator of a previous exercise. The second is a way to convert bits to letters. The method we choose is to take 26 consecutive bits from the Blub-Blub-Shum generator, form them into a number, and take the result mod 26, discarding the four largest numbers from the set in order to make all letters equally likely to be chosen, which is similar to the calculation of a previous exercise. Given those two things, it is easy to generate random letters and build a pad of any desired size.

Your task is to write a program to generate one-time pads. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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One Response to “One-Time Pad”

  1. mvaneerde said

    I should point out the cryptographic weakness of a one-time pad, which is that it quickly runs out, and then you have to figure out a way to get more of it to the party you’re communicating with; if this is intercepted, your plaintext can be eavesdropped upon.

    You’re consuming 26 bits per letter; can’t you get away with 5?

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