cryptography Archives - Stefano Ottolenghi's Place

The scope of this article is to present the one time pad cipher method and its biggest vulnerability: that of the many time pad.

The one time pad: what it is and how it works

The one time pad is the archetype of the idea of stream cipher. It’s very simple: if you want to make a message unintelligible to an eavesdropper, just change each character of the original message in a way that you can revert, but that looks random to another person.

The way the one time pad works is the following. Suppose $\mathcal{M}$ is the clear-text message you would like to send securely, of length $|\mathcal{M}| = s$ . First, you need to generate a string $\mathcal{K}$ of equal length $|\mathcal{K}| = s$ . Then, you can obtain a cipher-text version of your message by computing the bitwise XOR of the two strings:

$\mathcal{M} \oplus \mathcal{K}$

The best thing is that decoding is just the same as encoding, as the XOR operator has the property that $\mathcal{X} \oplus \mathcal{X} = 0 \ \forall X$ (and that $\mathcal{X} \oplus 0 = \mathcal{X} \ \forall \mathcal{X}$ ). The only difference is that the cipher-text is involved in the XOR, rather than the clear-text:

$\mathcal{C} \oplus \mathcal{K} = \mathcal{M} \oplus \mathcal{K} \oplus \mathcal{K} = \mathcal{M} \oplus 0 = \mathcal{M}$

Below is an example of the one time pad encoding achieved with Python, with a made-up pad string.

In the first section, *result* holds the XOR result. In the second part, the *result* and *one_time_pad* variables are XORed together to obtain the original plain-text message again.

It is not difficult to realize that the whole strength of the algorithm lies in the $\mathcal{K}$ pad. Of course, as an attacker, if you can obtain $\mathcal{K}$ in some way, then it is not difficult to get the clear-text message from the ciphered one as well.

Continue reading “The one time pad and the many time pad vulnerability”