Cryptography course projects

Here are some of the projects that I did as part of the cryptography course¹ in university. Perhaps, someone may find those useful.

Caesar

First and foremost, the basic Caesar cipher task with a bit of a caveat:

Write a program that implements Caesar cipher. Next, given an encrypted Ukrainian text, find a key using the estimate of the logarithmic likelihood function and decrypt it. Plot the log-likelihood function.

I used go-echarts to plot the function, it looks something like this:

To generate the HTML document from the screenshot, you can run

go run main.go caesar.go log.go chart.go

Vigenère

It is another historic cipher that is essentially running several Caesar ciphers according to the key. The task was to crack it by determining the key length using the Friedman test and then find the key content (I used the fact that space is the most frequent character in the Ukrainian text).

Simplified DES

Next task was to implement a simplistic algorithm somewhat resembling DES. It was described as such:

To see it in action, you can run

go run main.go keygen.go sdes.go

GOST 28147-89

An implementation of the famous Soviet algorithm and its modes of operation.

LFSR

An implementation of a simplistic stream cipher based on LFSR.

Here is a picture of a 16-bit LFSR with a polynomial $x^{16} + x^{14} + x^{13} + x^{11} + 1$, similar to the one in the lfsr/lfsr.go:

	var start, polynomial uint64 = 0xACE1, 0b1000000000101101

You can modify the code slightly (lol) or use a debugger to verify that 0xACE1 state is followed by 0x5670, as it should, or convey other tests that you prefer.

To see file encryption and decryption, just run

go run lfsr.go

SHA-1 CLI tool

There was such a task:

Write a CLI tool that implements a hashing algorithm in your preferred programming language.

Algorithm	Message source	Input	Output
SHA-1	File	Hex	Hash (in hex)

You can build it or just run

go run main.go password.txt

Footnotes

1. Криптографічні системи захисту інформації, i.e. Cryptographic information security systems ↩