vigenere-py 0.2.0

Vigenere cipher encryption for Python

vigenere-py

This is a Python implementation of the Vigenère cipher, where each letter of the plaintext is shifted according to each letter of the key.

Despite having been invented in the 16th century, the Vigenère cipher is still useful because it's simple enough for anyone to encipher and decipher using only pen and paper. See cipher-wheel/ for images of printable cipher wheels. (Pin the centers together with a brass brad.)

If the key is random and at least as long as the plaintext, the Vigenère cipher is effectively a one-time pad, which provide perfect secrecy when properly used.

However, if the key is reused, not fully random, or shorter than the plaintext, then the cipher can be easily broken through frequency analysis.

As a result, vigenere-py comes with a keygen command for generating suitable random keys, and it will refuse to encrypt text with a key that is too short unless the --insecure mode is specified.

Installation

Install this package with pipx for use as a standalone CLI:

pipx install vigenere-py

vigenere --help

Alternatively, you can install this package for use as a library via pip (ideally run this inside a virtualenv):

pip install vigenere-py

Usage

For help, run:

vigenere --help

You can also use:

python -m vigenere --help

Alphabets

Several different alphabets are available. The decimal alphabet expects keys and ciphertext to be 2-digit decimal numbers. This makes it convenient to compute by hand because encryption is just adding the numbers modulo 100.

The other alphabets are more traditional ciphers that can be computed on paper with the help of a table or a cipher wheel. The printable alphabet contains all printable ASCII characters with spaces but no other whitespace.

The other alphabets will pass through punctuation like spaces unchanged.

decimal: 100-char full ASCII, ciphertext written as digits aliases: (100|ascii) passthrough: none chars: ␀␉␊␌␍ !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]^_`abcdefghijklmnopqrstuvwxyz{|}~

printable: All printable characters and spaces aliases: (print|wheel) passthrough: other whitespace chars: !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]^_`abcdefghijklmnopqrstuvwxyz{|}~

letters: Uppercase letters only aliases: (upper|uppercase) passthrough: punctuation/whitespace chars: ABCDEFGHIJKLMNOPQRSTUVWXYZ

alpha-mixed: Mixed case letters and numbers aliases: (alpha|alphanumeric|alphanumeric-mixed) passthrough: punctuation/whitespace chars: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789

alpha-upper: Uppercase letters and numbers aliases: (alphanumeric-upper) passthrough: punctuation/whitespace chars: ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789

Examples

Generating a key and encrypting:

$ vigenere genkey -a letters 20 > key.txt

$ cat key.txt
RVRTCLIWHNPZAOJLXEWY

$ cat plain.txt
ATTACK AT DAWN

$ vigenere enc -a letters -k key.txt plain.txt
ROKTEV IP KNLM

Decrypting:

$ cat key.txt
RVRTCLIWHNPZAOJLXEWY

$ cat cipher.txt
ROKTEV IP KNLM

$ vigenere dec -a letters -k key.txt cipher.txt
ATTACK AT DAWN

Interactive mode, end the message with ctrl+d:

$ vigenere enc -a letters
Key: •••••••••••••••••
Text to encrypt:
SECRET MESSAGE
Ciphertext:
QSWIIT PXZWDUG

Decimal

The decimal alphabet (aka ascii or 100) expects keys and ciphertext to be encoded as two-digit numbers in base 10.

$ vigenere genkey -a ascii 14 > key.txt

$ cat key.txt
20 95 47 06 32 32 16 88 59 87

$ echo 'Hello, world!' > plain.txt

$ vigenere enc -a ascii -k key.txt plain.txt
18 32 84 77 37 76 86 89 86 97 95 30 76 36

To directly encode or decode from decimal, use vigenere decimal.

Note that decoded output for the ascii alphabet may contain control characters like \0!

$ export VIGENERE_ALPHABET=ascii

$ echo 'Hello!' | vigenere decimal -e
45 74 81 81 84 06 02

$ echo '45 74 81 81 84 06 02' | vigenere decimal -d
Hello!

$ echo '00 01 02 03 63' | vigenere decimal -d | xxd
00000000: 0009 0a0c 5a                             ....Z

Decimal encoding also works with other alphabets, if you want that for some reason.

$ echo 'ABCD' | vigenere decimal -a letters -e
00 01 02 03

$ echo '23 24 25 26 27' | vigenere decimal -a alpha -d
XYZab

Bash shell completions

_VIGENERE_COMPLETE=bash_source vigenere > ~/.local/share/bash-completion/completions/vigenere

Development

To contribute to this tool, first checkout the code.

Poetry

Poetry is used to manage dependencies and virtualenvs. So install poetry before proceeding.

I recommend installing poetry with pipx.

pipx install poetry

But if you don't want to use pipx, there are other installation instructions here: https://python-poetry.org/docs/#installation

Installing dependencies

cd vigenere-py
poetry install

Running the app

poetry run vigenere --help

Running tests

poetry run mypy .
poetry run pytest -v

Or, you can run these as a poe task:

Install poe:

pipx install poethepoet

Run tests:

poe test