Substitution Cipher Solver

Algorithm for solving simple, monoalphabetic substitution ciphers

This is Python implementation of the algorithm for solving simple, monoalphabetic substitution ciphers described in the paper “A Fast Method for the Cryptanalysis of Substitution Ciphers” by Thomas Jakobsen. The main difference from the paper is that random key swaps are used instead of a deterministic series of swaps since it yields better results, but the original method is included and can be used as an option.

Installing

pip install cipher_solver

API

class SimpleSolver:
    """Simple substitution cipher solver."""

    def __init__(self, ciphertext):
        """Create new solver.

        Creates a new cipher solver from an initial ciphertext.
        """

    def solve(self, method="random"):
        """Solve the cipher.

        Run the solver and save the resulting decryption key.
        """

    def plaintext(self):
        """Return a plaintext using the current decryption key."""

    def reset(self):
        """Reset the solver to its initial state.

        Set the decryption key to its initial state, effectively starting over.
        """

See the documentation for full description of methods and their parameters.

Requirements

• numpy

Usage

from cipher_solver.simple import SimpleSolver

# Solve a cipher.
s = SimpleSolver("U kn kgmhksz tkm exmpb xt Gxesxe.")
s.solve()
print(s.plaintext())  # "I am already far north of London."

s.reset()  # Discard current solution to start over.
s.solve()
print(s.plaintext())  # We have an alternative solution now.

print(s.decryption_key())  # "goaskbihxvrldepfwntmzqjucy"

# Solve using the original key swap method instead.
d = SimpleSolver("U kn kgmhksz tkm exmpb xt Gxesxe.", method="deterministic")
d.solve()
print(s.plaintext())

Note, however, that the above ciphertext is too short to give any meaningful results. A length of at least a few hundred letters is preferred to solve a cipher. See below for an example using an included sample text.

CLI

A simple command-line interface is included. To solve a cipher, put it into a text file and run:

cipher_solver <path_to_ciphertext_file>

Example:

cipher_solver texts/26_char_key/ciphertexts/ciphertext_frankenstein_sample.txt

Since the algorithm involves hill climbing and randomness you might sometimes end up with complete gibberish, just run the script again and the next result should be better.

Running tests

make test

Checking coverage

make coverage

(Requires the coverage package.)

Generating documentation

make docs

(Requires the pdoc3 package.)