Cryptanalysis & Cipher Decoding Engine

An automated cryptanalysis engine in pure Python 3 implementing letter frequency analysis, n-gram scoring, multi-restart stochastic hill-climbing optimization, and Kasiski / Index of Coincidence / Chi-squared statistical attacks to break classical mono-alphabetic and poly-alphabetic (Vigenère) ciphers.

Files

• mono-alphabetic.txt: your mono-substitution ciphertext
• poly-alphabetic.txt: Vigenere/poly-alphabetic ciphertext
• mono_decoder.py: Python script for mono frequency analysis and refinement
• poly_decoder.py: Python script for Vigenere key recovery and decryption

What the script does

1. Counts ciphertext letter frequencies
2. Optionally prints digraph/trigraph and double-letter frequency tables
3. Builds an initial substitution using standard English frequency order
4. Runs a lightweight hill-climbing swap search to improve readability
5. Prints both initial and refined plaintext guesses

Run

python mono_decoder.py --input mono-alphabetic.txt --show-ngrams --show-map --output mono-guess.txt

Useful options:

• --iterations 50000 for deeper search
• --seed 7 to try a different random route
• --restarts 20 for more hill-climb restarts
• --top 10 to print only top 10 letter frequencies
• --show-ngrams to include digraph/trigraph/double-letter tables
• --progress-every 1000 to print more frequent hill-climb progress updates
• --early-stop 0 to disable early stopping across restarts
• --quiet to suppress restart/progress log noise

Suggested run profiles:

• Quick check: python mono_decoder.py --iterations 8000 --restarts 6 --quiet
• Deeper pass: python mono_decoder.py --iterations 80000 --restarts 25 --progress-every 2000 --output mono-guess.txt
• Full diagnostic: python mono_decoder.py --show-ngrams --show-map --top 26

Next Step: Vigenere / Poly-Alphabetic Decoder

The poly decoder uses:

1. Kasiski repeated trigram distance factor votes (key-length hints)
2. Index of Coincidence ranking for key lengths
3. Per-column Caesar chi-squared matching to recover each key letter

Run auto key recovery:

python poly_decoder.py --input poly-alphabetic.txt --show-analysis --output poly-guess.txt

Useful options:

• --min-keylen 2 --max-keylen 25 to widen key-length search
• --top-lens 10 to evaluate more candidate lengths
• --key LEMON if you already know the key
• --show-analysis to print IC ranking and Kasiski vote hints
• --output poly-guess.txt to save decrypted plaintext to a file

Suggested run profiles:

• Quick auto-recovery: python poly_decoder.py --top-lens 6
• Wider key search: python poly_decoder.py --min-keylen 2 --max-keylen 30 --top-lens 12 --show-analysis
• Known key decrypt: python poly_decoder.py --key LEMON --output poly-guess.txt

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Research & Academic Context

This engine accompanies the cryptographic research and cryptanalysis coursework published on Code and Cypher:

• Cryptographic Controls & Kerckhoffs's Principle: Security Models and Kerckhoffs's Principle
• Breaking Alphabetic Ciphers: Breaking Alphabetic Ciphers
• Public-Key & Modern Cryptography: Public Key Cryptography