Cryptography

Implementations of classic and modern cryptographic algorithms in Python — symmetric ciphers, asymmetric encryption, and number-theoretic foundations.

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Structure

crypto-from-scratch/
├── symmetric/          # AES, DES, Feistel Cipher
├── asymmetric/         # RSA, ElGamal, ECC
└── math_foundations/   # Fermat test, Euler's totient, CRT

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Algorithms

Symmetric Encryption

File	Algorithm	Description
aes_encryption.py	AES (EAX mode)	Encrypt/decrypt using pycryptodome
des_encryption.py	DES from scratch	Full 16-round Feistel + S-boxes + permutations
feistel_cipher.py	Feistel Cipher	2-round Feistel structure on text

Asymmetric Encryption

File	Algorithm	Description
rsa_encryption.py	RSA	Key generation, encrypt/decrypt from scratch
elgamal_encryption.py	ElGamal	Discrete log based encryption
ecc_points.py	ECC	Point generation on elliptic curve over finite field

Math Foundations

File	Concept	Used In
fermat_primality_test.py	Fermat's Little Theorem	Primality testing for RSA
eulers_totient.py	Euler's φ(n)	RSA key generation
chinese_remainder_theorem.py	CRT	RSA decryption optimization

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Installation

git clone https://github.com/KRYSTALM7/crypto-from-scratch.git
cd crypto-from-scratch
pip install -r requirements.txt

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Usage

# Symmetric
python symmetric/aes_encryption.py
python symmetric/des_encryption.py
python symmetric/feistel_cipher.py

# Asymmetric
python asymmetric/rsa_encryption.py
python asymmetric/elgamal_encryption.py
python asymmetric/ecc_points.py

# Math foundations
python math_foundations/fermat_primality_test.py
python math_foundations/eulers_totient.py
python math_foundations/chinese_remainder_theorem.py

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How They Connect

Math Foundations
  └── Fermat Test      → checks primes used in RSA/ElGamal
  └── Euler's Totient  → φ(n) used in RSA key generation
  └── CRT              → speeds up RSA decryption

Symmetric
  └── Feistel Cipher   → building block used inside DES
  └── DES              → classic 16-round Feistel cipher
  └── AES              → modern standard (replaces DES)

Asymmetric
  └── RSA              → uses Euler totient + modular inverse
  └── ElGamal          → uses discrete log problem
  └── ECC              → elliptic curve over finite fields

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License

MIT License — Copyright (c) 2024 Sujan