crypt

Encrypts and decrypts any file using One-Time Pad (OTP)

Works on any file type — text, images, binaries, archives, etc...

---

How it works

Encryption

1. The file is read into memory as raw bytes.
2. A random pad of the exact same size is generated from /dev/urandom (kernel CSPRNG aka Cryptographically Secure Pseudorandom Number Generator).
3. Each byte of the file is XOR'd with the corresponding byte of the pad:

# for example the letter 'H'
Original:   01001000  (H)
Pad:        10110010
            --------
Ciphertext: 11111010

4. Two files are written to the current working directory:
   • pad — the secret key (same size as the input file)
   • encrypted_<filename> — the ciphertext

Decryption

XOR is applied again between the ciphertext and the pad, recovering the original bytes:

Ciphertext: 11111010
Pad:        10110010
            --------
Original:   01001000  (H)

The decrypted file is saved as decrypted_<filename> in the current working directory.

Why does this work? XOR is its own inverse — original XOR pad = ciphertext, and ciphertext XOR pad = original always.

A full visual breakdown of the OTP process is available in media/.

---

Build

chmod +x build.sh
./build.sh

---

Usage

Encrypt a file

./crypt secret
# Output: 
#   pad               
#   encrypted_secret

Decrypt a file

./decrypt pad encrypted_secret
# Output:
#   decrypted_secret

---

Some Notes

• Reusing a pad against two different files breaks the cipher completely.
• The pad must be bit-for-bit identical to the one used during encryption, so do not rename it.
• There is no integrity check, a modified ciphertext will decrypt garbage.