Small Plaintext Attack on RSA

A Python script to perform a small plaintext attack on RSA encryption. It recovers the plaintext (m) from a ciphertext (c) when the plaintext and public exponent (e) are small. Useful for CTF challenges or educational purposes.

Requirements

• Python 3.x
• gmpy2 library: pip install gmpy2

Usage

1. Set c (ciphertext), e (public exponent), and n (modulus) in the script.
2. Run: python small_plaintext_attack.py
3. Output: Plaintext as an integer and, if possible, an ASCII string (e.g., a CTF flag).

Example

from gmpy2 import iroot

c = 27  # Ciphertext
e = 3   # Exponent
n = 100 # Modulus

m, exact = iroot(c, e)
if exact and pow(m, e, n) == c:
    print("Plaintext:", m)
else:
    print("Attack failed.")