zakirkun Hardcoded IV / Nonce
Overview
A static or predictable Initialization Vector (IV) in block cipher modes (CBC, CTR, GCM) critically weakens encryption:
- CBC with fixed IV: Two identical plaintexts produce identical ciphertexts, leaking information
- CTR/GCM with reused nonce: Nonce reuse in CTR/GCM allows key recovery and plaintext XOR
The IV/nonce must be randomly generated for each encryption operation and stored alongside the ciphertext.
Detection Strategy
- Zero-filled IV:
\x00\x00...\x00oriv = bytes(16) - Hardcoded IV bytes:
iv = b'\x01\x02\x03...' - Derived from static string:
iv = "hardcoded_iv____".encode()
Remediation
Generate a cryptographically random IV for every encryption operation.
Vulnerable (Python):
from Crypto.Cipher import AES
iv = b'\x00' * 16 # Static zero IV!
cipher = AES.new(key, AES.MODE_CBC, iv)
Safe (Python):
import os
from Crypto.Cipher import AES
iv = os.urandom(16) # Random IV
cipher = AES.new(key, AES.MODE_CBC, iv)
ciphertext = iv + cipher.encrypt(plaintext) # Prepend IV to output