Python PyCryptodome Insecure Blowfish Cipher Usage

Medium Risk Cryptographic Weakness
PythonPyCryptodomeBlowfishEncryptionCryptographyBlock Cipher

What it is

Application uses the Blowfish cipher algorithm which has known vulnerabilities including a small block size (64 bits) that makes it susceptible to birthday attacks and collision issues.

from Crypto.Cipher import Blowfish from Crypto.Random import get_random_bytes from flask import Flask, request @app.route('/encrypt', methods=['POST']) def encrypt_data(): # Vulnerable: Blowfish with small block size key = get_random_bytes(16) cipher = Blowfish.new(key, Blowfish.MODE_CBC) data = request.form.get('data').encode() # Padding needed for block cipher padded_data = data + b'\0' * (8 - len(data) % 8) encrypted = cipher.encrypt(padded_data) return encrypted.hex() @app.route('/decrypt', methods=['POST']) def decrypt_data(): # Vulnerable: Blowfish decryption key = bytes.fromhex(request.form.get('key')) encrypted_hex = request.form.get('encrypted') cipher = Blowfish.new(key, Blowfish.MODE_CBC) decrypted = cipher.decrypt(bytes.fromhex(encrypted_hex)) return decrypted.decode()
from Crypto.Cipher import AES from Crypto.Random import get_random_bytes from Crypto.Util.Padding import pad, unpad from Crypto.Protocol.KDF import PBKDF2 from Crypto.Hash import SHA256 from flask import Flask, request import base64 @app.route('/encrypt', methods=['POST']) def encrypt_data(): """Secure encryption using AES-GCM.""" try: data = request.form.get('data', '').encode('utf-8') password = request.form.get('password', '') if not data or not password: return {'error': 'Data and password required'}, 400 # Generate random salt and nonce salt = get_random_bytes(16) nonce = get_random_bytes(12) # 96-bit nonce for GCM # Derive key using PBKDF2 key = PBKDF2(password, salt, 32, count=100000, hmac_hash_module=SHA256) # Secure: Use AES-GCM for authenticated encryption cipher = AES.new(key, AES.MODE_GCM, nonce=nonce) ciphertext, tag = cipher.encrypt_and_digest(data) # Combine all components result = salt + nonce + tag + ciphertext return { 'encrypted': base64.b64encode(result).decode(), 'algorithm': 'AES-256-GCM' } except Exception as e: return {'error': 'Encryption failed'}, 500 @app.route('/decrypt', methods=['POST']) def decrypt_data(): """Secure decryption using AES-GCM.""" try: encrypted_data = request.form.get('encrypted', '') password = request.form.get('password', '') if not encrypted_data or not password: return {'error': 'Encrypted data and password required'}, 400 # Decode and extract components data = base64.b64decode(encrypted_data) salt = data[:16] nonce = data[16:28] tag = data[28:44] ciphertext = data[44:] # Derive key using same parameters key = PBKDF2(password, salt, 32, count=100000, hmac_hash_module=SHA256) # Decrypt and verify cipher = AES.new(key, AES.MODE_GCM, nonce=nonce) plaintext = cipher.decrypt_and_verify(ciphertext, tag) return { 'decrypted': plaintext.decode('utf-8'), 'algorithm': 'AES-256-GCM' } except ValueError: return {'error': 'Decryption failed - invalid data or password'}, 400 except Exception as e: return {'error': 'Decryption failed'}, 500

💡 Why This Fix Works

See fix suggestions for detailed explanation.

Why it happens

Code uses Blowfish: from Crypto.Cipher import Blowfish; cipher = Blowfish.new(key, Blowfish.MODE_CBC). Blowfish has 64-bit block size. Small blocks vulnerable to birthday attacks with large data volumes. After ~32GB encrypted, collisions likely. Modern ciphers use 128-bit blocks.

Root causes

Using Blowfish Cipher from PyCryptodome or PyCrypto

Code uses Blowfish: from Crypto.Cipher import Blowfish; cipher = Blowfish.new(key, Blowfish.MODE_CBC). Blowfish has 64-bit block size. Small blocks vulnerable to birthday attacks with large data volumes. After ~32GB encrypted, collisions likely. Modern ciphers use 128-bit blocks.

Legacy Systems with Blowfish Encryption

Old applications using Blowfish for compatibility. File encryption systems from 1990s-2000s. Inherited codebases with Blowfish. Backward compatibility requirements. Historical algorithm choice persisting in modern applications. Migration difficult with encrypted legacy data.

Choosing Blowfish Based on Outdated Recommendations

Developers following old security guidance. Blowfish recommended in older books and tutorials. Not aware of modern alternatives like AES. Copying examples from legacy code. Blowfish was good choice in 1990s, inadequate now. Block size limitation not understood.

Using Blowfish for Large File or Database Encryption

Encrypting large volumes with Blowfish. Database field encryption. File storage encryption. Long-lived encryption keys. Large data volumes amplify block size weakness. Birthday bound reached sooner with active usage. 64-bit blocks insufficient for modern data volumes.

Misunderstanding Blowfish Security Status

Believing Blowfish still secure. Confusing lack of practical attacks with security. Not understanding birthday bound implications. Missing awareness of AES superiority. Blowfish not broken but deprecated. Modern security requires 128-bit block ciphers.

Fixes

1

Replace Blowfish with AES-256 for All Encryption

Use AES instead: from Crypto.Cipher import AES; cipher = AES.new(key, AES.MODE_GCM). AES-256 with GCM mode provides encryption and authentication. 128-bit blocks. NIST approved. Industry standard. Supported everywhere. Superior security and performance to Blowfish.

2

Use AES-GCM Mode for Authenticated Encryption

Authenticated encryption with AES-GCM: cipher = AES.new(key, AES.MODE_GCM); ciphertext, tag = cipher.encrypt_and_digest(plaintext). GCM provides confidentiality and integrity. Prevents tampering. Single operation. Nonce required for security. Modern applications should use authenticated encryption.

3

Migrate Legacy Blowfish Data to AES

Implement migration: decrypt with Blowfish, re-encrypt with AES. Gradual migration on access: when reading, convert to AES. Background migration jobs for inactive data. Maintain dual decryption support during transition. Complete migration plan for legacy encrypted data.

4

Use cryptography Library Instead of PyCryptodome

Modern cryptography library: from cryptography.fernet import Fernet; f = Fernet(key); encrypted = f.encrypt(data). Higher-level API. Better defaults. AES-128-CBC with HMAC. Handles key derivation. Prevents common mistakes. Recommended for new Python applications.

5

Generate Strong Random Keys with Proper Length

Use secure key generation: from Crypto.Random import get_random_bytes; key = get_random_bytes(32) for AES-256. Never use weak keys. 256-bit keys recommended. Use key derivation functions for passwords: PBKDF2, bcrypt, or Argon2. Proper key management essential.

6

Scan Codebase for Blowfish Usage and Replace

Find Blowfish: grep -r 'Blowfish' --include="*.py". Review all encryption code. Replace with AES. Update documentation. Security audit for cipher usage. Deprecation plan for Blowfish. Complete removal from codebase.

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