Insecure encryption algorithm - DSA - Python

Insecure encryption algorithm - DSA - Python

Need

Implementation of secure encryption algorithms

Context

  • Usage of Python 3 for scripting and application development
  • Usage of pycryptodome for cryptographic operations in Python

Description

Non compliant code

        from Crypto.PublicKey import DSA
from Crypto.Random import random
from Crypto.Hash import SHA

# Generate a public/private key pair using DSA (Digital Signature Algorithm)
key = DSA.generate(1024)

# Generate a random number
k = random.StrongRandom().randint(1,key.q-1)

# Hash the message
h = SHA.new("A message to be signed".encode('utf-8')).digest()

# Sign the message
sig = key.sign(h,k)

The above code is using the DSA (Digital Signature Algorithm) for encryption which is considered insecure. The DSA algorithm is based on the ElGamal algorithm and relies on the difficulty of the discrete logarithm problem. However, the security of DSA is not as strong as other algorithms such as RSA or ECC.

In this code, firstly a public/private key pair is generated using DSA. Then a random number k is generated and the message that needs to be signed is hashed using SHA (Secure Hash Algorithm). Finally, the message is signed using the private key and the random number k.

The vulnerability lies in the use of the DSA algorithm for encryption. If an attacker gets hold of the private key, they can easily decrypt any message encrypted with the public key. Furthermore, if the random number k is not truly random and an attacker can guess or calculate it, they can calculate the private key from the signature, the public key, the random number and the hash of the message. This is why the use of DSA is considered insecure and not recommended.

To fix this vulnerability, a more secure encryption algorithm like RSA or ECC should be used.

Steps

  • Replace the DSA (Digital Signature Algorithm) with a more secure encryption algorithm. RSA or ECC could be good alternatives.
  • Ensure the key length is sufficient. For RSA, a key length of at least 2048 bits is recommended. For ECC, a key length of 256 bits is usually sufficient.
  • Replace the SHA-1 hash function with a more secure one. SHA-256 or SHA-3 are good alternatives.
  • Ensure that the random number generator used is cryptographically secure. Python's os.urandom or secrets module can be used for generating secure random numbers.
  • Regularly update and patch the cryptographic libraries to ensure they are not susceptible to known vulnerabilities.

Compliant code

        from Crypto.PublicKey import RSA
from Crypto.Random import get_random_bytes
from Crypto.Hash import SHA256

# Generate a public/private key pair using RSA
key = RSA.generate(2048)

# Generate a random number
k = get_random_bytes(16)

# Hash the message
h = SHA256.new("A message to be signed".encode('utf-8')).digest()

# Sign the message
sig = key.sign(h, '')

The original code was using the DSA (Digital Signature Algorithm) for encryption, which is considered insecure. The updated code now uses RSA for encryption, which is currently more secure. The key length has been increased from 1024 bits to 2048 bits, which makes the encryption more difficult to break.

The original code was using a random number generator that may not be cryptographically secure. The updated code uses the get_random_bytes function from the Crypto.Random module, which is designed to be cryptographically secure.

The original code was using the SHA-1 hash function, which is considered insecure due to vulnerabilities that allow for collision attacks. The updated code uses the SHA-256 hash function, which is currently more secure.

The sign method of the RSA key object does not require a second parameter, so the random number k is not used in the signing process. This is a change from the DSA key object's sign method, which does require a second parameter.

It's important to regularly update and patch the cryptographic libraries to ensure they are not susceptible to known vulnerabilities.

References