Are there security vulnerabilities given the would a user be able to access the original text and the encrypted text with AES using an MD5 as a key?

In other words, if the user had both the original and encrypted text, is it possible for the user to encrypt and decrypt their own text or an altered version of the site's text?

I am specifically wondering about this based on the security of the MD5 hash.

Full Code:

class AESCipher:

def __init__(self, key):
    self.key = md5(key.encode('utf8')).hexdigest()

def encrypt(self, raw):
    raw = pad(raw)
    iv = Random.new().read(AES.block_size)
    cipher = AES.new(self.key, AES.MODE_CBC, iv)
    return b64encode(iv + cipher.encrypt(raw))

def decrypt(self, enc):
    enc = b64decode(enc)
    iv = enc[:16]
    cipher = AES.new(self.key, AES.MODE_CBC, iv)
    return unpad(cipher.decrypt(enc[16:])).decode('utf8')
  • $\begingroup$ Normally, MD5 has a collusion attack. What is the usage scenario? $\endgroup$
    – kelalaka
    Oct 3 '18 at 0:00
  • 1
    $\begingroup$ Why apply md5 to the key at all? If the key is a proper random string, there is no benefit. If the "key" passed to init is a password, then applying md5 to it is definitely a weakness. $\endgroup$
    – Ella Rose
    Oct 3 '18 at 1:04
  • 1
    $\begingroup$ @kelalaka Did you mean collision or collusion? The collision attack can be used for a collusion attack, but I think you meant collision :) $\endgroup$
    – Maarten Bodewes
    Oct 3 '18 at 1:15
  • $\begingroup$ @MaartenBodewes yes, Collision :) $\endgroup$
    – kelalaka
    Oct 3 '18 at 1:24
  • $\begingroup$ This is from PicoCTF... I don't think you're supposed to ask about those until it's finished $\endgroup$ Oct 3 '18 at 18:20

No, if the input key has enough entropy then using an MD5 hash will not reduce the security of AES. And AES protects the retrieval of the key from any combination of plaintext and ciphertext. MD5 can be seen as a poor man's (Key Based) Key Derivation Function or KDF which extracts (compresses) the entropy found in the input key material.

However, a lot of times MD5 is used instead of a Password Based Key Derivation Function or PBKDF (bcrypt, scrypt, PBKDF2 and the newer Argon2 variants are well known PBKDF's). If the input is a password or text then it is likely that a low amount of entropy is present. In that case it is easy to guess the input of MD5. The resulting AES key may seem secure and randomized, but the adversary may guess the input - for instance by using a dictionary attack - and perform the MD5 calculation to retrieve the AES key.

MD5 has been broken and the usage of MD5 is a red flag, often indicating that the developer hasn't got a clue about cryptography. This is true regardless if MD5 is used in a secure setting or not. The use of MD5 is often not by design, but because the developer copied another bad example of cryptography. In your case, the MD5 hash is hex encoded and then used as 256 bit key. Although using 128 bits (max) of entropy as 256 bit key doesn't break AES in practice, it does show that the developer wasn't a cryptographer himself.

If the quality is that bad, you can almost be certain that other errors have been made. In your example code, CBC mode could be used directly to achieve transport security, making padding oracle attacks a distinct possibility.

Please learn at least the basics of encoding and cryptography. Whatever you do, don't use self-made protocols like the one in your question. Try and use higher level frameworks or transport protocols such as TLS instead. You have been warned - and it is good you asked.

  • $\begingroup$ Reducing AES-256 to 128 bit security would definitely count as a rather serious break of the block cipher. However, having 128 bit security is still plenty until quantum computers can use Grover's algorithm for these kind of keys. Hence "in practice". $\endgroup$
    – Maarten Bodewes
    Oct 3 '18 at 1:21
  • $\begingroup$ this is from a CTF, its made to be vulnerable... the question is just fishing for a solution $\endgroup$ Oct 3 '18 at 18:21
  • $\begingroup$ @JannesBraet Unfortunately it is impossible for us to establish that kind of claim beforehand. As long as the questions are deemed on topic they are welcome here. $\endgroup$
    – Maarten Bodewes
    Nov 26 '19 at 21:40

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