RC4StaticKeyAttack

Attack on weak RC4 implementations that use static keys

Requirements

For this attack to work a few requirements have to be met.

1. The attacker must have the ability to encrypt a known plaintext

In order for this to work you need the ability to encrypt a known plaintext or you need a known plaintext and the corresponding ciphertext.

2. The cipher system must use a static key.

A vulnerable implementation should look something like this.

key = 'Never ever use the same key more than once!!!!'
encrypted = enc(key, plaintext)
print(base64.b64encode(encrypted)) 

Notice the static hardcoded key.

If there is no access to the sourcecode of the encrypting system, a static key can be identified by encrypting a known plaintext two times. If both resulting ciphertext are identical, it´s almost certain that the same encryption key is being used.

An example of such a vulnerable implementation can be found in crypt.py. Use the script crypt.py to encrypt a given plaintext twice.

python crypt.py testData/knownPlainTextSmall.txt
H4FXzKw=
python crypt.py testData/knownPlainTextSmall.txt
H4FXzKw=

Notice the identical ciphertexts.

Usage

Find a vulnerable encryptio system and let it encrypt a known plaintext. Store the corresponding ciphertext. Then pass the known plaintext, known ciphertext and the ciphertext you want to crack to rc4Cracker.py

python3 rc4Cracker.py testData/knownPlainText.txt testData/knownPlainText.rc4 testData/unknownPlainText.rc4

The script will return the previously unknown plaintext.

Disclaimer

This tool should only be used for educational purposes. It was build with the intention to make the lives of security researchers and pentersters a bit easier.