CODEWITHSUNDEEP
pythonencryptionrc4-cipher
Gal
Gal

Reputation: 719

How to decrypt a file that encrypted with rc4 using Python?

I got a file that encrypted with rc4 key.

I got that key and want to decrypt it with a python script.

How can I do this?

Upvotes: 4

Views: 29937

Answers (2)

BartBiczBoży
BartBiczBoży

Reputation: 2672

PyCryptodome does it nowadays: https://pycryptodome.readthedocs.io/en/latest/src/cipher/arc4.html

from Crypto.Cipher import ARC4
key = b'Very long and confidential key'
cipher = ARC4.new(key)
msg = cipher.encrypt(b'Open the pod bay doors, HAL')

Upvotes: 4

nivix zixer
nivix zixer

Reputation: 1651

Found the following after a 3 second Google search: http://www.emoticode.net/python/python-implementation-of-rc4-algorithm.html

Modified it thusly:

import base64

data = base64.b64decode("<encrypted file contents>")
key = "<rc4 key>"

S = range(256)
j = 0
out = []

#KSA Phase
for i in range(256):
    j = (j + S[i] + ord( key[i % len(key)] )) % 256
    S[i] , S[j] = S[j] , S[i]

#PRGA Phase
i = j = 0
for char in data:
    i = ( i + 1 ) % 256
    j = ( j + S[i] ) % 256
    S[i] , S[j] = S[j] , S[i]
    out.append(chr(ord(char) ^ S[(S[i] + S[j]) % 256]))

print ''.join(out)

Not sure if this will work, as you did not give us any data to work with.. next time please post an example of the data, the code you have already tried, and what errors you are getting.

Edit -- Working with files

import base64

with open("/path/to/file.txt", "r") as encrypted_file:
    data = base64.b64decode(encrypted_file.read())
key = "<rc4 key>"

S = range(256)
j = 0
out = []

#KSA Phase
for i in range(256):
    j = (j + S[i] + ord( key[i % len(key)] )) % 256
    S[i] , S[j] = S[j] , S[i]

#PRGA Phase
i = j = 0
for char in data:
    i = ( i + 1 ) % 256
    j = ( j + S[i] ) % 256
    S[i] , S[j] = S[j] , S[i]
    out.append(chr(ord(char) ^ S[(S[i] + S[j]) % 256]))

decrypted_text = ''.join(out)
with open('decrypted.txt', 'w') as decrypted_file:
    decrypted_file.write(decrypted_text)

Upvotes: 5

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