Helper script for decoding some .NET cryptor

msil_dec.py

#!/usr/bin/python2.7

import argparse

def decode(data, key, offset, extra_rounds):
    maxlen = len(data)
    keylen = len(key)
    j = 0 #key index
    num2 =  (maxlen - 1) * (extra_rounds + 1)
    decoded = bytearray()
    for i in range(offset, num2):
        dec = ((data[i % maxlen] ^ key[j % keylen]) - (data[(i + 1) % maxlen] + 0x100)) % 0x100
        j += 1
        decoded.append(dec) 
    return decoded

def main():
    parser = argparse.ArgumentParser(description="Helper script for decoding some .NET cryptor")
    parser.add_argument('--file', dest="file", default=None, help="Input file (dumped from resources)", required=True)
    parser.add_argument('--rounds', dest="rounds", default=0, help="Extra rounds", required=False, type=int)
    parser.add_argument('--key', dest="key", default=None, help="Key")
    parser.add_argument('--keyfile', dest="keyfile", default=None, help="File containing key (alternative to Key)")
    parser.add_argument('--offset',dest="offset", default=0,type=int, help="Offset in file from which XOR should start")
    args = parser.parse_args()


    data = bytearray(open(args.file, 'rb').read())
    if (args.key == None and args.keyfile == None):
        print "Supply key or keyfile"
        exit (-1)
    if args.keyfile:
        key = bytearray(open(args.keyfile, 'rb').read())
    else:
        key = bytearray(args.key)
    offset = args.offset
    
    print decode(data, key, offset, args.rounds)

if __name__ == "__main__":
    main()