decrypt.py

def encipher(num_rounds, v, key):
    
    v0=v[0]
    v1=v[1]
    sum=0 
    delta=0x9E3779B9
    for i in range(num_rounds):
        v0 += (((v1 << 4) ^ (v1 >> 5)) + v1) ^ (sum + key[sum & 3])
        sum += delta
        v0 &= 0xFFFFFFFF
        v1 += (((v0 << 4) ^ (v0 >> 5)) + v0) ^ (sum + key[(sum>>11) & 3])
        v1 &= 0xFFFFFFFF
    
    v[0]=v0; v[1]=v1;
    return (v[0],v[1])

def decipher(num_rounds, v, key):
 
    v0=v[0]
    v1=v[1]
    delta=0x9E3779B9
    sum=delta*num_rounds
    for i in range(num_rounds):
        v1 -= (((v0 << 4) ^ (v0 >> 5)) + v0) ^ (sum + key[(sum>>11) & 3]) 
        v1 &= 0xFFFFFFFF
        sum -= delta;
        v0 -= (((v1 << 4) ^ (v1 >> 5)) + v1) ^ (sum + key[sum & 3])
        v0 &= 0xFFFFFFFF
    
    v[0]=v0
    v[1]=v1
    return (v0,v1)

import struct
import os
import sys

def probe(data, key):
    #print(data)
    w = decipher(32, data, key)
    #print(hex(w[0]))
    return struct.pack("<I",w[0]) == b'GIF8'

def decipherFile(file, key):
    data = []
    with open(file,'rb') as f:
        b = os.path.getsize(file)
        for i in range(b //8):
            v1 = f.read(4)
            v1 = struct.unpack("<I", v1)[0]
            v2 = f.read(4)
            v2 = struct.unpack("<I", v2)[0]
            w = decipher(32, [v1,v2], key)
            #print(w)
            data.append(struct.pack("<I",w[0]))
            #print(data[0])
            #break
            data.append(struct.pack("<I",w[1]))
            #sys.stdout.write('.')
            #sys.stdout.flush()
    with open(file.replace('.Mugatu',''), 'wb') as f:
        for b in data:
        #print(data[0:1])
            f.write(b)

import itertools
if __name__ == "__main__":
    print("Start")
    if probe([0x4ee2f5c8,0x6b97f416], [0x0,0x0,0x0,0x0]) == True:
        print("Probe worked...")
    else:
        print("Probe didn't work")
        sys.exit(-1)
    print("Start search...")
    i = 0
    for (k1,k2,k3) in itertools.product(range(256), repeat=3):
        #print(hex(0x31)+hex(k1)+hex(k2)+hex(k3))
        if probe([0x50b08e24,0x6f68b2e8],[0x31,k1,k2,k3]) == True:
            print("Found key:"+hex(0x31)+hex(k1)+hex(k2)+hex(k3))
            decipherFile("best.gif.Mugatu", [0x31,k1,k2,k3])
            break
        i += 1
        if i % 100000 == 0:
            print("+")
        #sys.stdout.write('.')
        #sys.stdout.flush()
    #w = encipher(32, [0x361c1f24,0x1e252819],[0x85,0xb8,0xaf,0x3])
    #print(hex(w[0])+' - '+hex(w[1]))
    print("Finished")