For prog assignment 2, CTR method 2

[108anup]

def decrypt(msg, key):
        key = base64.b16decode(key, True)
        msg = base64.b16decode(msg, True)
        init_val = msg[:s]
        msg = msg[s:]
        ctr = int(base64.b16encode(init_val),16)
        cipher = AES.new(key, AES.MODE_ECB)
        pt = ""
        for i in range(0,len(msg)/s):           
            c = cipher.encrypt(ctr2str(ctr))
            ctr = (ctr + 1) % 2**128
            pt += xor(msg[i*s:(i+1)*s], c)
            
        c = cipher.encrypt(ctr2str(ctr))
        pt += xor(msg[len(msg) - len(msg)%s:], c)
        return pt