8.5. DONGLES
key=0x17
tmp=key
for i in msg:
sys.stdout.write ("%c" % (i^tmp))
tmp=tmp+3
sys.stdout.flush()
And it prints: “check security device connection”. So yes, this is the decrypted message.
There are also other encrypted messages with their corresponding keys. But needless to say, it is possible
to decrypt them without their keys. First, we can see that the key is in fact a byte. It is because the core
decryption instruction (XOR) works on byte level. The key is located in theESIregister, but only one byte
part ofESIis used. Hence, a key may be greater than 255, but its value is always to be rounded.
As a consequence, we can just try brute-force, trying all possible keys in the 0..255 range. We are also
going to skip the messages that has unprintable characters.
Listing 8.5: Python 3.x#!/usr/bin/python
import sys, curses.ascii
msgs=[
[0x74, 0x72, 0x78, 0x43, 0x48, 0x6, 0x5A, 0x49, 0x4C, 0x47, 0x47,
0x51, 0x4F, 0x47, 0x61, 0x20, 0x22, 0x3C, 0x24, 0x33, 0x36, 0x76,
0x3A, 0x33, 0x31, 0x0C, 0x0, 0x0B, 0x1F, 0x7, 0x1E, 0x1A],
[0x49, 0x65, 0x2D, 0x63, 0x76, 0x75, 0x6C, 0x6E, 0x76, 0x56, 0x5C,
8, 0x4F, 0x4B, 0x47, 0x5D, 0x54, 0x5F, 0x1D, 0x26, 0x2C, 0x33,
0x27, 0x28, 0x6F, 0x72, 0x75, 0x78, 0x7B, 0x7E, 0x41, 0x44],
[0x45, 0x61, 0x31, 0x67, 0x72, 0x79, 0x68, 0x52, 0x4A, 0x52, 0x50,
0x0C, 0x4B, 0x57, 0x43, 0x51, 0x58, 0x5B, 0x61, 0x37, 0x33, 0x2B,
0x39, 0x39, 0x3C, 0x38, 0x79, 0x3A, 0x30, 0x17, 0x0B, 0x0C],
[0x40, 0x64, 0x79, 0x75, 0x7F, 0x6F, 0x0, 0x4C, 0x40, 0x9, 0x4D, 0x5A,
0x46, 0x5D, 0x57, 0x49, 0x57, 0x3B, 0x21, 0x23, 0x6A, 0x38, 0x23,
0x36, 0x24, 0x2A, 0x7C, 0x3A, 0x1A, 0x6, 0x0D, 0x0E, 0x0A, 0x14,
0x10],
[0x72, 0x7C, 0x72, 0x79, 0x76, 0x0,
0x50, 0x43, 0x4A, 0x59, 0x5D, 0x5B, 0x41, 0x41, 0x1B, 0x5A,
0x24, 0x32, 0x2E, 0x29, 0x28, 0x70, 0x20, 0x22, 0x38, 0x28, 0x36,
0x0D, 0x0B, 0x48, 0x4B, 0x4E]]
def is_string_printable(s):
return all(list(map(lambda x: curses.ascii.isprint(x), s)))
cnt=1
for msg in msgs:
print ("message #%d" % cnt)
for key in range(0,256):
result=[]
tmp=key
for i in msg:
result.append (i^tmp)
tmp=tmp+3
if is_string_printable (result):
print ("key=", key, "value=", "".join(list(map(chr, result))))
cnt=cnt+1
And we get:
Listing 8.6: Resultsmessage #1
key= 20 value= `eb^h%|``hudw|_af{n~f%ljmSbnwlpk
key= 21 value= ajc]i"}cawtgv{^bgto}g"millcmvkqh
key= 22 value= bkd\j#rbbvsfuz!cduh|d#bhomdlujni