[Xortool] A tool to analyze multi-byte xor cipher


A tool to do some xor analysis:
  • guess the key length (based on count of equal chars)
  • guess the key (base on knowledge of most frequent char)

Usage

! python3 is not supported, use python 2.x

  xortool [-h|--help] [OPTIONS] [<filename>]
Options:
-l,--key-length length of the key (integer)
-c,--char most possible char (one char or hex code)
-m,--max-keylen=32 maximum key length to probe (integer)
-x,--hex input is hex-encoded str
-b,--brute-chars brute-force all possible characters
-o,--brute-printable same as -b but will only use printable
characters for keys

Example

# xor is some external script to encrypt files
tests $ xor -f /bin/ls -s "secret_key" > binary_xored

tests $ xortool binary_xored
The most probable key lengths:
2: 5.0%
5: 8.7%
8: 4.9%
10: 15.4%
12: 4.8%
15: 8.5%
18: 4.8%
20: 15.1%
25: 8.4%
30: 14.9%
Key-length can be 5*n
Most possible char is needed to guess the key!

# 00 is the most frequent byte in binaries
tests $ xortool binary_xored -l 10 -c 00
...
1 possible key(s) of length 10:
secret_key

# decrypted ciphertexts are placed in ./xortool_out/Number_<key repr>
# ( have no better idea )
tests $ md5sum xortool_out/0_secret_key /bin/ls
29942e290876703169e1b614d0b4340a xortool_out/0_secret_key
29942e290876703169e1b614d0b4340a /bin/ls

The most common use is to pass just the encrypted file and the most frequent character (usually 00 for binaries and 20 for text files) - length will be automatically chosen:

tests $ xortool tool_xored -c 20
The most probable key lengths:
2: 5.6%
5: 7.8%
8: 6.0%
10: 11.7%
12: 5.6%
15: 7.6%
20: 19.8%
25: 7.8%
28: 5.7%
30: 11.4%
Key-length can be 5*n
1 possible key(s) of length 20:
an0ther s3cret \xdd key

Here, the key is longer then default 32 limit:

tests $ xortool ls_xored -c 00 -m 64
The most probable key lengths:
3: 3.3%
6: 3.3%
9: 3.3%
11: 7.0%
22: 6.9%
24: 3.3%
27: 3.2%
33: 18.4%
44: 6.8%
55: 6.7%
Key-length can be 3*n
1 possible key(s) of length 33:
really long s3cr3t k3y... PADDING

So, if automated decryption fails, you can calibrate:
  • (-m) max length to try longer keys
  • (-l) selected length to see some interesting keys
  • (-c) the most frequent char to produce right plaintext

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