RandomStringUtils/nextInt Cracker

This is a tool to crack RandomStringUtils and Java's default java.util.Random.nextInt(bound) when bound is odd. See the accompanying blog post for some background and how it works.

Installation

Download and build the tool from source:

git clone https://github.com/elttam/rsu-cracker.git
cd rsu-cracker
cargo install --path .

Usage

Usage: rsu-cracker [COMMAND]

Commands:
  random-alphanumeric
  next-int
  help                 Print this message or the help of the given subcommand(s)

Options:
  -h, --help  Print help

Usage: rsu-cracker random-alphanumeric [OPTIONS] <TOKEN>

Arguments:
  <TOKEN>  Output of RandomStringUtils.randomAlphanumeric(n)

Options:
  -c, --count <COUNT>            Number of tokens to output [default: 10]
  -o, --output-len <OUTPUT_LEN>  Length of tokens to output [default: 0]
  -h, --help                     Print help

Usage: rsu-cracker next-int [OPTIONS] --n <N> [OUTPUTS]...

Arguments:
  [OUTPUTS]...  Outputs of random.nextInt(n)

Options:
  -n, --n <N>          Value of the bound n
  -c, --count <COUNT>  Number of values to output [default: 10]
  -h, --help           Print help

Examples

❯ rsu-cracker random-alphanumeric -c4 ygZDDnsAqDNRHT4FS5UrHN3vi6mEGTJr
[*] Attempting with skips = (0, 0, 0)
100%|███████████████████████████████████████████| 23598721/23598721 [00:05<00:00, 4467906.00it/s]
[*] Attempting with skips = (0, 0, 1)
100%|███████████████████████████████████████████| 23598721/23598721 [00:05<00:00, 4319475.50it/s]
[*] Attempting with skips = (0, 1, 0)
100%|███████████████████████████████████████████| 23598721/23598721 [00:05<00:00, 4287969.00it/s]
[*] Attempting with skips = (1, 0, 0)
100%|███████████████████████████████████████████| 23598721/23598721 [00:05<00:00, 4212314.50it/s]

[*] Finished running after 23.634s
[+] Java Random seed recovered: 1668247404
[+] The next 4 tokens are:
gcFPzMAEVIxlrpYx4NZedHoLFgjgHTeF
6KjEP8eTsNne85XWSgXaHS9C1rwmxbKI
dMZ0o9XO8RvTcoHjUT4WWcCUAUeehxmj
vgrdwxLmgzC4nUoZCw7CWi0hCMHbi3oA

❯ rsu-cracker random-alphanumeric -c1 -o 50 GxZRXq7BWQ
[*] Attempting with skips = (0, 0, 0)
100%|███████████████████████████████████████████| 23598721/23598721 [00:05<00:00, 4403258.50it/s]

[*] Finished running after 5.597s
[+] Java Random seed recovered: 175138514385774
[+] The next token is:
n103Z9XtEiGqQnwxh4ifc6dC2tjRaV7nm8Qi93cW7Z3QTOGJlK

❯ rsu-cracker next-int -n7 -c5 0 2 2 0 5 6 5 6 5 2 4 4 4 5 5 4 2 4 6 5
100%|███████████████████████████████████████████| 306783378/306783378 [02:50<00:00, 1798661.75it/s]

[*] Finished running after 362.326s
[+] Java Random seed recovered: 7955809
[+] The next 5 outputs are:
3
4
2
5
2

❯ rsu-cracker next-int -n1337 -c7 509 287 813 384 1013 1254
100%|███████████████████████████████████████████| 1606195/1606195 [00:00<00:00, 4161011.50it/s]

[*] Finished running after 0.45s
[+] Java Random seed recovered: 1337
[+] The next 7 outputs are:
1317
623
1123
457
201
1176
640

Known Issues

• The next-int command is not as effective for very small bounds (i.e. less than 7). In some cases it may not be able to recover the seed.
• If the random-alphanumeric command is not able to crack a string that you're confident comes from RandomStringUtils, try reversing the string as some older versions of RandomStringUtils build the string in reverse order.