v8_rand_buster

Breaks the following pattern in modern V8 javascript engine.

Math.floor(CONST * Math.random())

Credits to Douglas Goddard for the initial work and blog post that helped explain how to solve this problem initially.

Usage

Assuming you've watched the talk and understand the nuances of how this works, the following should serve as a simple explanation that can get you going with this tool.

Getting the seed

cat codes.txt | tac | python3 xs128p.py --multiple <MULTIPLE>

example codes.txt

12345
23451
34512
45123
51234
.
.
.

• codes.txt: an in-order list of codes you leaked from the Math.floor(Math.random() * CONST) invocations.
• tac: reverses the list (watch the talk for more details and how this might go wrong)
• : equivalent to CONST in the Math.floor(Math.random() * CONST) expression.

Getting the next secrets

Note: this gets the next outputs of the internal math.random calls after flooring. this will not get the next JS call to math.random. See the talk for more details

python3 xs128p.py --mutliple <MULTIPLE> --gen <SEEDS>,<NUMBER_OF_OUTPUTS>

• : Same as above ^
• : The seeds generated by the above step
• <NUMBER_OF_OUTPUTS>: the number of floor randoms you'd like generated. This should be greater than the number of codes you originally used to generate the seed if you want unknown values.