Defeat 2FA token because of bad randomness - rhme2 Twistword (Misc 400)
1,450Generating random numbers on computers is not easy. And while the intended solution was really hard, the challenge had a problem with the random number generation, which allowed me to solve it.
Clarification from Andres Moreno (riscure) on the challenge:
"The "official" challenge solution involved reading the tiny Mersenne twister (tinyMT) paper, writing some equations, and using a solver. The tinyMT is tricky to initialize. Giving a proper seed is not enough. You need to provide initial state matrices with certain properties (there is a generator for this). The challenge used improper initialized matrices (zeros) that reduced the PRNG period. During tests, we found that ~12hr were needed to solve the challenge (solver time only), but we did not test the amount of entropy reduction by improper state initialization. Fortunately, the problem was not in the PRNG."
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