Editorial for DMOJ Capture The Flag '20 B1 - Super Secure Pseudorandom Number Generator

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Submitting an official solution before solving the problem yourself is a bannable offence.

Author: Ninjaclasher

This is a simple integer overflow exploit. Notice that 500\,000\,000 \times x can overflow an integer relatively easily, so we can get enough money to generate the flag. Next, to get the flag, you had to enter the secret code. In this case, the secret code was the value of rand(), which is seeded with srand(). Notice that the seed is the time in seconds, meaning it is very easy to get the equivalent seed locally. Thus, you know the secret key, and can retrieve the flag.


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