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<article class="markdown-body entry-content container-lg" itemprop="text"><h1><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path fill-rule="evenodd" d="M7.775 3.275a.75.75 0 001.06 1.06l1.25-1.25a2 2 0 112.83 2.83l-2.5 2.5a2 2 0 01-2.83 0 .75.75 0 00-1.06 1.06 3.5 3.5 0 004.95 0l2.5-2.5a3.5 3.5 0 00-4.95-4.95l-1.25 1.25zm-4.69 9.64a2 2 0 010-2.83l2.5-2.5a2 2 0 012.83 0 .75.75 0 001.06-1.06 3.5 3.5 0 00-4.95 0l-2.5 2.5a3.5 3.5 0 004.95 4.95l1.25-1.25a.75.75 0 00-1.06-1.06l-1.25 1.25a2 2 0 01-2.83 0z"></path></svg>Crypto Warm up</h1>

We are given the encryption code and output.txt.

The script concatenates random string to the flag and then perform encryption algorithm using random prime number with nbit bits. However, we can get the generated random prime number (p) by looking at the output string length which is 19489.

We are not given the source code of the function is_valid, but we can guess that it is about primitive roots. So, using the primitive roots calculator, we can get all possible values of s.

Now, that we have p value and list of possible s values, we can bruteforce our way to the flag. (using solution.py)

ASIS{_how_d3CrYpt_Th1S_h0m3_m4dE_anD_wEird_CrYp70_5yST3M?!!!!!!}

~ Solution by Dusan and me.

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Crypto Warm up

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