[/MNT/AIN LIB] [REV] infinite_decoding_odyssey

infinite_decoding_odyssey
SRC: https://library.m0unt41n.ch/challenges/infinite-decoding-odyssey
This rev chall is pretty straight forward, with have a encryption or lets say a lil scrambling function which we should reverse. We have a scramble flag:
V1QWz9Xq8B_q11B9Q0sBQB}s7@B143WB9Y2UB9pB57q39CA

And a charset:
abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789{}_!@#$%^&*()-+=

The routine custom rot, implements a rot like cipher with a lil twist, it adds a mod too the rotated index, indicated by the % within the pseudo code gnerated by bninja. To be precise mod 0x4f -> so mod 79 is statically used everytime, which is the length of the alphabet.

And the rotation amount used is also statically set, but within the function call as argument -> 0x25 so 37.

Final cipher too reverse the scrambled flag: out = charset[(index(c) + R) mod L]

putchar((int32_t)custom_rot(scrambled_flag[(int64_t)i], 0x25));

00001169    uint64_t custom_rot(char arg1, int32_t arg2)

00001169    {
00001169        int32_t var_c = 0;
00001169       
000011c1        while (true)
000011c1        {
000011c1            if (var_c >= 0x4f)
000011c3                return (uint64_t)arg1;
000011c3           
00001198            if (arg1 == charset[(int64_t)var_c])
00001198                break;
00001198           
000011b7            var_c += 1;
000011c1        }
000011c1       
000011b1        return (uint64_t)charset[(int64_t)((int64_t)(arg2 + var_c) % 0x4f)];
00001169    }

The reversed cipher in py would be:

charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789{}_!@#$%^&*()-+="
rot = 37
scr  = "V1QWz9Xq8B_q11B9Q0sBQB}s7@B143WB9Y2UB9pB57q39CA"

flag = "".join(charset[(charset.index(c) + rot) % 79]for c in scr)
print(flag)

Resulting into: flag{th1s_w1ll_tak3_a_v3ry_long_time_t0_pr1nt!}

We could also just patch the sleep, but ye, we aint patchers we heavy thinkers.