A problem to exploit the input guard using math.
The problem statement is:
$ nc rivit.dev 10011
X * 212103456793011 = 183057226632645
X = ?
It seems we want to compute the number of x:
$$ x = \frac{183057226632645}{212103456793011}$$
If you compute it, it should be something like 0.8630563094088945586. However, it won’t give you the flag because x must be an integer. That said we need to overflow.
$$ 212103456793011x = 183057226632645 \mod{2^{64}} $$
or,
$$ 212103456793011x - 2^{64}q = 183057226632645$$
The solution to x is:
$$ x = \frac{183057226632645 + 2^{64} q}{212103456793011} $$
and it’s integer solutions:
$$ x = 18446744073709551616 n + 9585860797856392871, n \in \mathbb{Z} $$
which has the smallest solution:
$$ x = 9585860797856392871 $$
Takeaways
Even though it is categorized as programming, I’m sure it’s more like math problem. The last time I used modulo arithmetic in practice was analysis of RSA and that experience helped me to solve this a lot.