# Chosen Ciphertext Attack textbook RSA

I’m trying to perform a chosen-ciphertext attack against an RSA oracle.
I have $$c$$ as the ciphertext I want to decrypt, $$e$$ and $$n$$.
I already know that I could choose a number $$r$$, compute $$r^e cdot c$$, make the oracle decrypt, and return $$rcdot m$$.

The problem is that this particular oracle checks if $$m bmod mo = 0$$ where $$m$$ is the decrypted ciphertext I sent and $$mo$$ is the original message which I’m trying to get. If it’s equal to $$0$$ (like $$rn$$) it won’t print $$m$$ so I can’t use that attack.

I don’t really know what to do.

$$n$$ is pretty big (1024bit) so I can’t factorize it. Maybe there is a small adjustment to the mentioned attack, but I’m really stuck. If anybody could give me a hint I would really love it.