# How can public key not reverse encryption if the algorithm is known? [duplicate]

If I want to send a message from cell $A$ to cell $B$, since there's a public key, I will pass my message through this "algorithm" and send to the cell $B$.

My question is, knowing the public key doesn't automatically gives you power to reverse engineer the message sent? How is that possible that knowing the algorithm that created a code and the code itself doesn't get you the original information?