To add to the idea that crypto is hard, it is not just hard in the same way that, say, making a physics engine is hard. It is hard because there is no telltale

To add to the idea that crypto is hard, it is not just hard in the same way that, say, making a physics engine is hard. It is hard because there is no telltale sign you did it wrong. All crypto algorithms, even weak ones output what looks like random numbers that can be deciphered back into the original plaintext. Just by looking at it, there is no way to differentiate between secure and insecure crypto. Contrast to a physics engine, it is hard to get right, but at least, if you did it wrong, it tends to be obvious. Also, like everything security-related, it is adverserial. You may have some of the smartest and most resourceful guys on the planet working to break your thing. It is worse than even critical systems. Aircraft engine control is critical, people may die if it goes wrong, so robustness and correctness are crucial, but at least, pilots won't go out of their way to break it.