Essentially all internet security is based on the fundamental idea that if you have some really big randomly selected prime number, you can do math on it easily to get a final value, but doing the math backwards to find the original prime number is computationally infeasible. Your prime number is secret, the calculated final value is public. You can sign data with your prime number private key and someone can use the corresponding public key to verify that the data was signed with the corresponding private key.

It's really important that your private key stays private. It's really important that your private key can't be brute forced by trying a bunch of numbers to guess your secret prime number.

Your private key should be using a number like 11185240471355026063328096486023294694077671402173515074581823211722702563586240744645728206308310973537336052780084.

It would be really bad if your private key was something like 4. It would be REALLY BAD if that private key was used to sign a bunch of data used for internet security.

https://scotthelme.co.uk/eight-bits-of-trust-emudhras-ca-keys-cracked-in-milliseconds/

P.S. I don't actually know if all algorithms use only prime numbers or if some of them can use non-primes.