Although 16 or 20 characters is too less, I can't think of an efficient and user-friendly way to allow arbitrary lengths. What if a user inputs a password with

Although 16 or 20 characters is too less, I can't think of an efficient and user-friendly way to allow arbitrary lengths. What if a user inputs a password with 604462909807314587353087 characters? If your client is doing the hashing, it will get stuck or seem to take too much time to submit. If your backend is doing the hashing then you would need to send too much data over the internet, again killing the user experience. I think 256 or 512 characters should be a sensible limit. PS: 604462909807314587353087 is a Mersenne prime ;)