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The Argon2 memory-hard hashing function.

Argon2 summarizes the state of the art in the design of memory-hard functions.

It aims at the highest memory filling rate and effective use of multiple computing units, while still providing defense against tradeoff attacks.

It prevents ASICs from having a significant advantage over software implementations.

Start by determining how much memory the function can use. What will be the highest number of threads/processes evaluating the function simultaneously (ideally, no more than 1 per CPU core)? How much physical memory is guaranteed to be available?

Set memlimit to the amount of memory you want to reserve for password hashing.

Then, set opslimit to 3 and measure the time it takes to hash a password.

If this it is way too long for your application, reduce memlimit, but keep opslimit set to 3.

If the function is so fast that you can afford it to be more computationally intensive without any usability issues, increase opslimit.

For online use (e.g. login in on a website), a 1 second computation is likely to be the acceptable maximum.

For interactive use (e.g. a desktop application), a 5 second pause after having entered a password is acceptable if the password doesn't need to be entered more than once per session.

For non-interactive use and infrequent use (e.g. restoring an encrypted backup), an even slower computation can be an option.

This class depends upon the JNR-FFI library being available on the classpath, along with its dependencies. See https://github.com/jnr/jnr-ffi. JNR-FFI can be included using the gradle dependency 'com.github.jnr:jnr-ffi'.