Argon2 is a password-hashing function that summarizes the state of the art
in the design of memory-hard functions and can be used to hash passwords
for credential storage, key derivation, or other applications.

It has a simple design aimed at the highest memory filling rate and
effective use of multiple computing units, while still providing defense
against tradeoff attacks (by exploiting the cache and memory organization
of the recent processors).

Argon2 has three variants: Argon2i, Argon2d, and Argon2id.

* Argon2d is faster and uses data-depending memory access, which makes it
highly resistant against GPU cracking attacks and suitable for applications
with no threats from side-channel timing attacks (eg. cryptocurrencies).
* Argon2i instead uses data-independent memory access, which is preferred for
password hashing and password-based key derivation, but it is slower as it
makes more passes over the memory to protect from tradeoff attacks.
* Argon2id is a hybrid of Argon2i and Argon2d, using a combination of
data-depending and data-independent memory accesses, which gives some of
Argon2i's resistance to side-channel cache timing attacks and much of
Argon2d's resistance to GPU cracking attacks.