Nim implementation of Poseidon2
===============================

Experimental implementation of the [Poseidon 2][1] cryptographic hash function,
specialized to BN254 and t=3. Uses the [constantine][2] library for
cryptographic primitives. Neither completeness nor correctness are guaranteed at
this moment in time.

Installation
------------

Use the [Nimble][3] package manager to add `poseidon2` to an existing
project. Add the following to its .nimble file:

```nim
requires "poseidon2 >= 0.1.0 & < 0.2.0"
```

Usage
-----

Hashing bytes into a field element with the sponge construction:
```nim
import poseidon2

let input = [1'u8, 2'u8, 3'u8] # some bytes that you want to hash
let digest: F = Sponge.digest(input) # a field element
```

Converting a field element into bytes:
```nim
let output: array[32, byte] = digest.toBytes
```

Combining field elements, useful for constructing a binary Merkle tree:
```nim
let left = Sponge.digest([1'u8, 2'u8, 3'u8])
let right = Sponge.digest([4'u8, 5'u8, 6'u8])
let combination = compress(left, right)
```

Compatbility
------------

For Poseidon2, because of a historical accident, there are unfortunately TWO 
different sets of "standard" parameters, which is obviously bad for cross-project compatibility.

The switchover happened in commit #bb476b9ca38198cf5092487283c8b8c5d4317c4e in 
HorizenLab's reference repo. Both versions are safe to use though.

To resolve this issue, since version 0.1.1, we implement both sets (though the default
is the "old" set of round constants, so that we are backward compatible. This may
or may not change in the future.)


Links
-----

[1]: https://eprint.iacr.org/2023/323.pdf
[2]: https://github.com/mratsim/constantine
[3]: https://github.com/nim-lang/nimble