Hello everyone,

I’m working on a blockchain project in Elixir and I’m implementing some of the core cryptography in a Rust NIF using Rustler.

I’ve hit a wall trying to implement a function to calculate a key image (a component for preventing double-spending in a UTXO model).

The Objective

The goal is to create a NIF get_key_image_from_private_key. This function needs to calculate the key image I using the formula I = x * H_p(P), where:

• x is the one-time private key.

• P is the one-time public key (P = x * G).

• H_p is a hash-to-point function that hashes the public key P to a new point on the elliptic curve.

The Problem

To implement H_p, I’m trying to use the hash_from_bytes function, which seems to be provided by the GroupDigest trait in the elliptic-curve / k256 ecosystem.

Despite having the hash2curve feature enabled for k256 in my Cargo.toml, I cannot find the correct way to import and use this trait. This leads to the compilation error:

error[E0599]: no function or associated item named 'hash_from_bytes' found for struct 'ProjectivePoint' in the current scope

What I’ve Tried

I’ve tried multiple import paths, but none have worked:

• use k256::elliptic_curve::group::GroupDigest; (Error: unresolved import 'k256::group')

• use elliptic_curve::group::GroupDigest; (Error: use of unresolved module or unlinked crate 'elliptic_curve')

• use k256::group::GroupDigest; (Error: could not find 'group' in 'k256')

• use group::GroupDigest; (Error: no 'GroupDigest' in the root)