keyfork/keyfork-derive-util/src/public_key.rs

use crate::private_key::PrivateKeyBytes;

use digest::Digest;
use ripemd::Ripemd160;
use sha2::Sha256;
use thiserror::Error;

pub(crate) type PublicKeyBytes = [u8; 33];

/// Functions required to use an `ExtendedPublicKey`.
pub trait PublicKey: Sized {
    /// The error returned by [`PublicKey::derive_child()`].
    type Err: std::error::Error;

    /// Create a Self from bytes.
    fn from_bytes(b: &PublicKeyBytes) -> Self;

    /// Convert a &Self to bytse.
    fn to_bytes(&self) -> PublicKeyBytes;

    /// Derive a child [`PublicKey`] with given `PrivateKeyBytes`.
    ///
    /// # Errors
    ///
    /// An error may be returned if:
    /// * A nonzero `other` is provided.
    /// * An error specific to the given algorithm was encountered.
    fn derive_child(&self, other: PrivateKeyBytes) -> Result<Self, Self::Err>;

    /// Create a BIP-0032/SLIP-0010 fingerprint from the public key.
    fn fingerprint(&self) -> [u8; 4] {
        let hash = Sha256::new().chain_update(self.to_bytes()).finalize();
        let hash = Ripemd160::new().chain_update(hash).finalize();
        // Note: Safety assured by type returned from Ripemd160
        hash[..4].try_into().unwrap()
    }
}

/// Errors associated with creating and arithmetic on public keys. This specific error is only
/// intended to be used by the implementations in this crate.
#[derive(Clone, Debug, Error)]
pub enum PublicKeyError {
    /// For the given algorithm, the private key must be nonzero.
    #[error("The provided public key must be nonzero, but is not")]
    NonZero,

    /// Unable to convert a point to a key.
    #[error("Unable to convert point to key")]
    PointToKey(#[from] k256::elliptic_curve::Error),
}

#[cfg(feature = "secp256k1")]
use k256::{
    elliptic_curve::{group::prime::PrimeCurveAffine, sec1::ToEncodedPoint},
    AffinePoint, NonZeroScalar,
};

#[cfg(feature = "secp256k1")]
impl PublicKey for k256::PublicKey {
    type Err = PublicKeyError;

    fn from_bytes(b: &PublicKeyBytes) -> Self {
        Self::from_sec1_bytes(b).expect("Invalid public key bytes")
    }

    fn to_bytes(&self) -> PublicKeyBytes {
        // Note: Safety assured by type returned from EncodedPoint
        self.to_encoded_point(true).as_bytes().try_into().unwrap()
    }

    fn derive_child(&self, other: PrivateKeyBytes) -> Result<Self, Self::Err> {
        if other.iter().all(|n| n == &0) {
            return Err(PublicKeyError::NonZero);
        }
        // Checked: See above
        let scalar = Option::<NonZeroScalar>::from(NonZeroScalar::from_repr(other.into()))
            .expect("Should have been able to get a NonZeroScalar");

        let point = self.to_projective() + (AffinePoint::generator() * *scalar);
        Self::from_affine(point.into()).map_err(From::from)
    }
}