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

use crate::PublicKey;

use thiserror::Error;

pub type PrivateKeyBytes = [u8; 32];

/// Functions required to use an `ExtendedPrivateKey`.
pub trait PrivateKey: Sized {
    /// A type implementing [`PublicKey`] associated with Self.
    type PublicKey: PublicKey;

    /// The error returned by [`PrivateKey::derive_child()`].
    type Err: std::error::Error;

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

    /// Convert a &Self to bytes.
    fn to_bytes(&self) -> PrivateKeyBytes;

    /// Whether or not zero is a valid public key (such as with ed25519 keys).
    fn is_zero_valid_public_key() -> bool {
        false
    }

    /// The initial key for BIP-0032 and SLIP-0010 derivation, such as secp256k1's "Bitcoin seed".
    fn key() -> &'static str;

    /// Generate a [`Self::PublicKey`].
    fn public_key(&self) -> Self::PublicKey;

    /// Derive a child [`PrivateKey`] 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>;
}

/// Errors associated with creating and arithmetic on private keys. This specific error is only
/// intended to be used by the implementations in this crate.
#[derive(Clone, Debug, Error)]
pub enum PrivateKeyError {
    /// For the given algorithm, the private key must be nonzero.
    #[error("The provided private 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::NonZeroScalar;

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

    fn key() -> &'static str {
        "Bitcoin seed"
    }

    fn from_bytes(b: &PrivateKeyBytes) -> Self {
        Self::from_slice(b).expect("Invalid private key bytes")
    }

    fn to_bytes(&self) -> PrivateKeyBytes {
        // Note: Safety assured by type returned from EncodedPoint
        self.to_bytes().into()
    }

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

        let derived_scalar = self.to_nonzero_scalar().as_ref() + scalar.as_ref();
        Ok(
            Option::<NonZeroScalar>::from(NonZeroScalar::new(derived_scalar))
                .map(Into::into)
                .expect("Should be able to make Key"),
        )
    }

    fn public_key(&self) -> Self::PublicKey {
        self.public_key()
    }
}