pub const BlindKeyPair = struct {
    blind_public_key: BlindPublicKey,
    blind_secret_key: BlindSecretKey,

    /// Create an blind key pair from an existing key pair, a blinding seed and a context.
    pub fn init(key_pair: Ed25519.KeyPair, blind_seed: [blind_seed_length]u8, ctx: []const u8) (NonCanonicalError || IdentityElementError)!BlindKeyPair {
        var h: [Sha512.digest_length]u8 = undefined;
        Sha512.hash(&key_pair.secret_key.seed(), &h, .{});
        Curve.scalar.clamp(h[0..32]);
        const scalar = Curve.scalar.reduce(h[0..32].*);

        const blind_h = blindCtx(blind_seed, ctx);
        const blind_factor = Curve.scalar.reduce(blind_h[0..32].*);

        const blind_scalar = Curve.scalar.mul(scalar, blind_factor);
        const blind_public_key = BlindPublicKey{
            .key = try PublicKey.fromBytes((Curve.basePoint.mul(blind_scalar) catch return error.IdentityElement).toBytes()),
        };

        var prefix: [64]u8 = undefined;
        prefix[0..32].* = h[32..64].*;
        prefix[32..64].* = blind_h[32..64].*;

        const blind_secret_key = BlindSecretKey{
            .prefix = prefix,
            .blind_scalar = blind_scalar,
            .blind_public_key = blind_public_key,
        };
        return BlindKeyPair{
            .blind_public_key = blind_public_key,
            .blind_secret_key = blind_secret_key,
        };
    }

    /// Sign a message using a blind key pair, and optional random noise.
    /// Having noise creates non-standard, non-deterministic signatures,
    /// but has been proven to increase resilience against fault attacks.
    pub fn sign(key_pair: BlindKeyPair, msg: []const u8, noise: ?[noise_length]u8) (IdentityElementError || KeyMismatchError || NonCanonicalError || WeakPublicKeyError)!Signature {
        const scalar = key_pair.blind_secret_key.blind_scalar;
        const prefix = key_pair.blind_secret_key.prefix;

        return (try PublicKey.fromBytes(key_pair.blind_public_key.key.bytes))
            .computeNonceAndSign(msg, noise, scalar, &prefix);
    }
}