//! Tests for converting Ed25519 keys into X25519 (Montgomery form) keys.

use curve25519_dalek::scalar::{clamp_integer, Scalar};
use ed25519_dalek::SigningKey;
use hex_literal::hex;
use sha2::{Digest, Sha512};

/// Helper function to return the bytes corresponding to the input bytes after being clamped and
/// reduced mod 2^255 - 19
fn clamp_and_reduce(bytes: &[u8]) -> [u8; 32] {
    assert_eq!(bytes.len(), 32);
    Scalar::from_bytes_mod_order(clamp_integer(bytes.try_into().unwrap())).to_bytes()
}

/// Tests that X25519 Diffie-Hellman works when using keys converted from Ed25519.
// TODO: generate test vectors using another implementation of Ed25519->X25519
#[test]
fn ed25519_to_x25519_dh() {
    // Keys from RFC8032 test vectors (from section 7.1)
    let ed25519_secret_key_a =
        hex!("9d61b19deffd5a60ba844af492ec2cc44449c5697b326919703bac031cae7f60");
    let ed25519_secret_key_b =
        hex!("4ccd089b28ff96da9db6c346ec114e0f5b8a319f35aba624da8cf6ed4fb8a6fb");

    let ed25519_signing_key_a = SigningKey::from_bytes(&ed25519_secret_key_a);
    let ed25519_signing_key_b = SigningKey::from_bytes(&ed25519_secret_key_b);

    let scalar_a = ed25519_signing_key_a.to_scalar();
    let scalar_b = ed25519_signing_key_b.to_scalar();

    // Compare the scalar bytes to the first 32 bytes of SHA-512(secret_key). We have to clamp and
    // reduce the SHA-512 output because that's what the spec does before using the scalars for
    // anything.
    assert_eq!(
        scalar_a.to_bytes(),
        clamp_and_reduce(&Sha512::digest(ed25519_secret_key_a)[..32]),
    );
    assert_eq!(
        scalar_b.to_bytes(),
        clamp_and_reduce(&Sha512::digest(ed25519_secret_key_b)[..32]),
    );

    let x25519_public_key_a = ed25519_signing_key_a.verifying_key().to_montgomery();
    let x25519_public_key_b = ed25519_signing_key_b.verifying_key().to_montgomery();

    assert_eq!(
        x25519_public_key_a.to_bytes(),
        hex!("d85e07ec22b0ad881537c2f44d662d1a143cf830c57aca4305d85c7a90f6b62e")
    );
    assert_eq!(
        x25519_public_key_b.to_bytes(),
        hex!("25c704c594b88afc00a76b69d1ed2b984d7e22550f3ed0802d04fbcd07d38d47")
    );

    let expected_shared_secret =
        hex!("5166f24a6918368e2af831a4affadd97af0ac326bdf143596c045967cc00230e");

    assert_eq!(
        (x25519_public_key_a * scalar_b).to_bytes(),
        expected_shared_secret
    );
    assert_eq!(
        (x25519_public_key_b * scalar_a).to_bytes(),
        expected_shared_secret
    );
}