sanstun 0.1.0

use crate::PublicKey;
use crate::errors::Error;
use blake2::digest::consts::U16;
use blake2::digest::{FixedOutput, KeyInit};
use blake2::{Blake2s256, Blake2sMac, Digest};
use ring::aead::{Aad, CHACHA20_POLY1305, LessSafeKey, Nonce, UnboundKey};
use x25519_dalek::SharedSecret;
use zeroize::Zeroizing;

type HmacBlake2s = hmac::SimpleHmac<Blake2s256>;

#[inline]
pub(crate) fn b2s_hash(data1: &[u8], data2: &[u8]) -> [u8; 32] {
    let mut hash = Blake2s256::new();
    hash.update(data1);
    hash.update(data2);
    hash.finalize().into()
}

#[inline]
/// Computes HMAC-BLAKE2s, which differs from keyed BLAKE2s.
pub(crate) fn b2s_hmac(key: &[u8; 32], data1: &[u8]) -> Zeroizing<[u8; 32]> {
    use blake2::digest::Update;
    let mut hmac = HmacBlake2s::new_from_slice(key).unwrap();
    hmac.update(data1);
    let mut out = Zeroizing::new([0; 32]);
    hmac.finalize_into((&mut *out).into());
    out
}

#[inline]
/// Computes HMAC-BLAKE2s over `data1` followed by `data2`.
pub(crate) fn b2s_hmac2(key: &[u8; 32], data1: &[u8; 32], data2: &[u8]) -> Zeroizing<[u8; 32]> {
    use blake2::digest::Update;
    let mut hmac = HmacBlake2s::new_from_slice(key).unwrap();
    hmac.update(data1);
    hmac.update(data2);
    let mut out = Zeroizing::new([0; 32]);
    hmac.finalize_into((&mut *out).into());
    out
}

#[inline]
pub(crate) fn b2s_keyed_mac_16(key: &[u8], data1: &[u8]) -> [u8; 16] {
    let mut hmac = Blake2sMac::<U16>::new_from_slice(key).unwrap();
    blake2::digest::Update::update(&mut hmac, data1);
    hmac.finalize_fixed().into()
}

#[inline]
pub(crate) fn b2s_keyed_mac_16_2(key: &[u8], data1: &[u8], data2: &[u8]) -> [u8; 16] {
    let mut hmac = Blake2sMac::<U16>::new_from_slice(key).unwrap();
    blake2::digest::Update::update(&mut hmac, data1);
    blake2::digest::Update::update(&mut hmac, data2);
    hmac.finalize_fixed().into()
}

#[inline]
/// Encrypts `data` into `ciphertext` and appends the authentication tag.
pub(crate) fn aead_chacha20_seal(
    ciphertext: &mut [u8],
    key: &[u8; 32],
    counter: u64,
    data: &[u8],
    aad: &[u8],
) {
    aead_chacha20_seal_inner(ciphertext, key, nonce(counter), data, aad)
}

#[inline]
fn aead_chacha20_seal_inner(
    ciphertext: &mut [u8],
    key: &[u8; 32],
    nonce: [u8; 12],
    data: &[u8],
    aad: &[u8],
) {
    let key = LessSafeKey::new(UnboundKey::new(&CHACHA20_POLY1305, key).unwrap());

    ciphertext[..data.len()].copy_from_slice(data);

    let tag = key
        .seal_in_place_separate_tag(
            Nonce::assume_unique_for_key(nonce),
            Aad::from(aad),
            &mut ciphertext[..data.len()],
        )
        .unwrap();

    ciphertext[data.len()..].copy_from_slice(tag.as_ref());
}

#[inline]
/// Authenticates and decrypts `data` into `buffer`.
pub(crate) fn aead_chacha20_open(
    buffer: &mut [u8],
    key: &[u8; 32],
    counter: u64,
    data: &[u8],
    aad: &[u8],
) -> Result<(), Error> {
    aead_chacha20_open_inner(buffer, key, nonce(counter), data, aad)
        .map_err(|_| Error::InvalidAeadTag)?;
    Ok(())
}

#[inline]
fn aead_chacha20_open_inner(
    buffer: &mut [u8],
    key: &[u8; 32],
    nonce: [u8; 12],
    data: &[u8],
    aad: &[u8],
) -> Result<(), ring::error::Unspecified> {
    let key = LessSafeKey::new(UnboundKey::new(&CHACHA20_POLY1305, key).unwrap());

    let mut inner_buffer = data.to_owned();

    let plaintext = key.open_in_place(
        Nonce::assume_unique_for_key(nonce),
        Aad::from(aad),
        &mut inner_buffer,
    )?;

    buffer.copy_from_slice(plaintext);

    Ok(())
}

/// Encrypts `data` with XChaCha20-Poly1305 and appends the 16-byte authentication tag.
pub(crate) fn xaead_seal(
    ciphertext: &mut [u8],
    key: &[u8; 32],
    nonce: &[u8; 24],
    data: &[u8],
    aad: &[u8],
) {
    let (subkey, nonce) = xchacha20_subkey_and_nonce(key, nonce);

    aead_chacha20_seal_inner(ciphertext, &subkey, nonce, data, aad)
}

/// Authenticates and decrypts XChaCha20-Poly1305 `data` into `buffer`.
pub(crate) fn xaead_open(
    buffer: &mut [u8],
    key: &[u8; 32],
    nonce: &[u8; 24],
    data: &[u8],
    aad: &[u8],
) -> Result<(), Error> {
    let (subkey, nonce) = xchacha20_subkey_and_nonce(key, nonce);

    aead_chacha20_open_inner(buffer, &subkey, nonce, data, aad)
        .map_err(|_| Error::InvalidAeadTag)?;
    Ok(())
}

/// Derives the ChaCha20-Poly1305 key and nonce for XChaCha20-Poly1305.
fn xchacha20_subkey_and_nonce(key: &[u8; 32], nonce: &[u8; 24]) -> (Zeroizing<[u8; 32]>, [u8; 12]) {
    let (hchacha20_nonce, tail) = nonce.split_first_chunk::<16>().unwrap();

    let mut chacha20_nonce = [0u8; 12];
    chacha20_nonce[4..].copy_from_slice(tail);

    let subkey = chacha20::hchacha::<chacha20::R20>(key.into(), hchacha20_nonce.into());

    (Zeroizing::new(subkey.into()), chacha20_nonce)
}

/// Encodes `counter` as a ChaCha20-Poly1305 nonce: 32 zero bits followed by a little-endian counter.
pub(crate) fn nonce(counter: u64) -> [u8; 12] {
    let mut nonce = [0u8; 12];
    nonce[4..].copy_from_slice(&counter.to_le_bytes());
    nonce
}

/// Derives the X25519 shared secret for `private` and `public`.
///
/// Rejects low-order public keys that produce an all-zero shared secret.
pub(crate) fn dh(private: &impl x25519::Secret, public: &PublicKey) -> Result<SharedKey, Error> {
    let shared = private.diffie_hellman(&public.to_dalek());

    if !shared.was_contributory() {
        return Err(Error::LowOrderPoint);
    }

    Ok(SharedKey(shared))
}

/// An X25519 shared secret validated by [`dh`].
pub(crate) struct SharedKey(SharedSecret);

impl SharedKey {
    pub(crate) fn as_bytes(&self) -> &[u8; 32] {
        self.0.as_bytes()
    }
}

mod x25519 {
    use crate::StaticSecret;
    use x25519_dalek::{PublicKey, ReusableSecret, SharedSecret};

    pub trait Secret {
        fn diffie_hellman(&self, public: &PublicKey) -> SharedSecret;
    }

    impl Secret for StaticSecret {
        #[expect(
            clippy::disallowed_methods,
            reason = "Called through the validating `dh` wrapper."
        )]
        fn diffie_hellman(&self, public: &PublicKey) -> SharedSecret {
            self.as_dalek().diffie_hellman(public)
        }
    }

    impl Secret for ReusableSecret {
        #[expect(
            clippy::disallowed_methods,
            reason = "Called through the validating `dh` wrapper."
        )]
        fn diffie_hellman(&self, public: &PublicKey) -> SharedSecret {
            ReusableSecret::diffie_hellman(self, public)
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::StaticSecret;

    #[test]
    fn dh_rejects_low_order_points() {
        let private = StaticSecret::from(Zeroizing::new([1; 32]));
        let p_minus_1 = {
            let mut p = [0xff; 32];
            p[0] = 0xec;
            p[31] = 0x7f;
            p
        };
        let low_order_points = [
            [0; 32],
            {
                let mut one = [0; 32];
                one[0] = 1;
                one
            },
            p_minus_1,
            [
                0xe0, 0xeb, 0x7a, 0x7c, 0x3b, 0x41, 0xb8, 0xae, 0x16, 0x56, 0xe3, 0xfa, 0xf1, 0x9f,
                0xc4, 0x6a, 0xda, 0x09, 0x8d, 0xeb, 0x9c, 0x32, 0xb1, 0xfd, 0x86, 0x62, 0x05, 0x16,
                0x5f, 0x49, 0xb8, 0x00,
            ],
            [
                0x5f, 0x9c, 0x95, 0xbc, 0xa3, 0x50, 0x8c, 0x24, 0xb1, 0xd0, 0xb1, 0x55, 0x9c, 0x83,
                0xef, 0x5b, 0x04, 0x44, 0x5c, 0xc4, 0x58, 0x1c, 0x8e, 0x86, 0xd8, 0x22, 0x4e, 0xdd,
                0xd0, 0x9f, 0x11, 0x57,
            ],
        ];

        for point in low_order_points {
            let shared = dh(&private, &PublicKey::from(point));

            assert!(matches!(shared, Err(Error::LowOrderPoint)), "{point:x?}");
        }
    }

    #[test]
    fn dh_agrees_on_valid_keys() {
        let a = StaticSecret::from(Zeroizing::new([1; 32]));
        let b = StaticSecret::from(Zeroizing::new([2; 32]));

        let ab = dh(&a, &PublicKey::from(&b)).unwrap();
        let ba = dh(&b, &PublicKey::from(&a)).unwrap();

        assert_eq!(ab.as_bytes(), ba.as_bytes());
    }

    #[test]
    fn chacha20_seal_rfc7530_test_vector() {
        let plaintext = b"Ladies and Gentlemen of the class of '99: If I could offer you only one tip for the future, sunscreen would be it.";
        let aad: [u8; 12] = [
            0x50, 0x51, 0x52, 0x53, 0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7,
        ];
        let key: [u8; 32] = [
            0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d,
            0x8e, 0x8f, 0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9a, 0x9b,
            0x9c, 0x9d, 0x9e, 0x9f,
        ];
        let nonce: [u8; 12] = [
            0x07, 0x00, 0x00, 0x00, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47,
        ];
        let mut buffer = vec![0; plaintext.len() + 16];

        aead_chacha20_seal_inner(&mut buffer, &key, nonce, plaintext, &aad);

        const EXPECTED_CIPHERTEXT: [u8; 114] = [
            0xd3, 0x1a, 0x8d, 0x34, 0x64, 0x8e, 0x60, 0xdb, 0x7b, 0x86, 0xaf, 0xbc, 0x53, 0xef,
            0x7e, 0xc2, 0xa4, 0xad, 0xed, 0x51, 0x29, 0x6e, 0x08, 0xfe, 0xa9, 0xe2, 0xb5, 0xa7,
            0x36, 0xee, 0x62, 0xd6, 0x3d, 0xbe, 0xa4, 0x5e, 0x8c, 0xa9, 0x67, 0x12, 0x82, 0xfa,
            0xfb, 0x69, 0xda, 0x92, 0x72, 0x8b, 0x1a, 0x71, 0xde, 0x0a, 0x9e, 0x06, 0x0b, 0x29,
            0x05, 0xd6, 0xa5, 0xb6, 0x7e, 0xcd, 0x3b, 0x36, 0x92, 0xdd, 0xbd, 0x7f, 0x2d, 0x77,
            0x8b, 0x8c, 0x98, 0x03, 0xae, 0xe3, 0x28, 0x09, 0x1b, 0x58, 0xfa, 0xb3, 0x24, 0xe4,
            0xfa, 0xd6, 0x75, 0x94, 0x55, 0x85, 0x80, 0x8b, 0x48, 0x31, 0xd7, 0xbc, 0x3f, 0xf4,
            0xde, 0xf0, 0x8e, 0x4b, 0x7a, 0x9d, 0xe5, 0x76, 0xd2, 0x65, 0x86, 0xce, 0xc6, 0x4b,
            0x61, 0x16,
        ];
        const EXPECTED_TAG: [u8; 16] = [
            0x1a, 0xe1, 0x0b, 0x59, 0x4f, 0x09, 0xe2, 0x6a, 0x7e, 0x90, 0x2e, 0xcb, 0xd0, 0x60,
            0x06, 0x91,
        ];

        assert_eq!(buffer[..plaintext.len()], EXPECTED_CIPHERTEXT);
        assert_eq!(buffer[plaintext.len()..], EXPECTED_TAG);
    }

    #[test]
    fn symmetric_chacha20_seal_open() {
        let aad: [u8; 32] = Default::default();
        let key: [u8; 32] = Default::default();
        let counter = 0;

        let mut encrypted_nothing: [u8; 16] = Default::default();

        aead_chacha20_seal(&mut encrypted_nothing, &key, counter, &[], &aad);

        eprintln!("encrypted_nothing: {encrypted_nothing:?}");

        aead_chacha20_open(&mut [], &key, counter, &encrypted_nothing, &aad)
            .expect("freshly encrypted data must decrypt");
    }

    /// <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-xchacha-03#appendix-A.1>
    #[test]
    fn xaead_test_vector() {
        let plaintext = b"Ladies and Gentlemen of the class of '99: If I could offer you only one tip for the future, sunscreen would be it.";
        let aad = [
            0x50, 0x51, 0x52, 0x53, 0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7,
        ];
        let key = std::array::from_fn(|i| 0x80 + i as u8);
        let nonce = std::array::from_fn(|i| 0x40 + i as u8);
        let mut sealed = vec![0; plaintext.len() + 16];

        xaead_seal(&mut sealed, &key, &nonce, plaintext, &aad);

        const EXPECTED_CIPHERTEXT: [u8; 114] = [
            0xbd, 0x6d, 0x17, 0x9d, 0x3e, 0x83, 0xd4, 0x3b, 0x95, 0x76, 0x57, 0x94, 0x93, 0xc0,
            0xe9, 0x39, 0x57, 0x2a, 0x17, 0x00, 0x25, 0x2b, 0xfa, 0xcc, 0xbe, 0xd2, 0x90, 0x2c,
            0x21, 0x39, 0x6c, 0xbb, 0x73, 0x1c, 0x7f, 0x1b, 0x0b, 0x4a, 0xa6, 0x44, 0x0b, 0xf3,
            0xa8, 0x2f, 0x4e, 0xda, 0x7e, 0x39, 0xae, 0x64, 0xc6, 0x70, 0x8c, 0x54, 0xc2, 0x16,
            0xcb, 0x96, 0xb7, 0x2e, 0x12, 0x13, 0xb4, 0x52, 0x2f, 0x8c, 0x9b, 0xa4, 0x0d, 0xb5,
            0xd9, 0x45, 0xb1, 0x1b, 0x69, 0xb9, 0x82, 0xc1, 0xbb, 0x9e, 0x3f, 0x3f, 0xac, 0x2b,
            0xc3, 0x69, 0x48, 0x8f, 0x76, 0xb2, 0x38, 0x35, 0x65, 0xd3, 0xff, 0xf9, 0x21, 0xf9,
            0x66, 0x4c, 0x97, 0x63, 0x7d, 0xa9, 0x76, 0x88, 0x12, 0xf6, 0x15, 0xc6, 0x8b, 0x13,
            0xb5, 0x2e,
        ];
        const EXPECTED_TAG: [u8; 16] = [
            0xc0, 0x87, 0x59, 0x24, 0xc1, 0xc7, 0x98, 0x79, 0x47, 0xde, 0xaf, 0xd8, 0x78, 0x0a,
            0xcf, 0x49,
        ];

        assert_eq!(sealed[..plaintext.len()], EXPECTED_CIPHERTEXT);
        assert_eq!(sealed[plaintext.len()..], EXPECTED_TAG);

        let mut opened = vec![0; plaintext.len()];
        xaead_open(&mut opened, &key, &nonce, &sealed, &aad).unwrap();

        assert_eq!(opened, plaintext);
    }
}