ecdh.rs

// Bitcoin secp256k1 bindings
// Written in 2015 by
//   Andrew Poelstra
//
// To the extent possible under law, the author(s) have dedicated all
// copyright and related and neighboring rights to this software to
// the public domain worldwide. This software is distributed without
// any warranty.
//
// You should have received a copy of the CC0 Public Domain Dedication
// along with this software.
// If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
//

//! Support for shared secret computations.
//!

use core::ptr;
use core::borrow::Borrow;

use key::{SecretKey, PublicKey};
use ffi::{self, CPtr};
use secp256k1_sys::types::{c_int, c_uchar, c_void};
use constants;

// The logic for displaying shared secrets relies on this (see `secret.rs`).
const SHARED_SECRET_SIZE: usize = constants::SECRET_KEY_SIZE;

/// Enables two parties to create a shared secret without revealing their own secrets.
///
/// # Examples
///
/// ```
/// # #[cfg(all(feature = "std", feature = "rand-std"))] {
/// # use secp256k1::Secp256k1;
/// # use secp256k1::ecdh::SharedSecret;
/// # use secp256k1::rand::thread_rng;
/// let s = Secp256k1::new();
/// let (sk1, pk1) = s.generate_keypair(&mut thread_rng());
/// let (sk2, pk2) = s.generate_keypair(&mut thread_rng());
/// let sec1 = SharedSecret::new(&pk2, &sk1);
/// let sec2 = SharedSecret::new(&pk1, &sk2);
/// assert_eq!(sec1, sec2);
/// # }
// ```
#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct SharedSecret([u8; SHARED_SECRET_SIZE]);
impl_display_secret!(SharedSecret);

impl SharedSecret {
    /// Creates a new shared secret from a pubkey and secret key.
    #[inline]
    pub fn new(point: &PublicKey, scalar: &SecretKey) -> SharedSecret {
        let mut buf = [0u8; SHARED_SECRET_SIZE];
        let res = unsafe {
             ffi::secp256k1_ecdh(
                ffi::secp256k1_context_no_precomp,
                buf.as_mut_ptr(),
                point.as_c_ptr(),
                scalar.as_c_ptr(),
                ffi::secp256k1_ecdh_hash_function_default,
                ptr::null_mut(),
            )
        };
        debug_assert_eq!(res, 1);
        SharedSecret(buf)
    }

    /// Returns the shared secret as a byte value.
    #[inline]
    pub fn secret_bytes(&self) -> [u8; SHARED_SECRET_SIZE] {
        self.0
    }
}

impl Borrow<[u8]> for SharedSecret {
    fn borrow(&self) -> &[u8] {
        &self.0
    }
}

impl AsRef<[u8]> for SharedSecret {
    fn as_ref(&self) -> &[u8] {
        &self.0
    }
}

/// Creates a shared point from public key and secret key.
///
/// **Important: use of a strong cryptographic hash function may be critical to security! Do NOT use
/// unless you understand cryptographical implications.** If not, use SharedSecret instead.
///
/// Can be used like `SharedSecret` but caller is responsible for then hashing the returned buffer.
/// This allows for the use of a custom hash function since `SharedSecret` uses SHA256.
///
/// # Returns
///
/// 64 bytes representing the (x,y) co-ordinates of a point on the curve (32 bytes each).
///
/// # Examples
/// ```
/// # #[cfg(all(feature = "bitcoin_hashes", feature = "rand-std", feature = "std"))] {
/// # use secp256k1::{ecdh, Secp256k1, PublicKey, SecretKey};
/// # use secp256k1::hashes::{Hash, sha512};
/// # use secp256k1::rand::thread_rng;
///
/// let s = Secp256k1::new();
/// let (sk1, pk1) = s.generate_keypair(&mut thread_rng());
/// let (sk2, pk2) = s.generate_keypair(&mut thread_rng());
///
/// let point1 = ecdh::shared_secret_point(&pk2, &sk1);
/// let secret1 = sha512::Hash::hash(&point1);
/// let point2 = ecdh::shared_secret_point(&pk1, &sk2);
/// let secret2 = sha512::Hash::hash(&point2);
/// assert_eq!(secret1, secret2)
/// # }
/// ```
pub fn shared_secret_point(point: &PublicKey, scalar: &SecretKey) -> [u8; 64] {
    let mut xy = [0u8; 64];

    let res = unsafe {
        ffi::secp256k1_ecdh(
            ffi::secp256k1_context_no_precomp,
            xy.as_mut_ptr(),
            point.as_ptr(),
            scalar.as_ptr(),
            Some(c_callback),
            ptr::null_mut(),
        )
    };
    // Our callback *always* returns 1.
    // The scalar was verified to be valid (0 > scalar > group_order) via the type system.
    debug_assert_eq!(res, 1);
    xy
}

unsafe extern "C" fn c_callback(output: *mut c_uchar, x: *const c_uchar, y: *const c_uchar, _data: *mut c_void) -> c_int {
    ptr::copy_nonoverlapping(x, output, 32);
    ptr::copy_nonoverlapping(y, output.offset(32), 32);
    1
}

#[cfg(test)]
#[allow(unused_imports)]
mod tests {
    use super::*;
    use rand::thread_rng;
    use super::super::Secp256k1;

    #[cfg(target_arch = "wasm32")]
    use wasm_bindgen_test::wasm_bindgen_test as test;

    #[test]
    #[cfg(all(feature="rand-std", any(feature = "alloc", feature = "std")))]
    fn ecdh() {
        let s = Secp256k1::signing_only();
        let (sk1, pk1) = s.generate_keypair(&mut thread_rng());
        let (sk2, pk2) = s.generate_keypair(&mut thread_rng());

        let sec1 = SharedSecret::new(&pk2, &sk1);
        let sec2 = SharedSecret::new(&pk1, &sk2);
        let sec_odd = SharedSecret::new(&pk1, &sk1);
        assert_eq!(sec1, sec2);
        assert!(sec_odd != sec2);
    }

    #[test]
    fn test_c_callback() {
        let x = [5u8; 32];
        let y = [7u8; 32];
        let mut output = [0u8; 64];
        let res = unsafe { super::c_callback(output.as_mut_ptr(), x.as_ptr(), y.as_ptr(), ptr::null_mut()) };
        assert_eq!(res, 1);
        let mut new_x = [0u8; 32];
        let mut new_y = [0u8; 32];
        new_x.copy_from_slice(&output[..32]);
        new_y.copy_from_slice(&output[32..]);
        assert_eq!(x, new_x);
        assert_eq!(y, new_y);
    }

    #[test]
    #[cfg(not(fuzzing))]
    #[cfg(all(feature="rand-std", feature = "std", feature = "bitcoin_hashes"))]
    fn bitcoin_hashes_and_sys_generate_same_secret() {
        use hashes::{sha256, Hash, HashEngine};

        let s = Secp256k1::signing_only();
        let (sk1, _) = s.generate_keypair(&mut thread_rng());
        let (_, pk2) = s.generate_keypair(&mut thread_rng());

        let secret_sys = SharedSecret::new(&pk2, &sk1);

        let xy = shared_secret_point(&pk2, &sk1);

        // Mimics logic in `bitcoin-core/secp256k1/src/module/main_impl.h`
        let version = (xy[63] & 0x01) | 0x02;
        let mut engine = sha256::HashEngine::default();
        engine.input(&[version]);
        engine.input(&xy.as_ref()[..32]);
        let secret_bh = sha256::Hash::from_engine(engine);

        assert_eq!(secret_bh.as_inner(), secret_sys.as_ref());
    }
}

#[cfg(all(test, feature = "unstable"))]
mod benches {
    use rand::thread_rng;
    use test::{Bencher, black_box};

    use super::SharedSecret;
    use super::super::Secp256k1;

    #[bench]
    pub fn bench_ecdh(bh: &mut Bencher) {
        let s = Secp256k1::signing_only();
        let (sk, pk) = s.generate_keypair(&mut thread_rng());

        bh.iter( || {
            let res = SharedSecret::new(&pk, &sk);
            black_box(res);
        });
    }
}