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ecdh.rs
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ecdh.rs
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// 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::ops::Deref;
use key::{SecretKey, PublicKey};
use ffi::{self, CPtr};
use secp256k1_sys::types::{c_int, c_uchar, c_void};
/// 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(&pk1, &sk2);
/// let sec2 = SharedSecret::new(&pk2, &sk1);
/// assert_eq!(sec1, sec2);
/// # }
// ```
#[derive(Copy, Clone)]
pub struct SharedSecret {
data: [u8; 256],
len: usize,
}
impl_raw_debug!(SharedSecret);
// This implementes `From<N>` for all `[u8; N]` arrays from 128bits(16 byte) to 2048bits allowing known hash lengths.
// Lower than 128 bits isn't resistant to collisions any more.
impl_from_array_len!(SharedSecret, 256, (16 20 28 32 48 64 96 128 256));
impl SharedSecret {
/// Creates an empty `SharedSecret`.
pub(crate) fn empty() -> SharedSecret {
SharedSecret {
data: [0u8; 256],
len: 0,
}
}
/// Gets a pointer to the underlying data with the specified capacity.
pub(crate) fn get_data_mut_ptr(&mut self) -> *mut u8 {
self.data.as_mut_ptr()
}
/// Gets the capacity of the underlying data buffer.
pub fn capacity(&self) -> usize {
self.data.len()
}
/// Gets the len of the used data.
pub fn len(&self) -> usize {
self.len
}
/// Returns true if the underlying data buffer is empty.
pub fn is_empty(&self) -> bool {
self.data.is_empty()
}
/// Sets the length of the object.
pub(crate) fn set_len(&mut self, len: usize) {
debug_assert!(len <= self.data.len());
self.len = len;
}
}
impl PartialEq for SharedSecret {
fn eq(&self, other: &SharedSecret) -> bool {
self.as_ref() == other.as_ref()
}
}
impl AsRef<[u8]> for SharedSecret {
fn as_ref(&self) -> &[u8] {
&self.data[..self.len]
}
}
impl Deref for SharedSecret {
type Target = [u8];
fn deref(&self) -> &[u8] {
&self.data[..self.len]
}
}
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
}
impl SharedSecret {
/// Creates a new shared secret from a pubkey and secret key.
#[inline]
pub fn new(point: &PublicKey, scalar: &SecretKey) -> SharedSecret {
let mut ss = SharedSecret::empty();
let res = unsafe {
ffi::secp256k1_ecdh(
ffi::secp256k1_context_no_precomp,
ss.get_data_mut_ptr(),
point.as_c_ptr(),
scalar.as_c_ptr(),
ffi::secp256k1_ecdh_hash_function_default,
ptr::null_mut(),
)
};
// The default `secp256k1_ecdh_hash_function_default` should always return 1.
// and the scalar was verified to be valid(0 > scalar > group_order) via the type system
debug_assert_eq!(res, 1);
ss.set_len(32); // The default hash function is SHA256, which is 32 bytes long.
ss
}
}
/// Creates a shared point from public key and secret key.
///
/// 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
}
#[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(&pk1, &sk2);
let sec2 = SharedSecret::new(&pk2, &sk1);
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);
}
}
#[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);
});
}
}