Limit SharedSecret to 32 byte buffer

The `SharedSecret` uses sha256 to hash the secret, this implies the
secret is 32 bytes of data.

Currently we use a buffer of 256 bytes, this is unnecessary.

Change the implementation of `SharedSecret` to use a 32 byte buffer.

src/ecdh.rs

@@ -16,7 +16,7 @@
 //!
 
 use core::ptr;
-use core::ops::Deref;
+use core::borrow::Borrow;
 
 use key::{SecretKey, PublicKey};
 use ffi::{self, CPtr};
@@ -39,106 +39,46 @@ use secp256k1_sys::types::{c_int, c_uchar, c_void};
 /// 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
-}
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct SharedSecret([u8; 32]);
 
 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 mut buf = [0u8; 32];
         let res = unsafe {
              ffi::secp256k1_ecdh(
                 ffi::secp256k1_context_no_precomp,
-                ss.get_data_mut_ptr(),
+                buf.as_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
+        SharedSecret(buf)
+    }
+}
+
+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.
 ///
@@ -183,6 +123,12 @@ pub fn shared_secret_point(point: &PublicKey, scalar: &SecretKey) -> [u8; 64] {
     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 {
@@ -221,6 +167,28 @@ mod tests {
         assert_eq!(x, new_x);
         assert_eq!(y, new_y);
     }
+
+    #[test]
+    #[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 point = shared_secret_point(&pk2, &sk1);
+
+        let version = (point[63] & 0x01) | 0x02;
+        let mut engine = sha256::HashEngine::default();
+        engine.input(&[version]);
+        engine.input(&point.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"))]

src/macros.rs

@@ -26,20 +26,3 @@ macro_rules! impl_pretty_debug {
         }
      }
 }
-
-macro_rules! impl_from_array_len {
-    ($thing:ident, $capacity:tt, ($($N:tt)+)) => {
-        $(
-            impl From<[u8; $N]> for $thing {
-                fn from(arr: [u8; $N]) -> Self {
-                    let mut data = [0u8; $capacity];
-                    data[..$N].copy_from_slice(&arr);
-                    $thing {
-                        data,
-                        len: $N,
-                    }
-                }
-            }
-        )+
-    }
-}