Alternative: Passing custom hash functions to ECDH

secp256k1-sys/src/lib.rs

@@ -72,7 +72,7 @@ pub type EcdhHashFn = unsafe extern "C" fn(
     x: *const c_uchar,
     y: *const c_uchar,
     data: *mut c_void,
-);
+) -> c_int;
 
 /// A Secp256k1 context, containing various precomputed values and such
 /// needed to do elliptic curve computations. If you create one of these
@@ -134,25 +134,6 @@ impl Default for Signature {
     }
 }
 
-/// Library-internal representation of an ECDH shared secret
-#[repr(C)]
-pub struct SharedSecret([c_uchar; 32]);
-impl_array_newtype!(SharedSecret, c_uchar, 32);
-impl_raw_debug!(SharedSecret);
-
-impl SharedSecret {
-    /// Create a new (zeroed) signature usable for the FFI interface
-    pub fn new() -> SharedSecret { SharedSecret([0; 32]) }
-    /// Create a new (uninitialized) signature usable for the FFI interface
-    #[deprecated(since = "0.15.3", note = "Please use the new function instead")]
-    pub unsafe fn blank() -> SharedSecret { SharedSecret::new() }
-}
-
-impl Default for SharedSecret {
-    fn default() -> Self {
-        SharedSecret::new()
-    }
-}
 
 #[cfg(not(feature = "fuzztarget"))]
 extern "C" {
@@ -296,7 +277,7 @@ extern "C" {
     #[cfg_attr(not(feature = "external-symbols"), link_name = "rustsecp256k1_v0_1_0_ecdh")]
     pub fn secp256k1_ecdh(
         cx: *const Context,
-        output: *mut SharedSecret,
+        output: *mut c_uchar,
         pubkey: *const PublicKey,
         privkey: *const c_uchar,
         hashfp: EcdhHashFn,

secp256k1-sys/src/macros.rs

@@ -144,6 +144,7 @@ macro_rules! impl_array_newtype {
     }
 }
 
+#[macro_export]
 macro_rules! impl_raw_debug {
     ($thing:ident) => {
         impl ::core::fmt::Debug for $thing {

src/ecdh.rs

@@ -16,83 +16,149 @@
 //! Support for shared secret computations
 //!
 
-use core::{ops, ptr};
+use core::ptr;
+use core::ops::{FnMut, Deref};
 
 use key::{SecretKey, PublicKey};
 use ffi::{self, CPtr};
+use secp256k1_sys::types::{c_int, c_uchar, c_void};
 
 /// A tag used for recovering the public key from a compact signature
-#[derive(Copy, Clone, PartialEq, Eq, Debug)]
-pub struct SharedSecret(ffi::SharedSecret);
+#[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 a new shared secret from a pubkey and secret key
-    #[inline]
-    pub fn new(point: &PublicKey, scalar: &SecretKey) -> SharedSecret {
-        unsafe {
-            let mut ss = ffi::SharedSecret::new();
-            let res = ffi::secp256k1_ecdh(
-                ffi::secp256k1_context_no_precomp,
-                &mut ss,
-                point.as_c_ptr(),
-                scalar.as_c_ptr(),
-                ffi::secp256k1_ecdh_hash_function_default,
-                ptr::null_mut(),
-            );
-            debug_assert_eq!(res, 1);
-            SharedSecret(ss)
+
+    /// Create an empty SharedSecret
+    pub(crate) fn empty() ->  SharedSecret {
+        SharedSecret {
+            data: [0u8; 256],
+            len: 0,
         }
     }
 
-    /// Obtains a raw pointer suitable for use with FFI functions
-    #[inline]
-    pub fn as_ptr(&self) -> *const ffi::SharedSecret {
-        &self.0 as *const _
+    /// Get 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()
     }
-}
 
-/// Creates a new shared secret from a FFI shared secret
-impl From<ffi::SharedSecret> for SharedSecret {
-    #[inline]
-    fn from(ss: ffi::SharedSecret) -> SharedSecret {
-        SharedSecret(ss)
+    /// Get the capacity of the underlying data buffer.
+    pub fn capacity(&self) -> usize {
+        self.data.len()
     }
-}
 
+    /// Get the len of the used data.
+    pub fn len(&self) -> usize {
+        self.len
+    }
 
-impl ops::Index<usize> for SharedSecret {
-    type Output = u8;
+    /// Set the length of the object.
+    pub(crate) fn set_len(&mut self, len: usize) {
+        self.len = len;
+    }
+}
 
-    #[inline]
-    fn index(&self, index: usize) -> &u8 {
-        &self.0[index]
+impl PartialEq for SharedSecret {
+    fn eq(&self, other: &SharedSecret) -> bool {
+        self.as_ref() == other.as_ref()
     }
 }
 
-impl ops::Index<ops::Range<usize>> for SharedSecret {
-    type Output = [u8];
+impl AsRef<[u8]> for SharedSecret {
+    fn as_ref(&self) -> &[u8] {
+        &self.data[..self.len]
+    }
+}
 
-    #[inline]
-    fn index(&self, index: ops::Range<usize>) -> &[u8] {
-        &self.0[index]
+impl Deref for SharedSecret {
+    type Target = [u8];
+    fn deref(&self) -> &[u8] {
+        &self.data[..self.len]
     }
 }
 
-impl ops::Index<ops::RangeFrom<usize>> for SharedSecret {
-    type Output = [u8];
 
-    #[inline]
-    fn index(&self, index: ops::RangeFrom<usize>) -> &[u8] {
-        &self.0[index.start..]
-    }
+unsafe extern "C" fn hash_callback<F>(output: *mut c_uchar, x: *const c_uchar, y: *const c_uchar, data: *mut c_void) -> c_int
+    where F: FnMut([u8; 32], [u8; 32]) -> SharedSecret {
+    let callback: &mut F = &mut *(data as *mut F);
+
+    let mut x_arr = [0; 32];
+    let mut y_arr = [0; 32];
+    ptr::copy_nonoverlapping(x, x_arr.as_mut_ptr(), 32);
+    ptr::copy_nonoverlapping(y, y_arr.as_mut_ptr(), 32);
+
+    let secret = callback(x_arr, y_arr);
+    ptr::copy_nonoverlapping(secret.as_ptr(), output as *mut u8, secret.len());
+
+    secret.len() as c_int
 }
 
-impl ops::Index<ops::RangeFull> for SharedSecret {
-    type Output = [u8];
 
+impl SharedSecret {
+    /// Creates a new shared secret from a pubkey and secret key
     #[inline]
-    fn index(&self, _: ops::RangeFull) -> &[u8] {
-        &self.0[..]
+    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(),
+            )
+        };
+        debug_assert_eq!(res, 1); // The default `secp256k1_ecdh_hash_function_default` should always return 1.
+        ss.set_len(32); // The default hash function is SHA256, which is 32 bytes long.
+        ss
+    }
+
+    /// Creates a new shared secret from a pubkey and secret key with applied custom hash function
+    /// # Examples
+    /// ```
+    /// # use secp256k1::ecdh::SharedSecret;
+    /// # use secp256k1::{Secp256k1, PublicKey, SecretKey};
+    /// # fn sha2(_a: &[u8], _b: &[u8]) -> [u8; 32] {[0u8; 32]}
+    /// # let secp = Secp256k1::signing_only();
+    /// # let secret_key = SecretKey::from_slice(&[3u8; 32]).unwrap();
+    /// # let secret_key2 = SecretKey::from_slice(&[7u8; 32]).unwrap();
+    /// # let public_key = PublicKey::from_secret_key(&secp, &secret_key2);
+    ///
+    /// let secret = SharedSecret::new_with_hash(&public_key, &secret_key, |x,y| {
+    ///     let hash: [u8; 32] = sha2(&x,&y);
+    ///     hash.into()
+    /// });
+    ///
+    /// ```
+    pub fn new_with_hash<F>(point: &PublicKey, scalar: &SecretKey, mut hash_function: F) -> SharedSecret
+        where F: FnMut([u8; 32], [u8; 32]) -> SharedSecret
+    {
+        let mut ss = SharedSecret::empty();
+        let hashfp: ffi::EcdhHashFn = hash_callback::<F>;
+
+        let res =  unsafe {
+            ffi::secp256k1_ecdh(
+                ffi::secp256k1_context_no_precomp,
+                ss.get_data_mut_ptr(),
+                point.as_ptr(),
+                scalar.as_ptr(),
+                hashfp,
+                &mut hash_function as *mut F as *mut c_void,
+            )
+        };
+        debug_assert!(res >= 16); // 128 bit is the minimum for a secure hash function and the minimum we let users.
+        ss.set_len(res as usize);
+        ss
     }
 }
 

src/macros.rs

@@ -27,6 +27,23 @@ 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: $N,
+                    }
+                }
+            }
+        )+
+    }
+}
+
 #[cfg(feature="serde")]
 /// Implements `Serialize` and `Deserialize` for a type `$t` which represents
 /// a newtype over a byte-slice over length `$len`. Type `$t` must implement