Merge pull request #180 from elichai/2019-11-SharedSecret

Alternative: Passing custom hash functions to ECDH

no_std_test/src/main.rs

@@ -55,6 +55,7 @@ use core::panic::PanicInfo;
 use secp256k1::rand::{self, RngCore};
 use secp256k1::serde::Serialize;
 use secp256k1::*;
+use secp256k1::ecdh::SharedSecret;
 
 use serde_cbor::de;
 use serde_cbor::ser::SliceWrite;
@@ -102,6 +103,16 @@ fn start(_argc: isize, _argv: *const *const u8) -> isize {
     let new_sig: Signature = de::from_mut_slice(&mut cbor_ser[..size]).unwrap();
     assert_eq!(sig, new_sig);
 
+    let _ = SharedSecret::new(&public_key, &secret_key);
+    let mut x_arr = [0u8; 32];
+    let y_arr = unsafe { SharedSecret::new_with_hash_no_panic(&public_key, &secret_key, |x,y| {
+        x_arr = x;
+        y.into()
+    })}.unwrap();
+    assert_ne!(x_arr, [0u8; 32]);
+    assert_ne!(&y_arr[..], &[0u8; 32][..]);
+    
+
     unsafe { libc::printf("Verified Successfully!\n\0".as_ptr() as _) };
     0
 }

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,
@@ -459,7 +440,7 @@ mod fuzz_dummy {
     use self::std::{ptr, mem};
     use self::std::boxed::Box;
     use types::*;
-    use ::{Signature, Context, NonceFn, EcdhHashFn, PublicKey, SharedSecret,
+    use ::{Signature, Context, NonceFn, EcdhHashFn, PublicKey,
         SECP256K1_START_NONE, SECP256K1_START_VERIFY, SECP256K1_START_SIGN,
         SECP256K1_SER_COMPRESSED, SECP256K1_SER_UNCOMPRESSED};
 
@@ -788,7 +769,7 @@ mod fuzz_dummy {
     /// Sets out to point[0..16]||scalar[0..16]
     pub unsafe fn secp256k1_ecdh(
         cx: *const Context,
-        out: *mut SharedSecret,
+        out: *mut c_uchar,
         point: *const PublicKey,
         scalar: *const c_uchar,
         _hashfp: EcdhHashFn,
@@ -801,13 +782,13 @@ mod fuzz_dummy {
         ptr::copy(scalar, scalar_prefix[..].as_mut_ptr(), 16);
 
         if (*point).0[0..16] > scalar_prefix[0..16] {
-            (*out).0[0..16].copy_from_slice(&(*point).0[0..16]);
+            ptr::copy((*point).as_ptr(), out, 16);
-            ptr::copy(scalar, (*out).0[16..32].as_mut_ptr(), 16);
+            ptr::copy(scalar, out.offset(16), 16);
         } else {
-            ptr::copy(scalar, (*out).0[0..16].as_mut_ptr(), 16);
+            ptr::copy(scalar, out, 16);
-            (*out).0[16..32].copy_from_slice(&(*point).0[0..16]);
+            ptr::copy((*point).as_ptr(), out.offset(16), 16);
         }
-        (*out).0[16] = 0x00; // result should always be a valid secret key
+        (*out.offset(16)) = 0x00; // result should always be a valid secret key
         1
     }
 }

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 +16,209 @@
 //! 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};
+use Error;
 
 /// A tag used for recovering the public key from a compact signature
-#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+#[derive(Copy, Clone)]
-pub struct SharedSecret(ffi::SharedSecret);
+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 {
+
+    /// Create an empty SharedSecret
+    pub(crate) fn empty() ->  SharedSecret {
+        SharedSecret {
+            data: [0u8; 256],
+            len: 0,
+        }
+    }
+
+    /// 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()
+    }
+
+    /// 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
+    }
+
+    /// Set 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 fn callback_logic<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
+}
+
+#[cfg(feature = "std")]
+unsafe extern "C" fn hash_callback_catch_unwind<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 res = ::std::panic::catch_unwind(||callback_logic::<F>(output, x, y, data));
+    if let Ok(len) = res {
+        len
+    } else {
+        -1
+    }
+}
+
+unsafe extern "C" fn hash_callback_unsafe<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 {
+    callback_logic::<F>(output, x, y, data)
+}
+
 
 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 = SharedSecret::empty();
-            let mut ss = ffi::SharedSecret::new();
+        let res = unsafe {
-            let res = ffi::secp256k1_ecdh(
+             ffi::secp256k1_ecdh(
                 ffi::secp256k1_context_no_precomp,
-                &mut ss,
+                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);
+        };
-            SharedSecret(ss)
+        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
+    }
+
+    fn new_with_callback_internal<F>(point: &PublicKey, scalar: &SecretKey, mut closure: F, callback: ffi::EcdhHashFn) -> Result<SharedSecret, Error>
+        where F: FnMut([u8; 32], [u8; 32]) -> SharedSecret {
+        let mut ss = SharedSecret::empty();
+
+        let res =  unsafe {
+            ffi::secp256k1_ecdh(
+                ffi::secp256k1_context_no_precomp,
+                ss.get_data_mut_ptr(),
+                point.as_ptr(),
+                scalar.as_ptr(),
+                callback,
+                &mut closure as *mut F as *mut c_void,
+            )
+        };
+        if res == -1 {
+            return Err(Error::CallbackPanicked);
         }
+        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);
+        Ok(ss)
+
     }
 
-    /// Obtains a raw pointer suitable for use with FFI functions
+    /// Creates a new shared secret from a pubkey and secret key with applied custom hash function
-    #[inline]
+    /// # Examples
-    pub fn as_ptr(&self) -> *const ffi::SharedSecret {
+    /// ```
-        &self.0 as *const _
+    /// # 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()
+    /// });
+    ///
+    /// ```
+    #[cfg(feature = "std")]
+    pub fn new_with_hash<F>(point: &PublicKey, scalar: &SecretKey, hash_function: F) -> Result<SharedSecret, Error>
+        where F: FnMut([u8; 32], [u8; 32]) -> SharedSecret {
+            Self::new_with_callback_internal(point, scalar, hash_function, hash_callback_catch_unwind::<F>)
     }
-}
 
-/// Creates a new shared secret from a FFI shared secret
+    /// Creates a new shared secret from a pubkey and secret key with applied custom hash function
-impl From<ffi::SharedSecret> for SharedSecret {
+    /// Note that this function is the same as [`new_with_hash`]
-    #[inline]
+    ///
-    fn from(ss: ffi::SharedSecret) -> SharedSecret {
+    /// # Safety
-        SharedSecret(ss)
+    /// The function doesn't wrap the callback with [`catch_unwind`]
-    }
+    /// so if the callback panics it will panic through an FFI boundray which is [`Undefined Behavior`]
-}
+    /// If possible you should use [`new_with_hash`] which does wrap the callback with [`catch_unwind`] so is safe to use.
-
+    ///
-
+    /// [`catch_unwind`]: https://doc.rust-lang.org/std/panic/fn.catch_unwind.html
-impl ops::Index<usize> for SharedSecret {
+    /// [`Undefined Behavior`]: https://doc.rust-lang.org/nomicon/ffi.html#ffi-and-panics
-    type Output = u8;
+    /// [`new_with_hash`]: #method.new_with_hash
-
+    /// # Examples
-    #[inline]
+    /// ```
-    fn index(&self, index: usize) -> &u8 {
+    /// # use secp256k1::ecdh::SharedSecret;
-        &self.0[index]
+    /// # 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();
-impl ops::Index<ops::Range<usize>> for SharedSecret {
+    /// # let secret_key2 = SecretKey::from_slice(&[7u8; 32]).unwrap();
-    type Output = [u8];
+    /// # let public_key = PublicKey::from_secret_key(&secp, &secret_key2);
-
+    //
-    #[inline]
+    /// let secret = unsafe { SharedSecret::new_with_hash_no_panic(&public_key, &secret_key, |x,y| {
-    fn index(&self, index: ops::Range<usize>) -> &[u8] {
+    ///     let hash: [u8; 32] = sha2(&x,&y);
-        &self.0[index]
+    ///     hash.into()
-    }
+    /// })};
-}
+    ///
-
+    ///
-impl ops::Index<ops::RangeFrom<usize>> for SharedSecret {
+    /// ```
-    type Output = [u8];
+    pub unsafe fn new_with_hash_no_panic<F>(point: &PublicKey, scalar: &SecretKey, hash_function: F) -> Result<SharedSecret, Error>
-
+        where F: FnMut([u8; 32], [u8; 32]) -> SharedSecret {
-    #[inline]
+        Self::new_with_callback_internal(point, scalar, hash_function, hash_callback_unsafe::<F>)
-    fn index(&self, index: ops::RangeFrom<usize>) -> &[u8] {
-        &self.0[index.start..]
-    }
-}
-
-impl ops::Index<ops::RangeFull> for SharedSecret {
-    type Output = [u8];
-
-    #[inline]
-    fn index(&self, _: ops::RangeFull) -> &[u8] {
-        &self.0[..]
     }
 }
 
@@ -101,6 +227,7 @@ mod tests {
     use rand::thread_rng;
     use super::SharedSecret;
     use super::super::Secp256k1;
+    use Error;
 
     #[test]
     fn ecdh() {
@@ -114,6 +241,54 @@ mod tests {
         assert_eq!(sec1, sec2);
         assert!(sec_odd != sec2);
     }
+
+    #[test]
+    fn ecdh_with_hash() {
+        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_with_hash(&pk1, &sk2, |x,_| x.into()).unwrap();
+        let sec2 = SharedSecret::new_with_hash(&pk2, &sk1, |x,_| x.into()).unwrap();
+        let sec_odd = SharedSecret::new_with_hash(&pk1, &sk1, |x,_| x.into()).unwrap();
+        assert_eq!(sec1, sec2);
+        assert_ne!(sec_odd, sec2);
+    }
+
+    #[test]
+    fn ecdh_with_hash_callback() {
+        let s = Secp256k1::signing_only();
+        let (sk1, pk1) = s.generate_keypair(&mut thread_rng());
+        let expect_result: [u8; 64] = [123; 64];
+        let mut x_out = [0u8; 32];
+        let mut y_out = [0u8; 32];
+        let result = SharedSecret::new_with_hash(&pk1, &sk1, | x, y | {
+            x_out = x;
+            y_out = y;
+            expect_result.into()
+        }).unwrap();
+        let result_unsafe = unsafe {SharedSecret::new_with_hash_no_panic(&pk1, &sk1, | x, y | {
+            x_out = x;
+            y_out = y;
+            expect_result.into()
+        }).unwrap()};
+        assert_eq!(&expect_result[..], &result[..]);
+        assert_eq!(result, result_unsafe);
+        assert_ne!(x_out, [0u8; 32]);
+        assert_ne!(y_out, [0u8; 32]);
+    }
+
+    #[test]
+    fn ecdh_with_hash_callback_panic() {
+        let s = Secp256k1::signing_only();
+        let (sk1, pk1) = s.generate_keypair(&mut thread_rng());
+        let mut res = [0u8; 48];
+        let result = SharedSecret::new_with_hash(&pk1, &sk1, | x, _ | {
+            res.copy_from_slice(&x); // res.len() != x.len(). this will panic.
+            res.into()
+        });
+        assert_eq!(result, Err(Error::CallbackPanicked));
+    }
 }
 
 #[cfg(all(test, feature = "unstable"))]

src/lib.rs

@@ -496,7 +496,8 @@ pub enum Error {
     InvalidTweak,
     /// Didn't pass enough memory to context creation with preallocated memory
     NotEnoughMemory,
-
+    /// The callback has panicked.
+    CallbackPanicked,
 }
 
 impl Error {
@@ -510,6 +511,7 @@ impl Error {
             Error::InvalidRecoveryId => "secp: bad recovery id",
             Error::InvalidTweak => "secp: bad tweak",
             Error::NotEnoughMemory => "secp: not enough memory allocated",
+            Error::CallbackPanicked => "secp: a callback passed has panicked",
         }
     }
 }

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