feature gate new_with_hash with std only, added catch_unwind

src/ecdh.rs

@@ -22,6 +22,7 @@ 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)]
@@ -63,6 +64,7 @@ impl SharedSecret {
 
     /// Set the length of the object.
     pub(crate) fn set_len(&mut self, len: usize) {
+        debug_assert!(len <= self.data.len());
         self.len = len;
     }
 }
@@ -87,8 +89,12 @@ impl Deref for SharedSecret {
 }
 
 
+#[cfg(feature = "std")]
 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 {
+
+    use std::panic::catch_unwind;
+    let res = catch_unwind(|| {
         let callback: &mut F = &mut *(data as *mut F);
 
         let mut x_arr = [0; 32];
@@ -100,6 +106,12 @@ unsafe extern "C" fn hash_callback<F>(output: *mut c_uchar, x: *const c_uchar, y
         ptr::copy_nonoverlapping(secret.as_ptr(), output as *mut u8, secret.len());
 
         secret.len() as c_int
+    });
+    if let Ok(len) = res {
+        len
+    } else {
+        -1
+    }
 }
 
 
@@ -140,7 +152,8 @@ impl SharedSecret {
     /// });
     ///
     /// ```
-    pub fn new_with_hash<F>(point: &PublicKey, scalar: &SecretKey, mut hash_function: F) -> SharedSecret
+    #[cfg(feature = "std")]
+    pub fn new_with_hash<F>(point: &PublicKey, scalar: &SecretKey, mut hash_function: F) -> Result<SharedSecret, Error>
         where F: FnMut([u8; 32], [u8; 32]) -> SharedSecret
     {
         let mut ss = SharedSecret::empty();
@@ -156,9 +169,12 @@ impl SharedSecret {
                 &mut hash_function 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);
-        ss
+        Ok(ss)
     }
 }
 
@@ -167,6 +183,7 @@ mod tests {
     use rand::thread_rng;
     use super::SharedSecret;
     use super::super::Secp256k1;
+    use Error;
 
     #[test]
     fn ecdh() {
@@ -187,9 +204,9 @@ mod tests {
         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());
-        let sec2 = SharedSecret::new_with_hash(&pk2, &sk1, |x,_| x.into());
-        let sec_odd = SharedSecret::new_with_hash(&pk1, &sk1, |x,_| x.into());
+        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);
     }
@@ -205,11 +222,23 @@ mod tests {
             x_out = x;
             y_out = y;
             expect_result.into()
-        });
+        }).unwrap();
         assert_eq!(&expect_result[..], &result[..]);
         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",
         }
     }
 }