Add error type for combine keys + test and doc

Author: Tibo-lg

src/key.rs

@@ -20,7 +20,7 @@
 use core::{fmt, str};
 
 use super::{from_hex, Secp256k1};
-use super::Error::{self, InvalidPublicKey, InvalidSecretKey};
+use super::Error::{self, InvalidPublicKey, InvalidPublicKeySum, InvalidSecretKey};
 use Signing;
 use Verification;
 use constants;
@@ -395,12 +395,16 @@ impl PublicKey {
 
     /// Adds the keys in the provided slice together, returning the sum. Returns
     /// an error if the result would be the point at infinity, i.e. we are adding
-    /// a point to its own negation
+    /// a point to its own negation, if the provided slice has no element in it,
+    /// or if the number of element it contains is greater than i32::MAX.
     pub fn combine_keys(keys: &[&PublicKey]) -> Result<PublicKey, Error> {
         use core::mem::transmute;
         use core::i32::MAX;
 
-        debug_assert!(keys.len() < MAX as usize);
+        if keys.is_empty() || keys.len() > MAX as usize {
+            return Err(InvalidPublicKeySum);
+        }
+
         unsafe {
             let mut ret = ffi::PublicKey::new();
             let ptrs : &[*const ffi::PublicKey] =
@@ -414,7 +418,7 @@ impl PublicKey {
             {
                 Ok(PublicKey(ret))
             } else {
-                Err(InvalidPublicKey)
+                Err(InvalidPublicKeySum)
             }
         }
     }
@@ -893,6 +897,11 @@ mod test {
         assert_eq!(sum1.unwrap(), exp_sum);
     }
 
+    #[cfg_attr(not(fuzzing), test)]
+    fn pubkey_combine_keys_empty_slice() {
+        assert!(PublicKey::combine_keys(&[]).is_err());
+    }
+
     #[test]
     fn create_pubkey_combine() {
         let s = Secp256k1::new();

src/lib.rs

@@ -527,10 +527,10 @@ pub enum Error {
     InvalidRecoveryId,
     /// Invalid tweak for add_*_assign or mul_*_assign
     InvalidTweak,
-    /// `tweak_add_check` failed on an xonly public key
-    TweakCheckFailed,
     /// Didn't pass enough memory to context creation with preallocated memory
     NotEnoughMemory,
+    /// Bad set of public keys
+    InvalidPublicKeySum,
 }
 
 impl Error {
@@ -543,8 +543,8 @@ impl Error {
             Error::InvalidSecretKey => "secp: malformed or out-of-range secret key",
             Error::InvalidRecoveryId => "secp: bad recovery id",
             Error::InvalidTweak => "secp: bad tweak",
-            Error::TweakCheckFailed => "secp: xonly_pubkey_tewak_add_check failed",
             Error::NotEnoughMemory => "secp: not enough memory allocated",
+            Error::InvalidPublicKeySum => "secp: the sum of public keys was invalid or the input vector lengths was less than 1",
         }
     }
 }