Do not test secret->public derivation or pk validity in fuzzing cfg

In the next commit the secret->public key derivation in fuzzing cfg
is changed to be simpler, as well as the validity rules of public
keys relaxed.

This adds a new test to ensure random keys can be added, not just
the hard-coded keys test that exists today.

Author: TheBlueMatt

src/key.rs

@@ -690,7 +690,13 @@ mod test {
 
         let s = Secp256k1::signing_only();
         let sk = SecretKey::from_slice(&SK_BYTES).expect("sk");
+
+        // In fuzzing mode secret->public key derivation is different, so
+        // hard-code the epected result.
+        #[cfg(not(fuzzing))]
         let pk = PublicKey::from_secret_key(&s, &sk);
+        #[cfg(fuzzing)]
+        let pk = PublicKey::from_slice(&[0x02, 0x18, 0x84, 0x57, 0x81, 0xf6, 0x31, 0xc4, 0x8f, 0x1c, 0x97, 0x09, 0xe2, 0x30, 0x92, 0x06, 0x7d, 0x06, 0x83, 0x7f, 0x30, 0xaa, 0x0c, 0xd0, 0x54, 0x4a, 0xc8, 0x87, 0xfe, 0x91, 0xdd, 0xd1, 0x66]).expect("pk");
 
         assert_eq!(
             sk.to_string(),
@@ -733,6 +739,9 @@ mod test {
     }
 
     #[test]
+    // In fuzzing mode the Y coordinate is expected to match the X, so this
+    // test uses invalid public keys.
+    #[cfg(not(fuzzing))]
     fn test_pubkey_serialize() {
         struct DumbRng(u32);
         impl RngCore for DumbRng {
@@ -841,7 +850,7 @@ mod test {
         assert_eq!(set.len(), COUNT);
     }
 
-    #[test]
+    #[cfg_attr(not(fuzzing), test)]
     fn pubkey_combine() {
         let compressed1 = PublicKey::from_slice(
             &hex!("0241cc121c419921942add6db6482fb36243faf83317c866d2a28d8c6d7089f7ba"),
@@ -861,7 +870,7 @@ mod test {
         assert_eq!(sum1.unwrap(), exp_sum);
     }
 
-    #[test]
+    #[cfg_attr(not(fuzzing), test)]
     fn pubkey_combine_keys() {
         let compressed1 = PublicKey::from_slice(
             &hex!("0241cc121c419921942add6db6482fb36243faf83317c866d2a28d8c6d7089f7ba"),
@@ -884,6 +893,24 @@ mod test {
         assert_eq!(sum1.unwrap(), exp_sum);
     }
 
+    #[test]
+    fn create_pubkey_combine() {
+        let s = Secp256k1::new();
+
+        let (mut sk1, pk1) = s.generate_keypair(&mut thread_rng());
+        let (sk2, pk2) = s.generate_keypair(&mut thread_rng());
+
+        let sum1 = pk1.combine(&pk2);
+        assert!(sum1.is_ok());
+        let sum2 = pk2.combine(&pk1);
+        assert!(sum2.is_ok());
+        assert_eq!(sum1, sum2);
+
+        assert!(sk1.add_assign(&sk2.as_ref()[..]).is_ok());
+        let sksum = PublicKey::from_secret_key(&s, &sk1);
+        assert_eq!(Ok(sksum), sum1);
+    }
+
     #[test]
     fn pubkey_equal() {
         let pk1 = PublicKey::from_slice(

src/schnorrsig.rs

@@ -706,6 +706,9 @@ mod tests {
             PublicKey::from_slice(&[0xff; constants::SCHNORRSIG_PUBLIC_KEY_SIZE]),
             Err(InvalidPublicKey)
         );
+        // In fuzzing mode restrictions on public key validity are much more
+        // relaxed, thus the invalid check below is expected to fail.
+        #[cfg(not(fuzzing))]
         assert_eq!(
             PublicKey::from_slice(&[0x55; constants::SCHNORRSIG_PUBLIC_KEY_SIZE]),
             Err(InvalidPublicKey)
@@ -724,7 +727,13 @@ mod tests {
 
         let s = Secp256k1::signing_only();
         let sk = KeyPair::from_seckey_slice(&secp, &SK_BYTES).expect("sk");
+
+        // In fuzzing mode secret->public key derivation is different, so
+        // hard-code the epected result.
+        #[cfg(not(fuzzing))]
         let pk = PublicKey::from_keypair(&s, &sk);
+        #[cfg(fuzzing)]
+        let pk = PublicKey::from_slice(&[0x18, 0x84, 0x57, 0x81, 0xf6, 0x31, 0xc4, 0x8f, 0x1c, 0x97, 0x09, 0xe2, 0x30, 0x92, 0x06, 0x7d, 0x06, 0x83, 0x7f, 0x30, 0xaa, 0x0c, 0xd0, 0x54, 0x4a, 0xc8, 0x87, 0xfe, 0x91, 0xdd, 0xd1, 0x66]).expect("pk");
 
         assert_eq!(
             pk.to_string(),
@@ -762,6 +771,9 @@ mod tests {
     }
 
     #[test]
+    // In fuzzing mode secret->public key derivation is different, so
+    // this test will never correctly derive the static pubkey.
+    #[cfg(not(fuzzing))]
     fn test_pubkey_serialize() {
         struct DumbRng(u32);
         impl RngCore for DumbRng {