Return None if len is wrong for alg in PublicKey::from_vec

The docstring says this is the intended behavior. The only way a `Vec<u8>`
shouldn't be simply parsed into a `PublicKey` is if it's length is not as
expected for the `algorithm` specified.

src/lib.rs

@@ -88,6 +88,9 @@ impl PublicKey {
     pub fn from_vec(vec: Vec<u8>, algorithm: &'static Algorithm) -> Option<PublicKey> {
         let size = vec.len();
         let hash_output_size = algorithm.output_len;
+        if size != (hash_output_size * hash_output_size * 8 * 2) {
+            return None;
+        }
 
         let mut zero_values_merged = vec;
         let one_values_merged = zero_values_merged.split_off(size / 2);

src/tests.rs

@@ -65,6 +65,18 @@ fn test_serialization() {
     assert_eq!(pub_key.zero_values, recovered_pub_key.zero_values);
 }
 
+fn test_serialization_wrong_size_key() {
+    let pub_key = PrivateKey::new(digest_512).public_key();
+    let mut too_short = pub_key.to_bytes();
+    let extra = too_short.pop();
+    assert!(PublicKey::from_vec(too_short, digest_512).is_none());
+
+    let pub_key = PrivateKey::new(digest_512).public_key();
+    let mut too_long = pub_key.to_bytes();
+    too_long.extend(extra);
+    assert!(PublicKey::from_vec(too_long, digest_512).is_none());
+}
+
 #[test]
 #[should_panic]
 fn test_serialization_panic() {