Make contracthashtool check that the number of keys matches the template's requirement

This is easy to satisfy given that the template-to-script code takes a
slice of keys. Just do &keys[..n_keys] if you have too many keys. (If
you have too few you're SOL no matter what.) This way we can catch
likely configuration errors without putting much of a burden on users
who legitimately have more keys than the template requires.

Also add a method required_keys() to Template so that users can check
how many keys they ought to have.

Cargo.toml

@@ -1,7 +1,7 @@
 
 [package]
 name = "bitcoin"
-version = "0.3.6"
+version = "0.3.7"
 authors = ["Andrew Poelstra <apoelstra@wpsoftware.net>"]
 license = "CC0-1.0"
 homepage = "https://github.com/apoelstra/rust-bitcoin/"

src/util/contracthash.rs

@@ -49,7 +49,9 @@ pub enum Error {
     /// we got something else.
     ExpectedChecksig,
     /// Did not have enough keys to instantiate a script template
-    TooFewKeys(usize)
+    TooFewKeys(usize),
+    /// Had too many keys; template does not match key list
+    TooManyKeys(usize)
 }
 
 /// An element of a script template
@@ -81,7 +83,16 @@ impl Template {
                 }
             }
         }
+        if key_index == keys.len() {
             Ok(ret.into_script())
+        } else {
+            Err(Error::TooManyKeys(keys.len()))
+        }
+    }
+
+    /// Returns the number of keys this template requires to instantiate
+    pub fn required_keys(&self) -> usize {
+        self.0.iter().filter(|e| **e == TemplateElement::Key).count()
     }
 }
 
@@ -284,6 +295,22 @@ mod tests {
         assert_eq!(tweaked_pks[1], tweaked_pk2);
         assert_eq!(tweaked_pks[2], tweaked_pk3);
     }
+
+    #[test]
+    fn bad_key_number() {
+        let secp = Secp256k1::new();
+        let alpha_keys = alpha_keys!(&secp);
+        let template_short = Template::from(&hex!("55fefefefefefe57AE")[..]);
+        let template_long = Template::from(&hex!("55fefefefefefefefe57AE")[..]);
+        let template = Template::from(&hex!("55fefefefefefefe57AE")[..]);
+
+        assert_eq!(template_short.required_keys(), 6);
+        assert_eq!(template_long.required_keys(), 8);
+        assert_eq!(template.required_keys(), 7);
+        assert_eq!(template_short.to_script(alpha_keys), Err(Error::TooManyKeys(7)));
+        assert_eq!(template_long.to_script(alpha_keys), Err(Error::TooFewKeys(7)));
+        assert!(template.to_script(alpha_keys).is_ok());
+    }
 }