add check that n is a power of two for ipp

Author: cathieyun

src/inner_product_proof.rs

@@ -28,6 +28,9 @@ impl InnerProductProof {
     /// The `verifier` is passed in as a parameter so that the
     /// challenges depend on the *entire* transcript (including parent
     /// protocols).
+    ///
+    /// The lengths of the vectors must all be the same, and must all be
+    /// either 0 or a power of 2.
     pub fn create<I>(
         verifier: &mut ProofTranscript,
         Q: &RistrettoPoint,
@@ -57,6 +60,9 @@ impl InnerProductProof {
         assert_eq!(a.len(), n);
         assert_eq!(b.len(), n);
 
+        // All of the input vectors must have a length that is a power of two.
+        assert!(n.is_power_of_two());
+
         // XXX save these scalar mults by unrolling them into the
         // first iteration of the loop below
         for (H_i, h_i) in H.iter_mut().zip(Hprime_factors.into_iter()) {