From 8dca32504225aff57532aa44e0c583b762f94287 Mon Sep 17 00:00:00 2001
From: Weikeng Chen <w.k@berkeley.edu>
Date: Thu, 12 Nov 2020 01:18:43 -0800
Subject: [PATCH] Make ToBitsGadget for FpVar's output constant-length (#12)

This is related to a Zexe issue https://github.com/scipr-lab/zexe/issues/289

Basically, ToBitsGadget for FpVar may produce shorter output for a constant value, which is undesired for a few reasons.
---
 src/fields/fp/mod.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/fields/fp/mod.rs b/src/fields/fp/mod.rs
index 693093c..d6e035c 100644
--- a/src/fields/fp/mod.rs
+++ b/src/fields/fp/mod.rs
@@ -876,7 +876,8 @@ impl<F: PrimeField> ToBitsGadget<F> for FpVar<F> {
     fn to_non_unique_bits_le(&self) -> Result<Vec<Boolean<F>>, SynthesisError> {
         use ark_ff::BitIteratorLE;
         match self {
-            Self::Constant(c) => Ok(BitIteratorLE::without_trailing_zeros(&c.into_repr())
+            Self::Constant(c) => Ok(BitIteratorLE::new(&c.into_repr())
+                .take((F::Params::MODULUS_BITS) as usize)
                 .map(Boolean::constant)
                 .collect::<Vec<_>>()),
             Self::Var(v) => v.to_non_unique_bits_le(),