Change BigInt::{to_bits, from_bytes} to have big endian/ little endia…

…n variants, and add to_bytes_{be,le} (arkworks-rs#166)

* Add to_bytes to bigint

* Switch to *_be, *_le API

Co-authored-by: Pratyush Mishra <[email protected]>

CHANGELOG.md

@@ -37,6 +37,7 @@ The main features of this release are:
     a default value.
   Downstream users other than `ark-curves` should not see breakage unless they rely on these methods/traits explicitly.
 - #165 (ark-ff) Add `from_base_field_elements` as a method to the `Field` trait.
+- #166 (ark-ff) Change `BigInt::{from_bytes, to_bits}` to `from_bytes_le, from_bytes_be, to_bits_le, to_bits_be`.
 
 ### Features
 - #20 (ark-poly) Add structs/traits for multivariate polynomials
@@ -69,6 +70,7 @@ The main features of this release are:
 - #153 (ark-serialize) Add an impl of `CanonicalSerialize/Deserialize` for `Rc<T>`.
 - #157 (ark-ec) Speed up `variable_base_msm` by not relying on unnecessary normalization.
 - #158 (ark-serialize) Add an impl of `CanonicalSerialize/Deserialize` for `()`.
+- #166 (ark-ff) Add a `to_bytes_be()` and `to_bytes_le` methods to `BigInt`.
 
 ### Bug fixes
 - #36 (ark-ec) In Short-Weierstrass curves, include an infinity bit in `ToConstraintField`.

ec/src/msm/fixed_base.rs

@@ -65,8 +65,7 @@ impl FixedBaseMSM {
         scalar: &T::ScalarField,
     ) -> T {
         let modulus_size = <T::ScalarField as PrimeField>::Params::MODULUS_BITS as usize;
-        let mut scalar_val = scalar.into_repr().to_bits();
-        scalar_val.reverse();
+        let scalar_val = scalar.into_repr().to_bits_le();
 
         let mut res = multiples_of_g[0][0].into_projective();
         for outer in 0..outerc {

ff/src/biginteger/macros.rs

@@ -149,7 +149,7 @@ macro_rules! bigint_impl {
             }
 
             #[inline]
-            fn from_bits(bits: &[bool]) -> Self {
+            fn from_bits_be(bits: &[bool]) -> Self {
                 let mut res = Self::default();
                 let mut acc: u64 = 0;
 
@@ -166,11 +166,35 @@ macro_rules! bigint_impl {
                 res
             }
 
+            fn from_bits_le(bits: &[bool]) -> Self {
+                let mut res = Self::default();
+                let mut acc: u64 = 0;
+
+                let bits = bits.to_vec();
+                for (i, bits64) in bits.chunks(64).enumerate() {
+                    for bit in bits64.iter().rev() {
+                        acc <<= 1;
+                        acc += *bit as u64;
+                    }
+                    res.0[i] = acc;
+                    acc = 0;
+                }
+                res
+            }
+
+            #[inline]
+            fn to_bytes_be(&self) -> Vec<u8> {
+                let mut le_bytes = self.to_bytes_le();
+                le_bytes.reverse();
+                le_bytes
+            }
+
             #[inline]
-            fn to_bits(&self) -> Vec<bool> {
-                let mut res = Vec::with_capacity(256);
-                for b in BitIteratorBE::new(self.0) {
-                    res.push(b);
+            fn to_bytes_le(&self) -> Vec<u8> {
+                let array_map = self.0.iter().map(|limb| limb.to_le_bytes());
+                let mut res = Vec::<u8>::with_capacity($num_limbs * 8);
+                for limb in array_map {
+                    res.extend_from_slice(&limb);
                 }
                 res
             }

ff/src/biginteger/mod.rs

@@ -1,6 +1,6 @@
 use crate::{
     bytes::{FromBytes, ToBytes},
-    fields::BitIteratorBE,
+    fields::{BitIteratorBE, BitIteratorLE},
     UniformRand,
 };
 use ark_serialize::{CanonicalDeserialize, CanonicalSerialize, SerializationError};
@@ -96,11 +96,31 @@ pub trait BigInteger:
 
     /// Returns the big integer representation of a given big endian boolean
     /// array.
-    fn from_bits(bits: &[bool]) -> Self;
+    fn from_bits_be(bits: &[bool]) -> Self;
 
-    /// Returns the bit representation in a big endian boolean array, without
-    /// leading zeros.
-    fn to_bits(&self) -> Vec<bool>;
+    /// Returns the big integer representation of a given little endian boolean
+    /// array.
+    fn from_bits_le(bits: &[bool]) -> Self;
+
+    /// Returns the bit representation in a big endian boolean array,
+    /// with leading zeroes.
+    fn to_bits_be(&self) -> Vec<bool> {
+        BitIteratorBE::new(self).collect::<Vec<_>>()
+    }
+
+    /// Returns the bit representation in a little endian boolean array,
+    /// with trailing zeroes.
+    fn to_bits_le(&self) -> Vec<bool> {
+        BitIteratorLE::new(self).collect::<Vec<_>>()
+    }
+
+    /// Returns the byte representation in a big endian byte array,
+    /// with leading zeros.
+    fn to_bytes_be(&self) -> Vec<u8>;
+
+    /// Returns the byte representation in a little endian byte array,
+    /// with trailing zeros.
+    fn to_bytes_le(&self) -> Vec<u8>;
 
     /// Returns a vector for wnaf.
     fn find_wnaf(&self) -> Vec<i64>;