From f37eedca7f286187c71587797e08da189c4eee70 Mon Sep 17 00:00:00 2001
From: VolodymyrBg <aqdrgg19@gmail.com>
Date: Tue, 18 Feb 2025 21:53:43 +0200
Subject: [PATCH] feat(acir_field): Add little-endian byte serialization for
 FieldElement (#7258)

Co-authored-by: Akosh Farkash <aakoshh@gmail.com>
---
 acvm-repo/acir_field/src/field_element.rs | 61 +++++++++++++++++++++--
 acvm-repo/acir_field/src/generic_ark.rs   |  6 +++
 2 files changed, 63 insertions(+), 4 deletions(-)

diff --git a/acvm-repo/acir_field/src/field_element.rs b/acvm-repo/acir_field/src/field_element.rs
index 0249b410aa7..8afc76da9d8 100644
--- a/acvm-repo/acir_field/src/field_element.rs
+++ b/acvm-repo/acir_field/src/field_element.rs
@@ -274,12 +274,15 @@ impl<F: PrimeField> AcirField for FieldElement<F> {
     }
 
     fn to_be_bytes(self) -> Vec<u8> {
-        // to_be_bytes! uses little endian which is why we reverse the output
-        // TODO: Add a little endian equivalent, so the caller can use whichever one
-        // TODO they desire
+        let mut bytes = self.to_le_bytes();
+        bytes.reverse();
+        bytes
+    }
+
+    /// Converts the field element to a vector of bytes in little-endian order
+    fn to_le_bytes(self) -> Vec<u8> {
         let mut bytes = Vec::new();
         self.0.serialize_uncompressed(&mut bytes).unwrap();
-        bytes.reverse();
         bytes
     }
 
@@ -289,6 +292,12 @@ impl<F: PrimeField> AcirField for FieldElement<F> {
         FieldElement(F::from_be_bytes_mod_order(bytes))
     }
 
+    /// Converts bytes in little-endian order into a FieldElement and applies a
+    /// reduction if needed.
+    fn from_le_bytes_reduce(bytes: &[u8]) -> FieldElement<F> {
+        FieldElement(F::from_le_bytes_mod_order(bytes))
+    }
+
     /// Returns the closest number of bytes to the bits specified
     /// This method truncates
     fn fetch_nearest_bytes(&self, num_bits: usize) -> Vec<u8> {
@@ -405,6 +414,50 @@ mod tests {
         assert_eq!(max_num_bits_bn254, 254);
     }
 
+    proptest! {
+        #[test]
+        fn test_endianness_prop(value in any::<u64>()) {
+            let field = FieldElement::<ark_bn254::Fr>::from(value);
+            // Test serialization consistency
+            let le_bytes = field.to_le_bytes();
+            let be_bytes = field.to_be_bytes();
+
+            let mut reversed_le = le_bytes.clone();
+            reversed_le.reverse();
+            prop_assert_eq!(&be_bytes, &reversed_le, "BE bytes should be reverse of LE bytes");
+
+            // Test deserialization consistency
+            let from_le = FieldElement::from_le_bytes_reduce(&le_bytes);
+            let from_be = FieldElement::from_be_bytes_reduce(&be_bytes);
+            prop_assert_eq!(from_le, from_be, "Deserialization should be consistent between LE and BE");
+            prop_assert_eq!(from_le, field, "Deserialized value should match original");
+        }
+    }
+
+    #[test]
+    fn test_endianness() {
+        let field = FieldElement::<ark_bn254::Fr>::from(0x1234_5678_u32);
+        let le_bytes = field.to_le_bytes();
+        let be_bytes = field.to_be_bytes();
+
+        // Check that the bytes are reversed between BE and LE
+        let mut reversed_le = le_bytes.clone();
+        reversed_le.reverse();
+        assert_eq!(&be_bytes, &reversed_le);
+
+        // Verify we can reconstruct the same field element from either byte order
+        let from_le = FieldElement::from_le_bytes_reduce(&le_bytes);
+        let from_be = FieldElement::from_be_bytes_reduce(&be_bytes);
+        assert_eq!(from_le, from_be);
+        assert_eq!(from_le, field);
+
+        // Additional test with a larger number to ensure proper byte handling
+        let large_field = FieldElement::<ark_bn254::Fr>::from(0x0123_4567_89AB_CDEF_u64);
+        let large_le = large_field.to_le_bytes();
+        let reconstructed = FieldElement::from_le_bytes_reduce(&large_le);
+        assert_eq!(reconstructed, large_field);
+    }
+
     proptest! {
         // This currently panics due to the fact that we allow inputs which are greater than the field modulus,
         // automatically reducing them to fit within the canonical range.
diff --git a/acvm-repo/acir_field/src/generic_ark.rs b/acvm-repo/acir_field/src/generic_ark.rs
index 74927c07a36..04761dd1ed0 100644
--- a/acvm-repo/acir_field/src/generic_ark.rs
+++ b/acvm-repo/acir_field/src/generic_ark.rs
@@ -75,6 +75,12 @@ pub trait AcirField:
     /// Converts bytes into a FieldElement and applies a reduction if needed.
     fn from_be_bytes_reduce(bytes: &[u8]) -> Self;
 
+    /// Converts bytes in little-endian order into a FieldElement and applies a reduction if needed.
+    fn from_le_bytes_reduce(bytes: &[u8]) -> Self;
+
+    /// Converts the field element to a vector of bytes in little-endian order
+    fn to_le_bytes(self) -> Vec<u8>;
+
     /// Returns the closest number of bytes to the bits specified
     /// This method truncates
     fn fetch_nearest_bytes(&self, num_bits: usize) -> Vec<u8>;