Add read_le and write_le to PrimeFieldRepr

src/lib.rs

@@ -407,6 +407,28 @@ pub trait PrimeFieldRepr:
 
         Ok(())
     }
+
+    /// Writes this `PrimeFieldRepr` as a little endian integer.
+    fn write_le<W: Write>(&self, mut writer: W) -> io::Result<()> {
+        use byteorder::{LittleEndian, WriteBytesExt};
+
+        for digit in self.as_ref().iter() {
+            writer.write_u64::<LittleEndian>(*digit)?;
+        }
+
+        Ok(())
+    }
+
+    /// Reads a little endian integer into this representation.
+    fn read_le<R: Read>(&mut self, mut reader: R) -> io::Result<()> {
+        use byteorder::{LittleEndian, ReadBytesExt};
+
+        for digit in self.as_mut().iter_mut() {
+            *digit = reader.read_u64::<LittleEndian>()?;
+        }
+
+        Ok(())
+    }
 }
 
 #[derive(Debug, PartialEq)]

src/tests/repr.rs

@@ -12,6 +12,9 @@ fn random_encoding_tests<R: PrimeFieldRepr>() {
 
     for _ in 0..1000 {
         let r = R::rand(&mut rng);
+
+        // Big endian
+        {
             let mut rdecoded = R::default();
 
             let mut v: Vec<u8> = vec![];
@@ -20,6 +23,36 @@ fn random_encoding_tests<R: PrimeFieldRepr>() {
 
             assert_eq!(r, rdecoded);
         }
+
+        // Little endian
+        {
+            let mut rdecoded = R::default();
+
+            let mut v: Vec<u8> = vec![];
+            r.write_le(&mut v).unwrap();
+            rdecoded.read_le(&v[0..]).unwrap();
+
+            assert_eq!(r, rdecoded);
+        }
+
+        {
+            let mut rdecoded_le = R::default();
+            let mut rdecoded_be_flip = R::default();
+
+            let mut v: Vec<u8> = vec![];
+            r.write_le(&mut v).unwrap();
+
+            // This reads in little-endian, so we are done.
+            rdecoded_le.read_le(&v[..]).unwrap();
+
+            // This reads in big-endian, so we perform a swap of the
+            // bytes beforehand.
+            let v: Vec<u8> = v.into_iter().rev().collect();
+            rdecoded_be_flip.read_be(&v[..]).unwrap();
+
+            assert_eq!(rdecoded_le, rdecoded_be_flip);
+        }
+    }
 }
 
 fn random_shl_tests<R: PrimeFieldRepr>() {