to big endian for elments

src/arith.rs

@@ -195,6 +195,16 @@ impl U256 {
         Ok(U256(n))
     }
 
+    pub fn to_big_endian(&self, s: &mut [u8]) -> Result<(), Error> {
+        if s.len() != 32 { return Err(Error::InvalidLength { expected: 32, actual: s.len() }); }
+
+        for (l, i) in (0..4).rev().zip((0..4).map(|i| i * 8)) {
+            BigEndian::write_u64(&mut s[i..], self.0[l]);
+        }        
+
+        Ok(())
+    }
+
     #[inline]
     pub fn zero() -> U256 {
         U256([0, 0, 0, 0])
@@ -540,6 +550,21 @@ fn from_slice() {
     assert_eq!(num, tst);
 }
 
+#[test]
+fn to_big_endian() {
+    let num = U256::one();
+    let mut s = [0u8; 32];
+
+    num.to_big_endian(&mut s).expect("U256 should convert to bytes ok in `to_big_endian` test");
+    assert_eq!(
+        s,
+        [
+            0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8,
+            0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 0u8, 1u8,
+        ]
+    );
+}
+
 #[test]
 fn testing_divrem() {
     let rng = &mut ::rand::thread_rng();

src/fields/fp.rs

@@ -79,6 +79,10 @@ macro_rules! field_impl {
             pub fn modulus() -> U256 {
                 U256($modulus)
             }
+
+            pub fn raw(&self) -> &U256 {
+                &self.0
+            }
         }
 
         impl FieldElement for $name {

src/lib.rs

@@ -32,7 +32,10 @@ impl Fr {
             .map_err(|_| FieldError::InvalidSliceLength) // todo: maybe more sensful error handling
             .and_then(|x| fields::Fr::new(x).ok_or(FieldError::NotMember))
             .map(|x| Fr(x))
-    }    
+    }
+    pub fn to_big_endian(&self, slice: &mut [u8]) -> Result<(), FieldError> {
+        self.0.raw().to_big_endian(slice).map_err(|_| FieldError::InvalidSliceLength)
+    }
 }
 
 impl Add<Fr> for Fr {
@@ -86,6 +89,9 @@ impl Fq {
             .and_then(|x| fields::Fq::new(x).ok_or(FieldError::NotMember))
             .map(|x| Fq(x))
     }
+    pub fn to_big_endian(&self, slice: &mut [u8]) -> Result<(), FieldError> {
+        self.0.raw().to_big_endian(slice).map_err(|_| FieldError::InvalidSliceLength)
+    }    
 }
 
 impl Add<Fq> for Fq {