chore: add Bytes32ToBigInt

rln/utils.go

@@ -71,11 +71,30 @@ func ToBytes32LE(hexStr string) ([32]byte, error) {
 	return Bytes32(b), nil
 }
 
-func BigIntToBytes32(value *big.Int) [32]byte {
-	b := value.Bytes()
+func revert(b []byte) []byte {
 	bLen := len(b)
 	for i := 0; i < bLen/2; i++ {
 		b[i], b[bLen-i-1] = b[bLen-i-1], b[i]
 	}
-	return Bytes32(b)
+	return b
+}
+
+// BigIntToBytes32 takes a *big.Int (which uses big endian) and converts it into a little endian 32 byte array
+// Notice that is the *big.Int value contains an integer <= 2^248 - 1 (a 7 bytes value with all bits on), it will right-pad the result with 0s until
+// the result has 32 bytes, i.e.:
+// for a some bigInt whose `Bytes()` are {0x12, 0x34, 0x56, 0x78, 0x90, 0xAB, 0xCD, 0xEF}, using this function will return
+// {0xEF, 0xCD, 0xAB, 0x90, 0x78, 0x56, 0x34, 0x12, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}
+func BigIntToBytes32(value *big.Int) [32]byte {
+	b := revert(value.Bytes())
+	tmp := make([]byte, 32)
+	copy(tmp[0:len(b)], b)
+	return Bytes32(tmp)
+}
+
+// Bytes32ToBigInt takes a little endian 32 byte array and returns a *big.Int (which uses big endian)
+func Bytes32ToBigInt(value [32]byte) *big.Int {
+	b := revert(value[:])
+	result := new(big.Int)
+	result.SetBytes(b)
+	return result
 }

rln/utils_test.go

@@ -0,0 +1,21 @@
+package rln
+
+import (
+	"bytes"
+	"math/big"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestBigInt(t *testing.T) {
+	base := big.NewInt(2)
+	value := base.Exp(base, big.NewInt(248), nil)
+	value = value.Sub(value, big.NewInt(1)) // 2^248 - 1
+
+	b32Value := BigIntToBytes32(value)
+	require.True(t, bytes.Equal(b32Value[:], []byte{255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 0}))
+
+	newValue := Bytes32ToBigInt(b32Value)
+	require.True(t, bytes.Equal(newValue.Bytes(), value.Bytes()))
+}