Add FeEqual and feFromBig

internal/edwards25519/fe.go

@@ -0,0 +1,32 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Copyright 2019 George Tankersley. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package edwards25519
+
+import (
+	"crypto/subtle"
+	"math/big"
+
+	// This is exactly as horrible as it should be.
+	. "github.com/gtank/ristretto255/internal/edwards25519/internal/edwards25519"
+)
+
+// FeEqual returns 1 if a and b are equal, and 0 otherwise.
+func FeEqual(a, b *FieldElement) int {
+	var sa, sb [32]byte
+	FeToBytes(&sa, a)
+	FeToBytes(&sb, b)
+	return subtle.ConstantTimeCompare(sa[:], sb[:])
+}
+
+func feFromBig(dst *FieldElement, n *big.Int) {
+	var buf [32]byte
+	nn := n.Bytes()
+	copy(buf[len(buf)-len(nn):], nn)
+	for i := range buf[:len(buf)/2] {
+		buf[i], buf[len(buf)-1] = buf[len(buf)-1], buf[i]
+	}
+	FeFromBytes(dst, &buf)
+}