First ristretto255 function: EQUALS

ristretto255.go

@@ -0,0 +1,33 @@
+// 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 ristretto255 implements the ristretto255 prime-order group as
+// specified in draft-hdevalence-cfrg-ristretto-00.
+package ristretto255
+
+import (
+	"github.com/gtank/ristretto255/internal/edwards25519"
+)
+
+// Element is an element of the ristretto255 prime-order group.
+type Element struct {
+	r edwards25519.ExtendedGroupElement
+}
+
+// Equal returns 1 if e is equivalent to ee, and 0 otherwise.
+// Note that Elements must not be compared in any other way.
+func (e *Element) Equal(ee *Element) int {
+	var f0, f1 edwards25519.FieldElement
+
+	edwards25519.FeMul(&f0, &e.r.X, &ee.r.Y) // x1 * y2
+	edwards25519.FeMul(&f1, &e.r.Y, &ee.r.X) // y1 * x2
+	out := edwards25519.FeEqual(&f0, &f1)
+
+	edwards25519.FeMul(&f0, &e.r.Y, &ee.r.Y) // y1 * y2
+	edwards25519.FeMul(&f1, &e.r.X, &ee.r.X) // x1 * x2
+	out = out | edwards25519.FeEqual(&f0, &f1)
+
+	return out
+}