primitives/x25519: Remove the x/crypto/curve25519 fallback

Upstream got rid of the assembly.  This is marginally slower, but it
will use fiat, and it's only a few percent.

primitives/x25519/x25519.go

@@ -37,8 +37,6 @@ import (
 	"crypto/subtle"
 	"fmt"
 
-	xcurve "golang.org/x/crypto/curve25519"
-
 	"github.com/oasisprotocol/curve25519-voi/curve"
 	"github.com/oasisprotocol/curve25519-voi/curve/scalar"
 	_ "github.com/oasisprotocol/curve25519-voi/internal/toolchain"
@@ -55,11 +53,7 @@ const (
 // Basepoint is the canonical Curve25519 generator.
 var Basepoint []byte
 
-var (
-	basePoint = [32]byte{9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
-
-	debugNoXcurve bool
-)
+var basePoint = [32]byte{9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
 
 // ScalarMult sets dst to the product in*base where dst and base are the x
 // coordinates of group points and all values are in little-endian form.
@@ -68,14 +62,6 @@ var (
 // zeroes, irrespective of the scalar. Instead, use the X25519 function, which
 // will return an error.
 func ScalarMult(dst, in, base *[32]byte) {
-	// If the `x/crypto/curve25519` package would be faster, and we
-	// are not exercising the implementation provided by this package
-	// (eg: testing or benchmarking), use that instead.
-	if xcurveFaster && !debugNoXcurve {
-		xcurve.ScalarMult(dst, in, base)
-		return
-	}
-
 	var ec [ScalarSize]byte
 	copy(ec[:], in[:])
 	clampScalar(ec[:])

primitives/x25519/x25519_test.go

@@ -66,15 +66,6 @@ func TestScalarBaseMult(t *testing.T) {
 
 func TestX25519(t *testing.T) {
 	t.Run("voi", testX25519)
-	if xcurveFaster {
-		t.Run("voi/debugNoXcurve", func(t *testing.T) {
-			debugNoXcurve = true
-			defer func() {
-				debugNoXcurve = false
-			}()
-			testX25519(t)
-		})
-	}
 }
 
 func testX25519(t *testing.T) {
@@ -166,15 +157,6 @@ func testTestVectors(t *testing.T, scalarMult func(dst, scalar, point *[32]byte)
 
 func TestScalarMult(t *testing.T) {
 	t.Run("voi", testScalarMult)
-	if xcurveFaster {
-		t.Run("voi/debugNoXcurve", func(t *testing.T) {
-			debugNoXcurve = true
-			defer func() {
-				debugNoXcurve = false
-			}()
-			testScalarMult(t)
-		})
-	}
 }
 
 func testScalarMult(t *testing.T) {
@@ -245,15 +227,6 @@ func benchScalarBaseMult(b *testing.B, scalarBaseMult func(dst, scalar *[32]byte
 
 func BenchmarkScalarMult(b *testing.B) {
 	b.Run("voi", func(b *testing.B) { benchScalarMult(b, ScalarMult) })
-	if xcurveFaster {
-		b.Run("voi/debugNoXcurve", func(b *testing.B) {
-			debugNoXcurve = true
-			defer func() {
-				debugNoXcurve = false
-			}()
-			benchScalarMult(b, ScalarMult)
-		})
-	}
 	b.Run("xcrypto", func(b *testing.B) {
 		benchScalarMult(b, xcurve.ScalarMult) //nolint:staticcheck
 	})