Split constant time functions into crypto/subtle.

R=rsc
CC=go-dev
http://go/go-review/1018020

src/pkg/crypto/subtle/Makefile

diff --git a/src/pkg/crypto/subtle/Makefile b/src/pkg/crypto/subtle/Makefile
new file mode 100644
index 0000000..4d245c6
--- /dev/null
+++ b/src/pkg/crypto/subtle/Makefile
@@ -0,0 +1,11 @@
+# Copyright 2009 The Go Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+include $(GOROOT)/src/Make.$(GOARCH)
+
+TARG=crypto/subtle
+GOFILES=\
+	constant_time.go\
+
+include $(GOROOT)/src/Make.pkg

src/pkg/crypto/subtle/constant_time.go

diff --git a/src/pkg/crypto/subtle/constant_time.go b/src/pkg/crypto/subtle/constant_time.go
new file mode 100644
index 0000000..a1d2eaf
--- /dev/null
+++ b/src/pkg/crypto/subtle/constant_time.go
@@ -0,0 +1,59 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This package implements functions that are often useful in cryptographic
+// code but require careful thought to use correctly.
+package subtle
+
+// ConstantTimeCompare returns 1 iff the two equal length slices, x
+// and y, have equal contents. The time taken is a function of the length of
+// the slices and is independent of the contents.
+func ConstantTimeCompare(x, y []byte) int {
+	var v byte;
+
+	for i := 0; i < len(x); i++ {
+		v |= x[i]^y[i];
+	}
+
+	return ConstantTimeByteEq(v, 0);
+}
+
+// ConstantTimeSelect returns x if v is 1 and y if v is 0.
+// Its behavior is undefined if v takes any other value.
+func ConstantTimeSelect(v, x, y int) int {
+	return ^(v-1) & x | (v-1)&y;
+}
+
+// ConstantTimeByteEq returns 1 if x == x and 0 otherwise.
+func ConstantTimeByteEq(x, y uint8) int {
+	z := ^(x^y);
+	z &= z>>4;
+	z &= z>>2;
+	z &= z>>1;
+
+	return int(z);
+}
+
+// ConstantTimeEq returns 1 if x == y and 0 otherwise.
+func ConstantTimeEq(x, y int32) int {
+	z := ^(x^y);
+	z &= z>>16;
+	z &= z>>8;
+	z &= z>>4;
+	z &= z>>2;
+	z &= z>>1;
+
+	return int(z&1);
+}
+
+// ConstantTimeCopy copies the contents of y into x iff v == 1. If v == 0, x is left unchanged.
+// Its behavior is undefined if v takes any other value.
+func ConstantTimeCopy(v int, x, y []byte) {
+	xmask := byte(v-1);
+	ymask := byte(^(v-1));
+	for i := 0; i < len(x); i++ {
+		x[i] = x[i]&xmask | y[i]&ymask;
+	}
+	return;
+}

src/pkg/crypto/subtle/constant_time_test.go

diff --git a/src/pkg/crypto/subtle/constant_time_test.go b/src/pkg/crypto/subtle/constant_time_test.go
new file mode 100644
index 0000000..78aa677
--- /dev/null
+++ b/src/pkg/crypto/subtle/constant_time_test.go
@@ -0,0 +1,106 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package subtle
+
+import (
+	"bytes";
+	"testing";
+	"testing/quick";
+)
+
+type TestConstantTimeCompareStruct struct {
+	a, b	[]byte;
+	out	int;
+}
+
+var testConstandTimeCompareData = []TestConstantTimeCompareStruct{
+	TestConstantTimeCompareStruct{[]byte{}, []byte{}, 1},
+	TestConstantTimeCompareStruct{[]byte{0x11}, []byte{0x11}, 1},
+	TestConstantTimeCompareStruct{[]byte{0x12}, []byte{0x11}, 0},
+}
+
+func TestConstantTimeCompare(t *testing.T) {
+	for i, test := range testConstandTimeCompareData {
+		if r := ConstantTimeCompare(test.a, test.b); r != test.out {
+			t.Errorf("#%d bad result (got %x, want %x)", i, r, test.out);
+		}
+	}
+}
+
+type TestConstantTimeByteEqStruct struct {
+	a, b	uint8;
+	out	int;
+}
+
+var testConstandTimeByteEqData = []TestConstantTimeByteEqStruct{
+	TestConstantTimeByteEqStruct{0, 0, 1},
+	TestConstantTimeByteEqStruct{0, 1, 0},
+	TestConstantTimeByteEqStruct{1, 0, 0},
+	TestConstantTimeByteEqStruct{0xff, 0xff, 1},
+	TestConstantTimeByteEqStruct{0xff, 0xfe, 0},
+}
+
+func byteEq(a, b uint8) int {
+	if a == b {
+		return 1;
+	}
+	return 0;
+}
+
+func TestConstantTimeByteEq(t *testing.T) {
+	for i, test := range testConstandTimeByteEqData {
+		if r := ConstantTimeByteEq(test.a, test.b); r != test.out {
+			t.Errorf("#%d bad result (got %x, want %x)", i, r, test.out);
+		}
+	}
+	err := quick.CheckEqual(ConstantTimeByteEq, byteEq, nil);
+	if err != nil {
+		t.Error(err);
+	}
+}
+
+func eq(a, b int32) int {
+	if a == b {
+		return 1;
+	}
+	return 0;
+}
+
+func TestConstantTimeEq(t *testing.T) {
+	err := quick.CheckEqual(ConstantTimeEq, eq, nil);
+	if err != nil {
+		t.Error(err);
+	}
+}
+
+func copy(v int, x, y []byte) []byte {
+	if len(x) > len(y) {
+		x = x[0:len(y)];
+	} else {
+		y = y[0:len(x)];
+	}
+	if v == 1 {
+		bytes.Copy(x, y);
+	}
+	return x;
+}
+
+func constantTimeCopyWrapper(v int, x, y []byte) []byte {
+	if len(x) > len(y) {
+		x = x[0:len(y)];
+	} else {
+		y = y[0:len(x)];
+	}
+	v &= 1;
+	ConstantTimeCopy(v, x, y);
+	return x;
+}
+
+func TestConstantTimeCopy(t *testing.T) {
+	err := quick.CheckEqual(constantTimeCopyWrapper, copy, nil);
+	if err != nil {
+		t.Error(err);
+	}
+}