Split constant time functions into crypto/subtle.

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

8 files changed, 249 insertions, 216 deletions

diff --git a/src/pkg/Make.deps b/src/pkg/Make.deps
index 5ef36f5e2..614f48b66 100644
--- a/src/pkg/Make.deps
+++ b/src/pkg/Make.deps
@@ -17,6 +17,7 @@ crypto/hmac.install: crypto/md5.install crypto/sha1.install hash.install os.inst
 crypto/md5.install: hash.install os.install
 crypto/rc4.install: os.install strconv.install
 crypto/sha1.install: hash.install os.install
+crypto/subtle.install:
 debug/dwarf.install: encoding/binary.install os.install strconv.install
 debug/macho.install: bytes.install debug/dwarf.install encoding/binary.install fmt.install io.install os.install strconv.install
 debug/elf.install: debug/dwarf.install encoding/binary.install fmt.install io.install os.install strconv.install
diff --git a/src/pkg/Makefile b/src/pkg/Makefile
index 61bd325db..baf2122c8 100644
--- a/src/pkg/Makefile
+++ b/src/pkg/Makefile
@@ -31,6 +31,7 @@ DIRS=\
 	crypto/md5\
 	crypto/rc4\
 	crypto/sha1\
+	crypto/subtle\
 	debug/dwarf\
 	debug/macho\
 	debug/elf\
diff --git a/src/pkg/crypto/rsa/pkcs1v15.go b/src/pkg/crypto/rsa/pkcs1v15.go
index 9fb4584fe..2583f1911 100644
--- a/src/pkg/crypto/rsa/pkcs1v15.go
+++ b/src/pkg/crypto/rsa/pkcs1v15.go
@@ -6,6 +6,7 @@ package rsa
 
 import (
 	"bytes";
+	"crypto/subtle";
 	big "gmp";
 	"io";
 	"os";
@@ -27,7 +28,7 @@ func EncryptPKCS1v15(rand io.Reader, pub *PublicKey, msg []byte) (out []byte, er
 	// EM = 0x02 || PS || 0x00 || M
 	em := make([]byte, k-1);
 	em[0] = 2;
-	ps, mm := em[1:len(em)-len(msg)-1], em[len(em)-len(msg):len(em)];
+	ps, mm := em[1 : len(em)-len(msg)-1], em[len(em)-len(msg) : len(em)];
 	err = nonZeroRandomBytes(ps, rand);
 	if err != nil {
 		return;
@@ -77,8 +78,8 @@ func DecryptPKCS1v15SessionKey(rand io.Reader, priv *PrivateKey, ciphertext []by
 		return;
 	}
 
-	valid &= constantTimeEq(int32(len(msg)), int32(len(key)));
-	constantTimeCopy(valid, key, msg);
+	valid &= subtle.ConstantTimeEq(int32(len(msg)), int32(len(key)));
+	subtle.ConstantTimeCopy(valid, key, msg);
 	return;
 }
 
@@ -96,8 +97,8 @@ func decryptPKCS1v15(rand io.Reader, priv *PrivateKey, ciphertext []byte) (valid
 	}
 
 	em := leftPad(m.Bytes(), k);
-	firstByteIsZero := constantTimeByteEq(em[0], 0);
-	secondByteIsTwo := constantTimeByteEq(em[1], 2);
+	firstByteIsZero := subtle.ConstantTimeByteEq(em[0], 0);
+	secondByteIsTwo := subtle.ConstantTimeByteEq(em[1], 2);
 
 	// The remainder of the plaintext must be a string of non-zero random
 	// octets, followed by a 0, followed by the message.
@@ -107,9 +108,9 @@ func decryptPKCS1v15(rand io.Reader, priv *PrivateKey, ciphertext []byte) (valid
 	lookingForIndex = 1;
 
 	for i := 2; i < len(em); i++ {
-		equals0 := constantTimeByteEq(em[i], 0);
-		index = constantTimeSelect(lookingForIndex & equals0, i, index);
-		lookingForIndex = constantTimeSelect(equals0, 0, lookingForIndex);
+		equals0 := subtle.ConstantTimeByteEq(em[i], 0);
+		index = subtle.ConstantTimeSelect(lookingForIndex & equals0, i, index);
+		lookingForIndex = subtle.ConstantTimeSelect(equals0, 0, lookingForIndex);
 	}
 
 	valid = firstByteIsZero & secondByteIsTwo & (^lookingForIndex & 1);
@@ -126,7 +127,7 @@ func nonZeroRandomBytes(s []byte, rand io.Reader) (err os.Error) {
 
 	for i := 0; i < len(s); i++ {
 		for s[i] == 0 {
-			_, err = rand.Read(s[i:i+1]);
+			_, err = rand.Read(s[i : i+1]);
 			if err != nil {
 				return;
 			}
diff --git a/src/pkg/crypto/rsa/rsa.go b/src/pkg/crypto/rsa/rsa.go
index adc9c8f02..8ca87485a 100644
--- a/src/pkg/crypto/rsa/rsa.go
+++ b/src/pkg/crypto/rsa/rsa.go
@@ -8,11 +8,12 @@ package rsa
 // TODO(agl): Add support for PSS padding.
 
 import (
-		"bytes";
-	big	"gmp";
-		"hash";
-		"io";
-		"os";
+	"bytes";
+	"crypto/subtle";
+	big "gmp";
+	"hash";
+	"io";
+	"os";
 )
 
 var bigOne = big.NewInt(1)
@@ -92,7 +93,7 @@ type PublicKey struct {
 
 // A PrivateKey represents an RSA key
 type PrivateKey struct {
-	PublicKey;	// public part.
+	PublicKey;			// public part.
 	D		*big.Int;	// private exponent
 	P, Q		*big.Int;	// prime factors of N
 }
@@ -300,58 +301,6 @@ func modInverse(a, n *big.Int) (ia *big.Int) {
 	return x;
 }
 
-// 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 a if v is 1 and b if v is 0.
-// Its behaviour is undefined if v takes any other value.
-func constantTimeSelect(v, a, b int) int {
-	return ^(v-1)&a | (v-1)&b;
-}
-
-// constantTimeByteEq returns 1 if a == b and 0 otherwise.
-func constantTimeByteEq(a, b uint8) int {
-	x := ^(a^b);
-	x &= x>>4;
-	x &= x>>2;
-	x &= x>>1;
-
-	return int(x);
-}
-
-// constantTimeEq returns 1 if a == b and 0 otherwise.
-func constantTimeEq(a, b int32) int {
-	x := ^(a^b);
-	x &= x>>16;
-	x &= x>>8;
-	x &= x>>4;
-	x &= x>>2;
-	x &= x>>1;
-
-	return int(x&1);
-}
-
-// constantTimeCopy copies the contents of y into x iff v == 1. If v == 0, x is left unchanged.
-// Its behaviour 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;
-}
-
 // decrypt performs an RSA decryption, resulting in a plaintext integer. If a
 // random source is given, RSA blinding is used.
 func decrypt(rand io.Reader, priv *PrivateKey, c *big.Int) (m *big.Int, err os.Error) {
@@ -419,7 +368,7 @@ func DecryptOAEP(hash hash.Hash, rand io.Reader, priv *PrivateKey, ciphertext []
 	// anything about this.)
 	em := leftPad(m.Bytes(), k);
 
-	firstByteIsZero := constantTimeByteEq(em[0], 0);
+	firstByteIsZero := subtle.ConstantTimeByteEq(em[0], 0);
 
 	seed := em[1 : hash.Size() + 1];
 	db := em[hash.Size() + 1 : len(em)];
@@ -433,7 +382,7 @@ func DecryptOAEP(hash hash.Hash, rand io.Reader, priv *PrivateKey, ciphertext []
 	// attacks like: J. Manger. A Chosen Ciphertext Attack on RSA Optimal
 	// Asymmetric Encryption Padding (OAEP) as Standardized in PKCS #1
 	// v2.0. In J. Kilian, editor, Advances in Cryptology.
-	lHash2Good := constantTimeCompare(lHash, lHash2);
+	lHash2Good := subtle.ConstantTimeCompare(lHash, lHash2);
 
 	// The remainder of the plaintext must be zero or more 0x00, followed
 	// by 0x01, followed by the message.
@@ -445,11 +394,11 @@ func DecryptOAEP(hash hash.Hash, rand io.Reader, priv *PrivateKey, ciphertext []
 	rest := db[hash.Size() : len(db)];
 
 	for i := 0; i < len(rest); i++ {
-		equals0 := constantTimeByteEq(rest[i], 0);
-		equals1 := constantTimeByteEq(rest[i], 1);
-		index = constantTimeSelect(lookingForIndex & equals1, i, index);
-		lookingForIndex = constantTimeSelect(equals1, 0, lookingForIndex);
-		invalid = constantTimeSelect(lookingForIndex & ^equals0, 1, invalid);
+		equals0 := subtle.ConstantTimeByteEq(rest[i], 0);
+		equals1 := subtle.ConstantTimeByteEq(rest[i], 1);
+		index = subtle.ConstantTimeSelect(lookingForIndex & equals1, i, index);
+		lookingForIndex = subtle.ConstantTimeSelect(equals1, 0, lookingForIndex);
+		invalid = subtle.ConstantTimeSelect(lookingForIndex & ^equals0, 1, invalid);
 	}
 
 	if firstByteIsZero & lHash2Good & ^invalid & ^lookingForIndex != 1 {
diff --git a/src/pkg/crypto/rsa/rsa_test.go b/src/pkg/crypto/rsa/rsa_test.go
index df0d160dd..3bdfc66a7 100644
--- a/src/pkg/crypto/rsa/rsa_test.go
+++ b/src/pkg/crypto/rsa/rsa_test.go
@@ -5,12 +5,11 @@
 package rsa
 
 import (
-		"bytes";
-		"crypto/sha1";
-	big	"gmp";
-		"os";
-		"testing";
-		"testing/quick";
+	"bytes";
+	"crypto/sha1";
+	big "gmp";
+	"os";
+	"testing";
 )
 
 func TestKeyGeneration(t *testing.T) {
@@ -109,101 +108,6 @@ func TestDecryptOAEP(t *testing.T) {
 	}
 }
 
-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);
-	}
-}
-
 // testEncryptOAEPData contains a subset of the vectors from RSA's "Test vectors for RSA-OAEP".
 var testEncryptOAEPData = []testEncryptOAEPStruct{
 	// Key 1
@@ -305,47 +209,48 @@ var testEncryptOAEPData = []testEncryptOAEPStruct{
 		65537,
 		"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",
 		[]testEncryptOAEPMessage{
-		// Example 10.1
-		testEncryptOAEPMessage{
-			[]byte{0x8b, 0xba, 0x6b, 0xf8, 0x2a, 0x6c, 0x0f, 0x86,
-				0xd5, 0xf1, 0x75, 0x6e, 0x97, 0x95, 0x68, 0x70, 0xb0,
-				0x89, 0x53, 0xb0, 0x6b, 0x4e, 0xb2, 0x05, 0xbc, 0x16,
-				0x94, 0xee,
-			},
-			[]byte{0x47, 0xe1, 0xab, 0x71, 0x19, 0xfe, 0xe5, 0x6c,
-				0x95, 0xee, 0x5e, 0xaa, 0xd8, 0x6f, 0x40, 0xd0, 0xaa,
-				0x63, 0xbd, 0x33,
-			},
-			[]byte{0x53, 0xea, 0x5d, 0xc0, 0x8c, 0xd2, 0x60, 0xfb,
-				0x3b, 0x85, 0x85, 0x67, 0x28, 0x7f, 0xa9, 0x15, 0x52,
-				0xc3, 0x0b, 0x2f, 0xeb, 0xfb, 0xa2, 0x13, 0xf0, 0xae,
-				0x87, 0x70, 0x2d, 0x06, 0x8d, 0x19, 0xba, 0xb0, 0x7f,
-				0xe5, 0x74, 0x52, 0x3d, 0xfb, 0x42, 0x13, 0x9d, 0x68,
-				0xc3, 0xc5, 0xaf, 0xee, 0xe0, 0xbf, 0xe4, 0xcb, 0x79,
-				0x69, 0xcb, 0xf3, 0x82, 0xb8, 0x04, 0xd6, 0xe6, 0x13,
-				0x96, 0x14, 0x4e, 0x2d, 0x0e, 0x60, 0x74, 0x1f, 0x89,
-				0x93, 0xc3, 0x01, 0x4b, 0x58, 0xb9, 0xb1, 0x95, 0x7a,
-				0x8b, 0xab, 0xcd, 0x23, 0xaf, 0x85, 0x4f, 0x4c, 0x35,
-				0x6f, 0xb1, 0x66, 0x2a, 0xa7, 0x2b, 0xfc, 0xc7, 0xe5,
-				0x86, 0x55, 0x9d, 0xc4, 0x28, 0x0d, 0x16, 0x0c, 0x12,
-				0x67, 0x85, 0xa7, 0x23, 0xeb, 0xee, 0xbe, 0xff, 0x71,
-				0xf1, 0x15, 0x94, 0x44, 0x0a, 0xae, 0xf8, 0x7d, 0x10,
-				0x79, 0x3a, 0x87, 0x74, 0xa2, 0x39, 0xd4, 0xa0, 0x4c,
-				0x87, 0xfe, 0x14, 0x67, 0xb9, 0xda, 0xf8, 0x52, 0x08,
-				0xec, 0x6c, 0x72, 0x55, 0x79, 0x4a, 0x96, 0xcc, 0x29,
-				0x14, 0x2f, 0x9a, 0x8b, 0xd4, 0x18, 0xe3, 0xc1, 0xfd,
-				0x67, 0x34, 0x4b, 0x0c, 0xd0, 0x82, 0x9d, 0xf3, 0xb2,
-				0xbe, 0xc6, 0x02, 0x53, 0x19, 0x62, 0x93, 0xc6, 0xb3,
-				0x4d, 0x3f, 0x75, 0xd3, 0x2f, 0x21, 0x3d, 0xd4, 0x5c,
-				0x62, 0x73, 0xd5, 0x05, 0xad, 0xf4, 0xcc, 0xed, 0x10,
-				0x57, 0xcb, 0x75, 0x8f, 0xc2, 0x6a, 0xee, 0xfa, 0x44,
-				0x12, 0x55, 0xed, 0x4e, 0x64, 0xc1, 0x99, 0xee, 0x07,
-				0x5e, 0x7f, 0x16, 0x64, 0x61, 0x82, 0xfd, 0xb4, 0x64,
-				0x73, 0x9b, 0x68, 0xab, 0x5d, 0xaf, 0xf0, 0xe6, 0x3e,
-				0x95, 0x52, 0x01, 0x68, 0x24, 0xf0, 0x54, 0xbf, 0x4d,
-				0x3c, 0x8c, 0x90, 0xa9, 0x7b, 0xb6, 0xb6, 0x55, 0x32,
-				0x84, 0xeb, 0x42, 0x9f, 0xcc,
+			// Example 10.1
+			testEncryptOAEPMessage{
+				[]byte{0x8b, 0xba, 0x6b, 0xf8, 0x2a, 0x6c, 0x0f, 0x86,
+					0xd5, 0xf1, 0x75, 0x6e, 0x97, 0x95, 0x68, 0x70, 0xb0,
+					0x89, 0x53, 0xb0, 0x6b, 0x4e, 0xb2, 0x05, 0xbc, 0x16,
+					0x94, 0xee,
+				},
+				[]byte{0x47, 0xe1, 0xab, 0x71, 0x19, 0xfe, 0xe5, 0x6c,
+					0x95, 0xee, 0x5e, 0xaa, 0xd8, 0x6f, 0x40, 0xd0, 0xaa,
+					0x63, 0xbd, 0x33,
+				},
+				[]byte{0x53, 0xea, 0x5d, 0xc0, 0x8c, 0xd2, 0x60, 0xfb,
+					0x3b, 0x85, 0x85, 0x67, 0x28, 0x7f, 0xa9, 0x15, 0x52,
+					0xc3, 0x0b, 0x2f, 0xeb, 0xfb, 0xa2, 0x13, 0xf0, 0xae,
+					0x87, 0x70, 0x2d, 0x06, 0x8d, 0x19, 0xba, 0xb0, 0x7f,
+					0xe5, 0x74, 0x52, 0x3d, 0xfb, 0x42, 0x13, 0x9d, 0x68,
+					0xc3, 0xc5, 0xaf, 0xee, 0xe0, 0xbf, 0xe4, 0xcb, 0x79,
+					0x69, 0xcb, 0xf3, 0x82, 0xb8, 0x04, 0xd6, 0xe6, 0x13,
+					0x96, 0x14, 0x4e, 0x2d, 0x0e, 0x60, 0x74, 0x1f, 0x89,
+					0x93, 0xc3, 0x01, 0x4b, 0x58, 0xb9, 0xb1, 0x95, 0x7a,
+					0x8b, 0xab, 0xcd, 0x23, 0xaf, 0x85, 0x4f, 0x4c, 0x35,
+					0x6f, 0xb1, 0x66, 0x2a, 0xa7, 0x2b, 0xfc, 0xc7, 0xe5,
+					0x86, 0x55, 0x9d, 0xc4, 0x28, 0x0d, 0x16, 0x0c, 0x12,
+					0x67, 0x85, 0xa7, 0x23, 0xeb, 0xee, 0xbe, 0xff, 0x71,
+					0xf1, 0x15, 0x94, 0x44, 0x0a, 0xae, 0xf8, 0x7d, 0x10,
+					0x79, 0x3a, 0x87, 0x74, 0xa2, 0x39, 0xd4, 0xa0, 0x4c,
+					0x87, 0xfe, 0x14, 0x67, 0xb9, 0xda, 0xf8, 0x52, 0x08,
+					0xec, 0x6c, 0x72, 0x55, 0x79, 0x4a, 0x96, 0xcc, 0x29,
+					0x14, 0x2f, 0x9a, 0x8b, 0xd4, 0x18, 0xe3, 0xc1, 0xfd,
+					0x67, 0x34, 0x4b, 0x0c, 0xd0, 0x82, 0x9d, 0xf3, 0xb2,
+					0xbe, 0xc6, 0x02, 0x53, 0x19, 0x62, 0x93, 0xc6, 0xb3,
+					0x4d, 0x3f, 0x75, 0xd3, 0x2f, 0x21, 0x3d, 0xd4, 0x5c,
+					0x62, 0x73, 0xd5, 0x05, 0xad, 0xf4, 0xcc, 0xed, 0x10,
+					0x57, 0xcb, 0x75, 0x8f, 0xc2, 0x6a, 0xee, 0xfa, 0x44,
+					0x12, 0x55, 0xed, 0x4e, 0x64, 0xc1, 0x99, 0xee, 0x07,
+					0x5e, 0x7f, 0x16, 0x64, 0x61, 0x82, 0xfd, 0xb4, 0x64,
+					0x73, 0x9b, 0x68, 0xab, 0x5d, 0xaf, 0xf0, 0xe6, 0x3e,
+					0x95, 0x52, 0x01, 0x68, 0x24, 0xf0, 0x54, 0xbf, 0x4d,
+					0x3c, 0x8c, 0x90, 0xa9, 0x7b, 0xb6, 0xb6, 0x55, 0x32,
+					0x84, 0xeb, 0x42, 0x9f, 0xcc,
+				},
 			},
-		}},
+		},
 	},
 }
diff --git a/src/pkg/crypto/subtle/Makefile b/src/pkg/crypto/subtle/Makefile
new file mode 100644
index 000000000..4d245c683
--- /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
diff --git a/src/pkg/crypto/subtle/constant_time.go b/src/pkg/crypto/subtle/constant_time.go
new file mode 100644
index 000000000..a1d2eaf99
--- /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;
+}
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 000000000..78aa6771d
--- /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);
+	}
+}