Split constant time functions into crypto/subtle.
R=rsc
CC=go-dev
http://go/go-review/1018020
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;
+}
