crypto/internal/fips/subtle: move constant time functions from crypto/subtle

Change-Id: I267a3cac168fc0366fafac4c26e6a80ca545436a
Reviewed-on: https://go-review.googlesource.com/c/go/+/624755
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>

1 files changed, 60 insertions, 0 deletions

diff --git a/src/crypto/internal/fips/subtle/constant_time.go b/src/crypto/internal/fips/subtle/constant_time.go
new file mode 100644
index 0000000000..9fd3923e76
--- /dev/null
+++ b/src/crypto/internal/fips/subtle/constant_time.go
@@ -0,0 +1,60 @@
+// 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
+
+// ConstantTimeCompare returns 1 if the two slices, x and y, have equal contents
+// and 0 otherwise. The time taken is a function of the length of the slices and
+// is independent of the contents. If the lengths of x and y do not match it
+// returns 0 immediately.
+func ConstantTimeCompare(x, y []byte) int {
+	if len(x) != len(y) {
+		return 0
+	}
+
+	var v byte
+
+	for i := 0; i < len(x); i++ {
+		v |= x[i] ^ y[i]
+	}
+
+	return ConstantTimeByteEq(v, 0)
+}
+
+// ConstantTimeSelect returns x if v == 1 and y if v == 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 == y and 0 otherwise.
+func ConstantTimeByteEq(x, y uint8) int {
+	return int((uint32(x^y) - 1) >> 31)
+}
+
+// ConstantTimeEq returns 1 if x == y and 0 otherwise.
+func ConstantTimeEq(x, y int32) int {
+	return int((uint64(uint32(x^y)) - 1) >> 63)
+}
+
+// ConstantTimeCopy copies the contents of y into x (a slice of equal length)
+// if 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) {
+	if len(x) != len(y) {
+		panic("subtle: slices have different lengths")
+	}
+
+	xmask := byte(v - 1)
+	ymask := byte(^(v - 1))
+	for i := 0; i < len(x); i++ {
+		x[i] = x[i]&xmask | y[i]&ymask
+	}
+}
+
+// ConstantTimeLessOrEq returns 1 if x <= y and 0 otherwise.
+// Its behavior is undefined if x or y are negative or > 2**31 - 1.
+func ConstantTimeLessOrEq(x, y int) int {
+	x32 := int32(x)
+	y32 := int32(y)
+	return int(((x32 - y32 - 1) >> 31) & 1)
+}