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-rw-r--r--src/crypto/subtle/constant_time.go62
1 files changed, 62 insertions, 0 deletions
diff --git a/src/crypto/subtle/constant_time.go b/src/crypto/subtle/constant_time.go
new file mode 100644
index 0000000..4e0527f
--- /dev/null
+++ b/src/crypto/subtle/constant_time.go
@@ -0,0 +1,62 @@
+// 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 implements functions that are often useful in cryptographic
+// code but require careful thought to use correctly.
+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)
+}