1 files changed, 73 insertions, 0 deletions
diff --git a/replay.go b/replay.go
new file mode 100644
index 0000000..5d42860
--- /dev/null
+++ b/replay.go
@@ -0,0 +1,73 @@
+package main
+
+/* Copyright (C) 2015-2017 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved. */
+
+/* Implementation of RFC6479
+ * https://tools.ietf.org/html/rfc6479
+ *
+ * The implementation is not safe for concurrent use!
+ */
+
+const (
+	// See: https://golang.org/src/math/big/arith.go
+	_Wordm       = ^uintptr(0)
+	_WordLogSize = _Wordm>>8&1 + _Wordm>>16&1 + _Wordm>>32&1
+	_WordSize    = 1 << _WordLogSize
+)
+
+const (
+	CounterRedundantBitsLog = _WordLogSize + 3
+	CounterRedundantBits    = _WordSize * 8
+	CounterBitsTotal        = 2048
+	CounterWindowSize       = uint64(CounterBitsTotal - CounterRedundantBits)
+)
+
+const (
+	BacktrackWords = CounterBitsTotal / _WordSize
+)
+
+type ReplayFilter struct {
+	counter   uint64
+	backtrack [BacktrackWords]uintptr
+}
+
+func (filter *ReplayFilter) Init() {
+	filter.counter = 0
+	filter.backtrack[0] = 0
+}
+
+func (filter *ReplayFilter) ValidateCounter(counter uint64) bool {
+	if counter >= RejectAfterMessages {
+		return false
+	}
+
+	indexWord := counter >> CounterRedundantBitsLog
+
+	if counter > filter.counter {
+
+		// move window forward
+
+		current := filter.counter >> CounterRedundantBitsLog
+		diff := minUint64(indexWord-current, BacktrackWords)
+		for i := uint64(1); i <= diff; i++ {
+			filter.backtrack[(current+i)%BacktrackWords] = 0
+		}
+		filter.counter = counter
+
+	} else if filter.counter-counter > CounterWindowSize {
+
+		// behind current window
+
+		return false
+	}
+
+	indexWord %= BacktrackWords
+	indexBit := counter & uint64(CounterRedundantBits-1)
+
+	// check and set bit
+
+	oldValue := filter.backtrack[indexWord]
+	newValue := oldValue | (1 << indexBit)
+	filter.backtrack[indexWord] = newValue
+	return oldValue != newValue
+}