diff --git a/replay/replay.go b/replay/replay.go
new file mode 100644
index 0000000..9ad37be
--- /dev/null
+++ b/replay/replay.go
@@ -0,0 +1,78 @@
+package replay
+
+import (
+	"encoding/binary"
+	"errors"
+	"math/rand"
+	"sync"
+	"sync/atomic"
+)
+
+var ErrReplayedPacket = errors.New("replayed packet")
+
+const (
+	BlockBits  = 64
+	NumBlocks  = 128
+	WindowSize = (NumBlocks - 1) * BlockBits
+)
+
+type AntiReplay struct {
+	// for outbound packets
+	next uint64
+
+	// for inbound packets
+	mu     sync.Mutex
+	last   uint64
+	circle [1 << 7]uint64
+}
+
+func NewAntiReplay() *AntiReplay {
+	return &AntiReplay{next: rand.Uint64()}
+}
+
+func (a *AntiReplay) CodeLength() int {
+	return 8
+}
+
+func (a *AntiReplay) Generate([]byte) (out []byte) {
+	out = make([]byte, a.CodeLength())
+
+	s := atomic.AddUint64(&a.next, 1)
+	binary.LittleEndian.PutUint64(out, s)
+
+	return
+}
+
+func (a *AntiReplay) Verify(_, sum []byte) error {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+
+	s := binary.LittleEndian.Uint64(sum)
+
+	indexBlock := s >> 6
+	if s > a.last {
+		current := a.last >> 6
+		diff := indexBlock - current
+		if diff > NumBlocks {
+			diff = NumBlocks // number so far away the whole ring is fresh
+		}
+		for i := current; i <= current+diff; i++ {
+			a.circle[i&(NumBlocks-1)] = 0 // clear the ones skipped over
+		}
+		a.last = s
+	} else if a.last-s > WindowSize {
+		return ErrReplayedPacket
+	}
+
+	indexBlock &= NumBlocks - 1
+	indexBit := s & (BlockBits - 1)
+
+	prev := a.circle[indexBlock]
+	a.circle[indexBlock] = prev | 1<<indexBit
+
+	if prev == a.circle[indexBlock] {
+		return ErrReplayedPacket
+	}
+
+	return nil
+}