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/* SPDX-License-Identifier: MIT
*
* Copyright (C) 2017-2021 WireGuard LLC. All Rights Reserved.
*/
// Package replay implements an efficient anti-replay algorithm as specified in RFC 6479.
package replay
type block uint64
const (
blockBitLog = 6 // 1<<6 == 64 bits
blockBits = 1 << blockBitLog // must be power of 2
ringBlocks = 1 << 7 // must be power of 2
windowSize = (ringBlocks - 1) * blockBits
blockMask = ringBlocks - 1
bitMask = blockBits - 1
)
// A Filter rejects replayed messages by checking if message counter value is
// within a sliding window of previously received messages.
// The zero value for Filter is an empty filter ready to use.
// Filters are unsafe for concurrent use.
type Filter struct {
last uint64
ring [ringBlocks]block
}
// Reset resets the filter to empty state.
func (f *Filter) Reset() {
f.last = 0
f.ring[0] = 0
}
// ValidateCounter checks if the counter should be accepted.
// Overlimit counters (>= limit) are always rejected.
func (f *Filter) ValidateCounter(counter, limit uint64) bool {
if counter >= limit {
return false
}
indexBlock := counter >> blockBitLog
if counter > f.last { // move window forward
current := f.last >> blockBitLog
diff := indexBlock - current
if diff > ringBlocks {
diff = ringBlocks // cap diff to clear the whole ring
}
for i := current + 1; i <= current+diff; i++ {
f.ring[i&blockMask] = 0
}
f.last = counter
} else if f.last-counter > windowSize { // behind current window
return false
}
// check and set bit
indexBlock &= blockMask
indexBit := counter & bitMask
old := f.ring[indexBlock]
new := old | 1<<indexBit
f.ring[indexBlock] = new
return old != new
}
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