github.com/koomox/wireguard-go@v0.0.0-20230722134753-17a50b2f22a3/replay/replay.go (about)

     1  /* SPDX-License-Identifier: MIT
     2   *
     3   * Copyright (C) 2017-2023 WireGuard LLC. All Rights Reserved.
     4   */
     5  
     6  // Package replay implements an efficient anti-replay algorithm as specified in RFC 6479.
     7  package replay
     8  
     9  type block uint64
    10  
    11  const (
    12  	blockBitLog = 6                // 1<<6 == 64 bits
    13  	blockBits   = 1 << blockBitLog // must be power of 2
    14  	ringBlocks  = 1 << 7           // must be power of 2
    15  	windowSize  = (ringBlocks - 1) * blockBits
    16  	blockMask   = ringBlocks - 1
    17  	bitMask     = blockBits - 1
    18  )
    19  
    20  // A Filter rejects replayed messages by checking if message counter value is
    21  // within a sliding window of previously received messages.
    22  // The zero value for Filter is an empty filter ready to use.
    23  // Filters are unsafe for concurrent use.
    24  type Filter struct {
    25  	last uint64
    26  	ring [ringBlocks]block
    27  }
    28  
    29  // Reset resets the filter to empty state.
    30  func (f *Filter) Reset() {
    31  	f.last = 0
    32  	f.ring[0] = 0
    33  }
    34  
    35  // ValidateCounter checks if the counter should be accepted.
    36  // Overlimit counters (>= limit) are always rejected.
    37  func (f *Filter) ValidateCounter(counter, limit uint64) bool {
    38  	if counter >= limit {
    39  		return false
    40  	}
    41  	indexBlock := counter >> blockBitLog
    42  	if counter > f.last { // move window forward
    43  		current := f.last >> blockBitLog
    44  		diff := indexBlock - current
    45  		if diff > ringBlocks {
    46  			diff = ringBlocks // cap diff to clear the whole ring
    47  		}
    48  		for i := current + 1; i <= current+diff; i++ {
    49  			f.ring[i&blockMask] = 0
    50  		}
    51  		f.last = counter
    52  	} else if f.last-counter > windowSize { // behind current window
    53  		return false
    54  	}
    55  	// check and set bit
    56  	indexBlock &= blockMask
    57  	indexBit := counter & bitMask
    58  	old := f.ring[indexBlock]
    59  	new := old | 1<<indexBit
    60  	f.ring[indexBlock] = new
    61  	return old != new
    62  }