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/*
* Copyright (c) 2014, Yawning Angel <yawning at torproject dot org>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* * Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
// Package replayfilter implements a generic replay detection filter with a
// caller specifiable time-to-live. It only detects if a given byte sequence
// has been seen before based on the SipHash-2-4 digest of the sequence.
// Collisions are treated as positive matches, though the probability of this
// happening is negligible.
package replayfilter
import (
"container/list"
"encoding/binary"
"sync"
"time"
"github.com/dchest/siphash"
"github.com/OperatorFoundation/obfs4/common/csrand"
)
// maxFilterSize is the maximum capacity of a replay filter. This value is
// more as a safeguard to prevent runaway filter growth, and is sized to be
// serveral orders of magnitude greater than the number of connections a busy
// bridge sees in one day, so in practice should never be reached.
const maxFilterSize = 100 * 1024
type entry struct {
digest uint64
firstSeen time.Time
element *list.Element
}
// ReplayFilter is a simple filter designed only to detect if a given byte
// sequence has been seen before.
type ReplayFilter struct {
sync.Mutex
filter map[uint64]*entry
fifo *list.List
key [2]uint64
ttl time.Duration
}
// New creates a new ReplayFilter instance.
func New(ttl time.Duration) (filter *ReplayFilter, err error) {
// Initialize the SipHash-2-4 instance with a random key.
var key [16]byte
if err = csrand.Bytes(key[:]); err != nil {
return
}
filter = new(ReplayFilter)
filter.filter = make(map[uint64]*entry)
filter.fifo = list.New()
filter.key[0] = binary.BigEndian.Uint64(key[0:8])
filter.key[1] = binary.BigEndian.Uint64(key[8:16])
filter.ttl = ttl
return
}
// TestAndSet queries the filter for a given byte sequence, inserts the
// sequence, and returns if it was present before the insertion operation.
func (f *ReplayFilter) TestAndSet(now time.Time, buf []byte) bool {
digest := siphash.Hash(f.key[0], f.key[1], buf)
f.Lock()
defer f.Unlock()
f.compactFilter(now)
if e := f.filter[digest]; e != nil {
// Hit. Just return.
return true
}
// Miss. Add a new entry.
e := new(entry)
e.digest = digest
e.firstSeen = now
e.element = f.fifo.PushBack(e)
f.filter[digest] = e
return false
}
func (f *ReplayFilter) compactFilter(now time.Time) {
e := f.fifo.Front()
for e != nil {
ent, _ := e.Value.(*entry)
// If the filter is not full, only purge entries that exceed the TTL,
// otherwise purge at least one entry, then revert to TTL based
// compaction.
if f.fifo.Len() < maxFilterSize && f.ttl > 0 {
deltaT := now.Sub(ent.firstSeen)
if deltaT < 0 {
// Aeeeeeee, the system time jumped backwards, potentially by
// a lot. This will eventually self-correct, but "eventually"
// could be a long time. As much as this sucks, jettison the
// entire filter.
f.reset()
return
} else if deltaT < f.ttl {
return
}
}
// Remove the eldest entry.
eNext := e.Next()
delete(f.filter, ent.digest)
f.fifo.Remove(ent.element)
ent.element = nil
e = eNext
}
}
func (f *ReplayFilter) reset() {
f.filter = make(map[uint64]*entry)
f.fifo = list.New()
}
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