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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 obfs4
import (
"encoding/binary"
"fmt"
"github.com/yawning/obfs4/framing"
)
const (
packetOverhead = 2 + 1
maxPacketPayloadLength = framing.MaximumFramePayloadLength - packetOverhead
maxPacketPaddingLength = maxPacketPayloadLength
)
const (
packetTypePayload = iota
packetTypePrngSeed
)
// InvalidPacketLengthError is the error returned when decodePacket detects a
// invalid packet length/
type InvalidPacketLengthError int
func (e InvalidPacketLengthError) Error() string {
return fmt.Sprintf("packet: Invalid packet length: %d", int(e))
}
// InvalidPayloadLengthError is the error returned when decodePacket rejects the
// payload length.
type InvalidPayloadLengthError int
func (e InvalidPayloadLengthError) Error() string {
return fmt.Sprintf("packet: Invalid payload length: %d", int(e))
}
var zeroPadBytes [maxPacketPaddingLength]byte
func makePacket(pkt []byte, pktType uint8, data []byte, padLen uint16) int {
pktLen := packetOverhead + len(data) + int(padLen)
if len(data)+int(padLen) > maxPacketPayloadLength {
panic(fmt.Sprintf("BUG: makePacket() len(data) + padLen > maxPacketPayloadLength: %d + %d > %d",
len(data), padLen, maxPacketPayloadLength))
}
// Packets are:
// uint8_t type packetTypePayload (0x00)
// uint16_t length Length of the payload (Big Endian).
// uint8_t[] payload Data payload.
// uint8_t[] padding Padding.
pkt[0] = pktType
binary.BigEndian.PutUint16(pkt[1:], uint16(len(data)))
copy(pkt[3:], data[:])
copy(pkt[3+len(data):], zeroPadBytes[:padLen])
return pktLen
}
func (c *Obfs4Conn) decodePacket(pkt []byte) error {
if len(pkt) < packetOverhead {
return InvalidPacketLengthError(len(pkt))
}
pktType := pkt[0]
payloadLen := binary.BigEndian.Uint16(pkt[1:])
if int(payloadLen) > len(pkt)-packetOverhead {
return InvalidPayloadLengthError(int(payloadLen))
}
payload := pkt[3 : 3+payloadLen]
switch pktType {
case packetTypePayload:
// packetTypePayload
c.receiveDecodedBuffer.Write(payload)
default:
// Ignore unrecognised packet types.
}
return nil
}
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