344 lines
8.8 KiB
Go
344 lines
8.8 KiB
Go
// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package ssh
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import (
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"crypto/aes"
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"crypto/cipher"
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"crypto/rc4"
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"crypto/subtle"
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"encoding/binary"
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"errors"
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"fmt"
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"hash"
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"io"
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)
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const (
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packetSizeMultiple = 16 // TODO(huin) this should be determined by the cipher.
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// RFC 4253 section 6.1 defines a minimum packet size of 32768 that implementations
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// MUST be able to process (plus a few more kilobytes for padding and mac). The RFC
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// indicates implementations SHOULD be able to handle larger packet sizes, but then
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// waffles on about reasonable limits.
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//
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// OpenSSH caps their maxPacket at 256kB so we choose to do
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// the same. maxPacket is also used to ensure that uint32
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// length fields do not overflow, so it should remain well
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// below 4G.
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maxPacket = 256 * 1024
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)
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// noneCipher implements cipher.Stream and provides no encryption. It is used
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// by the transport before the first key-exchange.
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type noneCipher struct{}
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func (c noneCipher) XORKeyStream(dst, src []byte) {
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copy(dst, src)
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}
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func newAESCTR(key, iv []byte) (cipher.Stream, error) {
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c, err := aes.NewCipher(key)
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if err != nil {
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return nil, err
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}
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return cipher.NewCTR(c, iv), nil
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}
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func newRC4(key, iv []byte) (cipher.Stream, error) {
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return rc4.NewCipher(key)
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}
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type streamCipherMode struct {
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keySize int
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ivSize int
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skip int
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createFunc func(key, iv []byte) (cipher.Stream, error)
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}
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func (c *streamCipherMode) createStream(key, iv []byte) (cipher.Stream, error) {
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if len(key) < c.keySize {
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panic("ssh: key length too small for cipher")
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}
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if len(iv) < c.ivSize {
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panic("ssh: iv too small for cipher")
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}
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stream, err := c.createFunc(key[:c.keySize], iv[:c.ivSize])
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if err != nil {
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return nil, err
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}
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var streamDump []byte
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if c.skip > 0 {
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streamDump = make([]byte, 512)
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}
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for remainingToDump := c.skip; remainingToDump > 0; {
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dumpThisTime := remainingToDump
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if dumpThisTime > len(streamDump) {
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dumpThisTime = len(streamDump)
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}
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stream.XORKeyStream(streamDump[:dumpThisTime], streamDump[:dumpThisTime])
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remainingToDump -= dumpThisTime
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}
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return stream, nil
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}
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// cipherModes documents properties of supported ciphers. Ciphers not included
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// are not supported and will not be negotiated, even if explicitly requested in
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// ClientConfig.Crypto.Ciphers.
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var cipherModes = map[string]*streamCipherMode{
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// Ciphers from RFC4344, which introduced many CTR-based ciphers. Algorithms
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// are defined in the order specified in the RFC.
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"aes128-ctr": {16, aes.BlockSize, 0, newAESCTR},
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"aes192-ctr": {24, aes.BlockSize, 0, newAESCTR},
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"aes256-ctr": {32, aes.BlockSize, 0, newAESCTR},
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// Ciphers from RFC4345, which introduces security-improved arcfour ciphers.
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// They are defined in the order specified in the RFC.
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"arcfour128": {16, 0, 1536, newRC4},
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"arcfour256": {32, 0, 1536, newRC4},
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// Cipher defined in RFC 4253, which describes SSH Transport Layer Protocol.
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// Note that this cipher is not safe, as stated in RFC 4253: "Arcfour (and
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// RC4) has problems with weak keys, and should be used with caution."
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// RFC4345 introduces improved versions of Arcfour.
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"arcfour": {16, 0, 0, newRC4},
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// AES-GCM is not a stream cipher, so it is constructed with a
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// special case. If we add any more non-stream ciphers, we
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// should invest a cleaner way to do this.
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gcmCipherID: {16, 12, 0, nil},
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}
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// prefixLen is the length of the packet prefix that contains the packet length
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// and number of padding bytes.
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const prefixLen = 5
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// streamPacketCipher is a packetCipher using a stream cipher.
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type streamPacketCipher struct {
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mac hash.Hash
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cipher cipher.Stream
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// The following members are to avoid per-packet allocations.
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prefix [prefixLen]byte
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seqNumBytes [4]byte
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padding [2 * packetSizeMultiple]byte
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packetData []byte
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macResult []byte
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}
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// readPacket reads and decrypt a single packet from the reader argument.
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func (s *streamPacketCipher) readPacket(seqNum uint32, r io.Reader) ([]byte, error) {
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if _, err := io.ReadFull(r, s.prefix[:]); err != nil {
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return nil, err
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}
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s.cipher.XORKeyStream(s.prefix[:], s.prefix[:])
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length := binary.BigEndian.Uint32(s.prefix[0:4])
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paddingLength := uint32(s.prefix[4])
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var macSize uint32
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if s.mac != nil {
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s.mac.Reset()
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binary.BigEndian.PutUint32(s.seqNumBytes[:], seqNum)
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s.mac.Write(s.seqNumBytes[:])
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s.mac.Write(s.prefix[:])
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macSize = uint32(s.mac.Size())
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}
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if length <= paddingLength+1 {
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return nil, errors.New("ssh: invalid packet length, packet too small")
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}
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if length > maxPacket {
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return nil, errors.New("ssh: invalid packet length, packet too large")
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}
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// the maxPacket check above ensures that length-1+macSize
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// does not overflow.
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if uint32(cap(s.packetData)) < length-1+macSize {
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s.packetData = make([]byte, length-1+macSize)
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} else {
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s.packetData = s.packetData[:length-1+macSize]
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}
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if _, err := io.ReadFull(r, s.packetData); err != nil {
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return nil, err
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}
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mac := s.packetData[length-1:]
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data := s.packetData[:length-1]
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s.cipher.XORKeyStream(data, data)
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if s.mac != nil {
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s.mac.Write(data)
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s.macResult = s.mac.Sum(s.macResult[:0])
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if subtle.ConstantTimeCompare(s.macResult, mac) != 1 {
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return nil, errors.New("ssh: MAC failure")
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}
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}
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return s.packetData[:length-paddingLength-1], nil
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}
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// writePacket encrypts and sends a packet of data to the writer argument
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func (s *streamPacketCipher) writePacket(seqNum uint32, w io.Writer, rand io.Reader, packet []byte) error {
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if len(packet) > maxPacket {
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return errors.New("ssh: packet too large")
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}
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paddingLength := packetSizeMultiple - (prefixLen+len(packet))%packetSizeMultiple
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if paddingLength < 4 {
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paddingLength += packetSizeMultiple
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}
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length := len(packet) + 1 + paddingLength
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binary.BigEndian.PutUint32(s.prefix[:], uint32(length))
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s.prefix[4] = byte(paddingLength)
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padding := s.padding[:paddingLength]
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if _, err := io.ReadFull(rand, padding); err != nil {
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return err
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}
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if s.mac != nil {
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s.mac.Reset()
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binary.BigEndian.PutUint32(s.seqNumBytes[:], seqNum)
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s.mac.Write(s.seqNumBytes[:])
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s.mac.Write(s.prefix[:])
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s.mac.Write(packet)
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s.mac.Write(padding)
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}
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s.cipher.XORKeyStream(s.prefix[:], s.prefix[:])
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s.cipher.XORKeyStream(packet, packet)
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s.cipher.XORKeyStream(padding, padding)
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if _, err := w.Write(s.prefix[:]); err != nil {
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return err
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}
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if _, err := w.Write(packet); err != nil {
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return err
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}
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if _, err := w.Write(padding); err != nil {
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return err
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}
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if s.mac != nil {
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s.macResult = s.mac.Sum(s.macResult[:0])
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if _, err := w.Write(s.macResult); err != nil {
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return err
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}
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}
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return nil
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}
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type gcmCipher struct {
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aead cipher.AEAD
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prefix [4]byte
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iv []byte
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buf []byte
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}
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func newGCMCipher(iv, key, macKey []byte) (packetCipher, error) {
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c, err := aes.NewCipher(key)
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if err != nil {
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return nil, err
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}
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aead, err := cipher.NewGCM(c)
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if err != nil {
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return nil, err
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}
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return &gcmCipher{
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aead: aead,
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iv: iv,
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}, nil
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}
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const gcmTagSize = 16
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func (c *gcmCipher) writePacket(seqNum uint32, w io.Writer, rand io.Reader, packet []byte) error {
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// Pad out to multiple of 16 bytes. This is different from the
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// stream cipher because that encrypts the length too.
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padding := byte(packetSizeMultiple - (1+len(packet))%packetSizeMultiple)
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if padding < 4 {
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padding += packetSizeMultiple
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}
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length := uint32(len(packet) + int(padding) + 1)
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binary.BigEndian.PutUint32(c.prefix[:], length)
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if _, err := w.Write(c.prefix[:]); err != nil {
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return err
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}
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if cap(c.buf) < int(length) {
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c.buf = make([]byte, length)
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} else {
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c.buf = c.buf[:length]
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}
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c.buf[0] = padding
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copy(c.buf[1:], packet)
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if _, err := io.ReadFull(rand, c.buf[1+len(packet):]); err != nil {
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return err
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}
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c.buf = c.aead.Seal(c.buf[:0], c.iv, c.buf, c.prefix[:])
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if _, err := w.Write(c.buf); err != nil {
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return err
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}
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c.incIV()
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return nil
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}
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func (c *gcmCipher) incIV() {
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for i := 4 + 7; i >= 4; i-- {
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c.iv[i]++
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if c.iv[i] != 0 {
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break
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}
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}
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}
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func (c *gcmCipher) readPacket(seqNum uint32, r io.Reader) ([]byte, error) {
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if _, err := io.ReadFull(r, c.prefix[:]); err != nil {
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return nil, err
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}
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length := binary.BigEndian.Uint32(c.prefix[:])
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if length > maxPacket {
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return nil, errors.New("ssh: max packet length exceeded.")
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}
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if cap(c.buf) < int(length+gcmTagSize) {
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c.buf = make([]byte, length+gcmTagSize)
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} else {
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c.buf = c.buf[:length+gcmTagSize]
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}
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if _, err := io.ReadFull(r, c.buf); err != nil {
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return nil, err
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}
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plain, err := c.aead.Open(c.buf[:0], c.iv, c.buf, c.prefix[:])
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if err != nil {
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return nil, err
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}
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c.incIV()
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padding := plain[0]
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if padding < 4 || padding >= 20 {
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return nil, fmt.Errorf("ssh: illegal padding %d", padding)
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}
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if int(padding+1) >= len(plain) {
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return nil, fmt.Errorf("ssh: padding %d too large", padding)
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}
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plain = plain[1 : length-uint32(padding)]
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return plain, nil
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}
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