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- // Copyright 2011 The Go Authors. All rights reserved.
- // Use of this source code is governed by a BSD-style
- // license that can be found in the LICENSE file.
- package ssh
- import (
- "crypto/aes"
- "crypto/cipher"
- "crypto/des"
- "crypto/rc4"
- "crypto/subtle"
- "encoding/binary"
- "errors"
- "fmt"
- "hash"
- "io"
- "io/ioutil"
- )
- const (
- packetSizeMultiple = 16 // TODO(huin) this should be determined by the cipher.
- // RFC 4253 section 6.1 defines a minimum packet size of 32768 that implementations
- // MUST be able to process (plus a few more kilobytes for padding and mac). The RFC
- // indicates implementations SHOULD be able to handle larger packet sizes, but then
- // waffles on about reasonable limits.
- //
- // OpenSSH caps their maxPacket at 256kB so we choose to do
- // the same. maxPacket is also used to ensure that uint32
- // length fields do not overflow, so it should remain well
- // below 4G.
- maxPacket = 256 * 1024
- )
- // noneCipher implements cipher.Stream and provides no encryption. It is used
- // by the transport before the first key-exchange.
- type noneCipher struct{}
- func (c noneCipher) XORKeyStream(dst, src []byte) {
- copy(dst, src)
- }
- func newAESCTR(key, iv []byte) (cipher.Stream, error) {
- c, err := aes.NewCipher(key)
- if err != nil {
- return nil, err
- }
- return cipher.NewCTR(c, iv), nil
- }
- func newRC4(key, iv []byte) (cipher.Stream, error) {
- return rc4.NewCipher(key)
- }
- type streamCipherMode struct {
- keySize int
- ivSize int
- skip int
- createFunc func(key, iv []byte) (cipher.Stream, error)
- }
- func (c *streamCipherMode) createStream(key, iv []byte) (cipher.Stream, error) {
- if len(key) < c.keySize {
- panic("ssh: key length too small for cipher")
- }
- if len(iv) < c.ivSize {
- panic("ssh: iv too small for cipher")
- }
- stream, err := c.createFunc(key[:c.keySize], iv[:c.ivSize])
- if err != nil {
- return nil, err
- }
- var streamDump []byte
- if c.skip > 0 {
- streamDump = make([]byte, 512)
- }
- for remainingToDump := c.skip; remainingToDump > 0; {
- dumpThisTime := remainingToDump
- if dumpThisTime > len(streamDump) {
- dumpThisTime = len(streamDump)
- }
- stream.XORKeyStream(streamDump[:dumpThisTime], streamDump[:dumpThisTime])
- remainingToDump -= dumpThisTime
- }
- return stream, nil
- }
- // cipherModes documents properties of supported ciphers. Ciphers not included
- // are not supported and will not be negotiated, even if explicitly requested in
- // ClientConfig.Crypto.Ciphers.
- var cipherModes = map[string]*streamCipherMode{
- // Ciphers from RFC4344, which introduced many CTR-based ciphers. Algorithms
- // are defined in the order specified in the RFC.
- "aes128-ctr": {16, aes.BlockSize, 0, newAESCTR},
- "aes192-ctr": {24, aes.BlockSize, 0, newAESCTR},
- "aes256-ctr": {32, aes.BlockSize, 0, newAESCTR},
- // Ciphers from RFC4345, which introduces security-improved arcfour ciphers.
- // They are defined in the order specified in the RFC.
- "arcfour128": {16, 0, 1536, newRC4},
- "arcfour256": {32, 0, 1536, newRC4},
- // Cipher defined in RFC 4253, which describes SSH Transport Layer Protocol.
- // Note that this cipher is not safe, as stated in RFC 4253: "Arcfour (and
- // RC4) has problems with weak keys, and should be used with caution."
- // RFC4345 introduces improved versions of Arcfour.
- "arcfour": {16, 0, 0, newRC4},
- // AES-GCM is not a stream cipher, so it is constructed with a
- // special case. If we add any more non-stream ciphers, we
- // should invest a cleaner way to do this.
- gcmCipherID: {16, 12, 0, nil},
- // CBC mode is insecure and so is not included in the default config.
- // (See http://www.isg.rhul.ac.uk/~kp/SandPfinal.pdf). If absolutely
- // needed, it's possible to specify a custom Config to enable it.
- // You should expect that an active attacker can recover plaintext if
- // you do.
- aes128cbcID: {16, aes.BlockSize, 0, nil},
- // 3des-cbc is insecure and is disabled by default.
- tripledescbcID: {24, des.BlockSize, 0, nil},
- }
- // prefixLen is the length of the packet prefix that contains the packet length
- // and number of padding bytes.
- const prefixLen = 5
- // streamPacketCipher is a packetCipher using a stream cipher.
- type streamPacketCipher struct {
- mac hash.Hash
- cipher cipher.Stream
- // The following members are to avoid per-packet allocations.
- prefix [prefixLen]byte
- seqNumBytes [4]byte
- padding [2 * packetSizeMultiple]byte
- packetData []byte
- macResult []byte
- }
- // readPacket reads and decrypt a single packet from the reader argument.
- func (s *streamPacketCipher) readPacket(seqNum uint32, r io.Reader) ([]byte, error) {
- if _, err := io.ReadFull(r, s.prefix[:]); err != nil {
- return nil, err
- }
- s.cipher.XORKeyStream(s.prefix[:], s.prefix[:])
- length := binary.BigEndian.Uint32(s.prefix[0:4])
- paddingLength := uint32(s.prefix[4])
- var macSize uint32
- if s.mac != nil {
- s.mac.Reset()
- binary.BigEndian.PutUint32(s.seqNumBytes[:], seqNum)
- s.mac.Write(s.seqNumBytes[:])
- s.mac.Write(s.prefix[:])
- macSize = uint32(s.mac.Size())
- }
- if length <= paddingLength+1 {
- return nil, errors.New("ssh: invalid packet length, packet too small")
- }
- if length > maxPacket {
- return nil, errors.New("ssh: invalid packet length, packet too large")
- }
- // the maxPacket check above ensures that length-1+macSize
- // does not overflow.
- if uint32(cap(s.packetData)) < length-1+macSize {
- s.packetData = make([]byte, length-1+macSize)
- } else {
- s.packetData = s.packetData[:length-1+macSize]
- }
- if _, err := io.ReadFull(r, s.packetData); err != nil {
- return nil, err
- }
- mac := s.packetData[length-1:]
- data := s.packetData[:length-1]
- s.cipher.XORKeyStream(data, data)
- if s.mac != nil {
- s.mac.Write(data)
- s.macResult = s.mac.Sum(s.macResult[:0])
- if subtle.ConstantTimeCompare(s.macResult, mac) != 1 {
- return nil, errors.New("ssh: MAC failure")
- }
- }
- return s.packetData[:length-paddingLength-1], nil
- }
- // writePacket encrypts and sends a packet of data to the writer argument
- func (s *streamPacketCipher) writePacket(seqNum uint32, w io.Writer, rand io.Reader, packet []byte) error {
- if len(packet) > maxPacket {
- return errors.New("ssh: packet too large")
- }
- paddingLength := packetSizeMultiple - (prefixLen+len(packet))%packetSizeMultiple
- if paddingLength < 4 {
- paddingLength += packetSizeMultiple
- }
- length := len(packet) + 1 + paddingLength
- binary.BigEndian.PutUint32(s.prefix[:], uint32(length))
- s.prefix[4] = byte(paddingLength)
- padding := s.padding[:paddingLength]
- if _, err := io.ReadFull(rand, padding); err != nil {
- return err
- }
- if s.mac != nil {
- s.mac.Reset()
- binary.BigEndian.PutUint32(s.seqNumBytes[:], seqNum)
- s.mac.Write(s.seqNumBytes[:])
- s.mac.Write(s.prefix[:])
- s.mac.Write(packet)
- s.mac.Write(padding)
- }
- s.cipher.XORKeyStream(s.prefix[:], s.prefix[:])
- s.cipher.XORKeyStream(packet, packet)
- s.cipher.XORKeyStream(padding, padding)
- if _, err := w.Write(s.prefix[:]); err != nil {
- return err
- }
- if _, err := w.Write(packet); err != nil {
- return err
- }
- if _, err := w.Write(padding); err != nil {
- return err
- }
- if s.mac != nil {
- s.macResult = s.mac.Sum(s.macResult[:0])
- if _, err := w.Write(s.macResult); err != nil {
- return err
- }
- }
- return nil
- }
- type gcmCipher struct {
- aead cipher.AEAD
- prefix [4]byte
- iv []byte
- buf []byte
- }
- func newGCMCipher(iv, key, macKey []byte) (packetCipher, error) {
- c, err := aes.NewCipher(key)
- if err != nil {
- return nil, err
- }
- aead, err := cipher.NewGCM(c)
- if err != nil {
- return nil, err
- }
- return &gcmCipher{
- aead: aead,
- iv: iv,
- }, nil
- }
- const gcmTagSize = 16
- func (c *gcmCipher) writePacket(seqNum uint32, w io.Writer, rand io.Reader, packet []byte) error {
- // Pad out to multiple of 16 bytes. This is different from the
- // stream cipher because that encrypts the length too.
- padding := byte(packetSizeMultiple - (1+len(packet))%packetSizeMultiple)
- if padding < 4 {
- padding += packetSizeMultiple
- }
- length := uint32(len(packet) + int(padding) + 1)
- binary.BigEndian.PutUint32(c.prefix[:], length)
- if _, err := w.Write(c.prefix[:]); err != nil {
- return err
- }
- if cap(c.buf) < int(length) {
- c.buf = make([]byte, length)
- } else {
- c.buf = c.buf[:length]
- }
- c.buf[0] = padding
- copy(c.buf[1:], packet)
- if _, err := io.ReadFull(rand, c.buf[1+len(packet):]); err != nil {
- return err
- }
- c.buf = c.aead.Seal(c.buf[:0], c.iv, c.buf, c.prefix[:])
- if _, err := w.Write(c.buf); err != nil {
- return err
- }
- c.incIV()
- return nil
- }
- func (c *gcmCipher) incIV() {
- for i := 4 + 7; i >= 4; i-- {
- c.iv[i]++
- if c.iv[i] != 0 {
- break
- }
- }
- }
- func (c *gcmCipher) readPacket(seqNum uint32, r io.Reader) ([]byte, error) {
- if _, err := io.ReadFull(r, c.prefix[:]); err != nil {
- return nil, err
- }
- length := binary.BigEndian.Uint32(c.prefix[:])
- if length > maxPacket {
- return nil, errors.New("ssh: max packet length exceeded.")
- }
- if cap(c.buf) < int(length+gcmTagSize) {
- c.buf = make([]byte, length+gcmTagSize)
- } else {
- c.buf = c.buf[:length+gcmTagSize]
- }
- if _, err := io.ReadFull(r, c.buf); err != nil {
- return nil, err
- }
- plain, err := c.aead.Open(c.buf[:0], c.iv, c.buf, c.prefix[:])
- if err != nil {
- return nil, err
- }
- c.incIV()
- padding := plain[0]
- if padding < 4 || padding >= 20 {
- return nil, fmt.Errorf("ssh: illegal padding %d", padding)
- }
- if int(padding+1) >= len(plain) {
- return nil, fmt.Errorf("ssh: padding %d too large", padding)
- }
- plain = plain[1 : length-uint32(padding)]
- return plain, nil
- }
- // cbcCipher implements aes128-cbc cipher defined in RFC 4253 section 6.1
- type cbcCipher struct {
- mac hash.Hash
- macSize uint32
- decrypter cipher.BlockMode
- encrypter cipher.BlockMode
- // The following members are to avoid per-packet allocations.
- seqNumBytes [4]byte
- packetData []byte
- macResult []byte
- // Amount of data we should still read to hide which
- // verification error triggered.
- oracleCamouflage uint32
- }
- func newCBCCipher(c cipher.Block, iv, key, macKey []byte, algs directionAlgorithms) (packetCipher, error) {
- cbc := &cbcCipher{
- mac: macModes[algs.MAC].new(macKey),
- decrypter: cipher.NewCBCDecrypter(c, iv),
- encrypter: cipher.NewCBCEncrypter(c, iv),
- packetData: make([]byte, 1024),
- }
- if cbc.mac != nil {
- cbc.macSize = uint32(cbc.mac.Size())
- }
- return cbc, nil
- }
- func newAESCBCCipher(iv, key, macKey []byte, algs directionAlgorithms) (packetCipher, error) {
- c, err := aes.NewCipher(key)
- if err != nil {
- return nil, err
- }
- cbc, err := newCBCCipher(c, iv, key, macKey, algs)
- if err != nil {
- return nil, err
- }
- return cbc, nil
- }
- func newTripleDESCBCCipher(iv, key, macKey []byte, algs directionAlgorithms) (packetCipher, error) {
- c, err := des.NewTripleDESCipher(key)
- if err != nil {
- return nil, err
- }
- cbc, err := newCBCCipher(c, iv, key, macKey, algs)
- if err != nil {
- return nil, err
- }
- return cbc, nil
- }
- func maxUInt32(a, b int) uint32 {
- if a > b {
- return uint32(a)
- }
- return uint32(b)
- }
- const (
- cbcMinPacketSizeMultiple = 8
- cbcMinPacketSize = 16
- cbcMinPaddingSize = 4
- )
- // cbcError represents a verification error that may leak information.
- type cbcError string
- func (e cbcError) Error() string { return string(e) }
- func (c *cbcCipher) readPacket(seqNum uint32, r io.Reader) ([]byte, error) {
- p, err := c.readPacketLeaky(seqNum, r)
- if err != nil {
- if _, ok := err.(cbcError); ok {
- // Verification error: read a fixed amount of
- // data, to make distinguishing between
- // failing MAC and failing length check more
- // difficult.
- io.CopyN(ioutil.Discard, r, int64(c.oracleCamouflage))
- }
- }
- return p, err
- }
- func (c *cbcCipher) readPacketLeaky(seqNum uint32, r io.Reader) ([]byte, error) {
- blockSize := c.decrypter.BlockSize()
- // Read the header, which will include some of the subsequent data in the
- // case of block ciphers - this is copied back to the payload later.
- // How many bytes of payload/padding will be read with this first read.
- firstBlockLength := uint32((prefixLen + blockSize - 1) / blockSize * blockSize)
- firstBlock := c.packetData[:firstBlockLength]
- if _, err := io.ReadFull(r, firstBlock); err != nil {
- return nil, err
- }
- c.oracleCamouflage = maxPacket + 4 + c.macSize - firstBlockLength
- c.decrypter.CryptBlocks(firstBlock, firstBlock)
- length := binary.BigEndian.Uint32(firstBlock[:4])
- if length > maxPacket {
- return nil, cbcError("ssh: packet too large")
- }
- if length+4 < maxUInt32(cbcMinPacketSize, blockSize) {
- // The minimum size of a packet is 16 (or the cipher block size, whichever
- // is larger) bytes.
- return nil, cbcError("ssh: packet too small")
- }
- // The length of the packet (including the length field but not the MAC) must
- // be a multiple of the block size or 8, whichever is larger.
- if (length+4)%maxUInt32(cbcMinPacketSizeMultiple, blockSize) != 0 {
- return nil, cbcError("ssh: invalid packet length multiple")
- }
- paddingLength := uint32(firstBlock[4])
- if paddingLength < cbcMinPaddingSize || length <= paddingLength+1 {
- return nil, cbcError("ssh: invalid packet length")
- }
- // Positions within the c.packetData buffer:
- macStart := 4 + length
- paddingStart := macStart - paddingLength
- // Entire packet size, starting before length, ending at end of mac.
- entirePacketSize := macStart + c.macSize
- // Ensure c.packetData is large enough for the entire packet data.
- if uint32(cap(c.packetData)) < entirePacketSize {
- // Still need to upsize and copy, but this should be rare at runtime, only
- // on upsizing the packetData buffer.
- c.packetData = make([]byte, entirePacketSize)
- copy(c.packetData, firstBlock)
- } else {
- c.packetData = c.packetData[:entirePacketSize]
- }
- if n, err := io.ReadFull(r, c.packetData[firstBlockLength:]); err != nil {
- return nil, err
- } else {
- c.oracleCamouflage -= uint32(n)
- }
- remainingCrypted := c.packetData[firstBlockLength:macStart]
- c.decrypter.CryptBlocks(remainingCrypted, remainingCrypted)
- mac := c.packetData[macStart:]
- if c.mac != nil {
- c.mac.Reset()
- binary.BigEndian.PutUint32(c.seqNumBytes[:], seqNum)
- c.mac.Write(c.seqNumBytes[:])
- c.mac.Write(c.packetData[:macStart])
- c.macResult = c.mac.Sum(c.macResult[:0])
- if subtle.ConstantTimeCompare(c.macResult, mac) != 1 {
- return nil, cbcError("ssh: MAC failure")
- }
- }
- return c.packetData[prefixLen:paddingStart], nil
- }
- func (c *cbcCipher) writePacket(seqNum uint32, w io.Writer, rand io.Reader, packet []byte) error {
- effectiveBlockSize := maxUInt32(cbcMinPacketSizeMultiple, c.encrypter.BlockSize())
- // Length of encrypted portion of the packet (header, payload, padding).
- // Enforce minimum padding and packet size.
- encLength := maxUInt32(prefixLen+len(packet)+cbcMinPaddingSize, cbcMinPaddingSize)
- // Enforce block size.
- encLength = (encLength + effectiveBlockSize - 1) / effectiveBlockSize * effectiveBlockSize
- length := encLength - 4
- paddingLength := int(length) - (1 + len(packet))
- // Overall buffer contains: header, payload, padding, mac.
- // Space for the MAC is reserved in the capacity but not the slice length.
- bufferSize := encLength + c.macSize
- if uint32(cap(c.packetData)) < bufferSize {
- c.packetData = make([]byte, encLength, bufferSize)
- } else {
- c.packetData = c.packetData[:encLength]
- }
- p := c.packetData
- // Packet header.
- binary.BigEndian.PutUint32(p, length)
- p = p[4:]
- p[0] = byte(paddingLength)
- // Payload.
- p = p[1:]
- copy(p, packet)
- // Padding.
- p = p[len(packet):]
- if _, err := io.ReadFull(rand, p); err != nil {
- return err
- }
- if c.mac != nil {
- c.mac.Reset()
- binary.BigEndian.PutUint32(c.seqNumBytes[:], seqNum)
- c.mac.Write(c.seqNumBytes[:])
- c.mac.Write(c.packetData)
- // The MAC is now appended into the capacity reserved for it earlier.
- c.packetData = c.mac.Sum(c.packetData)
- }
- c.encrypter.CryptBlocks(c.packetData[:encLength], c.packetData[:encLength])
- if _, err := w.Write(c.packetData); err != nil {
- return err
- }
- return nil
- }
|