forgejo/modules/crypto/ssh/cipher.go
2015-09-24 17:55:01 -04:00

550 lines
15 KiB
Go
Executable file

// 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/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},
// insecure cipher, see http://www.isg.rhul.ac.uk/~kp/SandPfinal.pdf
// uncomment below to enable it.
// aes128cbcID: {16, aes.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 newAESCBCCipher(iv, key, macKey []byte, algs directionAlgorithms) (packetCipher, error) {
c, err := aes.NewCipher(key)
if err != nil {
return nil, err
}
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 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
}