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common.go
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common.go
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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"
"crypto/rand"
"fmt"
"io"
"sync"
_ "crypto/sha1"
_ "crypto/sha256"
_ "crypto/sha512"
)
// These are string constants in the SSH protocol.
const (
compressionNone = "none"
serviceUserAuth = "ssh-userauth"
serviceSSH = "ssh-connection"
)
// supportedCiphers specifies the supported ciphers in preference order.
var supportedCiphers = []string{
"aes128-ctr", "aes192-ctr", "aes256-ctr",
"aes128-gcm@openssh.com",
"arcfour256", "arcfour128",
}
// supportedKexAlgos specifies the supported key-exchange algorithms in
// preference order.
var supportedKexAlgos = []string{
// P384 and P521 are not constant-time yet, but since we don't
// reuse ephemeral keys, using them for ECDH should be OK.
kexAlgoECDH256, kexAlgoECDH384, kexAlgoECDH521,
kexAlgoDH14SHA1, kexAlgoDH1SHA1,
}
// supportedKexAlgos specifies the supported host-key algorithms (i.e. methods
// of authenticating servers) in preference order.
var supportedHostKeyAlgos = []string{
CertAlgoRSAv01, CertAlgoDSAv01, CertAlgoECDSA256v01,
CertAlgoECDSA384v01, CertAlgoECDSA521v01,
KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521,
KeyAlgoRSA, KeyAlgoDSA,
}
// supportedMACs specifies a default set of MAC algorithms in preference order.
// This is based on RFC 4253, section 6.4, but with hmac-md5 variants removed
// because they have reached the end of their useful life.
var supportedMACs = []string{
"hmac-sha1", "hmac-sha1-96",
}
var supportedCompressions = []string{compressionNone}
// hashFuncs keeps the mapping of supported algorithms to their respective
// hashes needed for signature verification.
var hashFuncs = map[string]crypto.Hash{
KeyAlgoRSA: crypto.SHA1,
KeyAlgoDSA: crypto.SHA1,
KeyAlgoECDSA256: crypto.SHA256,
KeyAlgoECDSA384: crypto.SHA384,
KeyAlgoECDSA521: crypto.SHA512,
CertAlgoRSAv01: crypto.SHA1,
CertAlgoDSAv01: crypto.SHA1,
CertAlgoECDSA256v01: crypto.SHA256,
CertAlgoECDSA384v01: crypto.SHA384,
CertAlgoECDSA521v01: crypto.SHA512,
}
// unexpectedMessageError results when the SSH message that we received didn't
// match what we wanted.
func unexpectedMessageError(expected, got uint8) error {
return fmt.Errorf("ssh: unexpected message type %d (expected %d)", got, expected)
}
// parseError results from a malformed SSH message.
func parseError(tag uint8) error {
return fmt.Errorf("ssh: parse error in message type %d", tag)
}
func findCommonAlgorithm(clientAlgos []string, serverAlgos []string) (commonAlgo string, ok bool) {
for _, clientAlgo := range clientAlgos {
for _, serverAlgo := range serverAlgos {
if clientAlgo == serverAlgo {
return clientAlgo, true
}
}
}
return
}
func findCommonCipher(clientCiphers []string, serverCiphers []string) (commonCipher string, ok bool) {
for _, clientCipher := range clientCiphers {
for _, serverCipher := range serverCiphers {
// reject the cipher if we have no cipherModes definition
if clientCipher == serverCipher && cipherModes[clientCipher] != nil {
return clientCipher, true
}
}
}
return
}
type directionAlgorithms struct {
Cipher string
MAC string
Compression string
}
type algorithms struct {
kex string
hostKey string
w directionAlgorithms
r directionAlgorithms
}
func findAgreedAlgorithms(clientKexInit, serverKexInit *kexInitMsg) (algs *algorithms) {
var ok bool
result := &algorithms{}
result.kex, ok = findCommonAlgorithm(clientKexInit.KexAlgos, serverKexInit.KexAlgos)
if !ok {
return
}
result.hostKey, ok = findCommonAlgorithm(clientKexInit.ServerHostKeyAlgos, serverKexInit.ServerHostKeyAlgos)
if !ok {
return
}
result.w.Cipher, ok = findCommonCipher(clientKexInit.CiphersClientServer, serverKexInit.CiphersClientServer)
if !ok {
return
}
result.r.Cipher, ok = findCommonCipher(clientKexInit.CiphersServerClient, serverKexInit.CiphersServerClient)
if !ok {
return
}
result.w.MAC, ok = findCommonAlgorithm(clientKexInit.MACsClientServer, serverKexInit.MACsClientServer)
if !ok {
return
}
result.r.MAC, ok = findCommonAlgorithm(clientKexInit.MACsServerClient, serverKexInit.MACsServerClient)
if !ok {
return
}
result.w.Compression, ok = findCommonAlgorithm(clientKexInit.CompressionClientServer, serverKexInit.CompressionClientServer)
if !ok {
return
}
result.r.Compression, ok = findCommonAlgorithm(clientKexInit.CompressionServerClient, serverKexInit.CompressionServerClient)
if !ok {
return
}
return result
}
// If rekeythreshold is too small, we can't make any progress sending
// stuff.
const minRekeyThreshold uint64 = 256
// Config contains configuration data common to both ServerConfig and
// ClientConfig.
type Config struct {
// Rand provides the source of entropy for cryptographic
// primitives. If Rand is nil, the cryptographic random reader
// in package crypto/rand will be used.
Rand io.Reader
// The maximum number of bytes sent or received after which a
// new key is negotiated. It must be at least 256. If
// unspecified, 1 gigabyte is used.
RekeyThreshold uint64
// The allowed key exchanges algorithms. If unspecified then a
// default set of algorithms is used.
KeyExchanges []string
// The allowed cipher algorithms. If unspecified then a sensible
// default is used.
Ciphers []string
// The allowed MAC algorithms. If unspecified then a sensible default
// is used.
MACs []string
}
// SetDefaults sets sensible values for unset fields in config. This is
// exported for testing: Configs passed to SSH functions are copied and have
// default values set automatically.
func (c *Config) SetDefaults() {
if c.Rand == nil {
c.Rand = rand.Reader
}
if c.Ciphers == nil {
c.Ciphers = supportedCiphers
}
var ciphers []string
for _, c := range c.Ciphers {
if cipherModes[c] != nil {
// reject the cipher if we have no cipherModes definition
ciphers = append(ciphers, c)
}
}
c.Ciphers = ciphers
if c.KeyExchanges == nil {
c.KeyExchanges = supportedKexAlgos
}
if c.MACs == nil {
c.MACs = supportedMACs
}
if c.RekeyThreshold == 0 {
// RFC 4253, section 9 suggests rekeying after 1G.
c.RekeyThreshold = 1 << 30
}
if c.RekeyThreshold < minRekeyThreshold {
c.RekeyThreshold = minRekeyThreshold
}
}
// buildDataSignedForAuth returns the data that is signed in order to prove
// possession of a private key. See RFC 4252, section 7.
func buildDataSignedForAuth(sessionId []byte, req userAuthRequestMsg, algo, pubKey []byte) []byte {
data := struct {
Session []byte
Type byte
User string
Service string
Method string
Sign bool
Algo []byte
PubKey []byte
}{
sessionId,
msgUserAuthRequest,
req.User,
req.Service,
req.Method,
true,
algo,
pubKey,
}
return Marshal(data)
}
func appendU16(buf []byte, n uint16) []byte {
return append(buf, byte(n>>8), byte(n))
}
func appendU32(buf []byte, n uint32) []byte {
return append(buf, byte(n>>24), byte(n>>16), byte(n>>8), byte(n))
}
func appendU64(buf []byte, n uint64) []byte {
return append(buf,
byte(n>>56), byte(n>>48), byte(n>>40), byte(n>>32),
byte(n>>24), byte(n>>16), byte(n>>8), byte(n))
}
func appendInt(buf []byte, n int) []byte {
return appendU32(buf, uint32(n))
}
func appendString(buf []byte, s string) []byte {
buf = appendU32(buf, uint32(len(s)))
buf = append(buf, s...)
return buf
}
func appendBool(buf []byte, b bool) []byte {
if b {
return append(buf, 1)
}
return append(buf, 0)
}
// newCond is a helper to hide the fact that there is no usable zero
// value for sync.Cond.
func newCond() *sync.Cond { return sync.NewCond(new(sync.Mutex)) }
// window represents the buffer available to clients
// wishing to write to a channel.
type window struct {
*sync.Cond
win uint32 // RFC 4254 5.2 says the window size can grow to 2^32-1
writeWaiters int
closed bool
}
// add adds win to the amount of window available
// for consumers.
func (w *window) add(win uint32) bool {
// a zero sized window adjust is a noop.
if win == 0 {
return true
}
w.L.Lock()
if w.win+win < win {
w.L.Unlock()
return false
}
w.win += win
// It is unusual that multiple goroutines would be attempting to reserve
// window space, but not guaranteed. Use broadcast to notify all waiters
// that additional window is available.
w.Broadcast()
w.L.Unlock()
return true
}
// close sets the window to closed, so all reservations fail
// immediately.
func (w *window) close() {
w.L.Lock()
w.closed = true
w.Broadcast()
w.L.Unlock()
}
// reserve reserves win from the available window capacity.
// If no capacity remains, reserve will block. reserve may
// return less than requested.
func (w *window) reserve(win uint32) (uint32, error) {
var err error
w.L.Lock()
w.writeWaiters++
w.Broadcast()
for w.win == 0 && !w.closed {
w.Wait()
}
w.writeWaiters--
if w.win < win {
win = w.win
}
w.win -= win
if w.closed {
err = io.EOF
}
w.L.Unlock()
return win, err
}
// waitWriterBlocked waits until some goroutine is blocked for further
// writes. It is used in tests only.
func (w *window) waitWriterBlocked() {
w.Cond.L.Lock()
for w.writeWaiters == 0 {
w.Cond.Wait()
}
w.Cond.L.Unlock()
}