ssh/server.go - crypto - Git at Google

 // 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 (
 	"bytes"
 	"errors"
 	"fmt"
 	"io"
 	"net"
 	"strings"
 )

 // The Permissions type holds fine-grained permissions that are
 // specific to a user or a specific authentication method for a
 // user. Permissions, except for "source-address", must be enforced in
 // the server application layer, after successful authentication. The
 // Permissions are passed on in ServerConn so a server implementation
 // can honor them.
 type Permissions struct {
 	// Critical options restrict default permissions. Common
 	// restrictions are "source-address" and "force-command". If
 	// the server cannot enforce the restriction, or does not
 	// recognize it, the user should not authenticate.
 	CriticalOptions map[string]string

 	// Extensions are extra functionality that the server may
 	// offer on authenticated connections. Common extensions are
 	// "permit-agent-forwarding", "permit-X11-forwarding". Lack of
 	// support for an extension does not preclude authenticating a
 	// user.
 	Extensions map[string]string
 }

 // ServerConfig holds server specific configuration data.
 type ServerConfig struct {
 	// Config contains configuration shared between client and server.
 	Config

 	hostKeys []Signer

 	// NoClientAuth is true if clients are allowed to connect without
 	// authenticating.
 	NoClientAuth bool

 	// MaxAuthTries specifies the maximum number of authentication attempts
 	// permitted per connection. If set to a negative number, the number of
 	// attempts are unlimited. If set to zero, the number of attempts are limited
 	// to 6.
 	MaxAuthTries int

 	// PasswordCallback, if non-nil, is called when a user
 	// attempts to authenticate using a password.
 	PasswordCallback func(conn ConnMetadata, password []byte) (*Permissions, error)

 	// PublicKeyCallback, if non-nil, is called when a client attempts public
 	// key authentication. It must return true if the given public key is
 	// valid for the given user. For example, see CertChecker.Authenticate.
 	PublicKeyCallback func(conn ConnMetadata, key PublicKey) (*Permissions, error)

 	// KeyboardInteractiveCallback, if non-nil, is called when
 	// keyboard-interactive authentication is selected (RFC
 	// 4256). The client object's Challenge function should be
 	// used to query the user. The callback may offer multiple
 	// Challenge rounds. To avoid information leaks, the client
 	// should be presented a challenge even if the user is
 	// unknown.
 	KeyboardInteractiveCallback func(conn ConnMetadata, client KeyboardInteractiveChallenge) (*Permissions, error)

 	// AuthLogCallback, if non-nil, is called to log all authentication
 	// attempts.
 	AuthLogCallback func(conn ConnMetadata, method string, err error)

 	// ServerVersion is the version identification string to announce in
 	// the public handshake.
 	// If empty, a reasonable default is used.
 	// Note that RFC 4253 section 4.2 requires that this string start with
 	// "SSH-2.0-".
 	ServerVersion string
 }

 // AddHostKey adds a private key as a host key. If an existing host
 // key exists with the same algorithm, it is overwritten. Each server
 // config must have at least one host key.
 func (s *ServerConfig) AddHostKey(key Signer) {
 	for i, k := range s.hostKeys {
 		if k.PublicKey().Type() == key.PublicKey().Type() {
 			s.hostKeys[i] = key
 			return
 		}
 	}

 	s.hostKeys = append(s.hostKeys, key)
 }

 // cachedPubKey contains the results of querying whether a public key is
 // acceptable for a user.
 type cachedPubKey struct {
 	user       string
 	pubKeyData []byte
 	result     error
 	perms      *Permissions
 }

 const maxCachedPubKeys = 16

 // pubKeyCache caches tests for public keys.  Since SSH clients
 // will query whether a public key is acceptable before attempting to
 // authenticate with it, we end up with duplicate queries for public
 // key validity.  The cache only applies to a single ServerConn.
 type pubKeyCache struct {
 	keys []cachedPubKey
 }

 // get returns the result for a given user/algo/key tuple.
 func (c *pubKeyCache) get(user string, pubKeyData []byte) (cachedPubKey, bool) {
 	for _, k := range c.keys {
 		if k.user == user && bytes.Equal(k.pubKeyData, pubKeyData) {
 			return k, true
 		}
 	}
 	return cachedPubKey{}, false
 }

 // add adds the given tuple to the cache.
 func (c *pubKeyCache) add(candidate cachedPubKey) {
 	if len(c.keys) < maxCachedPubKeys {
 		c.keys = append(c.keys, candidate)
 	}
 }

 // ServerConn is an authenticated SSH connection, as seen from the
 // server
 type ServerConn struct {
 	Conn

 	// If the succeeding authentication callback returned a
 	// non-nil Permissions pointer, it is stored here.
 	Permissions *Permissions
 }

 // NewServerConn starts a new SSH server with c as the underlying
 // transport.  It starts with a handshake and, if the handshake is
 // unsuccessful, it closes the connection and returns an error.  The
 // Request and NewChannel channels must be serviced, or the connection
 // will hang.
 func NewServerConn(c net.Conn, config *ServerConfig) (*ServerConn, <-chan NewChannel, <-chan *Request, error) {
 	if config.MaxAuthTries == 0 {
 		config.MaxAuthTries = 6
 	}

 	fullConf := *config
 	fullConf.SetDefaults()
 	s := &connection{
 		sshConn: sshConn{conn: c},
 	}
 	perms, err := s.serverHandshake(&fullConf)
 	if err != nil {
 		c.Close()
 		return nil, nil, nil, err
 	}
 	return &ServerConn{s, perms}, s.mux.incomingChannels, s.mux.incomingRequests, nil
 }

 // signAndMarshal signs the data with the appropriate algorithm,
 // and serializes the result in SSH wire format.
 func signAndMarshal(k Signer, rand io.Reader, data []byte) ([]byte, error) {
 	sig, err := k.Sign(rand, data)
 	if err != nil {
 		return nil, err
 	}

 	return Marshal(sig), nil
 }

 // handshake performs key exchange and user authentication.
 func (s *connection) serverHandshake(config *ServerConfig) (*Permissions, error) {
 	if len(config.hostKeys) == 0 {
 		return nil, errors.New("ssh: server has no host keys")
 	}

 	if !config.NoClientAuth && config.PasswordCallback == nil && config.PublicKeyCallback == nil && config.KeyboardInteractiveCallback == nil {
 		return nil, errors.New("ssh: no authentication methods configured but NoClientAuth is also false")
 	}

 	if config.ServerVersion != "" {
 		s.serverVersion = []byte(config.ServerVersion)
 	} else {
 		s.serverVersion = []byte(packageVersion)
 	}
 	var err error
 	s.clientVersion, err = exchangeVersions(s.sshConn.conn, s.serverVersion)
 	if err != nil {
 		return nil, err
 	}

 	tr := newTransport(s.sshConn.conn, config.Rand, false /* not client */)
 	s.transport = newServerTransport(tr, s.clientVersion, s.serverVersion, config)

 	if err := s.transport.waitSession(); err != nil {
 		return nil, err
 	}

 	// We just did the key change, so the session ID is established.
 	s.sessionID = s.transport.getSessionID()

 	var packet []byte
 	if packet, err = s.transport.readPacket(); err != nil {
 		return nil, err
 	}

 	var serviceRequest serviceRequestMsg
 	if err = Unmarshal(packet, &serviceRequest); err != nil {
 		return nil, err
 	}
 	if serviceRequest.Service != serviceUserAuth {
 		return nil, errors.New("ssh: requested service '" + serviceRequest.Service + "' before authenticating")
 	}
 	serviceAccept := serviceAcceptMsg{
 		Service: serviceUserAuth,
 	}
 	if err := s.transport.writePacket(Marshal(&serviceAccept)); err != nil {
 		return nil, err
 	}

 	perms, err := s.serverAuthenticate(config)
 	if err != nil {
 		return nil, err
 	}
 	s.mux = newMux(s.transport)
 	return perms, err
 }

 func isAcceptableAlgo(algo string) bool {
 	switch algo {
 	case KeyAlgoRSA, KeyAlgoDSA, KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521, KeyAlgoED25519,
 		CertAlgoRSAv01, CertAlgoDSAv01, CertAlgoECDSA256v01, CertAlgoECDSA384v01, CertAlgoECDSA521v01:
 		return true
 	}
 	return false
 }

 func checkSourceAddress(addr net.Addr, sourceAddrs string) error {
 	if addr == nil {
 		return errors.New("ssh: no address known for client, but source-address match required")
 	}

 	tcpAddr, ok := addr.(*net.TCPAddr)
 	if !ok {
 		return fmt.Errorf("ssh: remote address %v is not an TCP address when checking source-address match", addr)
 	}

 	for _, sourceAddr := range strings.Split(sourceAddrs, ",") {
 		if allowedIP := net.ParseIP(sourceAddr); allowedIP != nil {
 			if allowedIP.Equal(tcpAddr.IP) {
 				return nil
 			}
 		} else {
 			_, ipNet, err := net.ParseCIDR(sourceAddr)
 			if err != nil {
 				return fmt.Errorf("ssh: error parsing source-address restriction %q: %v", sourceAddr, err)
 			}

 			if ipNet.Contains(tcpAddr.IP) {
 				return nil
 			}
 		}
 	}

 	return fmt.Errorf("ssh: remote address %v is not allowed because of source-address restriction", addr)
 }

 func (s *connection) serverAuthenticate(config *ServerConfig) (*Permissions, error) {
 	sessionID := s.transport.getSessionID()
 	var cache pubKeyCache
 	var perms *Permissions

 	authFailures := 0

 userAuthLoop:
 	for {
 		if authFailures >= config.MaxAuthTries && config.MaxAuthTries > 0 {
 			discMsg := &disconnectMsg{
 				Reason:  2,
 				Message: "too many authentication failures",
 			}

 			if err := s.transport.writePacket(Marshal(discMsg)); err != nil {
 				return nil, err
 			}

 			return nil, discMsg
 		}

 		var userAuthReq userAuthRequestMsg
 		if packet, err := s.transport.readPacket(); err != nil {
 			return nil, err
 		} else if err = Unmarshal(packet, &userAuthReq); err != nil {
 			return nil, err
 		}

 		if userAuthReq.Service != serviceSSH {
 			return nil, errors.New("ssh: client attempted to negotiate for unknown service: " + userAuthReq.Service)
 		}

 		s.user = userAuthReq.User
 		perms = nil
 		authErr := errors.New("no auth passed yet")

 		switch userAuthReq.Method {
 		case "none":
 			if config.NoClientAuth {
 				authErr = nil
 			}

 			// allow initial attempt of 'none' without penalty
 			if authFailures == 0 {
 				authFailures--
 			}
 		case "password":
 			if config.PasswordCallback == nil {
 				authErr = errors.New("ssh: password auth not configured")
 				break
 			}
 			payload := userAuthReq.Payload
 			if len(payload) < 1 || payload[0] != 0 {
 				return nil, parseError(msgUserAuthRequest)
 			}
 			payload = payload[1:]
 			password, payload, ok := parseString(payload)
 			if !ok || len(payload) > 0 {
 				return nil, parseError(msgUserAuthRequest)
 			}

 			perms, authErr = config.PasswordCallback(s, password)
 		case "keyboard-interactive":
 			if config.KeyboardInteractiveCallback == nil {
 				authErr = errors.New("ssh: keyboard-interactive auth not configubred")
 				break
 			}

 			prompter := &sshClientKeyboardInteractive{s}
 			perms, authErr = config.KeyboardInteractiveCallback(s, prompter.Challenge)
 		case "publickey":
 			if config.PublicKeyCallback == nil {
 				authErr = errors.New("ssh: publickey auth not configured")
 				break
 			}
 			payload := userAuthReq.Payload
 			if len(payload) < 1 {
 				return nil, parseError(msgUserAuthRequest)
 			}
 			isQuery := payload[0] == 0
 			payload = payload[1:]
 			algoBytes, payload, ok := parseString(payload)
 			if !ok {
 				return nil, parseError(msgUserAuthRequest)
 			}
 			algo := string(algoBytes)
 			if !isAcceptableAlgo(algo) {
 				authErr = fmt.Errorf("ssh: algorithm %q not accepted", algo)
 				break
 			}

 			pubKeyData, payload, ok := parseString(payload)
 			if !ok {
 				return nil, parseError(msgUserAuthRequest)
 			}

 			pubKey, err := ParsePublicKey(pubKeyData)
 			if err != nil {
 				return nil, err
 			}

 			candidate, ok := cache.get(s.user, pubKeyData)
 			if !ok {
 				candidate.user = s.user
 				candidate.pubKeyData = pubKeyData
 				candidate.perms, candidate.result = config.PublicKeyCallback(s, pubKey)
 				if candidate.result == nil && candidate.perms != nil && candidate.perms.CriticalOptions != nil && candidate.perms.CriticalOptions[sourceAddressCriticalOption] != "" {
 					candidate.result = checkSourceAddress(
 						s.RemoteAddr(),
 						candidate.perms.CriticalOptions[sourceAddressCriticalOption])
 				}
 				cache.add(candidate)
 			}

 			if isQuery {
 				// The client can query if the given public key
 				// would be okay.

 				if len(payload) > 0 {
 					return nil, parseError(msgUserAuthRequest)
 				}

 				if candidate.result == nil {
 					okMsg := userAuthPubKeyOkMsg{
 						Algo:   algo,
 						PubKey: pubKeyData,
 					}
 					if err = s.transport.writePacket(Marshal(&okMsg)); err != nil {
 						return nil, err
 					}
 					continue userAuthLoop
 				}
 				authErr = candidate.result
 			} else {
 				sig, payload, ok := parseSignature(payload)
 				if !ok || len(payload) > 0 {
 					return nil, parseError(msgUserAuthRequest)
 				}
 				// Ensure the public key algo and signature algo
 				// are supported.  Compare the private key
 				// algorithm name that corresponds to algo with
 				// sig.Format.  This is usually the same, but
 				// for certs, the names differ.
 				if !isAcceptableAlgo(sig.Format) {
 					break
 				}
 				signedData := buildDataSignedForAuth(sessionID, userAuthReq, algoBytes, pubKeyData)

 				if err := pubKey.Verify(signedData, sig); err != nil {
 					return nil, err
 				}

 				authErr = candidate.result
 				perms = candidate.perms
 			}
 		default:
 			authErr = fmt.Errorf("ssh: unknown method %q", userAuthReq.Method)
 		}

 		if config.AuthLogCallback != nil {
 			config.AuthLogCallback(s, userAuthReq.Method, authErr)
 		}

 		if authErr == nil {
 			break userAuthLoop
 		}

 		authFailures++

 		var failureMsg userAuthFailureMsg
 		if config.PasswordCallback != nil {
 			failureMsg.Methods = append(failureMsg.Methods, "password")
 		}
 		if config.PublicKeyCallback != nil {
 			failureMsg.Methods = append(failureMsg.Methods, "publickey")
 		}
 		if config.KeyboardInteractiveCallback != nil {
 			failureMsg.Methods = append(failureMsg.Methods, "keyboard-interactive")
 		}

 		if len(failureMsg.Methods) == 0 {
 			return nil, errors.New("ssh: no authentication methods configured but NoClientAuth is also false")
 		}

 		if err := s.transport.writePacket(Marshal(&failureMsg)); err != nil {
 			return nil, err
 		}
 	}

 	if err := s.transport.writePacket([]byte{msgUserAuthSuccess}); err != nil {
 		return nil, err
 	}
 	return perms, nil
 }

 // sshClientKeyboardInteractive implements a ClientKeyboardInteractive by
 // asking the client on the other side of a ServerConn.
 type sshClientKeyboardInteractive struct {
 	*connection
 }

 func (c *sshClientKeyboardInteractive) Challenge(user, instruction string, questions []string, echos []bool) (answers []string, err error) {
 	if len(questions) != len(echos) {
 		return nil, errors.New("ssh: echos and questions must have equal length")
 	}

 	var prompts []byte
 	for i := range questions {
 		prompts = appendString(prompts, questions[i])
 		prompts = appendBool(prompts, echos[i])
 	}

 	if err := c.transport.writePacket(Marshal(&userAuthInfoRequestMsg{
 		Instruction: instruction,
 		NumPrompts:  uint32(len(questions)),
 		Prompts:     prompts,
 	})); err != nil {
 		return nil, err
 	}

 	packet, err := c.transport.readPacket()
 	if err != nil {
 		return nil, err
 	}
 	if packet[0] != msgUserAuthInfoResponse {
 		return nil, unexpectedMessageError(msgUserAuthInfoResponse, packet[0])
 	}
 	packet = packet[1:]

 	n, packet, ok := parseUint32(packet)
 	if !ok || int(n) != len(questions) {
 		return nil, parseError(msgUserAuthInfoResponse)
 	}

 	for i := uint32(0); i < n; i++ {
 		ans, rest, ok := parseString(packet)
 		if !ok {
 			return nil, parseError(msgUserAuthInfoResponse)
 		}

 		answers = append(answers, string(ans))
 		packet = rest
 	}
 	if len(packet) != 0 {
 		return nil, errors.New("ssh: junk at end of message")
 	}

 	return answers, nil
 }
	// 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 (
	"bytes"
	"errors"
	"fmt"
	"io"
	"net"
	"strings"
	)

	// The Permissions type holds fine-grained permissions that are
	// specific to a user or a specific authentication method for a
	// user. Permissions, except for "source-address", must be enforced in
	// the server application layer, after successful authentication. The
	// Permissions are passed on in ServerConn so a server implementation
	// can honor them.
	type Permissions struct {
	// Critical options restrict default permissions. Common
	// restrictions are "source-address" and "force-command". If
	// the server cannot enforce the restriction, or does not
	// recognize it, the user should not authenticate.
	CriticalOptions map[string]string

	// Extensions are extra functionality that the server may
	// offer on authenticated connections. Common extensions are
	// "permit-agent-forwarding", "permit-X11-forwarding". Lack of
	// support for an extension does not preclude authenticating a
	// user.
	Extensions map[string]string
	}

	// ServerConfig holds server specific configuration data.
	type ServerConfig struct {
	// Config contains configuration shared between client and server.
	Config

	hostKeys []Signer

	// NoClientAuth is true if clients are allowed to connect without
	// authenticating.
	NoClientAuth bool

	// MaxAuthTries specifies the maximum number of authentication attempts
	// permitted per connection. If set to a negative number, the number of
	// attempts are unlimited. If set to zero, the number of attempts are limited
	// to 6.
	MaxAuthTries int

	// PasswordCallback, if non-nil, is called when a user
	// attempts to authenticate using a password.
	PasswordCallback func(conn ConnMetadata, password []byte) (*Permissions, error)

	// PublicKeyCallback, if non-nil, is called when a client attempts public
	// key authentication. It must return true if the given public key is
	// valid for the given user. For example, see CertChecker.Authenticate.
	PublicKeyCallback func(conn ConnMetadata, key PublicKey) (*Permissions, error)

	// KeyboardInteractiveCallback, if non-nil, is called when
	// keyboard-interactive authentication is selected (RFC
	// 4256). The client object's Challenge function should be
	// used to query the user. The callback may offer multiple
	// Challenge rounds. To avoid information leaks, the client
	// should be presented a challenge even if the user is
	// unknown.
	KeyboardInteractiveCallback func(conn ConnMetadata, client KeyboardInteractiveChallenge) (*Permissions, error)

	// AuthLogCallback, if non-nil, is called to log all authentication
	// attempts.
	AuthLogCallback func(conn ConnMetadata, method string, err error)

	// ServerVersion is the version identification string to announce in
	// the public handshake.
	// If empty, a reasonable default is used.
	// Note that RFC 4253 section 4.2 requires that this string start with
	// "SSH-2.0-".
	ServerVersion string
	}

	// AddHostKey adds a private key as a host key. If an existing host
	// key exists with the same algorithm, it is overwritten. Each server
	// config must have at least one host key.
	func (s *ServerConfig) AddHostKey(key Signer) {
	for i, k := range s.hostKeys {
	if k.PublicKey().Type() == key.PublicKey().Type() {
	s.hostKeys[i] = key
	return
	}
	}

	s.hostKeys = append(s.hostKeys, key)
	}

	// cachedPubKey contains the results of querying whether a public key is
	// acceptable for a user.
	type cachedPubKey struct {
	user string
	pubKeyData []byte
	result error
	perms *Permissions
	}

	const maxCachedPubKeys = 16

	// pubKeyCache caches tests for public keys. Since SSH clients
	// will query whether a public key is acceptable before attempting to
	// authenticate with it, we end up with duplicate queries for public
	// key validity. The cache only applies to a single ServerConn.
	type pubKeyCache struct {
	keys []cachedPubKey
	}

	// get returns the result for a given user/algo/key tuple.
	func (c *pubKeyCache) get(user string, pubKeyData []byte) (cachedPubKey, bool) {
	for _, k := range c.keys {
	if k.user == user && bytes.Equal(k.pubKeyData, pubKeyData) {
	return k, true
	}
	}
	return cachedPubKey{}, false
	}

	// add adds the given tuple to the cache.
	func (c *pubKeyCache) add(candidate cachedPubKey) {
	if len(c.keys) < maxCachedPubKeys {
	c.keys = append(c.keys, candidate)
	}
	}

	// ServerConn is an authenticated SSH connection, as seen from the
	// server
	type ServerConn struct {
	Conn

	// If the succeeding authentication callback returned a
	// non-nil Permissions pointer, it is stored here.
	Permissions *Permissions
	}

	// NewServerConn starts a new SSH server with c as the underlying
	// transport. It starts with a handshake and, if the handshake is
	// unsuccessful, it closes the connection and returns an error. The
	// Request and NewChannel channels must be serviced, or the connection
	// will hang.
	func NewServerConn(c net.Conn, config ServerConfig) (ServerConn, <-chan NewChannel, <-chan *Request, error) {
	if config.MaxAuthTries == 0 {
	config.MaxAuthTries = 6
	}

	fullConf := *config
	fullConf.SetDefaults()
	s := &connection{
	sshConn: sshConn{conn: c},
	}
	perms, err := s.serverHandshake(&fullConf)
	if err != nil {
	c.Close()
	return nil, nil, nil, err
	}
	return &ServerConn{s, perms}, s.mux.incomingChannels, s.mux.incomingRequests, nil
	}

	// signAndMarshal signs the data with the appropriate algorithm,
	// and serializes the result in SSH wire format.
	func signAndMarshal(k Signer, rand io.Reader, data []byte) ([]byte, error) {
	sig, err := k.Sign(rand, data)
	if err != nil {
	return nil, err
	}

	return Marshal(sig), nil
	}

	// handshake performs key exchange and user authentication.
	func (s connection) serverHandshake(config ServerConfig) (*Permissions, error) {
	if len(config.hostKeys) == 0 {
	return nil, errors.New("ssh: server has no host keys")
	}

	if !config.NoClientAuth && config.PasswordCallback == nil && config.PublicKeyCallback == nil && config.KeyboardInteractiveCallback == nil {
	return nil, errors.New("ssh: no authentication methods configured but NoClientAuth is also false")
	}

	if config.ServerVersion != "" {
	s.serverVersion = []byte(config.ServerVersion)
	} else {
	s.serverVersion = []byte(packageVersion)
	}
	var err error
	s.clientVersion, err = exchangeVersions(s.sshConn.conn, s.serverVersion)
	if err != nil {
	return nil, err
	}

	tr := newTransport(s.sshConn.conn, config.Rand, false /* not client */)
	s.transport = newServerTransport(tr, s.clientVersion, s.serverVersion, config)

	if err := s.transport.waitSession(); err != nil {
	return nil, err
	}

	// We just did the key change, so the session ID is established.
	s.sessionID = s.transport.getSessionID()

	var packet []byte
	if packet, err = s.transport.readPacket(); err != nil {
	return nil, err
	}

	var serviceRequest serviceRequestMsg
	if err = Unmarshal(packet, &serviceRequest); err != nil {
	return nil, err
	}
	if serviceRequest.Service != serviceUserAuth {
	return nil, errors.New("ssh: requested service '" + serviceRequest.Service + "' before authenticating")
	}
	serviceAccept := serviceAcceptMsg{
	Service: serviceUserAuth,
	}
	if err := s.transport.writePacket(Marshal(&serviceAccept)); err != nil {
	return nil, err
	}

	perms, err := s.serverAuthenticate(config)
	if err != nil {
	return nil, err
	}
	s.mux = newMux(s.transport)
	return perms, err
	}

	func isAcceptableAlgo(algo string) bool {
	switch algo {
	case KeyAlgoRSA, KeyAlgoDSA, KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521, KeyAlgoED25519,
	CertAlgoRSAv01, CertAlgoDSAv01, CertAlgoECDSA256v01, CertAlgoECDSA384v01, CertAlgoECDSA521v01:
	return true
	}
	return false
	}

	func checkSourceAddress(addr net.Addr, sourceAddrs string) error {
	if addr == nil {
	return errors.New("ssh: no address known for client, but source-address match required")
	}

	tcpAddr, ok := addr.(*net.TCPAddr)
	if !ok {
	return fmt.Errorf("ssh: remote address %v is not an TCP address when checking source-address match", addr)
	}

	for _, sourceAddr := range strings.Split(sourceAddrs, ",") {
	if allowedIP := net.ParseIP(sourceAddr); allowedIP != nil {
	if allowedIP.Equal(tcpAddr.IP) {
	return nil
	}
	} else {
	_, ipNet, err := net.ParseCIDR(sourceAddr)
	if err != nil {
	return fmt.Errorf("ssh: error parsing source-address restriction %q: %v", sourceAddr, err)
	}

	if ipNet.Contains(tcpAddr.IP) {
	return nil
	}
	}
	}

	return fmt.Errorf("ssh: remote address %v is not allowed because of source-address restriction", addr)
	}

	func (s connection) serverAuthenticate(config ServerConfig) (*Permissions, error) {
	sessionID := s.transport.getSessionID()
	var cache pubKeyCache
	var perms *Permissions

	authFailures := 0

	userAuthLoop:
	for {
	if authFailures >= config.MaxAuthTries && config.MaxAuthTries > 0 {
	discMsg := &disconnectMsg{
	Reason: 2,
	Message: "too many authentication failures",
	}

	if err := s.transport.writePacket(Marshal(discMsg)); err != nil {
	return nil, err
	}

	return nil, discMsg
	}

	var userAuthReq userAuthRequestMsg
	if packet, err := s.transport.readPacket(); err != nil {
	return nil, err
	} else if err = Unmarshal(packet, &userAuthReq); err != nil {
	return nil, err
	}

	if userAuthReq.Service != serviceSSH {
	return nil, errors.New("ssh: client attempted to negotiate for unknown service: " + userAuthReq.Service)
	}

	s.user = userAuthReq.User
	perms = nil
	authErr := errors.New("no auth passed yet")

	switch userAuthReq.Method {
	case "none":
	if config.NoClientAuth {
	authErr = nil
	}

	// allow initial attempt of 'none' without penalty
	if authFailures == 0 {
	authFailures--
	}
	case "password":
	if config.PasswordCallback == nil {
	authErr = errors.New("ssh: password auth not configured")
	break
	}
	payload := userAuthReq.Payload
	if len(payload) < 1 \|\| payload[0] != 0 {
	return nil, parseError(msgUserAuthRequest)
	}
	payload = payload[1:]
	password, payload, ok := parseString(payload)
	if !ok \|\| len(payload) > 0 {
	return nil, parseError(msgUserAuthRequest)
	}

	perms, authErr = config.PasswordCallback(s, password)
	case "keyboard-interactive":
	if config.KeyboardInteractiveCallback == nil {
	authErr = errors.New("ssh: keyboard-interactive auth not configubred")
	break
	}

	prompter := &sshClientKeyboardInteractive{s}
	perms, authErr = config.KeyboardInteractiveCallback(s, prompter.Challenge)
	case "publickey":
	if config.PublicKeyCallback == nil {
	authErr = errors.New("ssh: publickey auth not configured")
	break
	}
	payload := userAuthReq.Payload
	if len(payload) < 1 {
	return nil, parseError(msgUserAuthRequest)
	}
	isQuery := payload[0] == 0
	payload = payload[1:]
	algoBytes, payload, ok := parseString(payload)
	if !ok {
	return nil, parseError(msgUserAuthRequest)
	}
	algo := string(algoBytes)
	if !isAcceptableAlgo(algo) {
	authErr = fmt.Errorf("ssh: algorithm %q not accepted", algo)
	break
	}

	pubKeyData, payload, ok := parseString(payload)
	if !ok {
	return nil, parseError(msgUserAuthRequest)
	}

	pubKey, err := ParsePublicKey(pubKeyData)
	if err != nil {
	return nil, err
	}

	candidate, ok := cache.get(s.user, pubKeyData)
	if !ok {
	candidate.user = s.user
	candidate.pubKeyData = pubKeyData
	candidate.perms, candidate.result = config.PublicKeyCallback(s, pubKey)
	if candidate.result == nil && candidate.perms != nil && candidate.perms.CriticalOptions != nil && candidate.perms.CriticalOptions[sourceAddressCriticalOption] != "" {
	candidate.result = checkSourceAddress(
	s.RemoteAddr(),
	candidate.perms.CriticalOptions[sourceAddressCriticalOption])
	}
	cache.add(candidate)
	}

	if isQuery {
	// The client can query if the given public key
	// would be okay.

	if len(payload) > 0 {
	return nil, parseError(msgUserAuthRequest)
	}

	if candidate.result == nil {
	okMsg := userAuthPubKeyOkMsg{
	Algo: algo,
	PubKey: pubKeyData,
	}
	if err = s.transport.writePacket(Marshal(&okMsg)); err != nil {
	return nil, err
	}
	continue userAuthLoop
	}
	authErr = candidate.result
	} else {
	sig, payload, ok := parseSignature(payload)
	if !ok \|\| len(payload) > 0 {
	return nil, parseError(msgUserAuthRequest)
	}
	// Ensure the public key algo and signature algo
	// are supported. Compare the private key
	// algorithm name that corresponds to algo with
	// sig.Format. This is usually the same, but
	// for certs, the names differ.
	if !isAcceptableAlgo(sig.Format) {
	break
	}
	signedData := buildDataSignedForAuth(sessionID, userAuthReq, algoBytes, pubKeyData)

	if err := pubKey.Verify(signedData, sig); err != nil {
	return nil, err
	}

	authErr = candidate.result
	perms = candidate.perms
	}
	default:
	authErr = fmt.Errorf("ssh: unknown method %q", userAuthReq.Method)
	}

	if config.AuthLogCallback != nil {
	config.AuthLogCallback(s, userAuthReq.Method, authErr)
	}

	if authErr == nil {
	break userAuthLoop
	}

	authFailures++

	var failureMsg userAuthFailureMsg
	if config.PasswordCallback != nil {
	failureMsg.Methods = append(failureMsg.Methods, "password")
	}
	if config.PublicKeyCallback != nil {
	failureMsg.Methods = append(failureMsg.Methods, "publickey")
	}
	if config.KeyboardInteractiveCallback != nil {
	failureMsg.Methods = append(failureMsg.Methods, "keyboard-interactive")
	}

	if len(failureMsg.Methods) == 0 {
	return nil, errors.New("ssh: no authentication methods configured but NoClientAuth is also false")
	}

	if err := s.transport.writePacket(Marshal(&failureMsg)); err != nil {
	return nil, err
	}
	}

	if err := s.transport.writePacket([]byte{msgUserAuthSuccess}); err != nil {
	return nil, err
	}
	return perms, nil
	}

	// sshClientKeyboardInteractive implements a ClientKeyboardInteractive by
	// asking the client on the other side of a ServerConn.
	type sshClientKeyboardInteractive struct {
	*connection
	}

	func (c *sshClientKeyboardInteractive) Challenge(user, instruction string, questions []string, echos []bool) (answers []string, err error) {
	if len(questions) != len(echos) {
	return nil, errors.New("ssh: echos and questions must have equal length")
	}

	var prompts []byte
	for i := range questions {
	prompts = appendString(prompts, questions[i])
	prompts = appendBool(prompts, echos[i])
	}

	if err := c.transport.writePacket(Marshal(&userAuthInfoRequestMsg{
	Instruction: instruction,
	NumPrompts: uint32(len(questions)),
	Prompts: prompts,
	})); err != nil {
	return nil, err
	}

	packet, err := c.transport.readPacket()
	if err != nil {
	return nil, err
	}
	if packet[0] != msgUserAuthInfoResponse {
	return nil, unexpectedMessageError(msgUserAuthInfoResponse, packet[0])
	}
	packet = packet[1:]

	n, packet, ok := parseUint32(packet)
	if !ok \|\| int(n) != len(questions) {
	return nil, parseError(msgUserAuthInfoResponse)
	}

	for i := uint32(0); i < n; i++ {
	ans, rest, ok := parseString(packet)
	if !ok {
	return nil, parseError(msgUserAuthInfoResponse)
	}

	answers = append(answers, string(ans))
	packet = rest
	}
	if len(packet) != 0 {
	return nil, errors.New("ssh: junk at end of message")
	}

	return answers, nil
	}