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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 (
"bytes"
"errors"
"fmt"
"io"
)
type authResult int
const (
authFailure authResult = iota
authPartialSuccess
authSuccess
)
// clientAuthenticate authenticates with the remote server. See RFC 4252.
func (c *connection) clientAuthenticate(config *ClientConfig) error {
// initiate user auth session
if err := c.transport.writePacket(Marshal(&serviceRequestMsg{serviceUserAuth})); err != nil {
return err
}
packet, err := c.transport.readPacket()
if err != nil {
return err
}
var serviceAccept serviceAcceptMsg
if err := Unmarshal(packet, &serviceAccept); err != nil {
return err
}
// during the authentication phase the client first attempts the "none" method
// then any untried methods suggested by the server.
tried := make(map[string]bool)
var lastMethods []string
sessionID := c.transport.getSessionID()
for auth := AuthMethod(new(noneAuth)); auth != nil; {
ok, methods, err := auth.auth(sessionID, config.User, c.transport, config.Rand)
if err != nil {
return err
}
if ok == authSuccess {
// success
return nil
} else if ok == authFailure {
tried[auth.method()] = true
}
if methods == nil {
methods = lastMethods
}
lastMethods = methods
auth = nil
findNext:
for _, a := range config.Auth {
candidateMethod := a.method()
if tried[candidateMethod] {
continue
}
for _, meth := range methods {
if meth == candidateMethod {
auth = a
break findNext
}
}
}
}
return fmt.Errorf("ssh: unable to authenticate, attempted methods %v, no supported methods remain", keys(tried))
}
func keys(m map[string]bool) []string {
s := make([]string, 0, len(m))
for key := range m {
s = append(s, key)
}
return s
}
// An AuthMethod represents an instance of an RFC 4252 authentication method.
type AuthMethod interface {
// auth authenticates user over transport t.
// Returns true if authentication is successful.
// If authentication is not successful, a []string of alternative
// method names is returned. If the slice is nil, it will be ignored
// and the previous set of possible methods will be reused.
auth(session []byte, user string, p packetConn, rand io.Reader) (authResult, []string, error)
// method returns the RFC 4252 method name.
method() string
}
// "none" authentication, RFC 4252 section 5.2.
type noneAuth int
func (n *noneAuth) auth(session []byte, user string, c packetConn, rand io.Reader) (authResult, []string, error) {
if err := c.writePacket(Marshal(&userAuthRequestMsg{
User: user,
Service: serviceSSH,
Method: "none",
})); err != nil {
return authFailure, nil, err
}
return handleAuthResponse(c)
}
func (n *noneAuth) method() string {
return "none"
}
// passwordCallback is an AuthMethod that fetches the password through
// a function call, e.g. by prompting the user.
type passwordCallback func() (password string, err error)
func (cb passwordCallback) auth(session []byte, user string, c packetConn, rand io.Reader) (authResult, []string, error) {
type passwordAuthMsg struct {
User string `sshtype:"50"`
Service string
Method string
Reply bool
Password string
}
pw, err := cb()
// REVIEW NOTE: is there a need to support skipping a password attempt?
// The program may only find out that the user doesn't have a password
// when prompting.
if err != nil {
return authFailure, nil, err
}
if err := c.writePacket(Marshal(&passwordAuthMsg{
User: user,
Service: serviceSSH,
Method: cb.method(),
Reply: false,
Password: pw,
})); err != nil {
return authFailure, nil, err
}
return handleAuthResponse(c)
}
func (cb passwordCallback) method() string {
return "password"
}
// Password returns an AuthMethod using the given password.
func Password(secret string) AuthMethod {
return passwordCallback(func() (string, error) { return secret, nil })
}
// PasswordCallback returns an AuthMethod that uses a callback for
// fetching a password.
func PasswordCallback(prompt func() (secret string, err error)) AuthMethod {
return passwordCallback(prompt)
}
type publickeyAuthMsg struct {
User string `sshtype:"50"`
Service string
Method string
// HasSig indicates to the receiver packet that the auth request is signed and
// should be used for authentication of the request.
HasSig bool
Algoname string
PubKey []byte
// Sig is tagged with "rest" so Marshal will exclude it during
// validateKey
Sig []byte `ssh:"rest"`
}
// publicKeyCallback is an AuthMethod that uses a set of key
// pairs for authentication.
type publicKeyCallback func() ([]Signer, error)
func (cb publicKeyCallback) method() string {
return "publickey"
}
func (cb publicKeyCallback) auth(session []byte, user string, c packetConn, rand io.Reader) (authResult, []string, error) {
// Authentication is performed by sending an enquiry to test if a key is
// acceptable to the remote. If the key is acceptable, the client will
// attempt to authenticate with the valid key. If not the client will repeat
// the process with the remaining keys.
signers, err := cb()
if err != nil {
return authFailure, nil, err
}
var methods []string
for _, signer := range signers {
ok, err := validateKey(signer.PublicKey(), user, c)
if err != nil {
return authFailure, nil, err
}
if !ok {
continue
}
pub := signer.PublicKey()
pubKey := pub.Marshal()
sign, err := signer.Sign(rand, buildDataSignedForAuth(session, userAuthRequestMsg{
User: user,
Service: serviceSSH,
Method: cb.method(),
}, []byte(pub.Type()), pubKey))
if err != nil {
return authFailure, nil, err
}
// manually wrap the serialized signature in a string
s := Marshal(sign)
sig := make([]byte, stringLength(len(s)))
marshalString(sig, s)
msg := publickeyAuthMsg{
User: user,
Service: serviceSSH,
Method: cb.method(),
HasSig: true,
Algoname: pub.Type(),
PubKey: pubKey,
Sig: sig,
}
p := Marshal(&msg)
if err := c.writePacket(p); err != nil {
return authFailure, nil, err
}
var success authResult
success, methods, err = handleAuthResponse(c)
if err != nil {
return authFailure, nil, err
}
// If authentication succeeds or the list of available methods does not
// contain the "publickey" method, do not attempt to authenticate with any
// other keys. According to RFC 4252 Section 7, the latter can occur when
// additional authentication methods are required.
if success == authSuccess || !containsMethod(methods, cb.method()) {
return success, methods, err
}
}
return authFailure, methods, nil
}
func containsMethod(methods []string, method string) bool {
for _, m := range methods {
if m == method {
return true
}
}
return false
}
// validateKey validates the key provided is acceptable to the server.
func validateKey(key PublicKey, user string, c packetConn) (bool, error) {
pubKey := key.Marshal()
msg := publickeyAuthMsg{
User: user,
Service: serviceSSH,
Method: "publickey",
HasSig: false,
Algoname: key.Type(),
PubKey: pubKey,
}
if err := c.writePacket(Marshal(&msg)); err != nil {
return false, err
}
return confirmKeyAck(key, c)
}
func confirmKeyAck(key PublicKey, c packetConn) (bool, error) {
pubKey := key.Marshal()
algoname := key.Type()
for {
packet, err := c.readPacket()
if err != nil {
return false, err
}
switch packet[0] {
case msgUserAuthBanner:
if err := handleBannerResponse(c, packet); err != nil {
return false, err
}
case msgUserAuthPubKeyOk:
var msg userAuthPubKeyOkMsg
if err := Unmarshal(packet, &msg); err != nil {
return false, err
}
if msg.Algo != algoname || !bytes.Equal(msg.PubKey, pubKey) {
return false, nil
}
return true, nil
case msgUserAuthFailure:
return false, nil
default:
return false, unexpectedMessageError(msgUserAuthSuccess, packet[0])
}
}
}
// PublicKeys returns an AuthMethod that uses the given key
// pairs.
func PublicKeys(signers ...Signer) AuthMethod {
return publicKeyCallback(func() ([]Signer, error) { return signers, nil })
}
// PublicKeysCallback returns an AuthMethod that runs the given
// function to obtain a list of key pairs.
func PublicKeysCallback(getSigners func() (signers []Signer, err error)) AuthMethod {
return publicKeyCallback(getSigners)
}
// handleAuthResponse returns whether the preceding authentication request succeeded
// along with a list of remaining authentication methods to try next and
// an error if an unexpected response was received.
func handleAuthResponse(c packetConn) (authResult, []string, error) {
for {
packet, err := c.readPacket()
if err != nil {
return authFailure, nil, err
}
switch packet[0] {
case msgUserAuthBanner:
if err := handleBannerResponse(c, packet); err != nil {
return authFailure, nil, err
}
case msgUserAuthFailure:
var msg userAuthFailureMsg
if err := Unmarshal(packet, &msg); err != nil {
return authFailure, nil, err
}
if msg.PartialSuccess {
return authPartialSuccess, msg.Methods, nil
}
return authFailure, msg.Methods, nil
case msgUserAuthSuccess:
return authSuccess, nil, nil
default:
return authFailure, nil, unexpectedMessageError(msgUserAuthSuccess, packet[0])
}
}
}
func handleBannerResponse(c packetConn, packet []byte) error {
var msg userAuthBannerMsg
if err := Unmarshal(packet, &msg); err != nil {
return err
}
transport, ok := c.(*handshakeTransport)
if !ok {
return nil
}
if transport.bannerCallback != nil {
return transport.bannerCallback(msg.Message)
}
return nil
}
// KeyboardInteractiveChallenge should print questions, optionally
// disabling echoing (e.g. for passwords), and return all the answers.
// Challenge may be called multiple times in a single session. After
// successful authentication, the server may send a challenge with no
// questions, for which the user and instruction messages should be
// printed. RFC 4256 section 3.3 details how the UI should behave for
// both CLI and GUI environments.
type KeyboardInteractiveChallenge func(user, instruction string, questions []string, echos []bool) (answers []string, err error)
// KeyboardInteractive returns an AuthMethod using a prompt/response
// sequence controlled by the server.
func KeyboardInteractive(challenge KeyboardInteractiveChallenge) AuthMethod {
return challenge
}
func (cb KeyboardInteractiveChallenge) method() string {
return "keyboard-interactive"
}
func (cb KeyboardInteractiveChallenge) auth(session []byte, user string, c packetConn, rand io.Reader) (authResult, []string, error) {
type initiateMsg struct {
User string `sshtype:"50"`
Service string
Method string
Language string
Submethods string
}
if err := c.writePacket(Marshal(&initiateMsg{
User: user,
Service: serviceSSH,
Method: "keyboard-interactive",
})); err != nil {
return authFailure, nil, err
}
for {
packet, err := c.readPacket()
if err != nil {
return authFailure, nil, err
}
// like handleAuthResponse, but with less options.
switch packet[0] {
case msgUserAuthBanner:
if err := handleBannerResponse(c, packet); err != nil {
return authFailure, nil, err
}
continue
case msgUserAuthInfoRequest:
// OK
case msgUserAuthFailure:
var msg userAuthFailureMsg
if err := Unmarshal(packet, &msg); err != nil {
return authFailure, nil, err
}
if msg.PartialSuccess {
return authPartialSuccess, msg.Methods, nil
}
return authFailure, msg.Methods, nil
case msgUserAuthSuccess:
return authSuccess, nil, nil
default:
return authFailure, nil, unexpectedMessageError(msgUserAuthInfoRequest, packet[0])
}
var msg userAuthInfoRequestMsg
if err := Unmarshal(packet, &msg); err != nil {
return authFailure, nil, err
}
// Manually unpack the prompt/echo pairs.
rest := msg.Prompts
var prompts []string
var echos []bool
for i := 0; i < int(msg.NumPrompts); i++ {
prompt, r, ok := parseString(rest)
if !ok || len(r) == 0 {
return authFailure, nil, errors.New("ssh: prompt format error")
}
prompts = append(prompts, string(prompt))
echos = append(echos, r[0] != 0)
rest = r[1:]
}
if len(rest) != 0 {
return authFailure, nil, errors.New("ssh: extra data following keyboard-interactive pairs")
}
answers, err := cb(msg.User, msg.Instruction, prompts, echos)
if err != nil {
return authFailure, nil, err
}
if len(answers) != len(prompts) {
return authFailure, nil, errors.New("ssh: not enough answers from keyboard-interactive callback")
}
responseLength := 1 + 4
for _, a := range answers {
responseLength += stringLength(len(a))
}
serialized := make([]byte, responseLength)
p := serialized
p[0] = msgUserAuthInfoResponse
p = p[1:]
p = marshalUint32(p, uint32(len(answers)))
for _, a := range answers {
p = marshalString(p, []byte(a))
}
if err := c.writePacket(serialized); err != nil {
return authFailure, nil, err
}
}
}
type retryableAuthMethod struct {
authMethod AuthMethod
maxTries int
}
func (r *retryableAuthMethod) auth(session []byte, user string, c packetConn, rand io.Reader) (ok authResult, methods []string, err error) {
for i := 0; r.maxTries <= 0 || i < r.maxTries; i++ {
ok, methods, err = r.authMethod.auth(session, user, c, rand)
if ok != authFailure || err != nil { // either success, partial success or error terminate
return ok, methods, err
}
}
return ok, methods, err
}
func (r *retryableAuthMethod) method() string {
return r.authMethod.method()
}
// RetryableAuthMethod is a decorator for other auth methods enabling them to
// be retried up to maxTries before considering that AuthMethod itself failed.
// If maxTries is <= 0, will retry indefinitely
//
// This is useful for interactive clients using challenge/response type
// authentication (e.g. Keyboard-Interactive, Password, etc) where the user
// could mistype their response resulting in the server issuing a
// SSH_MSG_USERAUTH_FAILURE (rfc4252 #8 [password] and rfc4256 #3.4
// [keyboard-interactive]); Without this decorator, the non-retryable
// AuthMethod would be removed from future consideration, and never tried again
// (and so the user would never be able to retry their entry).
func RetryableAuthMethod(auth AuthMethod, maxTries int) AuthMethod {
return &retryableAuthMethod{authMethod: auth, maxTries: maxTries}
}