1// Copyright 2011 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5package ssh
6
7import (
8	"encoding/asn1"
9	"errors"
10)
11
12var krb5OID []byte
13
14func init() {
15	krb5OID, _ = asn1.Marshal(krb5Mesh)
16}
17
18// GSSAPIClient provides the API to plug-in GSSAPI authentication for client logins.
19type GSSAPIClient interface {
20	// InitSecContext initiates the establishment of a security context for GSS-API between the
21	// ssh client and ssh server. Initially the token parameter should be specified as nil.
22	// The routine may return a outputToken which should be transferred to
23	// the ssh server, where the ssh server will present it to
24	// AcceptSecContext. If no token need be sent, InitSecContext will indicate this by setting
25	// needContinue to false. To complete the context
26	// establishment, one or more reply tokens may be required from the ssh
27	// server;if so, InitSecContext will return a needContinue which is true.
28	// In this case, InitSecContext should be called again when the
29	// reply token is received from the ssh server, passing the reply
30	// token to InitSecContext via the token parameters.
31	// See RFC 2743 section 2.2.1 and RFC 4462 section 3.4.
32	InitSecContext(target string, token []byte, isGSSDelegCreds bool) (outputToken []byte, needContinue bool, err error)
33	// GetMIC generates a cryptographic MIC for the SSH2 message, and places
34	// the MIC in a token for transfer to the ssh server.
35	// The contents of the MIC field are obtained by calling GSS_GetMIC()
36	// over the following, using the GSS-API context that was just
37	// established:
38	//  string    session identifier
39	//  byte      SSH_MSG_USERAUTH_REQUEST
40	//  string    user name
41	//  string    service
42	//  string    "gssapi-with-mic"
43	// See RFC 2743 section 2.3.1 and RFC 4462 3.5.
44	GetMIC(micFiled []byte) ([]byte, error)
45	// Whenever possible, it should be possible for
46	// DeleteSecContext() calls to be successfully processed even
47	// if other calls cannot succeed, thereby enabling context-related
48	// resources to be released.
49	// In addition to deleting established security contexts,
50	// gss_delete_sec_context must also be able to delete "half-built"
51	// security contexts resulting from an incomplete sequence of
52	// InitSecContext()/AcceptSecContext() calls.
53	// See RFC 2743 section 2.2.3.
54	DeleteSecContext() error
55}
56
57// GSSAPIServer provides the API to plug in GSSAPI authentication for server logins.
58type GSSAPIServer interface {
59	// AcceptSecContext allows a remotely initiated security context between the application
60	// and a remote peer to be established by the ssh client. The routine may return a
61	// outputToken which should be transferred to the ssh client,
62	// where the ssh client will present it to InitSecContext.
63	// If no token need be sent, AcceptSecContext will indicate this
64	// by setting the needContinue to false. To
65	// complete the context establishment, one or more reply tokens may be
66	// required from the ssh client. if so, AcceptSecContext
67	// will return a needContinue which is true, in which case it
68	// should be called again when the reply token is received from the ssh
69	// client, passing the token to AcceptSecContext via the
70	// token parameters.
71	// The srcName return value is the authenticated username.
72	// See RFC 2743 section 2.2.2 and RFC 4462 section 3.4.
73	AcceptSecContext(token []byte) (outputToken []byte, srcName string, needContinue bool, err error)
74	// VerifyMIC verifies that a cryptographic MIC, contained in the token parameter,
75	// fits the supplied message is received from the ssh client.
76	// See RFC 2743 section 2.3.2.
77	VerifyMIC(micField []byte, micToken []byte) error
78	// Whenever possible, it should be possible for
79	// DeleteSecContext() calls to be successfully processed even
80	// if other calls cannot succeed, thereby enabling context-related
81	// resources to be released.
82	// In addition to deleting established security contexts,
83	// gss_delete_sec_context must also be able to delete "half-built"
84	// security contexts resulting from an incomplete sequence of
85	// InitSecContext()/AcceptSecContext() calls.
86	// See RFC 2743 section 2.2.3.
87	DeleteSecContext() error
88}
89
90var (
91	// OpenSSH supports Kerberos V5 mechanism only for GSS-API authentication,
92	// so we also support the krb5 mechanism only.
93	// See RFC 1964 section 1.
94	krb5Mesh = asn1.ObjectIdentifier{1, 2, 840, 113554, 1, 2, 2}
95)
96
97// The GSS-API authentication method is initiated when the client sends an SSH_MSG_USERAUTH_REQUEST
98// See RFC 4462 section 3.2.
99type userAuthRequestGSSAPI struct {
100	N    uint32
101	OIDS []asn1.ObjectIdentifier
102}
103
104func parseGSSAPIPayload(payload []byte) (*userAuthRequestGSSAPI, error) {
105	n, rest, ok := parseUint32(payload)
106	if !ok {
107		return nil, errors.New("parse uint32 failed")
108	}
109	s := &userAuthRequestGSSAPI{
110		N:    n,
111		OIDS: make([]asn1.ObjectIdentifier, n),
112	}
113	for i := 0; i < int(n); i++ {
114		var (
115			desiredMech []byte
116			err         error
117		)
118		desiredMech, rest, ok = parseString(rest)
119		if !ok {
120			return nil, errors.New("parse string failed")
121		}
122		if rest, err = asn1.Unmarshal(desiredMech, &s.OIDS[i]); err != nil {
123			return nil, err
124		}
125
126	}
127	return s, nil
128}
129
130// See RFC 4462 section 3.6.
131func buildMIC(sessionID string, username string, service string, authMethod string) []byte {
132	out := make([]byte, 0, 0)
133	out = appendString(out, sessionID)
134	out = append(out, msgUserAuthRequest)
135	out = appendString(out, username)
136	out = appendString(out, service)
137	out = appendString(out, authMethod)
138	return out
139}
140