xref: /freebsd/crypto/openssl/doc/man3/SSL_CTX_set_verify.pod (revision db70ff37a051dfa19f6f3f0f0c5e3571aba91982)
1=pod
2
3=head1 NAME
4
5SSL_get_ex_data_X509_STORE_CTX_idx,
6SSL_CTX_set_verify, SSL_set_verify,
7SSL_CTX_set_verify_depth, SSL_set_verify_depth,
8SSL_verify_cb,
9SSL_verify_client_post_handshake,
10SSL_set_post_handshake_auth,
11SSL_CTX_set_post_handshake_auth
12- set peer certificate verification parameters
13
14=head1 SYNOPSIS
15
16 #include <openssl/ssl.h>
17
18 typedef int (*SSL_verify_cb)(int preverify_ok, X509_STORE_CTX *x509_ctx);
19
20 void SSL_CTX_set_verify(SSL_CTX *ctx, int mode, SSL_verify_cb verify_callback);
21 void SSL_set_verify(SSL *ssl, int mode, SSL_verify_cb verify_callback);
22 SSL_get_ex_data_X509_STORE_CTX_idx(void);
23
24 void SSL_CTX_set_verify_depth(SSL_CTX *ctx, int depth);
25 void SSL_set_verify_depth(SSL *ssl, int depth);
26
27 int SSL_verify_client_post_handshake(SSL *ssl);
28 void SSL_CTX_set_post_handshake_auth(SSL_CTX *ctx, int val);
29 void SSL_set_post_handshake_auth(SSL *ssl, int val);
30
31=head1 DESCRIPTION
32
33SSL_CTX_set_verify() sets the verification flags for B<ctx> to be B<mode> and
34specifies the B<verify_callback> function to be used. If no callback function
35shall be specified, the NULL pointer can be used for B<verify_callback>.
36
37SSL_set_verify() sets the verification flags for B<ssl> to be B<mode> and
38specifies the B<verify_callback> function to be used. If no callback function
39shall be specified, the NULL pointer can be used for B<verify_callback>. In
40this case last B<verify_callback> set specifically for this B<ssl> remains. If
41no special B<callback> was set before, the default callback for the underlying
42B<ctx> is used, that was valid at the time B<ssl> was created with
43L<SSL_new(3)>. Within the callback function,
44B<SSL_get_ex_data_X509_STORE_CTX_idx> can be called to get the data index
45of the current SSL object that is doing the verification.
46
47SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain
48verification that shall be allowed for B<ctx>.
49
50SSL_set_verify_depth() sets the maximum B<depth> for the certificate chain
51verification that shall be allowed for B<ssl>.
52
53SSL_CTX_set_post_handshake_auth() and SSL_set_post_handshake_auth() enable the
54Post-Handshake Authentication extension to be added to the ClientHello such that
55post-handshake authentication can be requested by the server. If B<val> is 0
56then the extension is not sent, otherwise it is. By default the extension is not
57sent. A certificate callback will need to be set via
58SSL_CTX_set_client_cert_cb() if no certificate is provided at initialization.
59
60SSL_verify_client_post_handshake() causes a CertificateRequest message to be
61sent by a server on the given B<ssl> connection. The SSL_VERIFY_PEER flag must
62be set; the SSL_VERIFY_POST_HANDSHAKE flag is optional.
63
64=head1 NOTES
65
66The verification of certificates can be controlled by a set of logically
67or'ed B<mode> flags:
68
69=over 4
70
71=item SSL_VERIFY_NONE
72
73B<Server mode:> the server will not send a client certificate request to the
74client, so the client will not send a certificate.
75
76B<Client mode:> if not using an anonymous cipher (by default disabled), the
77server will send a certificate which will be checked. The result of the
78certificate verification process can be checked after the TLS/SSL handshake
79using the L<SSL_get_verify_result(3)> function.
80The handshake will be continued regardless of the verification result.
81
82=item SSL_VERIFY_PEER
83
84B<Server mode:> the server sends a client certificate request to the client.
85The certificate returned (if any) is checked. If the verification process
86fails, the TLS/SSL handshake is
87immediately terminated with an alert message containing the reason for
88the verification failure.
89The behaviour can be controlled by the additional
90SSL_VERIFY_FAIL_IF_NO_PEER_CERT, SSL_VERIFY_CLIENT_ONCE and
91SSL_VERIFY_POST_HANDSHAKE flags.
92
93B<Client mode:> the server certificate is verified. If the verification process
94fails, the TLS/SSL handshake is
95immediately terminated with an alert message containing the reason for
96the verification failure. If no server certificate is sent, because an
97anonymous cipher is used, SSL_VERIFY_PEER is ignored.
98
99=item SSL_VERIFY_FAIL_IF_NO_PEER_CERT
100
101B<Server mode:> if the client did not return a certificate, the TLS/SSL
102handshake is immediately terminated with a "handshake failure" alert.
103This flag must be used together with SSL_VERIFY_PEER.
104
105B<Client mode:> ignored
106
107=item SSL_VERIFY_CLIENT_ONCE
108
109B<Server mode:> only request a client certificate once during the
110connection. Do not ask for a client certificate again during
111renegotiation or post-authentication if a certificate was requested
112during the initial handshake. This flag must be used together with
113SSL_VERIFY_PEER.
114
115B<Client mode:> ignored
116
117=item SSL_VERIFY_POST_HANDSHAKE
118
119B<Server mode:> the server will not send a client certificate request
120during the initial handshake, but will send the request via
121SSL_verify_client_post_handshake(). This allows the SSL_CTX or SSL
122to be configured for post-handshake peer verification before the
123handshake occurs. This flag must be used together with
124SSL_VERIFY_PEER. TLSv1.3 only; no effect on pre-TLSv1.3 connections.
125
126B<Client mode:> ignored
127
128=back
129
130If the B<mode> is SSL_VERIFY_NONE none of the other flags may be set.
131
132The actual verification procedure is performed either using the built-in
133verification procedure or using another application provided verification
134function set with
135L<SSL_CTX_set_cert_verify_callback(3)>.
136The following descriptions apply in the case of the built-in procedure. An
137application provided procedure also has access to the verify depth information
138and the verify_callback() function, but the way this information is used
139may be different.
140
141SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set a limit on the
142number of certificates between the end-entity and trust-anchor certificates.
143Neither the
144end-entity nor the trust-anchor certificates count against B<depth>. If the
145certificate chain needed to reach a trusted issuer is longer than B<depth+2>,
146X509_V_ERR_CERT_CHAIN_TOO_LONG will be issued.
147The depth count is "level 0:peer certificate", "level 1: CA certificate",
148"level 2: higher level CA certificate", and so on. Setting the maximum
149depth to 2 allows the levels 0, 1, 2 and 3 (0 being the end-entity and 3 the
150trust-anchor).
151The default depth limit is 100,
152allowing for the peer certificate, at most 100 intermediate CA certificates and
153a final trust anchor certificate.
154
155The B<verify_callback> function is used to control the behaviour when the
156SSL_VERIFY_PEER flag is set. It must be supplied by the application and
157receives two arguments: B<preverify_ok> indicates, whether the verification of
158the certificate in question was passed (preverify_ok=1) or not
159(preverify_ok=0). B<x509_ctx> is a pointer to the complete context used
160for the certificate chain verification.
161
162The certificate chain is checked starting with the deepest nesting level
163(the root CA certificate) and worked upward to the peer's certificate.
164At each level signatures and issuer attributes are checked. Whenever
165a verification error is found, the error number is stored in B<x509_ctx>
166and B<verify_callback> is called with B<preverify_ok>=0. By applying
167X509_CTX_store_* functions B<verify_callback> can locate the certificate
168in question and perform additional steps (see EXAMPLES). If no error is
169found for a certificate, B<verify_callback> is called with B<preverify_ok>=1
170before advancing to the next level.
171
172The return value of B<verify_callback> controls the strategy of the further
173verification process. If B<verify_callback> returns 0, the verification
174process is immediately stopped with "verification failed" state. If
175SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer and
176the TLS/SSL handshake is terminated. If B<verify_callback> returns 1,
177the verification process is continued. If B<verify_callback> always returns
1781, the TLS/SSL handshake will not be terminated with respect to verification
179failures and the connection will be established. The calling process can
180however retrieve the error code of the last verification error using
181L<SSL_get_verify_result(3)> or by maintaining its
182own error storage managed by B<verify_callback>.
183
184If no B<verify_callback> is specified, the default callback will be used.
185Its return value is identical to B<preverify_ok>, so that any verification
186failure will lead to a termination of the TLS/SSL handshake with an
187alert message, if SSL_VERIFY_PEER is set.
188
189After calling SSL_set_post_handshake_auth(), the client will need to add a
190certificate or certificate callback to its configuration before it can
191successfully authenticate. This must be called before SSL_connect().
192
193SSL_verify_client_post_handshake() requires that verify flags have been
194previously set, and that a client sent the post-handshake authentication
195extension. When the client returns a certificate the verify callback will be
196invoked. A write operation must take place for the Certificate Request to be
197sent to the client, this can be done with SSL_do_handshake() or SSL_write_ex().
198Only one certificate request may be outstanding at any time.
199
200When post-handshake authentication occurs, a refreshed NewSessionTicket
201message is sent to the client.
202
203=head1 BUGS
204
205In client mode, it is not checked whether the SSL_VERIFY_PEER flag
206is set, but whether any flags are set. This can lead to
207unexpected behaviour if SSL_VERIFY_PEER and other flags are not used as
208required.
209
210=head1 RETURN VALUES
211
212The SSL*_set_verify*() functions do not provide diagnostic information.
213
214The SSL_verify_client_post_handshake() function returns 1 if the request
215succeeded, and 0 if the request failed. The error stack can be examined
216to determine the failure reason.
217
218=head1 EXAMPLES
219
220The following code sequence realizes an example B<verify_callback> function
221that will always continue the TLS/SSL handshake regardless of verification
222failure, if wished. The callback realizes a verification depth limit with
223more informational output.
224
225All verification errors are printed; information about the certificate chain
226is printed on request.
227The example is realized for a server that does allow but not require client
228certificates.
229
230The example makes use of the ex_data technique to store application data
231into/retrieve application data from the SSL structure
232(see L<CRYPTO_get_ex_new_index(3)>,
233L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>).
234
235 ...
236 typedef struct {
237   int verbose_mode;
238   int verify_depth;
239   int always_continue;
240 } mydata_t;
241 int mydata_index;
242
243 ...
244 static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
245 {
246     char    buf[256];
247     X509   *err_cert;
248     int     err, depth;
249     SSL    *ssl;
250     mydata_t *mydata;
251
252     err_cert = X509_STORE_CTX_get_current_cert(ctx);
253     err = X509_STORE_CTX_get_error(ctx);
254     depth = X509_STORE_CTX_get_error_depth(ctx);
255
256     /*
257      * Retrieve the pointer to the SSL of the connection currently treated
258      * and the application specific data stored into the SSL object.
259      */
260     ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
261     mydata = SSL_get_ex_data(ssl, mydata_index);
262
263     X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256);
264
265     /*
266      * Catch a too long certificate chain. The depth limit set using
267      * SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so
268      * that whenever the "depth>verify_depth" condition is met, we
269      * have violated the limit and want to log this error condition.
270      * We must do it here, because the CHAIN_TOO_LONG error would not
271      * be found explicitly; only errors introduced by cutting off the
272      * additional certificates would be logged.
273      */
274     if (depth > mydata->verify_depth) {
275         preverify_ok = 0;
276         err = X509_V_ERR_CERT_CHAIN_TOO_LONG;
277         X509_STORE_CTX_set_error(ctx, err);
278     }
279     if (!preverify_ok) {
280         printf("verify error:num=%d:%s:depth=%d:%s\n", err,
281                X509_verify_cert_error_string(err), depth, buf);
282     } else if (mydata->verbose_mode) {
283         printf("depth=%d:%s\n", depth, buf);
284     }
285
286     /*
287      * At this point, err contains the last verification error. We can use
288      * it for something special
289      */
290     if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT)) {
291         X509_NAME_oneline(X509_get_issuer_name(err_cert), buf, 256);
292         printf("issuer= %s\n", buf);
293     }
294
295     if (mydata->always_continue)
296         return 1;
297     else
298         return preverify_ok;
299 }
300 ...
301
302 mydata_t mydata;
303
304 ...
305 mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL);
306
307 ...
308 SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER | SSL_VERIFY_CLIENT_ONCE,
309                    verify_callback);
310
311 /*
312  * Let the verify_callback catch the verify_depth error so that we get
313  * an appropriate error in the logfile.
314  */
315 SSL_CTX_set_verify_depth(verify_depth + 1);
316
317 /*
318  * Set up the SSL specific data into "mydata" and store it into th SSL
319  * structure.
320  */
321 mydata.verify_depth = verify_depth; ...
322 SSL_set_ex_data(ssl, mydata_index, &mydata);
323
324 ...
325 SSL_accept(ssl);       /* check of success left out for clarity */
326 if (peer = SSL_get_peer_certificate(ssl)) {
327     if (SSL_get_verify_result(ssl) == X509_V_OK) {
328         /* The client sent a certificate which verified OK */
329     }
330 }
331
332=head1 SEE ALSO
333
334L<ssl(7)>, L<SSL_new(3)>,
335L<SSL_CTX_get_verify_mode(3)>,
336L<SSL_get_verify_result(3)>,
337L<SSL_CTX_load_verify_locations(3)>,
338L<SSL_get_peer_certificate(3)>,
339L<SSL_CTX_set_cert_verify_callback(3)>,
340L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>,
341L<SSL_CTX_set_client_cert_cb(3)>,
342L<CRYPTO_get_ex_new_index(3)>
343
344=head1 HISTORY
345
346The SSL_VERIFY_POST_HANDSHAKE option, and the SSL_verify_client_post_handshake()
347and SSL_set_post_handshake_auth() functions were added in OpenSSL 1.1.1.
348
349=head1 COPYRIGHT
350
351Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.
352
353Licensed under the OpenSSL license (the "License").  You may not use
354this file except in compliance with the License.  You can obtain a copy
355in the file LICENSE in the source distribution or at
356L<https://www.openssl.org/source/license.html>.
357
358=cut
359