1=pod 2 3=head1 NAME 4 5SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth, SSL_set_verify_depth - set peer certificate verification parameters 6 7=head1 SYNOPSIS 8 9 #include <openssl/ssl.h> 10 11 void SSL_CTX_set_verify(SSL_CTX *ctx, int mode, 12 int (*verify_callback)(int, X509_STORE_CTX *)); 13 void SSL_set_verify(SSL *s, int mode, 14 int (*verify_callback)(int, X509_STORE_CTX *)); 15 void SSL_CTX_set_verify_depth(SSL_CTX *ctx,int depth); 16 void SSL_set_verify_depth(SSL *s, int depth); 17 18 int verify_callback(int preverify_ok, X509_STORE_CTX *x509_ctx); 19 20=head1 DESCRIPTION 21 22SSL_CTX_set_verify() sets the verification flags for B<ctx> to be B<mode> and 23specifies the B<verify_callback> function to be used. If no callback function 24shall be specified, the NULL pointer can be used for B<verify_callback>. 25 26SSL_set_verify() sets the verification flags for B<ssl> to be B<mode> and 27specifies the B<verify_callback> function to be used. If no callback function 28shall be specified, the NULL pointer can be used for B<verify_callback>. In 29this case last B<verify_callback> set specifically for this B<ssl> remains. If 30no special B<callback> was set before, the default callback for the underlying 31B<ctx> is used, that was valid at the the time B<ssl> was created with 32L<SSL_new(3)|SSL_new(3)>. 33 34SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain 35verification that shall be allowed for B<ctx>. (See the BUGS section.) 36 37SSL_set_verify_depth() sets the maximum B<depth> for the certificate chain 38verification that shall be allowed for B<ssl>. (See the BUGS section.) 39 40=head1 NOTES 41 42The verification of certificates can be controlled by a set of logically 43or'ed B<mode> flags: 44 45=over 4 46 47=item SSL_VERIFY_NONE 48 49B<Server mode:> the server will not send a client certificate request to the 50client, so the client will not send a certificate. 51 52B<Client mode:> if not using an anonymous cipher (by default disabled), the 53server will send a certificate which will be checked. The result of the 54certificate verification process can be checked after the TLS/SSL handshake 55using the L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> function. 56The handshake will be continued regardless of the verification result. 57 58=item SSL_VERIFY_PEER 59 60B<Server mode:> the server sends a client certificate request to the client. 61The certificate returned (if any) is checked. If the verification process 62fails, the TLS/SSL handshake is 63immediately terminated with an alert message containing the reason for 64the verification failure. 65The behaviour can be controlled by the additional 66SSL_VERIFY_FAIL_IF_NO_PEER_CERT and SSL_VERIFY_CLIENT_ONCE flags. 67 68B<Client mode:> the server certificate is verified. If the verification process 69fails, the TLS/SSL handshake is 70immediately terminated with an alert message containing the reason for 71the verification failure. If no server certificate is sent, because an 72anonymous cipher is used, SSL_VERIFY_PEER is ignored. 73 74=item SSL_VERIFY_FAIL_IF_NO_PEER_CERT 75 76B<Server mode:> if the client did not return a certificate, the TLS/SSL 77handshake is immediately terminated with a "handshake failure" alert. 78This flag must be used together with SSL_VERIFY_PEER. 79 80B<Client mode:> ignored 81 82=item SSL_VERIFY_CLIENT_ONCE 83 84B<Server mode:> only request a client certificate on the initial TLS/SSL 85handshake. Do not ask for a client certificate again in case of a 86renegotiation. This flag must be used together with SSL_VERIFY_PEER. 87 88B<Client mode:> ignored 89 90=back 91 92Exactly one of the B<mode> flags SSL_VERIFY_NONE and SSL_VERIFY_PEER must be 93set at any time. 94 95The actual verification procedure is performed either using the built-in 96verification procedure or using another application provided verification 97function set with 98L<SSL_CTX_set_cert_verify_callback(3)|SSL_CTX_set_cert_verify_callback(3)>. 99The following descriptions apply in the case of the built-in procedure. An 100application provided procedure also has access to the verify depth information 101and the verify_callback() function, but the way this information is used 102may be different. 103 104SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set the limit up 105to which depth certificates in a chain are used during the verification 106procedure. If the certificate chain is longer than allowed, the certificates 107above the limit are ignored. Error messages are generated as if these 108certificates would not be present, most likely a 109X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY will be issued. 110The depth count is "level 0:peer certificate", "level 1: CA certificate", 111"level 2: higher level CA certificate", and so on. Setting the maximum 112depth to 2 allows the levels 0, 1, and 2. The default depth limit is 9, 113allowing for the peer certificate and additional 9 CA certificates. 114 115The B<verify_callback> function is used to control the behaviour when the 116SSL_VERIFY_PEER flag is set. It must be supplied by the application and 117receives two arguments: B<preverify_ok> indicates, whether the verification of 118the certificate in question was passed (preverify_ok=1) or not 119(preverify_ok=0). B<x509_ctx> is a pointer to the complete context used 120for the certificate chain verification. 121 122The certificate chain is checked starting with the deepest nesting level 123(the root CA certificate) and worked upward to the peer's certificate. 124At each level signatures and issuer attributes are checked. Whenever 125a verification error is found, the error number is stored in B<x509_ctx> 126and B<verify_callback> is called with B<preverify_ok>=0. By applying 127X509_CTX_store_* functions B<verify_callback> can locate the certificate 128in question and perform additional steps (see EXAMPLES). If no error is 129found for a certificate, B<verify_callback> is called with B<preverify_ok>=1 130before advancing to the next level. 131 132The return value of B<verify_callback> controls the strategy of the further 133verification process. If B<verify_callback> returns 0, the verification 134process is immediately stopped with "verification failed" state. If 135SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer and 136the TLS/SSL handshake is terminated. If B<verify_callback> returns 1, 137the verification process is continued. If B<verify_callback> always returns 1381, the TLS/SSL handshake will not be terminated with respect to verification 139failures and the connection will be established. The calling process can 140however retrieve the error code of the last verification error using 141L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> or by maintaining its 142own error storage managed by B<verify_callback>. 143 144If no B<verify_callback> is specified, the default callback will be used. 145Its return value is identical to B<preverify_ok>, so that any verification 146failure will lead to a termination of the TLS/SSL handshake with an 147alert message, if SSL_VERIFY_PEER is set. 148 149=head1 BUGS 150 151In client mode, it is not checked whether the SSL_VERIFY_PEER flag 152is set, but whether SSL_VERIFY_NONE is not set. This can lead to 153unexpected behaviour, if the SSL_VERIFY_PEER and SSL_VERIFY_NONE are not 154used as required (exactly one must be set at any time). 155 156The certificate verification depth set with SSL[_CTX]_verify_depth() 157stops the verification at a certain depth. The error message produced 158will be that of an incomplete certificate chain and not 159X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected. 160 161=head1 RETURN VALUES 162 163The SSL*_set_verify*() functions do not provide diagnostic information. 164 165=head1 EXAMPLES 166 167The following code sequence realizes an example B<verify_callback> function 168that will always continue the TLS/SSL handshake regardless of verification 169failure, if wished. The callback realizes a verification depth limit with 170more informational output. 171 172All verification errors are printed, informations about the certificate chain 173are printed on request. 174The example is realized for a server that does allow but not require client 175certificates. 176 177The example makes use of the ex_data technique to store application data 178into/retrieve application data from the SSL structure 179(see L<SSL_get_ex_new_index(3)|SSL_get_ex_new_index(3)>, 180L<SSL_get_ex_data_X509_STORE_CTX_idx(3)|SSL_get_ex_data_X509_STORE_CTX_idx(3)>). 181 182 ... 183 typedef struct { 184 int verbose_mode; 185 int verify_depth; 186 int always_continue; 187 } mydata_t; 188 int mydata_index; 189 ... 190 static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx) 191 { 192 char buf[256]; 193 X509 *err_cert; 194 int err, depth; 195 SSL *ssl; 196 mydata_t *mydata; 197 198 err_cert = X509_STORE_CTX_get_current_cert(ctx); 199 err = X509_STORE_CTX_get_error(ctx); 200 depth = X509_STORE_CTX_get_error_depth(ctx); 201 202 /* 203 * Retrieve the pointer to the SSL of the connection currently treated 204 * and the application specific data stored into the SSL object. 205 */ 206 ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx()); 207 mydata = SSL_get_ex_data(ssl, mydata_index); 208 209 X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256); 210 211 /* 212 * Catch a too long certificate chain. The depth limit set using 213 * SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so 214 * that whenever the "depth>verify_depth" condition is met, we 215 * have violated the limit and want to log this error condition. 216 * We must do it here, because the CHAIN_TOO_LONG error would not 217 * be found explicitly; only errors introduced by cutting off the 218 * additional certificates would be logged. 219 */ 220 if (depth > mydata->verify_depth) { 221 preverify_ok = 0; 222 err = X509_V_ERR_CERT_CHAIN_TOO_LONG; 223 X509_STORE_CTX_set_error(ctx, err); 224 } 225 if (!preverify_ok) { 226 printf("verify error:num=%d:%s:depth=%d:%s\n", err, 227 X509_verify_cert_error_string(err), depth, buf); 228 } 229 else if (mydata->verbose_mode) 230 { 231 printf("depth=%d:%s\n", depth, buf); 232 } 233 234 /* 235 * At this point, err contains the last verification error. We can use 236 * it for something special 237 */ 238 if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT)) 239 { 240 X509_NAME_oneline(X509_get_issuer_name(ctx->current_cert), buf, 256); 241 printf("issuer= %s\n", buf); 242 } 243 244 if (mydata->always_continue) 245 return 1; 246 else 247 return preverify_ok; 248 } 249 ... 250 251 mydata_t mydata; 252 253 ... 254 mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL); 255 256 ... 257 SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER|SSL_VERIFY_CLIENT_ONCE, 258 verify_callback); 259 260 /* 261 * Let the verify_callback catch the verify_depth error so that we get 262 * an appropriate error in the logfile. 263 */ 264 SSL_CTX_set_verify_depth(verify_depth + 1); 265 266 /* 267 * Set up the SSL specific data into "mydata" and store it into th SSL 268 * structure. 269 */ 270 mydata.verify_depth = verify_depth; ... 271 SSL_set_ex_data(ssl, mydata_index, &mydata); 272 273 ... 274 SSL_accept(ssl); /* check of success left out for clarity */ 275 if (peer = SSL_get_peer_certificate(ssl)) 276 { 277 if (SSL_get_verify_result(ssl) == X509_V_OK) 278 { 279 /* The client sent a certificate which verified OK */ 280 } 281 } 282 283=head1 SEE ALSO 284 285L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, 286L<SSL_CTX_get_verify_mode(3)|SSL_CTX_get_verify_mode(3)>, 287L<SSL_get_verify_result(3)|SSL_get_verify_result(3)>, 288L<SSL_CTX_load_verify_locations(3)|SSL_CTX_load_verify_locations(3)>, 289L<SSL_get_peer_certificate(3)|SSL_get_peer_certificate(3)>, 290L<SSL_CTX_set_cert_verify_callback(3)|SSL_CTX_set_cert_verify_callback(3)>, 291L<SSL_get_ex_data_X509_STORE_CTX_idx(3)|SSL_get_ex_data_X509_STORE_CTX_idx(3)>, 292L<SSL_get_ex_new_index(3)|SSL_get_ex_new_index(3)> 293 294=cut 295