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Diffstat (limited to 'doc/man3/SSL_CTX_set_verify.pod')
-rw-r--r-- | doc/man3/SSL_CTX_set_verify.pod | 358 |
1 files changed, 358 insertions, 0 deletions
diff --git a/doc/man3/SSL_CTX_set_verify.pod b/doc/man3/SSL_CTX_set_verify.pod new file mode 100644 index 000000000000..21d9ae1018dc --- /dev/null +++ b/doc/man3/SSL_CTX_set_verify.pod @@ -0,0 +1,358 @@ +=pod + +=head1 NAME + +SSL_get_ex_data_X509_STORE_CTX_idx, +SSL_CTX_set_verify, SSL_set_verify, +SSL_CTX_set_verify_depth, SSL_set_verify_depth, +SSL_verify_cb, +SSL_verify_client_post_handshake, +SSL_set_post_handshake_auth, +SSL_CTX_set_post_handshake_auth +- set peer certificate verification parameters + +=head1 SYNOPSIS + + #include <openssl/ssl.h> + + typedef int (*SSL_verify_cb)(int preverify_ok, X509_STORE_CTX *x509_ctx); + + void SSL_CTX_set_verify(SSL_CTX *ctx, int mode, SSL_verify_cb verify_callback); + void SSL_set_verify(SSL *ssl, int mode, SSL_verify_cb verify_callback); + SSL_get_ex_data_X509_STORE_CTX_idx(void); + + void SSL_CTX_set_verify_depth(SSL_CTX *ctx, int depth); + void SSL_set_verify_depth(SSL *ssl, int depth); + + int SSL_verify_client_post_handshake(SSL *ssl); + void SSL_CTX_set_post_handshake_auth(SSL_CTX *ctx, int val); + void SSL_set_post_handshake_auth(SSL *ssl, int val); + +=head1 DESCRIPTION + +SSL_CTX_set_verify() sets the verification flags for B<ctx> to be B<mode> and +specifies the B<verify_callback> function to be used. If no callback function +shall be specified, the NULL pointer can be used for B<verify_callback>. + +SSL_set_verify() sets the verification flags for B<ssl> to be B<mode> and +specifies the B<verify_callback> function to be used. If no callback function +shall be specified, the NULL pointer can be used for B<verify_callback>. In +this case last B<verify_callback> set specifically for this B<ssl> remains. If +no special B<callback> was set before, the default callback for the underlying +B<ctx> is used, that was valid at the time B<ssl> was created with +L<SSL_new(3)>. Within the callback function, +B<SSL_get_ex_data_X509_STORE_CTX_idx> can be called to get the data index +of the current SSL object that is doing the verification. + +SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain +verification that shall be allowed for B<ctx>. + +SSL_set_verify_depth() sets the maximum B<depth> for the certificate chain +verification that shall be allowed for B<ssl>. + +SSL_CTX_set_post_handshake_auth() and SSL_set_post_handshake_auth() enable the +Post-Handshake Authentication extension to be added to the ClientHello such that +post-handshake authentication can be requested by the server. If B<val> is 0 +then the extension is not sent, otherwise it is. By default the extension is not +sent. A certificate callback will need to be set via +SSL_CTX_set_client_cert_cb() if no certificate is provided at initialization. + +SSL_verify_client_post_handshake() causes a CertificateRequest message to be +sent by a server on the given B<ssl> connection. The SSL_VERIFY_PEER flag must +be set; the SSL_VERIFY_POST_HANDSHAKE flag is optional. + +=head1 NOTES + +The verification of certificates can be controlled by a set of logically +or'ed B<mode> flags: + +=over 4 + +=item SSL_VERIFY_NONE + +B<Server mode:> the server will not send a client certificate request to the +client, so the client will not send a certificate. + +B<Client mode:> if not using an anonymous cipher (by default disabled), the +server will send a certificate which will be checked. The result of the +certificate verification process can be checked after the TLS/SSL handshake +using the L<SSL_get_verify_result(3)> function. +The handshake will be continued regardless of the verification result. + +=item SSL_VERIFY_PEER + +B<Server mode:> the server sends a client certificate request to the client. +The certificate returned (if any) is checked. If the verification process +fails, the TLS/SSL handshake is +immediately terminated with an alert message containing the reason for +the verification failure. +The behaviour can be controlled by the additional +SSL_VERIFY_FAIL_IF_NO_PEER_CERT, SSL_VERIFY_CLIENT_ONCE and +SSL_VERIFY_POST_HANDSHAKE flags. + +B<Client mode:> the server certificate is verified. If the verification process +fails, the TLS/SSL handshake is +immediately terminated with an alert message containing the reason for +the verification failure. If no server certificate is sent, because an +anonymous cipher is used, SSL_VERIFY_PEER is ignored. + +=item SSL_VERIFY_FAIL_IF_NO_PEER_CERT + +B<Server mode:> if the client did not return a certificate, the TLS/SSL +handshake is immediately terminated with a "handshake failure" alert. +This flag must be used together with SSL_VERIFY_PEER. + +B<Client mode:> ignored + +=item SSL_VERIFY_CLIENT_ONCE + +B<Server mode:> only request a client certificate once during the +connection. Do not ask for a client certificate again during +renegotiation or post-authentication if a certificate was requested +during the initial handshake. This flag must be used together with +SSL_VERIFY_PEER. + +B<Client mode:> ignored + +=item SSL_VERIFY_POST_HANDSHAKE + +B<Server mode:> the server will not send a client certificate request +during the initial handshake, but will send the request via +SSL_verify_client_post_handshake(). This allows the SSL_CTX or SSL +to be configured for post-handshake peer verification before the +handshake occurs. This flag must be used together with +SSL_VERIFY_PEER. TLSv1.3 only; no effect on pre-TLSv1.3 connections. + +B<Client mode:> ignored + +=back + +If the B<mode> is SSL_VERIFY_NONE none of the other flags may be set. + +The actual verification procedure is performed either using the built-in +verification procedure or using another application provided verification +function set with +L<SSL_CTX_set_cert_verify_callback(3)>. +The following descriptions apply in the case of the built-in procedure. An +application provided procedure also has access to the verify depth information +and the verify_callback() function, but the way this information is used +may be different. + +SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set a limit on the +number of certificates between the end-entity and trust-anchor certificates. +Neither the +end-entity nor the trust-anchor certificates count against B<depth>. If the +certificate chain needed to reach a trusted issuer is longer than B<depth+2>, +X509_V_ERR_CERT_CHAIN_TOO_LONG will be issued. +The depth count is "level 0:peer certificate", "level 1: CA certificate", +"level 2: higher level CA certificate", and so on. Setting the maximum +depth to 2 allows the levels 0, 1, 2 and 3 (0 being the end-entity and 3 the +trust-anchor). +The default depth limit is 100, +allowing for the peer certificate, at most 100 intermediate CA certificates and +a final trust anchor certificate. + +The B<verify_callback> function is used to control the behaviour when the +SSL_VERIFY_PEER flag is set. It must be supplied by the application and +receives two arguments: B<preverify_ok> indicates, whether the verification of +the certificate in question was passed (preverify_ok=1) or not +(preverify_ok=0). B<x509_ctx> is a pointer to the complete context used +for the certificate chain verification. + +The certificate chain is checked starting with the deepest nesting level +(the root CA certificate) and worked upward to the peer's certificate. +At each level signatures and issuer attributes are checked. Whenever +a verification error is found, the error number is stored in B<x509_ctx> +and B<verify_callback> is called with B<preverify_ok>=0. By applying +X509_CTX_store_* functions B<verify_callback> can locate the certificate +in question and perform additional steps (see EXAMPLES). If no error is +found for a certificate, B<verify_callback> is called with B<preverify_ok>=1 +before advancing to the next level. + +The return value of B<verify_callback> controls the strategy of the further +verification process. If B<verify_callback> returns 0, the verification +process is immediately stopped with "verification failed" state. If +SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer and +the TLS/SSL handshake is terminated. If B<verify_callback> returns 1, +the verification process is continued. If B<verify_callback> always returns +1, the TLS/SSL handshake will not be terminated with respect to verification +failures and the connection will be established. The calling process can +however retrieve the error code of the last verification error using +L<SSL_get_verify_result(3)> or by maintaining its +own error storage managed by B<verify_callback>. + +If no B<verify_callback> is specified, the default callback will be used. +Its return value is identical to B<preverify_ok>, so that any verification +failure will lead to a termination of the TLS/SSL handshake with an +alert message, if SSL_VERIFY_PEER is set. + +After calling SSL_set_post_handshake_auth(), the client will need to add a +certificate or certificate callback to its configuration before it can +successfully authenticate. This must be called before SSL_connect(). + +SSL_verify_client_post_handshake() requires that verify flags have been +previously set, and that a client sent the post-handshake authentication +extension. When the client returns a certificate the verify callback will be +invoked. A write operation must take place for the Certificate Request to be +sent to the client, this can be done with SSL_do_handshake() or SSL_write_ex(). +Only one certificate request may be outstanding at any time. + +When post-handshake authentication occurs, a refreshed NewSessionTicket +message is sent to the client. + +=head1 BUGS + +In client mode, it is not checked whether the SSL_VERIFY_PEER flag +is set, but whether any flags are set. This can lead to +unexpected behaviour if SSL_VERIFY_PEER and other flags are not used as +required. + +=head1 RETURN VALUES + +The SSL*_set_verify*() functions do not provide diagnostic information. + +The SSL_verify_client_post_handshake() function returns 1 if the request +succeeded, and 0 if the request failed. The error stack can be examined +to determine the failure reason. + +=head1 EXAMPLES + +The following code sequence realizes an example B<verify_callback> function +that will always continue the TLS/SSL handshake regardless of verification +failure, if wished. The callback realizes a verification depth limit with +more informational output. + +All verification errors are printed; information about the certificate chain +is printed on request. +The example is realized for a server that does allow but not require client +certificates. + +The example makes use of the ex_data technique to store application data +into/retrieve application data from the SSL structure +(see L<CRYPTO_get_ex_new_index(3)>, +L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>). + + ... + typedef struct { + int verbose_mode; + int verify_depth; + int always_continue; + } mydata_t; + int mydata_index; + + ... + static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx) + { + char buf[256]; + X509 *err_cert; + int err, depth; + SSL *ssl; + mydata_t *mydata; + + err_cert = X509_STORE_CTX_get_current_cert(ctx); + err = X509_STORE_CTX_get_error(ctx); + depth = X509_STORE_CTX_get_error_depth(ctx); + + /* + * Retrieve the pointer to the SSL of the connection currently treated + * and the application specific data stored into the SSL object. + */ + ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx()); + mydata = SSL_get_ex_data(ssl, mydata_index); + + X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256); + + /* + * Catch a too long certificate chain. The depth limit set using + * SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so + * that whenever the "depth>verify_depth" condition is met, we + * have violated the limit and want to log this error condition. + * We must do it here, because the CHAIN_TOO_LONG error would not + * be found explicitly; only errors introduced by cutting off the + * additional certificates would be logged. + */ + if (depth > mydata->verify_depth) { + preverify_ok = 0; + err = X509_V_ERR_CERT_CHAIN_TOO_LONG; + X509_STORE_CTX_set_error(ctx, err); + } + if (!preverify_ok) { + printf("verify error:num=%d:%s:depth=%d:%s\n", err, + X509_verify_cert_error_string(err), depth, buf); + } else if (mydata->verbose_mode) { + printf("depth=%d:%s\n", depth, buf); + } + + /* + * At this point, err contains the last verification error. We can use + * it for something special + */ + if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT)) { + X509_NAME_oneline(X509_get_issuer_name(err_cert), buf, 256); + printf("issuer= %s\n", buf); + } + + if (mydata->always_continue) + return 1; + else + return preverify_ok; + } + ... + + mydata_t mydata; + + ... + mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL); + + ... + SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER | SSL_VERIFY_CLIENT_ONCE, + verify_callback); + + /* + * Let the verify_callback catch the verify_depth error so that we get + * an appropriate error in the logfile. + */ + SSL_CTX_set_verify_depth(verify_depth + 1); + + /* + * Set up the SSL specific data into "mydata" and store it into th SSL + * structure. + */ + mydata.verify_depth = verify_depth; ... + SSL_set_ex_data(ssl, mydata_index, &mydata); + + ... + SSL_accept(ssl); /* check of success left out for clarity */ + if (peer = SSL_get_peer_certificate(ssl)) { + if (SSL_get_verify_result(ssl) == X509_V_OK) { + /* The client sent a certificate which verified OK */ + } + } + +=head1 SEE ALSO + +L<ssl(7)>, L<SSL_new(3)>, +L<SSL_CTX_get_verify_mode(3)>, +L<SSL_get_verify_result(3)>, +L<SSL_CTX_load_verify_locations(3)>, +L<SSL_get_peer_certificate(3)>, +L<SSL_CTX_set_cert_verify_callback(3)>, +L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>, +L<SSL_CTX_set_client_cert_cb(3)>, +L<CRYPTO_get_ex_new_index(3)> + +=head1 HISTORY + +The SSL_VERIFY_POST_HANDSHAKE option, and the SSL_verify_client_post_handshake() +and SSL_set_post_handshake_auth() functions were added in OpenSSL 1.1.1. + +=head1 COPYRIGHT + +Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved. + +Licensed under the OpenSSL license (the "License"). You may not use +this file except in compliance with the License. You can obtain a copy +in the file LICENSE in the source distribution or at +L<https://www.openssl.org/source/license.html>. + +=cut |