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294 lines
11 KiB
294 lines
11 KiB
=pod |
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=head1 NAME |
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SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth, SSL_set_verify_depth - set peer certificate verification parameters |
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=head1 SYNOPSIS |
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#include <openssl/ssl.h> |
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void SSL_CTX_set_verify(SSL_CTX *ctx, int mode, |
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int (*verify_callback)(int, X509_STORE_CTX *)); |
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void SSL_set_verify(SSL *s, int mode, |
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int (*verify_callback)(int, X509_STORE_CTX *)); |
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void SSL_CTX_set_verify_depth(SSL_CTX *ctx,int depth); |
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void SSL_set_verify_depth(SSL *s, int depth); |
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int verify_callback(int preverify_ok, X509_STORE_CTX *x509_ctx); |
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=head1 DESCRIPTION |
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SSL_CTX_set_verify() sets the verification flags for B<ctx> to be B<mode> and |
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specifies the B<verify_callback> function to be used. If no callback function |
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shall be specified, the NULL pointer can be used for B<verify_callback>. |
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SSL_set_verify() sets the verification flags for B<ssl> to be B<mode> and |
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specifies the B<verify_callback> function to be used. If no callback function |
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shall be specified, the NULL pointer can be used for B<verify_callback>. In |
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this case last B<verify_callback> set specifically for this B<ssl> remains. If |
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no special B<callback> was set before, the default callback for the underlying |
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B<ctx> is used, that was valid at the time B<ssl> was created with |
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L<SSL_new(3)|SSL_new(3)>. |
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SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain |
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verification that shall be allowed for B<ctx>. (See the BUGS section.) |
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SSL_set_verify_depth() sets the maximum B<depth> for the certificate chain |
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verification that shall be allowed for B<ssl>. (See the BUGS section.) |
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=head1 NOTES |
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The verification of certificates can be controlled by a set of logically |
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or'ed B<mode> flags: |
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=over 4 |
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=item SSL_VERIFY_NONE |
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B<Server mode:> the server will not send a client certificate request to the |
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client, so the client will not send a certificate. |
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B<Client mode:> if not using an anonymous cipher (by default disabled), the |
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server will send a certificate which will be checked. The result of the |
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certificate verification process can be checked after the TLS/SSL handshake |
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using the L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> function. |
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The handshake will be continued regardless of the verification result. |
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=item SSL_VERIFY_PEER |
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B<Server mode:> the server sends a client certificate request to the client. |
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The certificate returned (if any) is checked. If the verification process |
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fails, the TLS/SSL handshake is |
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immediately terminated with an alert message containing the reason for |
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the verification failure. |
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The behaviour can be controlled by the additional |
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SSL_VERIFY_FAIL_IF_NO_PEER_CERT and SSL_VERIFY_CLIENT_ONCE flags. |
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B<Client mode:> the server certificate is verified. If the verification process |
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fails, the TLS/SSL handshake is |
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immediately terminated with an alert message containing the reason for |
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the verification failure. If no server certificate is sent, because an |
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anonymous cipher is used, SSL_VERIFY_PEER is ignored. |
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=item SSL_VERIFY_FAIL_IF_NO_PEER_CERT |
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B<Server mode:> if the client did not return a certificate, the TLS/SSL |
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handshake is immediately terminated with a "handshake failure" alert. |
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This flag must be used together with SSL_VERIFY_PEER. |
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B<Client mode:> ignored |
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=item SSL_VERIFY_CLIENT_ONCE |
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B<Server mode:> only request a client certificate on the initial TLS/SSL |
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handshake. Do not ask for a client certificate again in case of a |
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renegotiation. This flag must be used together with SSL_VERIFY_PEER. |
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B<Client mode:> ignored |
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=back |
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Exactly one of the B<mode> flags SSL_VERIFY_NONE and SSL_VERIFY_PEER must be |
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set at any time. |
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The actual verification procedure is performed either using the built-in |
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verification procedure or using another application provided verification |
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function set with |
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L<SSL_CTX_set_cert_verify_callback(3)|SSL_CTX_set_cert_verify_callback(3)>. |
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The following descriptions apply in the case of the built-in procedure. An |
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application provided procedure also has access to the verify depth information |
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and the verify_callback() function, but the way this information is used |
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may be different. |
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SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set the limit up |
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to which depth certificates in a chain are used during the verification |
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procedure. If the certificate chain is longer than allowed, the certificates |
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above the limit are ignored. Error messages are generated as if these |
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certificates would not be present, most likely a |
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X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY will be issued. |
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The depth count is "level 0:peer certificate", "level 1: CA certificate", |
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"level 2: higher level CA certificate", and so on. Setting the maximum |
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depth to 2 allows the levels 0, 1, and 2. The default depth limit is 100, |
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allowing for the peer certificate and additional 100 CA certificates. |
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The B<verify_callback> function is used to control the behaviour when the |
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SSL_VERIFY_PEER flag is set. It must be supplied by the application and |
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receives two arguments: B<preverify_ok> indicates, whether the verification of |
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the certificate in question was passed (preverify_ok=1) or not |
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(preverify_ok=0). B<x509_ctx> is a pointer to the complete context used |
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for the certificate chain verification. |
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The certificate chain is checked starting with the deepest nesting level |
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(the root CA certificate) and worked upward to the peer's certificate. |
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At each level signatures and issuer attributes are checked. Whenever |
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a verification error is found, the error number is stored in B<x509_ctx> |
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and B<verify_callback> is called with B<preverify_ok>=0. By applying |
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X509_CTX_store_* functions B<verify_callback> can locate the certificate |
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in question and perform additional steps (see EXAMPLES). If no error is |
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found for a certificate, B<verify_callback> is called with B<preverify_ok>=1 |
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before advancing to the next level. |
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The return value of B<verify_callback> controls the strategy of the further |
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verification process. If B<verify_callback> returns 0, the verification |
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process is immediately stopped with "verification failed" state. If |
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SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer and |
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the TLS/SSL handshake is terminated. If B<verify_callback> returns 1, |
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the verification process is continued. If B<verify_callback> always returns |
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1, the TLS/SSL handshake will not be terminated with respect to verification |
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failures and the connection will be established. The calling process can |
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however retrieve the error code of the last verification error using |
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L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> or by maintaining its |
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own error storage managed by B<verify_callback>. |
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If no B<verify_callback> is specified, the default callback will be used. |
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Its return value is identical to B<preverify_ok>, so that any verification |
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failure will lead to a termination of the TLS/SSL handshake with an |
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alert message, if SSL_VERIFY_PEER is set. |
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=head1 BUGS |
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In client mode, it is not checked whether the SSL_VERIFY_PEER flag |
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is set, but whether SSL_VERIFY_NONE is not set. This can lead to |
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unexpected behaviour, if the SSL_VERIFY_PEER and SSL_VERIFY_NONE are not |
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used as required (exactly one must be set at any time). |
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The certificate verification depth set with SSL[_CTX]_verify_depth() |
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stops the verification at a certain depth. The error message produced |
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will be that of an incomplete certificate chain and not |
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X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected. |
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=head1 RETURN VALUES |
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The SSL*_set_verify*() functions do not provide diagnostic information. |
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=head1 EXAMPLES |
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The following code sequence realizes an example B<verify_callback> function |
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that will always continue the TLS/SSL handshake regardless of verification |
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failure, if wished. The callback realizes a verification depth limit with |
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more informational output. |
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All verification errors are printed; information about the certificate chain |
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is printed on request. |
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The example is realized for a server that does allow but not require client |
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certificates. |
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The example makes use of the ex_data technique to store application data |
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into/retrieve application data from the SSL structure |
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(see L<SSL_get_ex_new_index(3)|SSL_get_ex_new_index(3)>, |
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L<SSL_get_ex_data_X509_STORE_CTX_idx(3)|SSL_get_ex_data_X509_STORE_CTX_idx(3)>). |
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... |
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typedef struct { |
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int verbose_mode; |
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int verify_depth; |
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int always_continue; |
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} mydata_t; |
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int mydata_index; |
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... |
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static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx) |
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{ |
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char buf[256]; |
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X509 *err_cert; |
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int err, depth; |
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SSL *ssl; |
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mydata_t *mydata; |
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err_cert = X509_STORE_CTX_get_current_cert(ctx); |
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err = X509_STORE_CTX_get_error(ctx); |
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depth = X509_STORE_CTX_get_error_depth(ctx); |
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/* |
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* Retrieve the pointer to the SSL of the connection currently treated |
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* and the application specific data stored into the SSL object. |
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*/ |
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ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx()); |
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mydata = SSL_get_ex_data(ssl, mydata_index); |
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X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256); |
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/* |
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* Catch a too long certificate chain. The depth limit set using |
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* SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so |
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* that whenever the "depth>verify_depth" condition is met, we |
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* have violated the limit and want to log this error condition. |
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* We must do it here, because the CHAIN_TOO_LONG error would not |
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* be found explicitly; only errors introduced by cutting off the |
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* additional certificates would be logged. |
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*/ |
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if (depth > mydata->verify_depth) { |
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preverify_ok = 0; |
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err = X509_V_ERR_CERT_CHAIN_TOO_LONG; |
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X509_STORE_CTX_set_error(ctx, err); |
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} |
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if (!preverify_ok) { |
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printf("verify error:num=%d:%s:depth=%d:%s\n", err, |
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X509_verify_cert_error_string(err), depth, buf); |
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} |
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else if (mydata->verbose_mode) |
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{ |
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printf("depth=%d:%s\n", depth, buf); |
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} |
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/* |
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* At this point, err contains the last verification error. We can use |
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* it for something special |
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*/ |
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if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT)) |
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{ |
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X509_NAME_oneline(X509_get_issuer_name(ctx->current_cert), buf, 256); |
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printf("issuer= %s\n", buf); |
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} |
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if (mydata->always_continue) |
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return 1; |
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else |
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return preverify_ok; |
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} |
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... |
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mydata_t mydata; |
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... |
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mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL); |
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... |
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SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER|SSL_VERIFY_CLIENT_ONCE, |
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verify_callback); |
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/* |
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* Let the verify_callback catch the verify_depth error so that we get |
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* an appropriate error in the logfile. |
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*/ |
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SSL_CTX_set_verify_depth(verify_depth + 1); |
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/* |
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* Set up the SSL specific data into "mydata" and store it into th SSL |
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* structure. |
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*/ |
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mydata.verify_depth = verify_depth; ... |
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SSL_set_ex_data(ssl, mydata_index, &mydata); |
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... |
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SSL_accept(ssl); /* check of success left out for clarity */ |
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if (peer = SSL_get_peer_certificate(ssl)) |
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{ |
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if (SSL_get_verify_result(ssl) == X509_V_OK) |
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{ |
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/* The client sent a certificate which verified OK */ |
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} |
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} |
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=head1 SEE ALSO |
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L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, |
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L<SSL_CTX_get_verify_mode(3)|SSL_CTX_get_verify_mode(3)>, |
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L<SSL_get_verify_result(3)|SSL_get_verify_result(3)>, |
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L<SSL_CTX_load_verify_locations(3)|SSL_CTX_load_verify_locations(3)>, |
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L<SSL_get_peer_certificate(3)|SSL_get_peer_certificate(3)>, |
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L<SSL_CTX_set_cert_verify_callback(3)|SSL_CTX_set_cert_verify_callback(3)>, |
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L<SSL_get_ex_data_X509_STORE_CTX_idx(3)|SSL_get_ex_data_X509_STORE_CTX_idx(3)>, |
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L<SSL_get_ex_new_index(3)|SSL_get_ex_new_index(3)> |
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=cut
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