SSL_CTX_set_client_cert_cb.pod revision 100928
1=pod 2 3=head1 NAME 4 5SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb - handle client certificate callback function 6 7=head1 SYNOPSIS 8 9 #include <openssl/ssl.h> 10 11 void SSL_CTX_set_client_cert_cb(SSL_CTX *ctx, int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey)); 12 int (*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))(SSL *ssl, X509 **x509, EVP_PKEY **pkey); 13 int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey); 14 15=head1 DESCRIPTION 16 17SSL_CTX_set_client_cert_cb() sets the B<client_cert_cb()> callback, that is 18called when a client certificate is requested by a server. 19When B<client_cert_cb()> is NULL, not callback function is used. 20 21SSL_CTX_get_client_cert_cb() returns a pointer to the currently set callback 22function. 23 24client_cert_cb() is the application defined callback. If it wants to 25set a certificate, a certificate/private key combination must be set 26using the B<x509> and B<pkey> arguments and "1" must be returned. The 27certificate will be installed into B<ssl>, see the NOTES and BUGS sections. 28If no certificate should be set, "0" has to be returned and the default 29certificate will be sent. A fatal error can be indicated by returning 30a negative value, in which case the handshake will be canceled. 31 32=head1 NOTES 33 34During a handshake (or renegotiation) a server may request a certificate 35from the client. A client certificate must only be sent, when the server 36did send the request. 37 38When no callback function is set, an OpenSSL client will send the certificate 39that was set using the 40L<SSL_CTX_use_certificate(3)|SSL_CTX_use_certificate(3)> family of functions. 41The TLS standard requires that only a certificate is sent, if it matches 42the list of acceptable CAs sent by the server. This constraint is 43violated by the default behavior of the OpenSSL library. Using the 44callback function it is possible to implement a proper selection routine 45or to allow a user interaction to choose the certificate to be sent. 46The callback function can obtain the list of acceptable CAs using the 47L<SSL_get_client_CA_list(3)|SSL_get_client_CA_list(3)> function. 48 49If a callback function is defined, the callback function will be called. 50If the callback function returns a certificate, the OpenSSL library 51will try to load the private key and certificate data into the SSL 52object using SSL_use_certificate() and SSL_use_private_key() functions. 53Thus it will permanently override the certificate and key previously 54installed and will not be reset by calling L<SSL_clear(3)|SSL_clear(3)>. 55If the callback returns no certificate, the OpenSSL library will send 56the certificate previously installed for the SSL_CTX object or the specific 57certificate of the SSL object, if available. 58 59=head1 BUGS 60 61The client_cert_cb() cannot return a complete certificate chain, it can 62only return one client certificate. If the chain only has a length of 2, 63the root CA certificate may be omitted according to the TLS standard and 64thus a standard conforming answer can be sent to the server. For a 65longer chain, the client must send the complete chain (with the option 66to leave out the root CA certificate). This can only be accomplished by 67either adding the intermediate CA certificates into the trusted 68certificate store for the SSL_CTX object (resulting in having to add 69CA certificates that otherwise maybe would not be trusted), or by adding 70the chain certificates using the 71L<SSL_CTX_add_extra_chain_cert(3)|SSL_CTX_add_extra_chain_cert(3)> 72function, which is only available for the SSL_CTX object as a whole and that 73therefore probably can only apply for one client certificate, making 74the concept of the callback function (to allow the choice from several 75certificates) questionable. 76 77Once the SSL object has been used in conjunction with the callback function, 78the certificate will be set for the SSL object and will not be cleared 79even when L<SSL_clear(3)|SSL_clear(3)> is being called. It is therefore 80mandatory to destroy the SSL object using L<SSL_free(3)|SSL_free(3)> 81and create a new one to return to the previous state. 82 83=head1 SEE ALSO 84 85L<ssl(3)|ssl(3)>, L<SSL_CTX_use_certificate(3)|SSL_CTX_use_certificate(3)>, 86L<SSL_CTX_add_extra_chain_cert(3)|SSL_CTX_add_extra_chain_cert(3)>, 87L<SSL_get_client_CA_list(3)|SSL_get_client_CA_list(3)>, 88L<SSL_clear(3)|SSL_clear(3)>, L<SSL_free(3)|SSL_free(3)> 89 90=cut 91