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 and no certificate 19was yet set for the SSL object. 20 21When B<client_cert_cb()> is NULL, no callback function is used. 22 23SSL_CTX_get_client_cert_cb() returns a pointer to the currently set callback 24function. 25 26client_cert_cb() is the application defined callback. If it wants to 27set a certificate, a certificate/private key combination must be set 28using the B<x509> and B<pkey> arguments and "1" must be returned. The 29certificate will be installed into B<ssl>, see the NOTES and BUGS sections. 30If no certificate should be set, "0" has to be returned and no certificate 31will be sent. A negative return value will suspend the handshake and the 32handshake function will return immediately. L<SSL_get_error(3)|SSL_get_error(3)> 33will return SSL_ERROR_WANT_X509_LOOKUP to indicate, that the handshake was 34suspended. The next call to the handshake function will again lead to the call 35of client_cert_cb(). It is the job of the client_cert_cb() to store information 36about the state of the last call, if required to continue. 37 38=head1 NOTES 39 40During a handshake (or renegotiation) a server may request a certificate 41from the client. A client certificate must only be sent, when the server 42did send the request. 43 44When a certificate was set using the 45L<SSL_CTX_use_certificate(3)|SSL_CTX_use_certificate(3)> family of functions, 46it will be sent to the server. The TLS standard requires that only a 47certificate is sent, if it matches the list of acceptable CAs sent by the 48server. This constraint is violated by the default behavior of the OpenSSL 49library. Using the callback function it is possible to implement a proper 50selection routine or to allow a user interaction to choose the certificate to 51be sent. 52 53If a callback function is defined and no certificate was yet defined for the 54SSL object, the callback function will be called. 55If the callback function returns a certificate, the OpenSSL library 56will try to load the private key and certificate data into the SSL 57object using the SSL_use_certificate() and SSL_use_private_key() functions. 58Thus it will permanently install the certificate and key for this SSL 59object. It will not be reset by calling L<SSL_clear(3)|SSL_clear(3)>. 60If the callback returns no certificate, the OpenSSL library will not send 61a certificate. 62 63=head1 BUGS 64 65The client_cert_cb() cannot return a complete certificate chain, it can 66only return one client certificate. If the chain only has a length of 2, 67the root CA certificate may be omitted according to the TLS standard and 68thus a standard conforming answer can be sent to the server. For a 69longer chain, the client must send the complete chain (with the option 70to leave out the root CA certificate). This can only be accomplished by 71either adding the intermediate CA certificates into the trusted 72certificate store for the SSL_CTX object (resulting in having to add 73CA certificates that otherwise maybe would not be trusted), or by adding 74the chain certificates using the 75L<SSL_CTX_add_extra_chain_cert(3)|SSL_CTX_add_extra_chain_cert(3)> 76function, which is only available for the SSL_CTX object as a whole and that 77therefore probably can only apply for one client certificate, making 78the concept of the callback function (to allow the choice from several 79certificates) questionable. 80 81Once the SSL object has been used in conjunction with the callback function, 82the certificate will be set for the SSL object and will not be cleared 83even when L<SSL_clear(3)|SSL_clear(3)> is being called. It is therefore 84mandatory to destroy the SSL object using L<SSL_free(3)|SSL_free(3)> 85and create a new one to return to the previous state. 86 87=head1 SEE ALSO 88 89L<ssl(3)|ssl(3)>, L<SSL_CTX_use_certificate(3)|SSL_CTX_use_certificate(3)>, 90L<SSL_CTX_add_extra_chain_cert(3)|SSL_CTX_add_extra_chain_cert(3)>, 91L<SSL_get_client_CA_list(3)|SSL_get_client_CA_list(3)>, 92L<SSL_clear(3)|SSL_clear(3)>, L<SSL_free(3)|SSL_free(3)> 93 94=cut 95