#include
void SSL_CTX_set_client_cert_cb(SSL_CTX *ctx, int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey)); int (*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))(SSL *ssl, X509 **x509, EVP_PKEY **pkey); int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey);
When _cc_ll_ii_ee_nn_tt____cc_ee_rr_tt____cc_bb_((_)) is NULL, no callback function is used.
_S_S_L___C_T_X___g_e_t___c_l_i_e_n_t___c_e_r_t___c_b_(_) returns a pointer to the currently set callback function.
_c_l_i_e_n_t___c_e_r_t___c_b_(_) is the application defined callback. If it wants to set a certificate, a certificate/private key combination must be set using the xx550099 and ppkkeeyy arguments and "1" must be returned. The certificate will be installed into ssssll, see the NOTES and BUGS sections. If no certificate should be set, "0" has to be returned and no certificate will be sent. A negative return value will suspend the handshake and the handshake function will return immediatly. _S_S_L___g_e_t___e_r_r_o_r(3) will return SSL_ERROR_WANT_X509_LOOKUP to indicate, that the handshake was suspended. The next call to the handshake function will again lead to the call of _c_l_i_e_n_t___c_e_r_t___c_b_(_). It is the job of the _c_l_i_e_n_t___c_e_r_t___c_b_(_) to store information about the state of the last call, if required to continue.
When a certificate was set using the _S_S_L___C_T_X___u_s_e___c_e_r_t_i_f_i_c_a_t_e(3) family of functions, it will be sent to the server. The TLS standard requires that only a certificate is sent, if it matches the list of acceptable CAs sent by the server. This constraint is violated by the default behavior of the OpenSSL library. Using the callback function it is possible to implement a proper selection routine or to allow a user interaction to choose the certificate to be sent.
If a callback function is defined and no certificate was yet defined for the SSL object, the callback function will be called. If the callback function returns a certificate, the OpenSSL library will try to load the private key and certificate data into the SSL object using the _S_S_L___u_s_e___c_e_r_t_i_f_i_c_a_t_e_(_) and _S_S_L___u_s_e___p_r_i_v_a_t_e___k_e_y_(_) functions. Thus it will permanently install the certificate and key for this SSL object. It will not be reset by calling _S_S_L___c_l_e_a_r(3). If the callback returns no certificate, the OpenSSL library will not send a certificate.
Once the SSL object has been used in conjunction with the callback function, the certificate will be set for the SSL object and will not be cleared even when _S_S_L___c_l_e_a_r(3) is being called. It is therefore mandatory to destroy the SSL object using _S_S_L___f_r_e_e(3) and create a new one to return to the previous state.