SSL_CTX_set_client_cert_cb 3 2002-07-30 0.9.9-dev OpenSSL

NAME

SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb - handle client certificate callback function

LIBRARY

libcrypto, -lcrypto

SYNOPSIS


 #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);

DESCRIPTION

_S_S_L___C_T_X___s_e_t___c_l_i_e_n_t___c_e_r_t___c_b_(_) sets the _cc_ll_ii_ee_nn_tt____cc_ee_rr_tt____cc_bb_((_)) callback, that is called when a client certificate is requested by a server and no certificate was yet set for the SSL object.

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.

NOTES

During a handshake (or renegotiation) a server may request a certificate from the client. A client certificate must only be sent, when the server did send the request.

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.

BUGS

The _c_l_i_e_n_t___c_e_r_t___c_b_(_) cannot return a complete certificate chain, it can only return one client certificate. If the chain only has a length of 2, the root CA certificate may be omitted according to the TLS standard and thus a standard conforming answer can be sent to the server. For a longer chain, the client must send the complete chain (with the option to leave out the root CA certificate). This can only be accomplished by either adding the intermediate CA certificates into the trusted certificate store for the SSL_CTX object (resulting in having to add CA certificates that otherwise maybe would not be trusted), or by adding the chain certificates using the _S_S_L___C_T_X___a_d_d___e_x_t_r_a___c_h_a_i_n___c_e_r_t(3) function, which is only available for the SSL_CTX object as a whole and that therefore probably can only apply for one client certificate, making the concept of the callback function (to allow the choice from several certificates) questionable.

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.

SEE ALSO

_s_s_l(3), _S_S_L___C_T_X___u_s_e___c_e_r_t_i_f_i_c_a_t_e(3), _S_S_L___C_T_X___a_d_d___e_x_t_r_a___c_h_a_i_n___c_e_r_t(3), _S_S_L___g_e_t___c_l_i_e_n_t___C_A___l_i_s_t(3), _S_S_L___c_l_e_a_r(3), _S_S_L___f_r_e_e(3)