SSL_CTX_set_client_cert_cb(3)

NAME
SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb - handle client certificate callback function

SYNOPSIS

 #include <openssl/ssl.h>
 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
SSL_CTX_set_client_cert_cb() sets the client_cert_cb() callback, that is called when a client certificate is requested by a server. When client_cert_cb() is NULL, not callback function is used.

SSL_CTX_get_client_cert_cb() returns a pointer to the currently set callback function.

client_cert_cb() is the application defined callback. If it wants to set a certificate, a certificate/private key combination must be set using the x509 and pkey arguments and "1" must be returned. The certificate will be installed into ssl, see the NOTES and BUGS sections. If no certificate should be set, "0" has to be returned and the default certificate will be sent. A fatal error can be indicated by returning a negative value, in which case the handshake will be canceled.

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 no callback function is set, an OpenSSL client will send the certificate that was set using the SSL_CTX_use_certificate(3) family of functions. 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. The callback function can obtain the list of acceptable CAs using the SSL_get_client_CA_list(3) function.

If a callback function is defined, 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 SSL_use_certificate() and SSL_use_private_key() functions. Thus it will permanently override the certificate and key previously installed and will not be reset by calling SSL_clear(3). If the callback returns no certificate, the OpenSSL library will send the certificate previously installed for the SSL_CTX object or the specific certificate of the SSL object, if available.

BUGS
The client_cert_cb() 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 SSL_CTX_add_extra_chain_cert(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 SSL_clear(3) is being called. It is therefore mandatory to destroy the SSL object using SSL_free(3) and create a new one to return to the previous state.

SEE ALSO
ssl(3), SSL_CTX_use_certificate(3), SSL_CTX_add_extra_chain_cert(3), SSL_get_client_CA_list(3), SSL_clear(3), SSL_free(3)