#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);
When client_cert_cb() is NULL, no 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 no certificate will be sent. A negative return value will suspend the handshake and the handshake function will return immediately. SSL_get_error(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 client_cert_cb(). It is the job of the client_cert_cb() to store information about the state of the last call, if required to continue.
When a certificate was set using the SSL_CTX_use_certificate(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 SSL_use_certificate() and SSL_use_private_key() functions. Thus it will permanently install the certificate and key for this SSL object. It will not be reset by calling SSL_clear(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 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.