SSL_CTX_set_client_cert_cb3ssl

Usage

 #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);

Overview

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

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.

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 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.

Return values

SSL_CTX_get_client_cert_cb() returns function pointer of client_cert_cb or NULL if the callback is not set.