|SSL_CTX_SET_CLIENT_CERT_CB(3)||Library Functions Manual||SSL_CTX_SET_CLIENT_CERT_CB(3)|
*ctx, int (*client_cert_cb)(SSL *ssl, X509 **x509,
*ctx))(SSL *ssl, X509
**x509, EVP_PKEY **pkey);
*ssl, X509 **x509,
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. 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.
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 has been 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_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
SSL_CTX_get_client_cert_cb() first appeared in SSLeay 0.6.6 and have been available since OpenBSD 2.4.
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 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.
|March 27, 2018||OpenBSD-current|