SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb — handle client certificate callback function
SSL_CTX_set_client_cert_cb(SSL_CTX *ctx, int (*client_cert_cb)(SSL
*ssl, X509 **x509, EVP_PKEY **pkey));
*ctx))(SSL *ssl, X509 **x509, EVP_PKEY **pkey); int (*client_cert_cb)(SSL
*ssl, X509 **x509, EVP_PKEY **pkey);
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 Restrictions
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. 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
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.
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)
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.
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)