SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb -
handle client certificate callback function
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);
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.
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.
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.
SSL_get_client_CA_list(3), SSL_clear(3), SSL_free(3)
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