SSL_CTX_set_client_cert_cb(3)SSL_CTX_set_client_cert_cb(3)NAME
SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb - Handle client
certificate callback function
SYNOPSIS
#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 );
DESCRIPTION
The SSL_CTX_set_client_cert_cb() function sets the client_cert_cb()
callback that is called when a client certificate is requested by a
server and no certificate was set for the SSL object. When
client_cert_cb() is NULL, no callback function is used. The
SSL_CTX_get_client_cert_cb()function returns a pointer to the currently
set callback function. The 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 WARNINGS
sections.) If no certificate is set, ``0'' must be returned and no
certificate will be sent. A negative return value will suspend the
handshake and the handshake function will return immediately. The
SSL_get_error() function will return SSL_ERROR_WANT_X509_LOOKUP to
indicate that the handshake was suspended. The next call to the hand‐
shake function will lead to the call of client_cert_cb(). It is the
job of the client_cert_cb() function 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 certifi‐
cate from the client. A client certificate must only be sent when the
server sent a request. When a certificate was set using the
SSL_CTX_use_certificate() family of functions, it will be sent to the
server. The TLS standard requires that a certificate is sent only 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 selec‐
tion routine or to allow a user interaction to choose the certificate
to be sent. If a callback function is defined and no certificate was
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(). If the callback
returns no certificate, the OpenSSL library will not send a certifi‐
cate.
WARNINGS
The client_cert_cb() function 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 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 hav‐
ing 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()function. The
SSL_CTX_add_extra_chain_cert()function is only available for the
SSL_CTX object as a whole and 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() is called. It is mandatory to destroy the SSL object using
SSL_free() and create a new one to return to the previous state.
SEE ALSO
Files: ssl(3), SSL_CTX_use_certificate(3),
SSL_CTX_add_extra_chain_cert(3), (3), SSL_get_client_CA_list(3),
SSL_clear(3), SSL_free(3)SSL_CTX_set_client_cert_cb(3)
