SSL_CTX_set_client_cert_cb(3)OpenSSLSSL_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);
DESCRIPTIONSSL_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 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 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.
BUGS
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.
SEE ALSOssl(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)12/Jun/2002 0.9.6jSSL_CTX_set_client_cert_cb(3)
