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() call-
back, 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 combi-
nation 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 rou-
tine or to allow a user interaction to choose the certifi-
cate to be sent.
MirOS BSD #10-current 2005-02-05 1
SSL_CTX_SET_CLIENT_CERT_CB(3)OpenSSLSSL_CTX_SET_CLIENT_CERT_CB(3)
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 certi-
ficate 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 certi-
ficates into the trusted certificate store for the SSL_CTX
object (resulting in having to add CA certificates that oth-
erwise maybe would not be trusted), or by adding the chain
certificates using the SSL_CTX_add_extra_chain_cert(3) func-
tion, 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 func-
tion (to allow the choice from several certificates) ques-
tionable.
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)MirOS BSD #10-current 2005-02-05 2
