SSL_CTX_set_client_cert_cb(3) OpenSSL 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
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
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 cer‐
tificate will be installed into ssl, see the NOTES and BUGS sections.
If no certificate should be set, "0" has to be returned and no certifi‐
cate 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
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 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 pos‐
sible to implement a proper selection routine or to allow a user inter‐
action 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 call‐
back 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 cer‐
tificate 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) 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, mak‐
ing the concept of the callback function (to allow the choice from sev‐
eral 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)1.0.0h 2002-06-12 SSL_CTX_set_client_cert_cb(3)