Sortix 1.1dev nightly manual
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SSL_CTX_SET_CLIENT_CERT_CB(3) | Library Functions Manual | 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
SSL_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. 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 immediately.
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.
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 has been 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.
SEE ALSO
ssl(3), SSL_clear(3), SSL_CTX_add_extra_chain_cert(3), SSL_CTX_use_certificate(3), SSL_free(3), SSL_get_client_CA_list(3)HISTORY
SSL_CTX_set_client_cert_cb
() and
SSL_CTX_get_client_cert_cb
() first appeared
in SSLeay 0.6.6 and have been available since OpenBSD
2.4.
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 be accomplished only 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 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.March 27, 2018 | Debian |