1SSL_CTX_set_client_cert_cb(3)       OpenSSL      SSL_CTX_set_client_cert_cb(3)
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NAME

6       SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb - handle client
7       certificate callback function
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SYNOPSIS

10        #include <openssl/ssl.h>
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12        void SSL_CTX_set_client_cert_cb(SSL_CTX *ctx, int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey));
13        int (*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))(SSL *ssl, X509 **x509, EVP_PKEY **pkey);
14        int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey);
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DESCRIPTION

17       SSL_CTX_set_client_cert_cb() sets the client_cert_cb() callback, that
18       is called when a client certificate is requested by a server and no
19       certificate was yet set for the SSL object.
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21       When client_cert_cb() is NULL, no callback function is used.
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23       SSL_CTX_get_client_cert_cb() returns a pointer to the currently set
24       callback function.
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26       client_cert_cb() is the application defined callback. If it wants to
27       set a certificate, a certificate/private key combination must be set
28       using the x509 and pkey arguments and "1" must be returned. The
29       certificate will be installed into ssl, see the NOTES and BUGS
30       sections.  If no certificate should be set, "0" has to be returned and
31       no certificate will be sent. A negative return value will suspend the
32       handshake and the handshake function will return immediatly.
33       SSL_get_error(3) will return SSL_ERROR_WANT_X509_LOOKUP to indicate,
34       that the handshake was suspended. The next call to the handshake
35       function will again lead to the call of client_cert_cb(). It is the job
36       of the client_cert_cb() to store information about the state of the
37       last call, if required to continue.
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NOTES

40       During a handshake (or renegotiation) a server may request a
41       certificate from the client. A client certificate must only be sent,
42       when the server did send the request.
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44       When a certificate was set using the SSL_CTX_use_certificate(3) family
45       of functions, it will be sent to the server. The TLS standard requires
46       that only a certificate is sent, if it matches the list of acceptable
47       CAs sent by the server. This constraint is violated by the default
48       behavior of the OpenSSL library. Using the callback function it is
49       possible to implement a proper selection routine or to allow a user
50       interaction to choose the certificate to be sent.
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52       If a callback function is defined and no certificate was yet defined
53       for the SSL object, the callback function will be called.  If the
54       callback function returns a certificate, the OpenSSL library will try
55       to load the private key and certificate data into the SSL object using
56       the SSL_use_certificate() and SSL_use_private_key() functions.  Thus it
57       will permanently install the certificate and key for this SSL object.
58       It will not be reset by calling SSL_clear(3).  If the callback returns
59       no certificate, the OpenSSL library will not send a certificate.
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BUGS

62       The client_cert_cb() cannot return a complete certificate chain, it can
63       only return one client certificate. If the chain only has a length of
64       2, the root CA certificate may be omitted according to the TLS standard
65       and thus a standard conforming answer can be sent to the server. For a
66       longer chain, the client must send the complete chain (with the option
67       to leave out the root CA certificate). This can only be accomplished by
68       either adding the intermediate CA certificates into the trusted
69       certificate store for the SSL_CTX object (resulting in having to add CA
70       certificates that otherwise maybe would not be trusted), or by adding
71       the chain certificates using the SSL_CTX_add_extra_chain_cert(3)
72       function, which is only available for the SSL_CTX object as a whole and
73       that therefore probably can only apply for one client certificate,
74       making the concept of the callback function (to allow the choice from
75       several certificates) questionable.
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77       Once the SSL object has been used in conjunction with the callback
78       function, the certificate will be set for the SSL object and will not
79       be cleared even when SSL_clear(3) is being called. It is therefore
80       mandatory to destroy the SSL object using SSL_free(3) and create a new
81       one to return to the previous state.
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SEE ALSO

84       ssl(3), SSL_CTX_use_certificate(3), SSL_CTX_add_extra_chain_cert(3),
85       SSL_get_client_CA_list(3), SSL_clear(3), SSL_free(3)
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891.0.0e                            2002-06-12     SSL_CTX_set_client_cert_cb(3)
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