1=pod 2 3=head1 NAME 4 5SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb - handle client certificate callback function 6 7=head1 SYNOPSIS 8 9 #include <openssl/ssl.h> 10 11 void SSL_CTX_set_client_cert_cb(SSL_CTX *ctx, 12 int (*client_cert_cb)(SSL *ssl, X509 **x509, 13 EVP_PKEY **pkey)); 14 int (*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))(SSL *ssl, X509 **x509, 15 EVP_PKEY **pkey); 16 int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey); 17 18=head1 DESCRIPTION 19 20SSL_CTX_set_client_cert_cb() sets the client_cert_cb() callback, that is 21called when a client certificate is requested by a server and no certificate 22was yet set for the SSL object. 23 24When client_cert_cb() is NULL, no callback function is used. 25 26SSL_CTX_get_client_cert_cb() returns a pointer to the currently set callback 27function. 28 29client_cert_cb() is the application defined callback. If it wants to 30set a certificate, a certificate/private key combination must be set 31using the B<x509> and B<pkey> arguments and "1" must be returned. The 32certificate will be installed into B<ssl>, see the NOTES and BUGS sections. 33If no certificate should be set, "0" has to be returned and no certificate 34will be sent. A negative return value will suspend the handshake and the 35handshake function will return immediately. L<SSL_get_error(3)> 36will return SSL_ERROR_WANT_X509_LOOKUP to indicate, that the handshake was 37suspended. The next call to the handshake function will again lead to the call 38of client_cert_cb(). It is the job of the client_cert_cb() to store information 39about the state of the last call, if required to continue. 40 41=head1 NOTES 42 43During a handshake (or renegotiation) a server may request a certificate 44from the client. A client certificate must only be sent, when the server 45did send the request. 46 47When a certificate was set using the 48L<SSL_CTX_use_certificate(3)> family of functions, 49it will be sent to the server. The TLS standard requires that only a 50certificate is sent, if it matches the list of acceptable CAs sent by the 51server. This constraint is violated by the default behavior of the OpenSSL 52library. Using the callback function it is possible to implement a proper 53selection routine or to allow a user interaction to choose the certificate to 54be sent. 55 56If a callback function is defined and no certificate was yet defined for the 57SSL object, the callback function will be called. 58If the callback function returns a certificate, the OpenSSL library 59will try to load the private key and certificate data into the SSL 60object using the SSL_use_certificate() and SSL_use_private_key() functions. 61Thus it will permanently install the certificate and key for this SSL 62object. It will not be reset by calling L<SSL_clear(3)>. 63If the callback returns no certificate, the OpenSSL library will not send 64a certificate. 65 66=head1 RETURN VALUES 67 68SSL_CTX_get_client_cert_cb() returns function pointer of client_cert_cb() or 69NULL if the callback is not set. 70 71=head1 BUGS 72 73The client_cert_cb() cannot return a complete certificate chain, it can 74only return one client certificate. If the chain only has a length of 2, 75the root CA certificate may be omitted according to the TLS standard and 76thus a standard conforming answer can be sent to the server. For a 77longer chain, the client must send the complete chain (with the option 78to leave out the root CA certificate). This can only be accomplished by 79either adding the intermediate CA certificates into the trusted 80certificate store for the SSL_CTX object (resulting in having to add 81CA certificates that otherwise maybe would not be trusted), or by adding 82the chain certificates using the 83L<SSL_CTX_add_extra_chain_cert(3)> 84function, which is only available for the SSL_CTX object as a whole and that 85therefore probably can only apply for one client certificate, making 86the concept of the callback function (to allow the choice from several 87certificates) questionable. 88 89Once the SSL object has been used in conjunction with the callback function, 90the certificate will be set for the SSL object and will not be cleared 91even when L<SSL_clear(3)> is being called. It is therefore 92mandatory to destroy the SSL object using L<SSL_free(3)> 93and create a new one to return to the previous state. 94 95=head1 SEE ALSO 96 97L<ssl(7)>, L<SSL_CTX_use_certificate(3)>, 98L<SSL_CTX_add_extra_chain_cert(3)>, 99L<SSL_get_client_CA_list(3)>, 100L<SSL_clear(3)>, L<SSL_free(3)> 101 102=head1 COPYRIGHT 103 104Copyright 2002-2018 The OpenSSL Project Authors. All Rights Reserved. 105 106Licensed under the OpenSSL license (the "License"). You may not use 107this file except in compliance with the License. You can obtain a copy 108in the file LICENSE in the source distribution or at 109L<https://www.openssl.org/source/license.html>. 110 111=cut 112