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 17=head1 DESCRIPTION 18 19SSL_CTX_set_client_cert_cb() sets the I<client_cert_cb> callback, that is 20called when a client certificate is requested by a server and no certificate 21was yet set for the SSL object. 22 23When I<client_cert_cb> is NULL, no callback function is used. 24 25SSL_CTX_get_client_cert_cb() returns a pointer to the currently set callback 26function. 27 28I<client_cert_cb> is the application defined callback. If it wants to 29set a certificate, a certificate/private key combination must be set 30using the I<x509> and I<pkey> arguments and "1" must be returned. The 31certificate will be installed into I<ssl>, see the NOTES and BUGS sections. 32If no certificate should be set, "0" has to be returned and no certificate 33will be sent. A negative return value will suspend the handshake and the 34handshake function will return immediately. L<SSL_get_error(3)> 35will return SSL_ERROR_WANT_X509_LOOKUP to indicate, that the handshake was 36suspended. The next call to the handshake function will again lead to the call 37of I<client_cert_cb>. It is the job of the I<client_cert_cb> to store information 38about the state of the last call, if required to continue. 39 40=head1 NOTES 41 42During a handshake (or renegotiation) a server may request a certificate 43from the client. A client certificate must only be sent, when the server 44did send the request. 45 46When a certificate was set using the 47L<SSL_CTX_use_certificate(3)> family of functions, 48it will be sent to the server. The TLS standard requires that only a 49certificate is sent, if it matches the list of acceptable CAs sent by the 50server. This constraint is violated by the default behavior of the OpenSSL 51library. Using the callback function it is possible to implement a proper 52selection routine or to allow a user interaction to choose the certificate to 53be sent. 54 55If a callback function is defined and no certificate was yet defined for the 56SSL object, the callback function will be called. 57If the callback function returns a certificate, the OpenSSL library 58will try to load the private key and certificate data into the SSL 59object using the SSL_use_certificate() and SSL_use_private_key() functions. 60Thus it will permanently install the certificate and key for this SSL 61object. It will not be reset by calling L<SSL_clear(3)>. 62If the callback returns no certificate, the OpenSSL library will not send 63a certificate. 64 65=head1 RETURN VALUES 66 67SSL_CTX_get_client_cert_cb() returns function pointer of I<client_cert_cb> or 68NULL if the callback is not set. 69 70=head1 BUGS 71 72The I<client_cert_cb> cannot return a complete certificate chain, it can 73only return one client certificate. If the chain only has a length of 2, 74the root CA certificate may be omitted according to the TLS standard and 75thus a standard conforming answer can be sent to the server. For a 76longer chain, the client must send the complete chain (with the option 77to leave out the root CA certificate). This can only be accomplished by 78either adding the intermediate CA certificates into the trusted 79certificate store for the SSL_CTX object (resulting in having to add 80CA certificates that otherwise maybe would not be trusted), or by adding 81the chain certificates using the 82L<SSL_CTX_add_extra_chain_cert(3)> 83function, which is only available for the SSL_CTX object as a whole and that 84therefore probably can only apply for one client certificate, making 85the concept of the callback function (to allow the choice from several 86certificates) questionable. 87 88Once the SSL object has been used in conjunction with the callback function, 89the certificate will be set for the SSL object and will not be cleared 90even when L<SSL_clear(3)> is being called. It is therefore 91mandatory to destroy the SSL object using L<SSL_free(3)> 92and create a new one to return to the previous state. 93 94=head1 SEE ALSO 95 96L<ssl(7)>, L<SSL_CTX_use_certificate(3)>, 97L<SSL_CTX_add_extra_chain_cert(3)>, 98L<SSL_get_client_CA_list(3)>, 99L<SSL_clear(3)>, L<SSL_free(3)> 100 101=head1 COPYRIGHT 102 103Copyright 2002-2020 The OpenSSL Project Authors. All Rights Reserved. 104 105Licensed under the Apache License 2.0 (the "License"). You may not use 106this file except in compliance with the License. You can obtain a copy 107in the file LICENSE in the source distribution or at 108L<https://www.openssl.org/source/license.html>. 109 110=cut 111