1.\" $OpenBSD: SSL_CTX_set_client_cert_cb.3,v 1.4 2018/03/27 17:35:50 schwarze Exp $ 2.\" OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100 3.\" 4.\" This file was written by Lutz Jaenicke <jaenicke@openssl.org>. 5.\" Copyright (c) 2002 The OpenSSL Project. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in 16.\" the documentation and/or other materials provided with the 17.\" distribution. 18.\" 19.\" 3. 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Redistributions of any form whatsoever must retain the following 34.\" acknowledgment: 35.\" "This product includes software developed by the OpenSSL Project 36.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)" 37.\" 38.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY 39.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 40.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 41.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR 42.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 43.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 44.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 45.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 46.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 47.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 48.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED 49.\" OF THE POSSIBILITY OF SUCH DAMAGE. 50.\" 51.Dd $Mdocdate: March 27 2018 $ 52.Dt SSL_CTX_SET_CLIENT_CERT_CB 3 53.Os 54.Sh NAME 55.Nm SSL_CTX_set_client_cert_cb , 56.Nm SSL_CTX_get_client_cert_cb 57.Nd handle client certificate callback function 58.Sh SYNOPSIS 59.In openssl/ssl.h 60.Ft void 61.Fo SSL_CTX_set_client_cert_cb 62.Fa "SSL_CTX *ctx" 63.Fa "int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey)" 64.Fc 65.Ft int 66.Fo "(*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))" 67.Fa "SSL *ssl" "X509 **x509" "EVP_PKEY **pkey" 68.Fc 69.Ft int 70.Fn "(*client_cert_cb)" "SSL *ssl" "X509 **x509" "EVP_PKEY **pkey" 71.Sh DESCRIPTION 72.Fn SSL_CTX_set_client_cert_cb 73sets the 74.Fa client_cert_cb() 75callback that is called when a client certificate is requested by a server and 76no certificate was yet set for the SSL object. 77.Pp 78When 79.Fa client_cert_cb 80is 81.Dv NULL , 82no callback function is used. 83.Pp 84.Fn SSL_CTX_get_client_cert_cb 85returns a pointer to the currently set callback function. 86.Pp 87.Fn client_cert_cb 88is the application-defined callback. 89If it wants to set a certificate, 90a certificate/private key combination must be set using the 91.Fa x509 92and 93.Fa pkey 94arguments and 1 must be returned. 95The certificate will be installed into 96.Fa ssl . 97If no certificate should be set, 980 has to be returned and no certificate will be sent. 99A negative return value will suspend the handshake and the handshake function 100will return immediately. 101.Xr SSL_get_error 3 102will return 103.Dv SSL_ERROR_WANT_X509_LOOKUP 104to indicate that the handshake was suspended. 105The next call to the handshake function will again lead to the call of 106.Fa client_cert_cb() . 107It is the job of the 108.Fa client_cert_cb() 109to store information 110about the state of the last call, if required to continue. 111.Pp 112During a handshake (or renegotiation) 113a server may request a certificate from the client. 114A client certificate must only be sent when the server did send the request. 115.Pp 116When a certificate has been set using the 117.Xr SSL_CTX_use_certificate 3 118family of functions, 119it will be sent to the server. 120The TLS standard requires that only a certificate is sent if it matches the 121list of acceptable CAs sent by the server. 122This constraint is violated by the default behavior of the OpenSSL library. 123Using the callback function it is possible to implement a proper selection 124routine or to allow a user interaction to choose the certificate to be sent. 125.Pp 126If a callback function is defined and no certificate was yet defined for the 127.Vt SSL 128object, the callback function will be called. 129If the callback function returns a certificate, the OpenSSL library 130will try to load the private key and certificate data into the 131.Vt SSL 132object using the 133.Fn SSL_use_certificate 134and 135.Fn SSL_use_private_key 136functions. 137Thus it will permanently install the certificate and key for this SSL object. 138It will not be reset by calling 139.Xr SSL_clear 3 . 140If the callback returns no certificate, the OpenSSL library will not send a 141certificate. 142.Sh SEE ALSO 143.Xr ssl 3 , 144.Xr SSL_clear 3 , 145.Xr SSL_CTX_add_extra_chain_cert 3 , 146.Xr SSL_CTX_use_certificate 3 , 147.Xr SSL_free 3 , 148.Xr SSL_get_client_CA_list 3 149.Sh HISTORY 150.Fn SSL_CTX_set_client_cert_cb 151and 152.Fn SSL_CTX_get_client_cert_cb 153first appeared in SSLeay 0.6.6 and have been available since 154.Ox 2.4 . 155.Sh BUGS 156The 157.Fa client_cert_cb() 158cannot return a complete certificate chain; 159it can only return one client certificate. 160If the chain only has a length of 2, 161the root CA certificate may be omitted according to the TLS standard and 162thus a standard conforming answer can be sent to the server. 163For a longer chain, the client must send the complete chain 164(with the option to leave out the root CA certificate). 165This can be accomplished only by either adding the intermediate CA certificates 166into the trusted certificate store for the 167.Vt SSL_CTX 168object (resulting in having to add CA certificates that otherwise maybe would 169not be trusted), or by adding the chain certificates using the 170.Xr SSL_CTX_add_extra_chain_cert 3 171function, which is only available for the 172.Vt SSL_CTX 173object as a whole and that therefore probably can only apply for one client 174certificate, making the concept of the callback function 175(to allow the choice from several certificates) questionable. 176.Pp 177Once the 178.Vt SSL 179object has been used in conjunction with the callback function, 180the certificate will be set for the 181.Vt SSL 182object and will not be cleared even when 183.Xr SSL_clear 3 184is called. 185It is therefore 186.Em mandatory 187to destroy the 188.Vt SSL 189object using 190.Xr SSL_free 3 191and create a new one to return to the previous state. 192