.\" $OpenBSD: SSL_CTX_set_client_cert_cb.3,v 1.4 2018/03/27 17:35:50 schwarze Exp $ .\" OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100 .\" .\" This file was written by Lutz Jaenicke . .\" Copyright (c) 2002 The OpenSSL Project. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" .\" 3. 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IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: March 27 2018 $ .Dt SSL_CTX_SET_CLIENT_CERT_CB 3 .Os .Sh NAME .Nm SSL_CTX_set_client_cert_cb , .Nm SSL_CTX_get_client_cert_cb .Nd handle client certificate callback function .Sh SYNOPSIS .In openssl/ssl.h .Ft void .Fo SSL_CTX_set_client_cert_cb .Fa "SSL_CTX *ctx" .Fa "int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey)" .Fc .Ft int .Fo "(*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))" .Fa "SSL *ssl" "X509 **x509" "EVP_PKEY **pkey" .Fc .Ft int .Fn "(*client_cert_cb)" "SSL *ssl" "X509 **x509" "EVP_PKEY **pkey" .Sh DESCRIPTION .Fn SSL_CTX_set_client_cert_cb sets the .Fa 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. .Pp When .Fa client_cert_cb is .Dv NULL , no callback function is used. .Pp .Fn SSL_CTX_get_client_cert_cb returns a pointer to the currently set callback function. .Pp .Fn 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 .Fa x509 and .Fa pkey arguments and 1 must be returned. The certificate will be installed into .Fa 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. .Xr SSL_get_error 3 will return .Dv 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 .Fa client_cert_cb() . It is the job of the .Fa client_cert_cb() to store information about the state of the last call, if required to continue. .Pp 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. .Pp When a certificate has been set using the .Xr 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. .Pp If a callback function is defined and no certificate was yet defined for the .Vt 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 .Vt SSL object using the .Fn SSL_use_certificate and .Fn 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 .Xr SSL_clear 3 . If the callback returns no certificate, the OpenSSL library will not send a certificate. .Sh SEE ALSO .Xr ssl 3 , .Xr SSL_clear 3 , .Xr SSL_CTX_add_extra_chain_cert 3 , .Xr SSL_CTX_use_certificate 3 , .Xr SSL_free 3 , .Xr SSL_get_client_CA_list 3 .Sh HISTORY .Fn SSL_CTX_set_client_cert_cb and .Fn SSL_CTX_get_client_cert_cb first appeared in SSLeay 0.6.6 and have been available since .Ox 2.4 . .Sh BUGS The .Fa 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 .Vt 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 .Xr SSL_CTX_add_extra_chain_cert 3 function, which is only available for the .Vt 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. .Pp Once the .Vt SSL object has been used in conjunction with the callback function, the certificate will be set for the .Vt SSL object and will not be cleared even when .Xr SSL_clear 3 is called. It is therefore .Em mandatory to destroy the .Vt SSL object using .Xr SSL_free 3 and create a new one to return to the previous state.