SSL_CTX_SET_CLIENT_CE... BSD Library Functions Manual SSL_CTX_SET_CLIENT_CE...NAME
SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb — handle client
certificate callback function
SYNOPSIS
#include <openssl/ssl.h>
void
SSL_CTX_set_client_cert_cb(SSL_CTX *ctx,
int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey));
int
(*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))(SSL *ssl, X509 **x509,
EVP_PKEY **pkey);
int
(*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey);
DESCRIPTIONSSL_CTX_set_client_cert_cb() sets the client_cert_cb() callback that is
called when a client certificate is requested by a server and no certifi‐
cate was yet set for the SSL object.
When client_cert_cb is NULL, no callback function is used.
SSL_CTX_get_client_cert_cb() returns a pointer to the currently set call‐
back function.
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 x509 and pkey arguments and 1 must be returned. The certificate will
be installed into ssl; see the NOTES and BUGS sections. If no certifi‐
cate 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. SSL_get_error(3) will return
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
client_cert_cb(). It is the job of the client_cert_cb() to store infor‐
mation about the state of the last call, if required to continue.
NOTES
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.
When a certificate has been set using the SSL_CTX_use_certificate(3) fam‐
ily 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.
If a callback function is defined and no certificate was yet defined for
the 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 SSL object using the
SSL_use_certificate() and 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 SSL_clear(3). If the callback returns no cer‐
tificate, the OpenSSL library will not send a certificate.
SEE ALSOssl(3), SSL_clear(3), SSL_CTX_add_extra_chain_cert(3),
SSL_CTX_use_certificate(3), SSL_free(3), SSL_get_client_CA_list(3)BUGS
The 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 certifi‐
cate store for the SSL_CTX object (resulting in having to add CA certifi‐
cates that otherwise maybe would not be trusted), or by adding the chain
certificates using the SSL_CTX_add_extra_chain_cert(3) function, which is
only available for the SSL_CTX object as a whole and that therefore prob‐
ably can only apply for one client certificate, making the concept of the
callback function (to allow the choice from several certificates) ques‐
tionable.
Once the SSL object has been used in conjunction with the callback func‐
tion, the certificate will be set for the SSL object and will not be
cleared even when SSL_clear(3) is called. It is therefore mandatory to
destroy the SSL object using SSL_free(3) and create a new one to return
to the previous state.
BSD April 29, 2024 BSD
