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.\" ========================================================================
.\"
.IX Title "SSL_CTX_set_cert_cb 3"
.TH SSL_CTX_set_cert_cb 3 "2019-12-20" "1.0.2u" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH "NAME"
SSL_CTX_set_cert_cb, SSL_set_cert_cb \- handle certificate callback function
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/ssl.h>
\&
\& void SSL_CTX_set_cert_cb(SSL_CTX *c, int (*cert_cb)(SSL *ssl, void *arg), void *arg);
\& void SSL_set_cert_cb(SSL *s, int (*cert_cb)(SSL *ssl, void *arg), void *arg);
\&
\& int (*cert_cb)(SSL *ssl, void *arg);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBSSL_CTX_set_cert_cb()\fR and \fBSSL_set_cert_cb()\fR sets the \fB\fBcert_cb()\fB\fR callback,
\&\fBarg\fR value is pointer which is passed to the application callback.
.PP
When \fB\fBcert_cb()\fB\fR is \s-1NULL,\s0 no callback function is used.
.PP
\&\fBcert_cb()\fR is the application defined callback. It is called before a
certificate will be used by a client or server. The callback can then inspect
the passed \fBssl\fR structure and set or clear any appropriate certificates. If
the callback is successful it \fB\s-1MUST\s0\fR return 1 even if no certificates have
been set. A zero is returned on error which will abort the handshake with a
fatal internal error alert. A negative return value will suspend the handshake
and the handshake function will return immediately.
\&\fBSSL_get_error\fR\|(3) will return \s-1SSL_ERROR_WANT_X509_LOOKUP\s0 to
indicate, that the handshake was suspended. The next call to the handshake
function will again lead to the call of \fBcert_cb()\fR. It is the job of the
\&\fBcert_cb()\fR to store information about the state of the last call,
if required to continue.
.SH "NOTES"
.IX Header "NOTES"
An application will typically call \fBSSL_use_certificate()\fR and
\&\fBSSL_use_PrivateKey()\fR to set the end entity certificate and private key.
It can add intermediate and optionally the root \s-1CA\s0 certificates using
\&\fBSSL_add1_chain_cert()\fR.
.PP
It might also call \fBSSL_certs_clear()\fR to delete any certificates associated
with the \fB\s-1SSL\s0\fR object.
.PP
The certificate callback functionality supercedes the (largely broken)
functionality provided by the old client certificate callback interface.
It is \fBalways\fR called even is a certificate is already set so the callback
can modify or delete the existing certificate.
.PP
A more advanced callback might examine the handshake parameters and set
whatever chain is appropriate. For example a legacy client supporting only
\&\s-1TLS\s0 v1.0 might receive a certificate chain signed using \s-1SHA1\s0 whereas a
\&\s-1TLS\s0 v1.2 client which advertises support for \s-1SHA256\s0 could receive a chain
using \s-1SHA256.\s0
.PP
Normal server sanity checks are performed on any certificates set
by the callback. So if an \s-1EC\s0 chain is set for a curve the client does not
support it will \fBnot\fR be used.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBssl\fR\|(3), \fBSSL_use_certificate\fR\|(3),
\&\fBSSL_add1_chain_cert\fR\|(3),
\&\fBSSL_get_client_CA_list\fR\|(3),
\&\fBSSL_clear\fR\|(3), \fBSSL_free\fR\|(3)