.\" $OpenBSD: SSL_set_psk_use_session_callback.3,v 1.1 2021/09/14 14:30:57 schwarze Exp $
.\" OpenSSL man3/SSL_CTX_set_psk_client_callback.pod
.\" checked up to 24a535ea Sep 22 13:14:20 2020 +0100
.\"
.\" Copyright (c) 2021 Ingo Schwarze <schwarze@openbsd.org>
.\"
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.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
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.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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.Dd $Mdocdate: September 14 2021 $
.Dt SSL_SET_PSK_USE_SESSION_CALLBACK 3
.Os
.Sh NAME
.Nm SSL_set_psk_use_session_callback ,
.Nm SSL_psk_use_session_cb_func
.Nd set TLS pre-shared key client callback
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft typedef int
.Fo (*SSL_psk_use_session_cb_func)
.Fa "SSL *ssl"
.Fa "const EVP_MD *md"
.Fa "const unsigned char **id"
.Fa "size_t *idlen"
.Fa "SSL_SESSION **session"
.Fc
.Ft void
.Fo SSL_set_psk_use_session_callback
.Fa "SSL *ssl"
.Fa "SSL_psk_use_session_cb_func cb"
.Fc
.Sh DESCRIPTION
LibreSSL provides the stub function
.Fn SSL_set_psk_use_session_callback
to allow compiling application programs
that contain optional support for TLSv1.3 pre-shared keys.
.Pp
LibreSSL does not support TLS pre-shared keys,
and no action occurs when
.Fn SSL_set_psk_use_session_callback
is called.
In particular, both arguments are ignored.
During session negotiation,
LibreSSL never calls the callback
.Fa cb
and always behaves as if that callback succeeded and set the
.Pf * Fa session
pointer to
.Dv NULL .
That is, LibreSSL never sends a pre-shared key to the server
and never aborts the handshake for lack of a pre-shared key.
.Pp
With OpenSSL, a client application wishing to use TLSv1.3 pre-shared keys
can install a callback function
.Fa cb
using
.Fn SSL_set_psk_use_session_callback .
The OpenSSL library may call
.Fa cb
once or twice during session negotiation.
If the callback fails, OpenSSL aborts connection setup.
If the callback succeeds but sets the
.Pf * Fa session
pointer to
.Dv NULL ,
OpenSSL continues the handshake
but does not send a pre-shared key to the server.
.Sh RETURN VALUES
The
.Fn SSL_psk_use_session_cb_func
callback is expected to return 1 on success or 0 on failure.
.Sh HISTORY
.Fn SSL_set_psk_use_session_callback
and
.Fn SSL_psk_use_session_cb_func
first appeared in OpenSSL 1.1.1 and have been available since
.Ox 7.0 .