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SSL_CTX_SET_PSK_CLIENT_CALLBACK(3)  OpenSSL SSL_CTX_SET_PSK_CLIENT_CALLBACK(3)

NAME
       SSL_psk_client_cb_func, SSL_psk_use_session_cb_func,
       SSL_CTX_set_psk_client_callback,	SSL_set_psk_client_callback,
       SSL_CTX_set_psk_use_session_callback, SSL_set_psk_use_session_callback
       - set PSK client	callback

SYNOPSIS
	#include <openssl/ssl.h>

	typedef	int (*SSL_psk_use_session_cb_func)(SSL *ssl, const EVP_MD *md,
						   const unsigned char **id,
						   size_t *idlen,
						   SSL_SESSION **sess);

	void SSL_CTX_set_psk_use_session_callback(SSL_CTX *ctx,
						  SSL_psk_use_session_cb_func cb);
	void SSL_set_psk_use_session_callback(SSL *s, SSL_psk_use_session_cb_func cb);

	typedef	unsigned int (*SSL_psk_client_cb_func)(SSL *ssl,
						       const char *hint,
						       char *identity,
						       unsigned	int max_identity_len,
						       unsigned	char *psk,
						       unsigned	int max_psk_len);

	void SSL_CTX_set_psk_client_callback(SSL_CTX *ctx, SSL_psk_client_cb_func cb);
	void SSL_set_psk_client_callback(SSL *ssl, SSL_psk_client_cb_func cb);

DESCRIPTION
       A client	application wishing to use TLSv1.3 PSKs	should use either
       SSL_CTX_set_psk_use_session_callback() or
       SSL_set_psk_use_session_callback() as appropriate. These	functions
       cannot be used for TLSv1.2 and below PSKs.

       The callback function is	given a	pointer	to the SSL connection in ssl.

       The first time the callback is called for a connection the md parameter
       is NULL.	In some	circumstances the callback will	be called a second
       time. In	that case the server will have specified a ciphersuite to use
       already and the PSK must	be compatible with the digest for that
       ciphersuite. The	digest will be given in	md. The	PSK returned by	the
       callback	is allowed to be different between the first and second	time
       it is called.

       On successful completion	the callback must store	a pointer to an
       identifier for the PSK in *id. The identifier length in bytes should be
       stored in *idlen.  The memory pointed to	by *id remains owned by	the
       application and should be freed by it as	required at any	point after
       the handshake is	complete.

       Additionally the	callback should	store a	pointer	to an SSL_SESSION
       object in *sess.	This is	used as	the basis for the PSK, and should, at
       a minimum, have the following fields set:

       The master key
	   This	can be set via a call to SSL_SESSION_set1_master_key(3).

       A ciphersuite
	   Only	the handshake digest associated	with the ciphersuite is
	   relevant for	the PSK	(the server may	go on to negotiate any
	   ciphersuite which is	compatible with	the digest). The application
	   can use any TLSv1.3 ciphersuite. If md is not NULL the handshake
	   digest for the ciphersuite should be	the same.  The ciphersuite can
	   be set via a	call to	<SSL_SESSION_set_cipher(3)>. The handshake
	   digest of an	SSL_CIPHER object can be checked using
	   <SSL_CIPHER_get_handshake_digest(3)>.

       The protocol version
	   This	can be set via a call to SSL_SESSION_set_protocol_version(3)
	   and should be TLS1_3_VERSION.

       Additionally the	maximum	early data value should	be set via a call to
       SSL_SESSION_set_max_early_data(3) if the	PSK will be used for sending
       early data.

       Alternatively an	SSL_SESSION created from a previous non-PSK handshake
       may also	be used	as the basis for a PSK.

       Ownership of the	SSL_SESSION object is passed to	the OpenSSL library
       and so it should	not be freed by	the application.

       It is also possible for the callback to succeed but not supply a	PSK.
       In this case no PSK will	be sent	to the server but the handshake	will
       continue. To do this the	callback should	return successfully and	ensure
       that *sess is NULL. The contents	of *id and *idlen will be ignored.

       A client	application wishing to use PSK ciphersuites for	TLSv1.2	and
       below must provide a different callback function. This function will be
       called when the client is sending the ClientKeyExchange message to the
       server.

       The purpose of the callback function is to select the PSK identity and
       the pre-shared key to use during	the connection setup phase.

       The callback is set using functions SSL_CTX_set_psk_client_callback()
       or SSL_set_psk_client_callback(). The callback function is given	the
       connection in parameter ssl, a NULL-terminated PSK identity hint	sent
       by the server in	parameter hint,	a buffer identity of length
       max_identity_len	bytes where the	resulting NUL-terminated identity is
       to be stored, and a buffer psk of length	max_psk_len bytes where	the
       resulting pre-shared key	is to be stored.

       The callback for	use in TLSv1.2 will also work in TLSv1.3 although it
       is recommended to use SSL_CTX_set_psk_use_session_callback() or
       SSL_set_psk_use_session_callback() for this purpose instead. If TLSv1.3
       has been	negotiated then	OpenSSL	will first check to see	if a callback
       has been	set via	SSL_CTX_set_psk_use_session_callback() or
       SSL_set_psk_use_session_callback() and it will use that in preference.
       If no such callback is present then it will check to see	if a callback
       has been	set via	SSL_CTX_set_psk_client_callback() or
       SSL_set_psk_client_callback() and use that. In this case	the hint value
       will always be NULL and the handshake digest will default to SHA-256
       for any returned	PSK.

NOTES
       Note that parameter hint	given to the callback may be NULL.

       A connection established	via a TLSv1.3 PSK will appear as if session
       resumption has occurred so that SSL_session_reused(3) will return true.

       There are no known security issues with sharing the same	PSK between
       TLSv1.2 (or below) and TLSv1.3. However the RFC has this	note of
       caution:

       "While there is no known	way in which the same PSK might	produce
       related output in both versions,	only limited analysis has been done.
       Implementations can ensure safety from cross-protocol related output by
       not reusing PSKs	between	TLS 1.3	and TLS	1.2."

RETURN VALUES
       Return values from the SSL_psk_client_cb_func callback are interpreted
       as follows:

       On success (callback found a PSK	identity and a pre-shared key to use)
       the length (> 0)	of psk in bytes	is returned.

       Otherwise or on errors the callback should return 0. In this case the
       connection setup	fails.

       The SSL_psk_use_session_cb_func callback	should return 1	on success or
       0 on failure. In	the event of failure the connection setup fails.

SEE ALSO
       SSL_CTX_set_psk_find_session_callback(3),
       SSL_set_psk_find_session_callback(3)

HISTORY
       SSL_CTX_set_psk_use_session_callback() and
       SSL_set_psk_use_session_callback() were added in	OpenSSL	1.1.1.

COPYRIGHT
       Copyright 2006-2018 The OpenSSL Project Authors.	All Rights Reserved.

       Licensed	under the OpenSSL license (the "License").  You	may not	use
       this file except	in compliance with the License.	 You can obtain	a copy
       in the file LICENSE in the source distribution or at
       <https://www.openssl.org/source/license.html>.

1.1.1a				  2018-11-20SSL_CTX_SET_PSK_CLIENT_CALLBACK(3)

NAME | SYNOPSIS | DESCRIPTION | NOTES | RETURN VALUES | SEE ALSO | HISTORY | COPYRIGHT

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