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>.

1.1.1f                                             2025-02-05              SSL_CTX_SET_PSK_CLIENT_CALLBACK(3SSL)