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 NUL-terminated PSK identity hint sent
       by the server in parameter hint, a buffer  identity  of  length  max_identity_len  bytes  (including  the
       NUL-terminator)  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. TLSv1.3
       early data exchanges are possible in PSK connections only with the SSL_psk_use_session_cb_func  callback,
       and are not possible with the SSL_psk_client_cb_func callback.

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(7), 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 2006-2021 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (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>.

3.0.13                                             2025-02-05              SSL_CTX_SET_PSK_CLIENT_CALLBACK(3SSL)