=pod

=head1 NAME

SSL_psk_server_cb_func,

SSL_psk_find_session_cb_func,

SSL_CTX_use_psk_identity_hint,

SSL_use_psk_identity_hint,

SSL_CTX_set_psk_server_callback,

SSL_set_psk_server_callback,

SSL_CTX_set_psk_find_session_callback,

SSL_set_psk_find_session_callback

- set PSK identity hint to use

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 typedef int (*SSL_psk_find_session_cb_func)(SSL *ssl,

                                             const unsigned char *identity,

                                             size_t identity_len,

                                             SSL_SESSION **sess);

 void SSL_CTX_set_psk_find_session_callback(SSL_CTX *ctx,

                                            SSL_psk_find_session_cb_func cb);

 void SSL_set_psk_find_session_callback(SSL *s, SSL_psk_find_session_cb_func cb);

 typedef unsigned int (*SSL_psk_server_cb_func)(SSL *ssl,

                                                const char *identity,

                                                unsigned char *psk,

                                                unsigned int max_psk_len);

 int SSL_CTX_use_psk_identity_hint(SSL_CTX *ctx, const char *hint);

 int SSL_use_psk_identity_hint(SSL *ssl, const char *hint);

 void SSL_CTX_set_psk_server_callback(SSL_CTX *ctx, SSL_psk_server_cb_func cb);

 void SSL_set_psk_server_callback(SSL *ssl, SSL_psk_server_cb_func cb);

=head1 DESCRIPTION

A server application wishing to use TLSv1.3 PSKs should set a callback

using either SSL_CTX_set_psk_find_session_callback() or

SSL_set_psk_find_session_callback() as appropriate.

The callback function is given a pointer to the SSL connection in B<ssl> and

an identity in B<identity> of length B<identity_len>. The callback function

should identify an SSL_SESSION object that provides the PSK details and store it

in B<*sess>. The SSL_SESSION object should, as a minimum, set the master key,

the ciphersuite and the protocol version. See

L<SSL_CTX_set_psk_use_session_callback(3)> for details.

It is also possible for the callback to succeed but not supply a PSK. In this

case no PSK will be used but the handshake will continue. To do this the

callback should return successfully and ensure that B<*sess> is

NULL.

Identity hints are not relevant for TLSv1.3. A server application wishing to use

PSK ciphersuites for TLSv1.2 and below may call SSL_CTX_use_psk_identity_hint()

to set the given B<NUL>-terminated PSK identity hint B<hint> for SSL context

object B<ctx>. SSL_use_psk_identity_hint() sets the given B<NUL>-terminated PSK

identity hint B<hint> for the SSL connection object B<ssl>. If B<hint> is

B<NULL> the current hint from B<ctx> or B<ssl> is deleted.

In the case where PSK identity hint is B<NULL>, the server does not send the

ServerKeyExchange message to the client.

A server application wishing to use PSKs for TLSv1.2 and below must provide a

callback function which is called when the server receives the

ClientKeyExchange message from the client. The purpose of the callback function

is to validate the received PSK identity and to fetch the pre-shared key used

during the connection setup phase. The callback is set using the functions

SSL_CTX_set_psk_server_callback() or SSL_set_psk_server_callback(). The callback

function is given the connection in parameter B<ssl>, B<NUL>-terminated PSK

identity sent by the client in parameter B<identity>, and a buffer B<psk> of

length B<max_psk_len> bytes where the 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_find_session_callback()

or SSL_set_psk_find_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_find_session_callback() or SSL_set_psk_find_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_server_callback() or

SSL_set_psk_server_callback() and use that. In this case the handshake digest

will default to SHA-256 for any returned PSK.

=head1 NOTES

A connection established via a TLSv1.3 PSK will appear as if session resumption

has occurred so that L<SSL_session_reused(3)> will return true.

=head1 RETURN VALUES

B<SSL_CTX_use_psk_identity_hint()> and B<SSL_use_psk_identity_hint()> return

1 on success, 0 otherwise.

Return values from the TLSv1.2 and below server callback are interpreted as

follows:

=over 4

=item Z<>0

PSK identity was not found. An "unknown_psk_identity" alert message

will be sent and the connection setup fails.

=item E<gt>0

PSK identity was found and the server callback has provided the PSK

successfully in parameter B<psk>. Return value is the length of

B<psk> in bytes. It is an error to return a value greater than

B<max_psk_len>.

If the PSK identity was not found but the callback instructs the

protocol to continue anyway, the callback must provide some random

data to B<psk> and return the length of the random data, so the

connection will fail with decryption_error before it will be finished

completely.

=back

The B<SSL_psk_find_session_cb_func> callback should return 1 on success or 0 on

failure. In the event of failure the connection setup fails.

=head1 NOTES

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."

=head1 SEE ALSO

L<SSL_CTX_set_psk_use_session_callback(3)>,

L<SSL_set_psk_use_session_callback(3)>

=head1 HISTORY

SSL_CTX_set_psk_find_session_callback() and SSL_set_psk_find_session_callback()

were added in OpenSSL 1.1.1.

=head1 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

L<https://www.openssl.org/source/license.html>.

=cut