1=pod
2
3=head1 NAME
4
5SSL_psk_server_cb_func,
6SSL_psk_find_session_cb_func,
7SSL_CTX_use_psk_identity_hint,
8SSL_use_psk_identity_hint,
9SSL_CTX_set_psk_server_callback,
10SSL_set_psk_server_callback,
11SSL_CTX_set_psk_find_session_callback,
12SSL_set_psk_find_session_callback
13- set PSK identity hint to use
14
15=head1 SYNOPSIS
16
17 #include <openssl/ssl.h>
18
19 typedef int (*SSL_psk_find_session_cb_func)(SSL *ssl,
20                                             const unsigned char *identity,
21                                             size_t identity_len,
22                                             SSL_SESSION **sess);
23
24
25 void SSL_CTX_set_psk_find_session_callback(SSL_CTX *ctx,
26                                            SSL_psk_find_session_cb_func cb);
27 void SSL_set_psk_find_session_callback(SSL *s, SSL_psk_find_session_cb_func cb);
28
29 typedef unsigned int (*SSL_psk_server_cb_func)(SSL *ssl,
30                                                const char *identity,
31                                                unsigned char *psk,
32                                                unsigned int max_psk_len);
33
34 int SSL_CTX_use_psk_identity_hint(SSL_CTX *ctx, const char *hint);
35 int SSL_use_psk_identity_hint(SSL *ssl, const char *hint);
36
37 void SSL_CTX_set_psk_server_callback(SSL_CTX *ctx, SSL_psk_server_cb_func cb);
38 void SSL_set_psk_server_callback(SSL *ssl, SSL_psk_server_cb_func cb);
39
40=head1 DESCRIPTION
41
42A server application wishing to use TLSv1.3 PSKs should set a callback
43using either SSL_CTX_set_psk_find_session_callback() or
44SSL_set_psk_find_session_callback() as appropriate.
45
46The callback function is given a pointer to the SSL connection in B<ssl> and
47an identity in B<identity> of length B<identity_len>. The callback function
48should identify an SSL_SESSION object that provides the PSK details and store it
49in B<*sess>. The SSL_SESSION object should, as a minimum, set the master key,
50the ciphersuite and the protocol version. See
51L<SSL_CTX_set_psk_use_session_callback(3)> for details.
52
53It is also possible for the callback to succeed but not supply a PSK. In this
54case no PSK will be used but the handshake will continue. To do this the
55callback should return successfully and ensure that B<*sess> is
56NULL.
57
58Identity hints are not relevant for TLSv1.3. A server application wishing to use
59PSK ciphersuites for TLSv1.2 and below may call SSL_CTX_use_psk_identity_hint()
60to set the given B<NUL>-terminated PSK identity hint B<hint> for SSL context
61object B<ctx>. SSL_use_psk_identity_hint() sets the given B<NUL>-terminated PSK
62identity hint B<hint> for the SSL connection object B<ssl>. If B<hint> is
63B<NULL> the current hint from B<ctx> or B<ssl> is deleted.
64
65In the case where PSK identity hint is B<NULL>, the server does not send the
66ServerKeyExchange message to the client.
67
68A server application wishing to use PSKs for TLSv1.2 and below must provide a
69callback function which is called when the server receives the
70ClientKeyExchange message from the client. The purpose of the callback function
71is to validate the received PSK identity and to fetch the pre-shared key used
72during the connection setup phase. The callback is set using the functions
73SSL_CTX_set_psk_server_callback() or SSL_set_psk_server_callback(). The callback
74function is given the connection in parameter B<ssl>, B<NUL>-terminated PSK
75identity sent by the client in parameter B<identity>, and a buffer B<psk> of
76length B<max_psk_len> bytes where the pre-shared key is to be stored.
77
78The callback for use in TLSv1.2 will also work in TLSv1.3 although it is
79recommended to use SSL_CTX_set_psk_find_session_callback()
80or SSL_set_psk_find_session_callback() for this purpose instead. If TLSv1.3 has
81been negotiated then OpenSSL will first check to see if a callback has been set
82via SSL_CTX_set_psk_find_session_callback() or SSL_set_psk_find_session_callback()
83and it will use that in preference. If no such callback is present then it will
84check to see if a callback has been set via SSL_CTX_set_psk_server_callback() or
85SSL_set_psk_server_callback() and use that. In this case the handshake digest
86will default to SHA-256 for any returned PSK. TLSv1.3 early data exchanges are
87possible in PSK connections only with the B<SSL_psk_find_session_cb_func>
88callback, and are not possible with the B<SSL_psk_server_cb_func> callback.
89
90A connection established via a TLSv1.3 PSK will appear as if session resumption
91has occurred so that L<SSL_session_reused(3)> will return true.
92
93=head1 RETURN VALUES
94
95B<SSL_CTX_use_psk_identity_hint()> and B<SSL_use_psk_identity_hint()> return
961 on success, 0 otherwise.
97
98Return values from the TLSv1.2 and below server callback are interpreted as
99follows:
100
101=over 4
102
103=item Z<>0
104
105PSK identity was not found. An "unknown_psk_identity" alert message
106will be sent and the connection setup fails.
107
108=item E<gt>0
109
110PSK identity was found and the server callback has provided the PSK
111successfully in parameter B<psk>. Return value is the length of
112B<psk> in bytes. It is an error to return a value greater than
113B<max_psk_len>.
114
115If the PSK identity was not found but the callback instructs the
116protocol to continue anyway, the callback must provide some random
117data to B<psk> and return the length of the random data, so the
118connection will fail with decryption_error before it will be finished
119completely.
120
121=back
122
123The B<SSL_psk_find_session_cb_func> callback should return 1 on success or 0 on
124failure. In the event of failure the connection setup fails.
125
126=head1 NOTES
127
128There are no known security issues with sharing the same PSK between TLSv1.2 (or
129below) and TLSv1.3. However, the RFC has this note of caution:
130
131"While there is no known way in which the same PSK might produce related output
132in both versions, only limited analysis has been done.  Implementations can
133ensure safety from cross-protocol related output by not reusing PSKs between
134TLS 1.3 and TLS 1.2."
135
136=head1 SEE ALSO
137
138L<ssl(7)>,
139L<SSL_CTX_set_psk_use_session_callback(3)>,
140L<SSL_set_psk_use_session_callback(3)>
141
142=head1 HISTORY
143
144SSL_CTX_set_psk_find_session_callback() and SSL_set_psk_find_session_callback()
145were added in OpenSSL 1.1.1.
146
147=head1 COPYRIGHT
148
149Copyright 2006-2020 The OpenSSL Project Authors. All Rights Reserved.
150
151Licensed under the Apache License 2.0 (the "License").  You may not use
152this file except in compliance with the License.  You can obtain a copy
153in the file LICENSE in the source distribution or at
154L<https://www.openssl.org/source/license.html>.
155
156=cut
157