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1=pod
2
3=head1 NAME
4
5SSL_psk_client_cb_func,
6SSL_psk_use_session_cb_func,
7SSL_CTX_set_psk_client_callback,
8SSL_set_psk_client_callback,
9SSL_CTX_set_psk_use_session_callback,
10SSL_set_psk_use_session_callback
11- set PSK client callback
12
13=head1 SYNOPSIS
14
15 #include <openssl/ssl.h>
16
17 typedef int (*SSL_psk_use_session_cb_func)(SSL *ssl, const EVP_MD *md,
18                                            const unsigned char **id,
19                                            size_t *idlen,
20                                            SSL_SESSION **sess);
21
22
23 void SSL_CTX_set_psk_use_session_callback(SSL_CTX *ctx,
24                                           SSL_psk_use_session_cb_func cb);
25 void SSL_set_psk_use_session_callback(SSL *s, SSL_psk_use_session_cb_func cb);
26
27
28 typedef unsigned int (*SSL_psk_client_cb_func)(SSL *ssl,
29                                                const char *hint,
30                                                char *identity,
31                                                unsigned int max_identity_len,
32                                                unsigned char *psk,
33                                                unsigned int max_psk_len);
34
35 void SSL_CTX_set_psk_client_callback(SSL_CTX *ctx, SSL_psk_client_cb_func cb);
36 void SSL_set_psk_client_callback(SSL *ssl, SSL_psk_client_cb_func cb);
37
38
39=head1 DESCRIPTION
40
41A client application wishing to use TLSv1.3 PSKs should use either
42SSL_CTX_set_psk_use_session_callback() or SSL_set_psk_use_session_callback() as
43appropriate. These functions cannot be used for TLSv1.2 and below PSKs.
44
45The callback function is given a pointer to the SSL connection in B<ssl>.
46
47The first time the callback is called for a connection the B<md> parameter is
48NULL. In some circumstances the callback will be called a second time. In that
49case the server will have specified a ciphersuite to use already and the PSK
50must be compatible with the digest for that ciphersuite. The digest will be
51given in B<md>. The PSK returned by the callback is allowed to be different
52between the first and second time it is called.
53
54On successful completion the callback must store a pointer to an identifier for
55the PSK in B<*id>. The identifier length in bytes should be stored in B<*idlen>.
56The memory pointed to by B<*id> remains owned by the application and should
57be freed by it as required at any point after the handshake is complete.
58
59Additionally the callback should store a pointer to an SSL_SESSION object in
60B<*sess>. This is used as the basis for the PSK, and should, at a minimum, have
61the following fields set:
62
63=over 4
64
65=item The master key
66
67This can be set via a call to L<SSL_SESSION_set1_master_key(3)>.
68
69=item A ciphersuite
70
71Only the handshake digest associated with the ciphersuite is relevant for the
72PSK (the server may go on to negotiate any ciphersuite which is compatible with
73the digest). The application can use any TLSv1.3 ciphersuite. If B<md> is
74not NULL the handshake digest for the ciphersuite should be the same.
75The ciphersuite can be set via a call to <SSL_SESSION_set_cipher(3)>. The
76handshake digest of an SSL_CIPHER object can be checked using
77<SSL_CIPHER_get_handshake_digest(3)>.
78
79=item The protocol version
80
81This can be set via a call to L<SSL_SESSION_set_protocol_version(3)> and should
82be TLS1_3_VERSION.
83
84=back
85
86Additionally the maximum early data value should be set via a call to
87L<SSL_SESSION_set_max_early_data(3)> if the PSK will be used for sending early
88data.
89
90Alternatively an SSL_SESSION created from a previous non-PSK handshake may also
91be used as the basis for a PSK.
92
93Ownership of the SSL_SESSION object is passed to the OpenSSL library and so it
94should not be freed by the application.
95
96It is also possible for the callback to succeed but not supply a PSK. In this
97case no PSK will be sent to the server but the handshake will continue. To do
98this the callback should return successfully and ensure that B<*sess> is
99NULL. The contents of B<*id> and B<*idlen> will be ignored.
100
101A client application wishing to use PSK ciphersuites for TLSv1.2 and below must
102provide a different callback function. This function will be called when the
103client is sending the ClientKeyExchange message to the server.
104
105The purpose of the callback function is to select the PSK identity and
106the pre-shared key to use during the connection setup phase.
107
108The callback is set using functions SSL_CTX_set_psk_client_callback()
109or SSL_set_psk_client_callback(). The callback function is given the
110connection in parameter B<ssl>, a B<NULL>-terminated PSK identity hint
111sent by the server in parameter B<hint>, a buffer B<identity> of
112length B<max_identity_len> bytes where the resulting
113B<NUL>-terminated identity is to be stored, and a buffer B<psk> of
114length B<max_psk_len> bytes where the resulting pre-shared key is to
115be stored.
116
117The callback for use in TLSv1.2 will also work in TLSv1.3 although it is
118recommended to use SSL_CTX_set_psk_use_session_callback()
119or SSL_set_psk_use_session_callback() for this purpose instead. If TLSv1.3 has
120been negotiated then OpenSSL will first check to see if a callback has been set
121via SSL_CTX_set_psk_use_session_callback() or SSL_set_psk_use_session_callback()
122and it will use that in preference. If no such callback is present then it will
123check to see if a callback has been set via SSL_CTX_set_psk_client_callback() or
124SSL_set_psk_client_callback() and use that. In this case the B<hint> value will
125always be NULL and the handshake digest will default to SHA-256 for any returned
126PSK. TLSv1.3 early data exchanges are possible in PSK connections only with the
127B<SSL_psk_use_session_cb_func> callback, and are not possible with the
128B<SSL_psk_client_cb_func> callback.
129
130=head1 NOTES
131
132Note that parameter B<hint> given to the callback may be B<NULL>.
133
134A connection established via a TLSv1.3 PSK will appear as if session resumption
135has occurred so that L<SSL_session_reused(3)> will return true.
136
137There are no known security issues with sharing the same PSK between TLSv1.2 (or
138below) and TLSv1.3. However, the RFC has this note of caution:
139
140"While there is no known way in which the same PSK might produce related output
141in both versions, only limited analysis has been done.  Implementations can
142ensure safety from cross-protocol related output by not reusing PSKs between
143TLS 1.3 and TLS 1.2."
144
145=head1 RETURN VALUES
146
147Return values from the B<SSL_psk_client_cb_func> callback are interpreted as
148follows:
149
150On success (callback found a PSK identity and a pre-shared key to use)
151the length (> 0) of B<psk> in bytes is returned.
152
153Otherwise or on errors the callback should return 0. In this case
154the connection setup fails.
155
156The SSL_psk_use_session_cb_func callback should return 1 on success or 0 on
157failure. In the event of failure the connection setup fails.
158
159=head1 SEE ALSO
160
161L<SSL_CTX_set_psk_find_session_callback(3)>,
162L<SSL_set_psk_find_session_callback(3)>
163
164=head1 HISTORY
165
166SSL_CTX_set_psk_use_session_callback() and SSL_set_psk_use_session_callback()
167were added in OpenSSL 1.1.1.
168
169=head1 COPYRIGHT
170
171Copyright 2006-2020 The OpenSSL Project Authors. All Rights Reserved.
172
173Licensed under the OpenSSL license (the "License").  You may not use
174this file except in compliance with the License.  You can obtain a copy
175in the file LICENSE in the source distribution or at
176L<https://www.openssl.org/source/license.html>.
177
178=cut
179