1SSL_CTX_SET_PSK_CLIENT_CALLBACK(3)  OpenSSL SSL_CTX_SET_PSK_CLIENT_CALLBACK(3)
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3
4

NAME

6       SSL_psk_client_cb_func, SSL_psk_use_session_cb_func,
7       SSL_CTX_set_psk_client_callback, SSL_set_psk_client_callback,
8       SSL_CTX_set_psk_use_session_callback, SSL_set_psk_use_session_callback
9       - set PSK client callback
10

SYNOPSIS

12        #include <openssl/ssl.h>
13
14        typedef int (*SSL_psk_use_session_cb_func)(SSL *ssl, const EVP_MD *md,
15                                                   const unsigned char **id,
16                                                   size_t *idlen,
17                                                   SSL_SESSION **sess);
18
19
20        void SSL_CTX_set_psk_use_session_callback(SSL_CTX *ctx,
21                                                  SSL_psk_use_session_cb_func cb);
22        void SSL_set_psk_use_session_callback(SSL *s, SSL_psk_use_session_cb_func cb);
23
24
25        typedef unsigned int (*SSL_psk_client_cb_func)(SSL *ssl,
26                                                       const char *hint,
27                                                       char *identity,
28                                                       unsigned int max_identity_len,
29                                                       unsigned char *psk,
30                                                       unsigned int max_psk_len);
31
32        void SSL_CTX_set_psk_client_callback(SSL_CTX *ctx, SSL_psk_client_cb_func cb);
33        void SSL_set_psk_client_callback(SSL *ssl, SSL_psk_client_cb_func cb);
34

DESCRIPTION

36       A client application wishing to use TLSv1.3 PSKs should use either
37       SSL_CTX_set_psk_use_session_callback() or
38       SSL_set_psk_use_session_callback() as appropriate. These functions
39       cannot be used for TLSv1.2 and below PSKs.
40
41       The callback function is given a pointer to the SSL connection in ssl.
42
43       The first time the callback is called for a connection the md parameter
44       is NULL. In some circumstances the callback will be called a second
45       time. In that case the server will have specified a ciphersuite to use
46       already and the PSK must be compatible with the digest for that
47       ciphersuite. The digest will be given in md. The PSK returned by the
48       callback is allowed to be different between the first and second time
49       it is called.
50
51       On successful completion the callback must store a pointer to an
52       identifier for the PSK in *id. The identifier length in bytes should be
53       stored in *idlen.  The memory pointed to by *id remains owned by the
54       application and should be freed by it as required at any point after
55       the handshake is complete.
56
57       Additionally the callback should store a pointer to an SSL_SESSION
58       object in *sess. This is used as the basis for the PSK, and should, at
59       a minimum, have the following fields set:
60
61       The master key
62           This can be set via a call to SSL_SESSION_set1_master_key(3).
63
64       A ciphersuite
65           Only the handshake digest associated with the ciphersuite is
66           relevant for the PSK (the server may go on to negotiate any
67           ciphersuite which is compatible with the digest). The application
68           can use any TLSv1.3 ciphersuite. If md is not NULL the handshake
69           digest for the ciphersuite should be the same.  The ciphersuite can
70           be set via a call to <SSL_SESSION_set_cipher(3)>. The handshake
71           digest of an SSL_CIPHER object can be checked using
72           <SSL_CIPHER_get_handshake_digest(3)>.
73
74       The protocol version
75           This can be set via a call to SSL_SESSION_set_protocol_version(3)
76           and should be TLS1_3_VERSION.
77
78       Additionally the maximum early data value should be set via a call to
79       SSL_SESSION_set_max_early_data(3) if the PSK will be used for sending
80       early data.
81
82       Alternatively an SSL_SESSION created from a previous non-PSK handshake
83       may also be used as the basis for a PSK.
84
85       Ownership of the SSL_SESSION object is passed to the OpenSSL library
86       and so it should not be freed by the application.
87
88       It is also possible for the callback to succeed but not supply a PSK.
89       In this case no PSK will be sent to the server but the handshake will
90       continue. To do this the callback should return successfully and ensure
91       that *sess is NULL. The contents of *id and *idlen will be ignored.
92
93       A client application wishing to use PSK ciphersuites for TLSv1.2 and
94       below must provide a different callback function. This function will be
95       called when the client is sending the ClientKeyExchange message to the
96       server.
97
98       The purpose of the callback function is to select the PSK identity and
99       the pre-shared key to use during the connection setup phase.
100
101       The callback is set using functions SSL_CTX_set_psk_client_callback()
102       or SSL_set_psk_client_callback(). The callback function is given the
103       connection in parameter ssl, a NULL-terminated PSK identity hint sent
104       by the server in parameter hint, a buffer identity of length
105       max_identity_len bytes where the resulting NUL-terminated identity is
106       to be stored, and a buffer psk of length max_psk_len bytes where the
107       resulting pre-shared key is to be stored.
108
109       The callback for use in TLSv1.2 will also work in TLSv1.3 although it
110       is recommended to use SSL_CTX_set_psk_use_session_callback() or
111       SSL_set_psk_use_session_callback() for this purpose instead. If TLSv1.3
112       has been negotiated then OpenSSL will first check to see if a callback
113       has been set via SSL_CTX_set_psk_use_session_callback() or
114       SSL_set_psk_use_session_callback() and it will use that in preference.
115       If no such callback is present then it will check to see if a callback
116       has been set via SSL_CTX_set_psk_client_callback() or
117       SSL_set_psk_client_callback() and use that. In this case the hint value
118       will always be NULL and the handshake digest will default to SHA-256
119       for any returned PSK. TLSv1.3 early data exchanges are possible in PSK
120       connections only with the SSL_psk_use_session_cb_func callback, and are
121       not possible with the SSL_psk_client_cb_func callback.
122

NOTES

124       Note that parameter hint given to the callback may be NULL.
125
126       A connection established via a TLSv1.3 PSK will appear as if session
127       resumption has occurred so that SSL_session_reused(3) will return true.
128
129       There are no known security issues with sharing the same PSK between
130       TLSv1.2 (or below) and TLSv1.3. However, the RFC has this note of
131       caution:
132
133       "While there is no known way in which the same PSK might produce
134       related output in both versions, only limited analysis has been done.
135       Implementations can ensure safety from cross-protocol related output by
136       not reusing PSKs between TLS 1.3 and TLS 1.2."
137

RETURN VALUES

139       Return values from the SSL_psk_client_cb_func callback are interpreted
140       as follows:
141
142       On success (callback found a PSK identity and a pre-shared key to use)
143       the length (> 0) of psk in bytes is returned.
144
145       Otherwise or on errors the callback should return 0. In this case the
146       connection setup fails.
147
148       The SSL_psk_use_session_cb_func callback should return 1 on success or
149       0 on failure. In the event of failure the connection setup fails.
150

SEE ALSO

152       SSL_CTX_set_psk_find_session_callback(3),
153       SSL_set_psk_find_session_callback(3)
154

HISTORY

156       SSL_CTX_set_psk_use_session_callback() and
157       SSL_set_psk_use_session_callback() were added in OpenSSL 1.1.1.
158
160       Copyright 2006-2020 The OpenSSL Project Authors. All Rights Reserved.
161
162       Licensed under the OpenSSL license (the "License").  You may not use
163       this file except in compliance with the License.  You can obtain a copy
164       in the file LICENSE in the source distribution or at
165       <https://www.openssl.org/source/license.html>.
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1691.1.1i                            2021-07-22SSL_CTX_SET_PSK_CLIENT_CALLBACK(3)
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