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. 127 128=head1 NOTES 129 130Note that parameter B<hint> given to the callback may be B<NULL>. 131 132A connection established via a TLSv1.3 PSK will appear as if session resumption 133has occurred so that L<SSL_session_reused(3)> will return true. 134 135There are no known security issues with sharing the same PSK between TLSv1.2 (or 136below) and TLSv1.3. However the RFC has this note of caution: 137 138"While there is no known way in which the same PSK might produce related output 139in both versions, only limited analysis has been done. Implementations can 140ensure safety from cross-protocol related output by not reusing PSKs between 141TLS 1.3 and TLS 1.2." 142 143=head1 RETURN VALUES 144 145Return values from the B<SSL_psk_client_cb_func> callback are interpreted as 146follows: 147 148On success (callback found a PSK identity and a pre-shared key to use) 149the length (> 0) of B<psk> in bytes is returned. 150 151Otherwise or on errors the callback should return 0. In this case 152the connection setup fails. 153 154The SSL_psk_use_session_cb_func callback should return 1 on success or 0 on 155failure. In the event of failure the connection setup fails. 156 157=head1 SEE ALSO 158 159L<SSL_CTX_set_psk_find_session_callback(3)>, 160L<SSL_set_psk_find_session_callback(3)> 161 162=head1 HISTORY 163 164SSL_CTX_set_psk_use_session_callback() and SSL_set_psk_use_session_callback() 165were added in OpenSSL 1.1.1. 166 167=head1 COPYRIGHT 168 169Copyright 2006-2018 The OpenSSL Project Authors. All Rights Reserved. 170 171Licensed under the OpenSSL license (the "License"). You may not use 172this file except in compliance with the License. You can obtain a copy 173in the file LICENSE in the source distribution or at 174L<https://www.openssl.org/source/license.html>. 175 176=cut 177