Cryptography and SSL/TLS Toolkit



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


 #include <openssl/ssl.h>

 typedef unsigned int (*SSL_psk_server_cb_func)(SSL *ssl,
                                                const char *identity,
                                                unsigned char *psk,
                                                unsigned int max_psk_len);

 typedef int (*SSL_psk_find_session_cb_func)(SSL *ssl,
                                             const unsigned char *identity,
                                             size_t identity_len,
                                             SSL_SESSION **sess);

 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);

 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);


TLSv1.3 Pre-Shared Keys (PSKs) and PSKs for TLSv1.2 and below are not compatible.

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 NUL-terminated PSK identity hint hint for SSL context object ctx. SSL_use_psk_identity_hint() sets the given NUL-terminated PSK identity hint hint for the SSL connection object ssl. If hint is NULL the current hint from ctx or ssl is deleted.

In the case where PSK identity hint is NULL, the server does not send the ServerKeyExchange message to the client.

A server application 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 ssl, NUL-terminated PSK identity sent by the client in parameter identity, and a buffer psk of length max_psk_len bytes where the pre-shared key is to be stored.

A client application wishing to use TLSv1.3 PSKs must set a different callback using either SSL_CTX_set_psk_use_session_callback() or SSL_set_psk_use_session_callback() as appropriate.

The callback function is given a pointer to the SSL connection in ssl and an identity in identity of length identity_len. The callback function should identify an SSL_SESSION object that provides the PSK details and store it in *sess. The SSL_SESSION object should, as a minimum, set the master key, the ciphersuite and the protocol version. See 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 *sess is NULL.


A connection established via a TLSv1.3 PSK will appear as if session resumption has occurred so that SSL_session_reused(3) will return true.


SSL_CTX_use_psk_identity_hint() and 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:


PSK identity was not found. An "unknown_psk_identity" alert message will be sent and the connection setup fails.


PSK identity was found and the server callback has provided the PSK successfully in parameter psk. Return value is the length of psk in bytes. It is an error to return a value greater than 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 psk and return the length of the random data, so the connection will fail with decryption_error before it will be finished completely.

The 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.


SSL_CTX_set_psk_use_session_callback(3), SSL_set_psk_use_session_callback(3)


SSL_CTX_set_psk_find_session_callback() and SSL_set_psk_find_session_callback() were added in OpenSSL 1.1.1.

Copyright 2006-2017 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