Cryptography and SSL/TLS Toolkit



SSL_psk_client_cb_func, SSL_psk_use_session_cb_func, SSL_CTX_set_psk_client_callback, SSL_set_psk_client_callback, SSL_CTX_set_psk_use_session_callback, SSL_set_psk_use_session_callback - set PSK client callback


 #include <openssl/ssl.h>

 typedef unsigned int (*SSL_psk_client_cb_func)(SSL *ssl,
                                                const char *hint,
                                                char *identity,
                                                unsigned int max_identity_len,
                                                unsigned char *psk,
                                                unsigned int max_psk_len);
 typedef int (*SSL_psk_use_session_cb_func)(SSL *ssl, const EVP_MD *md,
                                            const unsigned char **id,
                                            size_t *idlen,
                                            SSL_SESSION **sess);

 void SSL_CTX_set_psk_client_callback(SSL_CTX *ctx, SSL_psk_client_cb_func cb);
 void SSL_set_psk_client_callback(SSL *ssl, SSL_psk_client_cb_func cb);

 void SSL_CTX_set_psk_use_session_callback(SSL_CTX *ctx,
                                           SSL_psk_use_session_cb_func cb);
 void SSL_set_psk_use_session_callback(SSL *s, SSL_psk_use_session_cb_func cb);


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

A client application wishing to use PSK ciphersuites for TLSv1.2 and below must provide a callback function. This function will be called when the client is sending the ClientKeyExchange message to the server.

The purpose of the callback function is to select the PSK identity and the pre-shared key to use during the connection setup phase.

The callback is set using functions SSL_CTX_set_psk_client_callback() or SSL_set_psk_client_callback(). The callback function is given the connection in parameter ssl, a NULL-terminated PSK identity hint sent by the server in parameter hint, a buffer identity of length max_identity_len bytes where the resulting NULL-terminated identity is to be stored, and a buffer psk of length max_psk_len bytes where the resulting 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.

The first time the callback is called for a connection the md parameter is NULL. In some circumstances the callback will be called a second time. In that case the server will have specified a ciphersuite to use already and the PSK must be compatible with the digest for that ciphersuite. The digest will be given in md. The PSK returned by the callback is allowed to be different between the first and second time it is called.

On successful completion the callback must store a pointer to an identifier for the PSK in *id. The identifier length in bytes should be stored in *idlen. The memory pointed to by *id remains owned by the application and should be freed by it as required at any point after the handshake is complete.

Additionally the callback should store a pointer to an SSL_SESSION object in *sess. This is used as the basis for the PSK, and should, at a minimum, have the following fields set:

The master key

This can be set via a call to SSL_SESSION_set1_master_key(3).

A ciphersuite

Only the handshake digest associated with the ciphersuite is relevant for the PSK (the server may go on to negotiate any ciphersuite which is compatible with the digest). The application can use any TLSv1.3 ciphersuite. If md is not NULL the handshake digest for the ciphersuite should be the same. The ciphersuite can be set via a call to <SSL_SESSION_set_cipher(3)>. The handshake digest of an SSL_CIPHER object can be checked using <SSL_CIPHER_get_handshake_digest(3)>.

The protocol version

This can be set via a call to SSL_SESSION_set_protocol_version(3) and should be TLS1_3_VERSION.

Additionally the maximum early data value should be set via a call to SSL_SESSION_set_max_early_data(3) if the PSK will be used for sending early data.

Alternatively an SSL_SESSION created from a previous non-PSK handshake may also be used as the basis for a PSK.

Ownership of the SSL_SESSION object is passed to the OpenSSL library and so it should not be freed by the application.

It is also possible for the callback to succeed but not supply a PSK. In this case no PSK will be sent to the server but the handshake will continue. To do this the callback should return successfully and ensure that *sess is NULL. The contents of *id and *idlen will be ignored.


Note that parameter hint given to the callback may be 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.


Return values from the SSL_psk_client_cb_func callback are interpreted as follows:

On success (callback found a PSK identity and a pre-shared key to use) the length (> 0) of psk in bytes is returned.

Otherwise or on errors the callback should return 0. In this case the connection setup fails.

The SSL_psk_use_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_find_session_callback(3), SSL_set_psk_find_session_callback(3)


SSL_CTX_set_psk_use_session_callback() and SSL_set_psk_use_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