SSL_CTX_SET_...IENT_CALLBACK(3ossl)

📦 version 3.5.0, in opensuse-leap 16.0

💅 SSL_CTX_SET_...IENT_CALLBACK — the 3ossl manual, but make it make sense 🫶

okay bestie, so this is the OpenSSL 3ossl page for the PSK client callback situation — it covers 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, and SSL_set_psk_use_session_callback. That's a lot of names but it all boils down to: set PSK client callback. Let's go ✨

🌸 NAME 🌸

  • 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 — all of these are here to set PSK client callback. Iconic, honestly. 👑

✨ SYNOPSIS ✨

#include <openssl/ssl.h>

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_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);

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);

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);

like, copy-paste those exactly, they are NOT vibes-based 💀

🦋 DESCRIPTION 🦋

so. A client app that wants to use TLSv1.3 PSKs should use either SSL_CTX_set_psk_use_session_callback() or SSL_set_psk_use_session_callback(), whichever one fits your life. Heads up though — these functions cannot be used for TLSv1.2 and below PSKs. Not a vibe, just a fact. 🚫

The callback function gets a pointer to the SSL connection in ssl. cute, love that for it.

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 — and in that case the server will have already specified a ciphersuite to use, so the PSK must be compatible with the digest for that ciphersuite. That digest shows up in md. And yes, the PSK returned by the callback is allowed to be different between the first and the second time it's called. glow-up permitted 💖

On successful completion the callback must store a pointer to an identifier for the PSK in *id. The identifier length in bytes should go 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. so like, don't lose track of it bestie 🙈

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. early data? we love an early bird 🐦

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

Ownership of the SSL_SESSION object is passed to the OpenSSL library, so it should not be freed by the application. hands off, it's the library's now 🫶

It's 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. ghosting the PSK, honestly iconic 👻

🌈 the TLSv1.2-and-below callback (a different beast) 🌈

A client application wishing to use PSK ciphersuites for TLSv1.2 and below must provide a different 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 NUL-terminated PSK identity hint sent by the server in parameter hint,
  • a buffer identity of length max_identity_len bytes (including the NUL-terminator) where the resulting NUL-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.

pop those in the right place, bestie 🎀

💜 wait, can the TLSv1.2 one do TLSv1.3? 💜

The callback for use in TLSv1.2 will also work in TLSv1.3, although it is recommended to use SSL_CTX_set_psk_use_session_callback() or SSL_set_psk_use_session_callback() for this purpose instead. (recommended! not mandatory! we're being chill about it ✨)

If TLSv1.3 has been negotiated then OpenSSL will first check to see if a callback has been set via SSL_CTX_set_psk_use_session_callback() or SSL_set_psk_use_session_callback() and it will use that in preference. If no such callback is present then it will check to see if a callback has been set via SSL_CTX_set_psk_client_callback() or SSL_set_psk_client_callback() and use that. In this case the hint value will always be NULL and the handshake digest will default to SHA-256 for any returned PSK.

🚨 TLSv1.3 early data exchanges are possible in PSK connections only with the SSL_psk_use_session_cb_func callback, and are not possible with the SSL_psk_client_cb_func callback. no early data for you, other callback 💀

🎀 NOTES 🎀

Note that parameter hint given to the callback may be NULL. so like, check it. please. 😭

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. surprise! it's a resumption now 🦋

There are no known security issues with sharing the same PSK between TLSv1.2 (or below) and TLSv1.3. However, the RFC has this note of caution:

"While there is no known way in which the same PSK might produce related output in both versions, only limited analysis has been done. Implementations can ensure safety from cross-protocol related output by not reusing PSKs between TLS 1.3 and TLS 1.2."

like they're not saying don't, but they're not not saying don't 👀

🌟 RETURN VALUES 🌟

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. slay 💅
  • 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.

🔥 SEE ALSO 🔥

ssl(7), SSL_CTX_set_psk_find_session_callback(3), SSL_set_psk_find_session_callback(3)

go read those too, iykyk 📚

⭐ HISTORY ⭐

SSL_CTX_set_psk_use_session_callback() and SSL_set_psk_use_session_callback() were added in OpenSSL 1.1.1. vintage, but in a good way 🍓

Copyright 2006-2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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 <https://www.openssl.org/source/license.html>.

and that's the whole page bestie, no cap 🧚 this was OpenSSL 3.5.0, dated 2026-09-2 — go forth and set those PSK client callbacks responsibly 💖✨🎀

📦 where this page comes from