SSL_CTX_SET_TMP_DH_CALLBACK(3ossl)

📦
version 3.5.9 · 8 versions
, in fedora 43, fedora 44

💖 SSL_CTX_SET_TMP_DH_CALLBACK 💖

okay bestie, this one lives in the OpenSSL manual, section 3ossl — yes that's the manual name, we're keeping it, it's part of the lore 🫶

🌸 NAME 🌸

SSL_CTX_set_dh_auto, SSL_set_dh_auto, SSL_CTX_set0_tmp_dh_pkey, SSL_set0_tmp_dh_pkey, SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_callback, SSL_set_tmp_dh — the whole girl squad that handles DH keys for ephemeral key exchange ✨

🎀 SYNOPSIS 🎀

#include <openssl/ssl.h>

long SSL_CTX_set_dh_auto(SSL_CTX *ctx, int onoff);
long SSL_set_dh_auto(SSL *s, int onoff);
int SSL_CTX_set0_tmp_dh_pkey(SSL_CTX *ctx, EVP_PKEY *dhpkey);
int SSL_set0_tmp_dh_pkey(SSL *s, EVP_PKEY *dhpkey);

The following functions have been deprecated since OpenSSL 3.0, and can be hidden entirely by defining OPENSSL_API_COMPAT with a suitable version value, see openssl_user_macros(7):

void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
                                 DH *(*tmp_dh_callback)(SSL *ssl, int is_export,
                                                        int keylength));
long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);

void SSL_set_tmp_dh_callback(SSL *ctx,
                             DH *(*tmp_dh_callback)(SSL *ssl, int is_export,
                                                    int keylength));
long SSL_set_tmp_dh(SSL *ssl, DH *dh);

🦋 DESCRIPTION 🦋

okay so first thing's first: everything on this page is for servers only. clients, this isn't your moment, sorry babe 💅

Some ciphersuites may use ephemeral Diffie-Hellman (DH) key exchange. When that happens, the session data gets negotiated using the ephemeral/temporary DH key, and the key that's supplied and certified by the certificate chain is only used for signing. Anonymous ciphers (the ones with no permanent server key) also use ephemeral DH keys. no cap.

Using ephemeral DH key exchange gives you forward secrecy 🔥 because the connection can only be decrypted when the DH key is known. If you generate a temporary DH key inside the server application that's lost when the application is left, it becomes literally impossible for an attacker to decrypt past sessions — even if they get hold of the normal (certified) key — because that key was only ever used for signing. iconic behaviour, honestly.

To actually perform a DH key exchange, the server must use a DH group (DH parameters) and generate a DH key. The server will always generate a new DH key during the negotiation. every single time. no shortcuts.

Now, here's the tea: generating DH parameters is extremely time consuming, so an application should not generate the parameters on the fly. 🛑 DH parameters can be reused, because the actual key is newly generated during the negotiation. so like, reuse the params, not the key. iykyk.

Typically applications should use well known DH parameters that have built-in support in OpenSSL. The macros SSL_CTX_set_dh_auto() and SSL_set_dh_auto() configure OpenSSL to use the default built-in DH parameters for the SSL_CTX and SSL objects respectively.

Passing a value of 2 or 1 in the onoff parameter switches it on. If the onoff parameter is set to 2, it will force the DH key size to 1024 if the SSL_CTX or SSL security level SSL_CTX_set_security_level(3) is 0 or 1. Passing a value of 0 switches it off. The default setting is off.

If "auto" DH parameters are switched on, then the parameters will be selected to be consistent with the size of the key associated with the server's certificate. If there is no certificate (e.g. for PSK ciphersuites), then it will be consistent with the size of the negotiated symmetric cipher key. ✨

Applications may supply their own DH parameters instead of using the built-in values. This approach is discouraged and applications should in preference use the built-in parameter support described above. 🙈

Applications wishing to supply their own DH parameters should call SSL_CTX_set0_tmp_dh_pkey() or SSL_set0_tmp_dh_pkey() to supply the parameters for the SSL_CTX or SSL respectively. The parameters should be supplied in the dhpkey argument as an EVP_PKEY containing DH parameters. Ownership of the dhpkey value is passed to the SSL_CTX or SSL object as a result of this call, and so the caller should not free it if the function call is successful. 🫶

The deprecated macros SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do the same thing as SSL_CTX_set0_tmp_dh_pkey() and SSL_set0_tmp_dh_pkey() — except that the DH parameters are supplied in a DH object instead, in the dh argument, and ownership of the DH object is retained by the application. Applications should use "auto" parameters instead, or call SSL_CTX_set0_tmp_dh_pkey() or SSL_set0_tmp_dh_pkey() as appropriate.

An application may instead specify the DH parameters via a callback function, using SSL_CTX_set_tmp_dh_callback() or SSL_set_tmp_dh_callback() to set the callback for the SSL_CTX or SSL object respectively. These functions are deprecated. Applications should instead use "auto" parameters, or specify the parameters via SSL_CTX_set0_tmp_dh_pkey() or SSL_set0_tmp_dh_pkey() as appropriate.

The callback will be invoked during a connection when DH parameters are required. The SSL object for the current connection is supplied as an argument. Previous versions of OpenSSL used the is_export and keylength arguments to control parameter generation for export and non-export cipher suites. Modern OpenSSL does not support export ciphersuites, so these arguments are unused and can be ignored by the callback. The callback should return the parameters to be used in a DH object. Ownership of the DH object is retained by the application and should later be freed. 💀 (free it, bestie, i'm begging)

⭐ RETURN VALUES ⭐

All of these functions/macros return 1 for success or 0 on error. that's it. that's the vibe check. 💅

🌈 SEE ALSO 🌈

Copyright 2001-2026 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 🎀 this was OpenSSL version 3.5.9, dated 2026-09-29, filed under SSL_CTX_SET_TMP_DH_CALLBACK(3ossl) — slay, go use "auto" parameters, ily, bye 💖✨🦋

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