SSL_CTX_SET_TMP_DH_CALLBACK(3ossl) OpenSSL SSL_CTX_SET_TMP_DH_CALLBACK(3ossl)
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 - handle 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
The functions described on this page are relevant for servers only.
Some ciphersuites may use ephemeral Diffie-Hellman (DH) key exchange. In
these cases, the session data is negotiated using the ephemeral/temporary
DH key and the key supplied and certified by the certificate chain is
only used for signing. Anonymous ciphers (without a permanent server key)
also use ephemeral DH keys.
Using ephemeral DH key exchange yields forward secrecy as the connection
can only be decrypted when the DH key is known. By generating a temporary
DH key inside the server application that is lost when the application is
left, it becomes impossible for an attacker to decrypt past sessions,
even if they get hold of the normal (certified) key, as this key was only
used for signing.
In order to 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.
As generating DH parameters is extremely time consuming, an application
should not generate the parameters on the fly. DH parameters can be
reused, as the actual key is newly generated during the negotiation.
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 the functions SSL_CTX_set_tmp_dh_callback(3) 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 and 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.
RETURN VALUES
All of these functions/macros return 1 for success or 0 on error.
SEE ALSO
ssl(7), SSL_CTX_set_cipher_list(3), SSL_CTX_set_options(3),
openssl-ciphers(1), openssl-dhparam(1)
COPYRIGHT
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>.
3.6.5 2026-09-29
SSL_CTX_SET_TMP_DH_CALLBACK(3ossl)
openssl 3.6.5 - Generated Sat Oct 3 08:37:55 CDT 2026