.\" $OpenBSD: SSL_CTX_set_tmp_dh_callback.3,v 1.10 2022/03/31 17:27:18 naddy Exp $ .\" OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100 .\" .\" This file was written by Lutz Jaenicke . .\" Copyright (c) 2001, 2014, 2015 The OpenSSL Project. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" .\" 3. 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IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: March 31 2022 $ .Dt SSL_CTX_SET_TMP_DH_CALLBACK 3 .Os .Sh NAME .Nm SSL_CTX_set_tmp_dh_callback , .Nm SSL_CTX_set_tmp_dh , .Nm SSL_set_tmp_dh_callback , .Nm SSL_set_tmp_dh .Nd handle DH keys for ephemeral key exchange .Sh SYNOPSIS .In openssl/ssl.h .Ft void .Fo SSL_CTX_set_tmp_dh_callback .Fa "SSL_CTX *ctx" .Fa "DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength)" .Fc .Ft long .Fn SSL_CTX_set_tmp_dh "SSL_CTX *ctx" "DH *dh" .Ft void .Fo SSL_set_tmp_dh_callback .Fa "SSL *ssl" .Fa "DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength" .Fc .Ft long .Fn SSL_set_tmp_dh "SSL *ssl" "DH *dh" .Sh DESCRIPTION .Fn SSL_CTX_set_tmp_dh_callback sets the callback function for .Fa ctx to be used when a DH parameters are required to .Fa tmp_dh_callback . The callback is inherited by all .Vt ssl objects created from .Fa ctx . .Pp .Fn SSL_CTX_set_tmp_dh sets DH parameters to be used by .Fa ctx . The key is inherited by all .Fa ssl objects created from .Fa ctx . .Pp .Fn SSL_set_tmp_dh_callback sets the callback only for .Fa ssl . .Pp .Fn SSL_set_tmp_dh sets the parameters only for .Fa ssl . .Pp These functions apply to SSL/TLS servers only. .Pp When using a cipher with RSA authentication, an ephemeral DH key exchange can take place. Ciphers with DSA keys always use ephemeral DH keys as well. In these cases, the session data are 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. .Pp 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 attackers to decrypt past sessions, even if they get hold of the normal (certified) key, as this key was only used for signing. .Pp 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. .Pp As generating DH parameters is extremely time consuming, an application should not generate the parameters on the fly but supply the parameters. DH parameters can be reused, as the actual key is newly generated during the negotiation. The risk in reusing DH parameters is that an attacker may specialize on a very often used DH group. Applications should therefore generate their own DH parameters during the installation process using the .Xr openssl 1 .Cm dhparam application. This application guarantees that "strong" primes are used. .Pp Files .Pa dh2048.pem and .Pa dh4096.pem in the .Pa apps directory of the current version of the OpenSSL distribution contain the .Sq SKIP DH parameters, which use safe primes and were generated verifiably pseudo-randomly. These files can be converted into C code using the .Fl C option of the .Xr openssl 1 .Cm dhparam application. Generation of custom DH parameters during installation should still be preferred to stop an attacker from specializing on a commonly used group. The file .Pa dh1024.pem contains old parameters that must not be used by applications. .Pp An application may either directly specify the DH parameters or can supply the DH parameters via a callback function. .Pp Previous versions of the callback used .Fa is_export and .Fa keylength parameters to control parameter generation for export and non-export cipher suites. Modern servers that do not support export ciphersuites are advised to either use .Fn SSL_CTX_set_tmp_dh or alternatively, use the callback but ignore .Fa keylength and .Fa is_export and simply supply at least 2048-bit parameters in the callback. .Sh RETURN VALUES .Fn SSL_CTX_set_tmp_dh and .Fn SSL_set_tmp_dh do return 1 on success and 0 on failure. Check the error queue to find out the reason of failure. .Sh EXAMPLES Set up DH parameters with a key length of 2048 bits. Error handling is partly left out. .Pp Command-line parameter generation: .Pp .Dl openssl dhparam -out dh_param_2048.pem 2048 .Pp Code for setting up parameters during server initialization: .Bd -literal SSL_CTX ctx = SSL_CTX_new(); \&... /* Set up ephemeral DH parameters. */ DH *dh_2048 = NULL; FILE *paramfile; paramfile = fopen("dh_param_2048.pem", "r"); if (paramfile) { dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL); fclose(paramfile); } else { /* Error. */ } if (dh_2048 == NULL) { /* Error. */ } if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) { /* Error. */ } .Ed .Sh SEE ALSO .Xr openssl 1 , .Xr ssl 3 , .Xr SSL_CTX_set_cipher_list 3 , .Xr SSL_CTX_set_options 3 , .Xr SSL_set_tmp_ecdh 3 .Sh HISTORY .Fn SSL_CTX_set_tmp_dh_callback and .Fn SSL_CTX_set_tmp_dh first appeared in SSLeay 0.8.0 and have been available since .Ox 2.4 . .Pp .Fn SSL_set_tmp_dh_callback and .Fn SSL_set_tmp_dh first appeared in OpenSSL 0.9.2b and have been available since .Ox 2.6 .