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Diffstat (limited to 'static/openbsd/man3/SSL_CTX_set_tmp_dh_callback.3')
| -rw-r--r-- | static/openbsd/man3/SSL_CTX_set_tmp_dh_callback.3 | 230 |
1 files changed, 230 insertions, 0 deletions
diff --git a/static/openbsd/man3/SSL_CTX_set_tmp_dh_callback.3 b/static/openbsd/man3/SSL_CTX_set_tmp_dh_callback.3 new file mode 100644 index 00000000..9fa83065 --- /dev/null +++ b/static/openbsd/man3/SSL_CTX_set_tmp_dh_callback.3 @@ -0,0 +1,230 @@ +.\" $OpenBSD: SSL_CTX_set_tmp_dh_callback.3,v 1.12 2025/06/08 22:52:00 schwarze Exp $ +.\" OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100 +.\" +.\" This file was written by Lutz Jaenicke <jaenicke@openssl.org>. +.\" 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. All advertising materials mentioning features or use of this +.\" software must display the following acknowledgment: +.\" "This product includes software developed by the OpenSSL Project +.\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)" +.\" +.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to +.\" endorse or promote products derived from this software without +.\" prior written permission. For written permission, please contact +.\" openssl-core@openssl.org. +.\" +.\" 5. Products derived from this software may not be called "OpenSSL" +.\" nor may "OpenSSL" appear in their names without prior written +.\" permission of the OpenSSL Project. +.\" +.\" 6. Redistributions of any form whatsoever must retain the following +.\" acknowledgment: +.\" "This product includes software developed by the OpenSSL Project +.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)" +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY +.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR +.\" PURPOSE ARE DISCLAIMED. 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: June 8 2025 $ +.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 +.Lb libssl libcrypto +.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. +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 . |