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138.\" ======================================================================
139.\"
140.IX Title "SSL_CTX_set_tmp_dh_callback 3"
141.TH SSL_CTX_set_tmp_dh_callback 3 "0.9.7a" "2003-02-19" "OpenSSL"
142.UC
143.SH "NAME"
144SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_callback, SSL_set_tmp_dh \- handle \s-1DH\s0 keys for ephemeral key exchange
145.SH "SYNOPSIS"
146.IX Header "SYNOPSIS"
147.Vb 1
148\& #include <openssl/ssl.h>
149.Ve
150.Vb 3
151\& void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
152\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
153\& long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
154.Ve
155.Vb 3
156\& void SSL_set_tmp_dh_callback(SSL_CTX *ctx,
157\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
158\& long SSL_set_tmp_dh(SSL *ssl, DH *dh)
159.Ve
160.Vb 1
161\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
162.Ve
163.SH "DESCRIPTION"
164.IX Header "DESCRIPTION"
165\&\fISSL_CTX_set_tmp_dh_callback()\fR sets the callback function for \fBctx\fR to be
166used when a \s-1DH\s0 parameters are required to \fBtmp_dh_callback\fR.
167The callback is inherited by all \fBssl\fR objects created from \fBctx\fR.
168.PP
169\&\fISSL_CTX_set_tmp_dh()\fR sets \s-1DH\s0 parameters to be used to be \fBdh\fR.
170The key is inherited by all \fBssl\fR objects created from \fBctx\fR.
171.PP
172\&\fISSL_set_tmp_dh_callback()\fR sets the callback only for \fBssl\fR.
173.PP
174\&\fISSL_set_tmp_dh()\fR sets the parameters only for \fBssl\fR.
175.PP
176These functions apply to \s-1SSL/TLS\s0 servers only.
177.SH "NOTES"
178.IX Header "NOTES"
179When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1DH\s0 key exchange
180can take place. Ciphers with \s-1DSA\s0 keys always use ephemeral \s-1DH\s0 keys as well.
181In these cases, the session data are negotiated using the
182ephemeral/temporary \s-1DH\s0 key and the key supplied and certified
183by the certificate chain is only used for signing.
184Anonymous ciphers (without a permanent server key) also use ephemeral \s-1DH\s0 keys.
185.PP
186Using ephemeral \s-1DH\s0 key exchange yields forward secrecy, as the connection
187can only be decrypted, when the \s-1DH\s0 key is known. By generating a temporary
188\&\s-1DH\s0 key inside the server application that is lost when the application
189is left, it becomes impossible for an attacker to decrypt past sessions,
190even if he gets hold of the normal (certified) key, as this key was
191only used for signing.
192.PP
193In order to perform a \s-1DH\s0 key exchange the server must use a \s-1DH\s0 group
194(\s-1DH\s0 parameters) and generate a \s-1DH\s0 key. The server will always generate a new
195\&\s-1DH\s0 key during the negotiation, when the \s-1DH\s0 parameters are supplied via
196callback and/or when the \s-1SSL_OP_SINGLE_DH_USE\s0 option of
197SSL_CTX_set_options(3) is set. It will
198immediately create a \s-1DH\s0 key, when \s-1DH\s0 parameters are supplied via
199\&\fISSL_CTX_set_tmp_dh()\fR and \s-1SSL_OP_SINGLE_DH_USE\s0 is not set. In this case,
200it may happen that a key is generated on initialization without later
201being needed, while on the other hand the computer time during the
202negotiation is being saved.
203.PP
204If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly
205necessary to generate a new key for each handshake but it does improve forward
206secrecy. If it is not assured, that \*(L"strong\*(R" primes were used (see especially
207the section about \s-1DSA\s0 parameters below), \s-1SSL_OP_SINGLE_DH_USE\s0 must be used
208in order to prevent small subgroup attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0
209has an impact on the computer time needed during negotiation, but it is not
210very large, so application authors/users should consider to always enable
211this option.
212.PP
213As generating \s-1DH\s0 parameters is extremely time consuming, an application
214should not generate the parameters on the fly but supply the parameters.
215\&\s-1DH\s0 parameters can be reused, as the actual key is newly generated during
216the negotiation. The risk in reusing \s-1DH\s0 parameters is that an attacker
217may specialize on a very often used \s-1DH\s0 group. Applications should therefore
218generate their own \s-1DH\s0 parameters during the installation process using the
219openssl dhparam(1) application. In order to reduce the computer
220time needed for this generation, it is possible to use \s-1DSA\s0 parameters
221instead (see dhparam(1)), but in this case \s-1SSL_OP_SINGLE_DH_USE\s0
222is mandatory.
223.PP
224Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem,
225dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of current
226version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters,
227which use safe primes and were generated verifiably pseudo-randomly.
228These files can be converted into C code using the \fB\-C\fR option of the
229dhparam(1) application.
230Authors may also generate their own set of parameters using
231dhparam(1), but a user may not be sure how the parameters were
232generated. The generation of \s-1DH\s0 parameters during installation is therefore
233recommended.
234.PP
235An application may either directly specify the \s-1DH\s0 parameters or
236can supply the \s-1DH\s0 parameters via a callback function. The callback approach
237has the advantage, that the callback may supply \s-1DH\s0 parameters for different
238key lengths.
239.PP
240The \fBtmp_dh_callback\fR is called with the \fBkeylength\fR needed and
241the \fBis_export\fR information. The \fBis_export\fR flag is set, when the
242ephemeral \s-1DH\s0 key exchange is performed with an export cipher.
243.SH "EXAMPLES"
244.IX Header "EXAMPLES"
245Handle \s-1DH\s0 parameters for key lengths of 512 and 1024 bits. (Error handling
246partly left out.)
247.PP
248.Vb 5
249\& ...
250\& /* Set up ephemeral DH stuff */
251\& DH *dh_512 = NULL;
252\& DH *dh_1024 = NULL;
253\& FILE *paramfile;
254.Ve
255.Vb 14
256\& ...
257\& /* "openssl dhparam -out dh_param_512.pem -2 512" */
258\& paramfile = fopen("dh_param_512.pem", "r");
259\& if (paramfile) {
260\& dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
261\& fclose(paramfile);
262\& }
263\& /* "openssl dhparam -out dh_param_1024.pem -2 1024" */
264\& paramfile = fopen("dh_param_1024.pem", "r");
265\& if (paramfile) {
266\& dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
267\& fclose(paramfile);
268\& }
269\& ...
270.Ve
271.Vb 3
272\& /* "openssl dhparam -C -2 512" etc... */
273\& DH *get_dh512() { ... }
274\& DH *get_dh1024() { ... }
275.Ve
276.Vb 3
277\& DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
278\& {
279\& DH *dh_tmp=NULL;
280.Ve
281.Vb 17
282\& switch (keylength) {
283\& case 512:
284\& if (!dh_512)
285\& dh_512 = get_dh512();
286\& dh_tmp = dh_512;
287\& break;
288\& case 1024:
289\& if (!dh_1024)
290\& dh_1024 = get_dh1024();
291\& dh_tmp = dh_1024;
292\& break;
293\& default:
294\& /* Generating a key on the fly is very costly, so use what is there */
295\& setup_dh_parameters_like_above();
296\& }
297\& return(dh_tmp);
298\& }
299.Ve
300.SH "RETURN VALUES"
301.IX Header "RETURN VALUES"
302\&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return
303diagnostic output.
304.PP
305\&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0
306on failure. Check the error queue to find out the reason of failure.
307.SH "SEE ALSO"
308.IX Header "SEE ALSO"
309ssl(3), SSL_CTX_set_cipher_list(3),
310SSL_CTX_set_tmp_rsa_callback(3),
311SSL_CTX_set_options(3),
312ciphers(1), dhparam(1)