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131 .IX Title "SSL_CTX_set_tmp_dh_callback 3"
132 .TH SSL_CTX_set_tmp_dh_callback 3 "2009-07-23" "0.9.8k" "OpenSSL"
133 .SH "NAME"
134 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
135 .SH "SYNOPSIS"
136 .IX Header "SYNOPSIS"
137 .Vb 1
138 \& #include <openssl/ssl.h>
139 .Ve
140 .PP
141 .Vb 3
142 \& void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
143 \& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
144 \& long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
145 .Ve
146 .PP
147 .Vb 3
148 \& void SSL_set_tmp_dh_callback(SSL_CTX *ctx,
149 \& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
150 \& long SSL_set_tmp_dh(SSL *ssl, DH *dh)
151 .Ve
152 .PP
153 .Vb 1
154 \& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
155 .Ve
156 .SH "DESCRIPTION"
157 .IX Header "DESCRIPTION"
158 \&\fISSL_CTX_set_tmp_dh_callback()\fR sets the callback function for \fBctx\fR to be
159 used when a \s-1DH\s0 parameters are required to \fBtmp_dh_callback\fR.
160 The callback is inherited by all \fBssl\fR objects created from \fBctx\fR.
161 .PP
162 \&\fISSL_CTX_set_tmp_dh()\fR sets \s-1DH\s0 parameters to be used to be \fBdh\fR.
163 The key is inherited by all \fBssl\fR objects created from \fBctx\fR.
164 .PP
165 \&\fISSL_set_tmp_dh_callback()\fR sets the callback only for \fBssl\fR.
166 .PP
167 \&\fISSL_set_tmp_dh()\fR sets the parameters only for \fBssl\fR.
168 .PP
169 These functions apply to \s-1SSL/TLS\s0 servers only.
170 .SH "NOTES"
171 .IX Header "NOTES"
172 When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1DH\s0 key exchange
173 can take place. Ciphers with \s-1DSA\s0 keys always use ephemeral \s-1DH\s0 keys as well.
174 In these cases, the session data are negotiated using the
175 ephemeral/temporary \s-1DH\s0 key and the key supplied and certified
176 by the certificate chain is only used for signing.
177 Anonymous ciphers (without a permanent server key) also use ephemeral \s-1DH\s0 keys.
178 .PP
179 Using ephemeral \s-1DH\s0 key exchange yields forward secrecy, as the connection
180 can only be decrypted, when the \s-1DH\s0 key is known. By generating a temporary
181 \&\s-1DH\s0 key inside the server application that is lost when the application
182 is left, it becomes impossible for an attacker to decrypt past sessions,
183 even if he gets hold of the normal (certified) key, as this key was
184 only used for signing.
185 .PP
186 In order to perform a \s-1DH\s0 key exchange the server must use a \s-1DH\s0 group
187 (\s-1DH\s0 parameters) and generate a \s-1DH\s0 key. The server will always generate a new
188 \&\s-1DH\s0 key during the negotiation, when the \s-1DH\s0 parameters are supplied via
189 callback and/or when the \s-1SSL_OP_SINGLE_DH_USE\s0 option of
190 \&\fISSL_CTX_set_options\fR\|(3) is set. It will
191 immediately create a \s-1DH\s0 key, when \s-1DH\s0 parameters are supplied via
192 \&\fISSL_CTX_set_tmp_dh()\fR and \s-1SSL_OP_SINGLE_DH_USE\s0 is not set. In this case,
193 it may happen that a key is generated on initialization without later
194 being needed, while on the other hand the computer time during the
195 negotiation is being saved.
196 .PP
197 If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly
198 necessary to generate a new key for each handshake but it does improve forward
199 secrecy. If it is not assured, that \*(L"strong\*(R" primes were used (see especially
200 the section about \s-1DSA\s0 parameters below), \s-1SSL_OP_SINGLE_DH_USE\s0 must be used
201 in order to prevent small subgroup attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0
202 has an impact on the computer time needed during negotiation, but it is not
203 very large, so application authors/users should consider to always enable
204 this option.
205 .PP
206 As generating \s-1DH\s0 parameters is extremely time consuming, an application
207 should not generate the parameters on the fly but supply the parameters.
208 \&\s-1DH\s0 parameters can be reused, as the actual key is newly generated during
209 the negotiation. The risk in reusing \s-1DH\s0 parameters is that an attacker
210 may specialize on a very often used \s-1DH\s0 group. Applications should therefore
211 generate their own \s-1DH\s0 parameters during the installation process using the
212 openssl \fIdhparam\fR\|(1) application. In order to reduce the computer
213 time needed for this generation, it is possible to use \s-1DSA\s0 parameters
214 instead (see \fIdhparam\fR\|(1)), but in this case \s-1SSL_OP_SINGLE_DH_USE\s0
215 is mandatory.
216 .PP
217 Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem,
218 dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of current
219 version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters,
220 which use safe primes and were generated verifiably pseudo\-randomly.
221 These files can be converted into C code using the \fB\-C\fR option of the
222 \&\fIdhparam\fR\|(1) application.
223 Authors may also generate their own set of parameters using
224 \&\fIdhparam\fR\|(1), but a user may not be sure how the parameters were
225 generated. The generation of \s-1DH\s0 parameters during installation is therefore
226 recommended.
227 .PP
228 An application may either directly specify the \s-1DH\s0 parameters or
229 can supply the \s-1DH\s0 parameters via a callback function. The callback approach
230 has the advantage, that the callback may supply \s-1DH\s0 parameters for different
231 key lengths.
232 .PP
233 The \fBtmp_dh_callback\fR is called with the \fBkeylength\fR needed and
234 the \fBis_export\fR information. The \fBis_export\fR flag is set, when the
235 ephemeral \s-1DH\s0 key exchange is performed with an export cipher.
236 .SH "EXAMPLES"
237 .IX Header "EXAMPLES"
238 Handle \s-1DH\s0 parameters for key lengths of 512 and 1024 bits. (Error handling
239 partly left out.)
240 .PP
241 .Vb 5
242 \& ...
243 \& /* Set up ephemeral DH stuff */
244 \& DH *dh_512 = NULL;
245 \& DH *dh_1024 = NULL;
246 \& FILE *paramfile;
247 .Ve
248 .PP
249 .Vb 14
250 \& ...
251 \& /* "openssl dhparam -out dh_param_512.pem -2 512" */
252 \& paramfile = fopen("dh_param_512.pem", "r");
253 \& if (paramfile) {
254 \& dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
255 \& fclose(paramfile);
256 \& }
257 \& /* "openssl dhparam -out dh_param_1024.pem -2 1024" */
258 \& paramfile = fopen("dh_param_1024.pem", "r");
259 \& if (paramfile) {
260 \& dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
261 \& fclose(paramfile);
262 \& }
263 \& ...
264 .Ve
265 .PP
266 .Vb 3
267 \& /* "openssl dhparam -C -2 512" etc... */
268 \& DH *get_dh512() { ... }
269 \& DH *get_dh1024() { ... }
270 .Ve
271 .PP
272 .Vb 3
273 \& DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
274 \& {
275 \& DH *dh_tmp=NULL;
276 .Ve
277 .PP
278 .Vb 17
279 \& switch (keylength) {
280 \& case 512:
281 \& if (!dh_512)
282 \& dh_512 = get_dh512();
283 \& dh_tmp = dh_512;
284 \& break;
285 \& case 1024:
286 \& if (!dh_1024)
287 \& dh_1024 = get_dh1024();
288 \& dh_tmp = dh_1024;
289 \& break;
290 \& default:
291 \& /* Generating a key on the fly is very costly, so use what is there */
292 \& setup_dh_parameters_like_above();
293 \& }
294 \& return(dh_tmp);
295 \& }
296 .Ve
297 .SH "RETURN VALUES"
298 .IX Header "RETURN VALUES"
299 \&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return
300 diagnostic output.
301 .PP
302 \&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0
303 on failure. Check the error queue to find out the reason of failure.
304 .SH "SEE ALSO"
305 .IX Header "SEE ALSO"
306 \&\fIssl\fR\|(3), \fISSL_CTX_set_cipher_list\fR\|(3),
307 \&\fISSL_CTX_set_tmp_rsa_callback\fR\|(3),
308 \&\fISSL_CTX_set_options\fR\|(3),
309 \&\fIciphers\fR\|(1), \fIdhparam\fR\|(1)