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140 .IX Title "SSL_CTX_SET_TMP_DH_CALLBACK 1"
141 .TH SSL_CTX_SET_TMP_DH_CALLBACK 1 "perl v5.6.1" "2002-01-27" "User Contributed Perl Documentation"
144 SSL_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
146 .IX Header "SYNOPSIS"
148 \& #include <openssl/ssl.h>
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);
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)
161 \& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
164 .IX Header "DESCRIPTION"
165 \&\fISSL_CTX_set_tmp_dh_callback()\fR sets the callback function for \fBctx\fR to be
166 used when a \s-1DH\s0 parameters are required to \fBtmp_dh_callback\fR.
167 The callback is inherited by all \fBssl\fR objects created from \fBctx\fR.
169 \&\fISSL_CTX_set_tmp_dh()\fR sets \s-1DH\s0 parameters to be used to be \fBdh\fR.
170 The key is inherited by all \fBssl\fR objects created from \fBctx\fR.
172 \&\fISSL_set_tmp_dh_callback()\fR sets the callback only for \fBssl\fR.
174 \&\fISSL_set_tmp_dh()\fR sets the parameters only for \fBssl\fR.
176 These functions apply to \s-1SSL/TLS\s0 servers only.
179 When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1DH\s0 key exchange
180 can take place. Ciphers with \s-1DSA\s0 keys always use ephemeral \s-1DH\s0 keys as well.
181 In these cases, the session data are negotiated using the
182 ephemeral/temporary \s-1DH\s0 key and the key supplied and certified
183 by the certificate chain is only used for signing.
184 Anonymous ciphers (without a permanent server key) also use ephemeral \s-1DH\s0 keys.
186 Using ephemeral \s-1DH\s0 key exchange yields forward secrecy, as the connection
187 can 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
189 is left, it becomes impossible for an attacker to decrypt past sessions,
190 even if he gets hold of the normal (certified) key, as this key was
191 only used for signing.
193 In 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
196 callback and/or when the \s-1SSL_OP_SINGLE_DH_USE\s0 option of
197 SSL_CTX_set_options(3) is set. It will
198 immediately 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,
200 it may happen that a key is generated on initialization without later
201 being needed, while on the other hand the computer time during the
202 negotiation is being saved.
204 If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly
205 necessary to generate a new key for each handshake but it does improve forward
206 secrecy. If it is not assured, that \*(L"strong\*(R" primes were used (see especially
207 the section about \s-1DSA\s0 parameters below), \s-1SSL_OP_SINGLE_DH_USE\s0 must be used
208 in order to prevent small subgroup attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0
209 has an impact on the computer time needed during negotiation, but it is not
210 very large, so application authors/users should consider to always enable
213 As generating \s-1DH\s0 parameters is extremely time consuming, an application
214 should 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
216 the negotiation. The risk in reusing \s-1DH\s0 parameters is that an attacker
217 may specialize on a very often used \s-1DH\s0 group. Applications should therefore
218 generate their own \s-1DH\s0 parameters during the installation process using the
219 openssl dhparam(1) application. In order to reduce the computer
220 time needed for this generation, it is possible to use \s-1DSA\s0 parameters
221 instead (see dhparam(1)), but in this case \s-1SSL_OP_SINGLE_DH_USE\s0
224 Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem,
225 dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of current
226 version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters,
227 which use safe primes and were generated verifiably pseudo-randomly.
228 These files can be converted into C code using the \fB\-C\fR option of the
229 dhparam(1) application.
230 Authors may also generate their own set of parameters using
231 dhparam(1), but a user may not be sure how the parameters were
232 generated. The generation of \s-1DH\s0 parameters during installation is therefore
235 An application may either directly specify the \s-1DH\s0 parameters or
236 can supply the \s-1DH\s0 parameters via a callback function. The callback approach
237 has the advantage, that the callback may supply \s-1DH\s0 parameters for different
240 The \fBtmp_dh_callback\fR is called with the \fBkeylength\fR needed and
241 the \fBis_export\fR information. The \fBis_export\fR flag is set, when the
242 ephemeral \s-1DH\s0 key exchange is performed with an export cipher.
244 .IX Header "EXAMPLES"
245 Handle \s-1DH\s0 parameters for key lengths of 512 and 1024 bits. (Error handling
250 \& /* Set up ephemeral DH stuff */
251 \& DH *dh_512 = NULL;
252 \& DH *dh_1024 = NULL;
257 \& /* "openssl dhparam -out dh_param_512.pem -2 512" */
258 \& paramfile = fopen("dh_param_512.pem", "r");
260 \& dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
261 \& fclose(paramfile);
263 \& /* "openssl dhparam -out dh_param_1024.pem -2 1024" */
264 \& paramfile = fopen("dh_param_1024.pem", "r");
266 \& dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
267 \& fclose(paramfile);
272 \& /* "openssl dhparam -C -2 512" etc... */
273 \& DH *get_dh512() { ... }
274 \& DH *get_dh1024() { ... }
277 \& DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
282 \& switch (keylength) {
285 \& dh_512 = get_dh512();
290 \& dh_1024 = get_dh1024();
294 \& /* Generating a key on the fly is very costly, so use what is there */
295 \& setup_dh_parameters_like_above();
301 .IX Header "RETURN VALUES"
302 \&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return
305 \&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0
306 on failure. Check the error queue to find out the reason of failure.
308 .IX Header "SEE ALSO"
309 ssl(3), SSL_CTX_set_cipher_list(3),
310 SSL_CTX_set_tmp_rsa_callback(3),
311 SSL_CTX_set_options(3),
312 ciphers(1), dhparam(1)