netinet{,6}: Assert in{,6}_inithead() are only used for system routing tables.
[dragonfly.git] / secure / lib / libssl / man / SSL_CTX_set_tmp_dh_callback.3
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133.\" ========================================================================
135.IX Title "SSL_CTX_set_tmp_dh_callback 3"
136.TH SSL_CTX_set_tmp_dh_callback 3 "2015-07-09" "1.0.1p" "OpenSSL"
137.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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139.if n .ad l
141.SH "NAME"
142SSL_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
144.IX Header "SYNOPSIS"
145.Vb 1
146\& #include <openssl/ssl.h>
148\& void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
149\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
150\& long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
152\& void SSL_set_tmp_dh_callback(SSL *ctx,
153\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
154\& long SSL_set_tmp_dh(SSL *ssl, DH *dh)
158\&\fISSL_CTX_set_tmp_dh_callback()\fR sets the callback function for \fBctx\fR to be
159used when a \s-1DH\s0 parameters are required to \fBtmp_dh_callback\fR.
160The callback is inherited by all \fBssl\fR objects created from \fBctx\fR.
162\&\fISSL_CTX_set_tmp_dh()\fR sets \s-1DH\s0 parameters to be used to be \fBdh\fR.
163The key is inherited by all \fBssl\fR objects created from \fBctx\fR.
165\&\fISSL_set_tmp_dh_callback()\fR sets the callback only for \fBssl\fR.
167\&\fISSL_set_tmp_dh()\fR sets the parameters only for \fBssl\fR.
169These functions apply to \s-1SSL/TLS\s0 servers only.
170.SH "NOTES"
171.IX Header "NOTES"
172When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1DH\s0 key exchange
173can take place. Ciphers with \s-1DSA\s0 keys always use ephemeral \s-1DH\s0 keys as well.
174In these cases, the session data are negotiated using the
175ephemeral/temporary \s-1DH\s0 key and the key supplied and certified
176by the certificate chain is only used for signing.
177Anonymous ciphers (without a permanent server key) also use ephemeral \s-1DH\s0 keys.
179Using ephemeral \s-1DH\s0 key exchange yields forward secrecy, as the connection
180can 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
182is left, it becomes impossible for an attacker to decrypt past sessions,
183even if he gets hold of the normal (certified) key, as this key was
184only used for signing.
186In 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.
188The server will always generate a new \s-1DH\s0 key during the negotiation
189if either the \s-1DH\s0 parameters are supplied via callback or the
190\&\s-1SSL_OP_SINGLE_DH_USE\s0 option of \fISSL_CTX_set_options\fR\|(3) is set (or both).
191It will immediately create a \s-1DH\s0 key if \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.
193In this case,
194it may happen that a key is generated on initialization without later
195being needed, while on the other hand the computer time during the
196negotiation is being saved.
198If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly
199necessary to generate a new key for each handshake but it does improve forward
200secrecy. If it is not assured that \*(L"strong\*(R" primes were used,
201\&\s-1SSL_OP_SINGLE_DH_USE\s0 must be used in order to prevent small subgroup
202attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0 has an impact on the
203computer time needed during negotiation, but it is not very large, so
204application authors/users should consider always enabling this option.
205The option is required to implement perfect forward secrecy (\s-1PFS\s0).
207As generating \s-1DH\s0 parameters is extremely time consuming, an application
208should not generate the parameters on the fly but supply the parameters.
209\&\s-1DH\s0 parameters can be reused, as the actual key is newly generated during
210the negotiation. The risk in reusing \s-1DH\s0 parameters is that an attacker
211may specialize on a very often used \s-1DH\s0 group. Applications should therefore
212generate their own \s-1DH\s0 parameters during the installation process using the
213openssl \fIdhparam\fR\|(1) application. This application
214guarantees that \*(L"strong\*(R" primes are used.
216Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current
217version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters,
218which use safe primes and were generated verifiably pseudo-randomly.
219These files can be converted into C code using the \fB\-C\fR option of the
220\&\fIdhparam\fR\|(1) application. Generation of custom \s-1DH\s0
221parameters during installation should still be preferred to stop an
222attacker from specializing on a commonly used group. Files dh1024.pem
223and dh512.pem contain old parameters that must not be used by
226An application may either directly specify the \s-1DH\s0 parameters or
227can supply the \s-1DH\s0 parameters via a callback function.
229Previous versions of the callback used \fBis_export\fR and \fBkeylength\fR
230parameters to control parameter generation for export and non-export
231cipher suites. Modern servers that do not support export ciphersuites
232are advised to either use \fISSL_CTX_set_tmp_dh()\fR in combination with
233\&\s-1SSL_OP_SINGLE_DH_USE,\s0 or alternatively, use the callback but ignore
234\&\fBkeylength\fR and \fBis_export\fR and simply supply at least 2048\-bit
235parameters in the callback.
237.IX Header "EXAMPLES"
238Setup \s-1DH\s0 parameters with a key length of 2048 bits. (Error handling
239partly left out.)
241.Vb 2
242\& Command\-line parameter generation:
243\& $ openssl dhparam \-out dh_param_2048.pem 2048
245\& Code for setting up parameters during server initialization:
247\& ...
248\& SSL_CTX ctx = SSL_CTX_new();
249\& ...
251\& /* Set up ephemeral DH parameters. */
252\& DH *dh_2048 = NULL;
253\& FILE *paramfile;
254\& paramfile = fopen("dh_param_2048.pem", "r");
255\& if (paramfile) {
256\& dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
257\& fclose(paramfile);
258\& } else {
259\& /* Error. */
260\& }
261\& if (dh_2048 == NULL) {
262\& /* Error. */
263\& }
264\& if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
265\& /* Error. */
266\& }
267\& SSL_CTX_set_options(ctx, SSL_OP_SINGLE_DH_USE);
268\& ...
272\&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return
273diagnostic output.
275\&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0
276on failure. Check the error queue to find out the reason of failure.
278.IX Header "SEE ALSO"
279\&\fIssl\fR\|(3), \fISSL_CTX_set_cipher_list\fR\|(3),
282\&\fIciphers\fR\|(1), \fIdhparam\fR\|(1)