- update OpenSSL to 0.9.8
[dragonfly.git] / secure / lib / libssl / man / SSL_CTX_set_tmp_dh_callback.3
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129.\" ========================================================================
130.\"
131.IX Title "SSL_CTX_set_tmp_dh_callback 3"
a561f9ff 132.TH SSL_CTX_set_tmp_dh_callback 3 "2005-07-06" "0.9.8" "OpenSSL"
984263bc 133.SH "NAME"
a7d27d5a 134SSL_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
984263bc 135.SH "SYNOPSIS"
e056f0e0 136.IX Header "SYNOPSIS"
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137.Vb 1
138\& #include <openssl/ssl.h>
139.Ve
e056f0e0 140.PP
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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
e056f0e0 146.PP
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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
e056f0e0 152.PP
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153.Vb 1
154\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
155.Ve
156.SH "DESCRIPTION"
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157.IX Header "DESCRIPTION"
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.
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160The callback is inherited by all \fBssl\fR objects created from \fBctx\fR.
161.PP
e056f0e0 162\&\fISSL_CTX_set_tmp_dh()\fR sets \s-1DH\s0 parameters to be used to be \fBdh\fR.
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163The key is inherited by all \fBssl\fR objects created from \fBctx\fR.
164.PP
e056f0e0 165\&\fISSL_set_tmp_dh_callback()\fR sets the callback only for \fBssl\fR.
984263bc 166.PP
e056f0e0 167\&\fISSL_set_tmp_dh()\fR sets the parameters only for \fBssl\fR.
984263bc 168.PP
e056f0e0 169These functions apply to \s-1SSL/TLS\s0 servers only.
984263bc 170.SH "NOTES"
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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.
984263bc 174In these cases, the session data are negotiated using the
e056f0e0 175ephemeral/temporary \s-1DH\s0 key and the key supplied and certified
984263bc 176by the certificate chain is only used for signing.
e056f0e0 177Anonymous ciphers (without a permanent server key) also use ephemeral \s-1DH\s0 keys.
984263bc 178.PP
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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
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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.
185.PP
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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. The server will always generate a new
188\&\s-1DH\s0 key during the negotiation, when the \s-1DH\s0 parameters are supplied via
189callback and/or when the \s-1SSL_OP_SINGLE_DH_USE\s0 option of
190\&\fISSL_CTX_set_options\fR\|(3) is set. It will
191immediately 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,
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193it may happen that a key is generated on initialization without later
194being needed, while on the other hand the computer time during the
195negotiation is being saved.
196.PP
e056f0e0 197If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly
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198necessary to generate a new key for each handshake but it does improve forward
199secrecy. If it is not assured, that \*(L"strong\*(R" primes were used (see especially
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200the section about \s-1DSA\s0 parameters below), \s-1SSL_OP_SINGLE_DH_USE\s0 must be used
201in order to prevent small subgroup attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0
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202has an impact on the computer time needed during negotiation, but it is not
203very large, so application authors/users should consider to always enable
204this option.
205.PP
e056f0e0 206As generating \s-1DH\s0 parameters is extremely time consuming, an application
984263bc 207should not generate the parameters on the fly but supply the parameters.
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208\&\s-1DH\s0 parameters can be reused, as the actual key is newly generated during
209the negotiation. The risk in reusing \s-1DH\s0 parameters is that an attacker
210may specialize on a very often used \s-1DH\s0 group. Applications should therefore
211generate their own \s-1DH\s0 parameters during the installation process using the
212openssl \fIdhparam\fR\|(1) application. In order to reduce the computer
213time needed for this generation, it is possible to use \s-1DSA\s0 parameters
214instead (see \fIdhparam\fR\|(1)), but in this case \s-1SSL_OP_SINGLE_DH_USE\s0
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215is mandatory.
216.PP
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217Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem,
218dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of current
219version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters,
220which use safe primes and were generated verifiably pseudo\-randomly.
984263bc 221These files can be converted into C code using the \fB\-C\fR option of the
e056f0e0 222\&\fIdhparam\fR\|(1) application.
984263bc 223Authors may also generate their own set of parameters using
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224\&\fIdhparam\fR\|(1), but a user may not be sure how the parameters were
225generated. The generation of \s-1DH\s0 parameters during installation is therefore
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226recommended.
227.PP
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228An application may either directly specify the \s-1DH\s0 parameters or
229can supply the \s-1DH\s0 parameters via a callback function. The callback approach
230has the advantage, that the callback may supply \s-1DH\s0 parameters for different
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231key lengths.
232.PP
233The \fBtmp_dh_callback\fR is called with the \fBkeylength\fR needed and
234the \fBis_export\fR information. The \fBis_export\fR flag is set, when the
e056f0e0 235ephemeral \s-1DH\s0 key exchange is performed with an export cipher.
984263bc 236.SH "EXAMPLES"
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237.IX Header "EXAMPLES"
238Handle \s-1DH\s0 parameters for key lengths of 512 and 1024 bits. (Error handling
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239partly 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
e056f0e0 248.PP
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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
e056f0e0 265.PP
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266.Vb 3
267\& /* "openssl dhparam -C -2 512" etc... */
268\& DH *get_dh512() { ... }
269\& DH *get_dh1024() { ... }
270.Ve
e056f0e0 271.PP
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272.Vb 3
273\& DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
274\& {
275\& DH *dh_tmp=NULL;
276.Ve
e056f0e0 277.PP
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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"
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298.IX Header "RETURN VALUES"
299\&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return
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300diagnostic output.
301.PP
e056f0e0 302\&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0
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303on failure. Check the error queue to find out the reason of failure.
304.SH "SEE ALSO"
a7d27d5a 305.IX Header "SEE ALSO"
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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)