Update files for OpenSSL-1.0.0f import.
[dragonfly.git] / secure / lib / libssl / man / SSL_CTX_set_tmp_dh_callback.3
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124.\" ========================================================================
125.\"
126.IX Title "SSL_CTX_set_tmp_dh_callback 3"
e3261593 127.TH SSL_CTX_set_tmp_dh_callback 3 "2012-01-04" "1.0.0f" "OpenSSL"
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128.\" For nroff, turn off justification. Always turn off hyphenation; it makes
129.\" way too many mistakes in technical documents.
130.if n .ad l
131.nh
984263bc 132.SH "NAME"
a7d27d5a 133SSL_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 134.SH "SYNOPSIS"
e056f0e0 135.IX Header "SYNOPSIS"
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136.Vb 1
137\& #include <openssl/ssl.h>
e257b235 138\&
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139\& void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
140\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
141\& long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
e257b235 142\&
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143\& void SSL_set_tmp_dh_callback(SSL_CTX *ctx,
144\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
145\& long SSL_set_tmp_dh(SSL *ssl, DH *dh)
e257b235 146\&
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147\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
148.Ve
149.SH "DESCRIPTION"
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150.IX Header "DESCRIPTION"
151\&\fISSL_CTX_set_tmp_dh_callback()\fR sets the callback function for \fBctx\fR to be
152used when a \s-1DH\s0 parameters are required to \fBtmp_dh_callback\fR.
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153The callback is inherited by all \fBssl\fR objects created from \fBctx\fR.
154.PP
e056f0e0 155\&\fISSL_CTX_set_tmp_dh()\fR sets \s-1DH\s0 parameters to be used to be \fBdh\fR.
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156The key is inherited by all \fBssl\fR objects created from \fBctx\fR.
157.PP
e056f0e0 158\&\fISSL_set_tmp_dh_callback()\fR sets the callback only for \fBssl\fR.
984263bc 159.PP
e056f0e0 160\&\fISSL_set_tmp_dh()\fR sets the parameters only for \fBssl\fR.
984263bc 161.PP
e056f0e0 162These functions apply to \s-1SSL/TLS\s0 servers only.
984263bc 163.SH "NOTES"
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164.IX Header "NOTES"
165When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1DH\s0 key exchange
166can take place. Ciphers with \s-1DSA\s0 keys always use ephemeral \s-1DH\s0 keys as well.
984263bc 167In these cases, the session data are negotiated using the
e056f0e0 168ephemeral/temporary \s-1DH\s0 key and the key supplied and certified
984263bc 169by the certificate chain is only used for signing.
e056f0e0 170Anonymous ciphers (without a permanent server key) also use ephemeral \s-1DH\s0 keys.
984263bc 171.PP
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172Using ephemeral \s-1DH\s0 key exchange yields forward secrecy, as the connection
173can only be decrypted, when the \s-1DH\s0 key is known. By generating a temporary
174\&\s-1DH\s0 key inside the server application that is lost when the application
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175is left, it becomes impossible for an attacker to decrypt past sessions,
176even if he gets hold of the normal (certified) key, as this key was
177only used for signing.
178.PP
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179In order to perform a \s-1DH\s0 key exchange the server must use a \s-1DH\s0 group
180(\s-1DH\s0 parameters) and generate a \s-1DH\s0 key. The server will always generate a new
181\&\s-1DH\s0 key during the negotiation, when the \s-1DH\s0 parameters are supplied via
182callback and/or when the \s-1SSL_OP_SINGLE_DH_USE\s0 option of
183\&\fISSL_CTX_set_options\fR\|(3) is set. It will
184immediately create a \s-1DH\s0 key, when \s-1DH\s0 parameters are supplied via
185\&\fISSL_CTX_set_tmp_dh()\fR and \s-1SSL_OP_SINGLE_DH_USE\s0 is not set. In this case,
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186it may happen that a key is generated on initialization without later
187being needed, while on the other hand the computer time during the
188negotiation is being saved.
189.PP
e056f0e0 190If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly
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191necessary to generate a new key for each handshake but it does improve forward
192secrecy. If it is not assured, that \*(L"strong\*(R" primes were used (see especially
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193the section about \s-1DSA\s0 parameters below), \s-1SSL_OP_SINGLE_DH_USE\s0 must be used
194in order to prevent small subgroup attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0
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195has an impact on the computer time needed during negotiation, but it is not
196very large, so application authors/users should consider to always enable
197this option.
198.PP
e056f0e0 199As generating \s-1DH\s0 parameters is extremely time consuming, an application
984263bc 200should not generate the parameters on the fly but supply the parameters.
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201\&\s-1DH\s0 parameters can be reused, as the actual key is newly generated during
202the negotiation. The risk in reusing \s-1DH\s0 parameters is that an attacker
203may specialize on a very often used \s-1DH\s0 group. Applications should therefore
204generate their own \s-1DH\s0 parameters during the installation process using the
205openssl \fIdhparam\fR\|(1) application. In order to reduce the computer
206time needed for this generation, it is possible to use \s-1DSA\s0 parameters
207instead (see \fIdhparam\fR\|(1)), but in this case \s-1SSL_OP_SINGLE_DH_USE\s0
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208is mandatory.
209.PP
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210Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem,
211dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of current
212version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters,
e257b235 213which use safe primes and were generated verifiably pseudo-randomly.
984263bc 214These files can be converted into C code using the \fB\-C\fR option of the
e056f0e0 215\&\fIdhparam\fR\|(1) application.
984263bc 216Authors may also generate their own set of parameters using
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217\&\fIdhparam\fR\|(1), but a user may not be sure how the parameters were
218generated. The generation of \s-1DH\s0 parameters during installation is therefore
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219recommended.
220.PP
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221An application may either directly specify the \s-1DH\s0 parameters or
222can supply the \s-1DH\s0 parameters via a callback function. The callback approach
223has the advantage, that the callback may supply \s-1DH\s0 parameters for different
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224key lengths.
225.PP
226The \fBtmp_dh_callback\fR is called with the \fBkeylength\fR needed and
227the \fBis_export\fR information. The \fBis_export\fR flag is set, when the
e056f0e0 228ephemeral \s-1DH\s0 key exchange is performed with an export cipher.
984263bc 229.SH "EXAMPLES"
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230.IX Header "EXAMPLES"
231Handle \s-1DH\s0 parameters for key lengths of 512 and 1024 bits. (Error handling
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232partly left out.)
233.PP
234.Vb 5
235\& ...
236\& /* Set up ephemeral DH stuff */
237\& DH *dh_512 = NULL;
238\& DH *dh_1024 = NULL;
239\& FILE *paramfile;
e257b235 240\&
984263bc 241\& ...
e257b235 242\& /* "openssl dhparam \-out dh_param_512.pem \-2 512" */
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243\& paramfile = fopen("dh_param_512.pem", "r");
244\& if (paramfile) {
245\& dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
246\& fclose(paramfile);
247\& }
e257b235 248\& /* "openssl dhparam \-out dh_param_1024.pem \-2 1024" */
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249\& paramfile = fopen("dh_param_1024.pem", "r");
250\& if (paramfile) {
251\& dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
252\& fclose(paramfile);
253\& }
254\& ...
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255\&
256\& /* "openssl dhparam \-C \-2 512" etc... */
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257\& DH *get_dh512() { ... }
258\& DH *get_dh1024() { ... }
e257b235 259\&
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260\& DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
261\& {
262\& DH *dh_tmp=NULL;
e257b235 263\&
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264\& switch (keylength) {
265\& case 512:
266\& if (!dh_512)
267\& dh_512 = get_dh512();
268\& dh_tmp = dh_512;
269\& break;
270\& case 1024:
271\& if (!dh_1024)
272\& dh_1024 = get_dh1024();
273\& dh_tmp = dh_1024;
274\& break;
275\& default:
276\& /* Generating a key on the fly is very costly, so use what is there */
277\& setup_dh_parameters_like_above();
278\& }
279\& return(dh_tmp);
280\& }
281.Ve
282.SH "RETURN VALUES"
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283.IX Header "RETURN VALUES"
284\&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return
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285diagnostic output.
286.PP
e056f0e0 287\&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0
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288on failure. Check the error queue to find out the reason of failure.
289.SH "SEE ALSO"
a7d27d5a 290.IX Header "SEE ALSO"
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291\&\fIssl\fR\|(3), \fISSL_CTX_set_cipher_list\fR\|(3),
292\&\fISSL_CTX_set_tmp_rsa_callback\fR\|(3),
293\&\fISSL_CTX_set_options\fR\|(3),
294\&\fIciphers\fR\|(1), \fIdhparam\fR\|(1)