Update build for OpenSSL-0.9.8j upgrade.
[dragonfly.git] / secure / lib / libssl / man / SSL_CTX_set_tmp_dh_callback.3
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132.\" ========================================================================
133.\"
134.IX Title "SSL_CTX_set_tmp_dh_callback 3"
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135.TH SSL_CTX_set_tmp_dh_callback 3 "2009-01-11" "0.9.8j" "OpenSSL"
136.\" For nroff, turn off justification. Always turn off hyphenation; it makes
137.\" way too many mistakes in technical documents.
138.if n .ad l
139.nh
984263bc 140.SH "NAME"
a7d27d5a 141SSL_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 142.SH "SYNOPSIS"
e056f0e0 143.IX Header "SYNOPSIS"
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144.Vb 1
145\& #include <openssl/ssl.h>
e257b235 146\&
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147\& void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
148\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
149\& long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
e257b235 150\&
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151\& void SSL_set_tmp_dh_callback(SSL_CTX *ctx,
152\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
153\& long SSL_set_tmp_dh(SSL *ssl, DH *dh)
e257b235 154\&
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155\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
156.Ve
157.SH "DESCRIPTION"
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158.IX Header "DESCRIPTION"
159\&\fISSL_CTX_set_tmp_dh_callback()\fR sets the callback function for \fBctx\fR to be
160used when a \s-1DH\s0 parameters are required to \fBtmp_dh_callback\fR.
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161The callback is inherited by all \fBssl\fR objects created from \fBctx\fR.
162.PP
e056f0e0 163\&\fISSL_CTX_set_tmp_dh()\fR sets \s-1DH\s0 parameters to be used to be \fBdh\fR.
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164The key is inherited by all \fBssl\fR objects created from \fBctx\fR.
165.PP
e056f0e0 166\&\fISSL_set_tmp_dh_callback()\fR sets the callback only for \fBssl\fR.
984263bc 167.PP
e056f0e0 168\&\fISSL_set_tmp_dh()\fR sets the parameters only for \fBssl\fR.
984263bc 169.PP
e056f0e0 170These functions apply to \s-1SSL/TLS\s0 servers only.
984263bc 171.SH "NOTES"
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172.IX Header "NOTES"
173When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1DH\s0 key exchange
174can take place. Ciphers with \s-1DSA\s0 keys always use ephemeral \s-1DH\s0 keys as well.
984263bc 175In these cases, the session data are negotiated using the
e056f0e0 176ephemeral/temporary \s-1DH\s0 key and the key supplied and certified
984263bc 177by the certificate chain is only used for signing.
e056f0e0 178Anonymous ciphers (without a permanent server key) also use ephemeral \s-1DH\s0 keys.
984263bc 179.PP
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180Using ephemeral \s-1DH\s0 key exchange yields forward secrecy, as the connection
181can only be decrypted, when the \s-1DH\s0 key is known. By generating a temporary
182\&\s-1DH\s0 key inside the server application that is lost when the application
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183is left, it becomes impossible for an attacker to decrypt past sessions,
184even if he gets hold of the normal (certified) key, as this key was
185only used for signing.
186.PP
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187In order to perform a \s-1DH\s0 key exchange the server must use a \s-1DH\s0 group
188(\s-1DH\s0 parameters) and generate a \s-1DH\s0 key. The server will always generate a new
189\&\s-1DH\s0 key during the negotiation, when the \s-1DH\s0 parameters are supplied via
190callback and/or when the \s-1SSL_OP_SINGLE_DH_USE\s0 option of
191\&\fISSL_CTX_set_options\fR\|(3) is set. It will
192immediately create a \s-1DH\s0 key, when \s-1DH\s0 parameters are supplied via
193\&\fISSL_CTX_set_tmp_dh()\fR and \s-1SSL_OP_SINGLE_DH_USE\s0 is not set. In this case,
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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.
197.PP
e056f0e0 198If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly
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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 (see especially
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201the section about \s-1DSA\s0 parameters below), \s-1SSL_OP_SINGLE_DH_USE\s0 must be used
202in order to prevent small subgroup attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0
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203has an impact on the computer time needed during negotiation, but it is not
204very large, so application authors/users should consider to always enable
205this option.
206.PP
e056f0e0 207As generating \s-1DH\s0 parameters is extremely time consuming, an application
984263bc 208should not generate the parameters on the fly but supply the parameters.
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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. In order to reduce the computer
214time needed for this generation, it is possible to use \s-1DSA\s0 parameters
215instead (see \fIdhparam\fR\|(1)), but in this case \s-1SSL_OP_SINGLE_DH_USE\s0
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216is mandatory.
217.PP
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218Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem,
219dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of current
220version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters,
e257b235 221which use safe primes and were generated verifiably pseudo-randomly.
984263bc 222These files can be converted into C code using the \fB\-C\fR option of the
e056f0e0 223\&\fIdhparam\fR\|(1) application.
984263bc 224Authors may also generate their own set of parameters using
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225\&\fIdhparam\fR\|(1), but a user may not be sure how the parameters were
226generated. The generation of \s-1DH\s0 parameters during installation is therefore
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227recommended.
228.PP
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229An application may either directly specify the \s-1DH\s0 parameters or
230can supply the \s-1DH\s0 parameters via a callback function. The callback approach
231has the advantage, that the callback may supply \s-1DH\s0 parameters for different
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232key lengths.
233.PP
234The \fBtmp_dh_callback\fR is called with the \fBkeylength\fR needed and
235the \fBis_export\fR information. The \fBis_export\fR flag is set, when the
e056f0e0 236ephemeral \s-1DH\s0 key exchange is performed with an export cipher.
984263bc 237.SH "EXAMPLES"
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238.IX Header "EXAMPLES"
239Handle \s-1DH\s0 parameters for key lengths of 512 and 1024 bits. (Error handling
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240partly left out.)
241.PP
242.Vb 5
243\& ...
244\& /* Set up ephemeral DH stuff */
245\& DH *dh_512 = NULL;
246\& DH *dh_1024 = NULL;
247\& FILE *paramfile;
e257b235 248\&
984263bc 249\& ...
e257b235 250\& /* "openssl dhparam \-out dh_param_512.pem \-2 512" */
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251\& paramfile = fopen("dh_param_512.pem", "r");
252\& if (paramfile) {
253\& dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
254\& fclose(paramfile);
255\& }
e257b235 256\& /* "openssl dhparam \-out dh_param_1024.pem \-2 1024" */
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257\& paramfile = fopen("dh_param_1024.pem", "r");
258\& if (paramfile) {
259\& dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
260\& fclose(paramfile);
261\& }
262\& ...
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263\&
264\& /* "openssl dhparam \-C \-2 512" etc... */
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265\& DH *get_dh512() { ... }
266\& DH *get_dh1024() { ... }
e257b235 267\&
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268\& DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
269\& {
270\& DH *dh_tmp=NULL;
e257b235 271\&
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272\& switch (keylength) {
273\& case 512:
274\& if (!dh_512)
275\& dh_512 = get_dh512();
276\& dh_tmp = dh_512;
277\& break;
278\& case 1024:
279\& if (!dh_1024)
280\& dh_1024 = get_dh1024();
281\& dh_tmp = dh_1024;
282\& break;
283\& default:
284\& /* Generating a key on the fly is very costly, so use what is there */
285\& setup_dh_parameters_like_above();
286\& }
287\& return(dh_tmp);
288\& }
289.Ve
290.SH "RETURN VALUES"
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291.IX Header "RETURN VALUES"
292\&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return
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293diagnostic output.
294.PP
e056f0e0 295\&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0
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296on failure. Check the error queue to find out the reason of failure.
297.SH "SEE ALSO"
a7d27d5a 298.IX Header "SEE ALSO"
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299\&\fIssl\fR\|(3), \fISSL_CTX_set_cipher_list\fR\|(3),
300\&\fISSL_CTX_set_tmp_rsa_callback\fR\|(3),
301\&\fISSL_CTX_set_options\fR\|(3),
302\&\fIciphers\fR\|(1), \fIdhparam\fR\|(1)