Merge branch 'vendor/OPENSSL'
[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"
ca2244c8 127.TH SSL_CTX_set_tmp_dh_callback 3 "2015-01-15" "1.0.1l" "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\&
ecf90583 143\& void SSL_set_tmp_dh_callback(SSL *ctx,
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144\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
145\& long SSL_set_tmp_dh(SSL *ssl, DH *dh)
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146.Ve
147.SH "DESCRIPTION"
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148.IX Header "DESCRIPTION"
149\&\fISSL_CTX_set_tmp_dh_callback()\fR sets the callback function for \fBctx\fR to be
150used when a \s-1DH\s0 parameters are required to \fBtmp_dh_callback\fR.
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151The callback is inherited by all \fBssl\fR objects created from \fBctx\fR.
152.PP
e056f0e0 153\&\fISSL_CTX_set_tmp_dh()\fR sets \s-1DH\s0 parameters to be used to be \fBdh\fR.
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154The key is inherited by all \fBssl\fR objects created from \fBctx\fR.
155.PP
e056f0e0 156\&\fISSL_set_tmp_dh_callback()\fR sets the callback only for \fBssl\fR.
984263bc 157.PP
e056f0e0 158\&\fISSL_set_tmp_dh()\fR sets the parameters only for \fBssl\fR.
984263bc 159.PP
e056f0e0 160These functions apply to \s-1SSL/TLS\s0 servers only.
984263bc 161.SH "NOTES"
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162.IX Header "NOTES"
163When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1DH\s0 key exchange
164can take place. Ciphers with \s-1DSA\s0 keys always use ephemeral \s-1DH\s0 keys as well.
984263bc 165In these cases, the session data are negotiated using the
e056f0e0 166ephemeral/temporary \s-1DH\s0 key and the key supplied and certified
984263bc 167by the certificate chain is only used for signing.
e056f0e0 168Anonymous ciphers (without a permanent server key) also use ephemeral \s-1DH\s0 keys.
984263bc 169.PP
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170Using ephemeral \s-1DH\s0 key exchange yields forward secrecy, as the connection
171can only be decrypted, when the \s-1DH\s0 key is known. By generating a temporary
172\&\s-1DH\s0 key inside the server application that is lost when the application
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173is left, it becomes impossible for an attacker to decrypt past sessions,
174even if he gets hold of the normal (certified) key, as this key was
175only used for signing.
176.PP
e056f0e0 177In order to perform a \s-1DH\s0 key exchange the server must use a \s-1DH\s0 group
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178(\s-1DH\s0 parameters) and generate a \s-1DH\s0 key.
179The server will always generate a new \s-1DH\s0 key during the negotiation
180if either the \s-1DH\s0 parameters are supplied via callback or the
181\&\s-1SSL_OP_SINGLE_DH_USE\s0 option of \fISSL_CTX_set_options\fR\|(3) is set (or both).
182It will immediately create a \s-1DH\s0 key if \s-1DH\s0 parameters are supplied via
183\&\fISSL_CTX_set_tmp_dh()\fR and \s-1SSL_OP_SINGLE_DH_USE\s0 is not set.
184In this case,
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185it may happen that a key is generated on initialization without later
186being needed, while on the other hand the computer time during the
187negotiation is being saved.
188.PP
e056f0e0 189If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly
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190necessary to generate a new key for each handshake but it does improve forward
191secrecy. If it is not assured, that \*(L"strong\*(R" primes were used (see especially
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192the section about \s-1DSA\s0 parameters below), \s-1SSL_OP_SINGLE_DH_USE\s0 must be used
193in order to prevent small subgroup attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0
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194has an impact on the computer time needed during negotiation, but it is not
195very large, so application authors/users should consider to always enable
196this option.
197.PP
e056f0e0 198As generating \s-1DH\s0 parameters is extremely time consuming, an application
984263bc 199should not generate the parameters on the fly but supply the parameters.
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200\&\s-1DH\s0 parameters can be reused, as the actual key is newly generated during
201the negotiation. The risk in reusing \s-1DH\s0 parameters is that an attacker
202may specialize on a very often used \s-1DH\s0 group. Applications should therefore
203generate their own \s-1DH\s0 parameters during the installation process using the
204openssl \fIdhparam\fR\|(1) application. In order to reduce the computer
205time needed for this generation, it is possible to use \s-1DSA\s0 parameters
206instead (see \fIdhparam\fR\|(1)), but in this case \s-1SSL_OP_SINGLE_DH_USE\s0
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207is mandatory.
208.PP
e056f0e0 209Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem,
ecf90583 210dh1024.pem, dh2048.pem, and dh4096.pem in the 'apps' directory of current
e056f0e0 211version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters,
e257b235 212which use safe primes and were generated verifiably pseudo-randomly.
984263bc 213These files can be converted into C code using the \fB\-C\fR option of the
e056f0e0 214\&\fIdhparam\fR\|(1) application.
984263bc 215Authors may also generate their own set of parameters using
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216\&\fIdhparam\fR\|(1), but a user may not be sure how the parameters were
217generated. The generation of \s-1DH\s0 parameters during installation is therefore
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218recommended.
219.PP
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220An application may either directly specify the \s-1DH\s0 parameters or
221can supply the \s-1DH\s0 parameters via a callback function. The callback approach
222has the advantage, that the callback may supply \s-1DH\s0 parameters for different
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223key lengths.
224.PP
225The \fBtmp_dh_callback\fR is called with the \fBkeylength\fR needed and
226the \fBis_export\fR information. The \fBis_export\fR flag is set, when the
e056f0e0 227ephemeral \s-1DH\s0 key exchange is performed with an export cipher.
984263bc 228.SH "EXAMPLES"
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229.IX Header "EXAMPLES"
230Handle \s-1DH\s0 parameters for key lengths of 512 and 1024 bits. (Error handling
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231partly left out.)
232.PP
233.Vb 5
234\& ...
235\& /* Set up ephemeral DH stuff */
236\& DH *dh_512 = NULL;
237\& DH *dh_1024 = NULL;
238\& FILE *paramfile;
e257b235 239\&
984263bc 240\& ...
e257b235 241\& /* "openssl dhparam \-out dh_param_512.pem \-2 512" */
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242\& paramfile = fopen("dh_param_512.pem", "r");
243\& if (paramfile) {
244\& dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
245\& fclose(paramfile);
246\& }
e257b235 247\& /* "openssl dhparam \-out dh_param_1024.pem \-2 1024" */
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248\& paramfile = fopen("dh_param_1024.pem", "r");
249\& if (paramfile) {
250\& dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
251\& fclose(paramfile);
252\& }
253\& ...
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254\&
255\& /* "openssl dhparam \-C \-2 512" etc... */
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256\& DH *get_dh512() { ... }
257\& DH *get_dh1024() { ... }
e257b235 258\&
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259\& DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
260\& {
261\& DH *dh_tmp=NULL;
e257b235 262\&
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263\& switch (keylength) {
264\& case 512:
265\& if (!dh_512)
266\& dh_512 = get_dh512();
267\& dh_tmp = dh_512;
268\& break;
269\& case 1024:
d95b477a 270\& if (!dh_1024)
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271\& dh_1024 = get_dh1024();
272\& dh_tmp = dh_1024;
273\& break;
274\& default:
275\& /* Generating a key on the fly is very costly, so use what is there */
276\& setup_dh_parameters_like_above();
277\& }
278\& return(dh_tmp);
279\& }
280.Ve
281.SH "RETURN VALUES"
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282.IX Header "RETURN VALUES"
283\&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return
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284diagnostic output.
285.PP
e056f0e0 286\&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0
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287on failure. Check the error queue to find out the reason of failure.
288.SH "SEE ALSO"
a7d27d5a 289.IX Header "SEE ALSO"
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290\&\fIssl\fR\|(3), \fISSL_CTX_set_cipher_list\fR\|(3),
291\&\fISSL_CTX_set_tmp_rsa_callback\fR\|(3),
292\&\fISSL_CTX_set_options\fR\|(3),
293\&\fIciphers\fR\|(1), \fIdhparam\fR\|(1)