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"
34240b21 127.TH SSL_CTX_set_tmp_dh_callback 3 "2014-06-05" "1.0.1h" "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
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177In order to perform a \s-1DH\s0 key exchange the server must use a \s-1DH\s0 group
178(\s-1DH\s0 parameters) and generate a \s-1DH\s0 key. The server will always generate a new
179\&\s-1DH\s0 key during the negotiation, when the \s-1DH\s0 parameters are supplied via
180callback and/or when the \s-1SSL_OP_SINGLE_DH_USE\s0 option of
181\&\fISSL_CTX_set_options\fR\|(3) is set. It will
182immediately create a \s-1DH\s0 key, when \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. In this case,
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184it may happen that a key is generated on initialization without later
185being needed, while on the other hand the computer time during the
186negotiation is being saved.
187.PP
e056f0e0 188If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly
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189necessary to generate a new key for each handshake but it does improve forward
190secrecy. If it is not assured, that \*(L"strong\*(R" primes were used (see especially
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191the section about \s-1DSA\s0 parameters below), \s-1SSL_OP_SINGLE_DH_USE\s0 must be used
192in order to prevent small subgroup attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0
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193has an impact on the computer time needed during negotiation, but it is not
194very large, so application authors/users should consider to always enable
195this option.
196.PP
e056f0e0 197As generating \s-1DH\s0 parameters is extremely time consuming, an application
984263bc 198should not generate the parameters on the fly but supply the parameters.
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199\&\s-1DH\s0 parameters can be reused, as the actual key is newly generated during
200the negotiation. The risk in reusing \s-1DH\s0 parameters is that an attacker
201may specialize on a very often used \s-1DH\s0 group. Applications should therefore
202generate their own \s-1DH\s0 parameters during the installation process using the
203openssl \fIdhparam\fR\|(1) application. In order to reduce the computer
204time needed for this generation, it is possible to use \s-1DSA\s0 parameters
205instead (see \fIdhparam\fR\|(1)), but in this case \s-1SSL_OP_SINGLE_DH_USE\s0
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206is mandatory.
207.PP
e056f0e0 208Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem,
ecf90583 209dh1024.pem, dh2048.pem, and dh4096.pem in the 'apps' directory of current
e056f0e0 210version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters,
e257b235 211which use safe primes and were generated verifiably pseudo-randomly.
984263bc 212These files can be converted into C code using the \fB\-C\fR option of the
e056f0e0 213\&\fIdhparam\fR\|(1) application.
984263bc 214Authors may also generate their own set of parameters using
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215\&\fIdhparam\fR\|(1), but a user may not be sure how the parameters were
216generated. The generation of \s-1DH\s0 parameters during installation is therefore
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217recommended.
218.PP
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219An application may either directly specify the \s-1DH\s0 parameters or
220can supply the \s-1DH\s0 parameters via a callback function. The callback approach
221has the advantage, that the callback may supply \s-1DH\s0 parameters for different
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222key lengths.
223.PP
224The \fBtmp_dh_callback\fR is called with the \fBkeylength\fR needed and
225the \fBis_export\fR information. The \fBis_export\fR flag is set, when the
e056f0e0 226ephemeral \s-1DH\s0 key exchange is performed with an export cipher.
984263bc 227.SH "EXAMPLES"
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228.IX Header "EXAMPLES"
229Handle \s-1DH\s0 parameters for key lengths of 512 and 1024 bits. (Error handling
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230partly left out.)
231.PP
232.Vb 5
233\& ...
234\& /* Set up ephemeral DH stuff */
235\& DH *dh_512 = NULL;
236\& DH *dh_1024 = NULL;
237\& FILE *paramfile;
e257b235 238\&
984263bc 239\& ...
e257b235 240\& /* "openssl dhparam \-out dh_param_512.pem \-2 512" */
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241\& paramfile = fopen("dh_param_512.pem", "r");
242\& if (paramfile) {
243\& dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
244\& fclose(paramfile);
245\& }
e257b235 246\& /* "openssl dhparam \-out dh_param_1024.pem \-2 1024" */
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247\& paramfile = fopen("dh_param_1024.pem", "r");
248\& if (paramfile) {
249\& dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
250\& fclose(paramfile);
251\& }
252\& ...
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253\&
254\& /* "openssl dhparam \-C \-2 512" etc... */
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255\& DH *get_dh512() { ... }
256\& DH *get_dh1024() { ... }
e257b235 257\&
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258\& DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
259\& {
260\& DH *dh_tmp=NULL;
e257b235 261\&
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262\& switch (keylength) {
263\& case 512:
264\& if (!dh_512)
265\& dh_512 = get_dh512();
266\& dh_tmp = dh_512;
267\& break;
268\& case 1024:
269\& if (!dh_1024)
270\& dh_1024 = get_dh1024();
271\& dh_tmp = dh_1024;
272\& break;
273\& default:
274\& /* Generating a key on the fly is very costly, so use what is there */
275\& setup_dh_parameters_like_above();
276\& }
277\& return(dh_tmp);
278\& }
279.Ve
280.SH "RETURN VALUES"
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281.IX Header "RETURN VALUES"
282\&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return
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283diagnostic output.
284.PP
e056f0e0 285\&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0
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286on failure. Check the error queue to find out the reason of failure.
287.SH "SEE ALSO"
a7d27d5a 288.IX Header "SEE ALSO"
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289\&\fIssl\fR\|(3), \fISSL_CTX_set_cipher_list\fR\|(3),
290\&\fISSL_CTX_set_tmp_rsa_callback\fR\|(3),
291\&\fISSL_CTX_set_options\fR\|(3),
292\&\fIciphers\fR\|(1), \fIdhparam\fR\|(1)