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| 124 | .\" ======================================================================== |
| 125 | .\" |
| 126 | .IX Title "SSL_CTX_set_tmp_dh_callback 3" |
| 127 | .TH SSL_CTX_set_tmp_dh_callback 3 "2013-02-05" "1.0.1d" "OpenSSL" |
| 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 |
| 132 | .SH "NAME" |
| 133 | SSL_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 |
| 134 | .SH "SYNOPSIS" |
| 135 | .IX Header "SYNOPSIS" |
| 136 | .Vb 1 |
| 137 | \& #include <openssl/ssl.h> |
| 138 | \& |
| 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); |
| 142 | \& |
| 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) |
| 146 | \& |
| 147 | \& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength)); |
| 148 | .Ve |
| 149 | .SH "DESCRIPTION" |
| 150 | .IX Header "DESCRIPTION" |
| 151 | \&\fISSL_CTX_set_tmp_dh_callback()\fR sets the callback function for \fBctx\fR to be |
| 152 | used when a \s-1DH\s0 parameters are required to \fBtmp_dh_callback\fR. |
| 153 | The callback is inherited by all \fBssl\fR objects created from \fBctx\fR. |
| 154 | .PP |
| 155 | \&\fISSL_CTX_set_tmp_dh()\fR sets \s-1DH\s0 parameters to be used to be \fBdh\fR. |
| 156 | The key is inherited by all \fBssl\fR objects created from \fBctx\fR. |
| 157 | .PP |
| 158 | \&\fISSL_set_tmp_dh_callback()\fR sets the callback only for \fBssl\fR. |
| 159 | .PP |
| 160 | \&\fISSL_set_tmp_dh()\fR sets the parameters only for \fBssl\fR. |
| 161 | .PP |
| 162 | These functions apply to \s-1SSL/TLS\s0 servers only. |
| 163 | .SH "NOTES" |
| 164 | .IX Header "NOTES" |
| 165 | When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1DH\s0 key exchange |
| 166 | can take place. Ciphers with \s-1DSA\s0 keys always use ephemeral \s-1DH\s0 keys as well. |
| 167 | In these cases, the session data are negotiated using the |
| 168 | ephemeral/temporary \s-1DH\s0 key and the key supplied and certified |
| 169 | by the certificate chain is only used for signing. |
| 170 | Anonymous ciphers (without a permanent server key) also use ephemeral \s-1DH\s0 keys. |
| 171 | .PP |
| 172 | Using ephemeral \s-1DH\s0 key exchange yields forward secrecy, as the connection |
| 173 | can 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 |
| 175 | is left, it becomes impossible for an attacker to decrypt past sessions, |
| 176 | even if he gets hold of the normal (certified) key, as this key was |
| 177 | only used for signing. |
| 178 | .PP |
| 179 | In 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 |
| 182 | callback and/or when the \s-1SSL_OP_SINGLE_DH_USE\s0 option of |
| 183 | \&\fISSL_CTX_set_options\fR\|(3) is set. It will |
| 184 | immediately 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, |
| 186 | it may happen that a key is generated on initialization without later |
| 187 | being needed, while on the other hand the computer time during the |
| 188 | negotiation is being saved. |
| 189 | .PP |
| 190 | If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly |
| 191 | necessary to generate a new key for each handshake but it does improve forward |
| 192 | secrecy. If it is not assured, that \*(L"strong\*(R" primes were used (see especially |
| 193 | the section about \s-1DSA\s0 parameters below), \s-1SSL_OP_SINGLE_DH_USE\s0 must be used |
| 194 | in order to prevent small subgroup attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0 |
| 195 | has an impact on the computer time needed during negotiation, but it is not |
| 196 | very large, so application authors/users should consider to always enable |
| 197 | this option. |
| 198 | .PP |
| 199 | As generating \s-1DH\s0 parameters is extremely time consuming, an application |
| 200 | should not generate the parameters on the fly but supply the parameters. |
| 201 | \&\s-1DH\s0 parameters can be reused, as the actual key is newly generated during |
| 202 | the negotiation. The risk in reusing \s-1DH\s0 parameters is that an attacker |
| 203 | may specialize on a very often used \s-1DH\s0 group. Applications should therefore |
| 204 | generate their own \s-1DH\s0 parameters during the installation process using the |
| 205 | openssl \fIdhparam\fR\|(1) application. In order to reduce the computer |
| 206 | time needed for this generation, it is possible to use \s-1DSA\s0 parameters |
| 207 | instead (see \fIdhparam\fR\|(1)), but in this case \s-1SSL_OP_SINGLE_DH_USE\s0 |
| 208 | is mandatory. |
| 209 | .PP |
| 210 | Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem, |
| 211 | dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of current |
| 212 | version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters, |
| 213 | which use safe primes and were generated verifiably pseudo-randomly. |
| 214 | These files can be converted into C code using the \fB\-C\fR option of the |
| 215 | \&\fIdhparam\fR\|(1) application. |
| 216 | Authors may also generate their own set of parameters using |
| 217 | \&\fIdhparam\fR\|(1), but a user may not be sure how the parameters were |
| 218 | generated. The generation of \s-1DH\s0 parameters during installation is therefore |
| 219 | recommended. |
| 220 | .PP |
| 221 | An application may either directly specify the \s-1DH\s0 parameters or |
| 222 | can supply the \s-1DH\s0 parameters via a callback function. The callback approach |
| 223 | has the advantage, that the callback may supply \s-1DH\s0 parameters for different |
| 224 | key lengths. |
| 225 | .PP |
| 226 | The \fBtmp_dh_callback\fR is called with the \fBkeylength\fR needed and |
| 227 | the \fBis_export\fR information. The \fBis_export\fR flag is set, when the |
| 228 | ephemeral \s-1DH\s0 key exchange is performed with an export cipher. |
| 229 | .SH "EXAMPLES" |
| 230 | .IX Header "EXAMPLES" |
| 231 | Handle \s-1DH\s0 parameters for key lengths of 512 and 1024 bits. (Error handling |
| 232 | partly 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; |
| 240 | \& |
| 241 | \& ... |
| 242 | \& /* "openssl dhparam \-out dh_param_512.pem \-2 512" */ |
| 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 | \& } |
| 248 | \& /* "openssl dhparam \-out dh_param_1024.pem \-2 1024" */ |
| 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 | \& ... |
| 255 | \& |
| 256 | \& /* "openssl dhparam \-C \-2 512" etc... */ |
| 257 | \& DH *get_dh512() { ... } |
| 258 | \& DH *get_dh1024() { ... } |
| 259 | \& |
| 260 | \& DH *tmp_dh_callback(SSL *s, int is_export, int keylength) |
| 261 | \& { |
| 262 | \& DH *dh_tmp=NULL; |
| 263 | \& |
| 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" |
| 283 | .IX Header "RETURN VALUES" |
| 284 | \&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return |
| 285 | diagnostic output. |
| 286 | .PP |
| 287 | \&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0 |
| 288 | on failure. Check the error queue to find out the reason of failure. |
| 289 | .SH "SEE ALSO" |
| 290 | .IX Header "SEE ALSO" |
| 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) |