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136 .IX Title "DH_set_method 3"
137 .TH DH_set_method 3 "2007-11-19" "1.1.0-dev" "OpenSSL"
138 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
139 .\" way too many mistakes in technical documents.
140 .if n .ad l
141 .nh
142 .SH "NAME"
143 DH_set_default_method, DH_get_default_method,
144 DH_set_method, DH_new_method, DH_OpenSSL \- select DH method
145 .SH "LIBRARY"
146 libcrypto, -lcrypto
147 .SH "SYNOPSIS"
148 .IX Header "SYNOPSIS"
149 .Vb 2
150 \& #include <openssl/dh.h>
151 \& #include <openssl/engine.h>
152 \&
153 \& void DH_set_default_method(const DH_METHOD *meth);
154 \&
155 \& const DH_METHOD *DH_get_default_method(void);
156 \&
157 \& int DH_set_method(DH *dh, const DH_METHOD *meth);
158 \&
159 \& DH *DH_new_method(ENGINE *engine);
160 \&
161 \& const DH_METHOD *DH_OpenSSL(void);
162 .Ve
163 .SH "DESCRIPTION"
164 .IX Header "DESCRIPTION"
165 A \fB\s-1DH_METHOD\s0\fR specifies the functions that OpenSSL uses for Diffie-Hellman
166 operations. By modifying the method, alternative implementations
167 such as hardware accelerators may be used. \s-1IMPORTANT:\s0 See the \s-1NOTES\s0 section for
168 important information about how these \s-1DH\s0 \s-1API\s0 functions are affected by the use
169 of \fB\s-1ENGINE\s0\fR \s-1API\s0 calls.
170 .PP
171 Initially, the default \s-1DH_METHOD\s0 is the OpenSSL internal implementation, as
172 returned by \fIDH_OpenSSL()\fR.
173 .PP
174 \&\fIDH_set_default_method()\fR makes \fBmeth\fR the default method for all \s-1DH\s0
175 structures created later. \fB\s-1NB\s0\fR: This is true only whilst no \s-1ENGINE\s0 has been set
176 as a default for \s-1DH\s0, so this function is no longer recommended.
177 .PP
178 \&\fIDH_get_default_method()\fR returns a pointer to the current default \s-1DH_METHOD\s0.
179 However, the meaningfulness of this result is dependent on whether the \s-1ENGINE\s0
180 \&\s-1API\s0 is being used, so this function is no longer recommended.
181 .PP
182 \&\fIDH_set_method()\fR selects \fBmeth\fR to perform all operations using the key \fBdh\fR.
183 This will replace the \s-1DH_METHOD\s0 used by the \s-1DH\s0 key and if the previous method
184 was supplied by an \s-1ENGINE\s0, the handle to that \s-1ENGINE\s0 will be released during the
185 change. It is possible to have \s-1DH\s0 keys that only work with certain \s-1DH_METHOD\s0
186 implementations (eg. from an \s-1ENGINE\s0 module that supports embedded
187 hardware-protected keys), and in such cases attempting to change the \s-1DH_METHOD\s0
188 for the key can have unexpected results.
189 .PP
190 \&\fIDH_new_method()\fR allocates and initializes a \s-1DH\s0 structure so that \fBengine\fR will
191 be used for the \s-1DH\s0 operations. If \fBengine\fR is \s-1NULL\s0, the default \s-1ENGINE\s0 for \s-1DH\s0
192 operations is used, and if no default \s-1ENGINE\s0 is set, the \s-1DH_METHOD\s0 controlled by
193 \&\fIDH_set_default_method()\fR is used.
194 .SH "THE DH_METHOD STRUCTURE"
195 .IX Header "THE DH_METHOD STRUCTURE"
196 .Vb 4
197 \& typedef struct dh_meth_st
198 \& {
199 \& /* name of the implementation */
200 \& const char *name;
201 \&
202 \& /* generate private and public DH values for key agreement */
203 \& int (*generate_key)(DH *dh);
204 \&
205 \& /* compute shared secret */
206 \& int (*compute_key)(unsigned char *key, BIGNUM *pub_key, DH *dh);
207 \&
208 \& /* compute r = a ^ p mod m (May be NULL for some implementations) */
209 \& int (*bn_mod_exp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p,
210 \& const BIGNUM *m, BN_CTX *ctx,
211 \& BN_MONT_CTX *m_ctx);
212 \&
213 \& /* called at DH_new */
214 \& int (*init)(DH *dh);
215 \&
216 \& /* called at DH_free */
217 \& int (*finish)(DH *dh);
218 \&
219 \& int flags;
220 \&
221 \& char *app_data; /* ?? */
222 \&
223 \& } DH_METHOD;
224 .Ve
225 .SH "RETURN VALUES"
226 .IX Header "RETURN VALUES"
227 \&\fIDH_OpenSSL()\fR and \fIDH_get_default_method()\fR return pointers to the respective
228 \&\fB\s-1DH_METHOD\s0\fRs.
229 .PP
230 \&\fIDH_set_default_method()\fR returns no value.
231 .PP
232 \&\fIDH_set_method()\fR returns non-zero if the provided \fBmeth\fR was successfully set as
233 the method for \fBdh\fR (including unloading the \s-1ENGINE\s0 handle if the previous
234 method was supplied by an \s-1ENGINE\s0).
235 .PP
236 \&\fIDH_new_method()\fR returns \s-1NULL\s0 and sets an error code that can be obtained by
237 \&\fIERR_get_error\fR\|(3) if the allocation fails. Otherwise it
238 returns a pointer to the newly allocated structure.
239 .SH "NOTES"
240 .IX Header "NOTES"
241 As of version 0.9.7, \s-1DH_METHOD\s0 implementations are grouped together with other
242 algorithmic APIs (eg. \s-1RSA_METHOD\s0, \s-1EVP_CIPHER\s0, etc) in \fB\s-1ENGINE\s0\fR modules. If a
243 default \s-1ENGINE\s0 is specified for \s-1DH\s0 functionality using an \s-1ENGINE\s0 \s-1API\s0 function,
244 that will override any \s-1DH\s0 defaults set using the \s-1DH\s0 \s-1API\s0 (ie.
245 \&\fIDH_set_default_method()\fR). For this reason, the \s-1ENGINE\s0 \s-1API\s0 is the recommended way
246 to control default implementations for use in \s-1DH\s0 and other cryptographic
247 algorithms.
248 .SH "SEE ALSO"
249 .IX Header "SEE ALSO"
250 \&\fIopenssl_dh\fR\|(3), \fIDH_new\fR\|(3)
251 .SH "HISTORY"
252 .IX Header "HISTORY"
253 \&\fIDH_set_default_method()\fR, \fIDH_get_default_method()\fR, \fIDH_set_method()\fR,
254 \&\fIDH_new_method()\fR and \fIDH_OpenSSL()\fR were added in OpenSSL 0.9.4.
255 .PP
256 \&\fIDH_set_default_openssl_method()\fR and \fIDH_get_default_openssl_method()\fR replaced
257 \&\fIDH_set_default_method()\fR and \fIDH_get_default_method()\fR respectively, and
258 \&\fIDH_set_method()\fR and \fIDH_new_method()\fR were altered to use \fB\s-1ENGINE\s0\fRs rather than
259 \&\fB\s-1DH_METHOD\s0\fRs during development of the engine version of OpenSSL 0.9.6. For
260 0.9.7, the handling of defaults in the \s-1ENGINE\s0 \s-1API\s0 was restructured so that this
261 change was reversed, and behaviour of the other functions resembled more closely
262 the previous behaviour. The behaviour of defaults in the \s-1ENGINE\s0 \s-1API\s0 now
263 transparently overrides the behaviour of defaults in the \s-1DH\s0 \s-1API\s0 without
264 requiring changing these function prototypes.