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137 .IX Title "DH_set_method 3"
138 .TH DH_set_method 3 "2009-07-19" "1.0.1n" "OpenSSL"
139 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
140 .\" way too many mistakes in technical documents.
141 .if n .ad l
142 .nh
143 .SH "NAME"
144 DH_set_default_method, DH_get_default_method,
145 DH_set_method, DH_new_method, DH_OpenSSL \- select DH method
146 .SH "LIBRARY"
147 libcrypto, -lcrypto
148 .SH "SYNOPSIS"
149 .IX Header "SYNOPSIS"
150 .Vb 2
151 \& #include <openssl/dh.h>
152 \& #include <openssl/engine.h>
153 \&
154 \& void DH_set_default_method(const DH_METHOD *meth);
155 \&
156 \& const DH_METHOD *DH_get_default_method(void);
157 \&
158 \& int DH_set_method(DH *dh, const DH_METHOD *meth);
159 \&
160 \& DH *DH_new_method(ENGINE *engine);
161 \&
162 \& const DH_METHOD *DH_OpenSSL(void);
163 .Ve
164 .SH "DESCRIPTION"
165 .IX Header "DESCRIPTION"
166 A \fB\s-1DH_METHOD\s0\fR specifies the functions that OpenSSL uses for Diffie-Hellman
167 operations. By modifying the method, alternative implementations
168 such as hardware accelerators may be used. \s-1IMPORTANT:\s0 See the \s-1NOTES\s0 section for
169 important information about how these \s-1DH API\s0 functions are affected by the use
170 of \fB\s-1ENGINE\s0\fR \s-1API\s0 calls.
171 .PP
172 Initially, the default \s-1DH_METHOD\s0 is the OpenSSL internal implementation, as
173 returned by \fIDH_OpenSSL()\fR.
174 .PP
175 \&\fIDH_set_default_method()\fR makes \fBmeth\fR the default method for all \s-1DH\s0
176 structures created later. \fB\s-1NB\s0\fR: This is true only whilst no \s-1ENGINE\s0 has been set
177 as a default for \s-1DH,\s0 so this function is no longer recommended.
178 .PP
179 \&\fIDH_get_default_method()\fR returns a pointer to the current default \s-1DH_METHOD.\s0
180 However, the meaningfulness of this result is dependent on whether the \s-1ENGINE
181 API\s0 is being used, so this function is no longer recommended.
182 .PP
183 \&\fIDH_set_method()\fR selects \fBmeth\fR to perform all operations using the key \fBdh\fR.
184 This will replace the \s-1DH_METHOD\s0 used by the \s-1DH\s0 key and if the previous method
185 was supplied by an \s-1ENGINE,\s0 the handle to that \s-1ENGINE\s0 will be released during the
186 change. It is possible to have \s-1DH\s0 keys that only work with certain \s-1DH_METHOD\s0
187 implementations (eg. from an \s-1ENGINE\s0 module that supports embedded
188 hardware-protected keys), and in such cases attempting to change the \s-1DH_METHOD\s0
189 for the key can have unexpected results.
190 .PP
191 \&\fIDH_new_method()\fR allocates and initializes a \s-1DH\s0 structure so that \fBengine\fR will
192 be used for the \s-1DH\s0 operations. If \fBengine\fR is \s-1NULL,\s0 the default \s-1ENGINE\s0 for \s-1DH\s0
193 operations is used, and if no default \s-1ENGINE\s0 is set, the \s-1DH_METHOD\s0 controlled by
194 \&\fIDH_set_default_method()\fR is used.
195 .SH "THE DH_METHOD STRUCTURE"
196 .IX Header "THE DH_METHOD STRUCTURE"
197 .Vb 4
198 \& typedef struct dh_meth_st
199 \& {
200 \& /* name of the implementation */
201 \& const char *name;
202 \&
203 \& /* generate private and public DH values for key agreement */
204 \& int (*generate_key)(DH *dh);
205 \&
206 \& /* compute shared secret */
207 \& int (*compute_key)(unsigned char *key, BIGNUM *pub_key, DH *dh);
208 \&
209 \& /* compute r = a ^ p mod m (May be NULL for some implementations) */
210 \& int (*bn_mod_exp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p,
211 \& const BIGNUM *m, BN_CTX *ctx,
212 \& BN_MONT_CTX *m_ctx);
213 \&
214 \& /* called at DH_new */
215 \& int (*init)(DH *dh);
216 \&
217 \& /* called at DH_free */
218 \& int (*finish)(DH *dh);
219 \&
220 \& int flags;
221 \&
222 \& char *app_data; /* ?? */
223 \&
224 \& } DH_METHOD;
225 .Ve
226 .SH "RETURN VALUES"
227 .IX Header "RETURN VALUES"
228 \&\fIDH_OpenSSL()\fR and \fIDH_get_default_method()\fR return pointers to the respective
229 \&\fB\s-1DH_METHOD\s0\fRs.
230 .PP
231 \&\fIDH_set_default_method()\fR returns no value.
232 .PP
233 \&\fIDH_set_method()\fR returns non-zero if the provided \fBmeth\fR was successfully set as
234 the method for \fBdh\fR (including unloading the \s-1ENGINE\s0 handle if the previous
235 method was supplied by an \s-1ENGINE\s0).
236 .PP
237 \&\fIDH_new_method()\fR returns \s-1NULL\s0 and sets an error code that can be obtained by
238 \&\fIERR_get_error\fR\|(3) if the allocation fails. Otherwise it
239 returns a pointer to the newly allocated structure.
240 .SH "NOTES"
241 .IX Header "NOTES"
242 As of version 0.9.7, \s-1DH_METHOD\s0 implementations are grouped together with other
243 algorithmic APIs (eg. \s-1RSA_METHOD, EVP_CIPHER,\s0 etc) in \fB\s-1ENGINE\s0\fR modules. If a
244 default \s-1ENGINE\s0 is specified for \s-1DH\s0 functionality using an \s-1ENGINE API\s0 function,
245 that will override any \s-1DH\s0 defaults set using the \s-1DH API \s0(ie.
246 \&\fIDH_set_default_method()\fR). For this reason, the \s-1ENGINE API\s0 is the recommended way
247 to control default implementations for use in \s-1DH\s0 and other cryptographic
248 algorithms.
249 .SH "SEE ALSO"
250 .IX Header "SEE ALSO"
251 \&\fIopenssl_dh\fR\|(3), \fIDH_new\fR\|(3)
252 .SH "HISTORY"
253 .IX Header "HISTORY"
254 \&\fIDH_set_default_method()\fR, \fIDH_get_default_method()\fR, \fIDH_set_method()\fR,
255 \&\fIDH_new_method()\fR and \fIDH_OpenSSL()\fR were added in OpenSSL 0.9.4.
256 .PP
257 \&\fIDH_set_default_openssl_method()\fR and \fIDH_get_default_openssl_method()\fR replaced
258 \&\fIDH_set_default_method()\fR and \fIDH_get_default_method()\fR respectively, and
259 \&\fIDH_set_method()\fR and \fIDH_new_method()\fR were altered to use \fB\s-1ENGINE\s0\fRs rather than
260 \&\fB\s-1DH_METHOD\s0\fRs during development of the engine version of OpenSSL 0.9.6. For
261 0.9.7, the handling of defaults in the \s-1ENGINE API\s0 was restructured so that this
262 change was reversed, and behaviour of the other functions resembled more closely
263 the previous behaviour. The behaviour of defaults in the \s-1ENGINE API\s0 now
264 transparently overrides the behaviour of defaults in the \s-1DH API\s0 without
265 requiring changing these function prototypes.