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140 .IX Title "DH_set_method 3"
141 .TH DH_set_method 3 "0.9.7a" "2003-02-19" "OpenSSL"
144 DH_set_default_method, DH_get_default_method,
145 DH_set_method, DH_new_method, DH_OpenSSL \- select \s-1DH\s0 method
147 .IX Header "SYNOPSIS"
149 \& #include <openssl/dh.h>
150 \& #include <openssl/engine.h>
153 \& void DH_set_default_method(const DH_METHOD *meth);
156 \& const DH_METHOD *DH_get_default_method(void);
159 \& int DH_set_method(DH *dh, const DH_METHOD *meth);
162 \& DH *DH_new_method(ENGINE *engine);
165 \& const DH_METHOD *DH_OpenSSL(void);
168 .IX Header "DESCRIPTION"
169 A \fB\s-1DH_METHOD\s0\fR specifies the functions that OpenSSL uses for Diffie-Hellman
170 operations. By modifying the method, alternative implementations
171 such as hardware accelerators may be used. \s-1IMPORTANT:\s0 See the \s-1NOTES\s0 section for
172 important information about how these \s-1DH\s0 \s-1API\s0 functions are affected by the use
173 of \fB\s-1ENGINE\s0\fR \s-1API\s0 calls.
175 Initially, the default \s-1DH_METHOD\s0 is the OpenSSL internal implementation, as
176 returned by \fIDH_OpenSSL()\fR.
178 \&\fIDH_set_default_method()\fR makes \fBmeth\fR the default method for all \s-1DH\s0
179 structures created later. \fB\s-1NB\s0\fR: This is true only whilst no \s-1ENGINE\s0 has been set
180 as a default for \s-1DH\s0, so this function is no longer recommended.
182 \&\fIDH_get_default_method()\fR returns a pointer to the current default \s-1DH_METHOD\s0.
183 However, the meaningfulness of this result is dependant on whether the \s-1ENGINE\s0
184 \&\s-1API\s0 is being used, so this function is no longer recommended.
186 \&\fIDH_set_method()\fR selects \fBmeth\fR to perform all operations using the key \fBdh\fR.
187 This will replace the \s-1DH_METHOD\s0 used by the \s-1DH\s0 key and if the previous method
188 was supplied by an \s-1ENGINE\s0, the handle to that \s-1ENGINE\s0 will be released during the
189 change. It is possible to have \s-1DH\s0 keys that only work with certain \s-1DH_METHOD\s0
190 implementations (eg. from an \s-1ENGINE\s0 module that supports embedded
191 hardware-protected keys), and in such cases attempting to change the \s-1DH_METHOD\s0
192 for the key can have unexpected results.
194 \&\fIDH_new_method()\fR allocates and initializes a \s-1DH\s0 structure so that \fBengine\fR will
195 be used for the \s-1DH\s0 operations. If \fBengine\fR is \s-1NULL\s0, the default \s-1ENGINE\s0 for \s-1DH\s0
196 operations is used, and if no default \s-1ENGINE\s0 is set, the \s-1DH_METHOD\s0 controlled by
197 \&\fIDH_set_default_method()\fR is used.
198 .SH "THE DH_METHOD STRUCTURE"
199 .IX Header "THE DH_METHOD STRUCTURE"
201 \& typedef struct dh_meth_st
203 \& /* name of the implementation */
207 \& /* generate private and public DH values for key agreement */
208 \& int (*generate_key)(DH *dh);
211 \& /* compute shared secret */
212 \& int (*compute_key)(unsigned char *key, BIGNUM *pub_key, DH *dh);
215 \& /* compute r = a ^ p mod m (May be NULL for some implementations) */
216 \& int (*bn_mod_exp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p,
217 \& const BIGNUM *m, BN_CTX *ctx,
218 \& BN_MONT_CTX *m_ctx);
221 \& /* called at DH_new */
222 \& int (*init)(DH *dh);
225 \& /* called at DH_free */
226 \& int (*finish)(DH *dh);
232 \& char *app_data; /* ?? */
238 .IX Header "RETURN VALUES"
239 \&\fIDH_OpenSSL()\fR and \fIDH_get_default_method()\fR return pointers to the respective
240 \&\fB\s-1DH_METHOD\s0\fRs.
242 \&\fIDH_set_default_method()\fR returns no value.
244 \&\fIDH_set_method()\fR returns non-zero if the provided \fBmeth\fR was successfully set as
245 the method for \fBdh\fR (including unloading the \s-1ENGINE\s0 handle if the previous
246 method was supplied by an \s-1ENGINE\s0).
248 \&\fIDH_new_method()\fR returns \s-1NULL\s0 and sets an error code that can be obtained by
249 ERR_get_error(3) if the allocation fails. Otherwise it
250 returns a pointer to the newly allocated structure.
253 As of version 0.9.7, \s-1DH_METHOD\s0 implementations are grouped together with other
254 algorithmic APIs (eg. \s-1RSA_METHOD\s0, \s-1EVP_CIPHER\s0, etc) in \fB\s-1ENGINE\s0\fR modules. If a
255 default \s-1ENGINE\s0 is specified for \s-1DH\s0 functionality using an \s-1ENGINE\s0 \s-1API\s0 function,
256 that will override any \s-1DH\s0 defaults set using the \s-1DH\s0 \s-1API\s0 (ie.
257 \&\fIDH_set_default_method()\fR). For this reason, the \s-1ENGINE\s0 \s-1API\s0 is the recommended way
258 to control default implementations for use in \s-1DH\s0 and other cryptographic
261 .IX Header "SEE ALSO"
265 \&\fIDH_set_default_method()\fR, \fIDH_get_default_method()\fR, \fIDH_set_method()\fR,
266 \&\fIDH_new_method()\fR and \fIDH_OpenSSL()\fR were added in OpenSSL 0.9.4.
268 \&\fIDH_set_default_openssl_method()\fR and \fIDH_get_default_openssl_method()\fR replaced
269 \&\fIDH_set_default_method()\fR and \fIDH_get_default_method()\fR respectively, and
270 \&\fIDH_set_method()\fR and \fIDH_new_method()\fR were altered to use \fB\s-1ENGINE\s0\fRs rather than
271 \&\fB\s-1DH_METHOD\s0\fRs during development of the engine version of OpenSSL 0.9.6. For
272 0.9.7, the handling of defaults in the \s-1ENGINE\s0 \s-1API\s0 was restructured so that this
273 change was reversed, and behaviour of the other functions resembled more closely
274 the previous behaviour. The behaviour of defaults in the \s-1ENGINE\s0 \s-1API\s0 now
275 transparently overrides the behaviour of defaults in the \s-1DH\s0 \s-1API\s0 without
276 requiring changing these function prototypes.