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126 .IX Title "DH_set_method 3"
127 .TH DH_set_method 3 "2012-03-14" "1.0.1" "OpenSSL"
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133 DH_set_default_method, DH_get_default_method,
134 DH_set_method, DH_new_method, DH_OpenSSL \- select DH method
136 .IX Header "SYNOPSIS"
138 \& #include <openssl/dh.h>
139 \& #include <openssl/engine.h>
141 \& void DH_set_default_method(const DH_METHOD *meth);
143 \& const DH_METHOD *DH_get_default_method(void);
145 \& int DH_set_method(DH *dh, const DH_METHOD *meth);
147 \& DH *DH_new_method(ENGINE *engine);
149 \& const DH_METHOD *DH_OpenSSL(void);
152 .IX Header "DESCRIPTION"
153 A \fB\s-1DH_METHOD\s0\fR specifies the functions that OpenSSL uses for Diffie-Hellman
154 operations. By modifying the method, alternative implementations
155 such as hardware accelerators may be used. \s-1IMPORTANT:\s0 See the \s-1NOTES\s0 section for
156 important information about how these \s-1DH\s0 \s-1API\s0 functions are affected by the use
157 of \fB\s-1ENGINE\s0\fR \s-1API\s0 calls.
159 Initially, the default \s-1DH_METHOD\s0 is the OpenSSL internal implementation, as
160 returned by \fIDH_OpenSSL()\fR.
162 \&\fIDH_set_default_method()\fR makes \fBmeth\fR the default method for all \s-1DH\s0
163 structures created later. \fB\s-1NB\s0\fR: This is true only whilst no \s-1ENGINE\s0 has been set
164 as a default for \s-1DH\s0, so this function is no longer recommended.
166 \&\fIDH_get_default_method()\fR returns a pointer to the current default \s-1DH_METHOD\s0.
167 However, the meaningfulness of this result is dependent on whether the \s-1ENGINE\s0
168 \&\s-1API\s0 is being used, so this function is no longer recommended.
170 \&\fIDH_set_method()\fR selects \fBmeth\fR to perform all operations using the key \fBdh\fR.
171 This will replace the \s-1DH_METHOD\s0 used by the \s-1DH\s0 key and if the previous method
172 was supplied by an \s-1ENGINE\s0, the handle to that \s-1ENGINE\s0 will be released during the
173 change. It is possible to have \s-1DH\s0 keys that only work with certain \s-1DH_METHOD\s0
174 implementations (eg. from an \s-1ENGINE\s0 module that supports embedded
175 hardware-protected keys), and in such cases attempting to change the \s-1DH_METHOD\s0
176 for the key can have unexpected results.
178 \&\fIDH_new_method()\fR allocates and initializes a \s-1DH\s0 structure so that \fBengine\fR will
179 be used for the \s-1DH\s0 operations. If \fBengine\fR is \s-1NULL\s0, the default \s-1ENGINE\s0 for \s-1DH\s0
180 operations is used, and if no default \s-1ENGINE\s0 is set, the \s-1DH_METHOD\s0 controlled by
181 \&\fIDH_set_default_method()\fR is used.
182 .SH "THE DH_METHOD STRUCTURE"
183 .IX Header "THE DH_METHOD STRUCTURE"
185 \& typedef struct dh_meth_st
187 \& /* name of the implementation */
190 \& /* generate private and public DH values for key agreement */
191 \& int (*generate_key)(DH *dh);
193 \& /* compute shared secret */
194 \& int (*compute_key)(unsigned char *key, BIGNUM *pub_key, DH *dh);
196 \& /* compute r = a ^ p mod m (May be NULL for some implementations) */
197 \& int (*bn_mod_exp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p,
198 \& const BIGNUM *m, BN_CTX *ctx,
199 \& BN_MONT_CTX *m_ctx);
201 \& /* called at DH_new */
202 \& int (*init)(DH *dh);
204 \& /* called at DH_free */
205 \& int (*finish)(DH *dh);
209 \& char *app_data; /* ?? */
214 .IX Header "RETURN VALUES"
215 \&\fIDH_OpenSSL()\fR and \fIDH_get_default_method()\fR return pointers to the respective
216 \&\fB\s-1DH_METHOD\s0\fRs.
218 \&\fIDH_set_default_method()\fR returns no value.
220 \&\fIDH_set_method()\fR returns non-zero if the provided \fBmeth\fR was successfully set as
221 the method for \fBdh\fR (including unloading the \s-1ENGINE\s0 handle if the previous
222 method was supplied by an \s-1ENGINE\s0).
224 \&\fIDH_new_method()\fR returns \s-1NULL\s0 and sets an error code that can be obtained by
225 \&\fIERR_get_error\fR\|(3) if the allocation fails. Otherwise it
226 returns a pointer to the newly allocated structure.
229 As of version 0.9.7, \s-1DH_METHOD\s0 implementations are grouped together with other
230 algorithmic APIs (eg. \s-1RSA_METHOD\s0, \s-1EVP_CIPHER\s0, etc) in \fB\s-1ENGINE\s0\fR modules. If a
231 default \s-1ENGINE\s0 is specified for \s-1DH\s0 functionality using an \s-1ENGINE\s0 \s-1API\s0 function,
232 that will override any \s-1DH\s0 defaults set using the \s-1DH\s0 \s-1API\s0 (ie.
233 \&\fIDH_set_default_method()\fR). For this reason, the \s-1ENGINE\s0 \s-1API\s0 is the recommended way
234 to control default implementations for use in \s-1DH\s0 and other cryptographic
237 .IX Header "SEE ALSO"
238 \&\fIdh\fR\|(3), \fIDH_new\fR\|(3)
241 \&\fIDH_set_default_method()\fR, \fIDH_get_default_method()\fR, \fIDH_set_method()\fR,
242 \&\fIDH_new_method()\fR and \fIDH_OpenSSL()\fR were added in OpenSSL 0.9.4.
244 \&\fIDH_set_default_openssl_method()\fR and \fIDH_get_default_openssl_method()\fR replaced
245 \&\fIDH_set_default_method()\fR and \fIDH_get_default_method()\fR respectively, and
246 \&\fIDH_set_method()\fR and \fIDH_new_method()\fR were altered to use \fB\s-1ENGINE\s0\fRs rather than
247 \&\fB\s-1DH_METHOD\s0\fRs during development of the engine version of OpenSSL 0.9.6. For
248 0.9.7, the handling of defaults in the \s-1ENGINE\s0 \s-1API\s0 was restructured so that this
249 change was reversed, and behaviour of the other functions resembled more closely
250 the previous behaviour. The behaviour of defaults in the \s-1ENGINE\s0 \s-1API\s0 now
251 transparently overrides the behaviour of defaults in the \s-1DH\s0 \s-1API\s0 without
252 requiring changing these function prototypes.