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Editing: DH_OpenSSL.3
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Always turn off hyphenation; it makes .\" way too many mistakes in technical documents. .if n .ad l .nh .SH "NAME" DH_set_default_method, DH_get_default_method, DH_set_method, DH_new_method, DH_OpenSSL \- select DH method .SH "SYNOPSIS" .IX Header "SYNOPSIS" .Vb 2 \& #include <openssl/dh.h> \& #include <openssl/engine.h> \& \& void DH_set_default_method(const DH_METHOD *meth); \& \& const DH_METHOD *DH_get_default_method(void); \& \& int DH_set_method(DH *dh, const DH_METHOD *meth); \& \& DH *DH_new_method(ENGINE *engine); \& \& const DH_METHOD *DH_OpenSSL(void); .Ve .SH "DESCRIPTION" .IX Header "DESCRIPTION" A \fB\s-1DH_METHOD\s0\fR specifies the functions that OpenSSL uses for Diffie-Hellman operations. By modifying the method, alternative implementations such as hardware accelerators may be used. \s-1IMPORTANT:\s0 See the \s-1NOTES\s0 section for important information about how these \s-1DH API\s0 functions are affected by the use of \fB\s-1ENGINE\s0\fR \s-1API\s0 calls. .PP Initially, the default \s-1DH_METHOD\s0 is the OpenSSL internal implementation, as returned by \fBDH_OpenSSL()\fR. .PP \&\fBDH_set_default_method()\fR makes \fBmeth\fR the default method for all \s-1DH\s0 structures created later. \fB\s-1NB\s0\fR: This is true only whilst no \s-1ENGINE\s0 has been set as a default for \s-1DH,\s0 so this function is no longer recommended. .PP \&\fBDH_get_default_method()\fR returns a pointer to the current default \s-1DH_METHOD.\s0 However, the meaningfulness of this result is dependent on whether the \s-1ENGINE API\s0 is being used, so this function is no longer recommended. .PP \&\fBDH_set_method()\fR selects \fBmeth\fR to perform all operations using the key \fBdh\fR. This will replace the \s-1DH_METHOD\s0 used by the \s-1DH\s0 key and if the previous method was supplied by an \s-1ENGINE,\s0 the handle to that \s-1ENGINE\s0 will be released during the change. It is possible to have \s-1DH\s0 keys that only work with certain \s-1DH_METHOD\s0 implementations (eg. from an \s-1ENGINE\s0 module that supports embedded hardware-protected keys), and in such cases attempting to change the \s-1DH_METHOD\s0 for the key can have unexpected results. .PP \&\fBDH_new_method()\fR allocates and initializes a \s-1DH\s0 structure so that \fBengine\fR will be used for the \s-1DH\s0 operations. If \fBengine\fR is \s-1NULL,\s0 the default \s-1ENGINE\s0 for \s-1DH\s0 operations is used, and if no default \s-1ENGINE\s0 is set, the \s-1DH_METHOD\s0 controlled by \&\fBDH_set_default_method()\fR is used. .SH "THE DH_METHOD STRUCTURE" .IX Header "THE DH_METHOD STRUCTURE" .Vb 4 \& typedef struct dh_meth_st \& { \& /* name of the implementation */ \& const char *name; \& \& /* generate private and public DH values for key agreement */ \& int (*generate_key)(DH *dh); \& \& /* compute shared secret */ \& int (*compute_key)(unsigned char *key, BIGNUM *pub_key, DH *dh); \& \& /* compute r = a ^ p mod m (May be NULL for some implementations) */ \& int (*bn_mod_exp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p, \& const BIGNUM *m, BN_CTX *ctx, \& BN_MONT_CTX *m_ctx); \& \& /* called at DH_new */ \& int (*init)(DH *dh); \& \& /* called at DH_free */ \& int (*finish)(DH *dh); \& \& int flags; \& \& char *app_data; /* ?? */ \& \& } DH_METHOD; .Ve .SH "RETURN VALUES" .IX Header "RETURN VALUES" \&\fBDH_OpenSSL()\fR and \fBDH_get_default_method()\fR return pointers to the respective \&\fB\s-1DH_METHOD\s0\fRs. .PP \&\fBDH_set_default_method()\fR returns no value. .PP \&\fBDH_set_method()\fR returns non-zero if the provided \fBmeth\fR was successfully set as the method for \fBdh\fR (including unloading the \s-1ENGINE\s0 handle if the previous method was supplied by an \s-1ENGINE\s0). .PP \&\fBDH_new_method()\fR returns \s-1NULL\s0 and sets an error code that can be obtained by \&\fBERR_get_error\fR\|(3) if the allocation fails. Otherwise it returns a pointer to the newly allocated structure. .SH "NOTES" .IX Header "NOTES" As of version 0.9.7, \s-1DH_METHOD\s0 implementations are grouped together with other algorithmic APIs (eg. \s-1RSA_METHOD, EVP_CIPHER,\s0 etc) in \fB\s-1ENGINE\s0\fR modules. If a default \s-1ENGINE\s0 is specified for \s-1DH\s0 functionality using an \s-1ENGINE API\s0 function, that will override any \s-1DH\s0 defaults set using the \s-1DH API\s0 (ie. \&\fBDH_set_default_method()\fR). For this reason, the \s-1ENGINE API\s0 is the recommended way to control default implementations for use in \s-1DH\s0 and other cryptographic algorithms. .SH "SEE ALSO" .IX Header "SEE ALSO" \&\fBdh\fR\|(3), \fBDH_new\fR\|(3) .SH "HISTORY" .IX Header "HISTORY" \&\fBDH_set_default_method()\fR, \fBDH_get_default_method()\fR, \fBDH_set_method()\fR, \&\fBDH_new_method()\fR and \fBDH_OpenSSL()\fR were added in OpenSSL 0.9.4. .PP \&\fBDH_set_default_openssl_method()\fR and \fBDH_get_default_openssl_method()\fR replaced \&\fBDH_set_default_method()\fR and \fBDH_get_default_method()\fR respectively, and \&\fBDH_set_method()\fR and \fBDH_new_method()\fR were altered to use \fB\s-1ENGINE\s0\fRs rather than \&\fB\s-1DH_METHOD\s0\fRs during development of the engine version of OpenSSL 0.9.6. For 0.9.7, the handling of defaults in the \s-1ENGINE API\s0 was restructured so that this change was reversed, and behaviour of the other functions resembled more closely the previous behaviour. The behaviour of defaults in the \s-1ENGINE API\s0 now transparently overrides the behaviour of defaults in the \s-1DH API\s0 without requiring changing these function prototypes.
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