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.\" ========================================================================
.\"
.IX Title "DH_set_method 3"
.TH DH_set_method 3 "2019-12-20" "1.0.2u" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.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 \fIDH_OpenSSL()\fR.
.PP
\&\fIDH_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
\&\fIDH_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
\&\fIDH_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
\&\fIDH_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
\&\fIDH_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"
\&\fIDH_OpenSSL()\fR and \fIDH_get_default_method()\fR return pointers to the respective
\&\fB\s-1DH_METHOD\s0\fRs.
.PP
\&\fIDH_set_default_method()\fR returns no value.
.PP
\&\fIDH_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
\&\fIDH_new_method()\fR returns \s-1NULL\s0 and sets an error code that can be obtained by
\&\fIERR_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.
\&\fIDH_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"
\&\fIdh\fR\|(3), \fIDH_new\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fIDH_set_default_method()\fR, \fIDH_get_default_method()\fR, \fIDH_set_method()\fR,
\&\fIDH_new_method()\fR and \fIDH_OpenSSL()\fR were added in OpenSSL 0.9.4.
.PP
\&\fIDH_set_default_openssl_method()\fR and \fIDH_get_default_openssl_method()\fR replaced
\&\fIDH_set_default_method()\fR and \fIDH_get_default_method()\fR respectively, and
\&\fIDH_set_method()\fR and \fIDH_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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