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.\" ========================================================================
.\"
.IX Title "dh 3SSL"
.TH dh 3SSL "2017-05-25" "1.0.2l" "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 \- Diffie\-Hellman key agreement
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 2
\& #include <openssl/dh.h>
\& #include <openssl/engine.h>
\&
\& DH *   DH_new(void);
\& void   DH_free(DH *dh);
\&
\& int    DH_size(const DH *dh);
\&
\& DH *   DH_generate_parameters(int prime_len, int generator,
\&                void (*callback)(int, int, void *), void *cb_arg);
\& int    DH_check(const DH *dh, int *codes);
\&
\& int    DH_generate_key(DH *dh);
\& int    DH_compute_key(unsigned char *key, BIGNUM *pub_key, DH *dh);
\&
\& 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);
\&
\& int DH_get_ex_new_index(long argl, char *argp, int (*new_func)(),
\&             int (*dup_func)(), void (*free_func)());
\& int DH_set_ex_data(DH *d, int idx, char *arg);
\& char *DH_get_ex_data(DH *d, int idx);
\&
\& DH *   d2i_DHparams(DH **a, unsigned char **pp, long length);
\& int    i2d_DHparams(const DH *a, unsigned char **pp);
\&
\& int    DHparams_print_fp(FILE *fp, const DH *x);
\& int    DHparams_print(BIO *bp, const DH *x);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
These functions implement the Diffie-Hellman key agreement protocol.
The generation of shared \s-1DH\s0 parameters is described in
\&\fIDH_generate_parameters\fR\|(3); \fIDH_generate_key\fR\|(3) describes how
to perform a key agreement.
.PP
The \fB\s-1DH\s0\fR structure consists of several \s-1BIGNUM\s0 components.
.PP
.Vb 9
\& struct
\&        {
\&        BIGNUM *p;              // prime number (shared)
\&        BIGNUM *g;              // generator of Z_p (shared)
\&        BIGNUM *priv_key;       // private DH value x
\&        BIGNUM *pub_key;        // public DH value g^x
\&        // ...
\&        };
\& DH
.Ve
.PP
Note that \s-1DH\s0 keys may use non-standard \fB\s-1DH_METHOD\s0\fR implementations,
either directly or by the use of \fB\s-1ENGINE\s0\fR modules. In some cases (eg. an
\&\s-1ENGINE\s0 providing support for hardware-embedded keys), these \s-1BIGNUM\s0 values
will not be used by the implementation or may be used for alternative data
storage. For this reason, applications should generally avoid using \s-1DH\s0
structure elements directly and instead use \s-1API\s0 functions to query or
modify keys.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fIdhparam\fR\|(1), \fIbn\fR\|(3), \fIdsa\fR\|(3), \fIerr\fR\|(3),
\&\fIrand\fR\|(3), \fIrsa\fR\|(3), \fIengine\fR\|(3),
\&\fIDH_set_method\fR\|(3), \fIDH_new\fR\|(3),
\&\fIDH_get_ex_new_index\fR\|(3),
\&\fIDH_generate_parameters\fR\|(3),
\&\fIDH_compute_key\fR\|(3), \fId2i_DHparams\fR\|(3),
\&\fIRSA_print\fR\|(3)