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.\" ========================================================================
.\"
.IX Title "DH_generate_parameters 3"
.TH DH_generate_parameters 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_generate_parameters_ex, DH_generate_parameters,
DH_check \- generate and check Diffie\-Hellman parameters
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/dh.h>
\&
\& int DH_generate_parameters_ex(DH *dh, int prime_len,int generator, BN_GENCB *cb);
\&
\& int DH_check(DH *dh, int *codes);
.Ve
.PP
Deprecated:
.PP
.Vb 2
\& DH *DH_generate_parameters(int prime_len, int generator,
\&     void (*callback)(int, int, void *), void *cb_arg);
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\fBDH_generate_parameters_ex()\fR generates Diffie-Hellman parameters that can
be shared among a group of users, and stores them in the provided \fBDH\fR
structure. The pseudo-random number generator must be
seeded prior to calling \fBDH_generate_parameters()\fR.
.PP
\&\fBprime_len\fR is the length in bits of the safe prime to be generated.
\&\fBgenerator\fR is a small number > 1, typically 2 or 5.
.PP
A callback function may be used to provide feedback about the progress
of the key generation. If \fBcb\fR is not \fBNULL\fR, it will be
called as described in \fBBN_generate_prime\fR\|(3) while a random prime
number is generated, and when a prime has been found, \fBBN_GENCB_call(cb, 3, 0)\fR
is called. See \fBBN_generate_prime\fR\|(3) for information on
the \fBBN_GENCB_call()\fR function.
.PP
\&\fBDH_check()\fR validates Diffie-Hellman parameters. It checks that \fBp\fR is
a safe prime, and that \fBg\fR is a suitable generator. In the case of an
error, the bit flags DH_CHECK_P_NOT_SAFE_PRIME or
DH_NOT_SUITABLE_GENERATOR are set in \fB*codes\fR.
DH_UNABLE_TO_CHECK_GENERATOR is set if the generator cannot be
checked, i.e. it does not equal 2 or 5.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBDH_generate_parameters_ex()\fR and \fBDH_check()\fR return 1 if the check could be
performed, 0 otherwise.
.PP
\&\fBDH_generate_parameters()\fR (deprecated) returns a pointer to the DH structure, or
NULL if the parameter generation fails.
.PP
The error codes can be obtained by \fBERR_get_error\fR\|(3).
.SH NOTES
.IX Header "NOTES"
\&\fBDH_generate_parameters_ex()\fR and \fBDH_generate_parameters()\fR may run for several
hours before finding a suitable prime.
.PP
The parameters generated by \fBDH_generate_parameters_ex()\fR and \fBDH_generate_parameters()\fR
are not to be used in signature schemes.
.SH BUGS
.IX Header "BUGS"
If \fBgenerator\fR is not 2 or 5, \fBdh\->g\fR=\fBgenerator\fR is not
a usable generator.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBdh\fR\|(3), \fBERR_get_error\fR\|(3), \fBrand\fR\|(3),
\&\fBDH_free\fR\|(3)
.SH HISTORY
.IX Header "HISTORY"
\&\fBDH_check()\fR is available in all versions of SSLeay and OpenSSL.
The \fBcb_arg\fR argument to \fBDH_generate_parameters()\fR was added in SSLeay 0.9.0.
.PP
In versions before OpenSSL 0.9.5, DH_CHECK_P_NOT_STRONG_PRIME is used
instead of DH_CHECK_P_NOT_SAFE_PRIME.