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.\" ========================================================================
.\"
.IX Title "DH_generate_parameters 3"
.TH DH_generate_parameters 3 "2007-10-24" "0.9.8g" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.SH "NAME"
DH_generate_parameters, DH_check \- generate and check Diffie\-Hellman parameters
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/dh.h>
\&
\& DH *DH_generate_parameters(int prime_len, int generator,
\&     void (*callback)(int, int, void *), void *cb_arg);
\&
\& int DH_check(DH *dh, int *codes);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fIDH_generate_parameters()\fR generates Diffie-Hellman parameters that can
be shared among a group of users, and returns them in a newly
allocated \fB\s-1DH\s0\fR structure. The pseudo-random number generator must be
seeded prior to calling \fIDH_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 \fBcallback\fR is not \fB\s-1NULL\s0\fR, it will be
called as described in \fIBN_generate_prime\fR\|(3) while a random prime
number is generated, and when a prime has been found, \fBcallback(3,
0, cb_arg)\fR is called.
.PP
\&\fIDH_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 \s-1DH_CHECK_P_NOT_SAFE_PRIME\s0 or
\&\s-1DH_NOT_SUITABLE_GENERATOR\s0 are set in \fB*codes\fR.
\&\s-1DH_UNABLE_TO_CHECK_GENERATOR\s0 is set if the generator cannot be
checked, i.e. it does not equal 2 or 5.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fIDH_generate_parameters()\fR returns a pointer to the \s-1DH\s0 structure, or
\&\s-1NULL\s0 if the parameter generation fails. The error codes can be
obtained by \fIERR_get_error\fR\|(3).
.PP
\&\fIDH_check()\fR returns 1 if the check could be performed, 0 otherwise.
.SH "NOTES"
.IX Header "NOTES"
\&\fIDH_generate_parameters()\fR may run for several hours before finding a
suitable prime.
.PP
The parameters generated by \fIDH_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"
\&\fIdh\fR\|(3), \fIERR_get_error\fR\|(3), \fIrand\fR\|(3),
\&\fIDH_free\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fIDH_check()\fR is available in all versions of SSLeay and OpenSSL.
The \fBcb_arg\fR argument to \fIDH_generate_parameters()\fR was added in SSLeay 0.9.0.
.PP
In versions before OpenSSL 0.9.5, \s-1DH_CHECK_P_NOT_STRONG_PRIME\s0 is used
instead of \s-1DH_CHECK_P_NOT_SAFE_PRIME\s0.