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=head1 NAME

DH_generate_parameters, DH_check - Generate and check Diffie-Hellman parameters

=head1 SYNOPSIS

 #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);

=head1 DESCRIPTION

DH_generate_parameters() generates Diffie-Hellman parameters that can
be shared among a group of users, and returns them in a newly
allocated B<DH> structure. The pseudo-random number generator must be
seeded prior to calling DH_generate_parameters().

B<prime_len> is the length in bits of the safe prime to be generated.
B<generator> is a small number E<gt> 1, typically 2 or 5. 

A callback function may be used to provide feedback about the progress
of the key generation. If B<callback> is not B<NULL>, it will be
called as described in L<BN_generate_prime(3)> while a random prime
number is generated, and when a prime has been found, B<callback(3,
0, cb_arg)> is called.

DH_check() validates Diffie-Hellman parameters. It checks that B<p> is
a safe prime, and that B<g> 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 B<*codes>.
DH_UNABLE_TO_CHECK_GENERATOR is set if the generator cannot be
checked, i.e. it does not equal 2 or 5.

=head1 RETURN VALUES

DH_generate_parameters() returns a pointer to the DH structure, or
NULL if the parameter generation fails. The error codes can be
obtained by ERR_get_error(3).

DH_check() returns 1 if the check could be performed, 0 otherwise.

=head1 NOTES

DH_generate_parameters() may run for several hours before finding a
suitable prime.

The parameters generated by DH_generate_parameters() are not to be
used in signature schemes.

=head1 SEE ALSO

dh(3), err(3), rand(3), DH_free(3)

=head1 HISTORY

DH_check() is available in all versions of SSLeay and OpenSSL.
The B<cb_arg> argument to DH_generate_parameters() was added in SSLeay 0.9.0.

In versions before OpenSSL 0.9.5, DH_CHECK_P_NOT_STRONG_PRIME is used
instead of DH_CHECK_P_NOT_SAFE_PRIME.

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