=pod

=head1 NAME

DH_generate_parameters_ex, DH_generate_parameters,
DH_check, DH_check_params,
DH_check_ex, DH_check_params_ex, DH_check_pub_key_ex
- generate and check Diffie-Hellman
parameters

=head1 SYNOPSIS

 #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);
 int DH_check_params(DH *dh, int *codes);

 int DH_check_ex(const DH *dh);
 int DH_check_params_ex(const DH *dh);
 int DH_check_pub_key_ex(const DH *dh, const BIGNUM *pub_key);

Deprecated:

 #if OPENSSL_API_COMPAT < 0x00908000L
 DH *DH_generate_parameters(int prime_len, int generator,
                            void (*callback)(int, int, void *), void *cb_arg);
 #endif

=head1 DESCRIPTION

DH_generate_parameters_ex() generates Diffie-Hellman parameters that can
be shared among a group of users, and stores them in the provided B<DH>
structure. The pseudo-random number generator must be
seeded before calling it.
The parameters generated by DH_generate_parameters_ex() should not be used in
signature schemes.

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<cb> 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<BN_GENCB_call(cb, 3, 0)>
is called. See L<BN_generate_prime_ex(3)> for information on
the BN_GENCB_call() function.

DH_generate_parameters() is similar to DH_generate_prime_ex() but
expects an old-style callback function; see
L<BN_generate_prime(3)> for information on the old-style callback.

DH_check_params() confirms that the B<p> and B<g> are likely enough to
be valid.
This is a lightweight check, if a more thorough check is needed, use
DH_check().
The value of B<*codes> is updated with any problems found.
If B<*codes> is zero then no problems were found, otherwise the
following bits may be set:

=over 4

=item DH_CHECK_P_NOT_PRIME

The parameter B<p> has been determined to not being an odd prime.
Note that the lack of this bit doesn't guarantee that B<p> is a
prime.

=item DH_NOT_SUITABLE_GENERATOR

The generator B<g> is not suitable.
Note that the lack of this bit doesn't guarantee that B<g> is
suitable, unless B<p> is known to be a strong prime.

=back

DH_check() confirms that the Diffie-Hellman parameters B<dh> are valid. The
value of B<*codes> is updated with any problems found. If B<*codes> is zero then
no problems were found, otherwise the following bits may be set:

=over 4

=item DH_CHECK_P_NOT_PRIME

The parameter B<p> is not prime.

=item DH_CHECK_P_NOT_SAFE_PRIME

The parameter B<p> is not a safe prime and no B<q> value is present.

=item DH_UNABLE_TO_CHECK_GENERATOR

The generator B<g> cannot be checked for suitability.

=item DH_NOT_SUITABLE_GENERATOR

The generator B<g> is not suitable.

=item DH_CHECK_Q_NOT_PRIME

The parameter B<q> is not prime.

=item DH_CHECK_INVALID_Q_VALUE

The parameter B<q> is invalid.

=item DH_CHECK_INVALID_J_VALUE

The parameter B<j> is invalid.

=back

DH_check_ex(), DH_check_params() and DH_check_pub_key_ex() are similar to
DH_check() and DH_check_params() respectively, but the error reasons are added
to the thread's error queue instead of provided as return values from the
function.

=head1 RETURN VALUES

DH_generate_parameters_ex(), DH_check() and DH_check_params() return 1
if the check could be performed, 0 otherwise.

DH_generate_parameters() returns a pointer to the DH structure or NULL if
the parameter generation fails.

DH_check_ex(), DH_check_params() and DH_check_pub_key_ex() return 1 if the
check is successful, 0 for failed.

The error codes can be obtained by L<ERR_get_error(3)>.

=head1 SEE ALSO

L<DH_new(3)>, L<ERR_get_error(3)>, L<RAND_bytes(3)>,
L<DH_free(3)>

=head1 HISTORY

DH_generate_parameters() was deprecated in OpenSSL 0.9.8; use
DH_generate_parameters_ex() instead.

=head1 COPYRIGHT

Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut