1=pod
2
3=head1 NAME
4
5openssl-dhparam,
6dhparam - DH parameter manipulation and generation
7
8=head1 SYNOPSIS
9
10B<openssl dhparam>
11[B<-help>]
12[B<-inform DER|PEM>]
13[B<-outform DER|PEM>]
14[B<-in> I<filename>]
15[B<-out> I<filename>]
16[B<-dsaparam>]
17[B<-check>]
18[B<-noout>]
19[B<-text>]
20[B<-C>]
21[B<-2>]
22[B<-5>]
23[B<-rand file...>]
24[B<-writerand file>]
25[B<-engine id>]
26[I<numbits>]
27
28=head1 DESCRIPTION
29
30This command is used to manipulate DH parameter files.
31
32=head1 OPTIONS
33
34=over 4
35
36=item B<-help>
37
38Print out a usage message.
39
40=item B<-inform DER|PEM>
41
42This specifies the input format. The B<DER> option uses an ASN1 DER encoded
43form compatible with the PKCS#3 DHparameter structure. The PEM form is the
44default format: it consists of the B<DER> format base64 encoded with
45additional header and footer lines.
46
47=item B<-outform DER|PEM>
48
49This specifies the output format, the options have the same meaning and default
50as the B<-inform> option.
51
52=item B<-in> I<filename>
53
54This specifies the input filename to read parameters from or standard input if
55this option is not specified.
56
57=item B<-out> I<filename>
58
59This specifies the output filename parameters to. Standard output is used
60if this option is not present. The output filename should B<not> be the same
61as the input filename.
62
63=item B<-dsaparam>
64
65If this option is used, DSA rather than DH parameters are read or created;
66they are converted to DH format.  Otherwise, "strong" primes (such
67that (p-1)/2 is also prime) will be used for DH parameter generation.
68
69DH parameter generation with the B<-dsaparam> option is much faster,
70and the recommended exponent length is shorter, which makes DH key
71exchange more efficient.  Beware that with such DSA-style DH
72parameters, a fresh DH key should be created for each use to
73avoid small-subgroup attacks that may be possible otherwise.
74
75=item B<-check>
76
77Performs numerous checks to see if the supplied parameters are valid and
78displays a warning if not.
79
80=item B<-2>, B<-5>
81
82The generator to use, either 2 or 5. If present then the
83input file is ignored and parameters are generated instead. If not
84present but B<numbits> is present, parameters are generated with the
85default generator 2.
86
87=item B<-rand file...>
88
89A file or files containing random data used to seed the random number
90generator.
91Multiple files can be specified separated by an OS-dependent character.
92The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for
93all others.
94
95=item [B<-writerand file>]
96
97Writes random data to the specified I<file> upon exit.
98This can be used with a subsequent B<-rand> flag.
99
100=item I<numbits>
101
102This option specifies that a parameter set should be generated of size
103I<numbits>. It must be the last option. If this option is present then
104the input file is ignored and parameters are generated instead. If
105this option is not present but a generator (B<-2> or B<-5>) is
106present, parameters are generated with a default length of 2048 bits.
107
108=item B<-noout>
109
110This option inhibits the output of the encoded version of the parameters.
111
112=item B<-text>
113
114This option prints out the DH parameters in human readable form.
115
116=item B<-C>
117
118This option converts the parameters into C code. The parameters can then
119be loaded by calling the get_dhNNNN() function.
120
121=item B<-engine id>
122
123Specifying an engine (by its unique B<id> string) will cause B<dhparam>
124to attempt to obtain a functional reference to the specified engine,
125thus initialising it if needed. The engine will then be set as the default
126for all available algorithms.
127
128=back
129
130=head1 WARNINGS
131
132The program B<dhparam> combines the functionality of the programs B<dh> and
133B<gendh> in previous versions of OpenSSL. The B<dh> and B<gendh>
134programs are retained for now but may have different purposes in future
135versions of OpenSSL.
136
137=head1 NOTES
138
139PEM format DH parameters use the header and footer lines:
140
141 -----BEGIN DH PARAMETERS-----
142 -----END DH PARAMETERS-----
143
144OpenSSL currently only supports the older PKCS#3 DH, not the newer X9.42
145DH.
146
147This program manipulates DH parameters not keys.
148
149=head1 BUGS
150
151There should be a way to generate and manipulate DH keys.
152
153=head1 SEE ALSO
154
155L<dsaparam(1)>
156
157=head1 COPYRIGHT
158
159Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.
160
161Licensed under the OpenSSL license (the "License").  You may not use
162this file except in compliance with the License.  You can obtain a copy
163in the file LICENSE in the source distribution or at
164L<https://www.openssl.org/source/license.html>.
165
166=cut
167