openssl-dhparam, dhparam - DH parameter manipulation and
generation
openssl dhparam [-help] [-inform DER|PEM]
[-outform DER|PEM] [-in filename] [-out
filename] [-dsaparam] [-check] [-noout]
[-text] [-C] [-2] [-5] [-rand file...]
[-writerand file] [-engine id] [numbits]
This command is used to manipulate DH parameter files.
- -help
- Print out a usage message.
- -inform
DER|PEM
- This specifies the input format. The DER option uses an ASN1 DER
encoded form compatible with the PKCS#3 DHparameter structure. The PEM
form is the default format: it consists of the DER format base64
encoded with additional header and footer lines.
- -outform
DER|PEM
- This specifies the output format, the options have the same meaning and
default as the -inform option.
- -in
filename
- This specifies the input filename to read parameters from or standard
input if this option is not specified.
- -out
filename
- This specifies the output filename parameters to. Standard output is used
if this option is not present. The output filename should not be
the same as the input filename.
- -dsaparam
- If this option is used, DSA rather than DH parameters are read or created;
they are converted to DH format. Otherwise, "strong" primes
(such that (p-1)/2 is also prime) will be used for DH parameter
generation.
DH parameter generation with the -dsaparam option is
much faster, and the recommended exponent length is shorter, which makes
DH key exchange more efficient. Beware that with such DSA-style DH
parameters, a fresh DH key should be created for each use to avoid
small-subgroup attacks that may be possible otherwise.
- -check
- Performs numerous checks to see if the supplied parameters are valid and
displays a warning if not.
- -2, -5
- The generator to use, either 2 or 5. If present then the input file is
ignored and parameters are generated instead. If not present but
numbits is present, parameters are generated with the default
generator 2.
- -rand file...
- A file or files containing random data used to seed the random number
generator. Multiple files can be specified separated by an OS-dependent
character. The separator is ; for MS-Windows, , for OpenVMS,
and : for all others.
- [-writerand file]
- Writes random data to the specified file upon exit. This can be
used with a subsequent -rand flag.
- numbits
- This option specifies that a parameter set should be generated of size
numbits. It must be the last option. If this option is present then
the input file is ignored and parameters are generated instead. If this
option is not present but a generator (-2 or -5) is present,
parameters are generated with a default length of 2048 bits.
- -noout
- This option inhibits the output of the encoded version of the
parameters.
- -text
- This option prints out the DH parameters in human readable form.
- -C
- This option converts the parameters into C code. The parameters can then
be loaded by calling the get_dhNNNN() function.
- -engine id
- Specifying an engine (by its unique id string) will cause
dhparam to attempt to obtain a functional reference to the
specified engine, thus initialising it if needed. The engine will then be
set as the default for all available algorithms.
The program dhparam combines the functionality of the
programs dh and gendh in previous versions of OpenSSL. The
dh and gendh programs are retained for now but may have
different purposes in future versions of OpenSSL.
PEM format DH parameters use the header and footer lines:
-----BEGIN DH PARAMETERS-----
-----END DH PARAMETERS-----
OpenSSL currently only supports the older PKCS#3 DH, not the newer
X9.42 DH.
This program manipulates DH parameters not keys.
There should be a way to generate and manipulate DH keys.
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
<https://www.openssl.org/source/license.html>.