openssl-dhparam

DHPARAM(1SSL)                       OpenSSL                      DHPARAM(1SSL)

NAME
       openssl-dhparam, dhparam - DH parameter manipulation and generation

SYNOPSIS
       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]

DESCRIPTION
       This command is used to manipulate DH parameter files.

OPTIONS
       -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.

WARNINGS
       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.

NOTES
       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.

BUGS
       There should be a way to generate and manipulate DH keys.

SEE ALSO
       dsaparam(1)

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
       <https://www.openssl.org/source/license.html>.

1.1.1f                            2023-10-10                     DHPARAM(1SSL)