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                                             2025-02-05                                      DHPARAM(1SSL)