NAME

       dhparam - DH parameter manipulation and generation

SYNOPSIS

       openssl dhparam [-inform DER|PEM] [-outform DER|PEM] [-in filename] [-out filename] [-dsaparam] [-check]
       [-noout] [-text] [-C] [-2] [-5] [-rand file(s)] [-engine id] [numbits]

DESCRIPTION

       This command is used to manipulate DH parameter files.

OPTIONS

       -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 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
           check if the parameters are valid primes and generator.

       -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(s)
           a file or files containing random data used to seed the random number generator,  or  an  EGD  socket
           (see  RAND_egd(3)).   Multiple  files  can  be  specified separated by a OS-dependent character.  The
           separator is ; for MS-Windows, , for OpenVMS, and : for all others.

       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_dhnumbits() 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 and SSLeay. 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)

HISTORY

       The dhparam command was added in OpenSSL 0.9.5.  The -dsaparam option was added in OpenSSL 0.9.6.

1.0.2g                                             2016-03-01                                      DHPARAM(1SSL)