NAME
    dhparam - DH parameter manipulation and generation

SYNOPSIS
    openssl dhparam [-inform DER|PEM] [-outform DER|PEM] [-in filename]
    [-out filename] [-dsaparam] [-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 stan-
   dard 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 param-
   eter 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 param-
   eters, a fresh DH key should be created for each use to avoid
   small-subgroup attacks that may be possible otherwise.

    -2, -5
   The generator to use, either 2 or 5. 2 is the default. If present
   then the input file is ignored and parameters are generated
   instead.

    -rand file(s)
   a file or files containing random data used to seed the random num-
   ber generator, or an EGD socket (see RAND_egd(3)). Multiple files
   can be specified separated by a OS-dependent character. The sepa-
   rator 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 not present then a
   value of 512 is used. If this option is present then the input file
   is ignored and parameters are generated instead.

    -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 it's unique id string) will cause req 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 pro-
    grams 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.