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)] [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.

       -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

           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 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.


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.



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