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 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 the random number generator, or an EGD socket (see (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.
- -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 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.