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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