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DHPARAM

Section: OpenSSL (1)
Updated: 2003-01-30
Index

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 encodedform compatible with the PKCS#3 DHparameter structure. The PEM form is thedefault format: it consists of the DER format base64 encoded withadditional 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 ifthis option is not specified.
-out filename: This specifies the output filename parameters to. Standard output is usedif this option is not present. The output filename should not be the sameas 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 (suchthat (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 keyexchange more efficient. Beware that with such DSA-style DHparameters, a fresh DH key should be created for each use toavoid 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 theinput file is ignored and parameters are generated instead.
-rand file(s): a file or files containing random data used to seed the random numbergenerator, 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 : forall others.
numbits: this option specifies that a parameter set should be generated of sizenumbits. It must be the last option. If not present then a value of 512is 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 thenbe loaded by calling the get_dhnumbits() function.
-engine id: specifying an engine (by it's unique id string) will cause reqto attempt to obtain a functional reference to the specified engine,thus initialising it if needed. The engine will then be set as the defaultfor all available algorithms.

WARNINGS

The program dhparam combines the functionality of the programs dh andgendh in previous versions of OpenSSL and SSLeay. The dh and gendhprograms 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 X9.42DH.

This program manipulates DH parameters not keys.

BUGS

There should be a way to generate and manipulate DH keys.

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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