openssl dhparam

DH parameter manipulation and generation 

openssl command

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 

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 

specifies the output format, the options have the same meaning as the -inform option.

-in filename 

specifies the input file name to read parameters from or standard input if this option is not specified.

-out filename 

specifies the output file name parameters to. Standard output is used if this option is not present. The output file name should not be the same as the input file name.

-dsaparam 

reads or creates DSA parameters rather than DH parameters; 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 

specifies 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) 

specifies a file or files containing random data used to seed the random number generator, or an EGD socket. Multiple files can be specified separated by a OS-dependent character. The separator is ; for MS-Windows, , for OpenVMS, and : for all others.

numbits 

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 

inhibits the output of the encoded version of the parameters.

-text 

prints out the DH parameters in human readable form.

-C 

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.

HISTORY

The dhparam command was added in OpenSSL 0.9.5. The -dsaparam option was added in OpenSSL 0.9.6.

AVAILABILITY

MKS Toolkit for System Administrators
MKS Toolkit for Developers
MKS Toolkit for Interoperability
MKS Toolkit for Professional Developers
MKS Toolkit for Enterprise Developers
MKS Toolkit for Enterprise Developers 64-Bit Edition
MKS AlertCentre

SEE ALSO

Commands:
openssl dsaparam