ssh-keygen may be used to generate groups for the Diffie-Hellman
Group Exchange (DH-GEX) protocol. Generating these groups is a
two-step process: first, candidate primes are generated using a
fast, but memory intensive process. These candidate primes are
then tested for suitability (a CPU-intensive process).
Generation of primes is performed using the -M generate option.
The desired length of the primes may be specified by the -O bits
option. For example:
# ssh-keygen -M generate -O bits=2048 moduli-2048.candidates
By default, the search for primes begins at a random point in the
desired length range. This may be overridden using the -O start
option, which specifies a different start point (in hex).
Once a set of candidates have been generated, they must be screened
for suitability. This may be performed using the -M screen option.
In this mode ssh-keygen will read candidates from standard input
(or a file specified using the -f option). For example:
# ssh-keygen -M screen -f moduli-2048.candidates moduli-2048
By default, each candidate will be subjected to 100 primality
tests. This may be overridden using the -O prime-tests option.
The DH generator value will be chosen automatically for the prime
under consideration. If a specific generator is desired, it may be
requested using the -O generator option. Valid generator values
are 2, 3, and 5.
Screened DH groups may be installed in /etc/moduli. It is
important that this file contains moduli of a range of bit lengths.
A number of options are available for moduli generation and
screening via the -O flag:
lines=number
Exit after screening the specified number of lines while
performing DH candidate screening.
start-line=line-number
Start screening at the specified line number while
performing DH candidate screening.
checkpoint=filename
Write the last line processed to the specified file while
performing DH candidate screening. This will be used to
skip lines in the input file that have already been
processed if the job is restarted.
memory=mbytes
Specify the amount of memory to use (in megabytes) when
generating candidate moduli for DH-GEX.
start=hex-value
Specify start point (in hex) when generating candidate
moduli for DH-GEX.
generator=value
Specify desired generator (in decimal) when testing
candidate moduli for DH-GEX.
