E.g. you might pass '--random-device=/dev/urandom'.
Mostly because I got sick of waiting for /dev/random to finish
blocking while I was trying to generate throwaway keys for testing bug
fixes in cmdgen itself. But it might also be useful on systems that
call their random device by a different name that we haven't
encountered.
(Since cmdgen also reads the saved PuTTY random seed file, setting
this option to /dev/zero will not render key generation deterministic.
It's tempting to provide _some_ way to do that, for testing purposes
and clearly marked as dangerous of course, but I think it would take
more faff than this.)
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