How can I mount a HDD with full encryption on another system?

[Felix Maschek]

Hi!

I've used a full encrypted HDD (created as described in the OpenBSD FAQ) 
on a broken system and want to backup some data from it.

I've assembled this HDD into an external USB case and want to mount the 
HDD on another system. How can I mount this HDD? Needless to say that I 
know the passphrase...

Kind regards
Felix