tarsnap-keyregen

[Gabriel Kerneis <kerneis@pps.jussieu.fr>]

Hi,

I do not understand the manpage of tarsnap-keyregen:

 tarsnap-keyregen generates a set of cryptographic keys which are
 compatible with an existing set of cryptographic keys, registers with
 the tarsnap server, and writes a key file for use with
 tarsnap-recrypt(1) and tarsnap(1).  The term "compatible" here means
 that it is possible to reencrypt archives stored with the first set of
 keys to be stored with the second set of keys.

Why is it necessary to use keyregen rather than keygen?  Is there some
cryptographic trick happening (some key signing another, for instance),
or is it just administrative stuff to associate several keys to a single
machine?

Best,
-- 
Gabriel