Mixmaster keys now have creation and expiration date.
It is not secured by any crypto voodoo, it's only
informational for clients to decide which keys to
use should they have more.
- on the client side we do not show remailers (and
therefore not use them) if their key is expired.
- the remailer refuses to decrypt messages to keys
that expired one month ago or earlier.
- the remailer automatically creates new mixmaster
keys if the current one are about to expire or
already are expired.
- the latest key from secring.mix is written to
key.txt. It used to be the first one. Since
creation of new mix key appends the key, this