The author of this bug (#880101) and #824827 is a user of mixmaster who has used an anonymizer to report the bugs.
As a result the bug reporter is *uncontactable*. As the Debian maintainer for mixmaster I am in agreement with removing mixmaster from Debian, although my reasons are not the same. The bug reporter is incorrect about the status of mixmaster, it's not designed to use 4k keys so it is hardly surprising that it fails when you try to use them. The project is effectively dead upstream and has been for some time. This is mostly because it is no longer secure. It is for this reason that I recommend it's removal. Colin