"losetup -e blowfish" broken across distributions

Apparently this has been an issue for a long time:

  1. create an encrypted disk image with "losetup -e blowfish"
  2. ...years pass...
  3. move the encrypted disk image to a machine with a
      different distribution of Linux
     -or-
     upgrade the installed version of Linux
  4. discover that all access to the encrypted data is lost!

This apparently has to do the hash being used in blowfish changing, usually for security reasons and usually without a backwards-compatibility option, as documented in loop-AES.README.

This is very bad news if you've been backing up your encrypted disk image. Better stick with a losetup-mount-tar-pgp backup procedure instead:

# to backup:
bash$ cd /media/encrypted_fs
bash$ tar cf - * | gpg --output /home/backup/encrypted_fs.gpg -r you@email.address -e - 
# to restore:
bash$ cd /media/encrypted_fs
bash$ gpg -d /home/backup/encrypted_fs.gp | tar xf -