aptitutde: "untrusted versions of the following packages will be installed!"

["Ryo" <furufuru@ccsr.u-tokyo.ac.jp>]

Hello all,

I got the following message from "aptitude dist-upgrade" all of a
sudden:

   . . . .
   Do you want to continue? [Y/n/?] y
   WARNING: untrusted versions of the following packages will
        be installed!
   . . . .

I've never seen this message, and googling has only told me that it's
related to public keys used by apt.  Could some one tell me how to fix
the problem?  Also, I'd appreciate it if some one could point me to
some
reference to learn these things from.

Regards,
Ryo