Btw, one interesting application of assymetric encryption is adding
signatures to files.

  Only the person who owns the secret key can sign files in that particular
way. The trick that makes this useful is that anyone can check using this
person's public key whether the signature is genuine or not. It's impossible
(to current knowledge) for anyone to fake the signature, so anyone can check
that the file has been indeed signed by that person and be pretty certain
that it hasn't been tampered with. Also this doesn't impede retrieving the
data from the file (ie. the data itself is not encrypted in a way that would
disallow retrieving it).

  One practical usage of this is, as far as I know, the PlayStation Portable:
It's ROM will refuse to run anything that hasn't been signed by Sony, and
since it's impossible for anyone to fake this signature (even though anyone
can read the public key stored in the ROM), it's not possible to run user-made
programs. (Of course this hasn't stopped from people successfully doing that
anyways, but not by cracking the protection, but instead by exploiting
programming errors in other parts of the ROM. Anyways, the technique itself
is quite sound.)

-- 
                                                          - Warp

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