Warp wrote:
>   Btw, one interesting application of assymetric encryption is adding
> signatures to files.

You can add them to the file itself, or have them as a seperate file. 
(Useful if the application that reads whatever it is you're trying to 
sign doesn't "understand" signatures, but you still want to prove 
authenticity.)

And, indeed, this is what SFTP is using keys for; the actual data 
transit uses regular symmetric encryption, and a key can be securely 
agreed upon without asymmetric encryption (which is also pretty mental, 
BTW...)

>   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.

The only glitch is proving that the thing is signed by who you think it is.

>   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.

Some people are predicting that in the future, regular computers will 
work in this way, to prevent anybody running anything that can 
circumvent DRM.

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