gumuz at looze.net
Thu Sep 5 15:19:41 CEST 2002
actually, i found a very good example for this at:
it's a java applet
only thing i'm still puzzling with is how to apply this properly
"Paul Rubin" <phr-n2002b at NOSPAMnightsong.com> wrote in message
news:7xsn0pzcol.fsf at ruckus.brouhaha.com...
> "Gumuz" <gumuz at looze.net> writes:
> > although this is just a play-learn-project and security is not really an
> > issue, i'd like to include encryption in it as well to get experience in
> > this, too.
> If you want to learn programming cryptography, the book you want is
> "Applied Cryptography" by Bruce Schneier. You might like to implement
> everything yourself rather than using a library.
> > Actually, I am trying to create a sort of simple instant messenger
> > server+client.
> I think this should use Diffie-Hellman key exchange to choose the
> decryption keys. DH is a way for two people to agree on a key based
> on random parameters they each choose. The traditional version goes:
> Alice and Bob agree beforehand on public parameters g and P,
> where P is a big prime number (like 300 digits) with certain
> properties and g is a generator of Z//p. If you don't understand
> that, don't worry too much--there are standard values of g and P
> that you can use.
> To start talking to each other, Alice chooses a secret random number x,
> and Bob chooses a secret random number y. They do not reveal these
> numbers to anyone. Alice instead computes the number X = g**x mod P
> and sends X to Bob. Bob computes Y = g**y mod P and sends Y to Alice.
> Computing these modular exponentials is trivial in python because
> of Python's built-in long integers and its 3-argument pow function.
> You can just say Y = pow(g,y,P).
> Since Alice receives Y from Bob and knows x already, she can compute
> K = Y**x mod P. Notice Y**x mod P == (g**y)**x mod P = g**(yx) mod P.
> Bob likewise can compute K = X**y mod P = (g**x)**y mod P = g**(xy) mod
> Since multiplication is commutative, xy==yx so both have found the same
> The coolness here is the computation required knowing at least one of
> the secrets, x or y. An eavesdropper knowing neither secret doesn't
> have any easy way to find K.
> At the end of the conversation, Alice and Bob should both erase their
> secret values x, y, and K from computer memory. That means the
> conversation can never be recovered by a third party, even by forcing
> Alice and Bob to reveal their passwords, turn over their computers,
> The keys are gone forever, like burning a document.
> Note you will also have to protect against "man in the middle"
> attacks, and deal with some other subtleties, to make DH secure--the
> description above is just to sketch the process. See Applied
> Cryptography for more info.
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