Re: Encryption question

["F.O. Bossert" <o.bossert () quapps org>]

Yoo,

But that is his point! Sending an Fake message with an fake hash, with 
real public key and fake private key,
does give the receiver the false impression that the message is from Alice!

Greetings O.


Marco Araujo wrote:

> Hi Tony,
>
> I think there's a mistake here. Digital signature is done by applying 
> the sender's private key at the message hash. The private key is known 
> only by the sender. The receiver knows that the message is not a fake 
> one because he (she) has the sender's public key to check. He (she) 
> applies the public key at the message hash to check. It's not possible 
> to make a new private key from the public key.
>
> Hope it helps.
>
> Marco Araujo
> MCSE
> Recife/PE - Brasil
>
>
>
>
>
> > From: "Preston, Tony" <Tony.Preston () acs-inc com>
> > To: security-basics () securityfocus com
> > Subject: Encryption question
> > Date: Tue, 24 Feb 2004 13:01:29 -0600
> >
> >
> >
> > Tony Preston
> > Systems Engineer, AS&T Inc.
> > Division of L3 Corporation
> > (609) 485-0205 x 181
> >
> > I have what is a rather basic question...  I probably am missing 
> > something
> > so I thought I would ask here.
> >
> > Alice and Bob both have a public and private key.
> >
> > Alice encrypts her email to Bob using his public key.  Sends the 
> > email and
> > Bob decrypts it using his keys..
> >
> > Since both Bob and Alice's public keys are known, Why can't I take 
> > Alice's
> > public key and create a key pair using any other private key.  Now, I 
> > fake
> > an electronic signature from Alice using the pair I created and send 
> > a bogus
> > encrypted message to Bob with my "fake" Alice signature.  Bob checks the
> > signature by using the public key and it is valid.   Bob assumes the 
> > message
> > is from Alice...
> >
> > What prevents me from spoofing someone's electronic signature this way?
> >
> >
> >
> > --------------------------------------------------------------------------- 
> >
> > ---------------------------------------------------------------------------- 
>
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