RE: Encryption question

["Marco Araujo" <amon87secfx () hotmail com>]

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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