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1997-08-13 - Re: Encrypting same data with many keys…

Header Data

From: nospam-seesignature@ceddec.com
To: amp@pobox.com
Message Hash: 515244384f3d53212ab8e1dafaccaf1409899c2a3f645e17bd31abad754ed1b4
Message ID: <97Aug13.114455edt.32260@brickwall.ceddec.com>
Reply To: <Chameleon.871421963.amp@ampugh.mcit.com>
UTC Datetime: 1997-08-13 15:53:57 UTC
Raw Date: Wed, 13 Aug 1997 23:53:57 +0800

Raw message

From: nospam-seesignature@ceddec.com
Date: Wed, 13 Aug 1997 23:53:57 +0800
To: amp@pobox.com
Subject: Re: Encrypting same data with many keys...
In-Reply-To: <Chameleon.871421963.amp@ampugh.mcit.com>
Message-ID: <97Aug13.114455edt.32260@brickwall.ceddec.com>
MIME-Version: 1.0
Content-Type: text/plain



On Tue, 12 Aug 1997 amp@pobox.com wrote:

> > > What if instead of using a private key cypher, we used a public key
> > > cypher?  Would that make any difference in attack methods?
> > 
> > Yes.
> > 
> > Having identical plaintexts raised to the same power modulo different
> > numbers makes the solution much easier.  If you have enough RSA
> > encryptions of the same number to the same power, you can solve it
> > outright by the remainder theorem.
> 
> So would that then be a possible weakness in encrypting to multiple 
> recipients with PGP? Probably not, since the actual data is encrypted with 
> idea.

PGP uses and E of 17 by default, but it would be a problem except that
there is a specification for random padding, so it *NEVER* encrypts
identical plaintext.  It always uses a number just a few bits shorter than 
N, starting with 0x02, then nonzero random bytes, then a zero byte, and
finally the message bytes you want to encrypt.

There was a man-in-the-middle or replay attack with SSL that they changed
the spec of the padding slightly (8 bytes before the zero byte must be
0x03), I think this is because you might be able to quickly find a random
cyphertext that decrypts to having a zero byte followed by something
useful as key material, but haven't read the details. 

--- reply to tzeruch - at - ceddec - dot - com ---

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