Re: Encrypting again an already encrypted file increase security ?

From: Splatter (me@me.net)
Date: 02/11/03 

From: "Splatter" <me@me.net>
Date: Tue, 11 Feb 2003 13:19:53 -0500

"Barry Margolin" <barry.margolin@level3.com> wrote in message
news:Laa2a.8$As2.501@paloalto-snr1.gtei.net...
> In article <wT92a.44$S46.20855@news.uswest.net>, Splatter <me@me.net>
wrote:
> >
> >"Sven Fischer" <sven_20@mail.com> wrote in message
> >news:b26gl0$q6r$02$1@news.t-online.com...
> >> Assume I have a file which is already encrypted with lets say a
Blowfish
> >tool.
> >> If I encrypt exactly this file again with another encryption algorithm
> >tool
> >> (e.g. IDEA or DES or RSA or whatever) does this improve the secureness
?
> >> Or is the crackability (read: the theoretical probability of being
> >cracking
> >> within a certain period of time) equal the security/safety of the best
of
> >the
> >> two used encryption methods ? That would mean that applying a less
secure
> >> encryption methdo would NOT improve the security.
> >
> >Humm,
> > I'm not sure about this but it seems to me that defining crackability as
> >the time needed to unencrypt or as you put it the probability of being
> >cracking within a certain period of time means that if two different
types
> >of encryption are layered on the same file that the file would be that
much
> >harder to render useful since the original version would have to be
gleamed
> >after the first & second round of encryption was broken.
>
> ISTM that breaking the extra encryptions is made harder because the
> cryptanalyst can't tell when he's succeeded. If a plain text file is
> encrypted once, you can tell if you've successfully decrypted it by
whether
> the result looks intelligible. But if you double-encrypt a file,
> successfully decrypting the second encryption results in a file that still
> looks pretty random (if the first encryption is any good).
>
> However, if the first encryption algorithm has a recognizable fingerprint,
> such as prefixing the result with a keyword or some known statistical
> properties of the ciphertext, this would be usable to determine if the
> second encryption has been successfully cracked.
>

Barry,
Does the fact that the result of the first round of encryption becomes the
input of the second round rather then plain text, make breaking that second
round harder then say using the same algorithm and plain text. Besides the
fact that you not sure if your successful of course :) . 

--
TIA
D.P. - CISSP
"To Alcohol, the cause of, and solution too,
 all of life's problems"
Homer Simpson
http://quiz.ravenblack.net/blood.pl?biter=splatter

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