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1997-08-12 - Re: Encrypting same data with many keys…

Header Data

From: ghio@temp0107.myriad.ml.org (Matthew Ghio)
To: Ray Arachelian <sunder@brainlink.com>
Message Hash: 2b83ded0a5ec442bb11fa40369ec255ce250cfea380e4f84d3489f1f8d0000b7
Message ID: <199708121945.MAA16770@myriad.alias.net>
Reply To: <Pine.SUN.3.96.970812111223.26326D-100000@beast.brainlink.com>
UTC Datetime: 1997-08-12 19:56:10 UTC
Raw Date: Wed, 13 Aug 1997 03:56:10 +0800

Raw message

From: ghio@temp0107.myriad.ml.org (Matthew Ghio)
Date: Wed, 13 Aug 1997 03:56:10 +0800
To: Ray Arachelian <sunder@brainlink.com>
Subject: Re: Encrypting same data with many keys...
In-Reply-To: <Pine.SUN.3.96.970812111223.26326D-100000@beast.brainlink.com>
Message-ID: <199708121945.MAA16770@myriad.alias.net>
MIME-Version: 1.0
Content-Type: text/plain



Ray Arachelian <sunder@brainlink.com> asked:
> 
> What are the dangers of taking a small block of data - say upto 1K in
> size, then producing many files, each being the same data encrypted by
> other keys?
...
> Assume we're using 128 bit Blowfish/Idea or better, and discarding
> weak keys.

For a standard block cipher there should be no problem.  For a stream
cipher, you would have the same type of problems as for OTP reuse, but
it would still be secure as long as you never reused a key.  However...

> 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.

Thread