From: daw@beijing.CS.Berkeley.EDU (David A Wagner)
Date: Wed, 24 Jan 1996 12:06:35 +0800
To: cypherpunks@toad.com
Subject: Re: Why is blowfish so slow?  Other fast algorithms?
In-Reply-To: <199601221851.NAA16938@amsterdam.lcs.mit.edu>
Message-ID: <199601240032.TAA16837@bb.hks.net>
MIME-Version: 1.0
Content-Type: text/plain


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In article <199601221851.NAA16938@amsterdam.lcs.mit.edu>,
David Mazieres <dm@amsterdam.lcs.mit.edu> wrote:
> The problem with RC4 is that it works in OFB only.  If I need data
> integrity in the face of known plaintext, I will need to compute a MAC
> in paralell with the encryption which could significantly slow things
> down.

If you want authentication, you must use a crypto-strength MAC.
Encryption (be it RC4, DES, etc.) is not enough.

>        With a block cypher in CFB, I can just re-encrypt the last
> block of data.

False.  CFB has limited error propagation, so if I modify any block
before the next-to-last, it will not show up with your method.



This seems to be a really common error.
If you want message integrity guarantees, you must use a MAC.  Always.
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