Why is blowfish so slow? Other fast algorithms?

[David A Wagner]

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In article <199601221851.NAA16938 at amsterdam.lcs.mit.edu>,
David Mazieres <dm at 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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