From: "James A. Donald" <jam...@echeque.com>
Subject: Re: Fast MAC algorithms?

Joseph Ashwood wrote:
RC-4 is broken when used as intended.
If you take these into consideration, can it be used "correctly"?

James A. Donald:
Hence "tricky"

Joseph Ashwood wrote:
By the same argument a Viginere cipher is "tricky" to use securely, same
with monoalphabetic and even Ceasar. Not that RC4 is anywhere near the
brokenness of Viginere, etc, but the same argument can be applied, so the
argument is flawed.

You cannot use a Viginere cipher securely. You can use an RC4 cipher
securely:  To use RC4 securely discard the first hundred bytes of output,
and renegotiate the key every gigabyte.

The way to use a Viginere securely is to apply an All-Or-Nothing-Transform to the plaintext, then encrypt, this results in the attacker entropy of the system that is in excess of the size, and therefore a OTP. There are other ways, but this method is not significantly more complex than the efforts necessary to secure RC4 and results in provable secrecy. It is just tricky to use a Vigenere securely. Joe
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