Re: How good an encryption algorithm is this?

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From: Roy Fine (rlfine_at_twt.obfuscate.net)
Date: 11/24/04 

Date: Tue, 23 Nov 2004 19:58:17 -0500

"Igor Tandetnik" <itandetnik@mvps.org> wrote in message
news:%23uHOnVb0EHA.3452@TK2MSFTNGP14.phx.gbl...
> "Roy Fine" <rlfine@twt.obfuscate.net> wrote in message
> news:eQtfZQb0EHA.2192@TK2MSFTNGP14.phx.gbl
> > You have essentially implemented a stream cipher - and they are much
> > easier to beat than are block ciphers.
>
> Not if they are implemented correctly - that is, with a key stream that
> is an output of a cryptographically strong random number generator. What
> Bonj has is a stream cipher with a repeated key, aka Vigenere cipher.
> Those are known to be weak and easily broken.
> --

stream cipers are easier to beat than are block ciphers - byte 0 of stream
cipher can be decoded with NO other information from any other bytes in the
stream, byte 1 plaintext can be discovered using only byte 1 decoded and
byte 0 plaintext - consider block ciphers that have to be broken one block
at a time - typically 16 bytes at a time.

In the absence of a salt value, these stream ciphers, even based on
"cryptographically strong random number generator" are trivial against a
dictionary attack.

You pointed out earlier, the XOR was a simple linear function - and that is
about as good as your will ever get on a stream cipher. You don't see
non-linear functions introduced (S-Box) until you get to block ciphers.

roy

> With best wishes,
> Igor Tandetnik
>
> With sufficient thrust, pigs fly just fine. However, this is not
> necessarily a good idea. It is hard to be sure where they are going to
> land, and it could be dangerous sitting under them as they fly
> overhead. -- RFC 1925
>
>

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