Re: How good an encryption algorithm is this?

From: Igor Tandetnik (itandetnik_at_mvps.org)
Date: 11/23/04 

Date: Tue, 23 Nov 2004 17:34:37 -0500

"Bonj" <benjtaylor at hotpop d0t com> wrote in message
news:O%23sMyaa0EHA.1300@TK2MSFTNGP14.phx.gbl
>> Thus, this modification of your algorithm is equivalent to a naive
>> XOR cipher - XORing the input data with a fixed key. If I happen to
>> know both a plaintext and a ciphertext for some message
>> (known-plaintext attack), I'll just XOR them together and get the key
>
> Right, I see what you mean here - yes. So you're saying that however
> many keys I invent, they will all 'boil down' to one master one,
> which even if not defined, will exist, such that it will just convert
> all cipher back to plaintext instantly?

This is pretty much what happens with any encryption algorithm that uses
XOR alone. This has something to do with XOR being a linear operation on
bit vectors, but that's about as much as I know about this stuff.

>> Even if I don't, a simple XOR cipher is
>> a particular case of polyalphabetic aka Vigenere cipher - all the
>> rage in 16-17th centuries until broken in 1863.
>
> Surely XOR requires binary, which was only invented in the1940s or
> something?
> Why would people before the age of computers understand binary?

They didn't. They drew (and presumably memorized) a 26x26 table showing
that if the plaintext letter is X and the key letter is Y, then the
ciphertext letter should be Z. A XOR on bytes may be thought of as a
256x256 table that explicitly spells the output given two inputs, and
thus is a particular case of Vigenere. The fact that you don't store
this table in memory but calculate the result on the fly does not change
the nature of the algorithm. 

-- 
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

Relevant Pages

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