The cipher period is diffused to N*(N-1) message elements (where N is key length.) This significantly reduces data available for frequency analysis so that a 2K textarea window can be protected with a 15 character key if it is assumed that more than 5 key periods are required for analysis.
Frequency analysis is not the only way to attack a cipher. A message encrypted with this system can be broken easily in only 2 key periods. Simply XOR the first N*(N-1) characters with the second N*(N-1), which removes the key entirely and leaves the difference of two chunks of plaintext. Using common words like "the" and "and" as a crib quickly produces enough of the original message for the key to be discovered.

What if you don't know the key length to start with? Some well-chosen shifts and xors expose the redundancy in the N*(N-1) period, allowing it to be attacked with frequency analysis as though it had a length of only N+(N-1). I'll post the details if anyone cares. Once you have the key length, proceed as described above.

And who am I? Not a professional cryptographer. I'm sure they have even better techniques. I'm a green amateur, armed only with what I've come up with over the course of the past week.

In reply to Re: Re: CipherText by no_slogan
in thread CipherText by NodeReaper

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