Re: NYT/CyberTimes on CWD article

At 9:17 AM 7/6/96, Declan McCullagh wrote:
>"We are writers, not crytographers."
>
>-Declan
....
This seems to be an application for Bloom filters.
See page bottom of page 561 in Knuth's "Searching and Sorting", First Edition.
(Vol 3 of Art of Computer Programming)

With a Bloom filter you can hide which URLs you reject yet quickly rejecting
particular URLs.

Compute SHA(URL) yielding 160 bits. Divide that into 16 ten bit quantities
b[i], for 0<=i< 10.
Reject the access if P[b[i]] = 1 for each i. P is an array of 1024 bits
computed by someone
with the index prohibitorum. (pardon my Latin)

Yes, this excludes 1/1024 "falsely accused" URLs, but you get the idea.

Actually, no matter what scheme you use, you are always vulnerable to a 
quite practical brute force attack- simply treat the filter as an oracle, 
and feed it the result of a 'web-crawl'. 

Simon


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