Talk:Avalanche effect
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There is a deep and nonobvious ambiguity where the definition refers to probability of flipping output bits: is it an average over specimens of a universal hash function family or over possible inputs or both? Are there subtly different meanings of avalanching?
Matt, I don't think that this is another case of 'the' as in 'the NSA' or not. It may be straighforward use. I agree that the conventional use is 'the av', but it may be misleading in such an article.
First, one may have a case in which the effect seen is more or less than in some other case. So my foo algorithm (for which I have high hopes) shows only 20% of bits flipping in rev 1, but 43% in rev 2. Does either rev exhiibt 'THE' avalanche effect. Or do both exhibit AN av? I think the latter. So 'the avalanche effect' is a pointer to a an effect with a range of outcomes, not a single operational behavior. Should not one distinguish in a discussion of goodness of avalanche effect in foo as opposed to bar, as still further opposed to baz the defined perfect example of av? I think so. The edits I made were intended in that direction. Thoughts?
- I've seen "rev 2 exhibits better avalanche", or "the avalanche is complete after 3 rounds", etc. — Matt 17:24, 1 Jul 2004 (UTC)
As for Shannon. I don't know that diffusion was the first use of the concept, but it certainly goes back that far. I suspect that the CI criterion Friedman was developing ca WWI was something mathematically homologous, but I didn't want to say so explicitly. It certainly developed in a different context than cypher design, of course.
- Can't remember: CI criterion? — Matt 17:24, 1 Jul 2004 (UTC)
- Matt, Sometimes quoted as Index of Coincidence, but I've just gone through a BP glossary, so my brain's stuck in 1945 usage. Sorry about that (and about verb agreement -- wooooolgathering. I've
corected it) . ww 17:31, 1 Jul 2004 (UTC)
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- Oh, OK. I don't see the connection between the Index of coincidence and the idea of avalanche, though? (The index is the probability that two letters picked at random from the entire text are the same. You can use it to gauge the rough length of the period of a polyalphabetic cipher.) BTW, is this BP glossary online? — Matt 17:37, 1 Jul 2004 (UTC)
- Matt, Note that this is dredged material and should be treated accordingly. IC is a statistical measure of the cyphertext and can be used to distinguish encryptions of English vs say Italian or whatever. It is in some sense an entropy measure. It was that I had in mind in speaking about possible homology. Nothing more. And yes, you can use it as you suggest during cryptanalysis.
- As for the glossary, yes. It's one of Tony Sales' scanning projects. It was produced ca 44 or 45 by an Oxford lexicographer (Hewett?) who worked in the same office as one of the famous crypto types (CHOD Alexander?). It's interesting that the word usage is characteristically just slightly skew (where there are comparable concepts, of course) to current use. Fascinating stuff. You can imagine how startled I was to find a paper (by Alexander I think) talking about IC this and IC that -- from 1944 or thereabouts!
- But recall that, in Vannevar Bush's time, it was computers he was trying to replace/supplement with his analog calculating widgets. (They were usually female, and young.) It's a living thing, this language (reification be damned!). It's just that the spelling is (and always has been) stinkin' rott'n. ww 17:57, 1 Jul 2004 (UTC)
- Oh, OK. I don't see the connection between the Index of coincidence and the idea of avalanche, though? (The index is the probability that two letters picked at random from the entire text are the same. You can use it to gauge the rough length of the period of a polyalphabetic cipher.) BTW, is this BP glossary online? — Matt 17:37, 1 Jul 2004 (UTC)
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Reactions? ww 17:10, 1 Jul 2004 (UTC)
