IACR News item: 21 June 2015
Maciej Skorski
ePrint ReportSome authors use a heuristic estimate obtained from the Asymptotic Equipartition Property, which yields roughly $n$ extractable bits, where $n$ is the total Shannon entropy amount. However the best known precise form gives only $n-O(\\sqrt{\\log(1/\\epsilon) n})$, where $\\epsilon$ is the distance of the extracted bits from uniform. In this paper we show a matching $ n-\\Omega(\\sqrt{\\log(1/\\epsilon) n})$ upper bound. Therefore, the loss of $\\Theta(\\sqrt{\\log(1/\\epsilon) n})$ bits is necessary. As we show, this theoretical bound is of practical relevance. Namely, applying the imprecise AEP heuristic to a mobile phone accelerometer one might overestimate extractable entropy even by $100\\%$, no matter what the extractor is. Thus, the ``AEP extracting heuristic\'\' should not be used without taking the precise error into account.
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