Paper 2015/384
Condensed Unpredictability
Maciej Skorski, Alexander Golovnev, and Krzysztof Pietrzak
Abstract
We consider the task of deriving a key with high HILL entropy (i.e., being computationally indistinguishable from a key with high minentropy) from an unpredictable source. Previous to this work, the only known way to transform unpredictability into a key that was $\eps$ indistinguishable from having minentropy was via pseudorandomness, for example by GoldreichLevin (GL) hardcore bits. This approach has the inherent limitation that from a source with $k$ bits of unpredictability entropy one can derive a key of length (and thus HILL entropy) at most $k2\log(1/\epsilon)$ bits. In many settings, e.g. when dealing with biometric data, such a $2\log(1/\epsilon)$ bit entropy loss in not an option. Our main technical contribution is a theorem that states that in the high entropy regime, unpredictability implies HILL entropy. Concretely, any variable $K$ with $Kd$ bits of unpredictability entropy has the same amount of so called metric entropy (against realvalued, deterministic distinguishers), which is known to imply the same amount of HILL entropy. The loss in circuit size in this argument is exponential in the entropy gap $d$, and thus this result only applies for small $d$ (i.e., where the size of distinguishers considered is exponential in $d$). To overcome the above restriction, we investigate if it's possible to first ``condense'' unpredictability entropy and make the entropy gap small. We show that any source with $k$ bits of unpredictability can be condensed into a source of length $k$ with $k3$ bits of unpredictability entropy. Our condenser simply ``abuses" the GL construction and derives a $k$ bit key from a source with $k$ bits of unpredicatibily. The original GL theorem implies nothing when extracting that many bits, but we show that in this regime, GL still behaves like a ``condenser" for unpredictability. This result comes with two caveats (1) the loss in circuit size is exponential in $k$ and (2) we require that the source we start with has \emph{no} HILL entropy (equivalently, one can efficiently check if a guess is correct). We leave it as an intriguing open problem to overcome these restrictions or to prove they're inherent.
Note: The paper accepted to ICALP 2015.
Metadata
 Available format(s)
 Category
 Foundations
 Publication info
 Published elsewhere. MINOR revision.ICALP 2015
 Keywords
 pseudoentropykey derivation
 Contact author(s)
 maciej skorski @ gmail com
 History
 20150428: received
 Short URL
 https://ia.cr/2015/384
 License

CC BY
BibTeX
@misc{cryptoeprint:2015/384, author = {Maciej Skorski and Alexander Golovnev and Krzysztof Pietrzak}, title = {Condensed Unpredictability}, howpublished = {Cryptology ePrint Archive, Paper 2015/384}, year = {2015}, note = {\url{https://eprint.iacr.org/2015/384}}, url = {https://eprint.iacr.org/2015/384} }