The complexity of online memory checking

  • Authors:
  • Moni Naor;Guy N. Rothblum

  • Affiliations:
  • Weizmann Institute of Science, Rehovot, Israel;MIT, Cambridge, Massachusetts

  • Venue:
  • Journal of the ACM (JACM)
  • Year:
  • 2009

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Abstract

We consider the problem of storing a large file on a remote and unreliable server. To verify that the file has not been corrupted, a user could store a small private (randomized) “fingerprint” on his own computer. This is the setting for the well-studied authentication problem in cryptography, and the required fingerprint size is well understood. We study the problem of sublinear authentication: suppose the user would like to encode and store the file in a way that allows him to verify that it has not been corrupted, but without reading the entire file. If the user only wants to read q bits of the file, how large does the size s of the private fingerprint need to be? We define this problem formally, and show a tight lower bound on the relationship between s and q when the adversary is not computationally bounded, namely: s × q = Ω(n), where n is the file size. This is an easier case of the online memory checking problem, introduced by Blum et al. [1991], and hence the same (tight) lower bound applies also to that problem. It was previously shown that, when the adversary is computationally bounded, under the assumption that one-way functions exist, it is possible to construct much better online memory checkers. The same is also true for sublinear authentication schemes. We show that the existence of one-way functions is also a necessary condition: even slightly breaking the s × q = Ω(n) lower bound in a computational setting implies the existence of one-way functions.