5
$\begingroup$

A verifier wants to ensure, with only little exchange of data with other systems, that a large block of data $M$ that the verifier holds is also available to some other system(s). It is not an objective to keep $M$ secret, or ensure that the other systems hold $M$ in some specified form (it is fine that $M$ is stored compressed or distributed in the other systems).

This seems ideal grounds for a protocol as follows, based on my limited understanding of Wegman-Carter hashes:

  • verifier chooses a random primitive polynomial $P$ of degree $k$ with binary coefficients, and publishes/broadcasts it as a message of $k-1$ bits (the coefficients of degree $k$ and $0$ are known to be $1$ and need not be sent, and we might compact this a little further by broadcasting the seed for some CSPRNG used to generate $P$);
  • verifier computes remainder of the polynomial $M$ (with the bits of $M$ defining the binary coefficients of the polynomial) by polynomial $P$ (in other words, the verifier computes a CRC of message $M$ per polynomial $P$);
  • verifier receives a message, and is content if it is of $k$ bits that match the remainder that it computed. Of course, that message was computed by the other system(s) proving they collectively hold $M$.

Questions:

  • Does this protocol meets the stated objective? Can we prove it under some appropriate definition of security, with quantitative bound as a function of $k$, and the number $n$ of iterations made for the same $M$ (and perhaps of the size of the broadcast message if that can be made much lower than $k$, and of the size $m$ of $M$ if that matters)?
  • What if we replace the condition that polynomial $P$ is primitive by some weaker condition?
  • What speed (in bit/second) is possible on an actual CPU, like a modern x86-64 or ARM CPU, with comparison to other means (perhaps, HMAC-MD5 or CBC-MAC-AES with challenge as key)?

Late update: it appears the scheme is Rabin fingerprinting, or closely related to that (a difference is that in Rabin fingerprinting, the polynomial is chosen irreducible, not necessarily primitive; and that Rabin fingerprints uses proper padding, which I forgot: my technique fails to prove that the number of initial 0 bits is known by the provers).

$\endgroup$
  • 1
    $\begingroup$ I suspect that working with polynomials will have similar properties to working with integers. $\hspace{.94 in}$ $\endgroup$ – user991 Nov 24 '14 at 8:10
  • $\begingroup$ @Ricky Demer: I guess you are suggesting that the challenge is (the seed of) some random prime $N$, and the response $M\bmod N$. That seems to works with odds of forgery $2^{-\log_2(N)}$ if $N<log_2(m)\log_2(\log(m))$ where $m$ is the bit size of $M$, or something on that tune; but I doubt this can be made computationally competitive. Using a smooth $N$ can be more efficient (by using the CRT) but is less secure, because an adversary might pre-compute $M$ modulo small primes, then discard $M$. $\endgroup$ – fgrieu Nov 24 '14 at 9:39
  • 1
    $\begingroup$ No; I just mean that a similar analysis may work. $\;$ $\endgroup$ – user991 Nov 24 '14 at 9:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.