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Fix security bug in proof-checking code #60
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added 4 commits
January 7, 2019 21:10
Per Appendix D.2 of the full version of the Prio paper, the servers
should evaluate the polynomials f, g, and h at a random point r in the
set {M+1, ..., modulus-1}, where M is the number of multiplication gates
in the "Valid" circuit. The implementation previously sampled the point
r from the larger set {0, ..., modulus-1}.
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Travis-CI is failing in some way that looks unrelated to this patch. I will take a look at that separately. |
Opened PR #61 to fix. |
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I'm going to tag version 1.3 so we can pull this change into Firefox. Thanks! |
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Short version: This PR fixes a small security bug in libprio. The bug would have taken quite a bit of work to exploit, but is a bug nonetheless.
Long version: Per Appendix D.2 of the full version of the Prio paper, the servers should evaluate the polynomials f, g, and h at a random point r in the set {M+1, ..., modulus-1}, where M is the number of multiplication gates in the "Valid" circuit. The implementation previously—and incorrectly—sampled the point r from the larger set {0, ..., modulus-1}.
The probability that a random point r would fall into the "bad set" {0, ..., M} is only (M+1)/(modulus size). Since we are using a 72-bit modulus, this probability would be no more than ~1/2^{60} for the data sizes that libprio currently supports. So, with probability ~1/2^{60}, the implementation could leak some bits of information about a client's private data to the servers. This is a pretty small failure probability, but this patch should nonetheless remove it entirely.