commit 52b1051fb14f4091d3e1e02b0497bad5b0d66ca6 Author: Karsten Loesing karsten.loesing@gmx.net Date: Fri May 18 16:29:42 2018 +0200

Fix flaw in noise-removing code of hidserv module.

In the technical report that the hidserv module is based on we write:

"When processing hidden-service statistics, we need to handle the fact that they have been obfuscated by relays. As first step, we're attempting to remove the additive Laplace-distributed noise by rounding up or down to the nearest multiple of bin_size. The idea is that it's most likely that noise was added to the closest right side of a bin than to the right side of another bin. In step two, we're subtracting half of bin_size, because the relay added between 0 and bin_size − 1 to the originally observed value."

However, our code has a flaw: we use integer truncation rather which always rounds toward zero, whereas we really want to use the floor function which rounds towards negative infinity.

The fix is to use Math.floorDiv() for the division rather than common integer division and truncation.

Fixes #26022. --- src/main/java/org/torproject/metrics/stats/hidserv/Parser.java | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/main/java/org/torproject/metrics/stats/hidserv/Parser.java b/src/main/java/org/torproject/metrics/stats/hidserv/Parser.java index 2423526..9c95db5 100644 --- a/src/main/java/org/torproject/metrics/stats/hidserv/Parser.java +++ b/src/main/java/org/torproject/metrics/stats/hidserv/Parser.java @@ -243,7 +243,7 @@ public class Parser { * right side of a bin and subtracting half of the bin size. */ private long removeNoise(long reportedNumber, long binSize) { long roundedToNearestRightSideOfTheBin = - ((reportedNumber + binSize / 2) / binSize) * binSize; + Math.floorDiv((reportedNumber + binSize / 2), binSize) * binSize; long subtractedHalfOfBinSize = roundedToNearestRightSideOfTheBin - binSize / 2; return subtractedHalfOfBinSize;