RAPPOR implementation

components/rappor/proto/rappor_metric.proto

Index: components/rappor/proto/rappor_metric.proto
diff --git a/components/rappor/proto/rappor_metric.proto b/components/rappor/proto/rappor_metric.proto
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+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// Contains information collected by the RAPPOR (Randomized Aggregatable
+// Privacy-Preserving Ordinal Responses) system.
+//
+// For a full description of the rappor metrics, see
+// http://www.chromium.org/developers/design-documents/rappor
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+
+package rappor;
+
+// Next tag: 4
+message RapporReports {
+  // Which version of the Rappor scheme generated these reports.
+  optional int32 version = 1;

[Ilya Sherman, 2014/02/13 01:39:03]

Hmm, is the Rappor scheme expected to change?  I'm wondering if it's really
worth including this field yet, or if we should wait to introduce it at the time
that the scheme does change (if ever).

[Steven Holte, 2014/02/13 05:11:12]

This was added in response to someone else's comment, but I don't feel strongly
either way.  We may add different types of Rappor metrics later (that may allow
correlations, numeric values etc.).

[Ilya Sherman, 2014/02/13 23:23:08]

I don't feel strongly either, but this does seem to be purely storage overhead
right now.

[Steven Holte, 2014/02/14 02:53:28]

I suppose protobufs have optional fields and default values for a reason. 
Removing.

+
+  // Which cohort these reports belong to.  The RAPPOR participants are
+  // partioned into cohorts in different ways, to allow better statistics and
+  // increased coverage.  In particular, the cohort will serve to choose the
+  // hash functions used for Bloom-filter-based reports.
+  optional int32 cohort = 2;

[Ilya Sherman, 2014/02/13 01:39:03]

It would be nice to document the expected amount of entropy in this value.  Does
this act as something of a unique identifier, or is it intended to be chosen
from a relatively small sample space, so that a large number of users appear in
any given cohort?  Is it a purely random number for each user, or is there some
additional structure to how it is generated?

[Steven Holte, 2014/02/13 05:11:12]

Done.

+
+  // Each Report contains the values generated by the RAPPOR process for one
+  // metric.
+  message Report {
+    // The name of the metric, hashed.

[Ilya Sherman, 2014/02/13 01:39:03]

Please document which hash function is used.

[Steven Holte, 2014/02/13 05:11:12]

Done.

+    optional fixed64 name_hash = 1;
+
+    // The sequence of bits produced by random coin flips in
+    // RapporMetric::GetReport().  For a complete description of RAPPOR
+    // metrics, refer to the design document at:
+    // http://www.chromium.org/developers/design-documents/rappor
+    optional bytes bits = 2;
+  }
+  repeated Report report = 3;
+}