RAPPOR C++ Client

We provide both a low level and high level client API. The low level API implements just the RAPPOR encoding algorithm on strings, with few dependencies.

The high level API provides wrappers that bundle encoded values into Protocol Buffer messages.

Build Instructions

You'll need a C++ compiler, the protobuf compiler, and a library that implements common hash functions (e.g. OpenSSL).

On Ubuntu or Debian, the protobuf compiler and header files can be installed with:

sudo apt-get install protobuf-compiler libprotobuf-dev

OpenSSL can be installed with:

sudo apt-get install libssl-dev

Test

After installing dependencies, You can test it out easily on your machine:

./demo.sh quick-cpp

This builds the test harness using a Makefile, and then runs the regtest.sh simulation. The last few lines of output will look like this:

Done running all test instances
Instances succeeded: 1  failed: 0  running: 0  total: 1
Wrote _tmp/cpp/results.html
URL: file:///usr/local/google/home/andychu/git/rappor/_tmp/cpp/results.html

Open the HTML file to see a plot and stats.

Encoder

The low level API is Encoder. You instantiatate it with RAPPOR encoding parameters and application dependencies. It has a method EncodeString() that takes an input string (no other types), sets an output parameter of type rappor::Bits, and returns success or failure.

#include <cassert>

#include "encoder.h"
#include "openssl_hash_impl.h"
#include "unix_kernel_rand_impl.h"

int main(int argc, char** argv) {
  FILE* fp = fopen("/dev/urandom", "r");
  rappor::UnixKernelRand irr_rand(fp);

  rappor::Deps deps(rappor::Md5, "client-secret", rappor::HmacSha256,
                    irr_rand);
  rappor::Params params(32,    // num_bits (k)
                        2,     // num_hashes (h)
                        128,   // num_cohorts (m)
                        0.25,  // probability f for PRR
                        0.75,  // probability p for IRR
                        0.5);  // probability q for IRR

  const char* encoder_id = "metric-name";
  rappor::Encoder encoder(encoder_id, params, deps);

  // Now use it to encode values.  The 'out' value can be sent over the
  // network.
  rappor::Bits out;
  assert(encoder.EncodeString("foo", &out));  // returns false on error
  printf("'foo' encoded with RAPPOR: %0x, cohort %d\n", out, encoder.cohort());

  // Raw bits
  assert(encoder.EncodeBits(0x123, &out));  // returns false on error
  printf("0x123 encoded with RAPPOR: %0x, cohort %d\n", out, encoder.cohort());
}

Dependencies

rappor::Deps is a struct-like object that holds the dependencies needed by the API.

The application must provide the following values:

• cohort: An integer between 0 and num_cohorts - 1. Each value is assigned with equal probability to a client process.
• client_secret: A persistent client secret (used for deterministic randomness in the PRR, i.e. “memoization” requirement).
• hash_func - string hash function implementation (e.g. MD5)
• hmac_func - HMAC-SHA256 implementation
• irr_rand - randomness for the IRR

We provide an implementation of hash_func and hmac_func and using OpenSSL. If your application already has a different implementation of these functions, you can implement the HashFunc and HmacFunc` interfaces.

We provide two example implementations of irr_rand: one based on libc rand() (insecure, for demo only), and one based on Unix /dev/urandom.

Error Handling

Note that incorrect usage of the SimpleEncoder and Protobuf constructors may cause runtime assertions (using assert()). For example, if Params.num_bits is more than 32, the process will crash.

Encoders should be initialized at application startup, with constant parameters, so this type of error should be seen early.

The various Encode() members do not raise assertions. If those are used incorrectly, then the return value will be false to indicate an error. These failures should be handled by the application.

Memory Management

The Encoder instances contain pointers to Params and Deps instances, but don‘t own them. In the examples, all instances live the stack of main(), so you don’t have to worry about them being destroyed.