[tor-commits] [tor/master] Add better support to obfuscate statistics.
nickm at torproject.org
nickm at torproject.org
Fri Dec 19 15:39:27 UTC 2014
commit 7cd53b75c10831e01e288b01f63cab069d3e3035
Author: Karsten Loesing <karsten.loesing at gmx.net>
Date: Mon Dec 8 15:00:58 2014 +0100
Add better support to obfuscate statistics.
---
src/common/util.c | 45 ++++++++++++++++++++++++++++++++++++++++++
src/common/util.h | 4 ++++
src/test/test_util.c | 53 ++++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 102 insertions(+)
diff --git a/src/common/util.c b/src/common/util.c
index 50097da..c52b279 100644
--- a/src/common/util.c
+++ b/src/common/util.c
@@ -513,6 +513,51 @@ round_uint64_to_next_multiple_of(uint64_t number, uint64_t divisor)
return number;
}
+/** Return the lowest x in [INT64_MIN, INT64_MAX] such that x is at least
+ * <b>number</b>, and x modulo <b>divisor</b> == 0. */
+int64_t
+round_int64_to_next_multiple_of(int64_t number, int64_t divisor)
+{
+ tor_assert(divisor > 0);
+ if (number >= 0 && INT64_MAX - divisor + 1 >= number)
+ number += divisor - 1;
+ number -= number % divisor;
+ return number;
+}
+
+/** Transform a random value <b>p</b> from the uniform distribution in
+ * [0.0, 1.0[ into a Laplace distributed value with location parameter
+ * <b>mu</b> and scale parameter <b>b</b> in [-Inf, Inf[. */
+double
+sample_laplace_distribution(double mu, double b, double p)
+{
+ tor_assert(p >= 0.0 && p < 1.0);
+ /* This is the "inverse cumulative distribution function" from:
+ * http://en.wikipedia.org/wiki/Laplace_distribution */
+ return mu - b * (p > 0.5 ? 1.0 : -1.0)
+ * tor_mathlog(1.0 - 2.0 * fabs(p - 0.5));
+}
+
+/** Add random noise between INT64_MIN and INT64_MAX coming from a
+ * Laplace distribution with mu = 0 and b = <b>delta_f</b>/<b>epsilon</b>
+ * to <b>signal</b> based on the provided <b>random</b> value in
+ * [0.0, 1.0[. */
+int64_t
+add_laplace_noise(int64_t signal, double random, double delta_f,
+ double epsilon)
+{
+ /* cast to int64_t intended */
+ int64_t noise = sample_laplace_distribution(
+ 0.0, /* just add noise, no further signal */
+ delta_f / epsilon, random);
+ if (noise > 0 && INT64_MAX - noise < signal)
+ return INT64_MAX;
+ else if (noise < 0 && INT64_MIN - noise > signal)
+ return INT64_MIN;
+ else
+ return signal + noise;
+}
+
/** Return the number of bits set in <b>v</b>. */
int
n_bits_set_u8(uint8_t v)
diff --git a/src/common/util.h b/src/common/util.h
index 921dd79..b8fd20f 100644
--- a/src/common/util.h
+++ b/src/common/util.h
@@ -172,6 +172,10 @@ uint64_t round_to_power_of_2(uint64_t u64);
unsigned round_to_next_multiple_of(unsigned number, unsigned divisor);
uint32_t round_uint32_to_next_multiple_of(uint32_t number, uint32_t divisor);
uint64_t round_uint64_to_next_multiple_of(uint64_t number, uint64_t divisor);
+int64_t round_int64_to_next_multiple_of(int64_t number, int64_t divisor);
+double sample_laplace_distribution(double mu, double b, double p);
+int64_t add_laplace_noise(int64_t signal, double random, double delta_f,
+ double epsilon);
int n_bits_set_u8(uint8_t v);
/* Compute the CEIL of <b>a</b> divided by <b>b</b>, for nonnegative <b>a</b>
diff --git a/src/test/test_util.c b/src/test/test_util.c
index b952bb2..6e73ccf 100644
--- a/src/test/test_util.c
+++ b/src/test/test_util.c
@@ -4619,6 +4619,58 @@ test_util_round_to_next_multiple_of(void *arg)
tt_assert(round_uint64_to_next_multiple_of(99,7) == 105);
tt_assert(round_uint64_to_next_multiple_of(99,9) == 99);
+ tt_assert(round_int64_to_next_multiple_of(0,1) == 0);
+ tt_assert(round_int64_to_next_multiple_of(0,7) == 0);
+
+ tt_assert(round_int64_to_next_multiple_of(99,1) == 99);
+ tt_assert(round_int64_to_next_multiple_of(99,7) == 105);
+ tt_assert(round_int64_to_next_multiple_of(99,9) == 99);
+
+ tt_assert(round_int64_to_next_multiple_of(-99,1) == -99);
+ tt_assert(round_int64_to_next_multiple_of(-99,7) == -98);
+ tt_assert(round_int64_to_next_multiple_of(-99,9) == -99);
+
+ tt_assert(round_int64_to_next_multiple_of(INT64_MIN,2) == INT64_MIN);
+ tt_assert(round_int64_to_next_multiple_of(INT64_MAX,2) ==
+ INT64_MAX-INT64_MAX%2);
+ done:
+ ;
+}
+
+static void
+test_util_laplace(void *arg)
+{
+ /* Sample values produced using Python's SciPy:
+ *
+ * >>> from scipy.stats import laplace
+ * >>> laplace.ppf([-0.01, 0.0, 0.01, 0.5, 0.51, 0.99, 1.0, 1.01],
+ ... loc = 24, scale = 24)
+ * array([ nan, -inf, -69.88855213, 24. ,
+ * 24.48486498, 117.88855213, inf, nan])
+ */
+ const double mu = 24.0, b = 24.0;
+ const double delta_f = 15.0, epsilon = 0.3; /* b = 15.0 / 0.3 = 50.0 */
+ (void)arg;
+
+ tt_assert(isinf(sample_laplace_distribution(mu, b, 0.0)));
+ test_feq(-69.88855213, sample_laplace_distribution(mu, b, 0.01));
+ test_feq(24.0, sample_laplace_distribution(mu, b, 0.5));
+ test_feq(24.48486498, sample_laplace_distribution(mu, b, 0.51));
+ test_feq(117.88855213, sample_laplace_distribution(mu, b, 0.99));
+
+ /* >>> laplace.ppf([0.0, 0.1, 0.25, 0.5, 0.75, 0.9, 0.99],
+ * ... loc = 0, scale = 50)
+ * array([ -inf, -80.47189562, -34.65735903, 0. ,
+ * 34.65735903, 80.47189562, 195.60115027])
+ */
+ tt_assert(LONG_MIN + 20 ==
+ add_laplace_noise(20, 0.0, delta_f, epsilon));
+ tt_assert(-60 == add_laplace_noise(20, 0.1, delta_f, epsilon));
+ tt_assert(-14 == add_laplace_noise(20, 0.25, delta_f, epsilon));
+ tt_assert(20 == add_laplace_noise(20, 0.5, delta_f, epsilon));
+ tt_assert(54 == add_laplace_noise(20, 0.75, delta_f, epsilon));
+ tt_assert(100 == add_laplace_noise(20, 0.9, delta_f, epsilon));
+ tt_assert(215 == add_laplace_noise(20, 0.99, delta_f, epsilon));
done:
;
}
@@ -4880,6 +4932,7 @@ struct testcase_t util_tests[] = {
UTIL_LEGACY(strtok),
UTIL_LEGACY(di_ops),
UTIL_TEST(round_to_next_multiple_of, 0),
+ UTIL_TEST(laplace, 0),
UTIL_TEST(strclear, 0),
UTIL_TEST(find_str_at_start_of_line, 0),
UTIL_TEST(string_is_C_identifier, 0),
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