crypto: add crypto.timingSafeEqual

PR-URL: https://github.com/nodejs/node/pull/8040
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>

1 files changed, 144 insertions, 0 deletions

diff --git a/test/sequential/test-crypto-timing-safe-equal.js b/test/sequential/test-crypto-timing-safe-equal.js
new file mode 100644
index 0000000000..a4312f86ad
--- /dev/null
+++ b/test/sequential/test-crypto-timing-safe-equal.js
@@ -0,0 +1,144 @@
+// Flags: --allow_natives_syntax
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+
+if (!common.hasCrypto) {
+  common.skip('missing crypto');
+  return;
+}
+
+const crypto = require('crypto');
+
+assert.strictEqual(
+  crypto.timingSafeEqual(Buffer.from('foo'), Buffer.from('foo')),
+  true,
+  'should consider equal strings to be equal'
+);
+
+assert.strictEqual(
+  crypto.timingSafeEqual(Buffer.from('foo'), Buffer.from('bar')),
+  false,
+  'should consider unequal strings to be unequal'
+);
+
+assert.throws(function() {
+  crypto.timingSafeEqual(Buffer.from([1, 2, 3]), Buffer.from([1, 2]));
+}, 'should throw when given buffers with different lengths');
+
+assert.throws(function() {
+  crypto.timingSafeEqual('not a buffer', Buffer.from([1, 2]));
+}, 'should throw if the first argument is not a buffer');
+
+assert.throws(function() {
+  crypto.timingSafeEqual(Buffer.from([1, 2]), 'not a buffer');
+}, 'should throw if the second argument is not a buffer');
+
+function runBenchmark(compareFunc, bufferA, bufferB, expectedResult) {
+  const startTime = process.hrtime();
+  const result = compareFunc(bufferA, bufferB);
+  const endTime = process.hrtime(startTime);
+
+  // Ensure that the result of the function call gets used, so that it doesn't
+  // get discarded due to engine optimizations.
+  assert.strictEqual(result, expectedResult);
+  return endTime[0] * 1e9 + endTime[1];
+}
+
+function getTValue(compareFunc) {
+  const numTrials = 10000;
+  const testBufferSize = 10000;
+  // Perform benchmarks to verify that timingSafeEqual is actually timing-safe.
+  const bufferA1 = Buffer.alloc(testBufferSize, 'A');
+  const bufferA2 = Buffer.alloc(testBufferSize, 'A');
+  const bufferB = Buffer.alloc(testBufferSize, 'B');
+  const bufferC = Buffer.alloc(testBufferSize, 'C');
+
+  const rawEqualBenches = Array(numTrials);
+  const rawUnequalBenches = Array(numTrials);
+
+  for (let i = 0; i < numTrials; i++) {
+    // First benchmark: comparing two equal buffers
+    rawEqualBenches[i] = runBenchmark(compareFunc, bufferA1, bufferA2, true);
+    // Second benchmark: comparing two unequal buffers
+    rawUnequalBenches[i] = runBenchmark(compareFunc, bufferB, bufferC, false);
+  }
+
+  const equalBenches = filterOutliers(rawEqualBenches);
+  const unequalBenches = filterOutliers(rawUnequalBenches);
+
+  // Use a two-sample t-test to determine whether the timing difference between
+  // the benchmarks is statistically significant.
+  // https://wikipedia.org/wiki/Student%27s_t-test#Independent_two-sample_t-test
+
+  const equalMean = mean(equalBenches);
+  const unequalMean = mean(unequalBenches);
+
+  const equalLen = equalBenches.length;
+  const unequalLen = unequalBenches.length;
+
+  const combinedStd = combinedStandardDeviation(equalBenches, unequalBenches);
+  const standardErr = combinedStd * Math.sqrt(1 / equalLen + 1 / unequalLen);
+
+  return (equalMean - unequalMean) / standardErr;
+}
+
+// Returns the mean of an array
+function mean(array) {
+  return array.reduce((sum, val) => sum + val, 0) / array.length;
+}
+
+// Returns the sample standard deviation of an array
+function standardDeviation(array) {
+  const arrMean = mean(array);
+  const total = array.reduce((sum, val) => sum + Math.pow(val - arrMean, 2), 0);
+  return Math.sqrt(total / (array.length - 1));
+}
+
+// Returns the common standard deviation of two arrays
+function combinedStandardDeviation(array1, array2) {
+  const sum1 = Math.pow(standardDeviation(array1), 2) * (array1.length - 1);
+  const sum2 = Math.pow(standardDeviation(array2), 2) * (array2.length - 1);
+  return Math.sqrt((sum1 + sum2) / (array1.length + array2.length - 2));
+}
+
+// Filter large outliers from an array. A 'large outlier' is a value that is at
+// least 50 times larger than the mean. This prevents the tests from failing
+// due to the standard deviation increase when a function unexpectedly takes
+// a very long time to execute.
+function filterOutliers(array) {
+  const arrMean = mean(array);
+  return array.filter((value) => value / arrMean < 50);
+}
+
+// Force optimization before starting the benchmark
+runBenchmark(crypto.timingSafeEqual, Buffer.from('A'), Buffer.from('A'), true);
+eval('%OptimizeFunctionOnNextCall(runBenchmark)');
+runBenchmark(crypto.timingSafeEqual, Buffer.from('A'), Buffer.from('A'), true);
+
+// t_(0.9995, ∞)
+// i.e. If a given comparison function is indeed timing-safe, the t-test result
+// has a 99.9% chance to be below this threshold. Unfortunately, this means that
+// this test will be a bit flakey and will fail 0.1% of the time even if
+// crypto.timingSafeEqual is working properly.
+// t-table ref: http://www.sjsu.edu/faculty/gerstman/StatPrimer/t-table.pdf
+// Note that in reality there are roughly `2 * numTrials - 2` degrees of
+// freedom, not ∞. However, assuming `numTrials` is large, this doesn't
+// significantly affect the threshold.
+const T_THRESHOLD = 3.291;
+
+const t = getTValue(crypto.timingSafeEqual);
+assert(
+  Math.abs(t) < T_THRESHOLD,
+  `timingSafeEqual should not leak information from its execution time (t=${t})`
+);
+
+// As a sanity check to make sure the statistical tests are working, run the
+// same benchmarks again, this time with an unsafe comparison function. In this
+// case the t-value should be above the threshold.
+const unsafeCompare = (bufA, bufB) => bufA.equals(bufB);
+const t2 = getTValue(unsafeCompare);
+assert(
+  Math.abs(t2) > T_THRESHOLD,
+  `Buffer#equals should leak information from its execution time (t=${t2})`
+);