crypto: add crypto.timingSafeEqual()

Reinstate crypto.timingSafeEqual() which was reverted due to test
issues. The flaky test issues are resolved in this new changeset.

PR-URL: #8304
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/crypto.md

@@ -1217,6 +1217,19 @@ keys:
 
 All paddings are defined in `crypto.constants`.
 
+### crypto.timingSafeEqual(a, b)
+
+Returns true if `a` is equal to `b`, without leaking timing information that
+would allow an attacker to guess one of the values. This is suitable for
+comparing HMAC digests or secret values like authentication cookies or
+[capability urls](https://www.w3.org/TR/capability-urls/).
+
+`a` and `b` must both be `Buffer`s, and they must have the same length.
+
+**Note**: Use of `crypto.timingSafeEqual` does not guarantee that the
+*surrounding* code is timing-safe. Care should be taken to ensure that the
+surrounding code does not introduce timing vulnerabilities.
+
 ### crypto.privateEncrypt(private_key, buffer)
 
 Encrypts `buffer` with `private_key`.

lib/crypto.js

@@ -16,6 +16,7 @@ const getHashes = binding.getHashes;
 const getCurves = binding.getCurves;
 const getFipsCrypto = binding.getFipsCrypto;
 const setFipsCrypto = binding.setFipsCrypto;
+const timingSafeEqual = binding.timingSafeEqual;
 
 const Buffer = require('buffer').Buffer;
 const stream = require('stream');
@@ -649,6 +650,8 @@ Object.defineProperty(exports, 'fips', {
   set: setFipsCrypto
 });
 
+exports.timingSafeEqual = timingSafeEqual;
+
 // Legacy API
 Object.defineProperty(exports, 'createCredentials', {
   configurable: true,

src/node_crypto.cc

@@ -5771,6 +5771,22 @@ void ExportChallenge(const FunctionCallbackInfo<Value>& args) {
   args.GetReturnValue().Set(outString);
 }
 
+void TimingSafeEqual(const FunctionCallbackInfo<Value>& args) {
+  Environment* env = Environment::GetCurrent(args);
+
+  THROW_AND_RETURN_IF_NOT_BUFFER(args[0], "First argument");
+  THROW_AND_RETURN_IF_NOT_BUFFER(args[1], "Second argument");
+
+  size_t buf_length = Buffer::Length(args[0]);
+  if (buf_length != Buffer::Length(args[1])) {
+    return env->ThrowTypeError("Input buffers must have the same length");
+  }
+
+  const char* buf1 = Buffer::Data(args[0]);
+  const char* buf2 = Buffer::Data(args[1]);
+
+  return args.GetReturnValue().Set(CRYPTO_memcmp(buf1, buf2, buf_length) == 0);
+}
 
 void InitCryptoOnce() {
   OPENSSL_config(NULL);
@@ -5903,6 +5919,7 @@ void InitCrypto(Local<Object> target,
   env->SetMethod(target, "setFipsCrypto", SetFipsCrypto);
   env->SetMethod(target, "PBKDF2", PBKDF2);
   env->SetMethod(target, "randomBytes", RandomBytes);
+  env->SetMethod(target, "timingSafeEqual", TimingSafeEqual);
   env->SetMethod(target, "getSSLCiphers", GetSSLCiphers);
   env->SetMethod(target, "getCiphers", GetCiphers);
   env->SetMethod(target, "getHashes", GetHashes);

test/sequential/test-crypto-timing-safe-equal.js

@@ -0,0 +1,166 @@
+'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 getTValue(compareFunc) {
+  const numTrials = 10000;
+  const testBufferSize = 10000;
+  // Perform benchmarks to verify that timingSafeEqual is actually timing-safe.
+
+  const rawEqualBenches = Array(numTrials);
+  const rawUnequalBenches = Array(numTrials);
+
+  for (let i = 0; i < numTrials; i++) {
+
+    // The `runEqualBenchmark` and `runUnequalBenchmark` functions are
+    // intentionally redefined on every iteration of this loop, to avoid
+    // timing inconsistency.
+    function runEqualBenchmark(compareFunc, bufferA, bufferB) {
+      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 it doesn't
+      // get discarded due to engine optimizations.
+      assert.strictEqual(result, true);
+      return endTime[0] * 1e9 + endTime[1];
+    }
+
+    // This is almost the same as the runEqualBenchmark function, but it's
+    // duplicated to avoid timing issues with V8 optimization/inlining.
+    function runUnequalBenchmark(compareFunc, bufferA, bufferB) {
+      const startTime = process.hrtime();
+      const result = compareFunc(bufferA, bufferB);
+      const endTime = process.hrtime(startTime);
+
+      assert.strictEqual(result, false);
+      return endTime[0] * 1e9 + endTime[1];
+    }
+
+    if (i % 2) {
+      const bufferA1 = Buffer.alloc(testBufferSize, 'A');
+      const bufferB = Buffer.alloc(testBufferSize, 'B');
+      const bufferA2 = Buffer.alloc(testBufferSize, 'A');
+      const bufferC = Buffer.alloc(testBufferSize, 'C');
+
+      // First benchmark: comparing two equal buffers
+      rawEqualBenches[i] = runEqualBenchmark(compareFunc, bufferA1, bufferA2);
+
+      // Second benchmark: comparing two unequal buffers
+      rawUnequalBenches[i] = runUnequalBenchmark(compareFunc, bufferB, bufferC);
+    } else {
+      // Swap the order of the benchmarks every second iteration, to avoid any
+      // patterns caused by memory usage.
+      const bufferB = Buffer.alloc(testBufferSize, 'B');
+      const bufferA1 = Buffer.alloc(testBufferSize, 'A');
+      const bufferC = Buffer.alloc(testBufferSize, 'C');
+      const bufferA2 = Buffer.alloc(testBufferSize, 'A');
+      rawUnequalBenches[i] = runUnequalBenchmark(compareFunc, bufferB, bufferC);
+      rawEqualBenches[i] = runEqualBenchmark(compareFunc, bufferA1, bufferA2);
+    }
+  }
+
+  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);
+}
+
+// t_(0.99995, ∞)
+// i.e. If a given comparison function is indeed timing-safe, the t-test result
+// has a 99.99% chance to be below this threshold. Unfortunately, this means
+// that this test will be a bit flakey and will fail 0.01% 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 = 3.892;
+
+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})`
+);