GH #1412: added Poco::DigestEngine::constantTimeEquals()

Foundation/include/Poco/DigestEngine.h

@@ -40,19 +40,19 @@ public:
 
 	DigestEngine();
 	virtual ~DigestEngine();
-		
+
 	void update(const void* data, std::size_t length);
 	void update(char data);
 	void update(const std::string& data);
 		/// Updates the digest with the given data.
-		
+
 	virtual std::size_t digestLength() const = 0;
 		/// Returns the length of the digest in bytes.
 
 	virtual void reset() = 0;
 		/// Resets the engine so that a new
 		/// digest can be computed.
-		
+
 	virtual const Digest& digest() = 0;
 		/// Finishes the computation of the digest and
 		/// returns the message digest. Resets the engine
@@ -66,11 +66,16 @@ public:
 	static Digest digestFromHex(const std::string& digest);
 		/// Converts a string created by digestToHex back to its Digest presentation
 
+	static bool constantTimeEquals(const Digest& d1, const Digest& d2);
+		/// Compares two Digest values using a constant-time comparison
+		/// algorithm. This can be used to prevent timing attacks
+		/// (as discussed in <https://codahale.com/a-lesson-in-timing-attacks/>).
+
 protected:
 	virtual void updateImpl(const void* data, std::size_t length) = 0;
 		/// Updates the digest with the given data. Must be implemented
 		/// by subclasses.
-		
+
 private:
 	DigestEngine(const DigestEngine&);
 	DigestEngine& operator = (const DigestEngine&);
@@ -96,7 +101,7 @@ inline void DigestEngine::update(char data)
 
 inline void DigestEngine::update(const std::string& data)
 {
-	updateImpl(data.data(), data.size());	
+	updateImpl(data.data(), data.size());
 }
 
 

Foundation/src/DigestEngine.cpp

@@ -79,5 +79,21 @@ DigestEngine::Digest DigestEngine::digestFromHex(const std::string& digest)
 }
 
 
+bool DigestEngine::constantTimeEquals(const Digest& d1, const Digest& d2)
+{
+	if (d1.size() != d2.size()) return false;
+
+	int result = 0;
+	Digest::const_iterator it1 = d1.begin();
+	Digest::const_iterator it2 = d2.begin();
+	Digest::const_iterator end1 = d1.end();
+	while (it1 != end1)
+	{
+		result |= *it1++ ^ *it2++;
+	}
+	return result == 0;
+}
+
+
 } // namespace Poco
 

Foundation/testsuite/src/MD5EngineTest.cpp

@@ -48,7 +48,7 @@ void MD5EngineTest::testMD5()
 
 	engine.update("abcdefghijklmnopqrstuvwxyz");
 	assert (DigestEngine::digestToHex(engine.digest()) == "c3fcd3d76192e4007dfb496cca67e13b");
-	
+
 	engine.update("ABCDEFGHIJKLMNOPQRSTUVWXYZ");
 	engine.update("abcdefghijklmnopqrstuvwxyz0123456789");
 	assert (DigestEngine::digestToHex(engine.digest()) == "d174ab98d277d9f5a5611c2c9f419d9f");
@@ -58,6 +58,17 @@ void MD5EngineTest::testMD5()
 }
 
 
+void MD5EngineTest::testConstantTimeEquals()
+{
+	DigestEngine::Digest d1 = DigestEngine::digestFromHex("d41d8cd98f00b204e9800998ecf8427e");
+	DigestEngine::Digest d2 = DigestEngine::digestFromHex("d41d8cd98f00b204e9800998ecf8427e");
+	DigestEngine::Digest d3 = DigestEngine::digestFromHex("0cc175b9c0f1b6a831c399e269772661");
+
+	assert (DigestEngine::constantTimeEquals(d1, d2));
+	assert (!DigestEngine::constantTimeEquals(d1, d3));
+}
+
+
 void MD5EngineTest::setUp()
 {
 }
@@ -73,6 +84,7 @@ CppUnit::Test* MD5EngineTest::suite()
 	CppUnit::TestSuite* pSuite = new CppUnit::TestSuite("MD5EngineTest");
 
 	CppUnit_addTest(pSuite, MD5EngineTest, testMD5);
+	CppUnit_addTest(pSuite, MD5EngineTest, testConstantTimeEquals);
 
 	return pSuite;
 }

Foundation/testsuite/src/MD5EngineTest.h

@@ -25,6 +25,7 @@ public:
 	~MD5EngineTest();
 
 	void testMD5();
+	void testConstantTimeEquals();
 
 	void setUp();
 	void tearDown();