Make RPC password resistant to timing attacks

Fixes issue#2838; this is a tweaked version of pull#2845 that
should not leak the length of the password and is more generic,
in case we run into other situations where we need
timing-attack-resistant comparisons.

src/bitcoinrpc.cpp

@@ -479,7 +479,7 @@ bool HTTPAuthorized(map<string, string>& mapHeaders)
         return false;
     string strUserPass64 = strAuth.substr(6); boost::trim(strUserPass64);
     string strUserPass = DecodeBase64(strUserPass64);
-    return strUserPass == strRPCUserColonPass;
+    return TimingResistantEqual(strUserPass, strRPCUserColonPass);
 }
 
 //

src/test/util_tests.cpp

@@ -323,4 +323,15 @@ BOOST_AUTO_TEST_CASE(util_seed_insecure_rand)
     }
 }
 
+BOOST_AUTO_TEST_CASE(util_TimingResistantEqual)
+{
+    BOOST_CHECK(TimingResistantEqual(std::string(""), std::string("")));
+    BOOST_CHECK(!TimingResistantEqual(std::string("abc"), std::string("")));
+    BOOST_CHECK(!TimingResistantEqual(std::string(""), std::string("abc")));
+    BOOST_CHECK(!TimingResistantEqual(std::string("a"), std::string("aa")));
+    BOOST_CHECK(!TimingResistantEqual(std::string("aa"), std::string("a")));
+    BOOST_CHECK(TimingResistantEqual(std::string("abc"), std::string("abc")));
+    BOOST_CHECK(!TimingResistantEqual(std::string("abc"), std::string("aba")));
+}
+
 BOOST_AUTO_TEST_SUITE_END()

src/util.h

@@ -438,6 +438,21 @@ static inline uint32_t insecure_rand(void)
  */
 void seed_insecure_rand(bool fDeterministic=false);
 
+/**
+ * Timing-attack-resistant comparison.
+ * Takes time proportional to length
+ * of first argument.
+ */
+template <typename T>
+bool TimingResistantEqual(const T& a, const T& b)
+{
+    if (b.size() == 0) return a.size() == 0;
+    size_t accumulator = a.size() ^ b.size();
+    for (size_t i = 0; i < a.size(); i++)
+        accumulator |= a[i] ^ b[i%b.size()];
+    return accumulator == 0;
+}
+
 /** Median filter over a stream of values.
  * Returns the median of the last N numbers
  */