Mitigate Timing Attacks On Basic RPC Authorization

See

bitcoin/bitcoin#2886
http://rdist.root.org/2010/01/07/timing-independent-array-comparison/

src/bitcoinrpc.cpp

@@ -457,7 +457,7 @@ 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/util.h

@@ -533,6 +533,20 @@ inline uint160 Hash160(const std::vector<unsigned char>& vch)
     return hash2;
 }
 
+/**
+ * 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