Also normalize the output of keccak_256.

keccak256.cpp

@@ -25,11 +25,12 @@ extern "C"
     Keccak_HashUpdate(&hi, (const unsigned char*)input, strlen(input) * 8);
     Keccak_HashFinal(&hi, (unsigned char*)out);
 
-    for (int i = 0; i < 32; ++i) {
-        unsigned char c = out[i];
-        out_str[2 + 2*i]     = num_to_hex(c >> 4);
-        out_str[2 + 2*i + 1] = num_to_hex(c & 0x0f);
-    }
+    // this part is needed to normalize the hexadecimal output
+    string str_result;
+    boost::algorithm::hex(out, out + 32, std::back_inserter(str_result));
+    transform(str_result.begin(), str_result.end(), str_result.begin(), ::tolower);
+    str_result.erase(0, str_result.find_first_not_of('0'));
+    strcpy(out_str+2, str_result.c_str());;
 
     return out_str;
   }

keccak256_test.cpp

@@ -27,6 +27,15 @@ BOOST_AUTO_TEST_CASE(test_hash_signature) {
     );
 }
 
+// output has leading zeros, we normalize that
+BOOST_AUTO_TEST_CASE(test_hash_signature_needs_normalization) {
+	BOOST_TEST(
+        keccak_256("approve(address,uint256)")
+	    ==
+	    "0x95ea7b334ae44009aa867bfb386f5c3b4b443ac6f0ee573fa91c4608fbadfba"
+    );
+}
+
 BOOST_AUTO_TEST_CASE(test_hex_to_str) {
 	BOOST_TEST(
         hex_to_str("0x72656365697665417070726f76616c28616464726573732c75696e743235362c616464726573732c627974657329")
@@ -59,6 +68,7 @@ BOOST_AUTO_TEST_CASE(test_hash_hex_keccak_256_two_bytes) {
     );
 }
 
+// output has leading zeros, we normalize that
 BOOST_AUTO_TEST_CASE(test_hash_hex_keccak_256_needs_normalization) {
 	BOOST_TEST(
         hex_keccak_256("0x0000000000000000000000000000000000000000000000000000000000000005")