Add an xtime implementation

aes/aes.cpp

@@ -100,7 +100,16 @@ void shiftRows(t_state s) {
 }
 
 uint8_t xtime(uint8_t a) {
-    return 0; /* ??? */
+    uint8_t mask;
+
+    if (a & 0b10000000) {
+        mask = 0x1B;
+    }
+    else {
+        mask = 0x00;
+    }
+
+    return ((a << 1) ^ mask) & 0xFF;
 }
 
 // not mandatory - mix a single column