It took some time, but I finally figured out that 503 is a simple code.

It would have to be a very simple code, and that increased the risk.

As always the locks were the usual simple codes, phone numbers, the wife's name, you know the kind of thing.

Such was the simple code that displayed itself in all his thoughts.

I think your friend lives by a very simple and ancient code.

Moreover, the written words were in a simple but effective code.

He discovered that three simple codes were used in every message.

The writing faded as he completed his reading of a simple code.

The words were in simple code, which Harry knew by heart.

Joe thought for a moment, quickly running down a sequence of simple codes he'd learned over the years.