Excel lets you hide formulas thanks to a password:

XLS Padlock lets you go further: it can protect any of your formulas in any worksheet by replacing it with a security function only available when your secure application runs.

In the compiled workbook, formulas will be replaced by XLS Padlock in the Formula Bar by anonymous functions such as =PLEvalFormD(0;0), =PLEvalFormD(1;COUNT($F7))… 

However, calculations will still work as expected!

Before protection, anyone can see your formula:

After protection with XLS Padlock, the formula is automatically replaced while remaining functional:

This increases the security of your workbook: protected cells can only work properly if the workbook is opened in the secure application made with XLS Padlock.

Although XLS Padlock automatically manages protected cells itself, you can see that the security functions as PLEvalForm and PLEvalFormD are also recognized by Excel at design time:

Protecting cells with XLS Padlock