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>We have a product that does exactly what you are looking for. My company invented an encryption algorithm which you can use to encrypt the text appearing in the grid. We have soft contact lenses that when you use them, will dynamically decrypt the text. Therefore, only those who use the lenses can see and read the text.
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>P.S. We give away a set of these lenses to everyone who buys from us a Brooklyn bridge

Those contact lenses the Rowdy Roddy Piper model?