Why don’t you use this extension only?
This extension was removed by owner 
I want this type of signature verification system in app so Niotron must think about app security… Or something else for making app better?
I remember this member, Cian. Unfortunately, it looks like he abandoned all the App Inventor Forums without leaving trace. It would have been nice if he had shared the source code of his extensions before that.
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Any other way ? For signature verification?!
If it is so important to you I suggest you to hire a developer to do it for you. Then you can use for your own purposes and if you want you can sell or share with the community and even share the source code.