I have done a lot of research in this area since I need a plugin in order to do the same thing as you.
I am currently using the plugin: http://willowsystems.github.com/jSignature/#/about/
In my case, it was necessary to be cross-platform, as we focus not only on Android, but also on iOS and WP. You can use it to save captions to images and other formats.
Hope this works for you, at least to capture the signature, since comparing the signature is a completely different need.
Vasco Costa
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