There are many JavaScript libraries that let a user draw using their finger on a CANVAS object and convert that to a PNG or JSON. We recommend fabric.js and you can see how we used it in this sample project:
https://bitbucket.org/vorenusventures/kazaamax-attendance

Inspect resources/assets/js/components/Edit-Attendee.vue to see how we made it work in that presentation. Fabric serializes the CANVAS object is serialized to a base-64 encoded PNG. When you view the event in the portal, the image of the signature is displayed:

The base64-encoded PNG of the signature is displayed in the portal and can be exported using the API.The base64-encoded PNG of the signature is displayed in the portal and can be exported using the API.

The base64-encoded PNG of the signature is displayed in the portal and can be exported using the API.

Other libraries you could use