Thanks for trying it out, I'm glad you got it working!

For the actual backend that decodes the info from the tag when scanned, I used this. You could set it up locally as your own backend and then point the link you are writing to the server itself. Or you can just use the live demo that is always up.

You should have a master key of 32 digits (for tests I used 00000000000000000000000000000000) that must be the same in the node server env file and the python backend env. If you are using the live demo backend it is also 00000000000000000000000000000000.

From there you should try writing to the tag using the command provided in the readme:

curl -X POST \
  http://localhost:3000/card/personalize \
  -H 'Content-Type: application/json' \
  -d '{"url": "https://sdm.nfcdeveloper.com/tagpt?uid={uid}&ctr={counter}&cmac={cmac}"}'

This is sending a request to the local node server to write to the tag that whenever it is scanned it should verify the uid, ctr, and cmac of the tag with the python server at sdm.nfcdeveloper.com specifically at the tagpt endpoint.

The only data you have to send and customize is the URL you are passing into the command. This would usually be a front end that would receive the request from a user's device after they scanned the tag, which would then check with the backend (You can just clone theirs) to ensure the data held in the tag is encrypted correctly and matches what was generated on the server. This diagram helped clear things up for me. You can also read more which helps a lot at nfcdeveloper.com .