I was wondering, how do you write the information to the NFC tag, do you need a special device or do you just use a mobile phone?
Yes . I think NFC tags are going to make bitcoin even better.....When I discovered the Blackseal app I knew It was it.... The blackseal app runs on my Nexus 7...and I just tapped it to the Satoshi Note to program it.. There are a few editing apps for writing and editing your own tag information too... all android that I know of..
I am curious how you have implemented the NFC tag security feature.
So you have written a signed message to the NFC tag, with the NFC tag ID as part of the message.
What key are used to sign the message, the bitcoin private key?
No ...Its signed by Trustpoint the makers of Blackseal app
Here is a demo...
https://www.youtube.com/watch?v=DouDT97LYI0&feature=youtu.be