Some store rewards cards have a NFC wireless chip inside them, you can wipe and rewrite this chip to hold a URL link to an expiring file which can be accessed wirelessly. In this tutorial I also added extra layers of trickiness by making the secret message file hidden in what looks like a plain picture and is only accessible by entering in the PIN in a Steganography decryption app.
Step 1: Components Needed
(I found that that thicker store rewards cards are often NFC chipped inside, some old conference ID tags might work too, also found one inside a security sticker inside some books my local library was selling!)
2. NFC Tools app (on Google Play App Store)
3. Tape to hide card with.
4. Access to dissipateapp.com
5. Access to a Steganography tool program or website.
(I used mobilefish.com/services/steganography/steganography.php)
6. NFC equipped Android device.
Step 2: Wipe NFC Card
Open the NFC Tools app, select "other" then select "erase tag" then place android device near rewards card as prompted. App should then say "Wipe successful" If it doesn't, try enabling Android Beam in your device's settings.
Step 3: Make File Hidden Inside a Plain Photo.
Go to mobilefish.com/services/steganography/steganography.php
Follow the instructions to encrypt secret message or file inside any plain image (.PNG or .jpg) make sure to write down the pin your entered!
Then download that file onto your device.
Step 4: Upload Photo to Dissipate
Go to dissipateapp.com and upload the steganographic photo. You can even set to file to expire after being viewed a selected number of times. (Like Snapchat, but for files.) Make sure you write down the URL it gives you!
Step 5: Write URL to NFC Card
In the NFC Tools app, select "write" then go to "add record" then select "URL" then pasted the URL you were given by dissipateapp.com for your secret message. Make sure you don't enter the "http://" part in, that is already assumed by the app. Then select "OK" then select "write" then follow the prompt to touch Android device near NFC card. It should say "write successful"!
Step 6: Hide Card With PIN Somewhere for Friends to Find
Hide the card in a clever place! I hid mine under a table. When your spy friend's Android phone is in proximity (about no more than 2 inches from card) it will automatically open his browser and download the seemingly generic photo from dissipateapp.com. Then all he or she has to do is decrypt the message hidden inside the picture file using the decrypt function on mobilefish.com/services/steganography/steganography.php.
You could always use an offline program for the steganography part, or just skip the expiring file hosting site part altogether and merely write the photo with hidden message inside directly to the NFC card. That would be better if you have enough memory on the NFC card to do so.
Step 7: Have Fun! Be Nice!
Please don't use this tutorial for any criminal activity!
I only intend this to be a fun game for you and your friends!