How to emulate a nfc-tag on an android phone

Question

I have an android phone (nexus s, sdk v15) which shall send a string to a development board running in nfc-reader/writer-mode by using tag-emulation. I know this is not official

Answer 1

If you root your device, it is possible to create an app that turns on the card emulation mode. It is not too difficult, see e.g. https://stackoverflow.com/a/10506299/1202968.

However, card emulation is completely handled by the Secure Element. Your app has no access to the data that is being transferred. The only way to get control over that would be to create and install a Java Card applet on the Secure Element. However, to be able to do that you need acces to secret authentication keys that are only known to Google.

PS: Don't bother with downgrading your phone to GB if it is running ICS. You cannot downgrade the NFC firmware to be compatible again with GB, so NFC will simply not work if you flash your device with GB.