I'm investigating options for building a USB key with NFC, and looking for some advice on parts and from people who have worked with NFC before.
The basic concept is a microcontroller that emulates a USB keyboard or simple NFC tag. A password can be programmed in to the dongle via USB and is "typed" out when a button is pressed, or transmitted via NFC (you can buy an NFC reader that acts as a USB keyboard). It needs to be very small, to fit on a keychain, and the NFC antenna takes up a lot of room.
A small battery or ultracapacitor will allow the password to be wiped at any time by holding the button down, as a defence against duress since the password can easily be read out. Normally the stored password would be appended to one you type in rather than used by itself.
The ST25DV seems like it could work reasonably well. I'm not sure about an MCU, the smallest 8 bitter with USB seems to be a 14 pin PIC16, or I'm sure there are some nice ARM M0 parts. Shame they don't make an XMEGA that will fit. Even better would be a physically small MCU with USB and the NFC transceiver built in.