The most notable thing about digital currencies are that they rarely exist in the real world. Instead they are stored as data in computers or smartphones. Firmcoin is a new project that is attempting to create tamper-proof cards that can hold an arbitrary amount of BTC and will be scannable via NFC.
The project was announced on the Bittalk forums by user Sergio_Demian_Lerner, who is from the company behind the idea, Certimix. The so-called Firmcoin is intended to be cheap enough (they hope as little as $5 USD) to be physically given to a payee just like any other 'real world' currency.
Here is an excerpt from the announcement:
When requested, a Firmcoin will send you its Bitcoin private key, but will automatically and atomically wipe the private key from memory, preventing double-spends.
There are several ways to verify the authenticity of a Firmcoin. For low value payments, you can just query the device if it has funds or not. Also the device can cryptographically prove that it is holding a certain private key, by signing user provided messages (but not transactions!)
If you want to verify the authenticity of a Firmcoin with more confidence, you must have connected to the Bitcoin network in the last 24 hours in order to download the list of Firmcoin addresses with funds. The Firmcoin can only receive funds if the transactions that load the funds are 24 hours old.
Carrying this small database, you can check the full validity of a Firmcoin without being online!
Of course, when dealing with any physical currency the problem of forgeries presents itself. Certimix claims it will be able to prevent this. It says you will be able to "download a physical description of random features of each manufactured token and check its physical authenticity by just taking a photo of the Firmcoin."
As for the software stored on the card that will manage transactions, Certimix have posted a long technical document on how it can protect firmware authenticity and defeat attempts to put malicious code onto a Firmcoin.
Firmcoins are a new kind of currency. They can work as Casascius coins or Bitbills, but they are reusable. You can load funds in a Firmcoin as if it were an hardware wallet, such as Trezor, and at any time you can unload the funds. Also you can check if a Firmcoin has associated funds by querying the device. A Firmcoin proof [sic] it is has a private key, without revealing it.
The technology behind Firmcoin is said to be open, however, there are elements which Certimax admit are patent pending.