Designed to balance on-the-go payments with secure storage, Sigsafe is an electronic key that allows bitcoin transactions between near-field communication (NFC) devices.

“Securely storing your bitcoins is too complicated right now!” Sigsafe’s architect Peter R told CoinDesk. It was for this reason that he designed a tool to make securing bitcoins easier – one that would "just work" for the average user.

NFC-enabled payments are often trumpeted as a revolutionary commercial tool. Users can store their digital money on a device – a smartphone for example – tap a point-of-sale (POS) terminal and their payment will flow through the connection, no wallet required.

The Sigsafe tag provides this same functionality, but with bitcoin. Starting off, Peter imagined:

Paypal

President David Marcus recently pronounced bitcoin a leader in revolutionizing commerce, but dismissed NFC as “technology for the sake of technology.” While rife with imaginative appeal, NFC hasn't made a significant dent in the market, while bitcoin remains in vogue.

However, by combining with bitcoin, Sigsafe is exploring a new route for the protocol.

First off, the device can be used as a hot or cold wallet system by locking in a single address the tag is allowed to sign transactions to and from.

It also can function as a debit card when paired with NFC-enabled devices. Once compatible POS terminals crop up – software updates for this compatibility are also forthcoming – the tag would provide the functionality of a debit card.

If HTML5 browsers incorporate NFC, users could even carry out e-payments with the touch of the key tag.

Key tag owners can lay down a variety of transaction rules. Among them, users can require a password, establish a maximum transaction amount, or feed it any number of whitelisted spending addresses.

An ECDSA signature takes approximately 0.2 to 0.6 seconds to sign. The tag wakes up when the presence of a NFC-supported device enters its field. A green LED flashes if the transaction is authorized, if not, it transmits an error to the host and flashes red.

Over the course of the device's life, it could sign approximately 80,000 transactions.

The tag can function without a battery, but it includes a battery and clock to enable time-dependent transactions. It could potentially support advanced features like multi-signature wallets and hierarchical deterministic wallets.

The white paper warns of potential attack vectors like side-channel attacks, electron microscopes, and NSA backdoors.

Peter designed the signing tag with deeper goals in mind. He said:

Although storage and easy transactions are the short-term uses, the key tag could build intuition for something more disruptive.

Sigsafe transaction images via Sigsafe.