Quote from: JstnPwll on August 07, 2014, 03:14:51 PM

I would love to see something like this, only with two additions:

1) A small LCD display
2) A hardware button

Sigsafe is designed to be an absolutely minimal bitcoin signing tag (hardware wallet). Like Stick pointed out, you are probably looking for something more like Trezor.

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The reason being (and correct me if I'm wrong here), with the current model you have to trust the POS terminal to request payment for the same amount that its display shows. What if a POS terminal is hacked / modified to request larger payments than the display shows? Or what if malicious persons set up secret hotspot terminals to steal bitcoin from accidental proximity?

The immediate use for something like Sigsafe is a sort of "paper wallet that can sign transactions" to your hot wallet. I find my prototype very useful because I can sign coins from cold storage to my hot wallet without needing to load the private key onto a computer and without needing to keep an "offline computer" around. The coins can be moved with a simple gesture. Soon I hope to use Sigsafe as one of the signers in a m-of-n multisig wallet.

The signing rules for Sigsafe make this type of use very secure. For example, the device can be configured to only sign transactions that move coins to a hot wallet, require a password from the user, or require cryptographic authentication from the interface device.

In the future, should bitcoin payments over NFC become widespread, you would only want to use a tap-and-pay tag like Sigsafe with a merchant/payment terminal that you reasonably trusted for the reasons you mentioned (although daily and per TX spending limits will likely be implemented be default with Sigsafe). But also remember: if a merchant wants to scam you, they can also just take your payment and then pretend that they never received it or that you sent it to the wrong address--a screen doesn't prevent this attack.

Every design must make trade-offs. Sigsafe doesn't have a screen or buttons, but because of that it gains simplicity (just tap and the device will sign if authorized) and minimizes cost (in high quantity these devices could be very inexpensive). This will be a benefit for some applications and to some users, and a hindrance to others.

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What I'd like to see is a two-pass system: one wave "receives" transaction data from the terminal, and displays the requested payment amount on the LCD screen suggested in point 1. If you'd like to confirm the payment, you simply press the button mentioned in point 2 and a second wave will "send".

Work is underway on an open NFC protocol to do exactly this. And you won't actually need special bitcoin hardware: you could use an Android phone with NFC. The interface device makes a payment request over NFC, the wallets signs it if authorized, relays the signed transaction back to the interface device which then pushes it to the bitcoin network. Whether the user authorizes the payment manually be pressing "send," or whether the wallet itself auto-authorizes payments using trust lists, certificates and signing rules, is a decision that would be application and user dependent. But we want this protocol to be flexible and open so that anyone can design compatible hardware to act as either the interface device that makes requests, or the wallet that receives and responds to requests (or both).