hardware_notes

Offline-first cryptocurrency transactions. (reference: https://ethresear.ch/t/hardware-notes-a-physical-asset-backed-by-cryptocurrancy/6471) Allows parties with outdated states to transfer assets offline, with assurance that they can be withdrawn online within an agreed time window.

To download the app and install it on your Android phone, please download the hardware_notes.apk file at https://github.com/therealyingtong/hardware_notes/releases/tag/v1.0
- By default, most phones do not allow app installations from unknown sources. Please change this in your phone settings / permissions (https://android.gadgethacks.com/how-to/android-101-sideload-apps-by-enabling-unknown-sources-install-unknown-apps-0161947/)
- We need Android 8.0 Oreo at least.
Enable NFC on your phone and hold the hardware note close to your phone. When the phone connects with the NFC tag, it should prompt you to open the app.
Once you're in the app, scan the hardware note again and press the SCAN NOTE button. After a slight delay (while we query the blockchain), you will see a page specifying details about the note.

User stories

registers a batch of hardware notes by specifying their public keys in the smart contract
deposits value into hardware notes on the smart contract. Note that deposits can be made by third parties, in ETH or any ERC20 token.

hardware notes can be used as currency, e.g. exchange for goods and services.
before accepting a hardware note, the recipient should scan it to verify that it has not been withdrawn. If the recipient is using an outdated state, they should verify at least that the withdrawDelay period has not passed.
the user should periodically sync their blockchain state, since the frequency of syncs decides which notes they can accept. As an illustration: if they never sync their state, they should accept notes with infinite withdrawDelay; and if they are constantly syncing their state, they can accept notes with very small withdrawDelay.
a card holder can withdraw the note's value to their own account by signing a transaction with the card. Given a valid signature, the smart contract will release assets to whichever recipient account is specified.

Please get in touch if you have further ideas or want to contribute.

we do have working offline state storage that persists between syncs; however it uses SharedPreferences. there's probably a more scalable way to do it (e.g. SQLite)
we should implement the withdraw function

See the smart contract (with published source code) on Kovan testnet at: https://kovan.etherscan.io/address/0xa2ff8dAEf58467b2Ac3c93c955449EE1342F6F9E.
As of Feb 2020 we are using Status Keycards with the Status Cash applet (https://status.im/keycard_api/sdk/cash.html). Please get in touch if you're interested in trying out some of these cards.