Quote from: niko on July 31, 2013, 04:46:41 PM

Quote from: neoranga on June 09, 2013, 12:38:49 PM

First I'll explain the problem and then my proposed solution:
The problem is that if you want to go on holidays or somewhere where you don't have data connection in your cellphone (ey! I'm on holidays in Berlin http://tinyurl.com/bwg33vf and I want to use my bitcoins), it's not possible to pay in any shop where they accept Bitcoins because you can't broadcast the transaction from your cellphone.

Of course, you can access WiFi or pay the data-roaming cost but I imagine a much easier scenario for the end user by using the shop's Point of Sell to broadcast the transaction. Here is the explanation:
1. When paying, the user scans the QR code of the shop address where the payment has to be sent to
2. The user includes the amount to pay and signs the transaction in the phone (android/iphone/etc) wallet
3. Now the transaction gets converted into a transaction QR code in the user's phone -> Offline broadcasting
4. The shop seller can now scan the transaction QR code in the shop's POS which has to be connected to internet
5. The shop's POS broadcast the client's transaction and immediately verifies the payment

Remarks:
- Size of transaction: QR codes can't handle too much data (less than 3Kb according to http://www.qrcode.com/en/faq.html) but if the user were to use a specific address only for this offline portable wallet, like the physical wallet you take with you, the transaction size should be quite small as there is only one address to take the funds from and one to send funds to (usually).
- Security: The portable wallet should have it's own address and private key separated from the rest of the wallets, therefore, if the phone gets compromised/stolen, only one address with the day to day cash amount is lost.

This is very similar to what Armory is currently doing for offline wallets but the transference method through usb stick is not as simple as the QR code, specially in the mobile environment. I haven't seen this already implemented in any of the wallets but maybe I'm mistaken and some android wallet is already able to do this transference of offline transactions.

Let me know what you think of the idea and if there is any extra trouble you can think of.

With Mycelium, I can move funds to one my addresses, then export its private key as an on-screen QR code. The merchant, assuming they are prepared for this, scans the code and spends it away into their system.

Other private keys are not exposed. Any problems with this solution?

How do you transfer the exact amount you have to pay to the merchant to one of your addresses if you don't have network connection in the shop?
Giving a merchant the private key of an address with a random amount of bitcoins is basically like opening your wallet to the merchant and asking him/her to take the money from your wallet while you look away...