Ok, I have a little bit of a dilemma: if we move to multiple OtherCoin denominations, each transaction would mean a transfer of multiple private keys (each key being 32 bytes secret on-card part and another 32 bytes known by the Android app). So it's 64 bytes/key. If we go all the way down to 0.1mBTC and assume people won't pay more than 10 BTC through the system (probably a safe bet at first), the maximum number of coins would be transferred when you pay 9.9999 BTC (45 keys, that's about 3 kilobytes including protocol overhead). That's a little too much for QR codes and could become a problem even for SMS chaining. It could be sent over NFC and over the Internet.

We could go with animated QR codes but I've used those before (I wrote this: www.visualbtc.com) and they're not exactly easy to use. So I wonder if I should drop QR codes altogether and keep SMS and Internet and NFC comms. SMS might be tricky (or expensive) for large transfers, it already uses concatenated messages but still, those are up to 140 bytes (160 characters) each, so you would need 20 or more to send a 45 key transfer. I've already implemented a small server that OtherCoin apps can send the info through (it's just a relay type of system, nothing fancy). Of course, QR codes and SMS have the advantage of not requiring and Internet connection, but we're going to need one anyway to split coins and do BTC transfers anyway.

Any thoughts would be appreciated. Thank you!
Razvan