[Bitcoin-development] Payment Protocol for Face-to-face Payments

[Andreas Schildbach]

On 03/20/2014 01:12 PM, Adam Back wrote:

> Whats a sensible limit on practical/convenient QR code size?

Technically 3 KB. In my experience codes above 1.5 KB become impossible
to scan (ZXing scanner, 3 years ago). You will want to stay below 500
bytes for convenient scanning. That said, I'm convinced there is a lot
of room for scanning improvements.

> How much of the payment protocol message size comes from use of x509?

As said in the OP, a minimal PR uses 50 bytes. X.509 seems to put about
4000 bytes on top of that.

As you can see, we have quite some room for improvements to PR payload
(PaymentDetails). X.509 certification will probably not be possible via
QR, at least not until specialized CA's will issue space-efficient certs
(using ECDSA?).