Very, very limited. The more data you stuff in them, the less reliable and
slower scanning becomes. A URL is about the limit of what's practically
achievable. Even with that, BitPay have been complaining about the
increased character length from adding the https url to download the
payment request (though not escaping reduces character count by a lot and
is valid).

X.509 is extremely bloated, partly due to the number of features it
supports, partly due to its history but mostly due to the widespread use of
RSA which generates giant keys and signatures. Of course you can get ECC
certs as well, but in practice most merchants don't seem to use them yet.
There's no way you can fit a cert chain into a QR code.

However, this is no big deal, because for the serverless PoS device case
Alex cares about you need a backchannel to submit the transaction and
refund address anyway, so Bluetooth is already useful/required. Downloading
the payment request via it as well as uploading the response is not a big
change and - as mentioned - already implemented by Andreas and myself some
time ago.



On Thu, Mar 20, 2014 at 1:12 PM, Adam Back <adam@cypherspace.org> wrote:

> Whats a sensible limit on practical/convenient QR code size?
>
> How much of the payment protocol message size comes from use of x509?
>
> (Just exploring what the options are).
>
> Adam
>
>
> On Thu, Mar 20, 2014 at 11:36:09AM +0100, Mike Hearn wrote:
>
>>   Encoding entire payment requests into qrcodes is definitely not the way
>>   to go. They can already be large when signed and we're just at the
>>   start of adding features.
>>   Finishing off and standardising the bluetooth support is the way to go
>>   (r=bt:mac). Andreas' app already has some support for this I believe,
>>   so Alex you could prototype with that, but we need to:
>>   1) Add an encryption/auth layer on top, because it runs over RFCOMM
>>   sockets. The authentication would require proof of owning the Bitcoin
>>   key that's in the address part of the URI (which is needed for
>>   backwards compat anyway).
>>   2) Write a BIP for it and make sure it's interoperable
>>   For the auth layer we could either use SSL and then just ignore the
>>   server certificate and require signing of the session public key with
>>   the Bitcoin key, which should be easy to code up but is rather heavy on
>>   the air, or roll a custom lightweight thing where we just do a basic
>>   ECDH, with the servers key being the same as the address key. But
>>   rolling such protocols is subtle and I guess it'd need to be reviewed
>>   by people familiar with such things.
>>   This feels like a good opportunity to grow the community - perhaps we
>>   can find a volunteer in the forums who enjoys crypto.
>>
>