Encode QR code for pairing

Encode the mailbox's hidden service address and the single-use auth token into a QR code.

Depends on #3 (closed), #42 (closed).

changed milestone to %Mailbox: Pairing

added Sponsor 6 T: task labels

marked this issue as related to #3 (closed)

marked this issue as related to #42 (closed)

mentioned in issue briar#2167 (closed)

marked this issue as related to briar#2167 (closed)

mentioned in issue #32 (closed)

marked this issue as related to #32 (closed)

added Mailbox label

In Briar, we use ISO 8859-1 to encode bytes directly as a string. We could do the same here. The hidden service address and the single-use auth token should have the same length, so we could just concatenate them. Maybe we'd need to add some sort of prefix and/or a version byte as well to be able to change the format in the future.

I think a version byte would be a good idea.

I also thought about whether we should split the first byte into n bits for a protocol ID and 8-n bits for the protocol version, so that we could distinguish mailbox pairing QR codes from BQP QR codes (and any other QR codes we might add in the future). (Protocol ID 0 would need to be assigned to BQP, as the initial bits are currently zero in BQP QR codes.) This would allow us to show a helpful message like "The QR code you scanned is meant for adding contacts, not for pairing a mailbox" (or vice versa).

But we'd still need to show a generic "This QR code is not valid" message if the protocol ID wasn't recognised, so I think the extra complexity of including a protocol ID probably isn't justified. I'm still in favour of a version byte though, so we can migrate to new formats.

We can let the version start at 32.

changed due date to November 15, 2021

added S: in progress label

mentioned in merge request !35 (merged)

mentioned in commit 193ba50d

closed with merge request !35 (merged)

Needs weight.

added R: fixed label and removed S: in progress label

assigned to @grote