allow meetup participation with SMS or USSD service only

[brenzi]

For legacy phones

• without wifi or cellular internet
• without camera
• without high-res display
• without bluetooth

1. How can we enable participation?
2. How can we enable hybrid meetups? smart phones and dumb phones?

some info on USSD (as used for M-pesa ??)

• https://youtu.be/5p9jfrl1MpY (about https://africastalking.com/)

non-custodial approach

Given

• the ability to install apps that can access and send SMS/USSD messages,
• a protocol modification described as B1 in improve meetup UX flow #113
• a non-custodial gateway bridging SMS to blockchain

we could implement an alternative flow:

1. user sends signed registration extrinsic via SMS to gateway who sends it to chain
2. gateway sends meetup assignment via SMS to user
3. send claimA and claimB between meetup participants via SMS
4. sign attestations on phone
5. send attestations via SMS to a gateway who will send attestations to the chain

questions:

• is data size of signed extrinsics suitable for SMS?
• are such old phones capable of running our app and keeping custody of private keys? crypto primitives aso.
• how to present a meetup location without a map? GPS coordinates are useless without capable devices.
• is it ok that users have to share their phone number to send claims to each other?

custodial approach

• manage private keys in gateway. Reduce SMS payloads to merely identify participants or use USSD
• no app needed, we can guide users through USSD prompts (much like M-Pesa does it)
• no need to share phone numbers and send claims. people can just exchange their (i.e. 4-digit) identifier