With the above, local chats would be encrypted but sometimes we want proof of locality for local plain text reports - reports/live footage/fotos/... known to have emerged from a specific location. Crypto can certainly help there, too. So some key hashes could be published for a geohash ahead of time but only the group members know the keys - again, only transmitted via local radio. Now, reports signed with those keys would have a high assurance of emerging from that location but other group members would have to check on those and confirm/deny having seen them on local radio.