I guess the Semux btc airdrop requires the distributors to check if the btc account had a balance at a given time, right? In the form people only provide the semux address and the signed address. How do you check if from the signature what btc address somebody used to sign? Can you reverse engineer it?
Yes, BTC balance is based on a snapshot of the bitcoin chain at block #480,000. From signed message, they know the BTC address that signs the message.