As BigO said (or implied at least) above any such device needs to be mine. I should get a transaction ID from the seller and use an encrypted text message or other interaction with my bank to forward the money to the seller - on completion they see their tx-ID as paid and I go on my way with my acquired goods.
The Square system is like me handing over a pot with all my money in it and saying "take what you like". Whilst the above is akin to me passing the money to an esscrow (my bank) and them checking it is OK and telling the receiver of the funds.
Sure a fraudster can get hold of a fake merchant ID and encode the transaction value, ID and their merchant ID with an acquired private key - but that merchant ID can just be locked when fraud is spotted and decrypting the data off the wire is going to require lots of resources.
The Square system is like me handing over a pot with all my money in it and saying "take what you like". Whilst the above is akin to me passing the money to an esscrow (my bank) and them checking it is OK and telling the receiver of the funds.
Sure a fraudster can get hold of a fake merchant ID and encode the transaction value, ID and their merchant ID with an acquired private key - but that merchant ID can just be locked when fraud is spotted and decrypting the data off the wire is going to require lots of resources.
What am I missing.