Fix or withdraw BIP120/121?

Hi

There is an inherent problem with BIP120, Proof of Payment: If there is a soft fork, a server that verifies PoPs will accept a PoP as valid without checking any of the new Bitcoin rules.

For example, a server will be fooled by a segwit transaction, because the server doesn't have a witness to verify and consequently will accept any PoP with an empty scriptSig.

Besides this problem, on-chain payments are not hot anymore and interest, or need, for PoP as a concept seems low.

I have no good solution for the soft fork problem. Requiring all software that uses PoP to upgrade to a new PoP specification on each soft-fork is not good enough. Do you have any ideas on how to fix it?

If there is no good solution to the soft-fork issue, I suggest that I withdraw BIP120 and BIP121.

As for current implementations: I know that Mycelium implements BIP120, but I'm not sure if there is any other software, besides my own, implementing it. If you know of any, please let me know so I can discuss it with them.

Regards, /Kalle