I am wondering how would switching from HTLCs to PTLCs change invoices? I assume there would not be a payment-hash anymore, but what would replace it. A payment-point?

Also, is there already support for this in BOLT11 invoices? or would it require a new/updated invoice spec?

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Yes the payment point would have to be included instead of the payment hash. I believe other than that nothing in the invoices would have to change.

Invoices have various fields with various semantics. One of them is the field for the payment hash. In order to stay backwards compatible I guess there will be a new field for payment points (public keys) instead of just putting a point inside the field for the hash. In this way someone decoding the invoice could already see if the invoice was for ptlcs instead of the hopefully soon to be legacy htlcs.

AFAIK until now noone has proposed a concrete invoice format yet as we still haven't agreed upon a specific way on how to implement ptlcs in lightning

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