user90366: I'm considering building a Lightning Network vending machine that is offline but has the capability to create LN invoices and verify payments.
As an example the scheme has:

One offline vending machine.
One online node to validate payments.
One user (online with a cellphone).

For that there must be:

A way for the vending machine to provide valid invoices that can be paid to the online node.
A way for the user to communicate a prof-of-payment to the vending machine. 

Is this currently viable? Is this something that can/should be standardized so every wallet can implement it?

user81940: Stepan Snigirev had basically provided an explanation how to do the tricky part described at https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-June/002009.html
The idea is that the vending machine creates an invoice with a routing hint that goes through the online node and the online node can construct the preimage from the onion that will be send. 
Another way would be to have a deterministic way of preimage generation as explained in this video:  https://youtu.be/bihYeAeNi0g - which was already inspired by Stepan but is not as elegant as his solution. 
Regarding the proof of payment the phone would just display a qr code of the preimage which a vending machine could scan

90366: <p>I'm considering building a Lightning Network vending machine that is offline but has the capability to create LN invoices and verify payments.</p>

<p>As an example the scheme has:</p>

<ul>
<li>One offline vending machine.</li>
<li>One online node to validate payments.</li>
<li>One user (online with a cellphone).</li>
</ul>

<p>For that there must be:</p>

<ul>
<li>A way for the vending machine to provide valid invoices that can be paid to the online node.</li>
<li>A way for the user to communicate a prof-of-payment to the vending machine. </li>
</ul>

<p>Is this currently viable? Is this something that can/should be standardized so every wallet can implement it?</p>
 81940: <p>Stepan Snigirev had basically provided an explanation how to do the tricky part described at <a href="https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-June/002009.html" rel="nofollow noreferrer">https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-June/002009.html</a></p>

<p>The idea is that the vending machine creates an invoice with a routing hint that goes through the online node and the online node can construct the preimage from the onion that will be send. </p>

<p>Another way would be to have a deterministic way of preimage generation as explained in this video:  <a href="https://youtu.be/bihYeAeNi0g" rel="nofollow noreferrer">https://youtu.be/bihYeAeNi0g</a> - which was already inspired by Stepan but is not as elegant as his solution. </p>

<p>Regarding the proof of payment the phone would just display a qr code of the preimage which a vending machine could scan</p>
