user-guides: add a proving payment howto

knowledge-base/user-guides/index.md

@@ -18,3 +18,4 @@ attribution: "<!-- Icon is based on work by Freepik (http://www.freepik.com) and
 <div class="text-left" style="padding-bottom: 15px;"><a style="color: #505050;" href="vps_run_node"><img src="//static.getmonero.org/images/icon_client.svg" class="title-icon"><h2 class="inline"><span class="green-kicks">How to run a node on VPS</span></h2></a></div>
 <div class="text-left" style="padding-bottom: 15px;"><a style="color: #505050;" href="monero_tools"><img src="//static.getmonero.org/images/icon_client.svg" class="title-icon"><h2 class="inline"><span class="green-kicks">Monero tools</span></h2></a></div>
 <div class="text-left" style="padding-bottom: 15px;"><a style="color: #505050;" href="simplewallet"><img src="//static.getmonero.org/images/icon_client.svg" class="title-icon"><h2 class="inline"><span class="green-kicks">Getting started with simplewallet</span></h2></a></div>
+<div class="text-left" style="padding-bottom: 15px;"><a style="color: #505050;" href="prove-payment"><img src="//static.getmonero.org/images/icon_client.svg" class="title-icon"><h2 class="inline"><span class="green-kicks">How to prove payment</span></h2></a></div>

knowledge-base/user-guides/prove-payment.md

+---
+layout: static_page
+title: "How to prove payment"
+title-pre-kick: "Prove "
+title-kick: "payment"
+title-post-kick: ""
+kick-class: "kicks"
+icon: "icon_about"
+attribution: "<!-- Icon is based on work by Freepik (http://www.freepik.com) and is licensed under Creative Commons BY 3.0 -->"
+---
+
+When you send money to a party who then disputes the payment was made, you need to be able to prove the payment was made.
+
+With Bitcoin, this is typically done by looking up the transaction ID, where the origin and destination addresses are
+shown, along with the amount transacted.
+
+Monero, however, is private: that information is not available publicly on the blockchain. The steps are therefore a bit
+more involved.
+
+To prove to Charlie that she made a payment to Bob, Alice must supply Charlie three pieces of information:
+
+- the transaction ID, as is done in Bitcoin
+- Bob's address, as is done with Bitcoin
+- the transaction's key, which is new with Monero and other Cryptonote currencies
+
+When Alice made the transaction, a one time key was automatically generated just for this transaction. Alice can
+query it thus in monero-wallet-cli (new name for the old simplewallet):
+
+> get_tx_key TXID
+
+Alice would plug in her actual transaction ID instead of this TXID placeholder. All being well, the one time transaction key
+will be displayed.
+
+Note that this will only work if monero-wallet-cli is set to save transaction keys. To double check:
+
+> set
+
+If it's set to 0, set it to 1:
+
+> set store-tx-info 1
+
+Alice can now send Charlie the transaction key along with transaction ID and Bob's address.
+
+Note: if several transactions were made, this needs repeating for each such transaction.
+
+
+---
+
+Charlie now received those three pieces of information, and wants to check Alice is telling the truth: on an up to date
+blockchain, Charlie types in monero-wallet-cli:
+
+> check_tx_key TXID TXKEY ADDRESS
+
+The information supplied by Alice plugs neatly instead of the placeholders. monero-wallet-cli will use the transaction
+key to decode the transaction, and display how much this particular transaction sent to this address. Obviously,
+Charlie will want to double check with Bob the address is really his - same as with Bitcoin.
+
+Note: if several transactions were made, this needs repeating for each such transaction.
+
+