gettxoutproof ["txid",...] ( blockhash ) Returns a hex-encoded proof that "txid" was included in a block. NOTE: By default this function only works sometimes. This is when there is an unspent output in the utxo for this transaction. To make it always work, you need to maintain a transaction index, using the -txindex command line option or specify the block in which the transaction is included manually (by blockhash). Arguments: 1. "txids" (string) A json array of txids to filter [ "txid" (string) A transaction hash ,... ] 2. "blockhash" (string, optional) If specified, looks for txid in the block with this hash Result: "data" (string) A string that is a serialized, hex-encoded data for the proof.