Making a blockchain verification program

[Was]

How trivial is it to make a program that interacts with a local bitcoin node, through a GUI, that would listen for a transaction that proves ownership of a private key (in order to allow access to an account, or permissions to do certain things) by requiring a transaction broadcast from a specific public key/hash ?

Would an API like blockchain's make this easier?

Thanks!

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[micaman]

You can use a websocket from an API to listen to new transactions and new blocks. Remember that the transactions you are listening to are still unconfirmed, and will have the first confirmation when it's hash shows up in a new block. In these transactions you can see both sender and receiving addresses.
You can also use also use RPC to interact with your bitcoind or bitcoin-qt, but in this case you will need to download the entire blockchain.

Check https://blockchain.info/api/api_websocket and https://en.bitcoin.it/wiki/API_reference_%28JSON-RPC%29