How does mymonero web wallet work?

Question

I understand that all the client code is running on the client computer, and the private keys never leave the client computer. The client creates transactions and transmits them to mymonero server, which then pushes it on the blockchain, correct?

Also, I understand mymonero server stores the private viewkey, and monitors the blockchain for any inputs.

How are outputs handled though? If the same wallet was used from simplewallet to spend an input, how can mymonero know it has been spent? Is this check done automatically on the client side every time the user logs in? Is it done every time from scratch, or does mymonero store this info as well?

Answer 1

There's a daemon (monero core) running on the MyMonero server. When you go to mymonero.com and make a new webwallet, there's a script in the browser that sorta functions like wallet software.

The browser script creates a new set of keys (spend and viewkey). The viewkey is transmitted to the mymonero server. The server uses the viewkey to find out which outputs on the blockchain are yours. It transmits that output information to your browser.

To create a transaction, the browser script creates and signs a transaction using your spend key.

I think the browser script also uses your spend key to find out which of the outputs received from the viewkey-identified outputs from the mymonero server are spent.

As you can see, the MyMonero server only ever gets your view key. Your spend key is created and stored locally when you login with your mnemonic word seed.

Answer 2

I don't know if it actually does that, but the client side code could generate key images when refreshing, and send them back to Mymonero, which can store them and look for them for then on. Alternatively, the client could send key images with a refresh request, and Mymonero doesn't need to store them.