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Comment by peter_l_downs | Hacker News Reader

    > COINBASE KEY: The only key that Coinbase stores.
    > SHARED KEY: Encrypted with your password and stored
    >             both by you and Coinbase.

Are these the same key, or is there an inconsistency with the language here? How many keys does Coinbase store?

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Coinbase stores one private key that we can access, and one private key that is encrypted with the user's vault password on the front-end and sent to us encrypted for storage. The third key is the user's and we never see that.

We have no ability to access multisig vault funds without the user passphrase, which never touches our server.

Is there a way to sign a transaction offline but still run it through Coinbase? When it comes to decrypting private keys in the browser, unless the user inspects the javascript each and every time they use the site, there is no guarantee that it hasn't been silently replaced by code that steals keys.

Yes, you can sign a transaction offline using our multisig vault recovery tool, which is open source:

https://github.com/coinbase/multisig-tool

You can use it to generate a transaction payload which can be broadcast from a client of your choice, including Toshi, our open source bitcoin node:

https://toshi.io/docs/#relay-transaction

Excellent! Thank you for putting forth the extra effort and thinking this through. Many comparable online solutions in the Bitcoin space do not offer similar offline tools, making them a non-starter for business use.

Yes. You can sign transactions completely independently from Coinbase, provided you have your private key, encrypted shared key, and passphrase (the two keys are provided on a sheet of paper which you print out during vault creation).

https://github.com/coinbase/multisig-tool

Different key. The other key is encrypted with BIP38 with your chosen password. I don't know how the actual interface works, if it is encrypted client-side or server-side. Anyway, on paper it sounds fantastic.

Yes, the shared key is generated and encrypted with your passphrase client-side, then sent to Coinbase. We never see the raw private key.

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