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Comment by Canada | Hacker News Reader

From Pretty Good Chat:

"We consider the case of the private key being stolen as very unlikely. The users themselves are not able to access it outside the app, and no other adversary will be able to either"

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Reminds me of Google Authenticator, hiding the private key (used for generating the one time codes) by hand-waving and wishful thinking. Most users probably don't realize that there is a shared key -- and are left wondering why there's no sane way to import/export keys/accounts (there's of course no good reason for this other than dumbing down/simplifying the UI/UX: I suppose opinions are divided on whether or not this is a good thing). When you don't understand that there's a shared secret, you don't realize that that secret could be backed up/leaked from the server side -- without you knowing. Pretending that it's a HSM type deal and hiding the complexity of key management obscures some pretty obvious attack vectors from the users.

All that said, (T)OTPs have some advantages versus traditional "provably shared-secret, by way of salt+hash+stretching" of traditional passwords.

Not a very good starting point to consider phone app storage a reliable secure black box...

True, but you can increase that reliability with encrypted sqlite. Gonna have to pay for it, though.

Why? https://www.zetetic.net/sqlcipher/

And where would you hide your encryption key exactly? (Honest question, maybe this is a solved problem, but I don't know the solution).

I would say in comparison to storing it on the server, storing the encrypted private key in Keychain probably offers a lot more security. An attacker would need direct access to the device and would need to either a) know the user's passphrase, b) be able to fool TouchID or c) be able to bypass Keychain's security.

However if iCloud Keychain is enabled then this opens up the attack surface a lot more.

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