How secure is a mobile e-banking authentication app that requires an NFC card plus a pin to authenticate the user?

Question

Many banks these days offer online banking (in a web browser). To increase security, most banks require a standard login with a username (account number) and password, plus a code that is sent to your mobile phone to two-factor authenticate the login and/or any transaction.

I am thinking of switching to a bank that uses a different scheme: In order to login online (say from your home computer), it just requires your account number (no password needed). The bank then provides a challenge (a few digits), which you must then enter into a dedicated mobile phone app. To increase security, the app requires you to hold a physical card (with a chip) to your phone (which I am guessing will use NFC) and to "unlock" the card with a pin (known only to you), which you type into your phone. If the pin is correct (matches the pin of the card), the phone will hash the challenge digits into a passcode, which you then type into your web browser. This then gives you access to the online banking platform. The same procedure is used when trying to issue a transaction.

I found a press release from when this technology was developed here. The article doesn't say much about the security, though.

I am wondering how secure this is. Let us assume we have an attacker that was able to completely compromise my phone to the root level (some exploits pop up from time to time that allow this, so it's not unheard of). So, we assume the attacker can intercept everything I type on the phone and can monitor everything that every app on the phone does (including memory accesses). We will further assume that the attacker knows the bank account number and who it belongs to, but does not have physical access to the card or the chip on it.

If I now used this app and activated the card's NFC and typed in the pin, would the attacker be able to copy everything he needs from the card to phone, thus allowing him to authenticate himself (in the future without the card) and gain complete access to my bank account?

Or am I wrong in worrying and does reading from the card change its state in a way that is unpredictable to the reading device, ensuring that even if I authenticate myself multiple times on a compromised phone, the attacker would still need physical access to the card itself before he can do any damage?

Answer 1

Usually NFC cards, as with EMV tap card payments use asymmetric encryption for an authentication challenge and response, and the card has a unique private key that is never output anywhere. Assuming this is how it works in your situation, and the bank doesn't authorize all phones when only one provides a the correct response it should be fine. To illustrate:

The attacker steals your phone and not the nfc card. You then get a new phone and install the same app. Lets say worst case scenario the code that the bank sends out is to your email and the attacker has access to your email. With your new phone you request a login. The attacker sees the email and attempts to log in. The bank then request both devices to use the NFC card to respond to a challenge. You succeed and the attacker does not. The bank authorizes only your session on your phone.

This should be secure as long as the bank clearly keeps its sessions correctly. And assuming that the NFC card does in fact use a secure scheme to generate and load private keys, the nfc card cannot be duplicated (short of gaining physical access and dissembling the IC's, or performing a successful DPA which I think is unlikely on an NFC chip because EM interference).

I personally would use this scheme accept I keep my cards and cell phone together!