The Use of SMS for Strong Authentication

NIST and others have discouraged the use of SMS for strong authentication.  This is another case of the perfect as the enemy of the good. 

First, strong authentication using a one time password sent via SMS is dramatically more secure than a replayable password. Second, if you get a one-time password when you ask for it, you are safe.

The problem is not so much with SMS but with the (cell) phone number. There is a risk that an attacker can either change the number in your account, to which the one time password will be sent, to a number other than yours, or get the phone company to associate, re-assign, your number with their phone. In either case, you will not get the one time password when you ask for it. In the latter case, you will not even get phone calls. Whenever the cell phone number in your profile is changed, you will get an e-mail message asking you if you really did it.

Carriers have controls in place to resist fraudulent reassignment of numbers to new phones.  However, the large number of agents and their desire to be accommodating, makes them vulnerable to ”social engineering” attacks. 

The difference in risk between a one-time password sent to your phone and one generated on board is small, particularly when compared to the difference in risk between either and a reuseable password.

In certain circumstances, the difference in convenience may be great. I have ten different accounts associated with my cell phone number. If I get a new phone, all my accounts continue to work as they did on the old phone. The number has moved to the new phone. If I used an on-board password generator, not portable to the new phone, I would have to register the new password generator with each of the ten accounts. I have to do that by calling support, authenticating myself, and registering the new generator. Until I have done that, I cannot logon to or use the account.

If you think about it, the real risk is in provisioning of the phone number or the registering of the on board generator (e.g., VIP Access, Google Authenticator, RSA SecurID Software Token).