Skip to main content

Reverse authentication for banking

A persistent problem with retail banks is that they phone you and then ask you for information.  A common scenario is that the bank's fraud department calls because of a suspicious debit or credit card transaction.  What follows is, from a security perspective, dangerous.

Either the bank just assumes that you'll accept that they really are your bank (and not some random person trying to get private information from you) or they'll go through some weak authentication (such as telling me half my postcode).  Sometimes they have the audacity to call me and then ask me to prove that I am me even though they just called me.

All this ridiculous nonsense can be fixed by use of the two factor authentication tokens that banks are now giving out.  In the UK Barclays has PINSentry, HSBC has Secure Key, NatWest has Card-Reader.  These tokens are usually used for logging in to online banking or authorizing a transaction, i.e. they are used so that you can prove to your bank that you are you.

But they can be used the other way around.

Imagine the phone ringing in your home:

Caller: Hello, Mr Foo it's Barclays Fraud Department calling.  We need to ask you about a transaction on your account?

You: OK

Caller: Do you have your PINSentry handy? I'd like to use it to prove that this is Barclays calling.

You: Yes, I have it right here.

Caller: Please switch it on.  A six digit number will appear on the screen.  I'm going to tell you the first three digits.

You: OK, it's on.

Caller: The first three digits are 4 7 2.  You should be able to see it on the screen.  That proves that this really is Barclays calling as only we would be able to predict the next three digits.

You: Yes, I see that.

Caller: And can you tell me the other three digits?  That way I'll know you really are Mr. Foo.

You: Yes, it reads 4 9 7.

Caller: Great.  Let's talk about the transaction our system has flagged...

With a simple conversation like that you've proved that you are you, and the bank has proved that they are who they say they are. Additional levels of authentication can be added (such as asking for personal information), but the key is that the two factor device contains a secret shared between your bank and you.


Anonymous said…
When the bank does this to me I:
a) tell them is bad security and they are training people to be phished. The length of this discussion depends on what kind of mood I am in.

b) Tell them I will ring them back to discuss the issue.
whoppie said…
This happened to me three times and the third time I asked to talk to the manager, then I told him to google the word phishing. Then I switched bank to another one and so far they haven't been stupid enough to do the same thing...
whoppie said…
This happened to me three times and the third time I asked to talk to the manager, then I told him to google the word phishing. Then I switched bank to another one and so far they haven't been stupid enough to do the same thing...
If the bank gives you three digits and the customer gives back three, you can then call 500 customers or so, successfully guess the digits for one of them, scam the other three out and log in to their account. I think this isn't quite the right protocol.

Popular posts from this blog

How to write a successful blog post

First, a quick clarification of 'successful'. In this instance, I mean a blog post that receives a large number of page views. For my, little blog the most successful post ever got almost 57,000 page views. Not a lot by some other standards, but I was pretty happy about it. Looking at the top 10 blog posts (by page views) on my site, I've tried to distill some wisdom about what made them successful. Your blog posting mileage may vary. 1. Avoid using the passive voice The Microsoft Word grammar checker has probably been telling you this for years, but the passive voice excludes the people involved in your blog post. And that includes you, the author, and the reader. By using personal pronouns like I, you and we, you will include the reader in your blog post. When I first started this blog I avoid using "I" because I thought I was being narcissistic. But we all like to read about other people, people help anchor a story in reality. Without people your bl

Your last name contains invalid characters

My last name is "Graham-Cumming". But here's a typical form response when I enter it: Does the web site have any idea how rude it is to claim that my last name contains invalid characters? Clearly not. What they actually meant is: our web site will not accept that hyphen in your last name. But do they say that? No, of course not. They decide to shove in my face the claim that there's something wrong with my name. There's nothing wrong with my name, just as there's nothing wrong with someone whose first name is Jean-Marie, or someone whose last name is O'Reilly. What is wrong is that way this is being handled. If the system can't cope with non-letters and spaces it needs to say that. How about the following error message: Our system is unable to process last names that contain non-letters, please replace them with spaces. Don't blame me for having a last name that your system doesn't like, whose fault is that? Saying "Your

The Elevator Button Problem

User interface design is hard. It's hard because people perceive apparently simple things very differently. For example, take a look at this interface to an elevator: From flickr Now imagine the following situation. You are on the third floor of this building and you wish to go to the tenth. The elevator is on the fifth floor and there's an indicator that tells you where it is. Which button do you press? Most people probably say: "press up" since they want to go up. Not long ago I watched someone do the opposite and questioned them about their behavior. They said: "well the elevator is on the fifth floor and I am on the third, so I want it to come down to me". Much can be learnt about the design of user interfaces by considering this, apparently, simple interface. If you think about the elevator button problem you'll find that something so simple has hidden depths. How do people learn about elevator calling? What's the right amount of