A colleague recently tried to convince me that "1.5 factor authentication" was better than 1 factor so I decided to look into it.
First some basics, generally speaking Authentication works at its most basic level on computer systems via a username and password. This is 1-factor authentication. It is something that is unprotected and possibly public your username and something that should be kept hidden and secret your password or passphrase.
The 2nd factor of authentication in 2 factor authentication is the combination of something you have, some kind of encrypted token (usb key, rfid token, smart card, numeric-alpha numeric token; ala RSA SecureID and Wikid soft tokens).
The 3rd factor of authentication is something new, but it requires the first two in addition to another something you are. Eg; thumbprint, voice print, etc. Basically the 3rd factor is the addition of biometrics. I am really not a fan of biometrics as the only method of authentication as you can reissue a security token but you can't reissue your thumb. I can see having it in addition though would be workable.
See here for a more in-depth PCI view of these three widely accepted Authentication factors; http://pciguru.wordpress.com/2010/05/01/one-two-and-three-factor-authentication/
There is also a not yet well supported but interesting idea for a 4th factor. So in addition to all the other factors the computer or website or what-have-you authenticates that you are where you say you are. This 4th factor is hard to implement at the moment, and they are obviously trying to make it transparent to the end user, so say you have an app on your phone that fires up GPS and sends it through to ensure you are logging in from areas you have pre-defined. I actually heard of someone using log correlation years ago to this effect, basically they watched logins from the internal network and VPN concentrators and if a user attempted to VPN in from a geographically remote IP when they had only recently been seen more geographically locally or even on network then they would shut down the geographically remote session. I can't find the article now, but this supposed shut down a hacker trying to get into this USA based company using an Execs credentials via the VPN from South America when the exec had been seen on the local network only minutes earlier.
See here for more on 4th factor; http://blog.dustintrammell.com/2008/11/21/four-factor-authentication/
Now to get to 1.5 factor auth. I couldn't find much ;
Comment decrying it for being touted as 2 factor auth; http://stackoverflow.com/questions/559639/what-is-two-factor-authentication
Market-speak, but interesting implementation; http://pingrid.org/
Very aptly named blog; http://www.ryanhicks.net/blog/2008/10/15-factor-authentication.html
But onto this colleagues definition: 1.5 factor auth is a password and a pin... So still two things that you know. Yes it maybe prettied up in the case of pingrid or horrible and easy to break as in the case of the below screenshot from a banking institution here in Australia that I used to use, but still two somethings that you know, by definition still one factor, aka one of the definitions of factors above.
Onto the example I mentioned earlier, I used to use a financial institution that I believe started using the below (this is a mock-up I no longer have an account there) "extra factor" in 2003, I laughed when I first saw it, realising it added no real security. The idea is that you pick three images and you have to click them in order, the images get shuffled each login.
As I watched after more logins I noticed that the pictures changed, every time, except the pictures I as a user had to click, so if a user had my username and password they could simply login several times see the picture auth, note down the pictures then exit, do this a large enough number of times and like a game of "guess who" you have narrowed down the pictures needed to authenticate in this step. As there are only three and you need to click them in order you have to only make 6 failed attempts and you will have it.
The problem with this 1.5 factor is depending on the implementation it could be almost 50% more security that 1 factor but in the case of the above image that is probably 1.0000000000000001 factor. The other issue is even if it is 50% better than 1 factor it is not 50% worse than 2 factor, 2 factor is insanely better than 1 factor, coming back to implementation of course but even the worst is orders of magnitude better.
Have a look at how complex pingrid is, I doubt that most end users would pick this up quickly and I would say 90% will write down what they have to do and what they do, do to get authenticated, this makes it no longer something that is kept secret, and may make authentication for legitimate users so hard that they fail more often, causing increased support calls and decreased productivity.
This half factor addition is bad market speak at best, and a false sense of security with a move to introducing vulnerabilities in the authentication chain at worst.
UPDATE: Being the security geek I am, I decided to email the venerable Bruce Schneier and his word from on high matches my own, "It doesn't (add security). It's a marketing ploy." Squeee I got a reply for Bruce Schneier... but yeah 1.5 factor is bs, coffin closed and put to bed.