>Take a look at Boojum Mobile -- it is precisely the idea of using the cell
>phone as an out-of-band chanel for an in-band transaction.

Banks here have been using it to authenticate higher-value electronic
transactions as well.  The way it works is that for transactions with a
combined value over the default floor limit of NZ$2.5K you have to use an
additional PIN sent via SMS to a pre-configured number to authenticate the
session.  The PIN authenticates that particular session (not just one
transaction), with a fee of NZ$0.25.  It's not perfect, obviously, but that
was seen as the best tradeoff between cost, user convenience, and security.

<grumble>A few years ago I wanted to do this out-of-band authentication as a
research project, and at the time couldn't find anyone interested in it; now
they've paid an arm and a leg for it themselves, sigh</grumble>.


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