One of the things I like about big events such as Mobile World Congress (and CeBIT, for that matter) is the serendipity they bring.

The flip-side of just how wearisome a huge trade-show can be is when you turn a corner in a far-flung hall and discover something you'd never even heard of before.

Once such was on my way back from visiting an MWC exhibitor, when I chanced upon the stand of the SD-Card Association and learnt that not only is the ubiquitous SD-Card gaining speed and capacity - the plan is to have 64GB SDHC cards and 32GB microSD cards by 2009 - it is also becoming a digital key.

The idea is that when you want to access content or applications on a device, you insert the SD-Card with the appropriate keys on it. It makes use of the format's existing ability to store protected content.

So why use this rather than a smartcard, say?

One reason is quite simply that many devices - especially portable ones such as media players or satnavs - already have SD-Card slots, according to one of the cryptography experts involved.

The SD-Card Association has also come up with a scheme for storing monetary value on the cards, turning them into a form of e-cash. It seems a bit less likely that this one will fly, however, as contactless smartcards already offer the same capability with greater flexibility.

It would in theory be feasible to build contactless comms into an SD-Card though, which could open up yet more interesting possibilities.