On the 1st of January 2010, many German bank customers found that their banking smart cards had stopped working. Details of why are still unclear, but indications are that the cards believed that the date was 2016, rather than 2010, and so refused to process a transaction supposedly after their expiry dates. This problem could turn out to be quite expensive for the cards’ manufacturer, Gemalto: their shares dropped almost 4%, and they have booked a €10 m charge to handle the consequences.
These cards implement the EMV protocol (the same one used for Chip and PIN in the UK). Here, the card is sent the current date in 3-byte YYMMDD binary-coded decimal (BCD) format, i.e. “100101″ on 1 January 2010. If however this was interpreted as hexadecimal, then the card will think the year is 2016 (in hexadecimal, 1 January 2010 should have actually been “0a0101″). Since the numbers 0–9 are the same in both BCD and hexadecimal, we can see why this problem only occurred in 2010*.
In one sense, this looks like a foolish error, and should have been caught in testing. However, before criticizing too harshly, one should remember that EMV is almost impossible to implement perfectly. I have written a fairly complete implementation of the protocol and frequently find edge cases which are insufficiently documented, making dealing with them error-prone. Not only is the specification vague, but it is also long — the first public version in 1996 was 201 pages, and it grew to 765 pages by 2008. Moreover, much of the complexity is unnecessary. In this article I will give just one example of this — the fact that there are nine different ways to encode integers.