21 January 2008

Security by Obscurity? I Don't Think So

Great post by Ed Felten about the complete mess the Dutch authorities have made of their new $2 billion transit card system, which, it seems, is wide open to cracking:


Kerckhoffs’s Principle, one of the bedrock maxims of cryptography, says that security should never rely on keeping an algorithm secret. It’s okay to have a secret key, if the key is randomly chosen and can be changed when needed, but you should never bank on an algorithm remaining secret.

Unfortunately the designers of Mifare Classic did not follow this principle. Instead, they chose to combine a secret algorithm with a relatively short 48-bit key. This is a problem because once you know the algorithm it’s possible for an attacker to search the entire 48-bit key space, and therefore to forge cards, in a matter or days or weeks.

More generally:

Now the Dutch authorities have a mess on their hands. About $2 billion have been invested in this project, but serious fraud seems likely if it is deployed as designed. This kind of disaster would have been more likely had the design process been more open. Secrecy was not only an engineering mistake (violating Kerckhoffs’s Principle) but also a policy mistake, as it allowed the project to get so far along before independent analysts had a chance to critique it. A more open process, like the one the U.S. government used in choosing the Advanced Encryption Standard (AES) would have been safer. Governments seem to have a hard time understanding that openness can make you more secure.

Let's hope other governments are listening...

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