Scientists from the Dutch Radboud University Nijmegen their findings during the Esorics security conference on Monday in Malaga, Spain. They also published an with cryptographic details.

Their presentations show how to circumvent the security mechanism of NXP Semiconductor's Mifare Classic RFID cards, which are widely used to provide access control to buildings and public transportation systems.

Researchers had previously reported that they had broken cryptography used in Mifare Classic, but NXP sued to prevent the information from being made public. The company asked for a lengthy delay to give customers enough time to secure their systems. However, a Dutch court ruled that free speech protected the researchers and that they shouldn't fall victim to mistakes made by the supplier.

Some details on the Mifare encryption were already public, but the publication of the Dutch paper marks the first time this information has been disclosed in a complete form, making it easy for a determined attacker to clone one of the cards.

In the paper, the researchers say they discovered the workings of the chip by analyzing communication between the chip and the reader. An RFID-compatible device, the Ghost, was made to function independently from a computer. Ultimately they were able to obtain the cryptographic protocol. One issue is that the reader has to communicate in a predictable way, which opened the path to data analysis.