The German passport serial-number format (previously 10-digit, all-numeric, sequentially assigned) was modified on 1 November 2007, in response to concerns about the low entropy of BAC session keys. The new 10-character serial number is alphanumeric and generated with the help of a specially-designed block cipher, to avoid a recognizable relationship with the expiry date and increase entropy. In addition, a public-key based extended access control mechanism is now used to protect any information in the RFID chip that goes beyond the minimum ICAO requirements, in particular fingerprint images.