Jun 15, 2011

The ePassport Standard and it's Cryptographic Scheme

ePassport is the name given for the new passport specification standardized by the International Civil Aviation Organization (ICAO). It was created to speed up and standardize the immigration procedures. This standard specifies RFID as the communication channel between the passport and the reader. The main feature present on it is the insertion of biometric data into the passport for biometric checks, including facial recognition, fingerprint and iris information. To keep this important data protected, an interesting cryptographic layer was added in addition to the usual document falsification mechanisms such as watermarks.
Brazilian ePassport. One of few which has full compatibility with the ICAO specification.
The name, nationality, birth date and other personal information as well as the biometric informations are held on a contactless chip inside the passport. This set of information are securely handled and is called the Machine Readable Zone (MRZ) and the chip asserts their security. These informations are transmitted to a ePassport reader at immigration process. As the embedded chip has low processing power, the biometric data is transmitted to a reader so it can be checked against the porter of the passport.
Anatomy of an ePassport

As the physical communication layer of the ePassport broadcasts the messages, even with the low range (less than a meter) RFID communication. But as we know an eavesdropper can be very determined to listen the communication ;-). I also has an electromagnetic shield on the passport cover, however it doesn't assure to block the communication between the chip and the outside world when it's closed, however it's an extra obstacle. Another issue to prevent is the passport and digital data forgery. Otherwise a falsary could get someone else passport and insert it's biometrics information and easily bypass the immigration. As we may see, the ePassport is a complete meal for cryptography specialists.

Starring at this cryptographic layer we have: 

A unique id used as a private key. This key is hardcoded in the chip manufacturing process and it's non traceable. Doing reverse engineering on this kind of chip it's really difficult because the equipment to do it is extremely expensive and hard to obtain, it is only available to large chip manufacturers.

An access control to prevent an attacker to force the ePassport to send information in the MRZ to readers unless the passport owner authorizes it. Otherwise an attacker could use a reader and simply request the biometric information to the passport. It uses a protocol called Basic Access Control (BAC) to give access for the reader. It consists on a OCR readable code inside the passport. With this code, the reader can say to the passport it is an authorized reader, and then transmit data to it. The BAC code is also used as key for the encrypted transmission of the data, making an eavesdropping attack more difficult. It's important to note that the BAC security entirely relies on the physical access to the passport. The BAC is inside the ICAO standard but doesn't obligates it's support.

BAC happy path's sequence diagram

The ICAO also standardizes a authentication for readers called Extended Access Control (EAC). It consists of sets of signatures of terminals stored in the passport's chip. These signatures are changed periodically by the countries (which signs them) on the terminals to avoid keys stolen from equipments being used for much time. Some MRZ sensitive data, such as iris and fingerprint, are just transmitted to readers if it is EAC authenticated. Other informations are optional.

EAC happy path's sequence diagram
    The Passive Authentication (PA) is a cryptographic mechanism  to ensure the data in the passport is valid by the authority emitting the passports. The PA is a kind of digital signature where the country who emits passports ensures the veracity of the data with a asymmetric Country Verification Certificate Authority (CVCA), a kind of message authentication code (MAC). This process is known as asymmetric cryptographic hashing or digital signing. The PA MAC is created by the country when emits a passport, which must keep a signing key safe, which is used to create the MAC code. Then it publish the public key (all readers must know the countries public keys) which can be used to check if the MAC is valid and it was really generated by the organization which possess the signing key. The PA scheme relies on mathematical functions which are easy to encrypt a MAC with the private key, but computationally hard without them. The public key can easily check if the MAC was generated by the ones who posses the private key, and detect forged data. This process makes the forgery of digital data a really hard computational problem, and ensures with negligible chance of failure that the data is valid. All the MRZ data is signed using such process. Using PA is mandatory by ICAO standards.
    PA happy path's sequence diagram

    The Active Authentication (AA) is a kind of challenge response authentication mechanism to prevent the cloning of passports. It relies on the untraceable chip key. Cloning the MRZ data is a simple task to be done, however you can't get a passport private key due to the difficulty of doing reverse engineering to obtain it. The AA works as a challenge to the passport, so you can verify if it really is. The reader encrypts a random message that only him knows it's content with the passport's public key. The message is then sent to the passport and it must decrypt and response to the reader what was the original message. This process works because of the duality principle of asymmetric cryptography. This concept of challenge is the same concept behind CAPTCHAs and the Turing Test.
    AA hapy path's sequence diagram


    1. Thanks for mentioning the definition of ePassport which is the name given for the new passport specification standardized by the International Civil Aviation Organization.this blog added much to my knowledge.
      digital certificates

    2. I really like the way you explained it.