chip and the reader by encrypting transmitted information. Before data can be read from a chip, the reader needs to provide a key which is derived from the Machine Readable Zone: the date of birth, the date of expiry and the document number. If BAC is used, an attacker cannot (easily) eavesdrop transferred information without knowing the correct key. Using BAC is optional. Passive Authentication (PA) PA is aimed at identifying modification of passport chip data. The chip contains a file, Document Security Object (SOD), that stores hash values of all files stored in the chip (picture, fingerprint, etc.) and a digital signature of these hashes. The digital signature is made using a document signing key which itself is signed by a country signing key. If a file in the chip (e.g., the picture) is changed, this can be detected since the hash value is incorrect. Readers need access to all used public country keys to check whether the digital signature is generated by a trusted country. Using PA is mandatory. According to a September 2011 United States Central Intelligence Agency document released by WikiLeaks in December 2014, "Although falsified e-passports will not have the correct digital signature, inspectors may not detect the fraud if the passports are from countries that do not participate in the International Civil Aviation Organization's Public Key Directory (ICAO PKD)." As of January 2017, 55 of over 60 e-passport-issuing countries belong to the PKD program. Active Authentication (AA) AA prevents cloning of passport chips. The chip contains a private key that cannot be read or copied, but its existence can easily be proven. Using AA is optional. Extended Access Control (EAC) EAC adds functionality to check the authenticity of both the chip (chip authentication) and the reader (terminal authentication). Furthermore, it uses stronger encryption than BAC. EAC is typically used to protect fingerprints and iris scans. Using EAC is optional. In the European Union, using EAC is mandatory for all documents issued starting 28 June 2009. Supplemental Access Control (SAC) SAC was introduced by ICAO in 2009 for addressing BAC weaknesses. It was introduced as a supplement to BAC (for keeping compatibility), but will replace it in the future. Shielding the chip This prevents unauthorised reading. Some countries – including at least the US – have integrated a very thin metal mesh into the passport's cover to act as a shield when the passport cover