What is NPKD Many countries have implemented Biometric Passports (or e-Passports), allowing their citizens to travel more secure and efficient. All efforts in standardization of travel documents are done under the umbrella of the International Civil Aviation Organization (ICAO). This makes travellers’ documents easy to recognize, read and validate by the foreign countries people visit. ICAO is in charge and operates a directory of certificates used to issue passports; each of the associated countries has its own certificate. The directory is called the Public Key Directory (PKD).