certificate issued to the U.S. Department of State by the ICAO Public Key Directory. Any and all data must be authentic and untampered, or else the signature will be invalidated. Data in a passport chip is scannable by electronic readers, a capability which is intended to speed up immigration processing. This data, along with the signature, is verified to either be valid or invalid. Like toll-road chips, data in passport chips can be read when passport chips are adjacent to readers. The passport cover contains a radio-frequency shield in the form of a wire mesh within the cover, so the cover must be opened for the data to be read. This cover acts as a Faraday cage. According to the Department of State, the Basic Access Control (BAC) security protocol prevents access to that data unless the printed information within the passport is also known or can be guessed. According to privacy advocates, the BAC and the shielded cover are ineffective when a passport is open, and a passport may have to be opened for inspection in a public place such as a hotel, a bank, or an Internet cafe. An open passport is subject to unwelcome reading of chip data, such as by a government agent who is tracking a passport holder's movements or by a criminal who is intending identity theft. Visa requirements Visa requirements for United States citizens are administrative entry restrictions by the authorities of other states placed on citizens of the United States. As of January 11, 2022, holders of a United States passport can visit 186 countries and territories without a visa or with a visa on arrival, ranking it sixth in terms of travel freedom (tied with Belgium, New Zealand, Norway, Switzerland and the United Kingdom) according to the Henley Passport Index. Additionally, Arton Capital's Passport Index ranked the United States passport fourth in the world in terms of travel freedom, with a visa-free score of 160 (tied with Belgium, New Zealand, Poland, Portugal, Spain and Switzerland), as of January 2022. United States Passport is ranked 3rd by the Global Passport Power Rank. The United States Government has prohibited all U.S. nationals from traveling to North Korea without special permission, making all United States passports invalid for travel to, in, or through the country. Foreign travel statistics These are the numbers of visits by U.S. nationals to various countries in 2015 (unless otherwise