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The secrecy of correspondence (German: Briefgeheimnis, Swedish: brevhemlighet, Finnish: kirjesalaisuus), or literally translated as secrecy of letters, is a fundamental legal principle enshrined in the constitutions of several European countries. It guarantees that the content of sealed letters is never revealed and letters in transit are not opened by government officials or any other third party. It is thus the main legal basis for the assumption of privacy of correspondence.

The principle has been naturally extended to other forms of communication, including telephony and electronic communications on the Internet as the constitutional guarantees are generally thought to cover also these forms of communication. However, national telecommunications privacy laws may allow lawful interception, i.e. wiretapping and monitoring of electronic communications in cases of suspicion of crime. Paper letters have in most jurisdictions remained outside the legal scope of law enforcement surveillance, even in cases of "reasonable searches and seizures".

When applied to electronic communication, the principle protects not only the content of the communication, but also the information on when and to whom any messages (if any) have been sent (see: Call detail records), and in the case of mobile communication, the location information of the mobile units. As a consequence in jurisdictions with a safeguard on secrecy of letters location data collected from mobile phone networks has a higher level of protection than data collected by vehicle telematics or transport tickets.

In the United States there is no specific constitutional guarantee on the privacy of correspondence. The secrecy of letters and correspondence is derived through litigation from the Fourth Amendment to the United States Constitution. In a 1877 case the U.S. Supreme Court stated:

"No law of Congress can place in the hands of officials connected with the Postal Service any authority to invade the secrecy of letters and such sealed packages in the mail; and all regulations adopted as to mail matter of this kind must be in subordination to the great principle embodied in the fourth amendment of the Constitution." [1]

The protection of the Fourth Amendment has been extended beyond the home in other instances. A protection similar to that of correspondence has even been argued to extend to the contents of trash cans outside one's house, although unsuccessfully. Like all rights derived through litigation, the secrecy of correspondence is subject to interpretations. Rights derived from the Fourth Amendment are limited by the legal requirement of a "reasonable expectation of privacy".

• Black room
• Data privacy
• Dead letter
• Electronic Communications Privacy Act
• Katz v. United States
• Lawful interception
• Telecommunications data retention

• U.S. Supreme Court: EX PARTE JACKSON, 96 U.S. 727 (1877)
• Act on the Protection of Privacy at the Finnish Communications Regulatory Authority site
• Netherlands - Constitution
• 1936 Constitution of the USSR
• Letter to the House On the Privacy of Outbound U.S. Mail (10/18/2000) by ACLU

• Back when spies played by the rules by David Kahn, New York Times - A history of the secrecy of letters