Black room

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For other uses, see Black room (disambiguation).

While a black room or black chamber is often now used to refer to any place or organisation dedicated to code-breaking, its more exact meaning is a secret room in a post office, and, later and by extension, a telecommunications center used by state officials to conduct clandestine interception and surveillance of communications.^[1] Typically all letters or communications would pass through the black room before being passed to the recipient.

This practice had been in vogue since the establishment of posts, and was frequently used in France by the ministers of Louis XIII and his followers as Cabinet noir (French for "black room").

In a modern American Network Operations Center, optical splitters divert a percentage of the laser light from all incoming and outgoing fiber-optic cables to the secret room.^[2] An example is Room 641A in the SBC Communications building in San Francisco.^[2] Activities in a black room do not fall under the concept of lawful interception, as all data is intercepted and no court order will be obtained for its interception as it is at least arguably an extralegal procedure.^{[citation needed]}