Black chamber

A Black Chamber (cabinet noir as it was originally rendered by the French) refers to the name given for secret letter-opening rooms for scrutinizing the letters of suspicious citizens without their knowledge as they went through the postal system, resealing the letters once the analysis was finished so the snooping would not be detected in the first place.

Many of these existed as part of the intelligence organs of many medieval monarchies, and many military organizations even in contemporary times employ mail censors who scrutinize outgoing mail for unauthorized disclosure of military secrets.

Originally, black chambers would intercept the letters they were ordered to scrutinize, make copies of any seals or bindings on the letter for later reapplication or resealing to avoid tipping off the intended recipient the original had been tampered with, then, after examining the letter, would send it forward to the recipient with the resealed letters not looking as if they had been tampered with.

In the modern day, this is but one of the many functions of modern intelligence agencies.