Computers can leak secrets without sending a packet.
The page collects material about compromising emanations: unintended electromagnetic or acoustic signals from information-processing equipment that can reveal what the machine is doing.
The history is wonderfully strange.
A declassified NSA account traces the problem back to World War II, when Bell Telephone engineers discovered that encrypted teletype equipment was radiating patterns correlated with the plaintext being processed. In a demonstration across the street from a Signal Corps crypto center, engineers were reportedly able to reconstruct much of the message from the leaked signal.
The machine was secure. The room was talking.
That is the conceptual punch of TEMPEST.
A cryptographic system can be mathematically strong while the hardware implementing it leaks information through a completely different channel.
The page follows the subject through Cold War countermeasures, intelligence collection, shielding standards, and later public research into display emissions. It also points to Wim van Eck’s 1985 work showing that information from computer monitors could be reconstructed with comparatively inexpensive equipment.
The site is not itself a modern security standard or operational guide. Its value is historical: it connects wartime teletype equipment, classified shielding programs, declassified NSA material, Cold War espionage, and the eventual public understanding that electronic devices radiate more than engineers intend.
That history matters because side-channel attacks are now a normal security concept. Timing, power use, sound, electromagnetic emissions, and other physical behavior can all betray information even when the cryptography is sound.
Old pages like this preserve how surprising that idea once seemed.
CacheRat’s 1,967 Ancient Web Domains research list includes many technical pages whose real value is contextual: they show when obscure specialist knowledge first escaped into the public web.
TEMPEST is a perfect example.
The secret was not necessarily in the cipher.
Sometimes it was humming out of the box.