Long before there was a National Security Agency, the U.S. Navy ran its own codebreaking, and the people who did the listening learned their trade on a rooftop. OP-20-G and its On-the-Roof Gang are the Navy’s entry in the long story of treating enemy communications as a problem to be solved by analysis.
The idea
Signals intelligence is applied information theory. Intercept a stream of enemy communication, and use pattern, frequency, and statistics to pull meaning out of what looks, at first, like noise.
OP-20-G
OP-20-G was the U.S. Navy’s signals intelligence and cryptanalysis group during World War II. Its mission was to intercept, decrypt, and analyze naval communications from the Japanese, German, and Italian navies. That work, breaking an enemy’s encrypted traffic, is one of the oldest and most demanding uses of computation, and in this era it was done largely by hand and mind.
The On-the-Roof Gang
Before you can break a message you have to catch it. The radiotelegraph operators who did the intercepting became known as the On-the-Roof Gang, after the rooftop where they trained. By June 1940, OP-20-G included 147 officers, enlisted men, and civilians, linked into a network of radio listening posts that fed the intercepted traffic to the analysts.
Why it belongs here
Codebreaking is one of the oldest drivers of computing power. The discipline OP-20-G practiced, intercept, analyze, exploit, is the human root of work that would later demand machines like Harvest, and it sits in the same family as the cipher and cryptanalysis notes in this cluster. For anyone who has worn a Navy cryptologic rating, it is also the start of a lineage that runs to the modern fleet.
Related Notes
- Cryptography, Codebreaking, and the NSA, the wider codebreaking story
- SIGABA, the machine that protected American traffic in the same war
- IBM Harvest and NSA Supercomputing, where this work later met the computer
- Shannon and Information Theory, the mathematics under signals analysis
- Computing and the U.S. Military, the cluster index
Sources
- “OP-20-G,” Wikipedia. https://en.wikipedia.org/wiki/OP-20-G . Supports OP-20-G as the U.S. Navy’s signals intelligence and cryptanalysis group during World War II, its mission to intercept, decrypt, and analyze naval communications from the Japanese, German, and Italian navies, the radiotelegraph operators known as the On-the-Roof Gang named for where they trained, and the figure of 147 officers, enlisted men, and civilians by June 1940 linked into a network of radio listening posts.