As is often the case with these finds, while researching something else, came across a document which records the history of SIGINT at the CIA. Many associate US SIGINT activities with the National Security Agency (NSA), but the CIA has also played a role in the signals intelligence game. The historical account, The History of SIGINT in the Central Intelligence Agency, 1947-1970 is readily available in the archive, but there’s an issue: the pages are completely out of order.
I started organizing the document, but after a bit of googling, turns out that the National Security Archive has already done the work, and the properly organized document is linked in an article on their website about the history of CIA’s signals intelligence activities.
SIGINT can be further broken down to communication intelligence (COMINT) meaning the interception of communication between people, and electronic (signals) intelligence (ELINT), which in turn focuses on the emissions and signatures of electronic systems themselves, such as radars. In addition to photographic intelligence, CIA-operated aircraft such as the U-2 also had an ELINT role. One tactic for inducing emissions from systems is called ferreting, where the system of interesting, e.g. a target acquisition radar, is teased to start emitting. This could be done for example by navigating in a way that forces an anti-aircraft system to activate and start tracking the ferret aircraft as a target.
One of the operations referenced in the history is the Berlin tunnel operation in the 1950s, where Soviet circuits were tapped by using a tunnel built specifically for the purpose. Declassified histories of the Berlin tunnel operation specifically are available.
Beyond that, when it comes to SIGINT operations, the history document released by the CIA is heavily redacted. Part of the reason appears to be that some of the discussion is on partnerships – information that is generally closely held in intelligence communities. Another sensitive subject is the use of diplomatic facilities for running SIGINT operations, as well as the interception of diplomatic message traffic, both of which can be inferred from the document.
The relationship between the CIA and the NSA in signals intelligence has been historically rocky, as is evident from the history account up to 1970. The online archive yields some additional data points to this, including a memo from 1976:
For an NSA perspective on code-breaking activities specifically, their three-part declassified history, American Cryptology during the Cold War, 1945-1989, is also recommended reading.