Van Eck Phreaking

The "requires a CRT screen" was, I think, referencing the much more fine grained ability to actually recreate what was displayed on a screen. Though that's been replicated for LCDs now also.

My opinion is digital signals are the very antithesis of "unpredictable", it's always an exact replication. Of course it may look like garbage (and sometimes even intentionally scrambled to garbage for electrical reasons) to the naked eyes without interpretation, but many things are possible after signal processing.

> manufacturer-specific ways

First, there are industrial standards on display interfaces, for example, there are well-defined ways to send a video signal, such as HDMI to the monitor, and LVDS (a specific version of LVDS) from the monitor to the LCD panel. The majority of LCD screens use these standard. Second, even when there's no standard, similar architectures are used across different systems, for example, most LCD drivers on the market are similar to each other. Discovering a way of doing it on a new system is not exactly a surprise.

> and certainly not sequentially enough to leak as recognizable audio.

All you need to do is doing something periodically. Even if the activity you're doing is at low frequency, often it influences some high frequency signals so that a signal can be modulated. As a thought experiment, if you write something to RAM with all 1s and all 0s once every few seconds, it's very reasonable to assume it creates a detectable burst of radiation on the RF spectrum even if the bus itself runs at a higher frequency. In practice it's more subtle and specific from system to system but there's often a way.

For example, it's possible to modulate a computer's memory bus to emit AM audio by accessing the memory in a specific pattern using a few lines of assembly code, whether on a PDP-11 or a MacBook, it's possible to modulate the mechanical vibration of inductor coils on the CPU power circuitry to emit audible music on a Thinkpad by rapidly changing the CPU power state, etc.

There are endless number of ingenious ways of doing it, if you have control over a process.

Probably you were right, in the past, because it has become much easier to do this with LCD screens as their resolution has increased into a range which demands fine control of higher-frequency digital data -- high enough (a function of the Nyquist frequency, I guess) to reproduce an analog radio signal with sufficient control to carry a recognizable audio signal

IIRC the protagonists actually do use Van Eck phreaking later in the novel to read out what's on their adversary's laptop screen from the next hotel room over.

It actually drives the climatic finale where Waterhouse performs an "epic hack" while in jail, modifying the linux kernel on his laptop to output the results of his cracking the encryption on the location of a cave full of gold as morse code in the form of his caps lock light going on and off. He also makes sure to write the code that will make these changes via hitting his space bar, which is translated to characters based on the amount of times he hits it, so that again the code to do the calculations is never exposed on screen. The only reason he is so careful about never exposing both the changes he is making to his laptop's code as well as the results of the calculations it runs is because Enoch Root visits him while he is in jail and basically warns him that people will be using Van Eck Phreaking to spy on the contents of his display.

So really, it is actually one of the most crucially import bits of technology in the whole book.