It was a hack to use "unused" control/signalling paths - see https://en.wikipedia.org/wiki/SMS

Effectively came "for free" for mobile operators...

That’s a common misconception. The signaling channels used aren’t “unused” – they’re literally set up and torn down to deliver SMS!

I think you meant to end that with "phone calls" rather than "to deliver SMS?" SS7 was used for that, but had a significant amount of idle time since most phone calls are much longer than the time needed to setup the connection.

I do indeed mean SMS, but I was focusing more on the air interface. There, SMS definitely consume resources in the same way that calls do (although of course at a very different rate: SDCCH uses 0.8 kbit/s, as opposed to 13 kbps for full rate voice/CSD traffic channels).

But the signaling channels weren't used to send SMS before SMS existed (which the original statement of "unused" implied).

Because it dates to a time when such attacks were infeasible. GPS is very similar in that regard. Even HTTPS was uncommon back when I was in university. NASA spacecraft still communicate over unencrypted channels.

Mindsets were different then.

not just mindsets, but the computing power available. These days, my smartphone is millions of times more powerful and the computation to do TLS encryption on every website I visit is trivial for a computer that fits in the palm of my hand. Way back when, the 1 or 2 kilobytes or so a modern RSA private key (PEM format) would take up on disk was meaningful when you only had 4 megabytes of RAM and CPUs ran in the megahertz range.

Also to a dumb phone it doesn't matter whether an SMS contains a phishing link because it has no way of accessing it. Until the advent of smart phones SMS phishing was a non-issue.

> GPS is very similar in that regard.

GPS originated as a military protocol and has some level of encryption and authentication, but this is not available to the general public.

GPS *had* no encryption or authentication; indeed, such security is only a recent addition to the L2 frequency.

USK: Galileo also has authentication available on its civilian frequencies.

> GPS had no encryption or authentication; indeed, such security is only a recent addition to the L2 frequency.

This was a long time ago, but I distinctly remember the encrypted P(Y)-code and anti-spoofing being operative in the mid 1990s.

> such security is only a recent addition to the L2 frequency

Is that already available? I thought GPS L2C didn't include authentication yet.

encryption was expensive in GSM days.

People seem to forget that dedicated TLS acceleration hardware was a thing not long ago.

SMS isn't a protocol. Attacks like these are done via 2G. Which is really why most people should disable it if they can.