What was the societal benefit of putting 20 years of monopoly on that algorithm? I don't think potential profit was a big motivator in that research work.
And that patent got invalidated in most of the world anyway.
You think they were just going to sit on that and do nothing if they couldn't get a patent? Or that they'd turn it into a product without the underlying math being revealed right away? I don't believe either of those for a second.
There is a rather long history of the workings of cryptography products kept secret, so yes, it is entirely possible that the underlying math would have been kept as a trade secret.
It is also possible that it would have never been created in the first place because resources were allocated to other patentable inventions.
Of course, in the case of RSA, a similar algorithm was developed separately by the British government and kept secret for 24 years.
> Given the history of RSA in particular, I'm extremely skeptical of that.
Well then you might want to read about RC4 which only became public after it was leaked. Prior to being leaked, it was RSA's cash cow and one of the most popular encryption algorithms worldwide due to it's speed and the fact that it was exportable (with a 40 bit key).
Indeed, RSA was rather notorious for keeping crypto algorithms as trade secrets (RC2, SecuriID OTP, etc.)
And that patent got invalidated in most of the world anyway.