Given the information that it was Ralph Merkle, that it was about his field (=cryptography), that it was intended to be a general overview (so about cryptography broadly construed and not a single result or breakthrough), aimed at outsiders (so little math), it was highly cited (more than most of his papers), and no co-author is mentioned (and you wouldn't expect one for an 'explainer' like that), you should be able to make a good guess with a few seconds in Google Scholar after sorting his papers by citation-count and skimming the titles & summaries of each paper: https://scholar.google.com/scholar?start=10&q=author:%22ralph+merkle%22&hl=en&as_sdt=0,21
My guess would be that it's hit #8 on page 1, "Secure communications over insecure channels", Merkle 1978, written in 1975 simultaneously with his breakthrough work on public-key crypto, which indeed would need to be explained to a lot of people at the time. It's written in a very conversational tone, with historical background and discussion of practical issues and solutions like sending secret keys in the mail, extremely few equations or math (but imperative program pseudocode instead), published in a more general interest publication than the usual cryptography journals, and despite not presenting any new results & being ancient, is still apparently his 8th most cited paper ever out of 162 hits for him as author.
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Followup to: Illusion of Transparency: Why No One Understands You, Expecting Short Inferential Distances
A few years ago, an eminent scientist once told me how he'd written an explanation of his field aimed at a much lower technical level than usual. He had thought it would be useful to academics outside the field, or even reporters. This ended up being one of his most popular papers within his field, cited more often than anything else he'd written.
The lesson was not that his fellow scientists were stupid, but that we tend to enormously underestimate the effort required to properly explain things.
He told me this, because I'd just told him about my experience publishing "An Intuitive Explanation of Bayesian Reasoning". This is still one of my most popular, most blogged, and most appreciated works today. I regularly get fan mail from formerly confused undergraduates taking statistics classes, and journalists, and professors from outside fields. In short, I successfully hit the audience the eminent scientist had thought he was aiming for.
I'd thought I was aiming for elementary school.
Today, when I look back at the Intuitive Explanation, it seems pretty silly as an attempt on grade school:
(Then again, I get a roughly equal number of complaints that the Intuitive Explanation is too long and drawn-out, as that it is too short. The current version does seem to be "just right" for a fair number of people.)
Explainers shoot way, way higher than they think they're aiming, thanks to the illusion of transparency and self-anchoring. We miss the mark by several major grades of expertise. Aiming for outside academics gets you an article that will be popular among specialists in your field. Aiming at grade school (admittedly, naively so) will hit undergraduates. This is not because your audience is more stupid than you think, but because your words are far less helpful than you think. You're way way overshooting the target. Aim several major gradations lower, and you may hit your mark.
PS: I know and do confess that I need to work on taking my own advice.
Addendum: With his gracious permission: The eminent scientist was Ralph Merkle.