The only mechanism I can think of that could have been used for that purpose, and was publicly known about (to at least some extent) in the late 1970s, would be RSA. That
is strong crypto, or at least we know it is when used properly today, but it's unlikely the authors of IP would have known about it. Even if they did, the logistical challenges of key distribution would have sunk its use, and they would almost certainly have fallen into one of the traps in implementing it that took years to discover, and the key sizes that would have been practical for use ca 1980 would be easy to break by the end of the 1990s.
Simply put, this isn't a road not taken, it's a road that didn't exist.