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In a private scenario (i.e. most B2B usage), you can communicate the key through a different channel. One option would be to write it down on a piece of paper, and fly someone to the other business to deliver it. Now both sides have a copy of the key, and transmission is guarded by your trust in your employee to not skim the key.

Mail is another option, depending on how secure you think that is.

It's much safer to just send the public key over whatever medium and then use an authenticated channel to verify the authenticity of said public key.

How often do you have a reliable tamper resistant authenticated channel that isn't also secret?

I might be confident that bad guys won't realise what's going on and somehow break into my email system replacing your public key with their own in, say, six hours it takes to receive and act on the message; whereas I can't be reasonably confident that all my mail systems and all backups of mail systems will remain unbreakable for the foreseeable future or until I've deleted all trace of the symmetric key.

Cryptography is often not about "uncrackable" but about confidence in security over a foreseen interval.

On the 4th of June 1943 the details of Operation Neptune ("D-Day") were a priceless secret of the Allied Command, a break that allowed Berlin to know the plan could have resulted in the invasion force being expected by a heavily prepared and well-reinforced German defence. Those who made it back to England alive would have no reward for their efforts.

By the 7th of June 1943 plans for Neptune were a historical curiosity of little military value.

Good comment, but I couldn't help notice that you got the date (6/6/1944 != 6/4/1943) wrong for Operation Neptune/Overlord: https://en.wikipedia.org/wiki/Operation_Overlord

D'oh. I swear I had the year correct (it's supposed to be a few days before the invasion because the Germans wouldn't be able to respond instantly) in an earlier draft. Thanks for the correction.

And I did mean Neptune, the Overlord plans would have remained valuable for several weeks as they included details of the immediate objectives in Normandy and how resupply would be done, Neptune was just the invasion and related actions.

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