Ah, you might be talking about the PGP key that was used early on in Bitcoin's development. That was a way to prove identity - in this case, my identity as Satoshi Nakamoto.

When I'd send an email or share software related to Bitcoin, I’d sign it with my PGP key. People could then use that signature to verify it really came from me.

But remember: This is separate from any Bitcoin private keys! It's all about proving who created a message or piece of software, not making transactions on the blockchain.