Yes it seems crazy, but besides the obvious "leak your own key" as other comments mentioned, this is actually possible. This is one of the biggest discoveries in cryptography in the last decades and its implications are still being researched. I dug around and found this article which seems to do a pretty good job describing the cryptographic concepts of "non-transferability" / "deniability" / "deniable authentication" for a lay audience: https://dinhtta.github.io/zkp/ Also: https://en.wikipedia.org/wiki/Deniable_authentication
I.e. a symmetric key is shared between you and me. If I receive a message with that key, I know it's from you because the key is only known by you and me, and I know I wasn't the sender, so it must be you. But any third party can only say that the message was by one of us.
