How would a public key suffice? Once the media has been published, the publisher could alter it, since they have the private key. I don't mean to encrypt the m

How would a public key suffice? Once the media has been published, the publisher could alter it, since they have the private key. I don't mean to encrypt the media, I mean to make a hash stream of the media stream, and store that in a way that cannot be adulterated.