An app could add a unique key signature as an invisible watermark to any picture captured by a user. That key could contain various meta-data, such as an ID of the app that created it, the date and time of creation, a unique user ID, etc. Before being embedded in the image, it would first be salted and hashed like any password should be, which guarantees that no software could spoof a particular key or reverse engineer the data it contains from the image containing it.

It could then be checked against a hash lookup by the app that created it (using a stored private key) to verify the authenticity of an image.

This could prove that a picture used as evidence in a court case is not a deepfake, though it does nothing to prevent people from creating deepfakes that do not have such watermarks.

But if this became ubiquitous, it could effectively stop deepfakes from being mistaken for authentic pictures. If every camera app automatically watermarked every photo it takes with a unique hashed key, then any photo taken by a particular app could be verified against some database somewhere containing the private key used to decrypt the key in the watermark.