Encrypt a photo, keep its thumbnail
Cloud photo services are convenient because they can see your photos. They build thumbnails, previews, and search out of them. That convenience is also the privacy problem. Encrypt everything and the service turns useless, with nothing to scroll. Leave it in the clear and you are trusting the provider with all of it.
Thumbnail-Preserving Encryption threads that needle. The ciphertext is itself an image with the same thumbnail as the original, and it provably leaks nothing beyond that thumbnail. You still recognize your own photos at a glance. The service, and anyone who breaches it, sees only the blur.
I worked on this at Oregon State: the construction, its security analysis, and a prototype that fits existing services, plus a user study on whether people can even tell TPE thumbnails apart. There’s a paper at NDSS 2019, and code and a demo at photoencryption.org.