I recall a court case where a police officer was convicted of possessing CSAM on his computer. The files were encrypted so the prosecution was not able to view

I recall a court case where a police officer was convicted of possessing CSAM on his computer. The files were encrypted so the prosecution was not able to view them, but they were able to demonstrate that the encrypted files contained known images, I believe by encrypting a known image with the same public key and demonstrating that the result was binary identical. Obviously this wouldn't work except for unmodified files including cropping, scaling and editing metadata.