Just to be sure this isn’t patentable.

1> Delicately manipulate a digital image to change it’s SHA1 hash and make bitwise comparison impossible. Options include taking a larger image and changing the subset of the image rendered down into a smaller image (i.e. moving the crop lines), adjusting brightness, contrast and other variables, like color balance, by an amount which alters the image but does not make it look all that different, tweaking compression parameters and so forth.

2> Checking a certificate revocation list or other centralized databases without GUIDS. Couple of notions in this direction. Firstly, have the GUID which is being checked on the CRL list be encrypted. Have a list of keys corresponding to persons of interest. If the GUID comes in can be decrypted, then this person is a POI. If it can’t be decrypted, the agency doing the checking has no idea who had their CRL checked.

A second approach is that the GUIDS for a given individual are one-use, but generated in a repeatable fashion (PRNG). So the POI database can be populated with the GUIDS of POIs – all of them – and if an GUID is in the database, a flag is raised.