It is said that what happens on the internet stays on the internet, forever. In some cases this may be considered a feature. Reports of human rights violations and corporate corruption, for example, should remain part of the public record. In other cases, however, digital immortality may be considered less desirable. Most would agree that terror-related content, child sexual abuse material, non-consensual intimate imagery, and dangerous disinformation, to name a few, should not be so easily found online. Neither human moderation nor artificial intelligence is currently able to contend with the spread of harmful content. Perceptual hashing has emerged as a powerful technology to limit the redistribution of multimedia content (including audio, images, and video). We review how this technology works, its advantages and disadvantages, and how it has been deployed on small- to large-scale platforms.
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