Perceptual Hashing

Intentionally maximizes the change of hash collision to detect similar images. Used by Google's Search by Image feature or Apple's CSAM. Perceptual Hashing

Algorithms and Their Performances

Zauner

• Fastest: block-mean-based perceptual image hash.
• Most discriminative: Marr-Hildreth operator-based hash function

Vulnerabilities

It's Not What It Looks Like: Manipulating Perceptual Hashing-based Applications

• TLDR: A delicately designed image can fool a perceptual hash application, making it classify two completely different images as the same.