I want to avoid using the term - even though they use it - because it's not a hash function like most people are familiar from computer science. Their "hash" function is resistant to alterations and is more like a "fingerprint".
Edit: to be completely clear, PhotoDNA isn't a cryptographic hash. It's a hash function that maximizes similarity of hashes based on inputs, and is probably closer to a bloom filter in some respects.
Edit: to be completely clear, PhotoDNA isn't a cryptographic hash. It's a hash function that maximizes similarity of hashes based on inputs, and is probably closer to a bloom filter in some respects.