Hacker News Reader

Parent Full thread

jjtheblunt

Cryptographic hashes are exactly not trivial to "dupe".

https://en.wikipedia.org/wiki/Cryptographic_hash_function

that said, it's not clear to me from

https://www.apple.com/child-safety/pdf/Apple_PSI_System_Secu...

how collision resistant what's to be used will be.

View on HN

They are not using cryptographic hashes. They are using perceptual hashes[1] which are fairly trivial to replicate.

[1]: https://en.wikipedia.org/wiki/Perceptual_hashing

This seems dumb. I'm sure that sophisticated bad people will just alter colors and things to defeat the hashes and meanwhile trolls will generate collisions to cause people to falsely be flagged.

The perceptual hashes are specifically designed to correlate images that are visually similar while not being exactly alike, even when the colors are altered. See:

https://blog.cloudflare.com/the-csam-scanning-tool/

Perceptual hashes aren't cryptographic.

Is there anything stopping them from using an actual cryptographic hash, though?

Even the smallest change to an image changes a cryptographic hash.

Ummm… the fact that changing a single pixel will let the baddies evade detection?

Yes, using cryptographic hashes prevents them from fuzzy matching and the algorithms that allow for efficient matching and fuzzy matching require comparing the bare hashes.

Reply on news.ycombinator.com