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Comment by jsjohnst | Hacker News Reader

Can you comment on how this compares to PhotoDNA? I looked at the readme and was surprised to not see mention of it.

Great question! PhotoDNA is the most well-known and supported hash function for this use case. And we do support hashing and matching with PhotoDNA in the Safer product. However, the PhotoDNA hash function is non-public so we cannot include it in an open source package. We support pHash as an open source alternative so that companies without PhotoDNA licenses can get started with hashing.

It’s been a long time since I had to implement PhotoDNA (helped write a PHP native version of the PhotoDNA hash function at Tumblr), can you indicate if pHash creates compatible hashes to PhotoDNA’s output?

Cool (re: writing a PHP version)! Generally speaking, hashes from different hash algorithms cannot be used with each other.

By the way, would be glad to connect and chat, especially if you have any thoughts on pain points associated with perceptual hashing (email in profile).

Does NCMEC support collection and distribution of hashes for alternative functions these days? Six years ago they only supported PhotoDNA hashes (hence why we ported Microsoft’s version of the hash function).

My email is in my profile too, feel free to reach out if I forget.

> hashes from different hash algorithms cannot be used with each other

That’s what I thought, but your mention of pHash alluded to it maybe being an open source drop in replacement for PhotoDNA, so I wanted to clarify instead of assuming.

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