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This seems like a pretty helpful document, and really shows how carefully the CSAM system has been designed. Two things that stuck out to me: 1) The CSAM hash

by cmsj 5y ago

This seems like a pretty helpful document, and really shows how carefully the CSAM system has been designed. Two things that stuck out to me: 1) The CSAM hash database is encrypted on-device 2) The device doesn't know the results of hash comparisons These suggest to me that the hash comparison is going to happen inside the Secure Enclave - give it the encrypted database and the NeuralHash of an image, it gives you back a Security Voucher to upload. I don't quite get how the hash database can stay secret though - the decryption key has to come from somewhere. AFAIK the hash database has never been made publicly available before, which probably makes it quite a high value target for security researchers and curious parties.

shuckles 5y ago
The hash database is blinded on the server before being embedded into iOS. You’d need to access Apple’s private key or break cryptography to get access to it from the client. For the less cryptographically savvy, the “Technical Summary” is a good document on their Child Safety page.
FabHK 5y ago
> These suggest to me that the hash comparison is going to happen inside the Secure Enclave - give it the encrypted database and the NeuralHash of an image, it gives you back a Security Voucher to upload. My understanding is that the hash comparison, or at any rate the counting of matches, is done in the cloud then. The way the crypto is set up is that the server has no access to the security envelope and its contents (and, after E2EE in the future, to any images) unless the number of matches exceeds the threshold.
nullc 5y ago
No secure enclave is needed, the system uses strong cryptography to protect Apple and their database providers from accountability. I gave a semi-technical description of how private set intersection works here: https://news.ycombinator.com/item?id=28124716 https://news.ycombinator.com/item?id=28124716 Assuming they implement it correctly and there are no apple-internal leaks we won't be discovering the content of the database.
stjohnswarts 5y ago
That's fine, but keep it on the server I don't want this crap on my phone. I don't want to be guilty until proven innocent.

by cmsj 5y ago

shuckles 5y ago

The hash database is blinded on the server before being embedded into iOS. You’d need to access Apple’s private key or break cryptography to get access to it from the client. For the less cryptographically savvy, the “Technical Summary” is a good document on their Child Safety page.

FabHK 5y ago

> These suggest to me that the hash comparison is going to happen inside the Secure Enclave - give it the encrypted database and the NeuralHash of an image, it gives you back a Security Voucher to upload. My understanding is that the hash comparison, or at any rate the counting of matches, is done in the cloud then. The way the crypto is set up is that the server has no access to the security envelope and its contents (and, after E2EE in the future, to any images) unless the number of matches exceeds the threshold.

nullc 5y ago

No secure enclave is needed, the system uses strong cryptography to protect Apple and their database providers from accountability. I gave a semi-technical description of how private set intersection works here: https://news.ycombinator.com/item?id=28124716 https://news.ycombinator.com/item?id=28124716 Assuming they implement it correctly and there are no apple-internal leaks we won't be discovering the content of the database.

stjohnswarts 5y ago

That's fine, but keep it on the server I don't want this crap on my phone. I don't want to be guilty until proven innocent.