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Apple gets the device-signed image and replaces with a PCC signature to preserve anonymity. > The final reference image is instead signed by Apple’s signing se

Apple gets the device-signed image and replaces with a PCC signature to preserve anonymity. > The final reference image is instead signed by Apple’s signing service, after validation by PCC. So, if compelled, Apple could theoretically tell someone if two images came from the same camera.

lxgr 10d ago
I wonder why they're not doing something like DAA [1], which achieves the same privacy properties without a centralized server. [1] https://en.wikipedia.org/wiki/Direct_Anonymous_Attestation https://en.wikipedia.org/wiki/Direct_Anonymous_Attestation
colejohnson66 10d ago
Still, "the same camera" isn't "this person" without more information.
- cobbzilla 10d ago
  sure but if you can tie “this person” to “this one photo”, then for any photo, Apple can (again, theoretically) identify all photos by that person. it’s one step removed from identity. fwiw i think this is an unambiguous improvement over current post-sensor attestations, it’s just good to explore the edges
alwillis 10d ago
> So, if compelled, Apple could theoretically tell someone if two images came from the same camera. No they couldn't. If you generate two SSH key pairs on your laptop, there's no way to confirm they were created on the same machine. There's no device identifying data in a reference image, which is the point. The factory signature, the image sensor key, the Secure Enclave Processor key and all of the signing that takes place on PCC are all device-agnostic. The reference image is processed and eventually signed by Private Cloud Compute's post-quantum signature using a hybrid MLDSA87-RSA-3072-PSS-SHA512 scheme. So… it's not possible for Apple to know if two images came from the same iPhone.
- cobbzilla 10d ago
  I stand corrected. If it’s truly only the signatures and zero other identifying info, then yes you’re as secure as the underlying algorithm. I think they implied there are multiple device-originated signatures for different parts of the data, this may open some possibilities for cryptanalysis
- microtonal 9d ago
  I am not sure I follow. The private keys in the image sensor and the SEP are static, so you can see that two images are signed with the same private key. Apple 'decorrelates' this by letting PCC verify the signature and then replacing it by their own signature: When the user initiates developing a reference image, the device uploads the secure digital negative to Private Cloud Compute. PCC recomputes the digest embedded in the frame and verifies the sensor's signature over the pixels and that digest, verifying the certificate chain back to the sensor CA. PCC also verifies the SEP signature and chains it to the BAA CA, and it verifies the signature on the device manifest and chains it to the CA that signs device manifests at the factory. It then confirms that the sensor and SEP named in those chains belong to the same device. [...] If these checks pass, PCC then submits the commitment to our signing service, which signs it with a composite post-quantum signature using a hybrid MLDSA87-RSA-3072-PSS-SHA512 scheme. The signature is embedded in the JPEG, and the reference image is returned to the device, which associates it with the main photo from the original capture. After the secure digital negative is successfully developed, it's automatically moved to the deleted photos folder." So in the end it all depends on how much you trust Apple's cloud and PCC nodes. If there is a weakness in their services, Apple could record both the original signatures and their signature, and could prove whether two photos were made using the same lens/device and they could even trace it back to a specific device (by looking up the original signature + signing identity given their signature).

lxgr 10d ago

I wonder why they're not doing something like DAA [1], which achieves the same privacy properties without a centralized server. [1] https://en.wikipedia.org/wiki/Direct_Anonymous_Attestation https://en.wikipedia.org/wiki/Direct_Anonymous_Attestation

colejohnson66 10d ago

Still, "the same camera" isn't "this person" without more information.

cobbzilla 10d ago
sure but if you can tie “this person” to “this one photo”, then for any photo, Apple can (again, theoretically) identify all photos by that person. it’s one step removed from identity. fwiw i think this is an unambiguous improvement over current post-sensor attestations, it’s just good to explore the edges

alwillis 10d ago

> So, if compelled, Apple could theoretically tell someone if two images came from the same camera. No they couldn't. If you generate two SSH key pairs on your laptop, there's no way to confirm they were created on the same machine. There's no device identifying data in a reference image, which is the point. The factory signature, the image sensor key, the Secure Enclave Processor key and all of the signing that takes place on PCC are all device-agnostic. The reference image is processed and eventually signed by Private Cloud Compute's post-quantum signature using a hybrid MLDSA87-RSA-3072-PSS-SHA512 scheme. So… it's not possible for Apple to know if two images came from the same iPhone.

cobbzilla 10d ago
I stand corrected. If it’s truly only the signatures and zero other identifying info, then yes you’re as secure as the underlying algorithm. I think they implied there are multiple device-originated signatures for different parts of the data, this may open some possibilities for cryptanalysis
microtonal 9d ago
I am not sure I follow. The private keys in the image sensor and the SEP are static, so you can see that two images are signed with the same private key. Apple 'decorrelates' this by letting PCC verify the signature and then replacing it by their own signature: When the user initiates developing a reference image, the device uploads the secure digital negative to Private Cloud Compute. PCC recomputes the digest embedded in the frame and verifies the sensor's signature over the pixels and that digest, verifying the certificate chain back to the sensor CA. PCC also verifies the SEP signature and chains it to the BAA CA, and it verifies the signature on the device manifest and chains it to the CA that signs device manifests at the factory. It then confirms that the sensor and SEP named in those chains belong to the same device. [...] If these checks pass, PCC then submits the commitment to our signing service, which signs it with a composite post-quantum signature using a hybrid MLDSA87-RSA-3072-PSS-SHA512 scheme. The signature is embedded in the JPEG, and the reference image is returned to the device, which associates it with the main photo from the original capture. After the secure digital negative is successfully developed, it's automatically moved to the deleted photos folder." So in the end it all depends on how much you trust Apple's cloud and PCC nodes. If there is a weakness in their services, Apple could record both the original signatures and their signature, and could prove whether two photos were made using the same lens/device and they could even trace it back to a specific device (by looking up the original signature + signing identity given their signature).