{"id":41756,"date":"2024-01-11T14:22:58","date_gmt":"2024-01-11T19:22:58","guid":{"rendered":"https:\/\/mjtsai.com\/blog\/?p=41756"},"modified":"2024-01-30T11:02:11","modified_gmt":"2024-01-30T16:02:11","slug":"airdrop-log-dehashing","status":"publish","type":"post","link":"https:\/\/mjtsai.com\/blog\/2024\/01\/11\/airdrop-log-dehashing\/","title":{"rendered":"AirDrop Log Dehashing"},"content":{"rendered":"

Lawrence Abrams<\/a> (MacRumors<\/a>):<\/p>\n

A Chinese state-backed research institute claims to have discovered how to decrypt device logs for Apple’s AirDrop feature, allowing the government to identify phone numbers or email addresses of those who shared content.<\/p>

[…]<\/p>

To get around censorship in the country, people turned to Apple’s AirDrop feature, which doesn’t require cellular service and uses Bluetooth and a private Wi-Fi network to send images and photos between devices.<\/p>

[…]<\/p>

The research institute says the sender’s device name, email address, and mobile phone number are hashed in the iOS device logs.<\/p>

Using rainbow tables, the researchers claim to have been able to dehash these fields to gain access to the sender’s information.<\/p><\/blockquote>\n

It’s not clear to me why this information needs to be logged at all, nor why the government would want to alert Apple to fix this.<\/p>\n\n

Matthew Green<\/a>:<\/p>\n

\n

To make a long story short: a Private Set Intersection protocol takes a set of strings from the Sender and a set from the Receiver. It gives one (or both parties) the intersection <\/em>of both sets: that is, the set of entries that appear on both<\/span> lists. Most critically, a good PSI protocol doesn’t reveal any other information about either of the sets.<\/em><\/p>\n

[…]<\/p>\n

For a variety of mildly defensible reasons — which I will come back to in a moment — Apple does not use<\/em> a secure PSI protocol to solve their AirDrop problem. Instead they did the thing that every software developer does when faced with the choice of doing complicated cryptography or “hacking something together in time for the next ship date”: they threw together their own solution using hash functions.<\/p>\n

[…]<\/p>\n

A second important issue here is that the hash identifiers are apparently stored in logs within the recipient’s phone, which means that to obtain them you don’t have to be physically present when the transfer happens. You can potentially scoop them out of someone else’s phone after the fact.<\/p>\n

[…]<\/p>\n

Hence there is a legitimate question about whether it’s politically wise for Apple to make a big technical improvement to their AirDrop privacy, right at the moment that the lack of privacy<\/em> is being viewed as an asset by authorities in China. Even if this attack isn’t really that critical to law enforcement within China, the decision to “fix” it could very well be seen as a slap in the face. <\/p>\n<\/blockquote>\n\n

Previously:<\/p>\n