4 ms·

The case I would be (slightly) concerned about would be where an attacker has obtained limited, read-only access to your database, so they have the current salt

The case I would be (slightly) concerned about would be where an attacker has obtained limited, read-only access to your database, so they have the current salt and the hashes related to the last 30 minutes of activity but not the ability to simply log whatever data they want. At that point brute-forcing the 32-bit address space and a small set of common user agent strings would allow them to determine the IP addresses for specific page views. Of course, if an attacker has access to the salt and is interested in the pages viewed by a specific known IP address then this all becomes much simpler. Then the only unknown is the user agent, which is relatively low-entropy. > So we'd not recycle a single salt, we'd recycle multiple salts that are based on, perhaps, the first 2-3 digits of a users IP address combined with the last 2-3 digits.... If an attacker is already brute forcing the IP address and has the ability to obtain the salt then they would just use the correct salt for each IP address. Making the salt depend on other data already included in the hash doesn't change the size of the search space.

JackWritesCode 7y ago
I think what’s been helpful for us with posting this here is to hear of different ideas for how someone might hack Fathom. When we came up with it, our starting point wasn’t “you have the salt and IP address”, go break a hash. It was on the assumption that you don’t have the salt or IP. I think we can improve what we’ve built. 720 salts improves resilience in a few areas but not in the scenario you are painting here. The scenario you’re painting here has made me think of additional ideas though. If they had the salt, IP and user agent, they’d have to also brute force every possible hostname and pathname, which would be insane. But I suppose they’d only have to do a few million based on the data we have on hostnames / pathnames... Lots of ideas for improvement are popping into my head and I love how this community keeps challenging you to improve things. We had feedback on Reddit but it was much angrier! The next step is to take your feedback and look into how we would defend against the scenario you’ve provided. Thank you!

JackWritesCode 7y ago

I think what’s been helpful for us with posting this here is to hear of different ideas for how someone might hack Fathom. When we came up with it, our starting point wasn’t “you have the salt and IP address”, go break a hash. It was on the assumption that you don’t have the salt or IP. I think we can improve what we’ve built. 720 salts improves resilience in a few areas but not in the scenario you are painting here. The scenario you’re painting here has made me think of additional ideas though. If they had the salt, IP and user agent, they’d have to also brute force every possible hostname and pathname, which would be insane. But I suppose they’d only have to do a few million based on the data we have on hostnames / pathnames... Lots of ideas for improvement are popping into my head and I love how this community keeps challenging you to improve things. We had feedback on Reddit but it was much angrier! The next step is to take your feedback and look into how we would defend against the scenario you’ve provided. Thank you!