5 ms·

I'm trying to figure out how my understanding of the paper went so horribly awry, but I'm just not seeing it. From the paper: The first family of attacks, ter

I'm trying to figure out how my understanding of the paper went so horribly awry, but I'm just not seeing it. From the paper: The first family of attacks, termed synchronous attacks, is applicable in scenarios where the plaintext or ciphertext is known and the attacker can operate synchronously with the encryption on the same processor, by using (or eavesdropping upon) some interface that triggers encryption under an unknown key. For example, a Virtual Private Network may allow an unprivileged user to send data packets through a secure channel. This lets the user trigger encryption of plaintexts that are mostly known (up to some uncertainties in the packet headers), and our attack would thus, under some circumstances, enable any such user to discover the key used by the VPN to protect all users’ packets. As another example, consider the Linux dm-crypt and cryptoloop services. These allow the administrator to create a virtual device which provides encrypted storage into an underlying physical device, and typically a normal filesystem is mounted on top of the virtual device. If a user has write permissions to any file on that filesystem, he can use it to trigger encryptions of known plaintext, and using our attack he is subsequently able to discover the encryption key used for the underlying device. We have experimentally demonstrated the latter attack, and showed it to reliably extract the full AES key using about 65ms of measurements (involving just 800 write operations) followed by 3 seconds of analysis. Questions: 1) In that example, the attacker "discovered the key used by the VPN to protect all users' packets." Does that mean the VPN reused the same AES key for each encryption? 2) If so, then if the VPN were to change the encryption key each time it encrypted a packet, wouldn't that protect against this attack? The attacker had to be able to trigger 800 encryptions that all used the same encryption key, right? Or does "write operation" refer to something other than "the attacker triggered an encryption of some known plaintext"?

tptacek 13y ago
The problem Osvik and Tromer are pointing out isn't that the VPN doesn't rekey; it's that the AES implementation it uses can't prevent a localhost attacker from learning its secrets by watching the footprints it leaves in the cache. There is no reasonable solution to that problem that doesn't involve eliminating the side channel. The problem isn't the key management; it's the broken AES deployment.
- sillysaurus2 13y ago
  What I think you're saying is, "the paper's point is that side channel attacks can be used to determine the key." I understand that... that's the whole point of the paper, after all. But wouldn't rekeying defend against this attack? And therefore how is what I originally said wrong (and hence how is it safe practice to reuse AES keys in general)? (I'm only trying to understand this.)
  - ghshephard 13y ago
    If you have a side channel attack then a single use of the key would be sufficient to break it-even a OTP would be vulnerable.
    - sillysaurus2 13y ago
      Then what does "800 write operations" refer to, if not 800 encryptions? How can a side channel be used to break a single encryption? Also, how would a one time pad be vulnerable to any side channel attack? That seems like it makes no sense. What am I missing here?
      - ghshephard 13y ago
        http://en.wikipedia.org/wiki/Side_channel_attack http://en.wikipedia.org/wiki/Side_channel_attack does a really good job Of describing it, but, in short, a side channel attack isn't a weakness in the algorithm - both a OTP and AES are strong. Rather, the side channel attack describes information leaked in a particular implementation. If your OTP implementation left some registers or cache lines in a state from which key information could be gleaned, then the attacker would be able to decrypt your message. Side channel attacks are about weaknesses in implementations, not algorithms.
        sillysaurus2 13y ago
        Yes, but it makes no sense that a one time pad would leak key information via cache timing, because the entire operation is just 'message xor key'." With a one time pad, the key's length has to be the length of the plaintext. There's no way for the key to be leaked through cache timing, because no memory location is ever read twice. The entire operation is simply: read a byte of plaintext; read a byte of key; XOR the two, writing the resulting byte" It makes no sense to use cache timing attacks against one time pads, because the speed of memory access is independent of the key and plaintext! If you have a side channel attack then a single use of the key would be sufficient to break it-even a OTP would be vulnerable. Let me try to put it another way: as far as I know, there are no reasonable side channel attacks against one time pad encryption, and certainly no cache timing attacks. So I don't understand why you're saying OTP is vulnerable to side channel attacks. Also, how is it possible for a single use of the key to be vulnerable to a timing-based side channel attack? If I'm wrong, please point out precisely what I've said that's wrong, so that I can learn from my mistake. I'll reword my original questions for clarity: does "800 write operations" mean "the attacker initiated 800 separate encryption operations, each using the same encryption key X, then analyzed the cache timing in order to infer X"? If so, doesn't that mean this attack can be defended against by changing the AES key for each encryption operation? And if that's correct, then by definition isn't it less safe (if not unsafe) to reuse your AES key?

tptacek 13y ago

The problem Osvik and Tromer are pointing out isn't that the VPN doesn't rekey; it's that the AES implementation it uses can't prevent a localhost attacker from learning its secrets by watching the footprints it leaves in the cache. There is no reasonable solution to that problem that doesn't involve eliminating the side channel. The problem isn't the key management; it's the broken AES deployment.

sillysaurus2 13y ago
What I think you're saying is, "the paper's point is that side channel attacks can be used to determine the key." I understand that... that's the whole point of the paper, after all. But wouldn't rekeying defend against this attack? And therefore how is what I originally said wrong (and hence how is it safe practice to reuse AES keys in general)? (I'm only trying to understand this.)
- ghshephard 13y ago
  If you have a side channel attack then a single use of the key would be sufficient to break it-even a OTP would be vulnerable.
  - sillysaurus2 13y ago
    Then what does "800 write operations" refer to, if not 800 encryptions? How can a side channel be used to break a single encryption? Also, how would a one time pad be vulnerable to any side channel attack? That seems like it makes no sense. What am I missing here?
    - ghshephard 13y ago
      http://en.wikipedia.org/wiki/Side_channel_attack http://en.wikipedia.org/wiki/Side_channel_attack does a really good job Of describing it, but, in short, a side channel attack isn't a weakness in the algorithm - both a OTP and AES are strong. Rather, the side channel attack describes information leaked in a particular implementation. If your OTP implementation left some registers or cache lines in a state from which key information could be gleaned, then the attacker would be able to decrypt your message. Side channel attacks are about weaknesses in implementations, not algorithms.
      - sillysaurus2 13y ago
        Yes, but it makes no sense that a one time pad would leak key information via cache timing, because the entire operation is just 'message xor key'." With a one time pad, the key's length has to be the length of the plaintext. There's no way for the key to be leaked through cache timing, because no memory location is ever read twice. The entire operation is simply: read a byte of plaintext; read a byte of key; XOR the two, writing the resulting byte" It makes no sense to use cache timing attacks against one time pads, because the speed of memory access is independent of the key and plaintext! If you have a side channel attack then a single use of the key would be sufficient to break it-even a OTP would be vulnerable. Let me try to put it another way: as far as I know, there are no reasonable side channel attacks against one time pad encryption, and certainly no cache timing attacks. So I don't understand why you're saying OTP is vulnerable to side channel attacks. Also, how is it possible for a single use of the key to be vulnerable to a timing-based side channel attack? If I'm wrong, please point out precisely what I've said that's wrong, so that I can learn from my mistake. I'll reword my original questions for clarity: does "800 write operations" mean "the attacker initiated 800 separate encryption operations, each using the same encryption key X, then analyzed the cache timing in order to infer X"? If so, doesn't that mean this attack can be defended against by changing the AES key for each encryption operation? And if that's correct, then by definition isn't it less safe (if not unsafe) to reuse your AES key?