3 ms·

Ciphertext leaks information via its size and its presence / absence.

by exo762 4y ago

dylan604 4y ago
yeah and so? how is this information useful? if the actual useful information is encrypted, then size is meaningless as it is just random text until decrypted. presence/absence? you have a cookie or you don't?
- recursive 4y ago
  You say the size is meaningless. But sometimes it may actually be meaningful. And presence/absence is information also. Not much, but it definitely exists.
  - dylan604 4y ago
    In what way? If I'm storing 255 bytes of username/email/credentials/settings/etc, but pad in some fun way to 512 bytes prior to encryption, what in the world does knowing that the cookie is 512 bytes do you? You have no idea what data is stored. You don't know if there are 512 bytes of actual data, 128 bytes with lots of fluff, or anything at all other than 512 bytes. Hell, it could be 0 bytes of data and 512 bytes of random data that I submit to every single person that visits the site just to fuck with people like you! You seem like you need something to do ;-)
    - recursive 4y ago
      If the contents of the cookie is a JSON array of recently viewed items, then the size is correlated to whether I've been actively viewing items recently. Adding random padding makes it harder to get a signal, but with a high enough sample size, it's still possible to get some information. If you always pad to a fixed size, then there's probably no useful information. At the moment, I think I have enough to do, but I'll probably be looking for something new in the next few weeks. Got any novel single player video game recommendations?
      - dylan604 4y ago
        >If That's are really big word in that sentence. You have NO idea, like 0, what is stored in an encrypted cookie. To even think you do is just pure folly.
        recursive 4y ago
        I do have ideas. I can strengthen them by reverse-engineering the behavior using my own account. I participate in follies regularly, so that's not a strong disincentive for me.
        vntok 4y ago
        This is a very simplistic way of looking at things. A lot of the times in security, having an idea of what's not in there can be almost as valuable as having an idea of what's in there. I will provide a very simple counterpoint to your argument: say I log in, log out and log in again, dumping my two encrypted session cookies in between. If they are perfectly identical, then knowing nothing else about the environment nor the encryption used, I already know that there's no timestamp value in the cookie acting as a time barrier, nor a challenge-response check, nor a nonce value. Knowing that, I can focus my attention on stealing session cookies from users for trying replay attacks.

Ciphertext leaks information via its size and its presence / absence.

by exo762 4y ago

Ciphertext leaks information via its size and its presence / absence.

yeah and so? how is this information useful? if the actual useful information is encrypted, then size is meaningless as it is just random text until decrypted. presence/absence? you have a cookie or you don't?

recursive 4y ago
You say the size is meaningless. But sometimes it may actually be meaningful. And presence/absence is information also. Not much, but it definitely exists.
- dylan604 4y ago
  In what way? If I'm storing 255 bytes of username/email/credentials/settings/etc, but pad in some fun way to 512 bytes prior to encryption, what in the world does knowing that the cookie is 512 bytes do you? You have no idea what data is stored. You don't know if there are 512 bytes of actual data, 128 bytes with lots of fluff, or anything at all other than 512 bytes. Hell, it could be 0 bytes of data and 512 bytes of random data that I submit to every single person that visits the site just to fuck with people like you! You seem like you need something to do ;-)
  - recursive 4y ago
    If the contents of the cookie is a JSON array of recently viewed items, then the size is correlated to whether I've been actively viewing items recently. Adding random padding makes it harder to get a signal, but with a high enough sample size, it's still possible to get some information. If you always pad to a fixed size, then there's probably no useful information. At the moment, I think I have enough to do, but I'll probably be looking for something new in the next few weeks. Got any novel single player video game recommendations?
    - dylan604 4y ago
      >If That's are really big word in that sentence. You have NO idea, like 0, what is stored in an encrypted cookie. To even think you do is just pure folly.
      - recursive 4y ago
        I do have ideas. I can strengthen them by reverse-engineering the behavior using my own account. I participate in follies regularly, so that's not a strong disincentive for me.
      - vntok 4y ago
        This is a very simplistic way of looking at things. A lot of the times in security, having an idea of what's not in there can be almost as valuable as having an idea of what's in there. I will provide a very simple counterpoint to your argument: say I log in, log out and log in again, dumping my two encrypted session cookies in between. If they are perfectly identical, then knowing nothing else about the environment nor the encryption used, I already know that there's no timestamp value in the cookie acting as a time barrier, nor a challenge-response check, nor a nonce value. Knowing that, I can focus my attention on stealing session cookies from users for trying replay attacks.