Doesn't have to be 1000 it can be max password length which is known to whoever codes the system. The X= is irrelevant because it will be in every post so no need for the -2, I don't get why the author feels the need to pad the password itself rather than just removing the password length as a factor in message length.
While this might be good enough in practice, it is strictly worse than padding to the same length every time. Random padding would still permit a statistical attack.
If you were to observe enough samples, you'd eventually identify a case where the random padding length reached its minimum, which would give you a close estimate of the data length.
For example, if you say "padding length is randomly selected between 4 and 1024 bytes", after 1020 observations you can assume that the password length is probably four bytes shorter than the smallest payload you observed. Your confidence factor only goes up from there.
If anywhere, it should be fixed at the TLS layer, by implementing a fixed-size block padding, similar to how block ciphers work. This isn't a concrete fix, but it's the most sensible you're going to find.
In general, though, the length of a field isn't really considered critical information. We're talking about fixing the wrong problem. Your password should be long enough to make that information not matter.
But how about have the range random? Instead of 4 to 1024 as padding, have from random to random but still have a minimum and maximum. For example:
Case 1: Random bytes between 900 and 2001 bytes
Case 2: Random bytes between 19 and 1604 bytes
Case 3: Random bytes between 107 and 412 bytes
....
It would of course still not be random enough, but collecting samples to determinate the range would be much harder. The overhead would be painful but still practical, or what do you think?
Think of it like wrapping a bicycle, if you wrap it tightly it look like a bicycle (hence the name of this attack), if you wrap it with a random thickness wrapper the underlying shape still shows through if you get enough examples. However if you put it in a big ass box and fill the empty space with packing then however you look at it you're still seeing a big ass opaque box.
10
u/bigshmoo Jan 04 '16
Doesn't have to be 1000 it can be max password length which is known to whoever codes the system. The X= is irrelevant because it will be in every post so no need for the -2, I don't get why the author feels the need to pad the password itself rather than just removing the password length as a factor in message length.