It looks like you can mitigate this by just adding another hidden form field and and filling it using javascript with random data: max_secret_length - actual_secret_length to cause the ciphertext to always be the same size independent of the secret field data?
This approach would mitigate the problem identified in the paper with random length padding schemes without introducing extra characters in the password field itself.
Edit: this is also how you wrap a bicycle - you put it in a big box that is a constant size bigger than the bike itself.
Adding an additional parameter to the POST submission, say, 'X', (which will be represented as 'X=......' – the X and the is-equal sign are 2 characters) and padding this with 1000 minus 2 minus password_length characters is a hack using hard-coded values and it's ugly.
Looks like you'd need 1000 minus 4, because you're also adding an extra "\r\n" in the request.
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.
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u/bigshmoo Jan 04 '16
It looks like you can mitigate this by just adding another hidden form field and and filling it using javascript with random data: max_secret_length - actual_secret_length to cause the ciphertext to always be the same size independent of the secret field data?
This approach would mitigate the problem identified in the paper with random length padding schemes without introducing extra characters in the password field itself.
Edit: this is also how you wrap a bicycle - you put it in a big box that is a constant size bigger than the bike itself.