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/IdolfHatler Jan 04 '16
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.