Eventually, when combined with linguistic models, it was shown possible to even identify individual words to some extent! Later, researchers developed techniques that could determine the language being spoken. Initially, it was shown possible to identify speakers (from a set of candidates). Attacks on these tools have been increasing in sophistication. VBR is also used in many encrypted voice chat programs.It is therefore rather straight-forward to determine which video is being watched, even on an encrypted connection, just by observing the packet lengths. When a user is watching a movie on Netflix or a Youtube video (as opposed to a live event stream), the bit-rate changes are consistent and predictable. In an encrypted video stream, the changes in bit rate are reflected as changes in packet length. When the stream is carrying less information (e.g., a scene with a fixed camera position, or a quiet section of audio), it is encoded at a lower bit rate, meaning that each unit of time is encoded in fewer bits. Variable-bit-rate (VBR) encoding is a common technique in audio/video encoding.1 The same idea applies to auto-complete suggestions in a search form. This can indeed be used to determine the region for which a user is requesting a map. So even though traffic to and from Google maps is encrypted, the sizes of the image tiles are leaked. Every region of the world has its own rather unique “fingerprint” of imagetile lengths. However, they are compressed to save resources, and not all images compress as significantly as others. Each image tile has the same pixel dimensions. When accessing Google maps, your browser receives many image tiles that comprise the map that you see.By observing only the length of encrypted network traffic, many serious attacks are possible. But that doesn’t mean it is consequence-free. If we want to truly support plaintexts of arbitrary length, then leaking the length is in fact unavoidable. We have just gone from requiring encryption to leak no partial information to casually allowing some specific information to leak. In the exercises, you are asked to prove that, with respect to these updated security definitions, CPA$ security implies CPA security as before. We can interpret the resulting security definition to mean that the scheme would hide all partial information about the plaintext apart from its length.įrom now on, when discussing encryption schemes that support variable-length plaintexts, CPA security will refer to the following updated libraries: This would allow the adversary to make the challenge plaintexts differ in any way of his/her choice, except in their length. The easiest way to avoid this situation is to simply insist that | m L| = m R|. But as we just described, it is necessary to modify our very conservative security definitions to allow any leakage at all.įor CPA security, we argued that if a distinguisher queries the CHALLENGE subroutine with plaintexts of different length in the CPA libraries, we cannot expect the libraries to be indistinguishable. Leaking the length of the plaintext (measured in blocks) seems like a rather minor concession. A successful distinguisher would break CPA security simply by choosing m L and m R of different lengths (measured in blocks) and then inspecting the length of the resulting ciphertext. Chapter 9: Block Cipher Modes of Operation Return to Table of Contentsīlock ciphers / PRPs can only act on a single block (element of blen)∗.
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