Category Archives: protocol

Protocol Cryptanalysis

Time is on my Side – Exploiting Timing Side Channel Vulnerabilities on the Web

Sebastian Schinzel gave an interesting talk today at 28C3, about timing side channel attacks against web applications. (Timing-) Side channel attacks are known in the cryptography world for a long time, and many algorithms like RSA or AES have been successfully attacked. In a nutshell, an attacker measures the time a device needs to process a request (usually an encryption or decryption), and can draw conclusions from that to the values of secret input parameters (a plaintext or a secret key).

(cc)Sebastian showed, that this can be used against none cryptography web applications as well. Instead of just presenting his attacks, he presented general methods how to do timing measurements against web applications first. For example, a web application could perform the following sequence of checks during a user login:

  1. Does the account exist?
  2. Is the account of the user locked?
  3. Has the account expired?
  4. Is the password correct?

If one of these checks fails, the procedure is aborted and an error page is send to the user. Of course, each of these steps requires some time, and from the time it takes from the request to the generation of the error message, one might guess, which of these steps went wrong.

The second attack presented in this talk was a timing attack on an implementation of the XML encryption standard using a PKCS#1.5 padding. Here, the server needs a longer time to process a request, depending on the padding inside the encrypted payload.

For me, my personal highlight was the extension of timing based side channel attacks to none cartographic web applications. I assume, if one would check some famous web applications, many of such timing leaks could be found, because web developers usually don’t care about timing side channels. The timing difference could also be used to assist blind SQL injection attacks, where the timing difference could be the only channel back to the attacker.

Unfortunately, the slides are not (yet) available, but a previous paper describing the methods can be found at http://sebastian-schinzel.de/_download/cosade-2011-extended-abstract.pdf.

Encrypted Traffic Mining (TM) – e.g. Leaks in Skype

Stefan Burschka presented a nice attack against Skype on 28C3. The attack allows you to detect a sentence or a sequence of words in an encrypted Skype call, without having to break the cryptography used in Skype. Skype is a famous internet telephony application, that offers free voice calls. To save bandwidth, Skype uses an advanced audio codec. The used bandwidth and the size of the packets send by Skype depends heavily on the audio, that is encoded. The packet length is no hidden by the encryption of Skype and visible to an eavesdropper.  Just using these information, Burschka could distinguish between difference sentences in an encrypted Skype call.

Two methods were used to improve these results:

  • Dynamic Time Warping (DTW) is an algorithm used by many speech recognition systems, to compare an audio sample with a set of reference samples of slightly different length.
  • A Kalman Filter, which is a method to remove noise from measurements. I think here, noise is a slight variation of the packet lengths.

That the amount of communication between two parties and the actual timing of packets can leak information about the content of an encrypted communication was known for a long time, and has been successfully used for an attack against the TOR onion router network. However, this is the first time, that I have seen this method applied against Skype. The methods here are so generic, that they might be applied as well against other VoIP systems like SIP, if they are encrypted or communicate over a VPN.

The full paper is available at http://www.csee.usf.edu/~labrador/Share/Globecom/DATA/01-038-02.PDF and the slides can be downloaded from http://events.ccc.de/congress/2011/Fahrplan/attachments/1985_CCC.pptx.

802.11 Packets in Packets – Standard-Compliant PHY Exploits

Travis Goodspeed presented a sneaky attack against WiFi networks at 28C3. The idea is simple: Assume we want to inject packets remotely into a wireless network. Assume that there is a user in the network visiting a malicious webpage. How can we trick him into injecting a packet of our choice into his network?

We can use a nice property of many radio protocols: They split their transmission in packets. A static radio preamble and/or sync-field is send at the beginning of a packet, followed by a packet header, payload, and a checksum. (Some protocols also use forward error correction.) The radio preamble is used by a receiver to detect the start of a transmission. A receiver will scan the frequency for a valid radio preamble, and if one was found, receive a packet. The end of a packet can either be determined by a length-field in the header, or some protocols use fixed length packets, so that the length of a packet is known in advance. After the packet has been received, a checksum in the packet can be used to find out, if the transmission has been damaged by radio interference.

So what would happen, if we embed the packet we want to inject, into a HTML-Page or another file, that is transferred to the client. When that file is downloaded, a perfectly valid packet, containing our packet as payout will be transmitted. As long as this packet is received correctly by the client, everything works as normal. However, if the radio preamble of this packet is damaged during the transmission, the receiver will continue to scan for a radio preamble, and start receiving our embedded packet. Of course, the odds that this specific radio preamble is damaged during the transmission might be quiet low, we can easily repeat that packet in the transmission, until it is decoded by a client. This is not a theoretical thing, from time to time, radio preambles of packets are really received incorrectly in practice. As a result, we manage to inject a packet into a WiFi network, that was never send. The packet send is also perfectly standard compliant(, if it is received without any error).

There are some practical problems when implementing this, like switching the transmission speed during a packet or different modulations. However, Travis managed to implement his attack against 802.11b WiFi networks.

The full paper is available at: http://www.usenix.org/events/woot11/tech/final_files/Goodspeed.pdf