28c3: Implementation of MITM Attack on HDCP-Secured Links
A system has been described that enables a man-in-the-middle attack upon HDCP secured links. The attack enables the overlay of video upon existing streams; an example of an application of the attack is the overlay of a personalized twitter feed over video programs. The attack relies upon the HDCP master key and a snooping mechanism implemented using an FPGA. The implementation of the attack never decrypts previously encrypted video, and it is incapable of operating without an existing, valid HDCP link. It is thus an embodiment of a bona-fide, non-infringing and commercially useful application of the HDCP master key. This embodiment impairs the equating of the HDCP master key with copyright circumvention purposes.
Implementation of MITM Attack on HDCP-Secured Links
Today, I gave a talk on an implementation of a man in the middle (MITM) attack on HDCP-secured video links. Here is a full copy of the slides that I presented (with explanatory diagrams), as well as the text-only of the paper which accompanies the slides, below.
Also, please note that the hardware disclosed in this talk is now available for purchase from the good folks at Adafruit. You can find more technical documentation about the NeTV at the kosagi.com wiki, and you can discuss at the kosagi.com forum.
A man-in-the-middle attack on HDCP-secured video links is demonstrated. The attack is implemented on an embedded Linux platform, with the help of a Spartan-6 FPGA, and is capable of operating real-time on HD video links. It utilizes the HDCP master key to derive the corresponding private keys of the video source and sink through observation and computation upon the exchanged public keys. The man-in-the-middle then genlocks its raster and cipher state to the incoming video stream, enabling it to do pixel by pixel swapping of encrypted data. Since the link does no CRC or hash verification of the data, one is able to forge video using this method.
Significantly, the attack enables forging of video data without decrypting original video data, so executing the attack does not constitute copyright circumvention. Therefore, this novel and commercially useful application of the HDCP master key impairs equating, in a legal sense, the master key with circumvention. Finally, the embodiment of the exploit is entirely open-source, including the hardware and the Verilog implementation of the FPGA.
BACKGROUND & CONTEXT
In September 2010, the HDCP master key was circulated via Pastebin. Speculation ensued around the application of the master key to create HDCP strippers, which would enable the circumvention of certain copyright control mechanisms put in place around video links. Unfortunately, this is a legally risky application, for a number of reasons, including potential conflicts with DMCA legislation that criminalizes the circumvention of copyright control mechanisms.
This talk discloses a new use for the HDCP master key that side-steps some of the potential legal issues. This hack never decrypts video; without decryption, there is no circumvention, and as a result the DMCA cannot apply to this hack. Significantly, by demonstrating a bona-fide commercially significant purpose for the HDCP master key that does not circumvent an access control measure, this hack impairs the equating of trafficking or possession of the HDCP master key to circumvention and/or circumvention-related crimes.
The main purpose of this hack is to enable the overlay of video content onto an HDCP encrypted stream. The simple fact that a trivial video overlay becomes an interesting topic is illustrative of the distortion of traditional rights and freedoms brought about by the DMCA. While the creation of derivative works of video through dynamic compositing and overlay (such as picture in picture) seems intuitively legal and natural in a pre-HDCP world, the introduction of HDCP made it difficult to build such in-line equipment. The putative purpose role of HDCP in the digital video ecosystem is to patch the plaintext-hole in the transmission of otherwise encrypted video from shiny disks (DVDs, BDs) to the glass (LCD, CRT). Since the implementation of video overlay would typically require manipulation of plaintext by intermediate processing elements, or at least the buffering of a plaintext frame where it can be vulnerable to readout, the creation of such devices has generally been very difficult to get past the body that controls the granting of HDCP keys, for fear that they can be hacked and/or repurposed to build an HDCP stripper. Also, while a manufacturer could implement such a feature without the controlling body’s blessing, they would have to live in constant fear that their device keys would be revoked.
While the applications of video overlay are numerous, the basic scenario is that while you may be enjoying content X, you would also like to be aware of content Y. To combine the two together would require a video overlay mechanism. Since video overlay mechanisms are effectively banned by the HDCP controlling organization, consumers are slaves to the video producers and distribution networks, because consumers have not been empowered to remix video at the consumption point.
The specific implementation of this hack enables the overlay of a WebKit browser over any video feed; a concrete example of the capability enabled by this technology is the overlay of twitter feeds as “news crawlers” across a TV program, so that one may watch community commentary in real-time on the same screen. While some TV programs have attempted to incorporate twitter feeds into the show, the incorporation has always been on the source side, and as such users are unable to pick their hashtags. Now, with this hack, the same broadcast program (say, a political debate) can have a very different viewing experience based on which hashtag is keyed into the viewer’s twitter crawler.
TECHNICAL IMPLEMENTATIONRead More...
Very interesting stuff. I figured it was just a matter of time...
- Overlaying video on encrypted HDMI connections
- Overlaying video on encrypted HDMI connections - Hack a Day
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