Thursday, February 20, 2014

DinoSec Challenge 0: Solution and Winners

This article provides details about the solution and winners of "DinoSec Challenge 0" (... and also explains how you can ruin a challenge trying to publish a nice blog post with images that fit on the web page ;)


The original goal of the challenge was to use the three images referenced by the "DinoSec Challenge 0", called "dinosec1.png", "dinosec2.jpg"and "dinosec3.jpg". However, due to the large size of some of these images, and in order to publish a nice looking blog post, I decided to use a reduced version of them for the challenge description, called "dinosec1_blog.png", "dinosec2_blog.jpg"and "dinosec3_blog.jpg", as you can see in the source code of the web page:

Google allows you to search for images by using terms (text) or 'Search By Image' capabilities, via URLs (pointing to image files) or by uploading local image files, both manually or using drag & drop. In the following video you can see how, depending on the web browser you are using, the Google Images search behavior varies. For example, when using Safari, the full URL of the image file pointed by the link (that is, the URL in the "<a href=…>" HTML tag, such as "http://.../dinosec1.png") is loaded into Google Images. As a result, Google cannot find any reference to any of the three images apart from the DinoSec challenge blog post (this was the expected behavior for this challenge). However, when using Chrome or Firefox, the reduced image used for the publication within the blog post (such as "dinosec1_blog.png", 320x212 pixels) is used instead. As a result, for images 1 and 3 Google is capable of finding direct references to the original images (in the case of the first one, even pointing to the original source, 1, 2 & 3) and, therefore, helping to directly and easily solve the challenge:

As a result, I ended up trying to identify the reasons behind the described behavior. When creating that reduced version of the images for publication, I probably used derivatives of the original images, an obviously, none of the SHA1 hashes of the different files match. On the one hand, Google is capable of visually matching image 1 (in PNG format, 320x212 pixels) with the original source image 1 (in JPEG format, 4,288x2,848 pixels). However, image 2 is not found by Google. On the other hand, image 3 (in JPEG format, 320x212 pixels) is also matched with the original image 3 on a third-party website (same format but 4,288x2,848 pixels) and with another JPEG image of a different size (780x518 pixels) on another website. You can read some of the details regarding reverse image and photo search from multiple articles around the web, based on mathematical models, computer vision and machine learning technologies, combined with EXIF metadata and web pages context.

Although this was designed as an introductory challenge, the idea for it was not to be so easily solved, unless you were really lucky searching for dinosaur images through the web. Instead, it was designed to be solved by inspecting the raw images, finding and decoding a few artifacts added on purpose.

Images 2 and 3 ("dinosec2.jpg" and "dinosec3.jpg") are JPEG files with 4,288x2,848 pixels. If you look inside their metadata you can find a couple of messages.

The "IPTC-NAA data (IIM)" metadata section for image 2 contains a "RAW File Info" field with the following obfuscated message: "NTQ0OTUwMzEzYTIwNTc2ODZmMjA2NDY5NjQyMDYzNzI2NTYxNzQ2NTIwNzQ2ODY5NzMyMDYzNjg2
MTZjNmM2NTZlNjc2NTNmMjA0ZDcyMmUyZTJl". If it is decoded as base64 and that output is decoded again as ASCII hex, you get the first ASCII message: "TIP1: Who did create this challenge? Mr...". The image bellow shows the decoding process using Burp Decoder, but other tools or scripting languages can be used to obtain the same result:

The "IPTC-NAA data (IIM)" metadata section for image 3 contains a "RAW File Info" field with the following obfuscated message: "56456c514d6a6f67535851675a57356b63794231634342336158526f4948526f5a53423362334a6b49
434a7a5958567964584d6949446f744b513d3d". If it is decoded as ASCII hex and that output is decoded again as base64, reverting the decoding process for image 2, you get the second ASCII message: "TIP2: It ends up with the word "saurus" :-)". The image bellow shows the decoding process using Burp Decoder, but again, other tools or scripting languages can be used to obtain the same result:

Putting together the answer for both tips you can solve the challenge: Silessaurus. Yes, indeed, it is my oldest ancestor I'm aware of :)

Image 1 ("dinosec1.png") is a JPEG file with 800x531 pixels, that still contains a hidden message. Although the message is not required to solve the challenge, I'm not going to disclose how to obtain it, so that the inquisitive reader can still play around with it :) (tip: stego)


This challenge winners are Juan Manuel Fernández (@TheXC3LL), with a very fast and correct answer using search engines, and @neosysforensics, with a correct technical answer based on decoding the image files metadata. Based on the details and epic fail previously explained, and although initially I thought on having a single winner, I decided it was fair this time to select two winners, one for the first correct answer (even when using the easy path :-), and a second one for the first technical answer. The winners books are on its way, "Apply Security Visualization" and "File System Forensic Analysis", both related somehow to the techniques used to solve the challenge.

Honorable mentions go to other participants like Ricardo, José Manuel, Román, Daniel, and David, that also provided a valid answer by using Google, TinEye (reverse image search engine), or technical analysis using base64 and Python. Thanks for all your submissions!

Lessons Learned

Designing a challenge is always tough, specially finding the right balance between difficulty and affordability. Even for introductory challenges, never underestimate the minor details, manage them as more advanced and complex challenges, and always give a much higher priority to the challenge than to its publication :-) Seriously, in order to solve this first challenge speed of submission was a key factor, especially if you followed the easy path. Thus, in this case the "luck" of been aware of the existence of the challenge influenced a fast and timely response. Therefore, future challenges will be published in advance, with a well known deadline, and all submissions will be evaluated based on their quality, accuracy, creativity, and technical contents.

Besides that, prizes will be announced in advance too, so that you can evaluate your participation based on them. Quite honestly, if you participate because of the prizes and not the enjoyable learning experience, I'm sure you will find wealthier challenges and CtFs out there... although I cannot think of an easiest way of winning a security book than dragging & dropping an image into Google :-D

Follow the @dinosec Twitter account and this blog... and get ready for the next challenge!!

No comments :

Post a Comment