I Smell Packets

Here is the solution to the hard version of the Crypto Kitchen Packet Challenge from Alec R Waters (@alecrwaters on twitter). There was some misdirection in this version and I think it threw a lot of people for a loop. Check this out.

Alec writes:

The first step is to load the capture into something like Wireshark for a quick look. The capture contains just one TCP session showing an email being sent from a mail client to a mail server for delivery. The easiest way to see everything that’s going on is to right-click on any row in the capture and say “Follow TCP Stream”. This will show you all of the printable characters in the TCP segment payloads, stripping off the headers:

We can see the SMTP server accepting an email from really.hungry@i.eat.packets addressed to newrecipe@recipes.on.line. The text that follows “354 OK, send” is a multi-part MIME formatted message.

The Content-Type header field tells us that it’s a multipart/mixed message, which is a way of sending several items of different types in a single email (for example, a plaintext message with an attachment). The content-type header also specifies a boundary, which is how the receiving mail client will know where one part stops and the next one begins.

As we look down the message, we can see the first instance of the boundary string (it’s hard to tell by eye, but it’s actually prefixed with two additional dashes). Each message part has its own header and body, and we can see that we’re looking of something of type text/plain – this is likely the body text of the email.

Looking further, we see the boundary again followed by another header. This time, we’re dealing with what appears to be a base64 encoded JPEG called “SecretIngredient.jpg”.

Finally, at the end of the message, we see the boundary for the last time. In this instance, it is both prefixed and suffixed with –, which tells the receiving mail client that this is the end of the message.

So, what the capture is showing us is the transmission of an email with a plaintext body and an attached image.

The object of the challenge was to discover the secret ingredient – everything needed to work out what it is is in the email, hidden in plain sight – all we have to do is look hard enough!

The first thing to notice is amongst the list of ingredients. The recipe calls for:

“30 ml Balsamic vigenere”

“Vigenere”, huh? That’s not an ingredient, that’s a cipher!

Now we have to look for something that could be ciphertext. There don’t seem to be any likely candidates in the body or header of the email, so let’s turn our attention to the attached image.

A quick-and-dirty way to extract it is to highlight all of the base64 encoded data and paste it into an online base64 decoder, like this one:

http://www.opinionatedgeek.com/dotnet/tools/Base64Decode/

Hit the “Decode” button, and you’ll download a file called DecodedBase64.bin. As we’ve already examined the structure of the raw email, we think that this is actually a file called SecretIngredient.jpg, and we can add weight to that with the file command:

thanatos:~# file DecodedBase64.bin
DecodedBase64.bin: JPEG image data, JFIF standard 1.01

Once we’ve satisfied ourselves that this isn’t a hostile JPEG that’s going to exploit a flaw in our imaging software, we can go ahead and load it up. It looks like this:

Hmm. There’s some swirly text here, hopefully telling us what we’re after. “Swirl” effects are often reversible, albeit with a degree of loss. If we load the image into an editor that can do swirls, we can try to reverse it. I used Paint.NET on Windows (http://www.getpaint.net/index.html), but I imagine there are many others that can do this.

First, highlight the swirled part of the image with the rectangle select tool:

(I’ve zoomed in a bit to make it easier)

Now select Effects->Distort->Twist… from the menu and play around with the settings. Set the Amount/Direction to -30 and squint a bit, and we find out that the secret ingredient is…

…drum roll…

…not in here! Messing around with the image was a wild goose chase.

So, we’re still looking for something that looks like ciphertext. Where else could it be?

JPEG images (amongst others) can have EXIF metadata attached to them:

http://en.wikipedia.org/wiki/Exif

This is intended to be used to store things like the camera make/model, the settings in use when you took the picture, geotagging information, etc. If we load up the image in an EXIF editor or viewer, we can see what’s there. There’s an online version here:

http://regex.info/exif.cgi

This tells us that there are three EXIF fields:

Document Name Pmjeyeglwfh7F
Resolution 96 pixels/inch
Software Paint.NET v3.36

The document name looks a bit odd. Perhaps that’s our ciphertext?

If we proceed with the theory that Pmjeyeglwfh7F is the product of a Vigenere cipher, all we need is the key (or enough time to brute-force it!)

Fortunately, the key is provided in the message too:

“discretion is the key”

We can decrypt our ciphertext online here:

http://sharkysoft.com/misc/vigenere/

Put Pmjeyeglwfh7F into the Input box, use discretion as the key, and hit Decode. The result is:

Merchandise7X

…which is apparently the secret ingredient in a certain brand of soft
drink 🙂

Chris writes:

The twist was a very cool idea. Again, Alec’s blog is http://wirewatcher.wordpress.com. Tomorrow, I’ll reveal the winner of the challenge and his solution.