In this post, I will write my solutions and through process when solving the challenges in the TryHackMe room c4ptur3-th3-fl4g.
Task 1
This section is titled Translations and Shifting and there are several messages that we have to decode. The format I will use for this section on of the writeup is a code block with the ciphertext, a small amount of text with my solution/thought process, and a comment with the solution below that.
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c4n y0u c4p7u23 7h3 f149?
Based on the first two words, we can easily assume that this is leet speak, or words with some letters replaced by numbers. We can logically guess that the first two words are can you. After looking at the other words for a few seconds and given the context of the challenge, it is safe to assume the message is:
can you capture the flag?
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01101100 01100101 01110100 01110011 00100000 01110100 01110010 01111001 00100000 01110011 01101111 01101101 01100101 00100000 01100010 01101001 01101110 01100001 01110010 01111001 00100000 01101111 01110101 01110100 00100001
This message contains all 1s and 0s, so we can assume it is binary. We can use a binary to ascii converter to get the message.
lets try some binary out!
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MJQXGZJTGIQGS4ZAON2XAZLSEBRW63LNN5XCA2LOEBBVIRRHOM======
The first thing I noticed here is the padding = at the end. This indicates that it is another base conversation question. However, there are only capital letters and numbers in the encoded message. Because of this, we can assume that it is base32.
I actually tried one base32 decode which did not spit out the right answer and claimed it was invalid. This confused me for a bit until I tried out the converter on dcode.fr which prompty spit out the correct answer.
base32 is super common in CTF’s
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RWFjaCBCYXNlNjQgZGlnaXQgcmVwcmVzZW50cyBleGFjdGx5IDYgYml0cyBvZiBkYXRhLg==
This message looks like the last one, except there are now lower case letters. If more characters are used to encode a message, then the base of the encoding goes up. Since 32 used less, we can assume the base used to encode this is greater than 32. base64 is the most popular base encoding, and it is the answer in this case.
Each Base64 digit represents exactly 6 bits of data.
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68 65 78 61 64 65 63 69 6d 61 6c 20 6f 72 20 62 61 73 65 31 36 3f
In this message, we can see numbers 0-9 and letters a-f. This is a clear indicator that this is base16/hexadecimal.
hexadecimal or base16?
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Ebgngr zr 13 cynprf!
Despite seeing the number 13 and it does not look as if there is any number/base conversion used here. Thus, we can assume the letters are shifted in some way. There is one popular shift called Rot 13 that only affects letters and not numbers, so I attempted to use that on this message.
Rotate me 13 places!
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*@F DA:? >6 C:89E C@F?5 323J C:89E C@F?5 Wcf E:>6DX
We know that it cannot be something like base conversation as there are spaces between words, similar to the last one. Thus, we can assume that this in a similar way. There is something called Rot 47 which would do exactly this.
You spin me right round baby right round (47 times)
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- . .-.. . -.-. --- -- -- ..- -. .. -.-. .- - .. --- -.
. -. -.-. --- -.. .. -. --.
This is clearly morse code. It took me 4-5 tries to find a website that would cleanly decode the message. The one I used is this: https://www.boxentriq.com/code-breaking/morse-code
TELECOMMUNICATIONENCODING
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85 110 112 97 99 107 32 116 104 105 115 32 66 67 68
This is a clear ascii numbers -> text conversion as the numbers are generally within the range of the alphabet. The website I used to convert it is this: https://convert.town/ascii-to-text
Unpack this BCD
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I see the padding at the end so I assume this is base64. After decoding, I get another message in morse. As it is 100 lines long, I will not include it in this post. I then get a series of 1s and 0s.
After decoding that message, I am given another string which looks similar to the rot47 string from above. After applying rot47, I see a series of numbers, which once I take the ascii value of I am given the plaintext message.
Let’s make this a bit trickier…
Task 2
A spectrogram is a visual representation of the spectrum of frequencies of a signal as it varies with time. When applied to an audio signal, spectrograms are sometimes called sonographs, voiceprints, or voicegrams. When the data is represented in a 3D plot they may be called waterfalls.
We are given an audio file titled secretaudio.wav and the prompt brings up a spectogram. With this info, I was able to find a website that lets you view spectograms and uploaded the file. After hitting run, I can clearly see the plaintext message.
Super Secret Message
Task 3
Steganography is the practice of concealing a file, message, image, or video within another file, message, image, or video.
There are several different tools that can be used to hide information in images. I checked the strings of the image, the metadata, and I ran through all the available options on stegsolve.jar. I was able to solve the challenge by using steghide with a blank password. This is not very logical, but it is a common CTF solution where once you see it once, you will know about and be able to solve it in the future.
SpaghettiSteg
Task 4
Security through obscurity is the reliance in security engineering on the secrecy of the design or implementation as the main method of providing security for a system or component of a system.
Questions:
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Download and get 'inside' the file. What is the first filename & extension?
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Get inside the archive and inspect the file carefully. Find the hidden text.
We are given an image file for the challenge. Based on the context of the questions, we can assume that there are some kinds of files hidden within the image. There is a tool called binwalk that would let us find and extract such files, given there are any.
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binwalk -e meme.jpg
DECIMAL HEXADECIMAL DESCRIPTION
--------------------------------------------------------------------------------
0 0x0 JPEG image data, EXIF standard
12 0xC TIFF image data, big-endian, offset of first image directory: 8
74389 0x12295 RAR archive data, version 5.x
74460 0x122DC PNG image, 147 x 37, 8-bit/color RGBA, non-interlaced
74611 0x12373 Zlib compressed data, default compression
It succeeeds! Let’s check out the folder with the extracted contents.
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cd _meme.jpg.extracted/
kali@kali:~/Desktop/thm/r00mc4pt4r3/_meme.jpg.extracted$ ls
12295.rar 12373 12373.zlib hackerchat.png
Looks like we can now answer the first question.
hackerchat.png
when inspecting the image, I decided to check the metadata associeted wit it by running the exiftool command.
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kali@kali:~/Desktop/thm/r00mc4pt4r3/_meme.jpg.extracted$ exiftool hackerchat.png
ExifTool Version Number : 12.04
File Name : hackerchat.png
Directory : .
File Size : 5.4 kB
File Modification Date/Time : 2019:05:27 22:30:36-04:00
File Access Date/Time : 2020:12:12 21:12:16-05:00
File Inode Change Date/Time : 2020:12:12 21:11:48-05:00
File Permissions : rw-r--r--
File Type : PNG
File Type Extension : png
MIME Type : image/png
Image Width : 147
Image Height : 37
Bit Depth : 8
Color Type : RGB with Alpha
Compression : Deflate/Inflate
Filter : Adaptive
Interlace : Noninterlaced
Significant Bits : 8 8 8 8
Software : gnome-screenshot
Creation Time : Wed 15 May 2019 09:09:10 PM CDT
Warning : [minor] Trailer data after PNG IEND chunk
Image Size : 147x37
Megapixels : 0.005
This line in particular is suspicious.
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> Warning : [minor] Trailer data after PNG IEND chunk
Let’s run strings on the file, which will output all printable strings that it can find within the file.
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kali@kali:~/Desktop/thm/r00mc4pt4r3/_meme.jpg.extracted$ strings hackerchat.png
<output cut as it is very long>
{@84
2$Es
i2Mc
IEND
"AHH_YOU_FOUND_ME!"
Found it!
AHH_YOU_FOUND_ME!
Conclusion
This was a good intro to CTF challenges. It is not worth doing if you are already familiar with common challenges and solutions, as it seems to mainly feature cheese solutions or things that you would only know if you have done a few CTFs before. To those who have no experience with CTFs, it shows off a few interesting tools and niche solutions, although they are not practical or applicable.