W4503 Capture the Flag III
From Coder Merlin
Revision as of 09:05, 19 January 2021 by MagicPizzaBox (talk | contribs) (added prime multiplication writeup)
Within these castle walls be forged Mavens of Computer Science ...
— Merlin, The Coder
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| This page will serve as the writeup for the CTF III competition problems that were released on January 12, 2021 |
Forensics[edit]
Prime Multiplication[edit]
[50 Points] There are 2 prime numbers associated with this image, multiply them together to get the flag.
Hint: flag format is ahsCTF{12345678}
The challenge also provides the file prime_multiplication.jpg.
We can examine the details of this image by using exiftool. Begin by downloading the image into the shell server using wget or SFTP. Then we can use exiftool to examine the metadata:
john-williams@codermerlin:~$ exiftool prime_multiplication.jpg
We get this output:
ExifTool Version Number : 11.88 File Name : prime_multiplicaton.jpg Directory : . File Size : 936 kB File Modification Date/Time : 2021:01:19 08:01:27-06:00 File Access Date/Time : 2021:01:19 08:01:26-06:00 File Inode Change Date/Time : 2021:01:19 08:01:27-06:00 File Permissions : rw-r--r-- File Type : JPEG File Type Extension : jpg MIME Type : image/jpeg JFIF Version : 1.01 Resolution Unit : inches X Resolution : 300 Y Resolution : 300 Exif Byte Order : Big-endian (Motorola, MM) Orientation : Horizontal (normal) Image Width : 2741 Image Height : 1901 Encoding Process : Baseline DCT, Huffman coding Bits Per Sample : 8 Color Components : 3 Y Cb Cr Sub Sampling : YCbCr4:2:0 (2 2) Image Size : 2741x1901 Megapixels : 5.2
Notice that the image size is 2741x1901. These are both prime numbers, so let's multiply them as the challenge instructs us. 2741 times 1901 is 5,210,641. When we format this in flag format, we get ahsCTF{5210641}.
