Update system-exploration directory

chapters/scratch-linux/system-exploration/drills/tasks/empty-files/README.md

@@ -0,0 +1,6 @@
+# Empty Files
+
+So many empty files...
+Nevertheless, you must find the flag!
+
+If you're having difficulties solving this exercise, go through [this](../../../reading/summary.md) reading material.

chapters/scratch-linux/system-exploration/drills/tasks/find-us-if-you-can/README.md

@@ -0,0 +1,7 @@
+# Find Us If You Can
+
+This is a two-stage challenge.
+The first flag is somewhere on the remote system.
+Use the hint it comes with, in order to figure out the second flag as well.
+
+If you're having difficulties solving this exercise, go through [this](../../../reading/summary.md) reading material.

chapters/scratch-linux/system-exploration/drills/tasks/find-us-if-you-can/solution/solution.sh

@@ -0,0 +1,9 @@
+#! /bin/bash
+# SPDX-License-Identifier: BSD-3-Clause
+
+echo "Get the first flag."
+# Use 2> /dev/null to ignore the error messages.
+find / -type f -name flag -print0 2> /dev/null | xargs -0 cat | grep SSS
+
+echo "Now get the second one."
+find / -name '*doc*' -print0 2> /dev/null | grep --null-data bugs | xargs -0 cat

chapters/scratch-linux/system-exploration/drills/tasks/not-your-doge/README.md

@@ -0,0 +1,8 @@
+# Not Your Doge
+
+The image is in `.pnm` format.
+It has a rather simple header, that you can find [here](https://en.wikipedia.org/wiki/Netpbm#PPM_example)(`.pnm`s are almost the same as `.ppm`s; it's just the data encoding that differs).
+But it's incomplete.
+Find a way to reveal it completely.
+
+If you're having difficulties solving this exercise, go through [this](../../../reading/summary.md) reading material.

chapters/scratch-linux/system-exploration/drills/tasks/not-your-doge/solution/solution.py

@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: BSD-3-Clause
+
+#! /usr/bin/python
+
+file = open("../support/not-doge.pnm", "rb")
+data = file.read()
+file.close()
+
+# The height is located at an offest of 7 bytes inside the header.
+new_data = data[:7] + b"688" + data[10:]
+
+out_file = open("not-doge.pnm", "wb")
+out_file.write(new_data)
+out_file.close()

chapters/scratch-linux/system-exploration/drills/tasks/surgical-precision/README.md

@@ -0,0 +1,13 @@
+# Surgical Precision
+
+There are many false flags out there.
+Answer the questions and find the **real** flags.
+The quizzes cover subjects discussed today and during the previous session.
+Think of them as a recap.
+
+The answer to each of the riddles in the files `question-*` from the `drills/surgical-precision/support` is the name of one of the given files.
+When you've found an answer, upload the flag in that file.
+
+Beware of [red herrings](https://en.wikipedia.org/wiki/Red_herring)!
+
+If you're having difficulties solving this exercise, go through [this](../../../reading/summary.md) reading material.

chapters/scratch-linux/system-exploration/guides/doge/README.md

@@ -0,0 +1,36 @@
+# Doge
+
+The best way to showcase the `strings` command is to use it in order to find our first flag for today.
+Head to the `guides/doge/support` folder and take a look at the image you've been given.
+
+Since this section is dedicated to the `strings` command, we'll run this command on our `doge.jpg` file:
+
+```bash
+root@kali:~/essentials-security/chapters/scratch-linux/system-exploration/guides/doge/public# strings doge.jpg
+JFIF
+[...]
+eP!_"
+```
+
+So there are lots of human-readable strings in this image, but very few, if any, actually make any sense.
+In order to filter them out, we'll use what we've learned today: `|` + `grep`.
+We'll try to find the flag itself.
+Maybe we get lucky.
+
+```bash
+root@kali:~/essentials/system-exploration/activities/doge/public# strings doge.jpg | grep SSS
+<there should be a flag here>
+```
+
+That's how you use `strings`: often in combination with some filtering mechanism, such as `grep`.
+
+Another way to get the flag is to run the `file` command:
+
+```bash
+root@kali:~/essentials/system-exploration# file activities/doge/public/doge.jpg
+activities/doge/public/doge.jpg: JPEG image data, JFIF standard 1.01, aspect ratio, density 1x1, segment length 16, comment: "SSS{grep_your_strings}", progressive, precision 8, 500x500, components 3
+```
+
+The flag is included in the file as a comment.
+Image comments are often used in CTFs in order to hide some more subtle information, such as hints.
+Always remember to check them out.