Step 250. ★ Mid-Track Check: Solving a Comprehensive Forensics Challenge Independently + Report — Becoming Someone Who Proves
Level 3 — Real-World CTF & Advanced Offensive Skills | Difficulty ★★★★☆ | Estimated time: 4 hours
Prerequisites: Step 240~249 (the entire Forensics track — disk, memory, network, logs, metadata, encrypted artifacts), Step 248 (the 5 comprehensive challenges), Step 249 (the six-section IR report structure).
- What you need: one comprehensive forensics challenge you haven’t solved before (see the selection criteria in 2-2), a Markdown file for your work journal, a folder for organizing evidence.
- ⚠️ All exercises in this chapter are for your own lab and legal platforms only. Applying them to unauthorized systems is a crime. Choose your practice challenge from legal CTF platforms (DreamHack, CTF archives, etc.).
- Chapter type: today is a [wargame] chapter and a mid-track check. Section 3 provides only the procedure and templates — solving the challenge, from start to finish, is entirely up to you. Because this chapter deals with external-platform challenges, its screens are screen examples.
This is the graduation exam of the Forensics track. In forensics, "done" isn’t "I solved it" — it’s "I proved it." Finding the flag isn’t the end; the work is finished only as a report where every claim carries evidence (commands, output, screenshots). Today has two completion conditions: solve an unknown comprehensive challenge independently, and leave behind a report a third party can reproduce.
If the chapters so far taught you techniques one by one, today you train at arranging those techniques yourself, in an order nobody tells you. Getting stuck isn’t failure — it’s part of the checkup. Where you got stuck is itself your diagnostic chart.
1. Learning Objectives
By the end of this chapter, you will be able to:
- Select and start a comprehensive forensics challenge without help
- Record the entire analysis process in a work journal (time, tool, finding, hypothesis)
- Construct the technique chain (evidence → technique → next evidence) yourself in a multi-stage challenge
- Write a reproducible report with the evidence list separated from the analysis process
- Self-verify your own report against the standard of "third-party reproducibility"
2. Background Knowledge — Today’s Tools and Concepts
Today’s Tools at a Glance
| Category | Details |
|---|---|
| Language/environment | Every forensics tool you’ve learned so far (binwalk, exiftool, tshark, Volatility, hashcat, etc.) |
| Today’s commands | No new commands — combining what you already have is today’s content |
| Concepts needed | Multi-stage, technique chain, work journal, reproducibility, playbook |
| Today’s deliverable | One work journal + one analysis report + an updated personal forensics playbook |
2-1. The Structure of a Comprehensive Challenge — Techniques Woven into a Chain
Realistic challenges mix techniques together. One classic arrangement:
pcap received → extract a file from the packets → the extracted file is encrypted
→ crack the password (the hint is in the metadata) → the flag is inside the opened document
It’s a structure where one piece of evidence is the key to the next. Each link is a technique you already know, but nobody tells you "what the next link is." The difficulty of a comprehensive challenge comes not from the individual techniques but from these connections.
2-2. Challenge Selection Criteria — What Makes It a Real Checkup
A mid-track-check challenge must satisfy these conditions:
- One you haven’t solved before — a checkup solved from memory is no checkup
- Multi-stage — at least two techniques connected (DreamHack Forensics intermediate, comprehensive challenges from CTF archives, etc.)
- A solution exists — the last resort. You open it only to compare against your own analysis after solving
- A size you can finish alone — 2~4 hours. For anything longer, split it and record the split
2-3. The Work Journal — Recording Is Analyzing
Today’s core habit. The moment you try something, write down these four cells.
[time] tool/command: (what you actually typed)
finding: (what stood out in the output — save the output itself in evidence/)
hypothesis: (what this finding suggests)
next: (what to do next)
"Record the moment you find it" is the principle. Records written in a batch later are reconstructions from memory, and memory gets embellished — exactly as you learned in Step 173. The 30 minutes you spent stuck and wandering is also a record. That time becomes the material for the report’s "breakthrough process" section.
2-4. The Stuck-Regression Checklist — Tools You Haven’t Tried Yet
Decide your regression points for when you get stuck, in advance: scan a list of "tools I haven’t tried yet."
□ It's a file and something's off → file, binwalk, strings
□ It's an image/photo → exiftool, zsteg, steghide
□ It's audio → spectrogram (Audacity), audio playback
□ It's packets → tshark statistics first (the Step 249 order)
□ It's memory → Volatility imageinfo → pslist → filescan
□ It's encrypted → look for the hint in other evidence (the key is in another link)
The suspicion "wouldn’t the password for this file be in that evidence over there?" — that connection suspicion is the core instinct of comprehensive challenges.
2-5. Report Structure — Extending Step 128·173·249
Today’s report has four axes.
① Evidence list — every file received, with each one's hash
② Analysis process — the techniques applied to each piece of evidence and their output (in command order)
③ Connections — which evidence was the key to which other evidence (a chain diagram)
④ Conclusion — the flag or the reconstructed incident result
You inherit Step 173’s standard as-is — every claim carries evidence, and a reader following the commands reaches the same conclusion.
3. Follow Along
Today’s Section 3 is not a solution but procedure and templates. The actual challenge is chosen by you and solved by you.
3-1. Starting — Challenge Selection and Evidence Sealing
Once you’ve picked a challenge, do three things before analyzing.
mkdir -p step250/evidence step250/notes
cd step250
# put the received evidence files into evidence/ and hash them
sha256sum evidence/* | tee notes/hashes.txt
How to read the output: there are two reasons for hashing — to show that the originals didn’t change during analysis, and to let the report point to evidence as "the file with this hash." From here on, leave all analysis output as files in notes/ (like command | tee notes/01-file.txt).
3-2. Kickoff — The First 30 Minutes’ Reconnaissance Template
Before opening the challenge files, write a plan. Template:
# Challenge: (name/source) — kickoff (date time)
### Evidence list and expected techniques
| file | first 8 chars of hash | apparent identity | technique to apply (hypothesis) |
|------|--------------|-------------|-------------------|
| ____ | ____ | ____ | ____ |
### First hypothesis
This challenge is probably of type ____. Basis: ____
How to read the output: the "technique to apply (hypothesis)" column is today’s training target. It’s fine to be wrong — the process of a wrong hypothesis being corrected into a right one stays in the journal, and that is the evidence of independent analysis ability.
3-3. Analyzing — The Rhythm That Keeps the Journal Rolling
During analysis, repeat the four cells from 2-3 (time, tool, finding, hypothesis). Each time a step ends, update your current state in one line.
[current state] secured: ____ / stuck on: ____ / next link candidates: ____
If there’s no progress for 30 minutes or more, open the regression checklist from 2-4 and walk through the "tools I haven’t tried yet" one by one. If you’re still stuck, ask the hypothesis backward — "why did the author give me this evidence?"
3-4. Drawing the Technique Chain — Visualizing the Connections
Once the solution starts coming into view, draw the links as a single chain.
[challenge.pcap] --tshark extraction--> [secret.zip] --password needed-->
[hint: EXIF comment in photo.jpg] --cracking--> [secret.txt] --> flag
How to read the output: this diagram becomes the report’s ③ Connections section. Mark every arrow with the basis — "which tool’s which output." An arrow without a basis is a guess.
3-5. Wrapping Up — Assembling the Report and Self-Verifying
When the solve is done, polish the journal into a report. Then ask the final questions.
① Do this report's evidence list and hashes match the actual files?
② Do the commands in ② Analysis process produce the same output when typed verbatim?
③ Does every arrow (connection) have its basis output attached?
④ Is there any part where the conclusion is claimed to hold without ①~③?
(if so, that part is an evidence-free claim)
⑤ Have I added the tools/options used for the first time in this challenge to my playbook?
If all five questions get a "yes," the report is ready to submit. If even one is "no," go back to that section.
4. Missions & Exercises
Mission — Solving a Comprehensive Forensics Challenge Independently and Completing the Report
- Select one multi-stage challenge you haven’t solved before, per the 2-2 criteria
- Start with the 3-1~3-2 templates, leaving a hash list behind
- Record the entire analysis process in the 3-3 journal format — including the time you spent stuck
- After solving, draw the 3-4 technique chain diagram
- Write the report along the four axes of 2-5, and self-verify with the five questions of 3-5
- Add the tools/techniques you used for the first time in this challenge to your personal forensics playbook
Exercises
Exercise 1. Explain the statement "in forensics, done isn’t ‘I solved it’ — it’s ‘I proved it,’" together with which elements of the report create this difference.
Exercise 2. Give two reasons why the work journal must not be written "in a batch later."
Exercise 3. What is "connection suspicion" in a multi-stage challenge, and why is it the core instinct of comprehensive challenges?
Exercise 4. Explain why you’re told to pick a challenge "whose solution exists" for the mid-track check, and when the correct time to use the solution is.
5. Model Answers & Completion Criteria
Mission Model Answer
Here’s an example of what the finished deliverables look like (contents differ per challenge — use the shape as your standard).
step250/
├── evidence/ # original evidence (untouched)
├── notes/
│ ├── hashes.txt # sha256 list — created at kickoff
│ ├── 01-file.txt # command+output for each step (saved via tee)
│ ├── 02-binwalk.txt
│ └── journal.md # journal of time, tool, finding, hypothesis
└── report.md # ① evidence list ② analysis process ③ connections ④ conclusion
Example of the report’s ③ Connections section:
challenge.pcap -(tshark: ftp-data extraction)-> secret.zip
-(password needed → searched other evidence)-> cover.jpg
cover.jpg -(exiftool: Comment field "try the year we met")->
4-digit password candidate -> (zip2john + john: 4-digit numeric rule)->
secret.txt -> flag confirmed
How to verify: ① Does sha256sum -c notes/hashes.txt come back all OK? ② Does every arrow in the chain have its basis (tool + finding) in parentheses? ③ Does the journal contain at least one stuck stretch and breakthrough? ④ Were items added to the playbook? All "yes" means done. If you opened the solution, append a comparison paragraph at the end of the report noting any links that differed from your own solve — that’s a learning deliverable too.
Exercise Answers
Answer 1. "I solved it" is proven by a single flag string, but "I proved it" is the state where every claim leading to that flag carries evidence. The elements that create this difference are ① the evidence list (with hashes), ② reproducible commands and output, ③ the basis of each connection. The flag is the result; the chain of evidence is the persuasiveness — in the field, the person reading the report examines not the flag but the chain.
Answer 2. First, memory distorts — a stretch where you wandered for 40 minutes shrinks to 5 minutes (Step 173’s retrospective principle). Second, intermediate artifacts (temporary files, intermediate output) get deleted or overwritten with time — only immediate recording preserves the evidence of that moment. The journal isn’t a byproduct of analysis; it’s part of the analysis.
Answer 3. It’s the habit of suspecting that the key opening one piece of evidence lies in another piece of evidence — "wouldn’t this zip’s password be in that jpg’s metadata?" Comprehensive challenges are designed to measure not individual techniques but the ability to find these connections. Tool knowledge for single-technique challenges was already verified in the track; what today verifies is the ability to arrange.
Answer 4. The purpose of a mid-track check is measurement, not frustration. A solution must exist so you can avoid being stuck forever on a truly unsolvable challenge (authoring error, environment dependency). The correct time to use it is "comparison after finishing," as a rule; if you must open it, ① record in the journal how many hours you tried alone, ② read only the minimal unit of the hint (the one next link), and ③ solve the rest alone again. And honestly mark the "solution-referenced stretch" in the report.
Completion Criteria Checklist
- [ ] I selected a multi-stage challenge I hadn’t solved before
- [ ] I recorded evidence hashes at kickoff time
- [ ] My work journal is filled out in the time·tool·finding·hypothesis format
- [ ] The journal contains a stuck stretch and the breakthrough process
- [ ] Every arrow in the technique chain diagram has a basis
- [ ] The report’s four axes (evidence·process·connections·conclusion) are complete
- [ ] I answered "yes" to all five self-verification questions
- [ ] I added newly used tools to my playbook
6. Common Pitfalls & Fixes
Wall 1. You’ve spent an hour just "deciding which challenge to pick"
Symptom: you keep browsing the challenge list and can’t start.
Cause: the desire to pick the perfect challenge.
Fix: pick the first challenge that meets the 2-2 criteria. The quality of finishing, not the quality of choosing, makes the score. If choosing takes more than 15 minutes, kill the timer and start with whatever is in front of you.
Wall 2. You overwrite intermediate artifacts and the chain breaks
Symptom: you extracted a file again under the same name and lost the original.
Cause: the absence of a working-folder rule.
Fix: use a new filename for each step (01_extract.zip, 02_cracked.txt). The original evidence folder is write-forbidden. You can verify original integrity anytime with the hash list (notes/hashes.txt).
Wall 3. You put off the journal until "how did I find this again?"
Symptom: the solve is done but there’s no material for the report.
Cause: you deferred recording to "later" — exactly the warning in 2-3.
Fix: this time, honestly write only what you can reconstruct from the terminal history and remaining output files, and mark what can’t be reconstructed as [no record — inferred]. And from the next challenge on, make tee the default. The honesty of marking blanks is also a checkup deliverable.
Wall 4. The report contains only the success path
Symptom: the two hours you spent wandering aren’t in the report.
Cause: embarrassment about failure.
Fix: remember Step 173’s sentence — the record of being stuck and breaking through is the reader’s trap-avoidance sentence, and the report’s very credibility. A report containing the sentence "I initially hypothesized X but rejected it on output Y" is more trustworthy than one without it.
Wall 5. After looking at the solution, you record it as "I solved it all"
Symptom: the report shows no sign of reference, but the journal has a gap of dozens of hours.
Cause: mistaking the checkup for an evaluation — a checkup is a self-diagnosis tool, not a report card for others.
Fix: marking the referenced stretch is actually to your advantage. The boundary "independent up to here, a hint from here" creates an accurate map of your own skill. Only when the map is accurate can the next training be decided.
7. Summary
Today’s Concepts
| Concept | One-line explanation |
|---|---|
| Multi-stage | A challenge structure where several techniques are woven into a chain |
| Technique chain | The connection diagram of evidence → technique → next evidence |
| Work journal | Immediate recording of time·tool·finding·hypothesis |
| Connection suspicion | The habit of suspecting the key is in another link |
| Reproducibility | The state where a third party following the commands reaches the same conclusion |
| Playbook | A cumulative document of the tools and techniques I’ve used |
Today’s Commands & Tools
| Tool | What it does |
|---|---|
sha256sum evidence/* |
Sealing the evidence — the first command at kickoff |
command | tee notes/NN-name.txt |
Send output to screen and file simultaneously |
sha256sum -c notes/hashes.txt |
Verify original integrity |
| Regression checklist (2-4) | The table that walks untried tools when stuck |
| Self-verification 5 questions (3-5) | The final gate before submission |
An Instinct More Important Than Commands
What this checkup measured is not tool knowledge but the ability to arrange — the power to build the chain yourself when nobody tells you the order. And what the report taught: in forensics, skill isn’t "what you know" but "what you can prove." The report you completed today is your first certificate of independent analysis. With this document in hand, you can ask yourself the same question in the next stretch (real-machine hacking) too — "what am I proving right now?"
Once every box is checked, Step 250 is complete. Click the checkbox in the sidebar to save your progress.