Hacktricks-skills crypto-ctf-esolangs

Use this skill whenever a CTF crypto challenge involves code that doesn't look like a standard programming language. This includes Brainfuck, Malbolge, Whitespace, Piet, and other esoteric languages. Trigger this skill when you see unusual syntax, strange characters, or code that appears to be a puzzle rather than normal programming. Also use when a challenge output needs decoding after running an esolang program.

install

source · Clone the upstream repo

git clone https://github.com/abelrguezr/hacktricks-skills

manifest: skills/crypto/ctf-misc/ctf-misc/SKILL.MD

source content

Crypto CTF: Esoteric Languages

This skill helps you solve crypto challenges where the actual task is to run an esoteric programming language (esolang) and decode its output.

When to use this skill

Use this skill when:

A challenge provides code with unusual syntax or strange characters
The code doesn't look like any standard programming language
You see tokens like
```
++++
```
,
```
[]
```
,
```
<>
```
, or other non-standard constructs
A challenge mentions or hints at an esoteric language
After running code, you get output that needs further decoding

Technique

Step 1: Identify the esolang

When you encounter unfamiliar code:

Look for distinctive tokens - Each esolang has characteristic syntax:
- Brainfuck:
```
+ - < > . , [ ]
```
- Malbolge:
```
defghijklmnopqrstuvwxyz0123456789()[]{}*
```
  with unusual operators
- Whitespace: Only spaces, tabs, newlines
- Piet: Image-based (you'll see a .png or .jpg)
- Unary: Only one character repeated
- Ook:
```
Ook. Ook? Ook! Ook.
```
- LOLCODE:
```
HAI
```
  ,
```
VISIBLE
```
  ,
```
KTHXBYE
```
Google distinctive tokens - Copy 2-3 unique characters or patterns and search
Check esolangs.org - The main reference for esoteric languages

Step 2: Run the program

Choose an execution method:

Option A: Online interpreter (fastest)

Search "[language name] online interpreter"
Paste the code and run
Copy the output

Option CTF: Docker image (for offline/air-gapped)

Many esolangs have Docker images

Run:

docker run -i [image-name] < program.txt

Option C: Local installation

Install the interpreter if available
Run locally for repeated testing

Step 3: Decode the output

Esolang outputs are often encoded:

Check for common encodings:

Base64:
```
echo "output" | base64 -d
```
Hex:
```
echo "output" | xxd -r -p
```

ROT13:

echo "output" | tr 'A-Za-z' 'N-ZA-Mn-za-m'

URL encoding:

echo "output" | python3 -c "import urllib.parse; print(urllib.parse.unquote(input()))"

Check for compression:

gzip:
```
gunzip -c file.gz
```

zlib:

python3 -c "import zlib; print(zlib.decompress(open('file','rb').read()))"

bzip2:
```
bunzip2 -c file.bz2
```

Look for layered encoding - Decode, then check if the result is still encoded

Common esolangs in CTFs

Language	Key Tokens	Typical Output
Brainfuck	`+ - < > . , [ ]`	ASCII characters
Malbolge	`defghijklmnopqrstuvwxyz0123456789()[]{}*`	Often encoded
Whitespace	Spaces/tabs/newlines only	ASCII characters
Piet	Image file	ASCII characters
Ook	`Ook. Ook? Ook! Ook.`	ASCII characters
LOLCODE	`HAI` , `VISIBLE` , `KTHXBYE`	ASCII characters
Unary	Single character repeated	Numbers

Resources

esolangs.org - https://esolangs.org/wiki/Main_Page
Esolang Sandbox - https://esolangs.org/wiki/Sandbox
CTF Esolang Collection - Various GitHub repos with CTF esolang challenges

Example workflow

Challenge: You receive a file with

++++[>++<-]>.

Identify: The
```
+ - < > [ ]
```
pattern indicates Brainfuck
Run: Use an online Brainfuck interpreter or
```
docker run -i brainfuck < file.bf
```
Output: Gets
```
BB
```
(two B characters)
Decode: Check if
```
BB
```
is encoded - might be base64, hex, or the final flag

Tips

Save the code - Keep the original esolang code for reference
Document the language - Note which esolang you identified
Try multiple decodings - If the first decode doesn't work, try others
Check for flags - Look for
```
flag{
```
,
```
CTF{
```
, or similar patterns in output
Layered encoding is common - Be prepared to decode multiple times

When this skill might not apply

Standard programming languages (Python, C, JavaScript, etc.)
Simple encoding challenges without esolang execution
Pure cryptography without code execution

If you're unsure whether something is an esolang, use this skill anyway - the identification step will help clarify.