Crypto Ctf Esolangs

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:

1. 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

2. Google distinctive tokens - Copy 2-3 unique characters or patterns and search

3. 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:

1. 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()))"

2. 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

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

Common esolangs in CTFs

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

1. Identify: The + - < > [ ] pattern indicates Brainfuck
2. Run: Use an online Brainfuck interpreter or docker run -i brainfuck < file.bf
3. Output: Gets BB (two B characters)
4. 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.