ctf-stego

CTF stego methodology

0. CRITICAL — never Read an unvalidated image

Per CLAUDE.md: reading a corrupt image poisons the conversation. Validate first:

run_tool("file", "/path/to/image")
run_tool("identify", "/path/to/image")     # ImageMagick
# or run_script:
# from PIL import Image; img = Image.open(path); print(img.size, img.mode)

Only Read the image after both checks pass.

1. Triage every input

file <file>
xxd <file> | head -30
exiftool <file>            # metadata — flag often hides here
strings <file> | grep -i "ctf\|flag\|key\|pass" | head
binwalk <file>             # multi-file polyglots

Always check metadata first. ~30% of intro stego is just exiftool.

2. Image (PNG / JPG / BMP / GIF)

Technique	Tool / approach
LSB in pixels	zsteg (PNG/BMP), stegsolve
EOF data after IEND/EOI	binwalk -e, manual hex
JPG specific	steghide extract -sf <file> (often with empty pass), stegseek for brute
PNG color planes	stegsolve (visual layer toggle)
LSB matching, custom	zsteg -a, then write custom Python with PIL
Polyglot (file with valid headers for multiple formats)	binwalk, foremost
File appended after IEND chunk (PNG)	pngcheck -v, manual carve
Modified/extra chunks	pngcheck -v, pngcsum
Width/height tampering (PNG)	edit IHDR width/height, recalc CRC — tweakpng style

# zsteg covers most LSB cases for PNG/BMP
zsteg -a image.png

# steghide brute (jpg/wav/bmp)
stegseek image.jpg rockyou.txt

# Multi-tool sweep
stegsolve image.png        # GUI, manual layer toggle

3. Audio (WAV / MP3 / FLAC / OGG)

Technique	Tool
LSB in samples	wavsteg, custom python
Steghide payload	steghide, stegseek
Spectrogram (visual hidden in freq domain)	audacity view spectrogram, sonic-visualiser
DTMF tones	multimon-ng -a DTMF, manual tone analysis
Morse	listen by ear, then morse-decoder or manual
SSTV (slow-scan TV in audio)	qsstv, slowrx

# Spectrogram via sox
sox audio.wav -n spectrogram -o spec.png
# Then validate spec.png before reading (see step 0)

# DTMF
multimon-ng -a DTMF audio.wav

4. Video / GIF

# Extract frames
ffmpeg -i video.mp4 -vf fps=1 frames/frame_%04d.png

# Per-frame stego
for f in frames/*.png; do zsteg -a "$f" 2>/dev/null | grep -i "flag\|ctf"; done

# Audio track
ffmpeg -i video.mp4 -vn audio.wav
# Then audio analysis

5. Text / Unicode

Technique	Tool
Zero-width characters	zwsp-steg, manual unicode inspect
Whitespace stego	snow, stegsnow
Homoglyph substitution	unicode normalize + diff
Base-N nested encodings	ciphey, manual
Bacon / null cipher	manual + dcode.fr

# Show every character including invisibles
python3 -c "import sys; [print(repr(c), hex(ord(c))) for c in open(sys.argv[1]).read()]" file.txt

6. Less common

• PDF: pdf-parser, peepdf, qpdf --qdf — check streams, annotations, JS, attachments
• Office docs: oletools (olevba, oleid), oledump, unzip the .docx/.xlsx
• QR / barcode: zbarimg, zxing — also try partial QR reconstruction
• DNA/protein sequences: ROT/encoding tricks, decode A/C/G/T as base-4

7. Default workflow

1. exiftool + strings + binwalk -e always first
2. If image → zsteg -a (PNG/BMP) OR stegseek (JPG)
3. If audio → spectrogram view
4. If nothing obvious → stegsolve GUI for visual analysis
5. If still nothing → metadata might encode the answer (look at GPS, comment, software field)

8. Don't waste time on

• LSB scripts when zsteg exists
• Manually viewing spectrograms in matplotlib when audacity/sonic-visualiser exist
• Reading the image with Read before validation (see step 0)

After solve

Use the writeup-template skill.