The Evidence

Author: Hait Patel

This is a steganography, forensics, and cryptography-based CTF challenge in which the flag is divided into two halves, F₁ and F₂. Both halves are protected using visual steganography, metadata pivot links, Word container structures, and custom PNG file chunks.

All true payload data and keys are hidden in the Bit 0 channel (LSB) across the primary color planes. Multiple decoy flags and trap keys exist across other bit planes to mislead automated tools and superficial visual inspection.

For the Tools Used:

StegSolve (or zsteg on Linux)
exiftool
CyberChef
A zip/archive utility

For Step 1 — Extract Key 1 and Decrypt Flag Part 1 (F₁):

Files provided: RV_challenge.png, Lab_challenge.png

For Recovering Key 1:

Open RV_challenge.png in StegSolve (or inspect via zsteg on Linux).
Cycle through the bit planes to Red Plane 0 (R0) to extract the plaintext key:

METH_LAB_RED_KEY

For Extracting the Encrypted F₁ Payload:

Open Lab_challenge.png in StegSolve.
Navigate to Analyse → Data Extract.
Select Blue 0 (B0) (MSB first, Row order) and copy the extracted hex stream:

0E 1C 07 33 1D 20 14 71 00 15 75

For Decrypting Part 1 in CyberChef:

Recipe:

From Hex (Delimiter: Auto)
XOR (Key: METH_LAB_RED_KEY, Type: UTF8, Scheme: Standard) → yields CYS{BlU3_G0

Output (F₁):

CYS{BlU3_G0

For Step 2 — Locate the Word File via Metadata:

Inspect the metadata of RV_challenge.png:

exiftool RV_challenge.png

The metadata contains a link pointing to an external Word document, challenge.docx. Download this file.

For Step 3 — Extract the Nested Document Media & Folder Links:

Microsoft Word’s UI strips ancillary PNG chunks and metadata when using “Save as Picture,” so the raw media must be unpacked directly from the .docx archive.

Rename challenge.docx → challenge.zip.
Unzip the file and navigate to the media folder:

challenge.zip/word/media/

Run exiftool on the second image (image2.png):

exiftool -Description -Comment image2.png

This reveals two separate links:
- Description field → link to Folder A (holds the file containing the encrypted F₂ payload)
- Comment field → link to Folder B (holds the file containing Key 2)
Download both files from their respective folders.

For Step 4 — Carve Embedded Files Using CyberChef:

Both downloaded files contain nested PNG structures injected into custom ancillary chunks.

Open CyberChef.
Drag and drop the file downloaded from Folder A into the Input panel.
Apply the Extract Files operation.
Save the carved inner image file: flaghere_carved.png.
Repeat the exact same steps for the file downloaded from Folder B to carve the key image file: key_carved.png.

For Step 5 — Extract Key 2 and Encrypted Flag Part 2 (F₂):

For Extracting Encrypted F₂:

Open flaghere_carved.png in StegSolve → Analyse → Data Extract.
Select Blue 0 (B0) (MSB, Row order) and scroll to the top of the output preview:

ENC_F2_HEX: 2F 74 05 32 3D 3F 6C 14 1C 7B 03 5C 39 1A 0E 64

For Extracting Key 2:

Open key_carved.png in StegSolve → Analyse → Data Extract.
Select Blue 0 (B0) and scroll to the top of the output preview:

REAL_KEY: M3TH_SUPR3M3_K3Y

For Step 6 — Decrypt Flag Part 2 (F₂) & Reassemble Flag:

Input:

2F 74 05 32 3D 3F 6C 14 1C 7B 03 5C 39 1A 0E 64

Recipe:

From Hex (Delimiter: Auto)
XOR (Key: M3TH_SUPR3M3_K3Y, Type: UTF8, Scheme: Standard) → yields bGQzbl9DNHNofQ==
From Base64

Output (F₂):

ld3n_C4sh}

For the Final Reassembly:

F1 + F2 = CYS{BlU3_G0 + ld3n_C4sh} = CYS{BlU3_G0ld3n_C4sh}

For the Final Flag:

CYS{BlU3_G0ld3n_C4sh}

Flag

CYS{BlU3_G0ld3n_C4sh}