CTF Event Info

During THJCC 2026, I participated in multiple challenges across forensics and steganography categories.

Due to guest participation constraints and limited time during the event, my contributions were focused on specific technical components rather than full end-to-end ownership of every challenge.

This post documents two challenges where I directly contributed to solving key technical portions:

1. CoLoR iS cOdE (500 pts)
2. SSTV Audio Challenge

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1. CoLoR iS cOdE (500 pts)

Challenge Information

• Category: Forensics / Steganography
• Points: 500
• Artifact: THJCC_CoLoR_iS_cOdE.zip
• Hint: colors can say a lot

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ZIP Exploitation

The archive was password-protected and contained a single encrypted PNG file: rainbow.png.

Since PNG files have a known header signature: 89504E470D0A1A0A0000000D49484452

I performed a known-plaintext attack against ZipCrypto using bkcrack.
This allowed recovery of the internal encryption keys and full decryption of the archive without knowing the original password.

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EXIF Payload & Ook → Brainfuck

Inside the extracted PNG, an eXIf chunk contained a long string of Ook. syntax.

Mapping Ook instruction pairs to Brainfuck operations and executing the decoded program revealed:

THJCC{c0lorfU1_col0rfu!\_c0}

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Pixel-Based Steganography

The top band of the image encoded ASCII characters via pixel frequency:

• 26 vertical columns
• Each column 12×12 pixels
• Count most frequent non-black color
• Convert count to ASCII

Recovered second half:

!0rful_img_m4d3_by_p1e7:>}

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Final Flag

THJCC{c0lorfU1_col0rfu!_c0!0rful_img_m4d3_by_p1e7:>}

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2. SSTV Audio Challenge

Challenge Information

• Artifact: output.flac
• Duration: ~115 seconds
• Sample Rate: 48kHz
• Channels: Mono

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Signal Analysis

Initial transcription attempts yielded nothing meaningful.

Generating a spectrogram (900--2500 Hz range) revealed SSTV-like signal patterns, indicating an encoded image within the audio.

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SSTV Mode Forcing

Automatic VIS detection failed, so I manually forced common SSTV modes:

• M1
• M2
• S1
• S2
• SDX
• R36
• R72

Mode M1 successfully rendered the hidden image.

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Final Flag

THJCC

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Contribution Context

My contributions during THJCC 2026 were focused and technical rather than massive in scale.

Due to guest participation limitations and time constraints, I concentrated on:

• ZIP cryptanalysis
• EXIF payload extraction
• Esoteric language decoding
• Signal analysis & SSTV reconstruction
• Automation scripting for forced decoding modes

Even when contributions aren't massive in volume, depth matters --- and this competition reinforced that lesson.