Khatrimaza-org-mkv - The

2 00:00:03,001 --> 00:00:07,000 Enjoy the movie. Nothing hidden in the subtitles – just a generic welcome message. We quickly glance at them with ffprobe just to be sure there’s nothing weird:

$ mediainfo khatrimaza-org.mkv General Complete name : khatrimaza-org.mkv Format : Matroska File size : 84.3 MiB Duration : 00:03:45.000 Overall bit rate : 2 028 kb/s

$ cat payload.bin | head -5 HTBmkv_5t34g_1s_4lw4ys_5urpr1s1ng Bingo! The flag is clearly visible. | Step | What we did | Tools / commands | |------|--------------|------------------| | 1️⃣ | Identified file type | file , mediainfo | | 2️⃣ | Listed container structure | mkvmerge -i , mkvextract attachments | | 3️⃣ | Extracted all tracks & attachments | mkvextract tracks , mkvextract attachments | | 4️⃣ | Looked for obvious clues in subtitles, video, audio | cat , ffprobe , strings | | 5️⃣ | Discovered a binary attachment ( hidden.bin ) | file , hexdump , ent , binwalk | | 6️⃣ | Searched MKV metadata for a possible key | mkvinfo | | 7️⃣ | Found comment field containing s3cr3t_k3y_4_f1ag | grep on mkvinfo output | | 8️⃣ | XOR‑decrypted the binary using the key | Small Python script | | 9️⃣ | Obtained the flag | cat payload.bin | The Khatrimaza-org-mkv

if __name__ == '__main__': if len(sys.argv) != 4: print(f'Usage: sys.argv[0] <input.bin> <key> <output.bin>') sys.exit(1)

Conclusion: the flag is in the video/audio tracks. 5. Deep dive into the suspicious attachment – hidden.bin 5.1 Basic inspection $ file hidden.bin hidden.bin: data 2 00:00:03,001 --> 00:00:07,000 Enjoy the movie

Our job is to that the challenge author has concealed somewhere inside the container. 2. Initial Recon $ file khatrimaza-org.mkv khatrimaza-org.mkv: Matroska data, video (V_MPEG4/ISO/AVC), audio (A_AAC), subtitle (S_TEXT/UTF8), 720p, 30 fps The file is a normal MKV with video, audio, and a subtitle track . Next we get a quick look at the container’s structure:

Video ID : 1 Format : AVC Format/Info : Advanced Video Coding Width : 1 280 pixels Height : 720 pixels Display aspect ratio : 16:9 Frame rate : 30.000 FPS Bit rate : 1 600 kb/s The flag is clearly visible

ffprobe -show_streams video.h264 ffprobe -show_streams audio.aac Both streams look clean (no extra data or unusual codec parameters). We also run strings on them, but no flag‑like patterns appear.