feat: add operation-silent-echo challenge

operation-silent-echo/README.md

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+# Operation Silent Echo
+## Text
+Classified Document: Operation Silent Echo
+TOP SECRET
+File No. 978-XB-ZD-344-FBIS
+Department of Space Anomalies and Threats (DSAT)
+Date: 2024/11/17
+Subject: Contingency Plan for Dr. Elara Hayes
+
+Have a look at this file.
+Kind regards,
+John
+## Files
+[fbi_protected.pdf](fbi_protected.pdf)
+## How to Deploy
+n/a

operation-silent-echo/SOLUTION.md

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+## Difficulty
+Medium
+## Category
+Steganography
+## How To Solve
+When you try to open the PDF, you see that it asks for a password. Simply starting to brute force the password won't be a good idea. Instead, use the `strings` command to find a hint: `johns_zipcode`. Now, we don't know John's zipcode. We don't even know who John is. In fact, John is a hint to use the brute forcing tool John the Ripper. First, you need to obtain the password hash, by using a tool like `pdf2john`. Online tools are also available. After you got the hash, you can use `john` to crack the password. Since the password is a zipcode, you can restrict the brute-force attack to digits only: `john --incremental=digits hash.txt` (where `hash.txt` contains you password hash). You should have obtained the password: 29641.
+
+You can now access the PDF file. Make an unlocked copy of the file to open it in an editor tool like Inkscape. After a bit of layer reordering and visibility toggeling, you should be able to see a string appear: `SUdDVEZ7U3AwdHQzZF9VRjB9`. This one is base64 encoded. Decode it with a tool like CyberChef to get the flag.
+## Flag
+`IGCTF{Sp0tt3d_UF0}`

operation-silent-echo/hash.txt

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+$pdf$2*3*128*-4*1*16*42ca010c954e6ccd0ad76fe119ff2e90*32*845d0dba0e85e21966f772e43062c37d28bf4e5e4e758a4164004e56fffa0108*32*da5fcdc527d79cc186016b00f36654810231ff9019179d398b25cfcd259519f7