Hidden Cipher 2 Writeup

Here is the exact writeup of how I solved it, step by step, including the tools I used.

Files and tools used

• Tools:
• nc
• python3

1) Connect and pass gate prompt

nc crystal-peak.picoctf.net 62957

Prompt asked:

• What is 8 - 1?
• answer: 7

2) Observe encoding rule

Service prints integer list like:

• 784, 735, 693, ...

Check first value:

• 784 / 7 = 112 -> ASCII p

So encoding is:

• encoded = ascii * 7

3) Decode all numbers

python3 - << 'PY'
vals=[784,735,693,777,469,588,490,861,763,364,812,728,665,686,357,728,343,770,700,665,693,343,784,728,357,798,665,357,343,392,371,707,700,350,707,875]
print(''.join(chr(v//7) for v in vals))
PY

Final flag

picoCTF{<redacted>}