Crypto

Secure Server

8/18/2025

by yeknom

scriptCTF

WriteUp

Secure Server

📋 Description

John Doe uses this secure server where plaintext is never shared. Our Forensics Analyst was able to capture this traffic and the source code for the server. Can you recover John Doe's secrets?

🔗 Given Files

server.py
capture.pcap

💡 Walkthough

Solve idea — Server flow:

Prompt 1: client sends enc = secret XOR key (hex).
Server computes enc2 = enc XOR key = secret XOR key XOR key = secret and prints it as “Double encrypted”.
Prompt 2: client returns dec = enc2 XOR key = secret XOR key.
Server reconstructs secret = dec XOR key.

In the PCAP we see:

enc  = bytes.fromhex("151e71ce4addf692d5bac83bb87911a20c39b71da3fa5e7ff05a2b2b0a83ba03")
enc2 = bytes.fromhex("e1930164280e44386b389f7e3bc02b707188ea70d9617e3ced989f15d8a10d70")
dec  = bytes.fromhex("87ee02c312a7f1fef8f92f75f1e60ba122df321925e8132068b0871ff303960e")
secret = bytes(a ^ b ^ c for a,b,c in zip(enc, enc2, dec))
print(secret.decode())