Silent Stream Writeup

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

Files and tools used

• Files: packets.pcap, encrypt.py
• Tools:
• file
• sed
• tshark
• python3

1) Inspect provided files

file packets.pcap encrypt.py
sed -n '1,260p' encrypt.py

encrypt.py shows byte encoding:

• encode_byte(b, key) = (b + key) % 256
• key used: 42

2) Extract TCP payload bytes

tshark -r packets.pcap -T fields -e tcp.payload

Concatenate non-empty payload lines as hex.

3) Reverse transform and rebuild file

python3 - << 'PY'
import subprocess
out = subprocess.check_output(['tshark','-r','packets.pcap','-T','fields','-e','tcp.payload'], text=True)
hexstr = ''.join(line.strip() for line in out.splitlines() if line.strip())
enc = bytes.fromhex(hexstr)
dec = bytes((b - 42) % 256 for b in enc)
open('/tmp/silent_stream_decoded.jpg','wb').write(dec)
print('decoded bytes:', len(dec))
PY

4) Verify output type

file /tmp/silent_stream_decoded.jpg

Then open image and read flag text.

Final flag

picoCTF{<redacted>}