# Roy's Randomness

## Problem

Roy found some suspicious network traffic, wireshark shows so many errors with it! Can you figure out what's happening

## Solution

This is a TCP covert channel, using morse code.

![](/files/-MYc5Np9ipVKB8aBtMaT)

The `PSH` packets are sent at regular intervals, serving as delimiters.

On the other hand, the `RST` and `SYN` packets act as '1' and '0' bits in the message. Using the international morse code, `RST -> -` and `SYN -> .`, we can obtain the message.

![](/files/-MYc5QhXsaTY6hKYwwwN)

Decoding from hex gives us the flag: UMDCTF-{r0y\_f0und\_m0r53}

```python
import json

MORSE_CODE_DICT = { 'A':'.-', 'B':'-...',
                    'C':'-.-.', 'D':'-..', 'E':'.',
                    'F':'..-.', 'G':'--.', 'H':'....',
                    'I':'..', 'J':'.---', 'K':'-.-',
                    'L':'.-..', 'M':'--', 'N':'-.',
                    'O':'---', 'P':'.--.', 'Q':'--.-',
                    'R':'.-.', 'S':'...', 'T':'-',
                    'U':'..-', 'V':'...-', 'W':'.--',
                    'X':'-..-', 'Y':'-.--', 'Z':'--..',
                    '1':'.----', '2':'..---', '3':'...--',
                    '4':'....-', '5':'.....', '6':'-....',
                    '7':'--...', '8':'---..', '9':'----.',
                    '0':'-----', ', ':'--..--', '.':'.-.-.-',
                    '?':'..--..', '/':'-..-.', '-':'-....-',
                    '(':'-.--.', ')':'-.--.-'}

data = json.loads(open('roy.json').read())
result = ''

curr = ''
for packet in data:
    flags = packet['_source']['layers']['tcp']['tcp.flags_tree']

    if flags['tcp.flags.syn'] == '1':
        curr += 'S'

    elif flags['tcp.flags.push'] == '1':

        if not curr:
            continue

        morse_code = ''
        for char in curr:
            if char == 'R':
                morse_code += '-'
            else:
                morse_code += '.'

        print(curr, morse_code)

        for key in MORSE_CODE_DICT:
            if MORSE_CODE_DICT[key] == morse_code:
                result += key

        curr = ''

    elif flags['tcp.flags.reset'] == '1':
        curr += 'R'

print(result)
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://ctf.zeyu2001.com/2021/umdctf-2021/roys-randomness.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.