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F11.9 Project: secure the robot

Cyber security · GCSE · OCR J277 1.4.2, AQA 8525 3.6.3, Edexcel 1CP2 4.2.1 · about 25 min

BugBotLab

What this lesson is about

Authenticate with a password, decrypt an encrypted command, and act on it.

Questions 5 marks in all

  1. [1 mark]Why does the robot authenticate before taking a command?

    1. ASo only someone with the password can control it
    2. BTo make it faster
    3. CTo save battery
    4. DTo encrypt the mat
    Answer: A. Authentication keeps out impostors.

  2. [1 mark]Why is the command encrypted?

    1. ASo a listener on the radio cannot understand it
    2. BTo make it shorter
    3. CSo it arrives faster
    4. DSo the robot can ignore it
    Answer: A. The radio is a broadcast; encryption keeps the command private.

  3. [1 mark]A listener hears the Base's encrypted reply, KTWBFWI 25. What can they read without the key?

    1. AOnly the number 25, because the Caesar cipher shifts letters and leaves digits alone
    2. BThe whole command
    3. CNothing at all
    4. DThe key
    Answer: A. KTWBFWI is FORWARD shifted by 5, but the digits are sent as they are. A cipher that only scrambles part of a message leaks the rest.

  4. [1 mark]Authentication and encryption together protect against what?

    1. AImpostors giving commands and listeners reading them
    2. BOnly power cuts
    3. COnly malware
    4. DOnly slow networks
    Answer: A. Authentication stops impostors; encryption stops eavesdroppers.

  5. [1 mark]If the wrong password is sent, what should the robot do?

    1. ANot move
    2. BDrive anyway
    3. CSend its password
    4. DTurn off the radio
    Answer: A. No valid authentication means no command is carried out.

The task: secure the robot

Authenticate with the Base using the password bugbot42, decrypt its reply with a Caesar cipher (key 5), and carry out the FORWARD <cm> command to reach the green depot, without collisions. Do not send any command before the password, and do not type the distance: read it from the decrypted message. The caesar helper is provided.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

def caesar(text, key):
    out = ""
    for ch in text:
        if ch.isalpha():
            base = ord("A")
            out = out + chr((ord(ch.upper()) - base + key) % 26 + base)
        else:
            out = out + ch
    return out

def ask(question, seconds=1.5):
    send(question)
    for tick in range(int(seconds * 10)):
        wait(0.1)
        for sender, text in messages():
            return text
    return None

The hint students can ask for: Send the password and wait for the reply, which is encrypted. Decrypt it with the key shifting the other way, split the command into its word and its number, and drive that far.

A solution

from bugbot import *
connect()
def caesar(text, key):
    out = ""
    for ch in text:
        if ch.isalpha():
            base = ord("A")
            out = out + chr((ord(ch.upper()) - base + key) % 26 + base)
        else:
            out = out + ch
    return out

def ask(question, seconds=1.5):
    send(question)
    for tick in range(int(seconds * 10)):
        wait(0.1)
        for sender, text in messages():
            return text
    return None

reply = ask("bugbot42")
if reply is None:
    led(255, 0, 0)
else:
    command = caesar(reply, -5)
    word, cm = command.split()
    forward(50, distance=int(cm))
    led(0, 255, 0)

Any program that meets the task's checks is marked correct in the simulator; this is one way, not the only way.