A7.10 Project: a secure sensor packet

The robot faces a wall. PAD is a list of four whole numbers from 0 to 255. 1. Read the sensor once with distance(), a reading in cm (a float, from 0 to 127.5 on this mat). 2. Build the four packet bytes as whole numbers from 0 to 255: the type byte ord("D"); the reading byte, the reading doubled with its fraction dropped (int); the flags byte with bit 7 always set and bit 0 set only when the reading is under 30 cm, built with shifts and OR; and the checksum, the sum of the first three bytes modulo 256. 3. Print packet: followed by the four bytes as two uppercase hex digits each, separated by single spaces. 4. Encrypt the packet by XORing each byte with the PAD byte in the same position, and print encrypted: followed by the four encrypted bytes in the same format. 5. Send PKT followed by the encrypted bytes as eight hex digits with no spaces, built by your program. 6. Now be the receiver, using only the text you sent: turn each pair of hex digits back into a byte, XOR with PAD, and check that the checksum matches and bit 7 of the flags is set. If both are true, print received: <cm> cm, checksum ok, where <cm> is the reading byte divided by 2 (for example 51.0), and set the LED to green. Otherwise print received: checksum failed and set the LED to red. Build every byte from the reading; do not type any of the packet in.

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

PAD = [0x3A, 0x91, 0x5C, 0xE7]

def hex_bytes(data, sep):
    return sep.join(format(b, "02X") for b in data)

d = distance()

The hint students can ask for: Build the four bytes one at a time: the type letter's code, the reading doubled to keep one fraction bit, the flags from shifted 1s ORed together, and the checksum of the first three. XOR with the pad, send, then do the whole thing in reverse as the receiver would and check the checksum before you trust the reading.

A solution

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

PAD = [0x3A, 0x91, 0x5C, 0xE7]

def hex_bytes(data, sep):
    return sep.join(format(b, "02X") for b in data)

def vernam(data, key):
    return [data[i] ^ key[i] for i in range(len(data))]

d = distance()
kind = ord("D")
reading = int(d * 2)
flags = 1 << 7
if d < 30:
    flags = flags | (1 << 0)
check = (kind + reading + flags) % 256
packet = [kind, reading, flags, check]
print("packet:", hex_bytes(packet, " "))

secret = vernam(packet, PAD)
print("encrypted:", hex_bytes(secret, " "))
text = "PKT " + hex_bytes(secret, "")
send(text)

digits = text.split(" ")[1]
arrived = [int(digits[i:i + 2], 16) for i in range(0, len(digits), 2)]
back = vernam(arrived, PAD)
if sum(back[:3]) % 256 == back[3] and back[2] & (1 << 7):
    print("received:", back[1] / 2, "cm, checksum ok")
    led("green")
else:
    print("received: checksum failed")
    led("red")

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