Has the message changed?

A bar code, a bank card number and a downloaded file all carry a small check number. If one digit is wrong, the scanner or the browser notices and refuses to trust it.

Radio can garble a message (a bit flips in the air), and a message can be changed on purpose. Encryption keeps a message private; it does not tell you it is unchanged. A checksum is the tool for that.

The sender works out a small number from the message (here: add up each letter's place in the alphabet, keep the last digit) and sends it with the message. The receiver works it out again. If the two differ, something changed.

send value sends a name and a number together (Agreeing a key in the open). Here the name is the message itself and the number is its checksum, so each message can be 8 characters at most: a longer one is cut short on the way and fails its own checksum.

PredictRead the code and say what it will do before you run anything. A wrong guess is fine: it shows you what to look for.

Predict

Show as code

checksum.ts

let ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
function checksum(message: string): number {
    let total = 0
    for (let i = 0; i < message.length; i++) {
        total += ALPHABET.indexOf(message.charAt(i))
    }
    return total % 10
}
radio.setGroup(5)
input.onButtonPressed(Button.A, function () {
    radio.sendValue("HELLO", checksum("HELLO"))
})
input.onButtonPressed(Button.B, function () {
    radio.sendValue("HELLP", checksum("HELLO"))
})
radio.onReceivedValue(function (name, value) {
    if (checksum(name) == value) {
        basic.showString(name)
    } else {
        basic.showIcon(IconNames.No)
    }
})

The receiver gets A's message, then B's message (HELLP with HELLO's checksum). What does it show?HELLP, then HELLOHELLO, then HELLPA cross, then HELLOHELLO, then a cross

RunRun it and compare the result with your prediction.

Run

B pretends the message was damaged on the way: the text changed but the checksum is still the old one.

Show as code

checksum.ts

let ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
function checksum(message: string): number {
    let total = 0
    for (let i = 0; i < message.length; i++) {
        total += ALPHABET.indexOf(message.charAt(i))
    }
    return total % 10
}
radio.setGroup(5)
input.onButtonPressed(Button.A, function () {
    radio.sendValue("HELLO", checksum("HELLO"))
})
input.onButtonPressed(Button.B, function () {
    radio.sendValue("HELLP", checksum("HELLO"))
})
radio.onReceivedValue(function (name, value) {
    if (checksum(name) == value) {
        basic.showString(name)
    } else {
        basic.showIcon(IconNames.No)
    }
})

Try it yourselfYour edits are saved in this browser.

It takes a few seconds to load. Your work in this step is kept.

I pressed A and B and saw that B's changed message was rejected.

InvestigateTrace how each line of code connects to what you see.

Investigate

Show as code

checksum.ts

let ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
function checksum(message: string): number {
    let total = 0
    for (let i = 0; i < message.length; i++) {
        total += ALPHABET.indexOf(message.charAt(i))
    }
    return total % 10
}
radio.setGroup(5)
input.onButtonPressed(Button.A, function () {
    radio.sendValue("HELLO", checksum("HELLO"))
})
input.onButtonPressed(Button.B, function () {
    radio.sendValue("HELLP", checksum("HELLO"))
})
radio.onReceivedValue(function (name, value) {
    if (checksum(name) == value) {
        basic.showString(name)
    } else {
        basic.showIcon(IconNames.No)
    }
})

What is the checksum of KEY? (K is 10, E is 4, Y is 24. Add them and keep the last digit.)

A message HELLO changes to EHLLO (two letters swap). Does this checksum notice?Yes, alwaysNo: the letters add to the same totalYes, because the order changedOnly for short messages

Someone changes the message in the air and also works out the new checksum with the same public method. What does the receiver think?It spots the changeNothing wrong: the checksum matches the changed messageIt asks for the keyIt shows an error

ModifyChange the code on purpose and check the result.

Modify

Mix a secret key into the checksum. Both micro:bits know it, and nobody else does, so only they can work out a matching number.

Try it yourselfYour edits are saved in this browser.

It takes a few seconds to load. Your work in this step is kept.

Need a hint?

Make a variable secretKey = 7, and start total from secretKey instead of 0.

The checksum starts from a secret key.

This ticks by itself when your blocks do what is asked.

MakeBuild something new using what you have learned. Expect a few tries; that is normal.

Make

Make a message checker: the receiver shows a tick for a good message and a cross for a bad one, and A+B shows how many bad messages have arrived.

Try it yourselfYour edits are saved in this browser.

It takes a few seconds to load. Your work in this step is kept.

Need a hint?

A sends MEET with its checksum; B sends a damaged MEAT with MEET's checksum.

Another hint

On receiving, show a tick when the checksum matches; otherwise add one to bad and show a cross. A+B shows bad.

A sends a good message and B a damaged one; the receiver shows a tick or a cross, and counts the bad ones.

This ticks by itself when your blocks do what is asked.

Has the message changed?

Predict

Run

Investigate

Modify

Make

Students will be able to

Before class

Running the lesson

The hard part (on purpose)

When students get stuck

Stretch and support

Where it leads

What each step is for