# Book Cipher Encryption 🔐


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## Materials

- Paper and pen  
- Sample text below (the Key)

------------------------------------------------------------------------

### Sample Text (Use for encryption/decryption)

**Page 1:**

<table>
<thead>
<tr>
<th>Line Number</th>
<th>Text</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>Alice went to the market to buy some apples.</td>
</tr>
<tr>
<td>2</td>
<td>Bob stayed home and read a book about coding.</td>
</tr>
<tr>
<td>3</td>
<td>They agreed to meet at the park after school.</td>
</tr>
<tr>
<td>4</td>
<td>Secret messages should never be sent in plain sight.</td>
</tr>
<tr>
<td>5</td>
<td>Always practice encryption to keep messages safe.</td>
</tr>
</tbody>
</table>

------------------------------------------------------------------------

## Process: Encrypt -\> Send -\> Decrypt

### Step 1: Agree which key to use (only needs to be done once or when changing the key)

For our **key**, we are using the sample text above.  
Rules:  
- Use **page number, line number, word number** to reference a word.

> Example: Page 1, Line 1, Word 1 → “Alice”

### Sample Text (Use for encryption/decryption)

**Page 1:**

<table>
<thead>
<tr>
<th>Line Number</th>
<th>Text</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>Alice went to the market to buy some apples.</td>
</tr>
<tr>
<td>2</td>
<td>Bob stayed home and read a book about coding.</td>
</tr>
<tr>
<td>3</td>
<td>They agreed to meet at the park after school.</td>
</tr>
<tr>
<td>4</td>
<td>Secret messages should never be sent in plain sight.</td>
</tr>
<tr>
<td>5</td>
<td>Always practice encryption to keep messages safe.</td>
</tr>
</tbody>
</table>

------------------------------------------------------------------------

### Step 2: Encrypt a message

1.  Write a short message (3–5 words) using words found in the sample
    text.  
2.  For each word, write down the **page–line–word numbers** instead of
    the word.

**Example Message:**  
`Alice meets Bob`

**Encryption using the sample text:**  
- Alice → 1-1-1  
- meets → 1-3-4 (“meet”)  
- Bob → 1-2-1

Encrypted message:  
`1-1-1 / 1-3-4 / 1-2-1`

> Everyone should use the same sample text so messages can be swapped
> and decrypted in class.

------------------------------------------------------------------------

### Sample Text (Use for encryption/decryption)

**Page 1:**

<table>
<thead>
<tr>
<th>Line Number</th>
<th>Text</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>Alice went to the market to buy some apples.</td>
</tr>
<tr>
<td>2</td>
<td>Bob stayed home and read a book about coding.</td>
</tr>
<tr>
<td>3</td>
<td>They agreed to meet at the park after school.</td>
</tr>
<tr>
<td>4</td>
<td>Secret messages should never be sent in plain sight.</td>
</tr>
<tr>
<td>5</td>
<td>Always practice encryption to keep messages safe.</td>
</tr>
</tbody>
</table>

### Send the encrypted message

Swap encrypted messages with a partner.

------------------------------------------------------------------------

### Step 3: Decrypt the message

1.  Use the same key (sample text) to decode.  
2.  Read the decrypted message.

------------------------------------------------------------------------

### Step 4: Discussion

- Why is the sample text called the **key**?  
- What happens if you don’t have the correct key?  
- How is this different from a hash? (once you have completed the lesson
  on hashing!)

------------------------------------------------------------------------

### Optional Stretch

- Create your own short “Alice & Bob” message for a partner to encrypt
  and decrypt.  
- Change the book lines or words — what happens if the key is
  different?  
- Compare to **hashing**: why could this not be reversed in the same
  way? (once you have completed the lesson on hashing!)