Encryption basics in Intro to Computing - Draw & Build Visually

Draw This - beginner

Draw a flowchart that shows the basic steps of encrypting a message using a simple substitution cipher. Start with the original message, then show how each letter is replaced by another letter, and end with the encrypted message.

10 minutes

Hint 1

Hint 2

Hint 3

Hint 4

Hint 5

Hint 6

Grading Criteria

Start and End symbols present

Process box for reading the original message

Loop structure to process each letter

Process box for replacing letters

Output box for encrypted message

Flow direction is clear and logical

All letters of the message are processed

Solution

  _______          ___________          ____________          ____________          _______
 / Start \        | Read Msg |        | For each  |        | Replace   |        | Output |
 \_______/        |__________|        | letter in |        | letter   |        | Encrypted|
      |                     |         | message   |        | with     |        | message |
      v                     |         |__________|        | substitute|        |_________|
  ___________               |               |               |               |
 | Initialize |<-------------               |<--------------               |
 | encrypted  |                                |                               |
 | message =  |                                |                               |
 | empty      |                                |                               |
 |____________|                                |                               |
      |                                        |                               |
      v                                        v                               v
  ___________        Yes?  letter left?  ---------> Replace letter ---------> Add to encrypted
 | Loop start|------------------------------->| message string             |
 |__________|                                |_____________________________|
      | No
      v
  _______
 / End   \
 \_______/

This flowchart starts with the Start symbol. Then it reads the original message. It initializes an empty string to hold the encrypted message. Next, it loops through each letter of the original message. For each letter, it replaces it with a substitute letter according to the cipher rules. The substituted letter is added to the encrypted message string. This loop continues until all letters are processed. Finally, the encrypted message is output, and the flowchart ends.

This shows the basic idea of encryption: transforming readable text into coded text by replacing letters.

Variations - 2 Challenges

[intermediate] Draw a flowchart for decrypting a message encrypted with a simple substitution cipher. Start with the encrypted message and end with the original message.

[advanced] Draw a flowchart for encrypting a message using a Caesar cipher with a shift of 3. Show how each letter is shifted by 3 positions in the alphabet.

Practice

1. What is the main purpose of encryption in computing?

easy

A. To speed up message delivery

B. To change readable messages into secret codes

C. To delete messages permanently

D. To make messages longer

Solution

1. Step 1: Understand what encryption does

   Encryption transforms readable messages into a form that cannot be understood without a key.

2. Step 2: Compare options with this definition

   Only To change readable messages into secret codes describes changing readable messages into secret codes, which matches encryption.

3. Final Answer:

   To change readable messages into secret codes -> Option B

4. Quick Check:

   Encryption = secret code conversion [OK]

Hint: Encryption means making messages secret, not deleting or speeding [OK]

Common Mistakes:

• Confusing encryption with deleting data
• Thinking encryption speeds up messages
• Believing encryption makes messages longer

2. Which of the following is the correct term for the secret information used to decrypt a message?

easy

A. Key

B. Hash

C. Algorithm

D. Cipher

Solution

1. Step 1: Identify the term for secret info to unlock messages

   The secret information used to decrypt messages is called a key.

2. Step 2: Check other options

   A cipher is the method, algorithm is the process, and hash is a fixed output, not the secret info.

3. Final Answer:

   Key -> Option A

4. Quick Check:

   Secret info to decrypt = Key [OK]

Hint: Secret info to unlock message = Key, not cipher or hash [OK]

Common Mistakes:

• Confusing cipher with key
• Thinking algorithm is the secret info
• Mixing up hash with key

3. Consider this simple encryption method: each letter in the message is shifted by 1 in the alphabet (A->B, B->C, ..., Z->A). What is the encrypted form of HELLO?

medium

A. GDKKN

B. IFNMP

C. HELLO

D. IFMMP

Solution

1. Step 1: Shift each letter by 1

   H->I, E->F, L->M, L->M, O->P

2. Step 2: Combine shifted letters

   The encrypted message is IFMMP.

3. Final Answer:

   IFMMP -> Option D

4. Quick Check:

   Shift each letter +1 = IFMMP [OK]

Hint: Shift letters by 1 to encrypt; H becomes I, E becomes F [OK]

Common Mistakes:

• Not wrapping Z to A
• Shifting letters backward
• Leaving message unchanged

4. A program uses a key to decrypt messages but the key is accidentally shared publicly. What is the main problem caused by this?

medium

A. Encryption speed slows down

B. Messages become longer

C. Messages can be read by anyone

D. Messages get deleted automatically

Solution

1. Step 1: Understand the role of the key

   The key keeps encrypted messages secret by allowing only authorized decryption.

2. Step 2: Consequence of sharing the key

   If the key is public, anyone can decrypt and read the messages.

3. Final Answer:

   Messages can be read by anyone -> Option C

4. Quick Check:

   Public key leaks = anyone reads messages [OK]

Hint: Keep keys secret; public keys let anyone read messages [OK]

Common Mistakes:

• Thinking messages get deleted
• Believing encryption speed changes
• Assuming message length changes

5. You want to send a secret message using a simple encryption that shifts letters by a key number. If your key is 3 and your message is CAT, what is the encrypted message? (Use A->B->C... and wrap Z->A)

hard

A. FDW

B. DBU

C. FDU

D. CDT

Solution

1. Step 1: Shift each letter by 3

   C->F, A->D, T->W (T->U->V->W)

2. Step 2: Combine shifted letters

   The encrypted message is FDW.

3. Final Answer:

   FDW -> Option A

4. Quick Check:

   Shift letters +3 = FDW [OK]

Hint: Shift letters by key number; C +3 = F [OK]

Common Mistakes:

• Not wrapping letters after Z
• Shifting letters by wrong number
• Mixing letter order