Encryption basics in Intro to Computing - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to encrypt a message by shifting each letter by 3 positions.

Intro to Computing

def encrypt(message):
    result = ""
    for char in message:
        if char.isalpha():
            shifted = chr((ord(char) - 65 + [1]) % 26 + 65)
            result += shifted
        else:
            result += char
    return result

Drag options to blanks, or click blank then click option'

A0

B1

C5

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 means no encryption happens.

Using 1 or 5 shifts letters incorrectly for this example.

Shifting letters by 3 positions is a common simple encryption called Caesar cipher.

2fill in blank

medium

Complete the code to decrypt a message encrypted by shifting letters by 3 positions.

Intro to Computing

def decrypt(message):
    result = ""
    for char in message:
        if char.isalpha():
            shifted = chr((ord(char) - 65 - [1]) % 26 + 65)
            result += shifted
        else:
            result += char
    return result

Drag options to blanks, or click blank then click option'

A3

B5

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 means no change, so message stays encrypted.

Using a different number than encryption causes wrong output.

Decrypting reverses the encryption by shifting letters backward by the same number.

3fill in blank

hard

Fix the error in the code to correctly check if a character is uppercase before encrypting.

Intro to Computing

def encrypt(message):
    result = ""
    for char in message:
        if [1]:
            shifted = chr((ord(char) - 65 + 3) % 26 + 65)
            result += shifted
        else:
            result += char
    return result

Drag options to blanks, or click blank then click option'

Achar.isalpha()

Bchar.isupper()

Cchar.isdigit()

Dchar.islower()

Attempts:

3 left

💡 Hint

Common Mistakes

Using isalpha() includes lowercase letters which may cause wrong shifts.

Using isdigit() or islower() is incorrect for uppercase check.

We only want to shift uppercase letters, so we check if the character is uppercase.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps each letter to its encrypted form by shifting 2 positions.

Intro to Computing

encrypted_dict = {char: chr((ord(char) - 65 [1] 2) % 26 + 65) for char in 'ABCDEFGHIJKLMNOPQRSTUVWXYZ' if char [2] 'M'}

Drag options to blanks, or click blank then click option'

A+

B-

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' in the first blank shifts letters backward.

Using '>' in the second blank selects letters after 'M'.

The first blank needs '+' to shift letters forward by 2. The second blank uses '<' to filter letters before 'M'.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps lowercase letters to uppercase encrypted letters shifted by 1, only for letters greater than 'f'.

Intro to Computing

encrypted_lower = { [1]: [2] for [3] in 'abcdefghijklmnopqrstuvwxyz' if [3] > 'f' }

Drag options to blanks, or click blank then click option'

A{char.upper()}

Bchar.upper()

Cchar

Dchr((ord(char.upper()) - 65 + 1) % 26 + 65)

Attempts:

3 left

💡 Hint

Common Mistakes

Using string interpolation in the key instead of a variable.

Not using the correct dictionary comprehension syntax.

The dictionary comprehension syntax requires a key-value pair separated by ':'. The key is 'char', the value is the encrypted uppercase letter shifted by 1.

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