Encryption basics in Intro to Computing - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the main purpose of encryption?

Encryption is a process used in computing and communication. What is its main purpose?

A To delete data permanently from a device

B To convert readable data into a secret code to protect it from unauthorized access

C To compress data so it takes less space on the disk

D To speed up the transmission of data over the internet

💡 Hint

Think about why secret messages are used in spy movies.

Encryption changes readable information into a coded form so only authorized people can read it. This protects privacy and security.

❓ trace

intermediate

Trace the output of a simple Caesar cipher

Given the text 'HELLO' and a Caesar cipher that shifts letters by 3 places forward, what is the encrypted output?

Intro to Computing

def caesar_encrypt(text, shift):
    result = ''
    for char in text:
        if char.isalpha():
            shifted = chr((ord(char) - 65 + shift) % 26 + 65)
            result += shifted
        else:
            result += char
    return result

print(caesar_encrypt('HELLO', 3))

A IFMMP

B EBIIL

C HELLO

D KHOOR

💡 Hint

Each letter moves 3 places forward in the alphabet. 'H' becomes 'K'.

The Caesar cipher shifts each letter by the given number. 'H' + 3 = 'K', 'E' + 3 = 'H', and so on, resulting in 'KHOOR'.

❓ identification

advanced

Identify the type of encryption used

Which type of encryption uses the same key for both encrypting and decrypting data?

A Steganography

B Asymmetric encryption

C Symmetric encryption

D Hashing

💡 Hint

Think about a single key that locks and unlocks a door.

Symmetric encryption uses one key for both encrypting and decrypting. Asymmetric uses two keys (public and private).

❓ Comparison

advanced

Compare encryption and hashing

Which statement correctly compares encryption and hashing?

A Encryption is reversible with a key; hashing is one-way and not reversible

B Both encryption and hashing are reversible processes

C Hashing requires a key to reverse; encryption does not

D Encryption and hashing both produce the same output for the same input

💡 Hint

Think about whether you can get the original message back.

Encryption can be reversed using a key to get original data. Hashing creates a fixed output that cannot be reversed.

🚀 Application

expert

Determine the output length of encrypted data

A message of 128 bytes is encrypted using AES with a block size of 16 bytes. What is the length of the encrypted output in bytes?

A 128 bytes

B 144 bytes

C 256 bytes

D 16 bytes

💡 Hint

AES encrypts data in fixed-size blocks. If input fits exactly, output size matches input size.

AES uses 16-byte blocks. 128 bytes is exactly 8 blocks (128/16). No padding needed, so output is 128 bytes.

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