Encryption basics in Intro to Computing - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is encryption?

Encryption is the process of changing information into a secret code so that only authorized people can read it.

beginner

What is the difference between plaintext and ciphertext?

Plaintext is the original readable message. Ciphertext is the scrambled, unreadable message after encryption.

beginner

What is a key in encryption?

A key is a secret value used to lock (encrypt) and unlock (decrypt) the message.

intermediate

What is symmetric encryption?

Symmetric encryption uses the same key to encrypt and decrypt the message, like a single key for a locked box.

intermediate

What is asymmetric encryption?

Asymmetric encryption uses two keys: a public key to encrypt and a private key to decrypt, like a mailbox where anyone can drop mail but only the owner can open it.

What does encryption do to a message?

A Turns it into a secret code

B Deletes the message

C Copies the message

D Prints the message

Encryption changes the message into a secret code so only authorized people can read it.

Which of these is the original readable message?

A Hash

B Ciphertext

C Key

D Plaintext

Plaintext is the original readable message before encryption.

In symmetric encryption, how many keys are used?

A Two keys

B One key

C No keys

D Three keys

Symmetric encryption uses one key for both encrypting and decrypting.

What is the role of the private key in asymmetric encryption?

A To create plaintext

B To encrypt messages

C To decrypt messages

D To generate ciphertext

The private key is used to decrypt messages encrypted with the public key.

Which analogy best describes asymmetric encryption?

A A mailbox where anyone can drop mail but only the owner can open it

B A single key for a locked box

C A book everyone can read

D A locked door with no key

Asymmetric encryption is like a mailbox: public key lets anyone encrypt (drop mail), private key lets owner decrypt (open mailbox).

Explain in your own words what encryption is and why it is important.

Think about how you keep your personal diary safe from others.

Describe the difference between symmetric and asymmetric encryption using simple analogies.

Imagine sharing a key with a friend versus having a public mailbox.

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