AP Cybersecurity
AP Cybersecurity / All lessons / Unit 5: Securing Applications and Data
5Unit 5: Securing Applications and Data
AP Cybersecurity topic 5.3

Protecting Stored Data with Cryptography

Encryption turns readable plaintext into ciphertext using a key. Symmetric encryption uses the same key to encrypt and decrypt; it is fast and used for files and disks (AES is the modern standard). Classic ciphers like Caesar show the idea but are easy to break by trying all keys or analyzing letter frequency.

Protecting Stored Data with Cryptography vocabulary

  • Plaintext / ciphertext: Readable data before encryption / scrambled data after.
  • Symmetric encryption: One shared secret key for both encrypting and decrypting.
  • AES: Advanced Encryption Standard: the widely used symmetric cipher.
  • Key space: The number of possible keys. Larger is harder to brute force.

AP-style practice questions

Try each one, then open the answer. To save your progress, take this quiz in the interactive course.

Skill: Mitigate Risk

1. A teacher encrypts a spreadsheet of student grades with a password before emailing it to a counselor. Which security goal does this protect?

  1. (A) Availability
  2. (B) Confidentiality
  3. (C) Physical security
  4. (D) Non-repudiation only
Show answer

(B) Confidentiality Encryption keeps the data unreadable to anyone without the key.

Skill: Mitigate Risk 
Caesar cipher, shift key = 3
Plaintext:  ATTACK
Ciphertext: ?

2. Which of the following is the ciphertext?

  1. (A) DWWDFN
  2. (B) XQQXZH
  3. (C) BUUBDL
  4. (D) ATTACK
Show answer

(A) DWWDFN Each letter shifts 3 places forward: A to D, T to W, C to F, K to N.

Skill: Analyze Risk

3. Which of the following best explains why symmetric encryption is commonly used to encrypt large files and whole disks?

  1. (A) It uses no key
  2. (B) It is fast and efficient for large amounts of data
  3. (C) It cannot be decrypted
  4. (D) It requires a different key for every byte
Show answer

(B) It is fast and efficient for large amounts of data Symmetric algorithms like AES are fast, which suits bulk data.

Skill: Analyze Risk

4. Two coworkers use symmetric encryption to share files. Which of the following is the main challenge?

  1. (A) They must share the secret key safely without an adversary seeing it
  2. (B) The files will become larger than a terabyte
  3. (C) They must use different keys to encrypt and decrypt
  4. (D) Symmetric encryption only works on images
Show answer

(A) They must share the secret key safely without an adversary seeing it Both sides need the same key, so key distribution is the weak point.

Open 5.3 in the interactive course