True or false: "Symmetric encryption can protect data integrity."

False — symmetric encryption provides only confidentiality, not integrity.

This is a common and dangerous misconception. Encryption scrambles data so it can't be read, but it does nothing to prevent modification.

Why not?

• An attacker can flip bits in the ciphertext without knowing the plaintext
• The receiver decrypts to a modified plaintext with no way to detect the change
• Stream ciphers and CTR mode are especially vulnerable: flipping bit $i$ in the ciphertext flips bit $i$ in the plaintext

To protect integrity, you need:

• A MAC (symmetric integrity protection), or
• A digital signature (asymmetric integrity protection)

Modern best practice: Use authenticated encryption (e.g., AES-GCM) which combines encryption + MAC in a single operation.

Go deeper:

• Block cipher mode of operation — how bit-flips and block reordering defeat unauthenticated encryption.