5.1 Encryption

What encryption does

Encryption transforms readable plaintext into unreadable ciphertext using an algorithm and a key, protecting confidentiality. Only someone with the correct key can reverse the process (decryption). Encryption protects data both at rest (stored files) and in transit (moving across networks).

Symmetric vs. asymmetric encryption

Symmetric encryption uses one shared key for both encrypting and decrypting — it's fast but requires securely distributing the key (e.g., AES). Asymmetric encryption uses a public key to encrypt and a mathematically linked private key to decrypt — slower, but solves the key-distribution problem (e.g., RSA).

Strength and use cases

Encryption strength depends on key length and algorithm quality; longer keys are harder to brute-force. Asymmetric encryption is often used to securely exchange a symmetric key, which then encrypts the bulk of the data — combining security with speed, as in HTTPS/TLS.

Key terms

Plaintext

Readable, unencrypted data before it is transformed by an encryption algorithm.

Ciphertext

Encrypted, unreadable data produced by applying an algorithm and key to plaintext.

Symmetric encryption

Uses one shared secret key for both encryption and decryption; fast but requires secure key distribution (e.g., AES).

Asymmetric encryption

Uses a public key to encrypt and a linked private key to decrypt; solves key distribution but is slower (e.g., RSA).

Data at rest vs. in transit

At rest = stored data (e.g., on a disk); in transit = data actively moving across a network. Both should be encrypted.

Try an AP-style question

In symmetric encryption, how many keys are used?

1. A public and private pair
2. No keys at all
3. A new key per letter
4. One shared key