5.2 Cryptography

Encryption Keeps Data Confidential

Cryptography protects data by transforming readable plaintext into unreadable ciphertext using an algorithm and a key. Symmetric encryption uses one shared key for both encrypting and decrypting, making it fast but requiring secure key exchange. Asymmetric encryption uses a public key to encrypt and a private key to decrypt, solving the key-sharing problem but running slower.

Hashing vs. Encryption

Hashing converts data into a fixed-length digest using a one-way function — it cannot be reversed, so it's used to verify integrity (e.g., checking if a file was altered) rather than to hide data for later retrieval. Unlike encryption, hashing has no key and the same input always produces the same output, which is useful for storing passwords securely.

Certificates and Trust

Digital certificates, issued by a Certificate Authority (CA), bind a public key to an identity, enabling secure communication like HTTPS. Digital signatures use a sender's private key to prove authenticity and integrity of a message — anyone with the public key can verify it. Together, encryption, hashing, and certificates provide confidentiality, integrity, and authentication.

Key terms

Plaintext vs. Ciphertext

Plaintext is readable original data; ciphertext is the scrambled result after encryption.

Symmetric Encryption

Uses one shared key for both encryption and decryption; fast but requires secure key exchange.

Asymmetric Encryption

Uses a public key to encrypt and a matching private key to decrypt; solves key-sharing problem, slower than symmetric.

Hashing

A one-way function that converts data into a fixed-length digest to verify integrity; cannot be reversed.

Digital Certificate

A document issued by a Certificate Authority that binds a public key to a verified identity, enabling trust (e.g., HTTPS).

Try an AP-style question

How does hashing differ from encryption?

1. Hashing is one-way
2. Hashing is reversible
3. They are the same
4. Hashing needs a key pair