Encryption &
Authentication

Symmetric · Asymmetric · Hashing · SSL/TLS · Two-Factor Authentication

CSZoneEdexcel GCSE Computer Science 1CP2

Encryption Basics

Scrambling Data to Keep it Private

Encryption converts plaintext into ciphertext using an encryption key. Only someone with the correct decryption key can read it. Protects data in transit and at rest.

Symmetric encryption: same key used to encrypt AND decrypt. Fast, but the key must be shared securely. e.g. AES.

Asymmetric encryption: uses a public key (encrypt) + private key (decrypt). Secure key exchange without sharing secrets. e.g. RSA. Used in HTTPS.

Hashing

One-way Scrambling

A hash function converts data into a fixed-length string (hash/digest). It is one-way — you cannot reverse a hash to get the original data. The same input always produces the same hash. Used for password storage.

password: "hello123"
SHA-256 hash: 2bd806c97f0e00af1a1fc3328fa763a9269723c8db8fac4f93af71db186d6e90

Databases store hashes, not plaintext passwords — if breached, actual passwords aren't exposed

Authentication

Proving Who You Are

Passwords: most common; should be long, complex, unique per site

Two-factor authentication (2FA): requires two separate proofs — e.g. password + SMS code, or password + fingerprint. Dramatically harder to compromise

Biometrics: fingerprint, face ID, iris scan — uses unique physical characteristics

SSL/TLS: protocol that provides encryption over the internet using asymmetric keys to establish a secure session — the basis of HTTPS

Exam Practice

Have a go at this question

Edexcel-style question

Explain why passwords should be stored as hashes rather than as plaintext in a database.

3 marks

If the database is hacked, plaintext passwords are immediately usable by attackers [1]. Hashes cannot be reversed to reveal the original password [1], so even if stolen, the attacker cannot log in with them [1].

Key Takeaways

What to Remember

Encryption: symmetric (same key) = fast; asymmetric (public/private keys) = secure key exchange

Hashing: one-way; fixed output; used to store passwords safely

2FA: two separate proofs of identity; much more secure than password alone

SSL/TLS: asymmetric encryption used to establish secure HTTPS connections