Encryption Methods

H446 · 1.3 Exchanging Data · A-Level Computer Science

Component 01

Caesar Cipher (Substitution Cipher)

Each letter is shifted by a fixed key value. With key=3: A→D, B→E, C→F, etc. Only 25 possible keys — trivial to brute-force. Educational/historical only.

Try It

3

Ciphertext:

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Why It's Insecure

  • Only 25 possible keys — an attacker can try all of them in seconds
  • Preserves letter frequency patterns — frequency analysis easily cracks it
  • e.g. 'E' is the most common letter in English; if 'H' is most common in ciphertext, key is probably 3

Vernam Cipher (One-Time Pad)

XOR each bit of the plaintext with the corresponding bit of a truly random key. If conditions are met, it is mathematically proven unbreakable.

Conditions for Perfect Security

  • Key must be as long as the plaintext
  • Key must be truly random (not pseudo-random)
  • Key must be used only once (hence "one-time")
  • Key must be kept completely secret

Why unbreakable: every possible plaintext is equally likely — even unlimited computation cannot determine the original message.

Practical Limitation

The key distribution problem: how do two parties securely share a key as long as their entire communication, before communicating?

If the same key is reused, XOR-ing two ciphertexts reveals information about both plaintexts. Used in practice by spies (cipher pads) and some military systems.

XOR Example

Plaintext:   A = 01000001
Key:             01101100
                 ─────────
Ciphertext:      00101101  (XOR)

Decryption:  00101101
Key (same):  01101100
             ─────────
Plaintext:   01000001 = A  ✓

Public Key Cryptography (Asymmetric)

Each party has a key pair: a public key (shared freely) and a private key (never shared). Data encrypted with the public key can only be decrypted with the corresponding private key.

Symmetric (same key)

Same key encrypts and decrypts. Fast. Problem: how to share the key securely?

✓ Fast for bulk data encryption

✗ Key distribution problem

Asymmetric (key pair)

Public key encrypts, private key decrypts. Solves key distribution. Slower.

✓ Solves key distribution

✗ Much slower than symmetric

HTTPS / TLS Hybrid Approach

1

Server sends its public key (in a certificate signed by a Certificate Authority).

2

Browser verifies the certificate. Generates a random symmetric session key.

3

Browser encrypts the session key with the server's public key and sends it.

4

Server decrypts with its private key — both now share the same symmetric session key.

5

All further communication uses fast symmetric encryption with the session key.

Hashing & Password Storage

A hash function maps data to a fixed-length output. It is a one-way function — you cannot reverse a hash to get the original. Same input always produces same output.

"password" → 5f4dcc3b5aa...

"Password" → 8cbf2b2e6d8...

"password1" → 7c6a180b36c...

Algorithms: MD5 (broken — do not use), SHA-256 (current standard), bcrypt (password-specific)

Rainbow Table Attack

Precomputed table of password→hash pairs. Attacker hashes all common passwords in advance. If database is breached, look up hashes to find plaintext passwords.

Salting (Defence)

Add a random value (salt) to each password before hashing. Hash("password" + "xK9p!3") is unique per user. Invalidates precomputed rainbow tables.

Brute Force Caesar Cipher

Enter ciphertext and see all 25 possible decryptions — showing why Caesar is trivially breakable.

Public Key Exchange Simulator

Step through Alice sending an encrypted message to Bob using asymmetric cryptography.