Encryption

This lesson covers encryption — the process of converting plaintext into ciphertext to protect data from unauthorised access. You need to understand the Caesar cipher, Vernam cipher (one-time pad), symmetric and asymmetric encryption, and their use cases for the OCR H446 specification.

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What is Encryption?

Encryption is the process of transforming readable data (plaintext) into unreadable data (ciphertext) using an algorithm and a key. Only someone with the correct key can reverse the process (decryption) to recover the plaintext.

Key Terms

Term	Definition
Plaintext	The original, readable message
Ciphertext	The encrypted, unreadable message
Key	A value used by the encryption algorithm to transform the data
Encryption	The process of converting plaintext to ciphertext
Decryption	The process of converting ciphertext back to plaintext
Cipher	The algorithm used to perform encryption/decryption

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Caesar Cipher

The Caesar cipher is a simple substitution cipher that shifts each letter in the plaintext by a fixed number of positions in the alphabet.

How It Works

• Choose a shift value (the key), e.g., 3.
• Each letter in the plaintext is replaced by the letter that is 3 positions further in the alphabet.
• The alphabet wraps around (Z + 1 = A).

Worked Example: Shift = 3

Plain	A	B	C	D	E	F	G	...	X	Y	Z
Cipher	D	E	F	G	H	I	J	...	A	B	C

Plaintext: HELLO Ciphertext: KHOOR

Decryption

To decrypt, shift each letter back by the key value.

KHOOR with shift -3 -> HELLO

Security Analysis

Aspect	Assessment
Key space	Only 25 possible keys (shifts 1-25)
Vulnerable to	Brute force (try all 25 shifts), frequency analysis
Security	Extremely weak — trivially breakable
Use	Educational only — never for real security

Frequency analysis: In English, the letter E is the most common. If the most common letter in the ciphertext is H, the shift is likely 3. This immediately breaks the cipher.

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Vernam Cipher (One-Time Pad)

The Vernam cipher is a theoretically unbreakable encryption scheme when used correctly.

How It Works

1. Generate a random key (one-time pad) that is at least as long as the plaintext.
2. Convert both the plaintext and key to binary.
3. Apply the XOR (exclusive OR) operation between each bit of the plaintext and the corresponding bit of the key.

XOR Truth Table

A	B	A XOR B
0	0	0
0	1	1
1	0	1
1	1	0

Key property: XOR is its own inverse. If C = P XOR K, then P = C XOR K.

Worked Example

Plaintext (ASCII):  H = 01001000
Key (random):           10110101
                        ────────
Ciphertext (XOR):       11111101

To decrypt: XOR the ciphertext with the same key:

Ciphertext:             11111101
Key:                    10110101
                        ────────
Plaintext:              01001000 = H

Requirements for Unbreakability

The Vernam cipher is only mathematically proven unbreakable if ALL of these conditions are met:

Requirement	Why
Key is truly random	Non-random patterns can be exploited
Key is at least as long as the plaintext	Short keys that are reused create patterns
Key is used only once	Reusing the key allows mathematical attacks
Key is kept completely secret	If the key is compromised, so is the message

If any condition is violated, the cipher is no longer provably secure.

Practical Limitations

• Generating truly random keys of sufficient length is difficult.
• Distributing the key securely is a major challenge (the "key distribution problem").
• The key must be as long as every message, making it impractical for large data volumes.

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Symmetric Encryption