Encryption and Digital Signatures

This lesson covers how data is protected during storage and transmission using encryption. At A-Level you must understand symmetric and asymmetric encryption, the role of keys, and how digital signatures and digital certificates ensure authenticity and integrity.

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What Is Encryption?

Encryption is the process of converting plaintext (readable data) into ciphertext (unreadable scrambled data) using an algorithm and a key. Only someone with the correct key can decrypt the ciphertext back to plaintext.

Plaintext ──→ [Encryption Algorithm + Key] ──→ Ciphertext
Ciphertext ──→ [Decryption Algorithm + Key] ──→ Plaintext

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Symmetric Encryption

In symmetric encryption, the same key is used for both encryption and decryption. Both the sender and receiver must share this secret key.

Sender                                     Receiver
Plaintext ──→ [Encrypt with Key K] ──→ Ciphertext ──→ [Decrypt with Key K] ──→ Plaintext

Examples

• AES (Advanced Encryption Standard) — the most widely used symmetric cipher (128, 192, or 256-bit keys).
• DES (Data Encryption Standard) — older, now considered insecure (56-bit key).
• 3DES (Triple DES) — applies DES three times for stronger security.

Advantages

Advantage	Explanation
Fast	Symmetric algorithms are computationally efficient — suitable for encrypting large amounts of data
Simple	Only one key to manage per communication pair

Disadvantages

Disadvantage	Explanation
Key distribution problem	The shared key must be transmitted securely to both parties. If intercepted, security is broken
Scalability	For n users to communicate securely in pairs, n(n-1)/2 keys are needed — this grows rapidly

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Asymmetric Encryption (Public Key Cryptography)

In asymmetric encryption, two mathematically related keys are used: a public key (shared openly) and a private key (kept secret).

• Data encrypted with the public key can only be decrypted with the corresponding private key.
• Data encrypted with the private key can only be decrypted with the corresponding public key (used for digital signatures).

Sender                                               Receiver
Plaintext ──→ [Encrypt with Receiver's Public Key] ──→ Ciphertext
                                                        ──→ [Decrypt with Receiver's Private Key] ──→ Plaintext

Examples

• RSA (Rivest-Shamir-Adleman) — widely used, based on the difficulty of factoring large prime numbers.
• Elliptic Curve Cryptography (ECC) — shorter keys for equivalent security, efficient for mobile/IoT.

Advantages

Advantage	Explanation
No key distribution problem	Only public keys need to be shared — private keys are never transmitted
Scalability	Each user has one key pair; 2n keys for n users
Enables digital signatures	The private key can sign data; the public key verifies the signature

Disadvantages

Disadvantage	Explanation
Slow	Asymmetric algorithms are computationally expensive — much slower than symmetric
Key management	Public keys must be verified as genuine (solved by digital certificates)

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Hybrid Encryption (Used in Practice)

In practice, both methods are combined: