Cryptography in High Level Design

Modern distributed systems constantly exchange sensitive data:

• User passwords
• Payment information
• API tokens
• Private messages
• Financial transactions

If this data travels across networks without protection, attackers could easily intercept or modify it.

This is where cryptography becomes essential.

Cryptography is the science of protecting information using mathematical techniques so that only authorized parties can read or verify it.

It ensures:

Large platforms like :contentReference[oaicite:0]{index=0}, :contentReference[oaicite:1]{index=1}, and **:contentReference[oaicite:2]{index=2} rely heavily on cryptography to secure billions of user interactions every day.

Why Cryptography Matters in System Design

Consider a simple login request.

User → Internet → Server

Without encryption:

• Password travels in plaintext
• Anyone intercepting traffic can steal it
• Attackers can impersonate users

With cryptography:

User → Encrypted Request → Server

Now even if someone intercepts the data:

• They cannot read it
• They cannot modify it
• They cannot impersonate the sender

Cryptography in Distributed Systems Architecture

Cryptography is used in multiple places inside large-scale systems.

Cryptography protects:

Two Main Types of Encryption

Encryption techniques are broadly divided into two categories.

Symmetric Encryption

Symmetric encryption uses a single shared secret key.

The same key is used to:

• Encrypt the data
• Decrypt the data

Example Flow

1. Sender and receiver share secret key K
2. Sender encrypts message using K
3. Receiver decrypts using the same key

Plaintext + Key → Encryption → Ciphertext
Ciphertext + Key → Decryption → Plaintext

Example Algorithm

One of the most widely used symmetric algorithms is AES (Advanced Encryption Standard).

AES is used by:

• Banking systems
• Cloud storage
• Secure messaging apps

Symmetric Encryption Architecture

Advantages

Disadvantages

This key distribution problem is why asymmetric cryptography exists.

Asymmetric Encryption (Public Key Cryptography)

Asymmetric encryption uses two different keys.

These keys are mathematically linked.

Data encrypted with one key can only be decrypted with the other.

Basic Architecture

Example Algorithm

One of the most famous asymmetric algorithms is RSA.

It is widely used in:

• TLS handshakes
• Secure emails
• Key exchange protocols

Example Flow

1. Server generates public/private key pair.
2. Server shares public key with clients.
3. Client encrypts message using public key.
4. Server decrypts using private key.

Diagram

Advantages

Disadvantages

Because of this, most systems combine both encryption types.

Hybrid Encryption (Real World Systems)

Modern protocols like **TLS use both encryption methods.

Process:

1. Asymmetric encryption exchanges a symmetric key.
2. Symmetric encryption encrypts actual data.

This provides:

• Secure key exchange
• Fast communication

Diffie-Hellman Key Exchange

The Diffie–Hellman Key Exchange is a cryptographic method that allows two parties to create a shared secret key over an insecure network.

The key insight:

Two parties can generate the same secret without ever sending the secret itself.

The Core Idea

Imagine two people mixing colors.

Public color → Yellow

Person A:

Yellow + Secret Red = Orange

Person B:

Yellow + Secret Blue = Green

They exchange results and mix again.

Both eventually arrive at the same final color.

An observer cannot determine the secret.

Diffie-Hellman Architecture

Both sides independently generate the same key.

Why Diffie-Hellman is Important

Diffie-Hellman enables:

• Secure session key creation
• No prior shared secret
• Protection against eavesdropping

It is used in modern protocols like **TLS and **SSH.

Digital Signatures

Encryption ensures confidentiality.

But systems also need:

• Authentication
• Data integrity
• Non-repudiation

This is where digital signatures come in.

What is a Digital Signature?

A digital signature proves:

• Who sent the message
• That the message was not modified

It works using asymmetric cryptography.

Signing Process

Steps:

1. Message hashed.
2. Hash encrypted using private key.
3. Result becomes digital signature.

Verification Process

If both hashes match:

Message is authentic

Example Algorithm

Common digital signature algorithms include:

• RSA
• ECDSA

Real World Use Cases

Digital signatures are used everywhere.

TLS Handshake Example

Modern HTTPS communication uses all these techniques together.

Cryptography in Modern System Design

Distributed architectures integrate cryptography at multiple layers.

Best Practices for Cryptography in HLD

Common Cryptographic Mistakes

Summary

Cryptography is fundamental for securing modern distributed systems.

Key techniques include:

Modern systems combine these techniques to ensure:

• secure communication
• trusted identities
• protected data

Without cryptography, the internet would not be secure enough to support banking, messaging, or large-scale platforms.

Understanding these concepts is essential for designing secure distributed systems in High Level Design.