The internet was NOT designed for privacy. When you send information online, it doesn’t travel directly from your device to its destination.
Instead, it is broken into small pieces called packets and sent across a shared network made up of routers, servers, and systems owned by many different organizations (and governments!).
Your data may pass through dozens of machines you do not own, cannot see, and do not control.
This is how large-scale networking works.
It’s efficient, resilient, and global, but it also means that information is constantly moving through infrastructure that is not private by default.
Without encryption, any system along that path could read the contents of those packets. That would include passwords, messages, financial information, and medical data.
Encryption exists to solve this exact problem.
Encryption is the process of mathematically transforming readable information, called plaintext, into an unreadable form called ciphertext.
Ciphertext looks like random data.
It does not contain recognizable words, structure, or meaning. Without the correct cryptographic key, reversing this transformation is not realistically possible.
This allows information to move freely across shared networks without exposing its contents to everyone who helps carry it.
Why Encryption Is Necessary on Shared Networks
In the physical world, secrecy often relied on controlling access.
You could hide a letter, choose a trusted courier, or restrict who could enter a room. That model does not work on the internet.
Modern communication depends on shared infrastructure.
Data must pass through systems operated by internet providers, cloud platforms, and network operators. There is no way to privately route data end to end without touching shared components.
Encryption accepts this reality.
Instead of trying to hide the path, it protects the message itself. The data can travel anywhere it needs to go, but only the intended recipient can understand it.
This is what allows private communication to exist on a public network.
How Encryption Protects Information
At a high level, encryption works by applying mathematical transformations to data using a secret value called a key.
You start with plaintext, such as a message or a file.
That plaintext is processed by an encryption algorithm together with a key. The algorithm scrambles the data in a precise way that depends on the key.
The result is ciphertext.
Ciphertext does not resemble the original data.
It does not leak meaning or structure. Even if someone intercepts it, they cannot recover the original information without the correct key.
To reverse the process, the recipient uses the corresponding key to run the ciphertext back through the algorithm. The original plaintext is restored.
The security of encryption comes from mathematics instead of secrecy of the algorithm.
Modern encryption algorithms are publicly known and widely studied.
What keeps the data secure is the key. Without it, decryption would require an infeasible amount of computation.
The Core Idea Behind Symmetric Encryption
The simplest form of encryption is called symmetric encryption. It uses the same secret key to encrypt and decrypt data.
You can think of it like a lockbox. The same key locks it and unlocks it.
Here is how symmetric encryption works in practice.
A message begins as plaintext.
That plaintext is fed into an encryption algorithm such as AES, the Advanced Encryption Standard, along with a secret key. The algorithm performs a series of mathematical operations that mix the data and the key together in a reversible way.
The output is ciphertext.
Anyone who intercepts the ciphertext sees only meaningless data.
To recover the original message, the recipient must have the same secret key and use it with the same algorithm.
Symmetric encryption is fast, efficient, and extremely secure when used correctly. It is widely used to protect data at rest and data in transit.
The Challenge of Sharing Secret Keys
Symmetric encryption works very well once both parties share the secret key.
The challenge is getting that key to the other party securely in the first place.
If you send the key in plaintext over the network, anyone who intercepts it can decrypt all future messages.
This creates a problem when communicating with someone you have never met, across a network you do not trust.
For a long time, this limitation made secure communication at scale very difficult.
What Encryption Makes Possible
Encryption doesn’t require private networks or trusted intermediaries. It allows confidentiality to exist even when communication passes through untrusted systems.
Your data can be routed across the internet, stored on remote servers, and transmitted through shared infrastructure without exposing its contents to everyone along the way.
This is what makes online banking, private messaging, secure websites, and modern digital life possible.
Coming Next: Solving the Key Exchange Problem
Symmetric encryption protects data, but it does not solve the problem of securely sharing secret keys. Public key cryptography provides the missing piece.
In the next lesson, we will explore how systems like RSA allow two parties to establish secure communication without ever sending a secret key across the network.
It is one of the most important ideas in cryptography, and it underpins nearly everything we rely on for security online today.
