Public and Private Keys, Using a Mailbox
Public-key cryptography can seem complicated at first, but the basic idea is simple:
One key can be shared with everyone, while the other key must be kept secret.
These are called a public key and a private key.
The Mailbox Analogy
Imagine a mailbox outside your house.
Anyone can walk up to the mailbox and put a letter inside.
But only you have the key that opens the mailbox and lets you retrieve what is inside.
This is similar to how public-key encryption works.
Public key → Can be shared with everyone
Private key → Must be kept secret
Someone can use your public key to encrypt information for you.
Only your private key can decrypt that information.
A Simple Example
Imagine Alice wants to send Bob a secret message.
Bob gives Alice his public key:
Bob's Public Key
↓
Alice
↓
Encrypts the message
↓
Encrypted message
↓
Bob
↓
Bob's Private Key
↓
Original message
Bob can safely share his public key with Alice because it is designed to be public.
His private key, however, should never be shared.
Why Have Two Keys?
With traditional symmetric encryption, the same secret key is used to encrypt and decrypt information.
That creates a problem:
How do you safely give the secret key to the other person in the first place?
Public-key cryptography helps solve this key distribution problem.
You can share your public key openly while keeping your private key secret.
Public Key vs Private Key
| Public Key | Private Key |
|---|---|
| Can be shared | Must be kept secret |
| Used by others to encrypt data for you | Used to decrypt that data |
| Can be published | Should never be published |
| Used in some digital-signature systems | Used to create digital signatures |
The exact use of each key depends on the cryptographic system and algorithm.
Public Keys Can Also Verify Signatures
Public and private keys aren’t only used for encryption.
They can also be used for digital signatures.
Imagine Bob signs a message using his private key.
Other people can use Bob’s public key to verify that the signature corresponds to Bob’s key.
Conceptually:
Bob's Private Key
↓
Creates signature
↓
Message
↓
Bob's Public Key
↓
Verifies signature
This helps provide authentication and integrity.
Where Are Public and Private Keys Used?
Public-key cryptography is used in many areas of cybersecurity, including:
- HTTPS/TLS for secure web communication
- SSH for secure remote access
- Digital signatures
- Secure email
- Software and certificate signing
- Cryptocurrency systems
It is one of the fundamental ideas behind modern digital security.
One Important Detail
Public-key cryptography is usually not used to encrypt huge amounts of data directly because asymmetric operations are generally more computationally expensive than symmetric encryption.
Instead, modern systems often combine both approaches.
For example:
Public-key cryptography
↓
Securely establish/share a session key
↓
Symmetric encryption
↓
Encrypt the actual data
This combination provides both secure key exchange and efficient data encryption.
Key Takeaways
- Public-key cryptography uses a key pair.
- The public key can be shared openly.
- The private key must remain secret.
- A public key can be used to encrypt information intended for the private-key holder.
- Private and public keys can also be used for digital signatures.
- Public-key cryptography is an important part of technologies such as HTTPS and SSH.
- Modern secure systems often combine asymmetric and symmetric cryptography.
Try It Yourself
You can see public-key cryptography in action by generating your own key pair in a Linux practice environment using SSH:
ssh-keygen -t ed25519
This creates a key pair consisting of:
Private key → Keep this secret
Public key → Safe to share
For learning, use your own computer or virtual machine. Never share a private key that protects a real account or system.
Next Lesson
Next, learn How Authentication Works to understand how websites verify who you are and how sessions keep you logged in.