🔑 Public Key & Private Key — The Basics
These are part of a cryptographic system called asymmetric encryption, where:
- Public Key: Can be shared with anyone.
- Private Key: Must be kept secret.
They are mathematically linked, but you can’t derive the private key from the public key easily (that’s the magic of cryptography 😄).
💡 How It Works
There are two major uses of this system:
1. Encryption / Decryption (for Confidentiality)
Imagine you want to send a secret message to someone.
- 🔐 You encrypt the message using their public key.
- 🗝️ Only they can decrypt it using their private key.
So:
Encrypt with Public Key ➡ Decrypt with Private Key
🧠 Only the person with the correct private key can unlock the message.
2. Digital Signatures (for Authenticity & Integrity)
This is to prove a message came from you and wasn’t changed.
- 🖋️ You sign a message with your private key.
- ✅ Anyone can verify it using your public key.
So:
Sign with Private Key ➡ Verify with Public Key
📍 Where Do We Use This?
-
SSL/TLS (HTTPS websites)
- Your browser uses a website’s public key to establish a secure connection.
-
SSH (for secure remote login)
- You use your private key to authenticate yourself on a remote server that has your public key.
-
Cryptocurrencies (like Bitcoin, Ethereum)
- Your private key gives access to your funds, and your public key is like your wallet address.
-
Email encryption (like PGP/GPG)
- Send secure emails using the recipient’s public key and decrypt using your private key.
-
JWTs and API Authentication
- Digital signatures ensure data or tokens haven’t been tampered with.
🔐 Summary
| Action | Uses Which Key? | Who Uses It? |
|---|---|---|
| Encrypt a message | Public key | Sender |
| Decrypt a message | Private key | Receiver |
| Sign a message | Private key | Sender |
| Verify a signature | Public key | Receiver |
🔁 SSH Authentication Flow
| Step | Action |
|---|---|
| 1️⃣ | Server has your public key |
| 2️⃣ | SSH client connects |
| 3️⃣ | Server sends encrypted challenge |
| 4️⃣ | Client decrypts using private key |
| 5️⃣ | Sends response |
| 6️⃣ | Server verifies and grants access |