Public vs. Private Key Encryption

🔐 Understanding Public vs. Private Key Encryption: What's the Difference?

In the world of cryptography, asymmetric encryption uses a pair of keys — a public key and a private key — to secure communication. While most people associate encryption with simply "locking" data, the use of public and private keys introduces two distinct use cases:

• Confidentiality (keeping messages secret)
• Authenticity (proving who sent the message)

But here's the twist:
Depending on which key you use to encrypt the data — public or private — you achieve very different goals.

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🔑 The Basics: Asymmetric Encryption in a Nutshell

Asymmetric cryptography uses two mathematically linked keys:

• Public Key: Can be shared freely.
• Private Key: Must be kept secret.

The keys are linked in such a way that:

• Data encrypted with the public key can only be decrypted with the private key.
• Data "encrypted" (signed) with the private key can only be verified using the public key.

But the intent of each is fundamentally different.

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🔒 Encrypting with the Public Key (for Confidentiality)

📬 Use Case: Sending a Private Message

If Alice wants to send Bob a secret message:

1. She encrypts it using Bob's public key.
2. Only Bob can decrypt it — using his private key.

This ensures:

• Confidentiality: Only Bob can read the message.
• Even if someone intercepts the encrypted message, they cannot decrypt it.

✅ Common Use Case: Secure messaging, TLS/SSL encryption, file encryption

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✍️ Encrypting (Signing) with the Private Key (for Authenticity)

✅ Use Case: Proving Authorship

If Bob wants to send a message and prove to Alice that he sent it:

1. Bob generates a hash of the message.
2. He encrypts (signs) that hash using his private key.
3. Alice receives the message + signature.
4. She decrypts the signature using Bob’s public key and compares it to a hash of the message.

If they match, Alice knows:

• The message came from Bob (only he has the private key).
• The message wasn't tampered with (hash matches).

✅ Common Use Case: Digital signatures, software signing, blockchain transactions

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⚠️ Key Insight: Intent Matters

Intent	Encrypt With	Decrypt/Verify With	Purpose
🔒 Confidentiality	Public Key	Private Key	Only the intended receiver can read the message
✍️ Authenticity	Private Key	Public Key	Anyone can verify the sender's identity

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🚫 What Not to Do: Misusing the Keys

• Don't encrypt whole messages with your private key thinking it secures them — anyone with your public key can read it.
• Don't rely on the public key for authentication — it’s public and doesn’t prove identity alone.

Instead:

• Use private key → for signing
• Use public key → for encrypting

In modern cryptosystems, we use a **hybrid approach**:

• Asymmetric encryption for secure key exchange
• Symmetric encryption (like AES) for fast, secure message transmission

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🔚 In Summary

• Public key encryption protects confidentiality.
• Private key signing proves authenticity.
• Combining both enables secure, trusted communication in protocols like TLS, PGP, JWT, and more.