PKE Encryption Tutorial
Hands-on guide to public-key encrypted note sharing.
Public Key Encryption: An Illustrated Primer
A visual guide to understanding how public key encryption works, featuring Alice and Bob.
Table of Contents
- Chapter 1: The Key Ceremony
- Chapter 2: Bob Wants to Send a Secret
- Chapter 3: The Magic Inside
- Chapter 4: Sending the File
- Chapter 5: Alice Decrypts
- Chapter 6: Multi-Recipient Encryption
- Summary
Introduction
Public Key Encryption (PKE) solves a fundamental problem: How can two people communicate securely without first meeting to exchange a secret password?
The answer is elegant: each person has TWO keys:
- A public key they share with everyone (like a mailing address)
- A private key they keep secret (like the key to their mailbox)
Anyone can drop a letter in the mailbox, but only the owner can open it.
Chapter 1: The Key Ceremony
Alice wants to receive encrypted messages. First, she generates her keypair:
┌─────────────────────────────────────────────────────────────┐
│ ALICE'S COMPUTER │
│ │
│ Generating keypair... │
│ │
│ ┌──────────────────────────────────────────────┐ │
│ │ │ │
│ │ [████████████████████████████] 100% │ │
│ │ │ │
│ │ ✓ Private key generated │ │
│ │ (stored encrypted with passphrase) │ │
│ │ │ │
│ │ ✓ Public key derived │ │
│ │ │ │
│ │ Your public address: │ │
│ │ ┌────────────────────────────────────────┐ │ │
│ │ │ pk:7Xq9KmPvR3nYhW2sT8uJcL4bN6aF5gD1eZ │ │ │
│ │ └────────────────────────────────────────┘ │ │
│ └──────────────────────────────────────────────┘ │
│ │
│ Alice's Keys: │
│ ├── private.key (secret! encrypted with passphrase) │
│ └── public.key (shareable with anyone) │
│ │
└─────────────────────────────────────────────────────────────┘Alice shares her public address on her website, like a cryptocurrency wallet:
┌─────────────────────────────────────────────────────────────┐
│ alice.example.com │
├─────────────────────────────────────────────────────────────┤
│ │
│ Alice's Secure Inbox │
│ │
│ Want to send me something private? │
│ Encrypt it using my public key: │
│ │
│ ╔════════════════════════════════════════════════════╗ │
│ ║ pk:7Xq9KmPvR3nYhW2sT8uJcL4bN6aF5gD1eZ ║ │
│ ╚════════════════════════════════════════════════════╝ │
│ │
│ [Copy to Clipboard] │
│ │
└─────────────────────────────────────────────────────────────┘Key Points:
- The public address (`pk:...`) is safe to share with the entire world
- The private key stays on Alice's computer, protected by a passphrase
- Anyone can encrypt data FOR Alice using just her public key
- Only Alice can decrypt it (with her private key)
Chapter 2: Bob Wants to Send a Secret
Bob has a confidential document for Alice. He visits her website and copies her public key:
┌─────────────────────────────────────────────────────────────┐
│ BOB'S COMPUTER │
│ │
│ secret-proposal.pdf │
│ ┌─────────────────────────────────────────────┐ │
│ │ │ │
│ │ CONFIDENTIAL: Project Moonshot │ │
│ │ │ │
│ │ Budget: $10,000,000 │ │
│ │ Timeline: Q3 2026 │ │
│ │ ... │ │
│ │ │ │
│ └─────────────────────────────────────────────┘ │
│ │
│ Bob doesn't have Alice's passphrase. │
│ Bob doesn't NEED it! He only needs her PUBLIC KEY. │
│ │
└─────────────────────────────────────────────────────────────┘Bob encrypts the file using Alice's public key:
┌─────────────────────────────────────────────────────────────┐
│ │
│ Encrypting for recipient: │
│ pk:7Xq9KmPvR3nYhW2sT8uJcL4bN6aF5gD1eZ │
│ │
│ ┌──────────────────────────────────────────────┐ │
│ │ │ │
│ │ ✓ Generating ephemeral keypair... │ │
│ │ ✓ Performing key exchange (ECDH)... │ │
│ │ ✓ Generating random data key... │ │
│ │ ✓ Encrypting data key for recipient... │ │
│ │ ✓ Encrypting document... │ │
│ │ │ │
│ │ Done! Created: secret-proposal.pdf.enc │ │
│ │ Size: 1.2 MB │ │
│ │ Recipients: 1 │ │
│ │ │ │
│ └──────────────────────────────────────────────┘ │
│ │
└─────────────────────────────────────────────────────────────┘Chapter 3: The Magic Inside
What actually happens during encryption? This is where it gets interesting.
The Problem with Simple Encryption
You might ask: "Why not just encrypt directly with Alice's public key?"
The answer: Public key encryption is slow. Encrypting large files directly would take forever.
The Solution: Hybrid Encryption
Modern PKE uses a clever two-layer approach:
1. Generate a random Data Encryption Key (DEK) - just 32 random bytes 2. Encrypt the actual data with the DEK (fast symmetric encryption) 3. Encrypt the DEK with the recipient's public key (slow but tiny)
THE ENCRYPTION PROCESS
┌─────────────────────────────────────────────────────────────┐
│ │
│ 1. Generate ephemeral keypair (one-time use) │
│ │
│ ┌──────────────┐ │
│ │ ephemeral_sk │──┐ (Secret: discarded after use) │
│ │ ephemeral_pk │ │ (Public: stored in output file) │
│ └──────────────┘ │ │
│ │ │
│ 2. ECDH key exchange │ │
│ │ │
│ ┌──────────────┐ │ ┌───────────────┐ │
│ │ ephemeral_sk │──┼───>│ shared_secret │ │
│ └──────────────┘ │ └───────┬───────┘ │
│ │ │ │
│ ┌──────────────┐ │ │ │
│ │ alice_pubkey │──┘ │ │
│ └──────────────┘ │ │
│ v │
│ 3. Derive KEK (Key Encryption Key) │
│ │
│ ┌───────────────┐ ┌──────────┐ │
│ │ shared_secret │───>│ HKDF-256 │───> KEK │
│ └───────────────┘ └──────────┘ │
│ │
│ 4. Generate random DEK (Data Encryption Key) │
│ │
│ ┌────────────────────────────┐ │
│ │ DEK = 32 random bytes │ │
│ └────────────────────────────┘ │
│ │
│ 5. Wrap (encrypt) DEK with KEK │
│ │
│ ┌─────┐ ┌──────────────┐ ┌──────────────┐ │
│ │ DEK │ + │ AES-256-GCM │ = │ wrapped_dek │ │
│ └─────┘ └──────────────┘ └──────────────┘ │
│ ^ │
│ │ │
│ KEK │
│ │
│ 6. Encrypt the document with DEK │
│ │
│ ┌───────────┐ ┌──────────────┐ ┌────────────┐ │
│ │ plaintext │ + │ AES-256-GCM │ = │ ciphertext │ │
│ └───────────┘ └──────────────┘ └────────────┘ │
│ ^ │
│ │ │
│ DEK │
│ │
└─────────────────────────────────────────────────────────────┘Why Ephemeral Keys?
Each encryption generates a fresh "ephemeral" keypair. Why?
Forward Secrecy: If Alice's private key is compromised later, past messages remain secure. Each message used a different ephemeral key that was discarded after encryption.
The Encrypted File Structure
The output file contains everything needed for decryption:
secret-proposal.pdf.enc
┌─────────────────────────────────────────────────────────────┐
│ HEADER │
├─────────────────────────────────────────────────────────────┤
│ │
│ Ephemeral Public Key: kL3mN9pQ2rS5tU8vW... │
│ (Bob threw away the private half) │
│ │
│ Recipients: │
│ ┌────────────────────────────────────────────────────┐ │
│ │ Address: pk:7Xq9KmPvR3nYhW2sT8uJcL4bN6aF5gD1eZ │ │
│ │ Wrapped DEK: xY4zA7bC2dE5fG8hI... │ │
│ │ Nonce: jK9lM0nO1pQ2rS3t │ │
│ └────────────────────────────────────────────────────┘ │
│ │
│ Data Nonce: uV4wX5yZ6aB7cD8e │
│ Original Filename: secret-proposal.pdf │
│ │
├─────────────────────────────────────────────────────────────┤
│ ENCRYPTED DATA │
├─────────────────────────────────────────────────────────────┤
│ │
│ a7f3b2c8d9e0f1a2b3c4d5e6f7a8b9c0d1e2f3a4b5c6d7e8f9... │
│ (AES-256-GCM ciphertext with authentication tag) │
│ │
│ The actual document, encrypted with the DEK │
│ (1.2 MB of unreadable ciphertext) │
│ │
└─────────────────────────────────────────────────────────────┘Chapter 4: Sending the File
Bob sends the encrypted file to Alice. Since it's encrypted, he can use ANY channel:
┌───────────────────────────────────────────────────────────┐
│ Email │
│ │
│ From: [email protected] │
│ To: [email protected] │
│ Subject: Project Moonshot Proposal │
│ │
│ ─────────────────────────────────────────────────────── │
│ │
│ Hi Alice, │
│ │
│ Please find the proposal attached. I encrypted it │
│ to your public key: │
│ pk:7Xq9KmPvR3nYhW2sT8uJcL4bN6aF5gD1eZ │
│ │
│ Only you can decrypt it. │
│ │
│ - Bob │
│ │
│ [Attachment] secret-proposal.pdf.enc (1.2 MB) │
│ │
└───────────────────────────────────────────────────────────┘
│
│ The Internet
│
│ (Email servers, ISPs, hackers,
│ government agencies can all
│ SEE the file... but they can't
│ READ its contents!)
│
v
┌───────────────────────────────────────────────────────────┐
│ Alice's Inbox │
│ │
│ 📨 New message from Bob │
│ [Attachment] secret-proposal.pdf.enc │
│ │
└───────────────────────────────────────────────────────────┘Security Note: The encrypted file can safely travel over insecure channels. Even if intercepted, attackers see only random-looking bytes. The contents are protected by military-grade AES-256-GCM encryption.
Chapter 5: Alice Decrypts
Alice receives the file and decrypts it with her private key:
┌─────────────────────────────────────────────────────────────┐
│ ALICE'S COMPUTER │
│ │
│ Decrypting: secret-proposal.pdf.enc │
│ Enter passphrase for private key: •••••••••••• │
│ │
│ ┌──────────────────────────────────────────────┐ │
│ │ │ │
│ │ ✓ Unlocking private key... │ │
│ │ ✓ Found my recipient block │ │
│ │ ✓ Performing key exchange (ECDH)... │ │
│ │ ✓ Unwrapping data key... │ │
│ │ ✓ Decrypting document... │ │
│ │ ✓ Verifying integrity... │ │
│ │ │ │
│ │ Success! Output: secret-proposal.pdf │ │
│ │ │ │
│ └──────────────────────────────────────────────┘ │
│ │
│ secret-proposal.pdf │
│ ┌─────────────────────────────────────────────┐ │
│ │ │ │
│ │ CONFIDENTIAL: Project Moonshot │ │
│ │ │ │
│ │ Budget: $10,000,000 │ │
│ │ Timeline: Q3 2026 │ │
│ │ ... │ │
│ │ │ │
│ └─────────────────────────────────────────────┘ │
│ │
└─────────────────────────────────────────────────────────────┘The Decryption Flow
ALICE'S DECRYPTION PROCESS
┌─────────────────────────────────────────────────────────────┐
│ │
│ 1. Parse header from encrypted file │
│ │
│ ┌──────────────────┐ │
│ │ ephemeral_pubkey │ (from Bob's encryption) │
│ │ recipients[] │ │
│ │ data_nonce │ │
│ └──────────────────┘ │
│ │
│ 2. Find my recipient block (matching my address) │
│ │
│ ┌──────────────────────────────────────────┐ │
│ │ address: pk:7Xq9KmPvR3nYhW2s... │ ◄─ Me! │
│ │ wrapped_dek: xY4zA7bC2dE5fG... │ │
│ │ nonce: jK9lM0nO1pQ2rS3t │ │
│ └──────────────────────────────────────────┘ │
│ │
│ 3. ECDH key exchange (reverse of Bob's process) │
│ │
│ ┌──────────────┐ ┌───────────────┐ │
│ │ my_private │──┐ │ │ │
│ └──────────────┘ ├───>│ shared_secret │ │
│ ┌──────────────┐ │ │ │ │
│ │ephemeral_pub │──┘ └───────┬───────┘ │
│ └──────────────┘ │ │
│ │ │
│ The magic: this produces the SAME shared secret │
│ that Bob computed, even though Alice never saw │
│ Bob's ephemeral private key! │
│ │ │
│ v │
│ 4. Derive KEK (same as Bob did) │
│ │
│ ┌───────────────┐ ┌──────────┐ │
│ │ shared_secret │───>│ HKDF-256 │───> KEK │
│ └───────────────┘ └──────────┘ │
│ │
│ 5. Unwrap DEK │
│ │
│ ┌─────────────┐ ┌──────────────┐ ┌─────┐ │
│ │ wrapped_dek │ + │ AES-256-GCM │ = │ DEK │ │
│ └─────────────┘ └──────────────┘ └─────┘ │
│ ^ │
│ │ │
│ KEK │
│ │
│ 6. Decrypt the document │
│ │
│ ┌────────────┐ ┌──────────────┐ ┌───────────┐ │
│ │ ciphertext │ + │ AES-256-GCM │ = │ plaintext │ │
│ └────────────┘ └──────────────┘ └───────────┘ │
│ ^ │
│ │ │
│ DEK │
│ │
└─────────────────────────────────────────────────────────────┘The Mathematical Magic
The core of PKE relies on Elliptic Curve Diffie-Hellman (ECDH):
Bob computes: ephemeral_private × Alice_public = shared_secret
Alice computes: Alice_private × ephemeral_public = shared_secret
↑
Same result!This works because of elliptic curve math. Neither party reveals their private key, yet they arrive at the same shared secret.
Chapter 6: Multi-Recipient Encryption
What if Bob needs to send the same document to both Alice AND Carol?
Carol also has a keypair:
┌────────────────────────────────────────────┐
│ Carol's public key: │
│ pk:8Yr0LnQwS4oZiX3uK7fMcN2dV9hJ6pE5wT │
└────────────────────────────────────────────┘Bob encrypts for BOTH recipients in a single operation:
Encrypting for recipients:
• pk:7Xq9KmPvR3nYhW2sT8uJcL4bN6aF5gD1eZ (Alice)
• pk:8Yr0LnQwS4oZiX3uK7fMcN2dV9hJ6pE5wT (Carol)How Multi-Recipient Works
The clever insight: encrypt the data once, wrap the key multiple times.
MULTI-RECIPIENT ENCRYPTION
┌─────────────────────────────────────────┐
│ Single DEK │
│ (32 random bytes) │
└───────────┬─────────────┬──────────────┘
│ │
┌────────v───────┐ ┌──v─────────────┐
│ Wrapped for │ │ Wrapped for │
│ Alice's key │ │ Carol's key │
│ │ │ │
│ (Different │ │ (Different │
│ ciphertext!) │ │ ciphertext!) │
└────────────────┘ └────────────────┘
The document is encrypted ONCE with the DEK.
The DEK is wrapped separately for EACH recipient.The Multi-Recipient File
secret-proposal.pdf.enc (multi-recipient)
┌─────────────────────────────────────────────────────────────┐
│ HEADER │
├─────────────────────────────────────────────────────────────┤
│ │
│ Ephemeral Public Key: kL3mN9pQ2rS5tU8vW... │
│ │
│ Recipients: │
│ ┌────────────────────────────────────────────────────┐ │
│ │ [1] Address: pk:7Xq9KmPvR3nYhW2s... ← Alice │ │
│ │ Wrapped DEK: xY4zA7bC2dE5fG... │ │
│ │ Nonce: jK9lM0nO1pQ2rS3t │ │
│ ├────────────────────────────────────────────────────┤ │
│ │ [2] Address: pk:8Yr0LnQwS4oZiX3u... ← Carol │ │
│ │ Wrapped DEK: aB1cD2eF3gH4iJ... (different!)│ │
│ │ Nonce: kL5mN6oP7qR8sT9u │ │
│ └────────────────────────────────────────────────────┘ │
│ │
├─────────────────────────────────────────────────────────────┤
│ ENCRYPTED DATA │
├─────────────────────────────────────────────────────────────┤
│ │
│ (Same encrypted document for all recipients) │
│ The data is encrypted ONCE with the DEK │
│ │
└─────────────────────────────────────────────────────────────┘Who Can Decrypt?
┌─────────────┐ ┌─────────────┐ ┌─────────────┐
│ ALICE │ │ CAROL │ │ EVE │
│ │ │ │ │ │
│ ✓ Can │ │ ✓ Can │ │ ✗ Cannot │
│ decrypt │ │ decrypt │ │ decrypt │
│ │ │ │ │ │
│ Has private │ │ Has private │ │ Not in │
│ key for │ │ key for │ │ recipient │
│ pk:7Xq9... │ │ pk:8Yr0... │ │ list │
└─────────────┘ └─────────────┘ └─────────────┘Efficiency
Adding more recipients barely increases file size:
| Recipients | Header Size | Data Size | Total Overhead |
|---|---|---|---|
| 1 | ~200 bytes | 1.2 MB | 0.02% |
| 10 | ~1 KB | 1.2 MB | 0.08% |
| 100 | ~10 KB | 1.2 MB | 0.8% |
The document is encrypted once. Only the DEK wrapping is repeated.
Summary
╔═══════════════════════════════════════════════════════════╗
║ ║
║ PUBLIC KEY ENCRYPTION: KEY CONCEPTS ║
║ ║
║ ┌─────────────────────────────────────────────────────┐ ║
║ │ │ ║
║ │ • Share your PUBLIC KEY freely (like an address) │ ║
║ │ │ ║
║ │ • Keep your PRIVATE KEY secret (like a password) │ ║
║ │ │ ║
║ │ • Anyone can encrypt FOR you using your public │ ║
║ │ key - they don't need your password │ ║
║ │ │ ║
║ │ • Only YOU can decrypt (with your private key) │ ║
║ │ │ ║
║ │ • No shared passwords to exchange! │ ║
║ │ │ ║
║ │ • Multiple recipients in a single file │ ║
║ │ │ ║
║ │ • Forward secrecy (ephemeral keys protect past │ ║
║ │ messages even if your key is compromised) │ ║
║ │ │ ║
║ └─────────────────────────────────────────────────────┘ ║
║ ║
╚═══════════════════════════════════════════════════════════╝Glossary
| Term | Description |
|---|---|
| Public Key | Your "address" - share it freely |
| Private Key | Your secret - never share, stored encrypted |
| Keypair | A matched public + private key |
| Ephemeral Key | One-time keypair, discarded after each encryption |
| DEK | Data Encryption Key - random key for encrypting data |
| KEK | Key Encryption Key - derived from ECDH, wraps the DEK |
| ECDH | Elliptic Curve Diffie-Hellman - the key exchange magic |
| Hybrid Encryption | Combining asymmetric (PKE) and symmetric (AES) encryption |
| Forward Secrecy | Past messages stay secure even if keys are later compromised |
Cryptographic Building Blocks
| Purpose | Common Algorithms |
|---|---|
| Key Exchange | X25519 (Curve25519), P-256, P-384 |
| Key Derivation | HKDF-SHA256, HKDF-SHA384 |
| Symmetric Encryption | AES-256-GCM, ChaCha20-Poly1305 |
| Password-Based Key Storage | Argon2id, scrypt, PBKDF2 |
The Security Properties
1. Confidentiality: Only recipients can read the data 2. Integrity: Any tampering is detected (GCM authentication) 3. Authenticity: The sender's ephemeral key is bound to the message 4. Forward Secrecy: Compromised long-term keys don't expose past messages
Appendix: Why Not Just Use Passwords?
You might wonder: "Why not just share a password?"
| Password Sharing | Public Key Encryption |
|---|---|
| Need to meet or use secure channel first | Just publish your public key |
| Same password for all senders | Each sender uses your public key |
| Compromise affects everyone | Compromise of one sender doesn't affect others |
| Can't prove who sent it | Ephemeral keys provide some sender binding |
| No forward secrecy | Fresh keys = forward secrecy |
PKE elegantly solves the key distribution problem that plagued cryptography for centuries.
This primer explains the concepts behind public key encryption. For implementation-specific details, consult your encryption software's documentation.