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RSA OAEP Padding

The Memo SDK uses RSA-OAEP (Optimal Asymmetric Encryption Padding) for encrypting AES keys. OAEP is a modern, secure padding scheme that enhances the security of RSA encryption.

What is OAEP?

Section titled “What is OAEP?”

OAEP stands for Optimal Asymmetric Encryption Padding:

  • OAEP = Optimal Asymmetric Encryption Padding
  • A padding scheme used with RSA encryption
  • More secure than older padding schemes like PKCS#1 v1.5

Why Padding is Needed

Section titled “Why Padding is Needed”

RSA encryption cannot directly encrypt raw data; it needs padding first:

  1. Fixed-size blocks: RSA can only encrypt fixed-size data blocks
  2. Size limitations: A 2048-bit RSA key can only encrypt 245 bytes (minus padding)
  3. Security enhancement: Padding increases randomness and prevents certain attacks

Padding Functions

Section titled “Padding Functions”
  • ✅ Increases randomness: Same plaintext produces different ciphertext
  • ✅ Prevents attacks: Protects against chosen ciphertext attacks
  • ✅ Ensures correct format: Makes data the correct size for encryption

OAEP vs PKCS#1 v1.5

Section titled “OAEP vs PKCS#1 v1.5”
FeatureOAEPPKCS#1 v1.5
Security✅ More secure⚠️ Older, known vulnerabilities
Randomness✅ Uses random numbers⚠️ Weaker randomness
Standard✅ Modern standard⚠️ Old standard
Recommended✅ Yes❌ Not recommended

Why Choose OAEP?

Section titled “Why Choose OAEP?”
  • ✅ More secure, resists more attacks
  • ✅ Uses hash functions (e.g., SHA-256) to increase security
  • ✅ Modern encryption standard recommendation

OAEP Workflow

Section titled “OAEP Workflow”

Here’s how OAEP padding works:

flowchart LR
    A[Raw Data] --> B[Add Padding<br/>OAEP Padding]
    B --> C[Use Random Numbers<br/>Increase Randomness]
    C --> D[Use Hash Function<br/>SHA-256]
    D --> E[RSA Encrypt]
    E --> F[Ciphertext]

    style A fill:#e1f5ff
    style F fill:#c8e6c9
    style B fill:#fff3e0
    style C fill:#fff3e0
    style D fill:#fff3e0

Implementation in Memo SDK

Section titled “Implementation in Memo SDK”

Here’s how RSA-OAEP is used in the Memo SDK:

Encryption

Section titled “Encryption”
// Encrypt AES key using OAEP
encryptedKey, err := rsa.EncryptOAEP(
    sha256.New(),        // Hash function: SHA-256
    rand.Reader,         // Random number generator
    s.PublicKey,         // RSA public key
    aesKey,              // Data to encrypt (AES key)
    nil,                 // Optional label (not used here)
)

Decryption

Section titled “Decryption”
// Decryption using corresponding OAEP
aesKey, err := rsa.DecryptOAEP(
    sha256.New(),        // Must use same hash function
    rand.Reader,         // Random number generator
    s.PrivateKey,        // RSA private key
    encryptedKey,        // Ciphertext to decrypt
    nil,                 // Optional label (must match encryption)
)

Key Points

Section titled “Key Points”
  • Same hash function: Encryption and decryption must use the same hash function (both SHA-256)
  • Randomness: OAEP ensures encrypting the same data produces different ciphertext (uses random numbers)
  • Security: OAEP provides better security than older padding schemes
Section titled “Related Topics”