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What Is RSA Encryption

RSA is the most widely used asymmetric encryption algorithm, invented by Rivest, Shamir, and Adleman in 1977. It uses a key pair: public key for encryption, private key for decryption.

How RSA Works

Key Pair Generation:
  Choose two large primes p, q
  Compute n = p × q
  Compute φ(n) = (p-1)(q-1)
  Choose e such that gcd(e, φ(n)) = 1
  Compute d such that e×d ≡ 1 (mod φ(n))

Public Key: (e, n)   ← Can be shared
Private Key: (d, n)   ← Must be kept secret

Encrypt: C = M^e mod n
Decrypt: M = C^d mod n

RSA vs AES

| Feature | RSA (Asymmetric) | AES (Symmetric) | |---------|------------------|-----------------| | Key count | Pair (public + private) | Single shared key | | Speed | Slow (1000x slower) | Fast | | Data size | Limited (< key length) | Unlimited | | Typical use | Key exchange, signatures | Data encryption | | Key length | 2048-4096 bits | 128-256 bits |

Why You Need RSA Encryption

1. Secure communication: Key exchange in HTTPS/TLS handshakes
2. Digital signatures: Verify data integrity and source
3. API authentication: JWT RS256 signature verification
4. Key exchange: Securely transmit AES session keys
5. Code signing: Verify software publisher identity

How to Use an Online Tool

Using DevToolkit Pro's RSA Encryption/Decryption Tool:

1. Generate RSA key pair (2048/4096 bits)
2. Encrypt data with public key
3. Decrypt data with private key
4. Create and verify digital signatures
5. Import/export in PEM format

RSA in Code

JavaScript: Web Crypto API

// Generate RSA key pair
async function generateRSAKeyPair() {
  const keyPair = await crypto.subtle.generateKey(
    {
      name: 'RSA-OAEP',
      modulusLength: 2048,
      publicExponent: new Uint8Array([1, 0, 1]),
      hash: 'SHA-256',
    },
    true,
    ['encrypt', 'decrypt']
  );
  return keyPair;
}

// Encrypt
async function rsaEncrypt(publicKey, data) {
  const encoder = new TextEncoder();
  const encrypted = await crypto.subtle.encrypt(
    { name: 'RSA-OAEP' },
    publicKey,
    encoder.encode(data)
  );
  return new Uint8Array(encrypted);
}

// Decrypt
async function rsaDecrypt(privateKey, encryptedData) {
  const decrypted = await crypto.subtle.decrypt(
    { name: 'RSA-OAEP' },
    privateKey,
    encryptedData
  );
  return new TextDecoder().decode(decrypted);
}

Python: Using cryptography Library

from cryptography.hazmat.primitives.asymmetric import rsa, padding
from cryptography.hazmat.primitives import hashes

# Generate key pair
private_key = rsa.generate_private_key(
    public_exponent=65537,
    key_size=2048,
)
public_key = private_key.public_key()

# Encrypt
ciphertext = public_key.encrypt(
    b"Hello, RSA!",
    padding.OAEP(
        mgf=padding.MGF1(algorithm=hashes.SHA256()),
        algorithm=hashes.SHA256(),
        label=None
    )
)

# Decrypt
plaintext = private_key.decrypt(
    ciphertext,
    padding.OAEP(
        mgf=padding.MGF1(algorithm=hashes.SHA256()),
        algorithm=hashes.SHA256(),
        label=None
    )
)

FAQ

How to Choose RSA Key Length?

• 2048 bits: Current standard, sufficiently secure
• 3072 bits: Higher security requirements
• 4096 bits: Maximum security level (lower performance) NIST recommends at least 2048 bits, 3072+ bits after 2030.

RSA vs ECC — Which Is Better?

ECC (elliptic curve) has shorter keys for the same security level (256-bit ECC ≈ 3072-bit RSA) and better performance. But RSA is more mature with better compatibility. New projects recommend ECC; existing systems keep RSA.

Why Can't RSA Directly Encrypt Large Files?

RSA has length limits (key length - padding overhead) and is slow. In practice, RSA encrypts AES keys, then AES encrypts the file (hybrid encryption). You can handle the symmetric AES side with our AES Encrypt/Decrypt tool.

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