How to Encrypt using RSA in iOS and Decrypt in Java

How can I encrypt data using RSA in an iOS application and then decrypt it using Java?

// Swift code snippet for RSA encryption in iOS:
let publicKey = "Your RSA Public Key Here"
let data = "Data to encrypt".data(using: .utf8)!
let encryptedData = try RSA.encrypt(data: data, withPublicKey: publicKey)

// Example Java code for RSA decryption:
String privateKey = "Your RSA Private Key Here";
byte[] decryptedBytes = RSA.decrypt(encryptedData, privateKey);

This guide will demonstrate how to perform RSA encryption in an iOS application using Swift and then decrypt that data in a Java application. RSA is a widely-used public key cryptosystem that enables secure data transmission.

// Swift example for encryption using SwiftyRSA:
import SwiftyRSA

let publicKey = try PublicKey(pemEncoded: "-----BEGIN PUBLIC KEY-----\n...")
let clear = try ClearMessage(string: "Hello, World!", using: .utf8)
let encrypted = try clear.encrypted(with: publicKey, padding: .pkcs1)

// Java example for decryption using Bouncy Castle:
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import java.security.Security;
import java.security.PrivateKey;
import java.security.Security;
import java.util.Base64;

Security.addProvider(new BouncyCastleProvider());
PrivateKey privateKey = getPrivateKey(); // Implement your method to retrieve the private key
byte[] original = decryptRSA(privateKey, Base64.getDecoder().decode(encryptedString));

Causes

• Mismatch between the encryption and decryption keys.
• Incorrect implementation of RSA algorithms in either platform.
• Data corruption during transmission.

Solutions

• Ensure the public key used in iOS matches the corresponding private key in Java.
• Use libraries like `SwiftyRSA` in Swift for encryption, and Bouncy Castle in Java for decryption.
• Consider encoding the encrypted data in Base64 before sending.

Mistake: Not using the correct key format (PEM vs DER).

Solution: Ensure the key format matches on both systems; use proper encoding and libraries.

Mistake: Forgetting to include padding in RSA operations.

Solution: Always specify padding when encrypting and decrypting, especially for non-trivial messages.