GenRsa.java

import java.io.FileWriter;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.util.Base64;

public class GenRsa {
    public static void main(String[] args) throws Exception {
        int keySize = 2048;
        String privateKeyPath = "private_key.pem";
        String publicKeyPath = "public_key.pem";

        if (args.length >= 1) {
            keySize = Integer.parseInt(args[0]);
        }
        if (args.length >= 2) {
            privateKeyPath = args[1];
        }
        if (args.length >= 3) {
            publicKeyPath = args[2];
        }

        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
        keyGen.initialize(keySize);
        KeyPair pair = keyGen.generateKeyPair();
        PrivateKey privateKey = pair.getPrivate();
        PublicKey publicKey = pair.getPublic();

        // Write private key in PKCS#8 PEM format
        String privatePem = "-----BEGIN PRIVATE KEY-----\n"
                + Base64.getMimeEncoder(64, new byte[]{'\n'}).encodeToString(privateKey.getEncoded())
                + "\n-----END PRIVATE KEY-----\n";
        try (FileWriter fw = new FileWriter(privateKeyPath)) {
            fw.write(privatePem);
        }

        // Write public key in X.509 PEM format
        String publicPem = "-----BEGIN PUBLIC KEY-----\n"
                + Base64.getMimeEncoder(64, new byte[]{'\n'}).encodeToString(publicKey.getEncoded())
                + "\n-----END PUBLIC KEY-----\n";
        try (FileWriter fw = new FileWriter(publicKeyPath)) {
            fw.write(publicPem);
        }

        System.out.println("RSA keypair generated.");
        System.out.println("Private key: " + privateKeyPath);
        System.out.println("Public key: " + publicKeyPath);
    }
}