RSAUtils

import java.io.ByteArrayOutputStream;
import java.util.Base64;
import javax.crypto.Cipher;
import java.security.Key;
import java.security.KeyFactory;
import java.security.spec.X509EncodedKeySpec;

/**
* RSA public key encryption/decryption utility class
* <p>
* Provides asymmetric encryption and decryption based on public key,
* suitable for encrypted data transmission in API communication.
*/
public class RSAUtils {
  // RSA encryption maximum block size (unit: bytes)
  private static final int MAX_ENCRYPT_BLOCK = 117;

  /**
   * Encrypt data using public key
   *
   * @param data Original binary data
   * @param publicKey Base64 encoded public key string
   * @return Encrypted binary data
   *
   * Usage scenario: Encrypt business data before sending API requests.
   * Automatically handles chunked encryption of large data (117 bytes/chunk).
   *
   * Example:
   * byte[] encrypted = RSAUtils.encryptByPublicKey(
   *     jsonData.getBytes(),
   *     "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ..."
   * );
   */
  public static byte[] encryptByPublicKey(byte[] data, String publicKey) throws Exception {
      // 1. Base64 decode public key
      byte[] keyBytes = Base64Util.decodeString(publicKey);

      // 2. Generate public key object
      KeyFactory keyFactory = KeyFactory.getInstance("RSA");
      Key publicK = keyFactory.generatePublic(
          new X509EncodedKeySpec(keyBytes)
      );

      // 3. Create and initialize cipher
      Cipher cipher = Cipher.getInstance(keyFactory.getAlgorithm());
      cipher.init(Cipher.ENCRYPT_MODE, publicK);

      // 4. Chunked encryption for large data
      int inputLen = data.length;
      int offset = 0;
      java.io.ByteArrayOutputStream out = new java.io.ByteArrayOutputStream();

      while (inputLen - offset > 0) {
          int blockSize = Math.min(inputLen - offset, MAX_ENCRYPT_BLOCK);
          byte[] encryptedBlock = cipher.doFinal(data, offset, blockSize);
          out.write(encryptedBlock, 0, encryptedBlock.length);
          offset += blockSize;
      }

      return out.toByteArray();
  }

  /**
   * Decrypt data using public key
   *
   * @param encryptedData Encrypted binary data
   * @param publicKey Base64 encoded public key string
   * @return Decrypted original binary data
   *
   * Usage scenario: Decrypt encrypted data returned from APIs.
   * Suitable for one-time decryption of small data chunks.
   *
   * Example:
   * byte[] decrypted = RSAUtils.decryptByPublicKey(
   *     encryptedBytes,
   *     "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ..."
   * );
   */
  public static byte[] decryptByPublicKey(byte[] encryptedData, String publicKey) throws Exception {

      final int MAX_DECRYPT_BLOCK = 128;

      // 1. Base64 decode public key
      byte[] keyBytes = Base64Util.decodeString(publicKey);

      // 2. Generate public key object
      KeyFactory keyFactory = KeyFactory.getInstance("RSA");
      Key publicK = keyFactory.generatePublic(
          new X509EncodedKeySpec(keyBytes)
      );

      // 3. Create and initialize decrypter
      Cipher cipher = Cipher.getInstance(keyFactory.getAlgorithm());
      cipher.init(Cipher.DECRYPT_MODE, publicK);

      // 4. One-time decryption (suitable for small data)
      // return cipher.doFinal(encryptedData);
      int inputLen = encryptedData.length;
      ByteArrayOutputStream out = new ByteArrayOutputStream();
      int offSet = 0;
      byte[] cache;
      int i = 0;
      // Decrypt data in segments
      while (inputLen - offSet > 0) {
          if (inputLen - offSet > MAX_DECRYPT_BLOCK) {
              cache = cipher.doFinal(encryptedData, offSet, MAX_DECRYPT_BLOCK);
          } else {
              cache = cipher.doFinal(encryptedData, offSet, inputLen - offSet);
          }
          out.write(cache, 0, cache.length);
          i++;
          offSet = i * MAX_DECRYPT_BLOCK;
      }
      byte[] decryptedData = out.toByteArray();
      out.close();
      return decryptedData;
  }
}