Refactor encryption system to support hybrid RSA-AES encryption per file

- Changed file upload logic to:
  - Generate random AES key per file
  - Encrypt AES key using user's RSA public key
  - Store encrypted AES key, IV, and salt in FileMetadata entity

- Changed file download logic to:
  - Decrypt AES key using user's RSA private key (encrypted with password-derived AES)
  - Use decrypted AES key and IV to decrypt file contents from HDFS

- Modified FileMetadata entity:
  - Changed `encryptedKey` to @Lob byte[] to support large encrypted AES keys

- Updated User entity:
  - Encrypted private RSA key with password-derived AES
  - Stored associated salt and IV for decryption

- Updated AuthenticationService:
  - Generate RSA keypair during sign-up
  - Encrypt and store private key with AES (salt, IV)
  - Create user folder in HDFS upon registration

- Updated FileService:
  - Rewrote upload and download logic to support hybrid encryption
  - Handled key wrapping and unwrapping securely
  - Added logging for upload/download events

- Fixed FileController upload to remove password from endpoint
  - Password now only required during download for private key decryption

- Updated EncryptionUtil and RSAKeyUtil:
  - Added RSA OAEP support and helper methods
  - Added AES key generation, encryption, decryption utilities

FILE UPLOAD AND ENCRYPTION WORKS! TESTED USING HEXDUMP.

src/main/java/com/skycrate/backend/skycrateBackend/services/AuthenticationService.java

@@ -5,6 +5,7 @@ import com.skycrate.backend.skycrateBackend.dto.LoginUserDto;
 import com.skycrate.backend.skycrateBackend.dto.RegisterUserDto;
 import com.skycrate.backend.skycrateBackend.entity.User;
 import com.skycrate.backend.skycrateBackend.repository.UserRepository;
+import com.skycrate.backend.skycrateBackend.utils.EncryptionUtil;
 import com.skycrate.backend.skycrateBackend.utils.RSAKeyUtil;
 import org.apache.hadoop.fs.FileSystem;
 import org.apache.hadoop.fs.Path;
@@ -13,6 +14,7 @@ import org.springframework.security.authentication.UsernamePasswordAuthenticatio
 import org.springframework.security.crypto.password.PasswordEncoder;
 import org.springframework.stereotype.Service;
 
+import javax.crypto.SecretKey;
 import java.security.KeyPair;
 import java.security.NoSuchAlgorithmException;
 
@@ -32,6 +34,7 @@ public class AuthenticationService {
     }
 
     public User signUp(RegisterUserDto inputUser) {
+        // Generate RSA key pair
         KeyPair keyPair;
         try {
             keyPair = RSAKeyUtil.generateKeyPair();
@@ -39,28 +42,43 @@ public class AuthenticationService {
             throw new RuntimeException("Failed to generate RSA key pair", e);
         }
 
+        // Encrypt private key using password-derived AES key
+        byte[] salt = EncryptionUtil.generateSalt();
+        byte[] iv = EncryptionUtil.generateIv();
+        byte[] encryptedPrivateKey;
+        try {
+            SecretKey aesKey = EncryptionUtil.deriveKey(inputUser.getPassword().toCharArray(), salt);
+            encryptedPrivateKey = EncryptionUtil.encrypt(keyPair.getPrivate().getEncoded(), aesKey, iv);
+        } catch (Exception e) {
+            throw new RuntimeException("Failed to encrypt private key", e);
+        }
+
+        // Create user entity with encrypted private key, salt, and iv
         User user = User.builder()
                 .fullname(inputUser.getFirstname() + " " + inputUser.getLastname())
                 .username(inputUser.getUsername())
                 .email(inputUser.getEmail())
                 .password(passwordEncoder.encode(inputUser.getPassword()))
                 .publicKey(keyPair.getPublic().getEncoded())
-                .privateKey(keyPair.getPrivate().getEncoded())
+                .privateKey(encryptedPrivateKey)
+                .privateKeySalt(salt)
+                .privateKeyIv(iv)
                 .build();
 
+        // Save user
         User savedUser = userRepository.save(user);
 
         // Create HDFS directory in root with username
         try {
             FileSystem fs = HDFSConfig.getHDFS();
-            String folderName = savedUser.getUsername();
-            Path userDir = new Path("/" + folderName);
+            Path userDir = new Path("/" + savedUser.getUsername());
             if (!fs.exists(userDir)) {
                 fs.mkdirs(userDir);
             }
         } catch (Exception e) {
             throw new RuntimeException("Failed to create HDFS directory for user: " + savedUser.getUsername(), e);
         }
+
         return savedUser;
     }