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/entity/User.java

@@ -38,25 +38,37 @@ public class User implements UserDetails {
     @Lob
     private byte[] privateKey;
 
+    @Lob
+    @Column(nullable = false)
+    private byte[] privateKeySalt;
+
+    @Lob
+    @Column(nullable = false)
+    private byte[] privateKeyIv;
+
     @Builder
-    public User(String email, String password, String username, String fullname, byte[] publicKey, byte[] privateKey) {
+    public User(String email, String password, String username, String fullname,
+                byte[] publicKey, byte[] privateKey,
+                byte[] privateKeySalt, byte[] privateKeyIv) {
         this.email = email;
         this.password = password;
         this.username = username;
         this.fullname = fullname;
         this.publicKey = publicKey;
         this.privateKey = privateKey;
+        this.privateKeySalt = privateKeySalt;
+        this.privateKeyIv = privateKeyIv;
     }
 
     // --- UserDetails interface methods ---
     @Override
     public Collection<? extends GrantedAuthority> getAuthorities() {
-        return List.of(); // Add roles/authorities if needed
+        return List.of(); // No roles assigned currently
     }
 
     @Override
     public String getUsername() {
-        return username; // or return username if that's your login key
+        return username;
     }
 
     @Override
@@ -65,22 +77,14 @@ public class User implements UserDetails {
     }
 
     @Override
-    public boolean isAccountNonExpired() {
-        return true;
-    }
+    public boolean isAccountNonExpired() { return true; }
 
     @Override
-    public boolean isAccountNonLocked() {
-        return true;
-    }
+    public boolean isAccountNonLocked() { return true; }
 
     @Override
-    public boolean isCredentialsNonExpired() {
-        return true;
-    }
+    public boolean isCredentialsNonExpired() { return true; }
 
     @Override
-    public boolean isEnabled() {
-        return true;
-    }
+    public boolean isEnabled() { return true; }
 }