diff --git a/.github/workflows/black.yml b/.github/workflows/black.yml
index af06272..cc31d1b 100644
--- a/.github/workflows/black.yml
+++ b/.github/workflows/black.yml
@@ -12,4 +12,4 @@ jobs:
           python -m pip install --upgrade pip
           pip install -r requirements.txt
       - run: |
-          black --check --verbose .
+          python3 -m black -t py38 --check .
diff --git a/server/fractionator.py b/server/fractionator.py
index 1985c9c..0e923f8 100644
--- a/server/fractionator.py
+++ b/server/fractionator.py
@@ -3,18 +3,19 @@
 import os
 from typing import Optional
 
-from cryptography.hazmat.primitives.ciphers import algorithms
+from cryptography.hazmat.primitives.ciphers import algorithms, modes
 
 from fraction import Fraction
 import utils
 
 
-class Fractionator(utils.AES_CFB_HELPER):
+class Fractionator(utils.AES_WITH_IV_HELPER):
     MAGIC: int = 0xDEADBEEF
     CHUNK_SIZE: int = 8192
     FRACTION_PATH_LEN: int = 16
     algorithm = algorithms.AES256
-
+    mode = modes.CBC
+    
     def __init__(self, file_path: str, out_path: str, key: bytes) -> None:
         """Prepare a Fractionator object for reading and generating fractions."""
         self.file_path: str = file_path
@@ -26,7 +27,7 @@ def __init__(self, file_path: str, out_path: str, key: bytes) -> None:
 
         self._buf_reader: Optional[io.BufferedReader] = None
 
-        super().__init__(key, self.algorithm)
+        super().__init__(key, self.algorithm, self.mode)
 
     def open_reading_stream(self) -> None:
         """Open a stream for reading the object file."""
diff --git a/server/utils.py b/server/utils.py
index 58654d5..ed0000c 100644
--- a/server/utils.py
+++ b/server/utils.py
@@ -1,36 +1,52 @@
 import random
 import string
 import secrets
-from typing import Optional
+from typing import Optional, Type
 
 from cryptography.hazmat.primitives.ciphers import Cipher, modes
+from cryptography.hazmat.primitives._cipheralgorithm import BlockCipherAlgorithm
+from cryptography.hazmat.primitives import padding
 
-
-class AES_CFB_HELPER:
+class AES_WITH_IV_HELPER:
     LENGTH_IV: int = 16
 
-    def __init__(self, key: bytes, algorithm) -> None:
-        self.algorithm = algorithm
-        self.mode = modes.CFB
-        self.key: bytes = key
+    def __init__(self, key: bytes, algorithm: Type[BlockCipherAlgorithm], mode: Type[modes.ModeWithInitializationVector]) -> None:
+        self.key = key
+        
+        self.algorithm = algorithm(self.key)
+        self.mode = mode
+        
+        self.padder_ctx = padding.PKCS7(self.algorithm.block_size)
+        
         self._iv: Optional[bytes] = None
+        
+    def pad(self, data: bytes) -> bytes:
+        padder = self.padder_ctx.padder()
+        return padder.update(data) + padder.finalize()
+    
+    def unpad(self, data: bytes) -> bytes:
+        unpadder = self.padder_ctx.unpadder()
+        return unpadder.update(data) + unpadder.finalize()
 
     def get_cipher(self, iv: bytes) -> Cipher:
         """Return a cipher instance."""
-        return Cipher(self.algorithm(self.key), self.mode(iv))
+        return Cipher(self.algorithm, self.mode(iv))
 
     def get_iv(self, new: bool = False) -> bytes:
         """Generate or reuse initialization vector (IV)."""
         if not self._iv or new:
-            return secrets.token_bytes(self.LENGTH_IV)
+            self._iv = secrets.token_bytes(self.LENGTH_IV)
         return self._iv
 
     def encrypt(self, data: bytes) -> bytes:
-        """Encrypt data using AES-CFB mode."""
-        cipher = self.get_cipher(self.get_iv(True))
-        operator = cipher.encryptor()
-        return operator.update(data) + operator.finalize()
-
+        """Encrypt data"""
+        encryptor = self.get_cipher(self.get_iv(True)).encryptor()
+        return encryptor.update(self.pad(data)) + encryptor.finalize()
+
+    def decrypt(self, data: bytes, iv: bytes) -> bytes:
+        """Decrypt data"""
+        decryptor = self.get_cipher(iv).decryptor()
+        return self.unpad(decryptor.update(data) + decryptor.finalize())
 
 def random_string(n: int = 16, sample: str = string.ascii_lowercase + string.digits):
     """Returns a random string using the characters defined in sample"""