MAINT: Add R parameter to generate_values (#1820)

Pass it through to compute_U_value / compute_O_value

Also fix a bug I (Martin) just introduced in
compute_O_value by setting the token_bytes=4 and not 16.

All credit for refactoring goes to exiledkingcc

Co-authored-by: exiledkingcc <exiledkingcc@gmail.com>

pypdf/_encryption.py

@@ -730,14 +730,15 @@ def verify_perms(
 
     @staticmethod
     def generate_values(
+        R: int,
         user_password: bytes,
         owner_password: bytes,
         key: bytes,
         p: int,
         metadata_encrypted: bool,
     ) -> Dict[Any, Any]:
-        u_value, ue_value = AlgV5.compute_U_value(user_password, key)
-        o_value, oe_value = AlgV5.compute_O_value(owner_password, key, u_value)
+        u_value, ue_value = AlgV5.compute_U_value(R, user_password, key)
+        o_value, oe_value = AlgV5.compute_O_value(R, owner_password, key, u_value)
         perms = AlgV5.compute_Perms_value(key, p, metadata_encrypted)
         return {
             "/U": u_value,
@@ -748,7 +749,7 @@ def generate_values(
         }
 
     @staticmethod
-    def compute_U_value(password: bytes, key: bytes) -> Tuple[bytes, bytes]:
+    def compute_U_value(R: int, password: bytes, key: bytes) -> Tuple[bytes, bytes]:
         """
         Algorithm 3.8 Computing the encryption dictionary’s U (user password)
         and UE (user encryption key) values.
@@ -766,6 +767,7 @@ def compute_U_value(password: bytes, key: bytes) -> Tuple[bytes, bytes]:
            as the UE key.
 
         Args:
+            R:
             password:
             key:
 
@@ -775,7 +777,6 @@ def compute_U_value(password: bytes, key: bytes) -> Tuple[bytes, bytes]:
         random_bytes = secrets.token_bytes(16)
         val_salt = random_bytes[:8]
         key_salt = random_bytes[8:]
-        R = 4  # only one supported so far
         u_value = AlgV5.calculate_hash(R, password, val_salt, b"") + val_salt + key_salt
 
         tmp_key = AlgV5.calculate_hash(R, password, key_salt, b"")
@@ -785,7 +786,7 @@ def compute_U_value(password: bytes, key: bytes) -> Tuple[bytes, bytes]:
 
     @staticmethod
     def compute_O_value(
-        password: bytes, key: bytes, u_value: bytes
+        R: int, password: bytes, key: bytes, u_value: bytes
     ) -> Tuple[bytes, bytes]:
         """
         Algorithm 3.9 Computing the encryption dictionary’s O (owner password)
@@ -807,6 +808,7 @@ def compute_O_value(
            The resulting 32-byte string is stored as the OE key.
 
         Args:
+            R:
             password:
             key:
             u_value: A 32-byte string, based on the user password, that shall be
@@ -816,10 +818,9 @@ def compute_O_value(
         Returns:
             A tuple (O value, OE value)
         """
-        random_bytes = secrets.token_bytes(4)
+        random_bytes = secrets.token_bytes(16)
         val_salt = random_bytes[:8]
         key_salt = random_bytes[8:]
-        R = 4  # only one supported so far
         o_value = (
             AlgV5.calculate_hash(R, password, val_salt, u_value) + val_salt + key_salt
         )
@@ -894,8 +895,6 @@ def __init__(
         self.StrF = StrF
         self.EFF = EFF
 
-        # 1 => owner password
-        # 2 => user password
         self._password_type = PasswordType.NOT_DECRYPTED
         self._key: Optional[bytes] = None
 

tests/test_encryption.py

@@ -214,6 +214,7 @@ def test_alg_v5_generate_values():
         return
     key = b"0123456789123451"
     values = AlgV5.generate_values(
+        R=4,
         user_password=b"foo",
         owner_password=b"bar",
         key=key,