Add decrypt option to decrypt body on demand (#360)

Fixes: #359

Co-authored-by: Jan-Michael Brummer <jan-michael.brummer1@volkswagen.de>

Author: janbrummer

README.rst

@@ -364,9 +364,9 @@ days after **credchange_date** that credential update is recommended
 
 Miscellaneous
 -------------
-**read** (filename=None, password=None, keyfile=None, transformed_key=None)
+**read** (filename=None, password=None, keyfile=None, transformed_key=None, decrypt=False)
 
-where ``filename``, ``password``, and ``keyfile`` are strings.  ``filename`` is the path to the database, ``password`` is the master password string, and ``keyfile`` is the path to the database keyfile.  At least one of ``password`` and ``keyfile`` is required.  Alternatively, the derived key can be supplied directly through ``transformed_key``.
+where ``filename``, ``password``, and ``keyfile`` are strings.  ``filename`` is the path to the database, ``password`` is the master password string, and ``keyfile`` is the path to the database keyfile.  At least one of ``password`` and ``keyfile`` is required.  Alternatively, the derived key can be supplied directly through ``transformed_key``. ``decrypt`` tells whether the file should be decrypted or not.
 
 Can raise ``CredentialsError``, ``HeaderChecksumError``, or ``PayloadChecksumError``.
 

pykeepass/kdbx_parsing/kdbx3.py

@@ -5,7 +5,7 @@
 from construct import (
     Byte, Bytes, Int16ul, Int32ul, Int64ul, RepeatUntil, GreedyBytes, Struct,
     this, Mapping, Switch, Prefixed, Padding, Checksum, Computed, IfThenElse,
-    Pointer, Tell, len_
+    Pointer, Tell, len_, If
 )
 from .common import (
     aes_kdf, AES256Payload, ChaCha20Payload, TwoFishPayload, Concatenated,
@@ -154,13 +154,15 @@ def compute_transformed(context):
 Body = Struct(
     "transformed_key" / Computed(compute_transformed),
     "master_key" / Computed(compute_master),
-    "payload" / UnpackedPayload(
-        Switch(
-            this._.header.value.dynamic_header.cipher_id.data,
-            {'aes256': AES256Payload(GreedyBytes),
-             'chacha20': ChaCha20Payload(GreedyBytes),
-             'twofish': TwoFishPayload(GreedyBytes),
-             }
+    "payload" / If(this._._.decrypt,
+        UnpackedPayload(
+            Switch(
+                this._.header.value.dynamic_header.cipher_id.data,
+                {'aes256': AES256Payload(GreedyBytes),
+                 'chacha20': ChaCha20Payload(GreedyBytes),
+                 'twofish': TwoFishPayload(GreedyBytes),
+                 }
+            )
         )
     ),
 )

pykeepass/kdbx_parsing/kdbx4.py

@@ -8,7 +8,7 @@
 from construct import (
     Byte, Bytes, Int32ul, RepeatUntil, GreedyBytes, Struct, this, Mapping,
     Switch, Flag, Prefixed, Int64ul, Int32sl, Int64sl, GreedyString, Padding,
-    Peek, Checksum, Computed, IfThenElse, Pointer, Tell
+    Peek, Checksum, Computed, IfThenElse, Pointer, Tell, If
 )
 from .common import (
     aes_kdf, Concatenated, AES256Payload, ChaCha20Payload, TwoFishPayload,
@@ -259,17 +259,21 @@ def compute_payload_block_hash(this):
         this._.header.data,
         # exception=HeaderChecksumError,
     ),
-    "cred_check" / Checksum(
-        Bytes(32),
-        compute_header_hmac_hash,
-        this,
-        # exception=CredentialsError,
+    "cred_check" / If(this._._.decrypt,
+        Checksum(
+            Bytes(32),
+            compute_header_hmac_hash,
+            this,
+            # exception=CredentialsError,
+        )
     ),
-    "payload" / UnpackedPayload(
-        IfThenElse(
-            this._.header.value.dynamic_header.compression_flags.data.compression,
-            Decompressed(DecryptedPayload),
-            DecryptedPayload
+    "payload" / If(this._._.decrypt,
+        UnpackedPayload(
+            IfThenElse(
+                this._.header.value.dynamic_header.compression_flags.data.compression,
+                Decompressed(DecryptedPayload),
+                DecryptedPayload
+            )
         )
     )
 )

pykeepass/pykeepass.py

@@ -43,6 +43,9 @@ class PyKeePass():
             database is assumed to have no keyfile
         transformed_key (:obj:`bytes`, optional): precomputed transformed
             key.
+        decrypt (:obj:`bool`, optional): whether to decrypt XML payload.
+            Set `False` to access outer header information without decrypting
+            database.
 
     Raises:
         CredentialsError: raised when password/keyfile or transformed key
@@ -57,13 +60,14 @@ class PyKeePass():
     """
 
     def __init__(self, filename, password=None, keyfile=None,
-                 transformed_key=None):
+                 transformed_key=None, decrypt=True):
 
         self.read(
             filename=filename,
             password=password,
             keyfile=keyfile,
-            transformed_key=transformed_key
+            transformed_key=transformed_key,
+            decrypt=decrypt
         )
 
     def __enter__(self):
@@ -74,7 +78,7 @@ def __exit__(self, typ, value, tb):
         pass
 
     def read(self, filename=None, password=None, keyfile=None,
-             transformed_key=None):
+             transformed_key=None, decrypt=True):
         """
         See class docstring.
 
@@ -94,14 +98,16 @@ def read(self, filename=None, password=None, keyfile=None,
                     filename,
                     password=password,
                     keyfile=keyfile,
-                    transformed_key=transformed_key
+                    transformed_key=transformed_key,
+                    decrypt=decrypt
                 )
             else:
                 self.kdbx = KDBX.parse_file(
                     filename,
                     password=password,
                     keyfile=keyfile,
-                    transformed_key=transformed_key
+                    transformed_key=transformed_key,
+                    decrypt=decrypt
                 )
 
         except CheckError as e:
@@ -152,7 +158,8 @@ def save(self, filename=None, transformed_key=None):
                 filename,
                 password=self.password,
                 keyfile=self.keyfile,
-                transformed_key=transformed_key
+                transformed_key=transformed_key,
+                decrypt=True
             )
         else:
             # save to temporary file to prevent database clobbering
@@ -164,7 +171,8 @@ def save(self, filename=None, transformed_key=None):
                     filename_tmp,
                     password=self.password,
                     keyfile=self.keyfile,
-                    transformed_key=transformed_key
+                    transformed_key=transformed_key,
+                    decrypt=True
                 )
             except Exception as e:
                 os.remove(filename_tmp)
@@ -207,6 +215,17 @@ def transformed_key(self):
         and passed to `open` for faster database opening"""
         return self.kdbx.body.transformed_key
 
+    @property
+    def database_salt(self):
+       """bytes: salt of database kdf. This can be used for adding additional
+       credentials which are used in extension to current keyfile."""
+
+       if self.version == (3, 1):
+            return self.kdbx.header.value.dynamic_header.transform_seed.data
+
+       kdf_parameters = self.kdbx.header.value.dynamic_header.kdf_parameters.data.dict
+       return kdf_parameters['S'].value
+
     @property
     def tree(self):
         """lxml.etree._ElementTree: database XML payload"""

tests/tests.py

@@ -1206,6 +1206,30 @@ def test_open_error(self):
                     os.path.join(base_dir, keyfile)
                 )
 
+
+    def test_open_no_decrypt(self):
+
+        databases = [
+            'test3.kdbx',
+            'test4.kdbx',
+        ]
+        passwords = [
+            'invalid_password',
+            'invalid_password',
+        ]
+        salts = [
+            b'\x82\xef\xf1\x05\x13\xbcQ\xa7\x8aG\x04b\xc7^o(\xf2R[\xc0\x0f\xa4?\xaa\xf9 Gi\xcf\xaf6\x0f',
+            b'\x82\xb0\xab/Bbn\x93\x90\xe0\x02m\x82\xaa\x9a\x9a\xd1\xc0k\x95\xbb\xc5kn\xe3\xeb\xd6GHg<$'
+        ]
+        for database, password, salt in zip(databases, passwords, salts):
+            kp = PyKeePass(
+                os.path.join(base_dir, database),
+                password,
+                decrypt=False
+            )
+
+            self.assertEqual(kp.database_salt, salt)
+
 if __name__ == '__main__':
     unittest.main()