Add hash and length verification

Extend MetaFile and TargetFile classes with methods
for length and hash verification.  The common functionality
is implemented as static methods of the base class while
MetaFile and TargetFile implement the user API based on it.

Define LengthOrHasheMismathError.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Author: sechkova

tuf/api/metadata.py

@@ -16,10 +16,23 @@
 
 """
 import abc
+import io
 import tempfile
 from datetime import datetime, timedelta
-from typing import Any, ClassVar, Dict, List, Mapping, Optional, Tuple, Type
+from typing import (
+    Any,
+    BinaryIO,
+    ClassVar,
+    Dict,
+    List,
+    Mapping,
+    Optional,
+    Tuple,
+    Type,
+    Union,
+)
 
+from securesystemslib import hash as sslib_hash
 from securesystemslib import keys as sslib_keys
 from securesystemslib.signer import Signature, Signer
 from securesystemslib.storage import FilesystemBackend, StorageBackendInterface
@@ -622,7 +635,53 @@ def remove_key(self, role: str, keyid: str) -> None:
         del self.keys[keyid]
 
 
-class MetaFile:
+class BaseFile:
+    """A base class of MetaFile and TargetFile.
+
+    Encapsulates common static methods for length and hash verification.
+    """
+
+    @staticmethod
+    def _verify_hashes(
+        data: Union[bytes, BinaryIO], expected_hashes: Dict[str, str]
+    ) -> None:
+        """Verifies that the hash of 'data' matches 'expected_hashes'"""
+        is_bytes = isinstance(data, bytes)
+        for algo, exp_hash in expected_hashes.items():
+            if is_bytes:
+                digest_object = sslib_hash.digest(algo)
+                digest_object.update(data)
+            else:
+                # if data is not bytes, assume it is a file object
+                digest_object = sslib_hash.digest_fileobject(data, algo)
+
+            observed_hash = digest_object.hexdigest()
+            if observed_hash != exp_hash:
+                raise exceptions.LengthOrHashMismatchError(
+                    f"Observed hash {observed_hash} does not match"
+                    f"expected hash {exp_hash}"
+                )
+
+    @staticmethod
+    def _verify_length(
+        data: Union[bytes, BinaryIO], expected_length: int
+    ) -> None:
+        """Verifies that the length of 'data' matches 'expected_length'"""
+        if isinstance(data, bytes):
+            observed_length = len(data)
+        else:
+            # if data is not bytes, assume it is a file object
+            data.seek(0, io.SEEK_END)
+            observed_length = data.tell()
+
+        if observed_length != expected_length:
+            raise exceptions.LengthOrHashMismatchError(
+                f"Observed length {observed_length} does not match"
+                f"expected length {expected_length}"
+            )
+
+
+class MetaFile(BaseFile):
     """A container with information about a particular metadata file.
 
     Attributes:
@@ -678,6 +737,22 @@ def to_dict(self) -> Dict[str, Any]:
 
         return res_dict
 
+    def verify_length_and_hashes(self, data: Union[bytes, BinaryIO]):
+        """Verifies that the length and hashes of "data" match expected
+            values.
+        Args:
+            data: File object or its content in bytes.
+        Raises:
+            LengthOrHashMismatchError: Calculated length or hashes do not
+                match expected values.
+        """
+        if self.length is not None:
+            self._verify_length(data, self.length)
+
+        # Skip the check in case of an empty dictionary too
+        if self.hashes:
+            self._verify_hashes(data, self.hashes)
+
 
 class Timestamp(Signed):
     """A container for the signed part of timestamp metadata.
@@ -905,7 +980,7 @@ def to_dict(self) -> Dict[str, Any]:
         }
 
 
-class TargetFile:
+class TargetFile(BaseFile):
     """A container with information about a particular target file.
 
     Attributes:
@@ -923,12 +998,6 @@ class TargetFile:
 
     """
 
-    @property
-    def custom(self):
-        if self.unrecognized_fields is None:
-            return None
-        return self.unrecognized_fields.get("custom", None)
-
     def __init__(
         self,
         length: int,
@@ -939,6 +1008,12 @@ def __init__(
         self.hashes = hashes
         self.unrecognized_fields = unrecognized_fields or {}
 
+    @property
+    def custom(self):
+        if self.unrecognized_fields is None:
+            return None
+        return self.unrecognized_fields.get("custom", None)
+
     @classmethod
     def from_dict(cls, target_dict: Dict[str, Any]) -> "TargetFile":
         """Creates TargetFile object from its dict representation."""
@@ -955,6 +1030,18 @@ def to_dict(self) -> Dict[str, Any]:
             **self.unrecognized_fields,
         }
 
+    def verify_length_and_hashes(self, data: Union[bytes, BinaryIO]):
+        """Verifies that the length and hashes of "data" match expected
+            values.
+        Args:
+            data: File object or its content in bytes.
+        Raises:
+            LengthOrHashMismatchError: Calculated length or hashes do not
+                match expected values.
+        """
+        self._verify_length(data, self.length)
+        self._verify_hashes(data, self.hashes)
+
 
 class Targets(Signed):
     """A container for the signed part of targets metadata.

tuf/exceptions.py

@@ -65,6 +65,8 @@ def __repr__(self):
 class UnsupportedAlgorithmError(Error):
   """Indicate an error while trying to identify a user-specified algorithm."""
 
+class LengthOrHashMismatchError(Error):
+  """Indicate an error while checking the length and hash values of an object"""
 
 class BadHashError(Error):
   """Indicate an error while checking the value of a hash object."""
@@ -88,9 +90,6 @@ def __repr__(self):
     #     self.__class__.__name__ + '(' + repr(self.expected_hash) + ', ' +
     #     repr(self.observed_hash) + ')')
 
-
-
-
 class BadVersionNumberError(Error):
   """Indicate an error for metadata that contains an invalid version number."""