Add SuccinctRole class

Add SuccinctRoles class containing the information from the
succint_roles dict described in TAP 15.
This allows for easy mypy checks on the types, easy enforcement on
TAP 15 restrictions (as for example that "bit_length" must be between 1
and 32) and support for unrecognized fields inside succinct_roles
without much of a hassle.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

tests/test_metadata_eq_.py

@@ -23,6 +23,7 @@
     Metadata,
     MetaFile,
     Role,
+    SuccinctRoles,
     TargetFile,
 )
 
@@ -55,6 +56,7 @@ def setUpClass(cls) -> None:
         cls.objects["Role"] = Role(["keyid1", "keyid2"], 3)
         cls.objects["MetaFile"] = MetaFile(1, 12, {"sha256": "abc"})
         cls.objects["DelegatedRole"] = DelegatedRole("a", [], 1, False, ["d"])
+        cls.objects["SuccinctRoles"] = SuccinctRoles(["keyid"], 1, 8, "foo")
         cls.objects["Delegations"] = Delegations(
             {"keyid": cls.objects["Key"]}, {"a": cls.objects["DelegatedRole"]}
         )
@@ -79,6 +81,7 @@ def setUpClass(cls) -> None:
             "paths": [""],
             "path_hash_prefixes": [""],
         },
+        "SuccinctRoles": {"bit_length": 0, "name_prefix": ""},
         "Delegations": {"keys": {}, "roles": {}},
         "TargetFile": {"length": 0, "hashes": {}, "path": ""},
         "Targets": {"targets": {}, "delegations": []},

tests/test_metadata_serialization.py

@@ -24,6 +24,7 @@
     Role,
     Root,
     Snapshot,
+    SuccinctRoles,
     TargetFile,
     Targets,
     Timestamp,
@@ -408,6 +409,36 @@ def test_invalid_delegated_role_serialization(
         with self.assertRaises(ValueError):
             DelegatedRole.from_dict(case_dict)
 
+    valid_succinct_roles: utils.DataSet = {
+        # SuccinctRoles inherits Role and some use cases can be found in the valid_roles.
+        "standard succinct_roles information": '{"keyids": ["keyid"], "threshold": 1, \
+            "bit_length": 8, "name_prefix": "foo"}',
+        "succinct_roles with unrecognized fields": '{"keyids": ["keyid"], "threshold": 1, \
+            "bit_length": 8, "name_prefix": "foo", "foo": "bar"}',
+    }
+
+    @utils.run_sub_tests_with_dataset(valid_succinct_roles)
+    def test_succinct_roles_serialization(self, test_case_data: str) -> None:
+        case_dict = json.loads(test_case_data)
+        succinct_roles = SuccinctRoles.from_dict(copy.copy(case_dict))
+        self.assertDictEqual(case_dict, succinct_roles.to_dict())
+
+    invalid_succinct_roles: utils.DataSet = {
+        # SuccinctRoles inherits Role and some use cases can be found in the invalid_roles.
+        "missing bit_length from succinct_roles": '{"keyids": ["keyid"], "threshold": 1, "name_prefix": "foo"}',
+        "missing name_prefix from succinct_roles": '{"keyids": ["keyid"], "threshold": 1, "bit_length": 8}',
+        "succinct_roles with invalid bit_length type": '{"keyids": ["keyid"], "threshold": 1, "bit_length": "a", "name_prefix": "foo"}',
+        "succinct_roles with invalid name_prefix type": '{"keyids": ["keyid"], "threshold": 1, "bit_length": 8, "name_prefix": 1}',
+        "succinct_roles with high bit_length value": '{"keyids": ["keyid"], "threshold": 1, "bit_length": 50, "name_prefix": "foo"}',
+        "succinct_roles with low bit_length value": '{"keyids": ["keyid"], "threshold": 1, "bit_length": 0, "name_prefix": "foo"}',
+    }
+
+    @utils.run_sub_tests_with_dataset(invalid_succinct_roles)
+    def test_invalid_succinct_roles_serialization(self, test_data: str) -> None:
+        case_dict = json.loads(test_data)
+        with self.assertRaises((ValueError, KeyError, TypeError)):
+            SuccinctRoles.from_dict(case_dict)
+
     invalid_delegations: utils.DataSet = {
         "empty delegations": "{}",
         "missing keys": '{ "roles": [ \

tuf/api/metadata.py

@@ -1435,6 +1435,109 @@ def is_delegated_path(self, target_filepath: str) -> bool:
         return False
 
 
+class SuccinctRoles(Role):
+    """Succinctly defines a hash bin delegation graph.
+
+    A ``SuccinctRoles`` object describes a delegation graph that covers all
+    targets, distributing them uniformly over the delegated roles (i.e. bins)
+    in the graph.
+
+    The total number of bins is 2 to the power of the passed ``bit_length``.
+    Targets are assigned to bins by casting the left-most ``bit_length`` of
+    bits of the file path hash digest to int, using it as bin index between 0
+    and ``2**bit_length - 1``.
+
+    Bin names are the concatenation of the passed ``name_prefix`` and a hex
+    representation of the bin index between separated by a hyphen.
+
+    The passed ``keyids`` and ``threshold`` is used for each bin, and each bin
+    is 'terminating'.
+
+    For details: https://github.com/theupdateframework/taps/blob/master/tap15.md
+
+    Args:
+        keyids: Signing key identifiers for any bin metadata.
+        threshold: Number of keys required to sign any bin metadata.
+        bit_length: Number of bits between 1 and 32.
+        name_prefix: Prefix of all bin names.
+        unrecognized_fields: Dictionary of all attributes that are not managed
+            by TUF Metadata API.
+
+    Raises:
+            ValueError, TypeError, AttributeError: Invalid arguments.
+    """
+
+    def __init__(
+        self,
+        keyids: List[str],
+        threshold: int,
+        bit_length: int,
+        name_prefix: str,
+        unrecognized_fields: Optional[Dict[str, Any]] = None,
+    ) -> None:
+        super().__init__(keyids, threshold, unrecognized_fields)
+
+        if bit_length <= 0 or bit_length > 32:
+            raise ValueError("bit_length must be between 1 and 32")
+        if not isinstance(name_prefix, str):
+            raise ValueError("name_prefix must be a string")
+
+        self.bit_length = bit_length
+        self.name_prefix = name_prefix
+
+    def __eq__(self, other: Any) -> bool:
+        if not isinstance(other, SuccinctRoles):
+            return False
+
+        return (
+            super().__eq__(other)
+            and self.bit_length == other.bit_length
+            and self.name_prefix == other.name_prefix
+        )
+
+    @classmethod
+    def from_dict(cls, role_dict: Dict[str, Any]) -> "SuccinctRoles":
+        """Creates ``SuccinctRoles`` object from its json/dict representation.
+
+        Raises:
+            ValueError, KeyError, AttributeError, TypeError: Invalid arguments.
+        """
+        keyids = role_dict.pop("keyids")
+        threshold = role_dict.pop("threshold")
+        bit_length = role_dict.pop("bit_length")
+        name_prefix = role_dict.pop("name_prefix")
+        # All fields left in the role_dict are unrecognized.
+        return cls(keyids, threshold, bit_length, name_prefix, role_dict)
+
+    def to_dict(self) -> Dict[str, Any]:
+        """Returns the dict representation of self."""
+        base_role_dict = super().to_dict()
+        return {
+            "bit_length": self.bit_length,
+            "name_prefix": self.name_prefix,
+            **base_role_dict,
+        }
+
+    def get_role_for_target(self, target_filepath: str) -> str:
+        """Calculates the name of the delegated role responsible for
+        ``target_filepath``.
+
+        Args:
+            target_filepath: URL path to a target file, relative to a base
+                targets URL.
+        """
+        hasher = sslib_hash.digest(algorithm="sha256")
+        hasher.update(target_filepath.encode("utf-8"))
+
+        # We can't ever need more than 4 bytes (32 bits).
+        hash_bytes = hasher.digest()[:4]
+        # Right shift hash bytes, so that we only have the leftmost
+        # bit_length bits that we care about.
+        shift_value = 32 - self.bit_length
+        bin_number = int.from_bytes(hash_bytes, byteorder="big") >> shift_value
+        return f"{self.name_prefix}-{bin_number}"
+
+
 class Delegations:
     """A container object storing information about all delegations.