metadata.py

# Copyright New York University and the TUF contributors
# SPDX-License-Identifier: MIT OR Apache-2.0

"""
The low-level Metadata API in ``tuf.api.metadata`` module contains:

* Safe de/serialization of metadata to and from files.
* Access to and modification of signed metadata content.
* Signing metadata and verifying signatures.

Metadata API implements functionality at the metadata file level, it does
not provide TUF repository or client functionality on its own (but can be used
to implement them).

The API design is based on the file format defined in the `TUF specification
<https://theupdateframework.github.io/specification/latest/>`_ and the object
attributes generally follow the JSON format used in the specification.

The above principle means that a ``Metadata`` object represents a single
metadata file, and has a ``signed`` attribute that is an instance of one of the
four top level signed classes (``Root``, ``Timestamp``, ``Snapshot`` and ``Targets``).
To make Python type annotations useful ``Metadata`` can be type constrained: e.g. the
signed attribute of ``Metadata[Root]`` is known to be ``Root``.

Currently Metadata API supports JSON as the file format.

A basic example of repository implementation using the Metadata is available in
`examples/repo_example <https://github.com/theupdateframework/python-tuf/tree/develop/examples/repo_example>`_.
"""
import abc
import fnmatch
import io
import logging
from datetime import datetime
from typing import (
    IO,
    Any,
    ClassVar,
    Dict,
    Generic,
    Iterator,
    List,
    Mapping,
    Optional,
    Tuple,
    Type,
    TypeVar,
    Union,
    cast,
)

from securesystemslib import exceptions as sslib_exceptions
from securesystemslib import hash as sslib_hash
from securesystemslib.metadata import Envelope as BaseEnvelope
from securesystemslib.serialization import JSONSerializable
from securesystemslib.signer import Key, Signature, Signer

from tuf.api.exceptions import LengthOrHashMismatchError, UnsignedMetadataError
from tuf.api.serialization import (
    BaseDeserializer,
    BaseSerializer,
    SerializationMixin,
    SignedDeserializer,
    SignedSerializer,
)

_ROOT = "root"
_SNAPSHOT = "snapshot"
_TARGETS = "targets"
_TIMESTAMP = "timestamp"

# pylint: disable=too-many-lines

logger = logging.getLogger(__name__)

# We aim to support SPECIFICATION_VERSION and require the input metadata
# files to have the same major version (the first number) as ours.
SPECIFICATION_VERSION = ["1", "0", "31"]
TOP_LEVEL_ROLE_NAMES = {_ROOT, _TIMESTAMP, _SNAPSHOT, _TARGETS}

# T is a Generic type constraint for Metadata.signed
T = TypeVar("T", "Root", "Timestamp", "Snapshot", "Targets")


class BaseMetadata(SerializationMixin, JSONSerializable, metaclass=abc.ABCMeta):
    """A common metadata interface for Envelope (DSSE) and Metadata objects."""

    @staticmethod
    def _default_deserializer() -> BaseDeserializer:
        """Default deserializer for Serialization Mixin."""
        # pylint: disable=import-outside-toplevel
        from tuf.api.serialization.json import JSONDeserializer

        return JSONDeserializer()

    @staticmethod
    def _default_serializer() -> BaseSerializer:
        """Default serializer for Serialization Mixin."""
        # pylint: disable=import-outside-toplevel
        from tuf.api.serialization.json import JSONSerializer

        return JSONSerializer(compact=True)

    @staticmethod
    def _get_role_and_keys(
        signed: "Signed", delegated_role: str
    ) -> Tuple["Role", Dict[str, Key]]:
        """Return the keys and role for delegated_role"""

        role: Optional[Role] = None
        if isinstance(signed, Root):
            keys = signed.keys
            role = signed.roles.get(delegated_role)
        elif isinstance(signed, Targets):
            if signed.delegations is None:
                raise ValueError(f"No delegation found for {delegated_role}")

            keys = signed.delegations.keys
            if signed.delegations.roles is not None:
                role = signed.delegations.roles.get(delegated_role)
            elif signed.delegations.succinct_roles is not None:
                if signed.delegations.succinct_roles.is_delegated_role(
                    delegated_role
                ):
                    role = signed.delegations.succinct_roles
        else:
            raise TypeError("Call is valid only on delegator metadata")

        if role is None:
            raise ValueError(f"No delegation found for {delegated_role}")

        return (role, keys)

    @abc.abstractmethod
    def get_signed(self) -> "Signed":
        raise NotImplementedError

    @abc.abstractmethod
    def sign(
        self,
        signer: Signer,
    ) -> Signature:
        raise NotImplementedError

    @abc.abstractmethod
    def verify_delegate(
        self,
        delegated_role: str,
        delegated_metadata: "BaseMetadata",
    ) -> None:
        raise NotImplementedError


class Envelope(Generic[T], BaseMetadata, BaseEnvelope):
    """DSSE Envelope for tuf payloads."""

    DEFAULT_PAYLOAD_TYPE = "application/vnd.tuf"

    @classmethod
    def from_signed(
        cls, signed: "Signed", serializer: SignedSerializer = None
    ) -> "Envelope":
        """Creates DSSE envelope with signed bytes as payload."""

        if serializer is None:
            # Use local scope import to avoid circular import errors
            # pylint: disable=import-outside-toplevel
            from tuf.api.serialization.json import JSONSerializer

            serializer = JSONSerializer(compact=True)

        return cls(
            payload=serializer.serialize(signed),
            payload_type=cls.DEFAULT_PAYLOAD_TYPE,
            signatures=[],
        )

    def get_signed(self, deserializer: SignedDeserializer = None) -> "Signed":
        if deserializer is None:
            # Use local scope import to avoid circular import errors
            # pylint: disable=import-outside-toplevel
            from tuf.api.serialization.json import SignedJSONDeserializer

            deserializer = SignedJSONDeserializer()

        return BaseEnvelope.get_payload(self, deserializer)

    def sign(
        self,
        signer: Signer,
        append: bool = False,
    ) -> Signature:

        if not append:
            self.signatures.clear()

        return BaseEnvelope.sign(self, signer)

    def verify_delegate(
        self,
        delegated_role: str,
        delegated_metadata: "BaseMetadata",
    ) -> None:
        signed = self.get_signed(None)
        role, keys = self._get_role_and_keys(signed, delegated_role)

        try:
            _ = BaseEnvelope.verify(
                delegated_metadata, keys.values(), role.threshold
            )

        except sslib_exceptions.UnverifiedSignatureError as e:
            raise UnsignedMetadataError from e


class Metadata(Generic[T], BaseMetadata):
    """A container for signed TUF metadata.

    Provides methods to convert to and from dictionary, read and write to and
    from file and to create and verify metadata signatures.

    ``Metadata[T]`` is a generic container type where T can be any one type of
    [``Root``, ``Timestamp``, ``Snapshot``, ``Targets``]. The purpose of this
    is to allow static type checking of the signed attribute in code using
    Metadata::

        root_md = Metadata[Root].from_file("root.json")
        # root_md type is now Metadata[Root]. This means signed and its
        # attributes like consistent_snapshot are now statically typed and the
        # types can be verified by static type checkers and shown by IDEs
        print(root_md.signed.consistent_snapshot)

    Using a type constraint is not required but not doing so means T is not a
    specific type so static typing cannot happen. Note that the type constraint
    ``[Root]`` is not validated at runtime (as pure annotations are not available
    then).

    New Metadata instances can be created from scratch with::

        one_day = datetime.utcnow() + timedelta(days=1)
        timestamp = Metadata(Timestamp(expires=one_day))

    Apart from ``expires`` all of the arguments to the inner constructors have
    reasonable default values for new metadata.

    *All parameters named below are not just constructor arguments but also
    instance attributes.*

    Args:
        signed: Actual metadata payload, i.e. one of ``Targets``,
            ``Snapshot``, ``Timestamp`` or ``Root``.
        signatures: Ordered dictionary of keyids to ``Signature`` objects, each
            signing the canonical serialized representation of ``signed``.
            Default is an empty dictionary.
        unrecognized_fields: Dictionary of all attributes that are not managed
            by TUF Metadata API. These fields are NOT signed and it's preferable
            if unrecognized fields are added to the Signed derivative classes.
    """

    def __init__(
        self,
        signed: T,
        signatures: Optional[Dict[str, Signature]] = None,
        unrecognized_fields: Optional[Dict[str, Any]] = None,
    ):
        self.signed: T = signed
        self.signatures = signatures if signatures is not None else {}
        if unrecognized_fields is None:
            unrecognized_fields = {}

        self.unrecognized_fields = unrecognized_fields

    def __eq__(self, other: Any) -> bool:
        if not isinstance(other, Metadata):
            return False

        return (
            self.signatures == other.signatures
            # Order of the signatures matters (see issue #1788).
            and list(self.signatures.items()) == list(other.signatures.items())
            and self.signed == other.signed
            and self.unrecognized_fields == other.unrecognized_fields
        )

    def get_signed(self) -> "Signed":
        return self.signed

    @classmethod
    def from_dict(cls, metadata: Dict[str, Any]) -> "Metadata[T]":
        """Creates ``Metadata`` object from its json/dict representation.

        Args:
            metadata: TUF metadata in dict representation.

        Raises:
            ValueError, KeyError, TypeError: Invalid arguments.

        Side Effect:
            Destroys the metadata dict passed by reference.

        Returns:
            TUF ``Metadata`` object.
        """

        # Dispatch to contained metadata class on metadata _type field.
        _type = metadata["signed"]["_type"]

        if _type == _TARGETS:
            inner_cls: Type[Signed] = Targets
        elif _type == _SNAPSHOT:
            inner_cls = Snapshot
        elif _type == _TIMESTAMP:
            inner_cls = Timestamp
        elif _type == _ROOT:
            inner_cls = Root
        else:
            raise ValueError(f'unrecognized metadata type "{_type}"')

        # Make sure signatures are unique
        signatures: Dict[str, Signature] = {}
        for sig_dict in metadata.pop("signatures"):
            sig = Signature.from_dict(sig_dict)
            if sig.keyid in signatures:
                raise ValueError(
                    f"Multiple signatures found for keyid {sig.keyid}"
                )
            signatures[sig.keyid] = sig

        return cls(
            # Specific type T is not known at static type check time: use cast
            signed=cast(T, inner_cls.from_dict(metadata.pop("signed"))),
            signatures=signatures,
            # All fields left in the metadata dict are unrecognized.
            unrecognized_fields=metadata,
        )

    def to_dict(self) -> Dict[str, Any]:
        """Returns the dict representation of self."""

        signatures = [sig.to_dict() for sig in self.signatures.values()]

        return {
            "signatures": signatures,
            "signed": self.signed.to_dict(),
            **self.unrecognized_fields,
        }

    # Signatures.
    def sign(
        self,
        signer: Signer,
        append: bool = False,
        signed_serializer: Optional[SignedSerializer] = None,
    ) -> Signature:
        """Creates signature over ``signed`` and assigns it to ``signatures``.

        Args:
            signer: A ``securesystemslib.signer.Signer`` object that provides a private
                key and signing implementation to generate the signature. A standard
                implementation is available in ``securesystemslib.signer.SSlibSigner``.
            append: ``True`` if the signature should be appended to
                the list of signatures or replace any existing signatures. The
                default behavior is to replace signatures.
            signed_serializer: ``SignedSerializer`` that implements the desired
                serialization format. Default is ``CanonicalJSONSerializer``.

        Raises:
            tuf.api.serialization.SerializationError:
                ``signed`` cannot be serialized.
            UnsignedMetadataError: Signing errors.

        Returns:
            ``securesystemslib.signer.Signature`` object that was added into
            signatures.
        """

        if signed_serializer is None:
            # Use local scope import to avoid circular import errors
            # pylint: disable=import-outside-toplevel
            from tuf.api.serialization.json import CanonicalJSONSerializer

            signed_serializer = CanonicalJSONSerializer()

        bytes_data = signed_serializer.serialize(self.signed)

        try:
            signature = signer.sign(bytes_data)
        except Exception as e:
            raise UnsignedMetadataError("Problem signing the metadata") from e

        if not append:
            self.signatures.clear()

        self.signatures[signature.keyid] = signature

        return signature

    def verify_delegate(
        self,
        delegated_role: str,
        delegated_metadata: "Metadata",
        signed_serializer: Optional[SignedSerializer] = None,
    ) -> None:
        """Verifies that ``delegated_metadata`` is signed with the required
        threshold of keys for the delegated role ``delegated_role``.

        Args:
            delegated_role: Name of the delegated role to verify
            delegated_metadata: ``Metadata`` object for the delegated role
            signed_serializer: Serializer used for delegate
                serialization. Default is ``CanonicalJSONSerializer``.

        Raises:
            UnsignedMetadataError: ``delegated_role`` was not signed with
                required threshold of keys for ``role_name``.
            ValueError: no delegation was found for ``delegated_role``.
            TypeError: called this function on non-delegating metadata class.
        """

        if signed_serializer is None:
            # pylint: disable=import-outside-toplevel
            from tuf.api.serialization.json import CanonicalJSONSerializer

            signed_serializer = CanonicalJSONSerializer()

        data = signed_serializer.serialize(delegated_metadata.signed)
        role, keys = self._get_role_and_keys(self.signed, delegated_role)

        # verify that delegated_metadata is signed by threshold of unique keys
        signing_keys = set()
        for keyid in role.keyids:
            if keyid not in keys:
                logger.info("No key for keyid %s", keyid)
                continue

            if keyid not in delegated_metadata.signatures:
                logger.info("No signature for keyid %s", keyid)
                continue

            sig = delegated_metadata.signatures[keyid]
            try:
                keys[keyid].verify_signature(sig, data)
                signing_keys.add(keyid)
            except sslib_exceptions.UnverifiedSignatureError:
                logger.info("Key %s failed to verify %s", keyid, delegated_role)

        if len(signing_keys) < role.threshold:
            raise UnsignedMetadataError(
                f"{delegated_role} was signed by {len(signing_keys)}/"
                f"{role.threshold} keys",
            )


class Signed(metaclass=abc.ABCMeta):
    """A base class for the signed part of TUF metadata.

    Objects with base class Signed are usually included in a ``Metadata`` object
    on the signed attribute. This class provides attributes and methods that
    are common for all TUF metadata types (roles).

    *All parameters named below are not just constructor arguments but also
    instance attributes.*

    Args:
        version: Metadata version number. If None, then 1 is assigned.
        spec_version: Supported TUF specification version. If None, then the
            version currently supported by the library is assigned.
        expires: Metadata expiry date. If None, then current date and time is
            assigned.
        unrecognized_fields: Dictionary of all attributes that are not managed
            by TUF Metadata API

    Raises:
        ValueError: Invalid arguments.
    """

    # type is required for static reference without changing the API
    type: ClassVar[str] = "signed"

    # _type and type are identical: 1st replicates file format, 2nd passes lint
    @property
    def _type(self) -> str:
        return self.type

    @property
    def expires(self) -> datetime:
        """The metadata expiry date::

        # Use 'datetime' module to e.g. expire in seven days from now
        obj.expires = utcnow() + timedelta(days=7)
        """
        return self._expires

    @expires.setter
    def expires(self, value: datetime) -> None:
        self._expires = value.replace(microsecond=0)

    # NOTE: Signed is a stupid name, because this might not be signed yet, but
    # we keep it to match spec terminology (I often refer to this as "payload",
    # or "inner metadata")
    def __init__(
        self,
        version: Optional[int],
        spec_version: Optional[str],
        expires: Optional[datetime],
        unrecognized_fields: Optional[Dict[str, Any]],
    ):
        if spec_version is None:
            spec_version = ".".join(SPECIFICATION_VERSION)
        # Accept semver (X.Y.Z) but also X.Y for legacy compatibility
        spec_list = spec_version.split(".")
        if len(spec_list) not in [2, 3] or not all(
            el.isdigit() for el in spec_list
        ):
            raise ValueError(f"Failed to parse spec_version {spec_version}")

        # major version must match
        if spec_list[0] != SPECIFICATION_VERSION[0]:
            raise ValueError(f"Unsupported spec_version {spec_version}")

        self.spec_version = spec_version

        self.expires = expires or datetime.utcnow()

        if version is None:
            version = 1
        elif version <= 0:
            raise ValueError(f"version must be > 0, got {version}")
        self.version = version

        if unrecognized_fields is None:
            unrecognized_fields = {}

        self.unrecognized_fields = unrecognized_fields

    def __eq__(self, other: Any) -> bool:
        if not isinstance(other, Signed):
            return False

        return (
            self.type == other.type
            and self.version == other.version
            and self.spec_version == other.spec_version
            and self.expires == other.expires
            and self.unrecognized_fields == other.unrecognized_fields
        )

    @abc.abstractmethod
    def to_dict(self) -> Dict[str, Any]:
        """Serialization helper that returns dict representation of self"""
        raise NotImplementedError

    @classmethod
    @abc.abstractmethod
    def from_dict(cls, signed_dict: Dict[str, Any]) -> "Signed":
        """Deserialization helper, creates object from json/dict representation"""
        raise NotImplementedError

    @classmethod
    def _common_fields_from_dict(
        cls, signed_dict: Dict[str, Any]
    ) -> Tuple[int, str, datetime]:
        """Returns common fields of ``Signed`` instances from the passed dict
        representation, and returns an ordered list to be passed as leading
        positional arguments to a subclass constructor.

        See ``{Root, Timestamp, Snapshot, Targets}.from_dict`` methods for usage.

        """
        _type = signed_dict.pop("_type")
        if _type != cls.type:
            raise ValueError(f"Expected type {cls.type}, got {_type}")

        version = signed_dict.pop("version")
        spec_version = signed_dict.pop("spec_version")
        expires_str = signed_dict.pop("expires")
        # Convert 'expires' TUF metadata string to a datetime object, which is
        # what the constructor expects and what we store. The inverse operation
        # is implemented in '_common_fields_to_dict'.
        expires = datetime.strptime(expires_str, "%Y-%m-%dT%H:%M:%SZ")

        return version, spec_version, expires

    def _common_fields_to_dict(self) -> Dict[str, Any]:
        """Returns dict representation of common fields of ``Signed`` instances.

        See ``{Root, Timestamp, Snapshot, Targets}.to_dict`` methods for usage.

        """
        return {
            "_type": self._type,
            "version": self.version,
            "spec_version": self.spec_version,
            "expires": self.expires.isoformat() + "Z",
            **self.unrecognized_fields,
        }

    def is_expired(self, reference_time: Optional[datetime] = None) -> bool:
        """Checks metadata expiration against a reference time.

        Args:
            reference_time: Time to check expiration date against. A naive
                datetime in UTC expected. Default is current UTC date and time.

        Returns:
            ``True`` if expiration time is less than the reference time.
        """
        if reference_time is None:
            reference_time = datetime.utcnow()

        return reference_time >= self.expires


class Role:
    """Container that defines which keys are required to sign roles metadata.

    Role defines how many keys are required to successfully sign the roles
    metadata, and which keys are accepted.

    *All parameters named below are not just constructor arguments but also
    instance attributes.*

    Args:
        keyids: Roles signing key identifiers.
        threshold: Number of keys required to sign this role's metadata.
        unrecognized_fields: Dictionary of all attributes that are not managed
            by TUF Metadata API

    Raises:
        ValueError: Invalid arguments.
    """

    def __init__(
        self,
        keyids: List[str],
        threshold: int,
        unrecognized_fields: Optional[Dict[str, Any]] = None,
    ):
        if len(set(keyids)) != len(keyids):
            raise ValueError(f"Nonunique keyids: {keyids}")
        if threshold < 1:
            raise ValueError("threshold should be at least 1!")
        self.keyids = keyids
        self.threshold = threshold
        if unrecognized_fields is None:
            unrecognized_fields = {}

        self.unrecognized_fields = unrecognized_fields

    def __eq__(self, other: Any) -> bool:
        if not isinstance(other, Role):
            return False

        return (
            self.keyids == other.keyids
            and self.threshold == other.threshold
            and self.unrecognized_fields == other.unrecognized_fields
        )

    @classmethod
    def from_dict(cls, role_dict: Dict[str, Any]) -> "Role":
        """Creates ``Role`` object from its json/dict representation.

        Raises:
            ValueError, KeyError: Invalid arguments.
        """
        keyids = role_dict.pop("keyids")
        threshold = role_dict.pop("threshold")
        # All fields left in the role_dict are unrecognized.
        return cls(keyids, threshold, role_dict)

    def to_dict(self) -> Dict[str, Any]:
        """Returns the dictionary representation of self."""
        return {
            "keyids": self.keyids,
            "threshold": self.threshold,
            **self.unrecognized_fields,
        }


class Root(Signed):
    """A container for the signed part of root metadata.

    Parameters listed below are also instance attributes.

    Args:
        version: Metadata version number. Default is 1.
        spec_version: Supported TUF specification version. Default is the
            version currently supported by the library.
        expires: Metadata expiry date. Default is current date and time.
        keys: Dictionary of keyids to Keys. Defines the keys used in ``roles``.
            Default is empty dictionary.
        roles: Dictionary of role names to Roles. Defines which keys are
            required to sign the metadata for a specific role. Default is
            a dictionary of top level roles without keys and threshold of 1.
        consistent_snapshot: ``True`` if repository supports consistent snapshots.
            Default is True.
        unrecognized_fields: Dictionary of all attributes that are not managed
            by TUF Metadata API

    Raises:
        ValueError: Invalid arguments.
    """

    type = _ROOT

    # pylint: disable=too-many-arguments
    def __init__(
        self,
        version: Optional[int] = None,
        spec_version: Optional[str] = None,
        expires: Optional[datetime] = None,
        keys: Optional[Dict[str, Key]] = None,
        roles: Optional[Mapping[str, Role]] = None,
        consistent_snapshot: Optional[bool] = True,
        unrecognized_fields: Optional[Dict[str, Any]] = None,
    ):
        super().__init__(version, spec_version, expires, unrecognized_fields)
        self.consistent_snapshot = consistent_snapshot
        self.keys = keys if keys is not None else {}

        if roles is None:
            roles = {r: Role([], 1) for r in TOP_LEVEL_ROLE_NAMES}
        elif set(roles) != TOP_LEVEL_ROLE_NAMES:
            raise ValueError("Role names must be the top-level metadata roles")
        self.roles = roles

    def __eq__(self, other: Any) -> bool:
        if not isinstance(other, Root):
            return False

        return (
            super().__eq__(other)
            and self.keys == other.keys
            and self.roles == other.roles
            and self.consistent_snapshot == other.consistent_snapshot
        )

    @classmethod
    def from_dict(cls, signed_dict: Dict[str, Any]) -> "Root":
        """Creates ``Root`` object from its json/dict representation.

        Raises:
            ValueError, KeyError, TypeError: Invalid arguments.
        """
        common_args = cls._common_fields_from_dict(signed_dict)
        consistent_snapshot = signed_dict.pop("consistent_snapshot", None)
        keys = signed_dict.pop("keys")
        roles = signed_dict.pop("roles")

        for keyid, key_dict in keys.items():
            keys[keyid] = Key.from_dict(keyid, key_dict)
        for role_name, role_dict in roles.items():
            roles[role_name] = Role.from_dict(role_dict)

        # All fields left in the signed_dict are unrecognized.
        return cls(*common_args, keys, roles, consistent_snapshot, signed_dict)

    def to_dict(self) -> Dict[str, Any]:
        """Returns the dict representation of self."""
        root_dict = self._common_fields_to_dict()
        keys = {keyid: key.to_dict() for (keyid, key) in self.keys.items()}
        roles = {}
        for role_name, role in self.roles.items():
            roles[role_name] = role.to_dict()
        if self.consistent_snapshot is not None:
            root_dict["consistent_snapshot"] = self.consistent_snapshot

        root_dict.update(
            {
                "keys": keys,
                "roles": roles,
            }
        )
        return root_dict

    def add_key(self, key: Key, role: str) -> None:
        """Adds new signing key for delegated role ``role``.

        Args:
            key: Signing key to be added for ``role``.
            role: Name of the role, for which ``key`` is added.

        Raises:
            ValueError: If the argument order is wrong or if ``role`` doesn't
                exist.
        """
        # Verify that our users are not using the old argument order.
        if isinstance(role, Key):
            raise ValueError("Role must be a string, not a Key instance")

        if role not in self.roles:
            raise ValueError(f"Role {role} doesn't exist")
        if key.keyid not in self.roles[role].keyids:
            self.roles[role].keyids.append(key.keyid)
        self.keys[key.keyid] = key

    def revoke_key(self, keyid: str, role: str) -> None:
        """Revoke key from ``role`` and updates the key store.

        Args:
            keyid: Identifier of the key to be removed for ``role``.
            role: Name of the role, for which a signing key is removed.

        Raises:
            ValueError: If ``role`` doesn't exist or if ``role`` doesn't include
                the key.
        """
        if role not in self.roles:
            raise ValueError(f"Role {role} doesn't exist")
        if keyid not in self.roles[role].keyids:
            raise ValueError(f"Key with id {keyid} is not used by {role}")
        self.roles[role].keyids.remove(keyid)
        for keyinfo in self.roles.values():
            if keyid in keyinfo.keyids:
                return

        del self.keys[keyid]


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, IO[bytes]], 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():
            try:
                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)
            except (
                sslib_exceptions.UnsupportedAlgorithmError,
                sslib_exceptions.FormatError,
            ) as e:
                raise LengthOrHashMismatchError(
                    f"Unsupported algorithm '{algo}'"
                ) from e

            observed_hash = digest_object.hexdigest()
            if observed_hash != exp_hash:
                raise LengthOrHashMismatchError(
                    f"Observed hash {observed_hash} does not match "
                    f"expected hash {exp_hash}"
                )

    @staticmethod
    def _verify_length(
        data: Union[bytes, IO[bytes]], 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 LengthOrHashMismatchError(
                f"Observed length {observed_length} does not match "
                f"expected length {expected_length}"
            )

    @staticmethod
    def _validate_hashes(hashes: Dict[str, str]) -> None:
        if not hashes:
            raise ValueError("Hashes must be a non empty dictionary")
        for key, value in hashes.items():
            if not (isinstance(key, str) and isinstance(value, str)):
                raise TypeError("Hashes items must be strings")

    @staticmethod
    def _validate_length(length: int) -> None:
        if length < 0:
            raise ValueError(f"Length must be >= 0, got {length}")


class MetaFile(BaseFile):
    """A container with information about a particular metadata file.

    *All parameters named below are not just constructor arguments but also
    instance attributes.*

    Args:
        version: Version of the metadata file.
        length: Length of the metadata file in bytes.
        hashes: Dictionary of hash algorithm names to hashes of the metadata
            file content.
        unrecognized_fields: Dictionary of all attributes that are not managed
            by TUF Metadata API

    Raises:
        ValueError, TypeError: Invalid arguments.
    """

    def __init__(
        self,
        version: int = 1,
        length: Optional[int] = None,
        hashes: Optional[Dict[str, str]] = None,
        unrecognized_fields: Optional[Dict[str, Any]] = None,
    ):

        if version <= 0:
            raise ValueError(f"Metafile version must be > 0, got {version}")
        if length is not None:
            self._validate_length(length)
        if hashes is not None:
            self._validate_hashes(hashes)

        self.version = version
        self.length = length
        self.hashes = hashes
        if unrecognized_fields is None:
            unrecognized_fields = {}

        self.unrecognized_fields = unrecognized_fields

    def __eq__(self, other: Any) -> bool:
        if not isinstance(other, MetaFile):
            return False

        return (
            self.version == other.version
            and self.length == other.length
            and self.hashes == other.hashes
            and self.unrecognized_fields == other.unrecognized_fields
        )

    @classmethod
    def from_dict(cls, meta_dict: Dict[str, Any]) -> "MetaFile":
        """Creates ``MetaFile`` object from its json/dict representation.

        Raises:
            ValueError, KeyError: Invalid arguments.
        """
        version = meta_dict.pop("version")
        length = meta_dict.pop("length", None)
        hashes = meta_dict.pop("hashes", None)

        # All fields left in the meta_dict are unrecognized.
        return cls(version, length, hashes, meta_dict)

    def to_dict(self) -> Dict[str, Any]:
        """Returns the dictionary representation of self."""
        res_dict: Dict[str, Any] = {
            "version": self.version,
            **self.unrecognized_fields,
        }

        if self.length is not None:
            res_dict["length"] = self.length

        if self.hashes is not None:
            res_dict["hashes"] = self.hashes

        return res_dict

    def verify_length_and_hashes(self, data: Union[bytes, IO[bytes]]) -> None:
        """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 or hash algorithm is not supported.
        """
        if self.length is not None:
            self._verify_length(data, self.length)

        if self.hashes is not None:
            self._verify_hashes(data, self.hashes)


class Timestamp(Signed):
    """A container for the signed part of timestamp metadata.

    TUF file format uses a dictionary to contain the snapshot information:
    this is not the case with ``Timestamp.snapshot_meta`` which is a ``MetaFile``.

    *All parameters named below are not just constructor arguments but also
    instance attributes.*

    Args:
        version: Metadata version number. Default is 1.
        spec_version: Supported TUF specification version. Default is the
            version currently supported by the library.
        expires: Metadata expiry date. Default is current date and time.
        unrecognized_fields: Dictionary of all attributes that are not managed
            by TUF Metadata API
        snapshot_meta: Meta information for snapshot metadata. Default is a
            MetaFile with version 1.

    Raises:
        ValueError: Invalid arguments.
    """

    type = _TIMESTAMP

    def __init__(
        self,
        version: Optional[int] = None,
        spec_version: Optional[str] = None,
        expires: Optional[datetime] = None,
        snapshot_meta: Optional[MetaFile] = None,
        unrecognized_fields: Optional[Dict[str, Any]] = None,
    ):
        super().__init__(version, spec_version, expires, unrecognized_fields)
        self.snapshot_meta = snapshot_meta or MetaFile(1)

    def __eq__(self, other: Any) -> bool:
        if not isinstance(other, Timestamp):
            return False

        return (
            super().__eq__(other) and self.snapshot_meta == other.snapshot_meta
        )

    @classmethod
    def from_dict(cls, signed_dict: Dict[str, Any]) -> "Timestamp":
        """Creates ``Timestamp`` object from its json/dict representation.

        Raises:
            ValueError, KeyError: Invalid arguments.
        """
        common_args = cls._common_fields_from_dict(signed_dict)
        meta_dict = signed_dict.pop("meta")
        snapshot_meta = MetaFile.from_dict(meta_dict["snapshot.json"])
        # All fields left in the timestamp_dict are unrecognized.
        return cls(*common_args, snapshot_meta, signed_dict)

    def to_dict(self) -> Dict[str, Any]:
        """Returns the dict representation of self."""
        res_dict = self._common_fields_to_dict()
        res_dict["meta"] = {"snapshot.json": self.snapshot_meta.to_dict()}
        return res_dict


class Snapshot(Signed):
    """A container for the signed part of snapshot metadata.

    Snapshot contains information about all target Metadata files.

    *All parameters named below are not just constructor arguments but also
    instance attributes.*

    Args:
        version: Metadata version number. Default is 1.
        spec_version: Supported TUF specification version. Default is the
            version currently supported by the library.
        expires: Metadata expiry date. Default is current date and time.
        unrecognized_fields: Dictionary of all attributes that are not managed
            by TUF Metadata API
        meta: Dictionary of targets filenames to ``MetaFile`` objects. Default
            is a dictionary with a Metafile for "snapshot.json" version 1.

    Raises:
        ValueError: Invalid arguments.
    """

    type = _SNAPSHOT

    def __init__(
        self,
        version: Optional[int] = None,
        spec_version: Optional[str] = None,
        expires: Optional[datetime] = None,
        meta: Optional[Dict[str, MetaFile]] = None,
        unrecognized_fields: Optional[Dict[str, Any]] = None,
    ):
        super().__init__(version, spec_version, expires, unrecognized_fields)
        self.meta = meta if meta is not None else {"targets.json": MetaFile(1)}

    def __eq__(self, other: Any) -> bool:
        if not isinstance(other, Snapshot):
            return False

        return super().__eq__(other) and self.meta == other.meta

    @classmethod
    def from_dict(cls, signed_dict: Dict[str, Any]) -> "Snapshot":
        """Creates ``Snapshot`` object from its json/dict representation.

        Raises:
            ValueError, KeyError: Invalid arguments.
        """
        common_args = cls._common_fields_from_dict(signed_dict)
        meta_dicts = signed_dict.pop("meta")
        meta = {}
        for meta_path, meta_dict in meta_dicts.items():
            meta[meta_path] = MetaFile.from_dict(meta_dict)
        # All fields left in the snapshot_dict are unrecognized.
        return cls(*common_args, meta, signed_dict)

    def to_dict(self) -> Dict[str, Any]:
        """Returns the dict representation of self."""
        snapshot_dict = self._common_fields_to_dict()
        meta_dict = {}
        for meta_path, meta_info in self.meta.items():
            meta_dict[meta_path] = meta_info.to_dict()

        snapshot_dict["meta"] = meta_dict
        return snapshot_dict


class DelegatedRole(Role):
    """A container with information about a delegated role.

    A delegation can happen in two ways:

        - ``paths`` is set: delegates targets matching any path pattern in ``paths``
        - ``path_hash_prefixes`` is set: delegates targets whose target path hash
          starts with any of the prefixes in ``path_hash_prefixes``

        ``paths`` and ``path_hash_prefixes`` are mutually exclusive: both cannot be
        set, at least one of them must be set.

    *All parameters named below are not just constructor arguments but also
    instance attributes.*

    Args:
        name: Delegated role name.
        keyids: Delegated role signing key identifiers.
        threshold: Number of keys required to sign this role's metadata.
        terminating: ``True`` if this delegation terminates a target lookup.
        paths: Path patterns. See note above.
        path_hash_prefixes: Hash prefixes. See note above.
        unrecognized_fields: Dictionary of all attributes that are not managed
            by TUF Metadata API.

    Raises:
        ValueError: Invalid arguments.
    """

    def __init__(
        self,
        name: str,
        keyids: List[str],
        threshold: int,
        terminating: bool,
        paths: Optional[List[str]] = None,
        path_hash_prefixes: Optional[List[str]] = None,
        unrecognized_fields: Optional[Dict[str, Any]] = None,
    ):
        super().__init__(keyids, threshold, unrecognized_fields)
        self.name = name
        self.terminating = terminating
        exclusive_vars = [paths, path_hash_prefixes]
        if sum(1 for var in exclusive_vars if var is not None) != 1:
            raise ValueError(
                "Only one of (paths, path_hash_prefixes) must be set"
            )

        if paths is not None and any(not isinstance(p, str) for p in paths):
            raise ValueError("Paths must be strings")
        if path_hash_prefixes is not None and any(
            not isinstance(p, str) for p in path_hash_prefixes
        ):
            raise ValueError("Path_hash_prefixes must be strings")

        self.paths = paths
        self.path_hash_prefixes = path_hash_prefixes

    def __eq__(self, other: Any) -> bool:
        if not isinstance(other, DelegatedRole):
            return False

        return (
            super().__eq__(other)
            and self.name == other.name
            and self.terminating == other.terminating
            and self.paths == other.paths
            and self.path_hash_prefixes == other.path_hash_prefixes
        )

    @classmethod
    def from_dict(cls, role_dict: Dict[str, Any]) -> "DelegatedRole":
        """Creates ``DelegatedRole`` object from its json/dict representation.

        Raises:
            ValueError, KeyError, TypeError: Invalid arguments.
        """
        name = role_dict.pop("name")
        keyids = role_dict.pop("keyids")
        threshold = role_dict.pop("threshold")
        terminating = role_dict.pop("terminating")
        paths = role_dict.pop("paths", None)
        path_hash_prefixes = role_dict.pop("path_hash_prefixes", None)
        # All fields left in the role_dict are unrecognized.
        return cls(
            name,
            keyids,
            threshold,
            terminating,
            paths,
            path_hash_prefixes,
            role_dict,
        )

    def to_dict(self) -> Dict[str, Any]:
        """Returns the dict representation of self."""
        base_role_dict = super().to_dict()
        res_dict = {
            "name": self.name,
            "terminating": self.terminating,
            **base_role_dict,
        }
        if self.paths is not None:
            res_dict["paths"] = self.paths
        elif self.path_hash_prefixes is not None:
            res_dict["path_hash_prefixes"] = self.path_hash_prefixes
        return res_dict

    @staticmethod
    def _is_target_in_pathpattern(targetpath: str, pathpattern: str) -> bool:
        """Determines whether ``targetpath`` matches the ``pathpattern``."""
        # We need to make sure that targetpath and pathpattern are pointing to
        # the same directory as fnmatch doesn't threat "/" as a special symbol.
        target_parts = targetpath.split("/")
        pattern_parts = pathpattern.split("/")
        if len(target_parts) != len(pattern_parts):
            return False

        # Every part in the pathpattern could include a glob pattern, that's why
        # each of the target and pathpattern parts should match.
        for target_dir, pattern_dir in zip(target_parts, pattern_parts):
            if not fnmatch.fnmatch(target_dir, pattern_dir):
                return False

        return True

    def is_delegated_path(self, target_filepath: str) -> bool:
        """Determines whether the given ``target_filepath`` is in one of
        the paths that ``DelegatedRole`` is trusted to provide.

        The ``target_filepath`` and the ``DelegatedRole`` paths are expected to be
        in their canonical forms, so e.g. "a/b" instead of "a//b" . Only "/" is
        supported as target path separator. Leading separators are not handled
        as special cases (see `TUF specification on targetpath
        <https://theupdateframework.github.io/specification/latest/#targetpath>`_).

        Args:
            target_filepath: URL path to a target file, relative to a base
                targets URL.
        """

        if self.path_hash_prefixes is not None:
            # Calculate the hash of the filepath
            # to determine in which bin to find the target.
            digest_object = sslib_hash.digest(algorithm="sha256")
            digest_object.update(target_filepath.encode("utf-8"))
            target_filepath_hash = digest_object.hexdigest()

            for path_hash_prefix in self.path_hash_prefixes:
                if target_filepath_hash.startswith(path_hash_prefix):
                    return True

        elif self.paths is not None:
            for pathpattern in self.paths:
                # A delegated role path may be an explicit path or glob
                # pattern (Unix shell-style wildcards).
                if self._is_target_in_pathpattern(target_filepath, pathpattern):
                    return True

        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``.

    Bin names are the concatenation of the passed ``name_prefix`` and a
    zero-padded hex representation of the bin index 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

        # Calculate the suffix_len value based on the total number of bins in
        # hex. If bit_length = 10 then number_of_bins = 1024 or bin names will
        # have a suffix between "000" and "3ff" in hex and suffix_len will be 3
        # meaning the third bin will have a suffix of "003".
        self.number_of_bins = 2**bit_length
        # suffix_len is calculated based on "number_of_bins - 1" as the name
        # of the last bin contains the number "number_of_bins -1" as a suffix.
        self.suffix_len = len(f"{self.number_of_bins-1:x}")

    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``.

        The target at path ``target_filepath`` is assigned to a bin 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``.

        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
        # Add zero padding if necessary and cast to hex the suffix.
        suffix = f"{bin_number:0{self.suffix_len}x}"
        return f"{self.name_prefix}-{suffix}"

    def get_roles(self) -> Iterator[str]:
        """Yield the names of all different delegated roles one by one."""
        for i in range(0, self.number_of_bins):
            suffix = f"{i:0{self.suffix_len}x}"
            yield f"{self.name_prefix}-{suffix}"

    def is_delegated_role(self, role_name: str) -> bool:
        """Determines whether the given ``role_name`` is in one of
        the delegated roles that ``SuccinctRoles`` represents.

        Args:
            role_name: The name of the role to check against.
        """
        desired_prefix = self.name_prefix + "-"

        if not role_name.startswith(desired_prefix):
            return False

        suffix = role_name[len(desired_prefix) :]
        if len(suffix) != self.suffix_len:
            return False

        try:
            # make sure suffix is hex value
            num = int(suffix, 16)
        except ValueError:
            return False

        return 0 <= num < self.number_of_bins


class Delegations:
    """A container object storing information about all delegations.

    *All parameters named below are not just constructor arguments but also
    instance attributes.*

    Args:
        keys: Dictionary of keyids to Keys. Defines the keys used in ``roles``.
        roles: Ordered dictionary of role names to DelegatedRoles instances. It
            defines which keys are required to sign the metadata for a specific
            role. The roles order also defines the order that role delegations
            are considered during target searches.
        succinct_roles: Contains succinct information about hash bin
            delegations. Note that succinct roles is not a TUF specification
            feature yet and setting `succinct_roles` to a value makes the
            resulting metadata non-compliant. The metadata will not be accepted
            as valid by specification compliant clients such as those built with
            python-tuf <= 1.1.0. For more information see: https://github.com/theupdateframework/taps/blob/master/tap15.md
        unrecognized_fields: Dictionary of all attributes that are not managed
            by TUF Metadata API

    Exactly one of ``roles`` and ``succinct_roles`` must be set.

    Raises:
        ValueError: Invalid arguments.
    """

    def __init__(
        self,
        keys: Dict[str, Key],
        roles: Optional[Dict[str, DelegatedRole]] = None,
        succinct_roles: Optional[SuccinctRoles] = None,
        unrecognized_fields: Optional[Dict[str, Any]] = None,
    ):
        self.keys = keys
        if sum(1 for v in [roles, succinct_roles] if v is not None) != 1:
            raise ValueError("One of roles and succinct_roles must be set")

        if roles is not None:
            for role in roles:
                if not role or role in TOP_LEVEL_ROLE_NAMES:
                    raise ValueError(
                        "Delegated roles cannot be empty or use top-level "
                        "role names"
                    )

        self.roles = roles
        self.succinct_roles = succinct_roles
        if unrecognized_fields is None:
            unrecognized_fields = {}

        self.unrecognized_fields = unrecognized_fields

    def __eq__(self, other: Any) -> bool:
        if not isinstance(other, Delegations):
            return False

        all_attributes_check = (
            self.keys == other.keys
            and self.roles == other.roles
            and self.succinct_roles == other.succinct_roles
            and self.unrecognized_fields == other.unrecognized_fields
        )

        if self.roles is not None and other.roles is not None:
            all_attributes_check = (
                all_attributes_check
                # Order of the delegated roles matters (see issue #1788).
                and list(self.roles.items()) == list(other.roles.items())
            )

        return all_attributes_check

    @classmethod
    def from_dict(cls, delegations_dict: Dict[str, Any]) -> "Delegations":
        """Creates ``Delegations`` object from its json/dict representation.

        Raises:
            ValueError, KeyError, TypeError: Invalid arguments.
        """
        keys = delegations_dict.pop("keys")
        keys_res = {}
        for keyid, key_dict in keys.items():
            keys_res[keyid] = Key.from_dict(keyid, key_dict)
        roles = delegations_dict.pop("roles", None)
        roles_res: Optional[Dict[str, DelegatedRole]] = None

        if roles is not None:
            roles_res = {}
            for role_dict in roles:
                new_role = DelegatedRole.from_dict(role_dict)
                if new_role.name in roles_res:
                    raise ValueError(f"Duplicate role {new_role.name}")
                roles_res[new_role.name] = new_role

        succinct_roles_dict = delegations_dict.pop("succinct_roles", None)
        succinct_roles_info = None
        if succinct_roles_dict is not None:
            succinct_roles_info = SuccinctRoles.from_dict(succinct_roles_dict)

        # All fields left in the delegations_dict are unrecognized.
        return cls(keys_res, roles_res, succinct_roles_info, delegations_dict)

    def to_dict(self) -> Dict[str, Any]:
        """Returns the dict representation of self."""
        keys = {keyid: key.to_dict() for keyid, key in self.keys.items()}
        res_dict: Dict[str, Any] = {
            "keys": keys,
            **self.unrecognized_fields,
        }
        if self.roles is not None:
            roles = [role_obj.to_dict() for role_obj in self.roles.values()]
            res_dict["roles"] = roles
        elif self.succinct_roles is not None:
            res_dict["succinct_roles"] = self.succinct_roles.to_dict()

        return res_dict

    def get_roles_for_target(
        self, target_filepath: str
    ) -> Iterator[Tuple[str, bool]]:
        """Given ``target_filepath`` get names and terminating status of all
        delegated roles who are responsible for it.

        Args:
            target_filepath: URL path to a target file, relative to a base
                targets URL.
        """
        if self.roles is not None:
            for role in self.roles.values():
                if role.is_delegated_path(target_filepath):
                    yield role.name, role.terminating

        elif self.succinct_roles is not None:
            # We consider all succinct_roles as terminating.
            # For more information read TAP 15.
            yield self.succinct_roles.get_role_for_target(target_filepath), True


class TargetFile(BaseFile):
    """A container with information about a particular target file.

    *All parameters named below are not just constructor arguments but also
    instance attributes.*

    Args:
        length: Length of the target file in bytes.
        hashes: Dictionary of hash algorithm names to hashes of the target
            file content.
        path: URL path to a target file, relative to a base targets URL.
        unrecognized_fields: Dictionary of all attributes that are not managed
            by TUF Metadata API

    Raises:
        ValueError, TypeError: Invalid arguments.
    """

    def __init__(
        self,
        length: int,
        hashes: Dict[str, str],
        path: str,
        unrecognized_fields: Optional[Dict[str, Any]] = None,
    ):

        self._validate_length(length)
        self._validate_hashes(hashes)

        self.length = length
        self.hashes = hashes
        self.path = path
        if unrecognized_fields is None:
            unrecognized_fields = {}

        self.unrecognized_fields = unrecognized_fields

    @property
    def custom(self) -> Any:
        """Can be used to provide implementation specific data related to the
        target. python-tuf does not use or validate this data."""
        return self.unrecognized_fields.get("custom")

    def __eq__(self, other: Any) -> bool:
        if not isinstance(other, TargetFile):
            return False

        return (
            self.length == other.length
            and self.hashes == other.hashes
            and self.path == other.path
            and self.unrecognized_fields == other.unrecognized_fields
        )

    @classmethod
    def from_dict(cls, target_dict: Dict[str, Any], path: str) -> "TargetFile":
        """Creates ``TargetFile`` object from its json/dict representation.

        Raises:
            ValueError, KeyError, TypeError: Invalid arguments.
        """
        length = target_dict.pop("length")
        hashes = target_dict.pop("hashes")

        # All fields left in the target_dict are unrecognized.
        return cls(length, hashes, path, target_dict)

    def to_dict(self) -> Dict[str, Any]:
        """Returns the JSON-serializable dictionary representation of self."""
        return {
            "length": self.length,
            "hashes": self.hashes,
            **self.unrecognized_fields,
        }

    @classmethod
    def from_file(
        cls,
        target_file_path: str,
        local_path: str,
        hash_algorithms: Optional[List[str]] = None,
    ) -> "TargetFile":
        """Creates ``TargetFile`` object from a file.

        Args:
            target_file_path: URL path to a target file, relative to a base
                targets URL.
            local_path: Local path to target file content.
            hash_algorithms: Hash algorithms to calculate hashes with. If not
                specified the securesystemslib default hash algorithm is used.
        Raises:
            FileNotFoundError: The file doesn't exist.
            ValueError: The hash algorithms list contains an unsupported
                algorithm.
        """
        with open(local_path, "rb") as file:
            return cls.from_data(target_file_path, file, hash_algorithms)

    @classmethod
    def from_data(
        cls,
        target_file_path: str,
        data: Union[bytes, IO[bytes]],
        hash_algorithms: Optional[List[str]] = None,
    ) -> "TargetFile":
        """Creates ``TargetFile`` object from bytes.

        Args:
            target_file_path: URL path to a target file, relative to a base
                targets URL.
            data: Target file content.
            hash_algorithms: Hash algorithms to create the hashes with. If not
                specified the securesystemslib default hash algorithm is used.

        Raises:
            ValueError: The hash algorithms list contains an unsupported
                algorithm.
        """
        if isinstance(data, bytes):
            length = len(data)
        else:
            data.seek(0, io.SEEK_END)
            length = data.tell()

        hashes = {}

        if hash_algorithms is None:
            hash_algorithms = [sslib_hash.DEFAULT_HASH_ALGORITHM]

        for algorithm in hash_algorithms:
            try:
                if isinstance(data, bytes):
                    digest_object = sslib_hash.digest(algorithm)
                    digest_object.update(data)
                else:
                    digest_object = sslib_hash.digest_fileobject(
                        data, algorithm
                    )
            except (
                sslib_exceptions.UnsupportedAlgorithmError,
                sslib_exceptions.FormatError,
            ) as e:
                raise ValueError(f"Unsupported algorithm '{algorithm}'") from e

            hashes[algorithm] = digest_object.hexdigest()

        return cls(length, hashes, target_file_path)

    def verify_length_and_hashes(self, data: Union[bytes, IO[bytes]]) -> None:
        """Verifies that length and hashes of ``data`` match expected values.

        Args:
            data: Target file object or its content in bytes.

        Raises:
            LengthOrHashMismatchError: Calculated length or hashes do not
                match expected values or hash algorithm is not supported.
        """
        self._verify_length(data, self.length)
        self._verify_hashes(data, self.hashes)


class Targets(Signed):
    """A container for the signed part of targets metadata.

    Targets contains verifying information about target files and also
    delegates responsibility to other Targets roles.

    *All parameters named below are not just constructor arguments but also
    instance attributes.*

    Args:
        version: Metadata version number. Default is 1.
        spec_version: Supported TUF specification version. Default is the
            version currently supported by the library.
        expires: Metadata expiry date. Default is current date and time.
        targets: Dictionary of target filenames to TargetFiles. Default is an
            empty dictionary.
        delegations: Defines how this Targets delegates responsibility to other
            Targets Metadata files. Default is None.
        unrecognized_fields: Dictionary of all attributes that are not managed
            by TUF Metadata API

    Raises:
        ValueError: Invalid arguments.
    """

    type = _TARGETS

    # pylint: disable=too-many-arguments
    def __init__(
        self,
        version: Optional[int] = None,
        spec_version: Optional[str] = None,
        expires: Optional[datetime] = None,
        targets: Optional[Dict[str, TargetFile]] = None,
        delegations: Optional[Delegations] = None,
        unrecognized_fields: Optional[Dict[str, Any]] = None,
    ) -> None:
        super().__init__(version, spec_version, expires, unrecognized_fields)
        self.targets = targets if targets is not None else {}
        self.delegations = delegations

    def __eq__(self, other: Any) -> bool:
        if not isinstance(other, Targets):
            return False

        return (
            super().__eq__(other)
            and self.targets == other.targets
            and self.delegations == other.delegations
        )

    @classmethod
    def from_dict(cls, signed_dict: Dict[str, Any]) -> "Targets":
        """Creates ``Targets`` object from its json/dict representation.

        Raises:
            ValueError, KeyError, TypeError: Invalid arguments.
        """
        common_args = cls._common_fields_from_dict(signed_dict)
        targets = signed_dict.pop(_TARGETS)
        try:
            delegations_dict = signed_dict.pop("delegations")
        except KeyError:
            delegations = None
        else:
            delegations = Delegations.from_dict(delegations_dict)
        res_targets = {}
        for target_path, target_info in targets.items():
            res_targets[target_path] = TargetFile.from_dict(
                target_info, target_path
            )
        # All fields left in the targets_dict are unrecognized.
        return cls(*common_args, res_targets, delegations, signed_dict)

    def to_dict(self) -> Dict[str, Any]:
        """Returns the dict representation of self."""
        targets_dict = self._common_fields_to_dict()
        targets = {}
        for target_path, target_file_obj in self.targets.items():
            targets[target_path] = target_file_obj.to_dict()
        targets_dict[_TARGETS] = targets
        if self.delegations is not None:
            targets_dict["delegations"] = self.delegations.to_dict()
        return targets_dict

    def add_key(self, key: Key, role: Optional[str] = None) -> None:
        """Adds new signing key for delegated role ``role``.

        If succinct_roles is used then the ``role`` argument is not required.

        Args:
            key: Signing key to be added for ``role``.
            role: Name of the role, for which ``key`` is added.

        Raises:
            ValueError: If the argument order is wrong or if there are no
                delegated roles or if ``role`` is not delegated by this Target.
        """
        # Verify that our users are not using the old argument order.
        if isinstance(role, Key):
            raise ValueError("Role must be a string, not a Key instance")

        if self.delegations is None:
            raise ValueError(f"Delegated role {role} doesn't exist")

        if self.delegations.roles is not None:
            if role not in self.delegations.roles:
                raise ValueError(f"Delegated role {role} doesn't exist")
            if key.keyid not in self.delegations.roles[role].keyids:
                self.delegations.roles[role].keyids.append(key.keyid)

        elif self.delegations.succinct_roles is not None:
            if key.keyid not in self.delegations.succinct_roles.keyids:
                self.delegations.succinct_roles.keyids.append(key.keyid)

        self.delegations.keys[key.keyid] = key

    def revoke_key(self, keyid: str, role: Optional[str] = None) -> None:
        """Revokes key from delegated role ``role`` and updates the delegations
        key store.

        If succinct_roles is used then the ``role`` argument is not required.

        Args:
            keyid: Identifier of the key to be removed for ``role``.
            role: Name of the role, for which a signing key is removed.

        Raises:
            ValueError: If there are no delegated roles or if ``role`` is not
                delegated by this ``Target`` or if key is not used by ``role``
                or if key with id ``keyid`` is not used by succinct roles.
        """
        if self.delegations is None:
            raise ValueError(f"Delegated role {role} doesn't exist")

        if self.delegations.roles is not None:
            if role not in self.delegations.roles:
                raise ValueError(f"Delegated role {role} doesn't exist")
            if keyid not in self.delegations.roles[role].keyids:
                raise ValueError(f"Key with id {keyid} is not used by {role}")

            self.delegations.roles[role].keyids.remove(keyid)
            for keyinfo in self.delegations.roles.values():
                if keyid in keyinfo.keyids:
                    return

        elif self.delegations.succinct_roles is not None:
            if keyid not in self.delegations.succinct_roles.keyids:
                raise ValueError(
                    f"Key with id {keyid} is not used by succinct_roles"
                )

            self.delegations.succinct_roles.keyids.remove(keyid)

        del self.delegations.keys[keyid]