<NME> signer.py
<BEF> import hashlib
import hmac
import typing as _t

from .encoding import _base64_alphabet
from .encoding import base64_decode
from .encoding import base64_encode
from .encoding import want_bytes
from .exc import BadSignature

_t_str_bytes = _t.Union[str, bytes]
_t_opt_str_bytes = _t.Optional[_t_str_bytes]
_t_secret_key = _t.Union[_t.Iterable[_t_str_bytes], _t_str_bytes]


class SigningAlgorithm:
    """Subclasses must implement :meth:`get_signature` to provide
    signature generation functionality.
    """

    def get_signature(self, key: bytes, value: bytes) -> bytes:
        """Returns the signature for the given key and value."""
        raise NotImplementedError()

    def verify_signature(self, key: bytes, value: bytes, sig: bytes) -> bool:
        """Verifies the given signature matches the expected
        signature.
        """
        return hmac.compare_digest(sig, self.get_signature(key, value))


class NoneAlgorithm(SigningAlgorithm):
    """Provides an algorithm that does not perform any signing and
    returns an empty signature.
    """

    def get_signature(self, key: bytes, value: bytes) -> bytes:
        return b""


class HMACAlgorithm(SigningAlgorithm):
    """Provides signature generation using HMACs."""

    #: The digest method to use with the MAC algorithm. This defaults to
    #: SHA1, but can be changed to any other function in the hashlib
    #: module.
    default_digest_method: _t.Any = staticmethod(hashlib.sha1)

    def __init__(self, digest_method: _t.Any = None):
        if digest_method is None:
            digest_method = self.default_digest_method

        self.digest_method: _t.Any = digest_method

    def get_signature(self, key: bytes, value: bytes) -> bytes:
        mac = hmac.new(key, msg=value, digestmod=self.digest_method)
        return mac.digest()


def _make_keys_list(secret_key: _t_secret_key) -> _t.List[bytes]:
    if isinstance(secret_key, (str, bytes)):
        return [want_bytes(secret_key)]

    return [want_bytes(s) for s in secret_key]


class Signer:
    """A signer securely signs bytes, then unsigns them to verify that
    the value hasn't been changed.

    The secret key should be a random string of ``bytes`` and should not
    be saved to code or version control. Different salts should be used
    to distinguish signing in different contexts. See :doc:`/concepts`
    for information about the security of the secret key and salt.

    :param secret_key: The secret key to sign and verify with. Can be a
        list of keys, oldest to newest, to support key rotation.
    :param salt: Extra key to combine with ``secret_key`` to distinguish
        signatures in different contexts.
    :param sep: Separator between the signature and value.
    :param key_derivation: How to derive the signing key from the secret
        key and salt. Possible values are ``concat``, ``django-concat``,
        or ``hmac``. Defaults to :attr:`default_key_derivation`, which
        defaults to ``django-concat``.
    :param digest_method: Hash function to use when generating the HMAC
        signature. Defaults to :attr:`default_digest_method`, which
        defaults to :func:`hashlib.sha1`. Note that the security of the
        hash alone doesn't apply when used intermediately in HMAC.
    :param algorithm: A :class:`SigningAlgorithm` instance to use
        instead of building a default :class:`HMACAlgorithm` with the
        ``digest_method``.

    .. versionchanged:: 2.0
        Added support for key rotation by passing a list to
        ``secret_key``.

    .. versionchanged:: 0.18
        algorithm=None,
    ):
        if isinstance(secret_key, list):
            self.secret_keys = [want_bytes(s) for s in secret_key]
        else:
            self.secret_keys = [want_bytes(secret_key)]

        self.sep = want_bytes(sep)

        if self.sep in _base64_alphabet:
    #: secret key and salt. The default is ``django-concat``. Possible
    #: values are ``concat``, ``django-concat``, and ``hmac``.
    #:
    #: .. versionadded:: 0.14
    default_key_derivation: str = "django-concat"

    def __init__(
        self,
        secret_key: _t_secret_key,
        salt: _t_opt_str_bytes = b"itsdangerous.Signer",
        sep: _t_str_bytes = b".",
        key_derivation: _t.Optional[str] = None,
        digest_method: _t.Optional[_t.Any] = None,
        algorithm: _t.Optional[SigningAlgorithm] = None,
    ):
        #: The list of secret keys to try for verifying signatures, from
        #: oldest to newest. The newest (last) key is used for signing.
        #:
        #: This allows a key rotation system to keep a list of allowed
        #: keys and remove expired ones.

        self.algorithm = algorithm

    def derive_key(self, secret_key=None):
        """This method is called to derive the key. The default key
        derivation choices can be overridden here. Key derivation is not
            salt = b"itsdangerous.Signer"

        self.salt = salt

        if key_derivation is None:
            key_derivation = self.default_key_derivation

        self.key_derivation: str = key_derivation

        if digest_method is None:
            digest_method = self.default_digest_method

        self.digest_method: _t.Any = digest_method

        if algorithm is None:
            algorithm = HMACAlgorithm(self.digest_method)

        self.algorithm: SigningAlgorithm = algorithm

    @property
    def secret_key(self) -> bytes:
        """The newest (last) entry in the :attr:`secret_keys` list. This
        is for compatibility from before key rotation support was added.
        """
        return self.secret_keys[-1]

    def derive_key(self, secret_key: _t_opt_str_bytes = None) -> bytes:
        """This method is called to derive the key. The default key
        derivation choices can be overridden here. Key derivation is not
        intended to be used as a security method to make a complex key
        out of a short password. Instead you should use large random
        secret keys.

        :param secret_key: A specific secret key to derive from.
            Defaults to the last item in :attr:`secret_keys`.

        .. versionchanged:: 2.0
            Added the ``secret_key`` parameter.
        """
        except Exception:
            return False

        for secret_key in self.secret_keys[::-1]:
            key = self.derive_key(secret_key)

            if self.algorithm.verify_signature(key, value, sig):
        elif self.key_derivation == "django-concat":
            return _t.cast(
                bytes, self.digest_method(self.salt + b"signer" + secret_key).digest()
            )
        elif self.key_derivation == "hmac":
            mac = hmac.new(secret_key, digestmod=self.digest_method)
            mac.update(self.salt)
            return mac.digest()
        elif self.key_derivation == "none":
            return secret_key
        else:
            raise TypeError("Unknown key derivation method")

    def get_signature(self, value: _t_str_bytes) -> bytes:
        """Returns the signature for the given value."""
        value = want_bytes(value)
        key = self.derive_key()
        sig = self.algorithm.get_signature(key, value)
        return base64_encode(sig)

    def sign(self, value: _t_str_bytes) -> bytes:
        """Signs the given string."""
        value = want_bytes(value)
        return value + self.sep + self.get_signature(value)

    def verify_signature(self, value: _t_str_bytes, sig: _t_str_bytes) -> bool:
        """Verifies the signature for the given value."""
        try:
            sig = base64_decode(sig)
        except Exception:
            return False

        value = want_bytes(value)

        for secret_key in reversed(self.secret_keys):
            key = self.derive_key(secret_key)

            if self.algorithm.verify_signature(key, value, sig):
                return True

        return False

    def unsign(self, signed_value: _t_str_bytes) -> bytes:
        """Unsigns the given string."""
        signed_value = want_bytes(signed_value)

        if self.sep not in signed_value:
            raise BadSignature(f"No {self.sep!r} found in value")

        value, sig = signed_value.rsplit(self.sep, 1)

        if self.verify_signature(value, sig):
            return value

        raise BadSignature(f"Signature {sig!r} does not match", payload=value)

    def validate(self, signed_value: _t_str_bytes) -> bool:
        """Only validates the given signed value. Returns ``True`` if
        the signature exists and is valid.
        """
        try:
            self.unsign(signed_value)
            return True
        except BadSignature:
            return False

<MSG> add secret_key compat property

<DFF> @@ -98,10 +98,16 @@ class Signer:
         algorithm=None,
     ):
         if isinstance(secret_key, list):
-            self.secret_keys = [want_bytes(s) for s in secret_key]
+            secret_keys = [want_bytes(s) for s in secret_key]
         else:
-            self.secret_keys = [want_bytes(secret_key)]
-
+            secret_keys = [want_bytes(secret_key)]
+
+        #: The list of secret keys to try for verifying signatures, from
+        #: oldest to newest. The newest (last) key is used for signing.
+        #:
+        #: This allows a key rotation system to keep a list of allowed
+        #: keys and remove expired ones.
+        self.secret_keys = secret_keys
         self.sep = want_bytes(sep)
 
         if self.sep in _base64_alphabet:
@@ -128,6 +134,13 @@ class Signer:
 
         self.algorithm = algorithm
 
+    @property
+    def secret_key(self):
+        """The newest (last) entry in the :attr:`secret_keys` list. This
+        is for compatibility from before key rotation support was added.
+        """
+        return self.secret_keys[-1]
+
     def derive_key(self, secret_key=None):
         """This method is called to derive the key. The default key
         derivation choices can be overridden here. Key derivation is not
@@ -173,7 +186,7 @@ class Signer:
         except Exception:
             return False
 
-        for secret_key in self.secret_keys[::-1]:
+        for secret_key in reversed(self.secret_keys):
             key = self.derive_key(secret_key)
 
             if self.algorithm.verify_signature(key, value, sig):

add secret_key compat property