diff --git a/src/cryptography/hazmat/backends/openssl/ciphers.py b/src/cryptography/hazmat/backends/openssl/ciphers.py
index 760774079..2b10681b3 100644
--- a/src/cryptography/hazmat/backends/openssl/ciphers.py
+++ b/src/cryptography/hazmat/backends/openssl/ciphers.py
@@ -118,12 +118,12 @@ class _CipherContext(object):
         self._backend._lib.EVP_CIPHER_CTX_set_padding(ctx, 0)
         self._ctx = ctx
 
-    def update(self, data):
+    def update(self, data: bytes) -> bytes:
         buf = bytearray(len(data) + self._block_size_bytes - 1)
         n = self.update_into(data, buf)
         return bytes(buf[:n])
 
-    def update_into(self, data, buf):
+    def update_into(self, data: bytes, buf) -> int:
         total_data_len = len(data)
         if len(buf) < (total_data_len + self._block_size_bytes - 1):
             raise ValueError(
@@ -151,7 +151,7 @@ class _CipherContext(object):
 
         return total_out
 
-    def finalize(self):
+    def finalize(self) -> bytes:
         if (
             self._operation == self._DECRYPT
             and isinstance(self._mode, modes.ModeWithAuthenticationTag)
@@ -202,7 +202,7 @@ class _CipherContext(object):
         self._backend.openssl_assert(res == 1)
         return self._backend._ffi.buffer(buf)[: outlen[0]]
 
-    def finalize_with_tag(self, tag):
+    def finalize_with_tag(self, tag: bytes) -> bytes:
         if len(tag) < self._mode._min_tag_length:
             raise ValueError(
                 "Authentication tag must be {} bytes or longer.".format(
@@ -216,7 +216,7 @@ class _CipherContext(object):
         self._tag = tag
         return self.finalize()
 
-    def authenticate_additional_data(self, data):
+    def authenticate_additional_data(self, data: bytes) -> None:
         outlen = self._backend._ffi.new("int *")
         res = self._backend._lib.EVP_CipherUpdate(
             self._ctx,
diff --git a/src/cryptography/hazmat/primitives/_cipheralgorithm.py b/src/cryptography/hazmat/primitives/_cipheralgorithm.py
index f13d02d2d..04ca5a987 100644
--- a/src/cryptography/hazmat/primitives/_cipheralgorithm.py
+++ b/src/cryptography/hazmat/primitives/_cipheralgorithm.py
@@ -3,6 +3,7 @@
 # for complete details.
 
 import abc
+import typing
 
 
 # This exists to break an import cycle. It is normally accessible from the
@@ -11,13 +12,19 @@ import abc
 
 class CipherAlgorithm(metaclass=abc.ABCMeta):
     @abc.abstractproperty
-    def name(self):
+    def name(self) -> str:
         """
         A string naming this mode (e.g. "AES", "Camellia").
         """
 
     @abc.abstractproperty
-    def key_size(self):
+    def key_sizes(self) -> typing.FrozenSet[int]:
+        """
+        Valid key sizes for this algorithm in bits
+        """
+
+    @abc.abstractproperty
+    def key_size(self) -> int:
         """
         The size of the key being used as an integer in bits (e.g. 128, 256).
         """
@@ -25,7 +32,7 @@ class CipherAlgorithm(metaclass=abc.ABCMeta):
 
 class BlockCipherAlgorithm(metaclass=abc.ABCMeta):
     @abc.abstractproperty
-    def block_size(self):
+    def block_size(self) -> int:
         """
         The size of a block as an integer in bits (e.g. 64, 128).
         """
diff --git a/src/cryptography/hazmat/primitives/ciphers/aead.py b/src/cryptography/hazmat/primitives/ciphers/aead.py
index b539c6a46..5c7cdc25b 100644
--- a/src/cryptography/hazmat/primitives/ciphers/aead.py
+++ b/src/cryptography/hazmat/primitives/ciphers/aead.py
@@ -136,14 +136,14 @@ class AESCCM(object):
             backend, self, nonce, data, associated_data, self._tag_length
         )
 
-    def _validate_lengths(self, nonce, data_len):
+    def _validate_lengths(self, nonce: bytes, data_len: int):
         # For information about computing this, see
         # https://tools.ietf.org/html/rfc3610#section-2.1
         l_val = 15 - len(nonce)
         if 2 ** (8 * l_val) < data_len:
             raise ValueError("Data too long for nonce")
 
-    def _check_params(self, nonce, data, associated_data):
+    def _check_params(self, nonce: bytes, data: bytes, associated_data: bytes):
         utils._check_byteslike("nonce", nonce)
         utils._check_bytes("data", data)
         utils._check_bytes("associated_data", associated_data)
diff --git a/src/cryptography/hazmat/primitives/ciphers/algorithms.py b/src/cryptography/hazmat/primitives/ciphers/algorithms.py
index cb1f252d4..ed7251661 100644
--- a/src/cryptography/hazmat/primitives/ciphers/algorithms.py
+++ b/src/cryptography/hazmat/primitives/ciphers/algorithms.py
@@ -11,7 +11,7 @@ from cryptography.hazmat.primitives.ciphers import (
 from cryptography.hazmat.primitives.ciphers.modes import ModeWithNonce
 
 
-def _verify_key_size(algorithm, key):
+def _verify_key_size(algorithm: CipherAlgorithm, key: bytes):
     # Verify that the key is instance of bytes
     utils._check_byteslike("key", key)
 
diff --git a/src/cryptography/hazmat/primitives/ciphers/base.py b/src/cryptography/hazmat/primitives/ciphers/base.py
index 1b9863721..6f02597a7 100644
--- a/src/cryptography/hazmat/primitives/ciphers/base.py
+++ b/src/cryptography/hazmat/primitives/ciphers/base.py
@@ -4,6 +4,7 @@
 
 
 import abc
+import typing
 
 from cryptography import utils
 from cryptography.exceptions import (
@@ -21,7 +22,7 @@ from cryptography.hazmat.primitives.ciphers import modes
 
 class BlockCipherAlgorithm(metaclass=abc.ABCMeta):
     @abc.abstractproperty
-    def block_size(self):
+    def block_size(self) -> int:
         """
         The size of a block as an integer in bits (e.g. 64, 128).
         """
@@ -29,21 +30,21 @@ class BlockCipherAlgorithm(metaclass=abc.ABCMeta):
 
 class CipherContext(metaclass=abc.ABCMeta):
     @abc.abstractmethod
-    def update(self, data):
+    def update(self, data: bytes) -> bytes:
         """
         Processes the provided bytes through the cipher and returns the results
         as bytes.
         """
 
     @abc.abstractmethod
-    def update_into(self, data, buf):
+    def update_into(self, data: bytes, buf) -> int:
         """
         Processes the provided bytes and writes the resulting data into the
         provided buffer. Returns the number of bytes written.
         """
 
     @abc.abstractmethod
-    def finalize(self):
+    def finalize(self) -> bytes:
         """
         Returns the results of processing the final block as bytes.
         """
@@ -51,7 +52,7 @@ class CipherContext(metaclass=abc.ABCMeta):
 
 class AEADCipherContext(metaclass=abc.ABCMeta):
     @abc.abstractmethod
-    def authenticate_additional_data(self, data):
+    def authenticate_additional_data(self, data: bytes) -> None:
         """
         Authenticates the provided bytes.
         """
@@ -59,7 +60,7 @@ class AEADCipherContext(metaclass=abc.ABCMeta):
 
 class AEADDecryptionContext(metaclass=abc.ABCMeta):
     @abc.abstractmethod
-    def finalize_with_tag(self, tag):
+    def finalize_with_tag(self, tag: bytes) -> bytes:
         """
         Returns the results of processing the final block as bytes and allows
         delayed passing of the authentication tag.
@@ -68,7 +69,7 @@ class AEADDecryptionContext(metaclass=abc.ABCMeta):
 
 class AEADEncryptionContext(metaclass=abc.ABCMeta):
     @abc.abstractproperty
-    def tag(self):
+    def tag(self) -> bytes:
         """
         Returns tag bytes. This is only available after encryption is
         finalized.
@@ -76,7 +77,12 @@ class AEADEncryptionContext(metaclass=abc.ABCMeta):
 
 
 class Cipher(object):
-    def __init__(self, algorithm, mode, backend=None):
+    def __init__(
+        self,
+        algorithm: CipherAlgorithm,
+        mode: typing.Optional[modes.Mode],
+        backend=None,
+    ):
         backend = _get_backend(backend)
         if not isinstance(backend, CipherBackend):
             raise UnsupportedAlgorithm(
@@ -126,17 +132,17 @@ class _CipherContext(object):
     def __init__(self, ctx):
         self._ctx = ctx
 
-    def update(self, data):
+    def update(self, data: bytes) -> bytes:
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
         return self._ctx.update(data)
 
-    def update_into(self, data, buf):
+    def update_into(self, data: bytes, buf) -> int:
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
         return self._ctx.update_into(data, buf)
 
-    def finalize(self):
+    def finalize(self) -> bytes:
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
         data = self._ctx.finalize()
@@ -155,7 +161,7 @@ class _AEADCipherContext(object):
         self._tag = None
         self._updated = False
 
-    def _check_limit(self, data_size):
+    def _check_limit(self, data_size: int):
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
         self._updated = True
@@ -167,15 +173,15 @@ class _AEADCipherContext(object):
                 )
             )
 
-    def update(self, data):
+    def update(self, data: bytes) -> bytes:
         self._check_limit(len(data))
         return self._ctx.update(data)
 
-    def update_into(self, data, buf):
+    def update_into(self, data: bytes, buf) -> int:
         self._check_limit(len(data))
         return self._ctx.update_into(data, buf)
 
-    def finalize(self):
+    def finalize(self) -> bytes:
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
         data = self._ctx.finalize()
@@ -183,7 +189,7 @@ class _AEADCipherContext(object):
         self._ctx = None
         return data
 
-    def finalize_with_tag(self, tag):
+    def finalize_with_tag(self, tag: bytes) -> bytes:
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
         data = self._ctx.finalize_with_tag(tag)
@@ -191,7 +197,7 @@ class _AEADCipherContext(object):
         self._ctx = None
         return data
 
-    def authenticate_additional_data(self, data):
+    def authenticate_additional_data(self, data: bytes) -> None:
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
         if self._updated:
@@ -211,9 +217,10 @@ class _AEADCipherContext(object):
 @utils.register_interface(AEADEncryptionContext)
 class _AEADEncryptionContext(_AEADCipherContext):
     @property
-    def tag(self):
+    def tag(self) -> bytes:
         if self._ctx is not None:
             raise NotYetFinalized(
                 "You must finalize encryption before " "getting the tag."
             )
+        assert self._tag is not None
         return self._tag
diff --git a/tests/doubles.py b/tests/doubles.py
index dd9bb9f0a..de79fbe8c 100644
--- a/tests/doubles.py
+++ b/tests/doubles.py
@@ -12,7 +12,8 @@ from cryptography.hazmat.primitives.ciphers.modes import Mode
 class DummyCipherAlgorithm(CipherAlgorithm):
     name = "dummy-cipher"
     block_size = 128
-    key_size = None
+    key_size = 256
+    key_sizes = frozenset([256])
 
 
 class DummyMode(Mode):
diff --git a/tests/hazmat/primitives/test_block.py b/tests/hazmat/primitives/test_block.py
index 50f9fbb91..1e01628ad 100644
--- a/tests/hazmat/primitives/test_block.py
+++ b/tests/hazmat/primitives/test_block.py
@@ -45,7 +45,9 @@ class TestCipher(object):
     def test_instantiate_with_non_algorithm(self, backend):
         algorithm = object()
         with pytest.raises(TypeError):
-            Cipher(algorithm, mode=None, backend=backend)
+            Cipher(
+                algorithm, mode=None, backend=backend  # type: ignore[arg-type]
+            )
 
 
 @pytest.mark.requires_backend_interface(interface=CipherBackend)
diff --git a/tests/hazmat/primitives/test_ciphers.py b/tests/hazmat/primitives/test_ciphers.py
index 694141821..e82e3c26d 100644
--- a/tests/hazmat/primitives/test_ciphers.py
+++ b/tests/hazmat/primitives/test_ciphers.py
@@ -205,7 +205,7 @@ def test_invalid_backend():
     pretend_backend = object()
 
     with raises_unsupported_algorithm(_Reasons.BACKEND_MISSING_INTERFACE):
-        ciphers.Cipher(AES(b"AAAAAAAAAAAAAAAA"), modes.ECB, pretend_backend)
+        ciphers.Cipher(AES(b"AAAAAAAAAAAAAAAA"), modes.ECB(), pretend_backend)
 
 
 @pytest.mark.supported(