diff --git a/src/cryptography/fernet.py b/src/cryptography/fernet.py
index ac2dd0b..b990def 100644
--- a/src/cryptography/fernet.py
+++ b/src/cryptography/fernet.py
@@ -12,6 +12,7 @@
 
 import six
 
+from cryptography import utils
 from cryptography.exceptions import InvalidSignature
 from cryptography.hazmat.backends import default_backend
 from cryptography.hazmat.primitives import hashes, padding
@@ -51,8 +52,7 @@
         return self._encrypt_from_parts(data, current_time, iv)
 
     def _encrypt_from_parts(self, data, current_time, iv):
-        if not isinstance(data, bytes):
-            raise TypeError("data must be bytes.")
+        utils._check_bytes("data", data)
 
         padder = padding.PKCS7(algorithms.AES.block_size).padder()
         padded_data = padder.update(data) + padder.finalize()
@@ -82,9 +82,7 @@
 
     @staticmethod
     def _get_unverified_token_data(token):
-        if not isinstance(token, bytes):
-            raise TypeError("token must be bytes.")
-
+        utils._check_bytes("token", token)
         try:
             data = base64.urlsafe_b64decode(token)
         except (TypeError, binascii.Error):
diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index 5a22a55..ae966cd 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -1203,8 +1203,8 @@
     def _load_key(self, openssl_read_func, convert_func, data, password):
         mem_bio = self._bytes_to_bio(data)
 
-        if password is not None and not isinstance(password, bytes):
-            raise TypeError("Password must be bytes")
+        if password is not None:
+            utils._check_bytes("password", password)
 
         userdata = self._ffi.new("CRYPTOGRAPHY_PASSWORD_DATA *")
         if password is not None:
@@ -2132,8 +2132,8 @@
     def load_key_and_certificates_from_pkcs12(self, data, password):
         if password is None:
             password = self._ffi.NULL
-        elif not isinstance(password, bytes):
-            raise TypeError("Password must be a byte string or None")
+        else:
+            utils._check_bytes("password", password)
 
         bio = self._bytes_to_bio(data)
         p12 = self._lib.d2i_PKCS12_bio(bio.bio, self._ffi.NULL)
diff --git a/src/cryptography/hazmat/backends/openssl/dsa.py b/src/cryptography/hazmat/backends/openssl/dsa.py
index 48886e4..de61f08 100644
--- a/src/cryptography/hazmat/backends/openssl/dsa.py
+++ b/src/cryptography/hazmat/backends/openssl/dsa.py
@@ -211,8 +211,7 @@
 
     def verifier(self, signature, signature_algorithm):
         _warn_sign_verify_deprecated()
-        if not isinstance(signature, bytes):
-            raise TypeError("signature must be bytes.")
+        utils._check_bytes("signature", signature)
 
         _check_not_prehashed(signature_algorithm)
         return _DSAVerificationContext(
diff --git a/src/cryptography/hazmat/backends/openssl/ec.py b/src/cryptography/hazmat/backends/openssl/ec.py
index 69da234..852b491 100644
--- a/src/cryptography/hazmat/backends/openssl/ec.py
+++ b/src/cryptography/hazmat/backends/openssl/ec.py
@@ -244,8 +244,7 @@
 
     def verifier(self, signature, signature_algorithm):
         _warn_sign_verify_deprecated()
-        if not isinstance(signature, bytes):
-            raise TypeError("signature must be bytes.")
+        utils._check_bytes("signature", signature)
 
         _check_signature_algorithm(signature_algorithm)
         _check_not_prehashed(signature_algorithm.algorithm)
diff --git a/src/cryptography/hazmat/backends/openssl/rsa.py b/src/cryptography/hazmat/backends/openssl/rsa.py
index 00f5e37..b7d2173 100644
--- a/src/cryptography/hazmat/backends/openssl/rsa.py
+++ b/src/cryptography/hazmat/backends/openssl/rsa.py
@@ -434,8 +434,7 @@
 
     def verifier(self, signature, padding, algorithm):
         _warn_sign_verify_deprecated()
-        if not isinstance(signature, bytes):
-            raise TypeError("signature must be bytes.")
+        utils._check_bytes("signature", signature)
 
         _check_not_prehashed(algorithm)
         return _RSAVerificationContext(
diff --git a/src/cryptography/hazmat/primitives/ciphers/algorithms.py b/src/cryptography/hazmat/primitives/ciphers/algorithms.py
index 68a9e33..21d9ecf 100644
--- a/src/cryptography/hazmat/primitives/ciphers/algorithms.py
+++ b/src/cryptography/hazmat/primitives/ciphers/algorithms.py
@@ -153,8 +153,7 @@
 
     def __init__(self, key, nonce):
         self.key = _verify_key_size(self, key)
-        if not isinstance(nonce, bytes):
-            raise TypeError("nonce must be bytes")
+        utils._check_bytes("nonce", nonce)
 
         if len(nonce) != 16:
             raise ValueError("nonce must be 128-bits (16 bytes)")
diff --git a/src/cryptography/hazmat/primitives/ciphers/modes.py b/src/cryptography/hazmat/primitives/ciphers/modes.py
index e82c1a8..d244458 100644
--- a/src/cryptography/hazmat/primitives/ciphers/modes.py
+++ b/src/cryptography/hazmat/primitives/ciphers/modes.py
@@ -88,9 +88,7 @@
     name = "CBC"
 
     def __init__(self, initialization_vector):
-        if not isinstance(initialization_vector, bytes):
-            raise TypeError("initialization_vector must be bytes")
-
+        utils._check_bytes("initialization_vector", initialization_vector)
         self._initialization_vector = initialization_vector
 
     initialization_vector = utils.read_only_property("_initialization_vector")
@@ -103,8 +101,7 @@
     name = "XTS"
 
     def __init__(self, tweak):
-        if not isinstance(tweak, bytes):
-            raise TypeError("tweak must be bytes")
+        utils._check_bytes("tweak", tweak)
 
         if len(tweak) != 16:
             raise ValueError("tweak must be 128-bits (16 bytes)")
@@ -134,9 +131,7 @@
     name = "OFB"
 
     def __init__(self, initialization_vector):
-        if not isinstance(initialization_vector, bytes):
-            raise TypeError("initialization_vector must be bytes")
-
+        utils._check_bytes("initialization_vector", initialization_vector)
         self._initialization_vector = initialization_vector
 
     initialization_vector = utils.read_only_property("_initialization_vector")
@@ -149,9 +144,7 @@
     name = "CFB"
 
     def __init__(self, initialization_vector):
-        if not isinstance(initialization_vector, bytes):
-            raise TypeError("initialization_vector must be bytes")
-
+        utils._check_bytes("initialization_vector", initialization_vector)
         self._initialization_vector = initialization_vector
 
     initialization_vector = utils.read_only_property("_initialization_vector")
@@ -164,9 +157,7 @@
     name = "CFB8"
 
     def __init__(self, initialization_vector):
-        if not isinstance(initialization_vector, bytes):
-            raise TypeError("initialization_vector must be bytes")
-
+        utils._check_bytes("initialization_vector", initialization_vector)
         self._initialization_vector = initialization_vector
 
     initialization_vector = utils.read_only_property("_initialization_vector")
@@ -179,9 +170,7 @@
     name = "CTR"
 
     def __init__(self, nonce):
-        if not isinstance(nonce, bytes):
-            raise TypeError("nonce must be bytes")
-
+        utils._check_bytes("nonce", nonce)
         self._nonce = nonce
 
     nonce = utils.read_only_property("_nonce")
@@ -206,14 +195,12 @@
         # len(initialization_vector) must in [1, 2 ** 64), but it's impossible
         # to actually construct a bytes object that large, so we don't check
         # for it
-        if not isinstance(initialization_vector, bytes):
-            raise TypeError("initialization_vector must be bytes")
+        utils._check_bytes("initialization_vector", initialization_vector)
         if len(initialization_vector) == 0:
             raise ValueError("initialization_vector must be at least 1 byte")
         self._initialization_vector = initialization_vector
         if tag is not None:
-            if not isinstance(tag, bytes):
-                raise TypeError("tag must be bytes or None")
+            utils._check_bytes("tag", tag)
             if min_tag_length < 4:
                 raise ValueError("min_tag_length must be >= 4")
             if len(tag) < min_tag_length:
diff --git a/src/cryptography/hazmat/primitives/cmac.py b/src/cryptography/hazmat/primitives/cmac.py
index 77537f0..1404eac 100644
--- a/src/cryptography/hazmat/primitives/cmac.py
+++ b/src/cryptography/hazmat/primitives/cmac.py
@@ -36,8 +36,8 @@
     def update(self, data):
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
-        if not isinstance(data, bytes):
-            raise TypeError("data must be bytes.")
+
+        utils._check_bytes("data", data)
         self._ctx.update(data)
 
     def finalize(self):
@@ -48,8 +48,7 @@
         return digest
 
     def verify(self, signature):
-        if not isinstance(signature, bytes):
-            raise TypeError("signature must be bytes.")
+        utils._check_bytes("signature", signature)
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
 
diff --git a/src/cryptography/hazmat/primitives/hashes.py b/src/cryptography/hazmat/primitives/hashes.py
index 259a2c0..89e66ab 100644
--- a/src/cryptography/hazmat/primitives/hashes.py
+++ b/src/cryptography/hazmat/primitives/hashes.py
@@ -82,8 +82,7 @@
     def update(self, data):
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
-        if not isinstance(data, bytes):
-            raise TypeError("data must be bytes.")
+        utils._check_bytes("data", data)
         self._ctx.update(data)
 
     def copy(self):
diff --git a/src/cryptography/hazmat/primitives/hmac.py b/src/cryptography/hazmat/primitives/hmac.py
index 2e9a4e2..590555d 100644
--- a/src/cryptography/hazmat/primitives/hmac.py
+++ b/src/cryptography/hazmat/primitives/hmac.py
@@ -38,8 +38,7 @@
     def update(self, data):
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
-        if not isinstance(data, bytes):
-            raise TypeError("data must be bytes.")
+        utils._check_bytes("data", data)
         self._ctx.update(data)
 
     def copy(self):
@@ -60,8 +59,7 @@
         return digest
 
     def verify(self, signature):
-        if not isinstance(signature, bytes):
-            raise TypeError("signature must be bytes.")
+        utils._check_bytes("signature", signature)
         if self._ctx is None:
             raise AlreadyFinalized("Context was already finalized.")
 
diff --git a/src/cryptography/hazmat/primitives/kdf/concatkdf.py b/src/cryptography/hazmat/primitives/kdf/concatkdf.py
index c6399e4..89c3b28 100644
--- a/src/cryptography/hazmat/primitives/kdf/concatkdf.py
+++ b/src/cryptography/hazmat/primitives/kdf/concatkdf.py
@@ -27,14 +27,12 @@
             "Can not derive keys larger than {0} bits.".format(
                 max_length
             ))
-    if not (otherinfo is None or isinstance(otherinfo, bytes)):
-        raise TypeError("otherinfo must be bytes.")
+    if otherinfo is not None:
+        utils._check_bytes("otherinfo", otherinfo)
 
 
 def _concatkdf_derive(key_material, length, auxfn, otherinfo):
-    if not isinstance(key_material, bytes):
-        raise TypeError("key_material must be bytes.")
-
+    utils._check_bytes("key_material", key_material)
     output = [b""]
     outlen = 0
     counter = 1
@@ -96,10 +94,11 @@
         if self._otherinfo is None:
             self._otherinfo = b""
 
-        if not (salt is None or isinstance(salt, bytes)):
-            raise TypeError("salt must be bytes.")
         if salt is None:
             salt = b"\x00" * algorithm.block_size
+        else:
+            utils._check_bytes("salt", salt)
+
         self._salt = salt
 
         if not isinstance(backend, HMACBackend):
diff --git a/src/cryptography/hazmat/primitives/kdf/hkdf.py b/src/cryptography/hazmat/primitives/kdf/hkdf.py
index 917b4e9..27dc9c9 100644
--- a/src/cryptography/hazmat/primitives/kdf/hkdf.py
+++ b/src/cryptography/hazmat/primitives/kdf/hkdf.py
@@ -26,11 +26,10 @@
 
         self._algorithm = algorithm
 
-        if not (salt is None or isinstance(salt, bytes)):
-            raise TypeError("salt must be bytes.")
-
         if salt is None:
             salt = b"\x00" * self._algorithm.digest_size
+        else:
+            utils._check_bytes("salt", salt)
 
         self._salt = salt
 
@@ -44,9 +43,7 @@
         return h.finalize()
 
     def derive(self, key_material):
-        if not isinstance(key_material, bytes):
-            raise TypeError("key_material must be bytes.")
-
+        utils._check_bytes("key_material", key_material)
         return self._hkdf_expand.derive(self._extract(key_material))
 
     def verify(self, key_material, expected_key):
@@ -77,11 +74,10 @@
 
         self._length = length
 
-        if not (info is None or isinstance(info, bytes)):
-            raise TypeError("info must be bytes.")
-
         if info is None:
             info = b""
+        else:
+            utils._check_bytes("info", info)
 
         self._info = info
 
@@ -102,9 +98,7 @@
         return b"".join(output)[:self._length]
 
     def derive(self, key_material):
-        if not isinstance(key_material, bytes):
-            raise TypeError("key_material must be bytes.")
-
+        utils._check_bytes("key_material", key_material)
         if self._used:
             raise AlreadyFinalized
 
diff --git a/src/cryptography/hazmat/primitives/kdf/kbkdf.py b/src/cryptography/hazmat/primitives/kdf/kbkdf.py
index 14de56e..74fe9e2 100644
--- a/src/cryptography/hazmat/primitives/kdf/kbkdf.py
+++ b/src/cryptography/hazmat/primitives/kdf/kbkdf.py
@@ -73,10 +73,8 @@
         if context is None:
             context = b''
 
-        if (not isinstance(label, bytes) or
-                not isinstance(context, bytes)):
-            raise TypeError('label and context must be of type bytes')
-
+        utils._check_bytes("label", label)
+        utils._check_bytes("context", context)
         self._algorithm = algorithm
         self._mode = mode
         self._length = length
@@ -102,8 +100,7 @@
         if self._used:
             raise AlreadyFinalized
 
-        if not isinstance(key_material, bytes):
-            raise TypeError('key_material must be bytes')
+        utils._check_bytes("key_material", key_material)
         self._used = True
 
         # inverse floor division (equivalent to ceiling)
diff --git a/src/cryptography/hazmat/primitives/kdf/pbkdf2.py b/src/cryptography/hazmat/primitives/kdf/pbkdf2.py
index f8ce7a3..fbe8964 100644
--- a/src/cryptography/hazmat/primitives/kdf/pbkdf2.py
+++ b/src/cryptography/hazmat/primitives/kdf/pbkdf2.py
@@ -31,8 +31,7 @@
         self._used = False
         self._algorithm = algorithm
         self._length = length
-        if not isinstance(salt, bytes):
-            raise TypeError("salt must be bytes.")
+        utils._check_bytes("salt", salt)
         self._salt = salt
         self._iterations = iterations
         self._backend = backend
@@ -42,8 +41,7 @@
             raise AlreadyFinalized("PBKDF2 instances can only be used once.")
         self._used = True
 
-        if not isinstance(key_material, bytes):
-            raise TypeError("key_material must be bytes.")
+        utils._check_bytes("key_material", key_material)
         return self._backend.derive_pbkdf2_hmac(
             self._algorithm,
             self._length,
diff --git a/src/cryptography/hazmat/primitives/kdf/scrypt.py b/src/cryptography/hazmat/primitives/kdf/scrypt.py
index 77dcf9a..44e369f 100644
--- a/src/cryptography/hazmat/primitives/kdf/scrypt.py
+++ b/src/cryptography/hazmat/primitives/kdf/scrypt.py
@@ -30,9 +30,7 @@
             )
 
         self._length = length
-        if not isinstance(salt, bytes):
-            raise TypeError("salt must be bytes.")
-
+        utils._check_bytes("salt", salt)
         if n < 2 or (n & (n - 1)) != 0:
             raise ValueError("n must be greater than 1 and be a power of 2.")
 
@@ -54,8 +52,7 @@
             raise AlreadyFinalized("Scrypt instances can only be used once.")
         self._used = True
 
-        if not isinstance(key_material, bytes):
-            raise TypeError("key_material must be bytes.")
+        utils._check_bytes("key_material", key_material)
         return self._backend.derive_scrypt(
             key_material, self._salt, self._length, self._n, self._r, self._p
         )
diff --git a/src/cryptography/hazmat/primitives/kdf/x963kdf.py b/src/cryptography/hazmat/primitives/kdf/x963kdf.py
index 83789b3..a8c0775 100644
--- a/src/cryptography/hazmat/primitives/kdf/x963kdf.py
+++ b/src/cryptography/hazmat/primitives/kdf/x963kdf.py
@@ -27,8 +27,9 @@
         if length > max_len:
             raise ValueError(
                 "Can not derive keys larger than {0} bits.".format(max_len))
-        if not (sharedinfo is None or isinstance(sharedinfo, bytes)):
-            raise TypeError("sharedinfo must be bytes.")
+        if sharedinfo is not None:
+            utils._check_bytes("sharedinfo", sharedinfo)
+
         self._algorithm = algorithm
         self._length = length
         self._sharedinfo = sharedinfo
@@ -45,10 +46,7 @@
         if self._used:
             raise AlreadyFinalized
         self._used = True
-
-        if not isinstance(key_material, bytes):
-            raise TypeError("key_material must be bytes.")
-
+        utils._check_bytes("key_material", key_material)
         output = [b""]
         outlen = 0
         counter = 1
diff --git a/src/cryptography/hazmat/primitives/padding.py b/src/cryptography/hazmat/primitives/padding.py
index a081976..170c802 100644
--- a/src/cryptography/hazmat/primitives/padding.py
+++ b/src/cryptography/hazmat/primitives/padding.py
@@ -40,8 +40,7 @@
     if buffer_ is None:
         raise AlreadyFinalized("Context was already finalized.")
 
-    if not isinstance(data, bytes):
-        raise TypeError("data must be bytes.")
+    utils._check_bytes("data", data)
 
     buffer_ += data
 
@@ -65,8 +64,7 @@
     if buffer_ is None:
         raise AlreadyFinalized("Context was already finalized.")
 
-    if not isinstance(data, bytes):
-        raise TypeError("data must be bytes.")
+    utils._check_bytes("data", data)
 
     buffer_ += data