use _get_backend everywhere (#5408)

* use _get_backend everywhere

* black

Author: reaperhulk

src/cryptography/fernet.py

@@ -14,7 +14,7 @@
 
 from cryptography import utils
 from cryptography.exceptions import InvalidSignature
-from cryptography.hazmat.backends import default_backend
+from cryptography.hazmat.backends import _get_backend
 from cryptography.hazmat.primitives import hashes, padding
 from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
 from cryptography.hazmat.primitives.hmac import HMAC
@@ -29,8 +29,7 @@ class InvalidToken(Exception):
 
 class Fernet(object):
     def __init__(self, key, backend=None):
-        if backend is None:
-            backend = default_backend()
+        backend = _get_backend(backend)
 
         key = base64.urlsafe_b64decode(key)
         if len(key) != 32:

src/cryptography/hazmat/primitives/serialization/pkcs12.py

@@ -5,7 +5,7 @@
 from __future__ import absolute_import, division, print_function
 
 from cryptography import x509
-from cryptography.hazmat.backends import _get_backend, default_backend
+from cryptography.hazmat.backends import _get_backend
 from cryptography.hazmat.primitives import serialization
 from cryptography.hazmat.primitives.asymmetric import dsa, ec, rsa
 
@@ -44,6 +44,7 @@ def serialize_key_and_certificates(name, key, cert, cas, encryption_algorithm):
     if key is None and cert is None and not cas:
         raise ValueError("You must supply at least one of key, cert, or cas")
 
-    return default_backend().serialize_key_and_certificates_to_pkcs12(
+    backend = _get_backend(None)
+    return backend.serialize_key_and_certificates_to_pkcs12(
         name, key, cert, cas, encryption_algorithm
     )

src/cryptography/hazmat/primitives/serialization/pkcs7.py

@@ -4,12 +4,14 @@
 
 from __future__ import absolute_import, division, print_function
 
-from cryptography.hazmat.backends import default_backend
+from cryptography.hazmat.backends import _get_backend
 
 
 def load_pem_pkcs7_certificates(data):
-    return default_backend().load_pem_pkcs7_certificates(data)
+    backend = _get_backend(None)
+    return backend.load_pem_pkcs7_certificates(data)
 
 
 def load_der_pkcs7_certificates(data):
-    return default_backend().load_der_pkcs7_certificates(data)
+    backend = _get_backend(None)
+    return backend.load_der_pkcs7_certificates(data)

src/cryptography/hazmat/primitives/serialization/ssh.py

@@ -13,7 +13,7 @@
 
 from cryptography import utils
 from cryptography.exceptions import UnsupportedAlgorithm
-from cryptography.hazmat.backends import _get_backend, default_backend
+from cryptography.hazmat.backends import _get_backend
 from cryptography.hazmat.primitives.asymmetric import dsa, ec, ed25519, rsa
 from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
 from cryptography.hazmat.primitives.serialization import (
@@ -579,9 +579,8 @@ def serialize_ssh_private_key(private_key, password=None):
         salt = os.urandom(16)
         f_kdfoptions.put_sshstr(salt)
         f_kdfoptions.put_u32(rounds)
-        ciph = _init_cipher(
-            ciphername, password, salt, rounds, default_backend()
-        )
+        backend = _get_backend(None)
+        ciph = _init_cipher(ciphername, password, salt, rounds, backend)
     else:
         ciphername = kdfname = _NONE
         blklen = 8