make multibackend error with an empty list. fixes #2717

2 files changed, 22 insertions, 5 deletions

diff --git a/src/cryptography/hazmat/backends/multibackend.py b/src/cryptography/hazmat/backends/multibackend.py
index 65f18531..95382450 100644
--- a/src/cryptography/hazmat/backends/multibackend.py
+++ b/src/cryptography/hazmat/backends/multibackend.py
@@ -28,6 +28,13 @@ class MultiBackend(object):
     name = "multibackend"
 
     def __init__(self, backends):
+        if len(backends) == 0:
+            raise ValueError(
+                "Multibackend cannot be initialized with no backends. If you "
+                "are seeing this error when trying to use default_backend "
+                "please try uninstalling and reinstalling cryptography."
+            )
+
         self._backends = backends
 
     def _filtered_backends(self, interface):
diff --git a/tests/hazmat/backends/test_multibackend.py b/tests/hazmat/backends/test_multibackend.py
index 74835716..bf54d5ce 100644
--- a/tests/hazmat/backends/test_multibackend.py
+++ b/tests/hazmat/backends/test_multibackend.py
@@ -4,6 +4,8 @@
 
 from __future__ import absolute_import, division, print_function
 
+import pytest
+
 from cryptography import utils
 from cryptography.exceptions import (
     UnsupportedAlgorithm, _Reasons
@@ -21,6 +23,10 @@ from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
 from ...utils import raises_unsupported_algorithm
 
 
+class DummyBackend(object):
+    pass
+
+
 @utils.register_interface(CipherBackend)
 class DummyCipherBackend(object):
     def __init__(self, supported_ciphers):
@@ -226,6 +232,10 @@ class DummyX509Backend(object):
 
 
 class TestMultiBackend(object):
+    def test_raises_error_with_empty_list(self):
+        with pytest.raises(ValueError):
+            MultiBackend([])
+
     def test_ciphers(self):
         backend = MultiBackend([
             DummyHashBackend([]),
@@ -310,7 +320,7 @@ class TestMultiBackend(object):
 
         backend.load_rsa_public_numbers("public_numbers")
 
-        backend = MultiBackend([])
+        backend = MultiBackend([DummyBackend()])
         with raises_unsupported_algorithm(
             _Reasons.UNSUPPORTED_PUBLIC_KEY_ALGORITHM
         ):
@@ -353,7 +363,7 @@ class TestMultiBackend(object):
         backend.load_dsa_public_numbers("numbers")
         backend.load_dsa_parameter_numbers("numbers")
 
-        backend = MultiBackend([])
+        backend = MultiBackend([DummyBackend()])
         with raises_unsupported_algorithm(
             _Reasons.UNSUPPORTED_PUBLIC_KEY_ALGORITHM
         ):
@@ -491,7 +501,7 @@ class TestMultiBackend(object):
         backend.load_pem_private_key(b"keydata", None)
         backend.load_pem_public_key(b"keydata")
 
-        backend = MultiBackend([])
+        backend = MultiBackend([DummyBackend()])
         with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_SERIALIZATION):
             backend.load_pem_private_key(b"keydata", None)
         with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_SERIALIZATION):
@@ -503,7 +513,7 @@ class TestMultiBackend(object):
         backend.load_der_private_key(b"keydata", None)
         backend.load_der_public_key(b"keydata")
 
-        backend = MultiBackend([])
+        backend = MultiBackend([DummyBackend()])
         with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_SERIALIZATION):
             backend.load_der_private_key(b"keydata", None)
         with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_SERIALIZATION):
@@ -523,7 +533,7 @@ class TestMultiBackend(object):
         backend.create_x509_crl(object(), b"privatekey", hashes.SHA1())
         backend.create_x509_revoked_certificate(object())
 
-        backend = MultiBackend([])
+        backend = MultiBackend([DummyBackend()])
         with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_X509):
             backend.load_pem_x509_certificate(b"certdata")
         with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_X509):