Merge pull request #2360 from obi1kenobi/master

SubjectKeyIdentifier equality now uses constant-time digest comparison.
diff --git a/src/cryptography/x509/extensions.py b/src/cryptography/x509/extensions.py
index 803d7ec..cd75ecd 100644
--- a/src/cryptography/x509/extensions.py
+++ b/src/cryptography/x509/extensions.py
@@ -15,7 +15,7 @@
 import six
 
 from cryptography import utils
-from cryptography.hazmat.primitives import serialization
+from cryptography.hazmat.primitives import constant_time, serialization
 from cryptography.x509.general_name import GeneralName, IPAddress, OtherName
 from cryptography.x509.name import Name
 from cryptography.x509.oid import (
@@ -193,9 +193,7 @@
         if not isinstance(other, SubjectKeyIdentifier):
             return NotImplemented
 
-        return (
-            self.digest == other.digest
-        )
+        return constant_time.bytes_eq(self.digest, other.digest)
 
     def __ne__(self, other):
         return not self == other