Refs #2578 -- implement __hash__ on SubjectKeyIdentifier
diff --git a/src/cryptography/x509/extensions.py b/src/cryptography/x509/extensions.py
index 2363d2b..5ef7a28 100644
--- a/src/cryptography/x509/extensions.py
+++ b/src/cryptography/x509/extensions.py
@@ -234,6 +234,9 @@
     def __ne__(self, other):
         return not self == other
 
+    def __hash__(self):
+        return hash(self.digest)
+
 
 @utils.register_interface(ExtensionType)
 class AuthorityInformationAccess(object):