add aki hash (#3910)

* Implement __hash__ on AuthorityKeyIdentifier

* Adding dirname to fix build issue on AuthorityKeyIdentifier test

* .authority_cert_issuer to str

* use a tuple and not a str repr

2 files changed, 19 insertions, 1 deletions

diff --git a/src/cryptography/x509/extensions.py b/src/cryptography/x509/extensions.py
index 01378b38..8880f744 100644
--- a/src/cryptography/x509/extensions.py
+++ b/src/cryptography/x509/extensions.py
@@ -213,6 +213,15 @@ class AuthorityKeyIdentifier(object):
     def __ne__(self, other):
         return not self == other
 
+    def __hash__(self):
+        if self.authority_cert_issuer is None:
+            aci = None
+        else:
+            aci = tuple(self.authority_cert_issuer)
+        return hash((
+            self.key_identifier, aci, self.authority_cert_serial_number
+        ))
+
     key_identifier = utils.read_only_property("_key_identifier")
     authority_cert_issuer = utils.read_only_property("_authority_cert_issuer")
     authority_cert_serial_number = utils.read_only_property(
diff --git a/tests/x509/test_x509_ext.py b/tests/x509/test_x509_ext.py
index 6010e77d..22b4cc5b 100644
--- a/tests/x509/test_x509_ext.py
+++ b/tests/x509/test_x509_ext.py
@@ -907,7 +907,6 @@ class TestSubjectKeyIdentifier(object):
         ski3 = x509.SubjectKeyIdentifier(
             binascii.unhexlify(b"aa8098456f6ff7ff3ac9092384932230498bc980")
         )
-
         assert hash(ski1) == hash(ski2)
         assert hash(ski1) != hash(ski3)
 
@@ -1025,6 +1024,16 @@ class TestAuthorityKeyIdentifier(object):
         assert aki != aki5
         assert aki != object()
 
+    def test_hash(self):
+        dirname = x509.DirectoryName(
+            x509.Name([x509.NameAttribute(NameOID.COMMON_NAME, u'myCN')])
+        )
+        aki1 = x509.AuthorityKeyIdentifier(b"digest", [dirname], 1234)
+        aki2 = x509.AuthorityKeyIdentifier(b"digest", [dirname], 1234)
+        aki3 = x509.AuthorityKeyIdentifier(b"digest", None, None)
+        assert hash(aki1) == hash(aki2)
+        assert hash(aki1) != hash(aki3)
+
 
 class TestBasicConstraints(object):
     def test_ca_not_boolean(self):