cert_audit: Calculate identifier for X.509 objects

The identifier is calculated SHA1 hex string from
the DER encoding of each X.509 objects. It's useful
for finding out the identical X.509 objects.

Signed-off-by: Pengyu Lv <pengyu.lv@arm.com>

tests/scripts/audit-validity-dates.py

@@ -31,6 +31,7 @@ import argparse
 import datetime
 import glob
 import logging
+import hashlib
 from enum import Enum
 
 # The script requires cryptography >= 35.0.0 which is only available
@@ -69,10 +70,20 @@ class AuditData:
         self.locations = [] # type: typing.List[str]
         self.fill_validity_duration(x509_obj)
         self._obj = x509_obj
+        encoding = cryptography.hazmat.primitives.serialization.Encoding.DER
+        self._identifier = hashlib.sha1(self._obj.public_bytes(encoding)).hexdigest()
 
     def __eq__(self, __value) -> bool:
         return self._obj == __value._obj
 
+    @property
+    def identifier(self):
+        """
+        Identifier of the underlying X.509 object, which is consistent across
+        different runs.
+        """
+        return self._identifier
+
     def fill_validity_duration(self, x509_obj):
         """Read validity period from an X.509 object."""
         # Certificate expires after "not_valid_after"