refactor: move nested imports to top of files and improve docs

README.md

@@ -189,6 +189,27 @@ mdoc.disclosure_map
 >> ... dictionary containing all the disclosed attributes ...
 ````
 
+### Verify with Certificate Chain and Element Hashes
+
+For production use, verify both the X.509 certificate chain and element hashes:
+
+````python
+# skip in doc examples (requires your_ca_cert.pem and device_response_bytes)
+from pymdoccbor.mdoc.verifier import MdocCbor
+from cryptography import x509
+from cryptography.hazmat.backends import default_backend
+
+# Load trusted root certificates
+with open('your_ca_cert.pem', 'rb') as f:
+    iaca_cert = x509.load_pem_x509_certificate(f.read(), default_backend())
+
+mdoc = MdocCbor()
+mdoc.loads(device_response_bytes)
+is_valid = mdoc.verify(trusted_root_certs=[iaca_cert], verify_hashes=True)
+````
+
+For complete documentation on certificate chain verification and hash verification, see [docs/CERTIFICATE-CHAIN-VERIFICATION.md](docs/CERTIFICATE-CHAIN-VERIFICATION.md).
+
 ### Verify the Mobile Security Object
 
 ````

pymdoccbor/mso/verifier.py

@@ -1,4 +1,6 @@
+import hashlib
 import logging
+from datetime import datetime, timezone
 from typing import Union
 
 import cbor2
@@ -156,7 +158,6 @@ def attest_public_key(self, trusted_root_certs: list = None):
             raise ValueError("DS certificate not signed by any trusted root")
 
         # Verify certificate validity dates
-        from datetime import datetime, timezone
         now = datetime.now(timezone.utc)
 
         if ds_cert.not_valid_before_utc > now:
@@ -221,8 +222,6 @@ def verify_element_hashes(self, namespaces: dict) -> dict:
         Returns:
             dict: Results with 'valid' (bool), 'total' (int), 'verified' (int), 'failed' (list)
         """
-        import hashlib
-
         mso_data = self.payload_as_dict
         value_digests = mso_data.get('valueDigests', {})
 

pymdoccbor/tests/test_09_errors_field.py

@@ -5,6 +5,8 @@
 an 'errors' field describing which elements were not available.
 """
 
+import cbor2
+
 from pymdoccbor.mdoc.issuer import MdocCborIssuer
 from pymdoccbor.mdoc.verifier import MobileDocument
 from pymdoccbor.tests.cert_data import CERT_DATA
@@ -103,7 +105,6 @@ def test_mobile_document_dump_with_errors():
     assert isinstance(dump, bytes)
 
     # Decode and verify errors field is present
-    import cbor2
     decoded = cbor2.loads(dump)
     # The dump is wrapped in a CBORTag, so we need to access .value
     if hasattr(decoded, 'value'):
@@ -138,7 +139,6 @@ def test_mobile_document_dump_without_errors():
     assert isinstance(dump, bytes)
 
     # Decode and verify errors field is NOT present
-    import cbor2
     decoded = cbor2.loads(dump)
     if hasattr(decoded, 'value'):
         decoded = decoded.value