add: detach and attach test to COSEMessage

Author: kentakayama

tests/test_cose_message.py

@@ -460,3 +460,49 @@ def test_cose_message_counterverify_with_different_abbreviated_countersignature(
             COSEMessage.loads(countersigned).counterverify(pub_key)
             pytest.fail("counterverify() should not fail.")
         assert "Failed to verify." in str(err.value)
+
+    def test_cose_message_detach_and_attach(self):
+        """
+        Detach the payload from a COSE message.
+        For example, [an example message](https://github.com/cose-wg/Examples/blob/master/ecdsa-examples/ecdsa-sig-01.json)
+        ```
+        18([
+          / protected: / h'A201260300',
+          / unprotected: / {4: h'3131'},
+          / payload: / h'546869732069732074686520636F6E74656E742E',
+          / signature: / h'6520BBAF2081D7E0ED0F95F76EB0733D667005F7467CEC4B87B9381A6BA1EDE8E00DF29F32A37230F39A842A54821FDD223092819D7728EFB9D3A0080B75380B'
+        ])
+        ```
+        would be separated into
+        ```
+        18([
+          / protected: / h'A201260300',
+          / unprotected: / {4: h'3131'},
+          / payload: / null / detached /,
+          / signature: / h'6520BBAF2081D7E0ED0F95F76EB0733D667005F7467CEC4B87B9381A6BA1EDE8E00DF29F32A37230F39A842A54821FDD223092819D7728EFB9D3A0080B75380B'
+        ])
+        ```
+        and
+        ```
+        546869732069732074686520636F6E74656E742E
+        ```
+
+        """
+        ecdsa_cose_sign1_example = COSEMessage.loads(
+            bytes.fromhex(
+                "D28445A201260300A10442313154546869732069732074686520636F6E74656E742E58406520BBAF2081D7E0ED0F95F76EB0733D667005F7467CEC4B87B9381A6BA1EDE8E00DF29F32A37230F39A842A54821FDD223092819D7728EFB9D3A0080B75380B"
+            )
+        )
+        expected_detached_cose_message = COSEMessage.loads(
+            bytes.fromhex(
+                "D28445A201260300A104423131F658406520BBAF2081D7E0ED0F95F76EB0733D667005F7467CEC4B87B9381A6BA1EDE8E00DF29F32A37230F39A842A54821FDD223092819D7728EFB9D3A0080B75380B"
+            )
+        )
+        expected_payload = bytes.fromhex("546869732069732074686520636F6E74656E742E")
+
+        detached_content_cose_message, detached_payload = ecdsa_cose_sign1_example.to_detached()
+        assert expected_detached_cose_message == detached_content_cose_message
+        assert expected_payload == detached_payload
+
+        reverted_cose_message = detached_content_cose_message.from_detached(detached_payload)
+        assert ecdsa_cose_sign1_example == reverted_cose_message