diff --git a/src/sigstore_tsp/tsp.py b/src/sigstore_tsp/tsp.py
index a3f551b..a3d68d4 100644
--- a/src/sigstore_tsp/tsp.py
+++ b/src/sigstore_tsp/tsp.py
@@ -140,6 +140,11 @@ def version(self) -> int:
     def policy(self) -> ObjectIdentifier:
         """Returns the policy OID."""
 
+    @property
+    @abc.abstractmethod
+    def message_imprint(self) -> MessageImprint:
+        """Returns the Message Imprint."""
+
     @property
     @abc.abstractmethod
     def serial_number(self) -> int: