diff --git a/src/grung/types.py b/src/grung/types.py
index cdba6ec..34e2653 100644
--- a/src/grung/types.py
+++ b/src/grung/types.py
@@ -1,7 +1,9 @@
 from __future__ import annotations
 
 import hashlib
+import hmac
 import os
+import re
 from collections import namedtuple
 from dataclasses import dataclass, field
 from typing import Dict, List
@@ -57,6 +59,21 @@ class Password(Field):
     salt_size = 4
     digest_size = 16
 
+    @classmethod
+    def is_digest(cls, passwd: str):
+        if not passwd:
+            return False
+        offset = 2 * cls.salt_size  # each byte is 2 hex chars
+        try:
+            if passwd[offset] != ":":
+                return False
+            digest = passwd[offset + 1 :]
+            if len(digest) != cls.digest_size * 2:
+                return False
+            return re.match(r"^[0-9a-f]+$", digest)
+        except IndexError:
+            return False
+
     @classmethod
     def get_digest(cls, passwd: str, salt: bytes = None):
         if not salt:
@@ -68,10 +85,10 @@ class Password(Field):
     def compare(cls, passwd: value_type, stored: value_type):
         stored_salt, stored_digest = stored.split(":")
         input_digest, input_salt = cls.get_digest(passwd, bytes.fromhex(stored_salt))
-        return input_digest == stored_digest
+        return hmac.compare_digest(input_digest, stored_digest)
 
     def before_insert(self, value: value_type, db: TinyDB, record: Record) -> None:
-        if value:
+        if value and not self.__class__.is_digest(value):
             digest, salt = self.__class__.get_digest(value)
             record[self.name] = f"{salt}:{digest}"
 
diff --git a/test/test_db.py b/test/test_db.py
index eb06005..ddfc49b 100644
--- a/test/test_db.py
+++ b/test/test_db.py
@@ -123,6 +123,9 @@ def test_search(db):
 def test_password(db):
     user = db.save(examples.User(name="john", email="john@foo", password="fnord"))
 
+    # make sure we don't compute the digest on an existing digest
+    user = db.save(user)
+
     assert ":" in user.password
     assert user.password != "fnord"