diff --git a/src/envelope_crypto/envelope_crypto.py b/src/envelope_crypto/envelope_crypto.py
index 4fa98b6..ea76224 100644
--- a/src/envelope_crypto/envelope_crypto.py
+++ b/src/envelope_crypto/envelope_crypto.py
@@ -21,7 +21,6 @@ variants are the same functions - use whichever fits your storage.
 """
 
 import os
-import re
 import copy
 import json
 import base64
@@ -631,50 +630,72 @@ def decrypt_record(crypto: EnvelopeCrypto, record, traversal_level: int = 2) ->
     return result
 
 
-def _fingerprint_default(value: Any) -> str:
+_DETERMINISTIC_SCALARS = (str, int, float, bool, type(None))
+
+
+def _fingerprint_default(value: Any) -> Any:
     """json.dumps default= handler for values fingerprint_data can't natively serialize
 
-    raises TypeError on an object with the identity-based default repr (`<X at 0x...>`),
-    whose address is non-deterministic across processes - fingerprinting it would return an
-    unstable digest, so it fails loud instead
+    handles datetime (isoformat) and bytes (hex) deterministically, returning a 2-element
+    ``["type", value]`` list so the encoding is a JSON ARRAY that no genuine str value can
+    ever equal (a str encodes as a JSON string) - collision-resistant by construction.
+    anything else - a callable, an arbitrary object, a generator - has no guaranteed stable
+    representation across processes (its repr usually embeds a memory address), so it fails
+    loud with a TypeError rather than yield an unstable digest.
     """
     if hasattr(value, "isoformat"):
-        return f"isoformat:{value.isoformat()}"
+        return ["datetime", value.isoformat()]
     if isinstance(value, (bytes, bytearray)):
-        return f"hex:{value.hex()}"
-    text = repr(value)
-    if re.search(r" object at 0x[0-9a-fA-F]+>$", text):
-        raise TypeError(
-            f"fingerprint_data: {type(value).__name__} has no deterministic representation "
-            "(identity-based repr); give it a stable __repr__ or serialize it before fingerprinting"
-        )
-    return f"{type(value).__name__}:{text}"
+        return ["bytes", value.hex()]
+    raise TypeError(
+        f"fingerprint_data: {type(value).__name__} has no deterministic representation; "
+        "serialize it to a stable str/bytes/datetime before fingerprinting"
+    )
+
+
+def _fingerprint_key(key: Any) -> str:
+    """derive a deterministic, type-qualified tag for a dict key
+
+    scalars tag by type + value; set/frozenset keys canonically sort their members; any other
+    key type (a callable, an identity-repr object, a nested container) has no stable key form
+    and raises TypeError, mirroring the value-side determinism guard.
+    """
+    if isinstance(key, _DETERMINISTIC_SCALARS):
+        return f"{type(key).__name__}:{key!r}"
+    if isinstance(key, (set, frozenset)):
+        normalized = _fingerprint_normalize(key)
+        return f"{type(key).__name__}:" + json.dumps(normalized, sort_keys=True, default=_fingerprint_default)
+    raise TypeError(
+        f"fingerprint_data: dict key of type {type(key).__name__} has no deterministic "
+        "representation; use a scalar or set/frozenset key"
+    )
 
 
 def _fingerprint_normalize(value: Any) -> Any:
-    """recursively tag dict keys with their type and canonically sort sets for determinism"""
+    """recursively normalize dict keys and sets so the encoding is deterministic across processes"""
     if isinstance(value, dict):
         return {
-            f"{type(key).__name__}:{key!r}": _fingerprint_normalize(sub_value)
+            _fingerprint_key(key): _fingerprint_normalize(sub_value)
             for key, sub_value in value.items()
         }
     if isinstance(value, (set, frozenset)):
         members = [_fingerprint_normalize(item) for item in value]
         members.sort(key=lambda item: json.dumps(item, sort_keys=True, default=_fingerprint_default))
-        return {f"{type(value).__name__}:members": members}
+        return {"\x00set:members": members}
     if isinstance(value, (list, tuple)):
         return [_fingerprint_normalize(item) for item in value]
     return value
 
 
 def fingerprint_data(data: dict) -> str:
-    """return a deterministic, collision-free SHA-256 hex fingerprint of a dict
+    """return a deterministic, collision-resistant SHA-256 hex fingerprint of a dict
 
     keys of different types that would coerce to the same JSON string are kept distinct via
-    a type-tagged pre-pass; set/frozenset values are canonically sorted so their unordered
-    members hash stably; non-JSON-native values (datetime/bytes/etc.) go through a stable
-    default= handler. an object with only an identity-based repr raises TypeError rather than
-    yield an unstable digest. never logs the data being fingerprinted.
+    a type-tagged pre-pass; set/frozenset keys and values are canonically sorted so their
+    unordered members hash stably; datetime/bytes go through a type-qualified default= handler
+    whose ``\\x00``-prefixed tags cannot collide with a genuine str value. any key or value
+    with no deterministic representation (a callable, an identity-repr object) raises TypeError
+    rather than yield a digest that varies across processes. never logs the data.
     """
     normalized = _fingerprint_normalize(data)
     encoded = json.dumps(normalized, sort_keys=True, default=_fingerprint_default)