Validate Base32 quantum lengths and padding

src/validators/encoding.py

@@ -46,7 +46,15 @@ def base32(value: str, /):
         (Literal[True]): If `value` is a valid base32 encoding.
         (ValidationError): If `value` is an invalid base32 encoding.
     """
-    return re.fullmatch(r"^[A-Z2-7]+=*$", value) if value else False
+    return (
+        re.fullmatch(
+            r"(?:[A-Z2-7]{8})*(?:[A-Z2-7]{2}(?:={6})?|[A-Z2-7]{4}(?:={4})?|"
+            r"[A-Z2-7]{5}(?:={3})?|[A-Z2-7]{7}=?)?",
+            value,
+        )
+        if value
+        else False
+    )
 
 
 @validator

tests/test_encoding.py

@@ -130,3 +130,16 @@ def test_encoding_rejects_trailing_characters(
 ):
     """Reject characters outside the encoded value."""
     assert isinstance(validate(value + suffix), ValidationError)
+
+
+@pytest.mark.parametrize("value", ["A", "AAA", "AAAAAA", "AB=", "AB===", "AB=======", "ABCDEFGH="])
+def test_base32_rejects_invalid_length_and_padding(value: str):
+    """Reject symbol counts and padding that cannot encode whole bytes."""
+    assert isinstance(base32(value), ValidationError)
+
+
+@pytest.mark.parametrize("value", ["MY======", "MZXQ====", "MZXW6===", "MZXW6YQ=", "MZXW6YTB"])
+def test_base32_accepts_valid_final_quanta(value: str):
+    """Accept each legal final quantum with or without padding."""
+    assert base32(value) is True
+    assert base32(value.rstrip("=")) is True