FIX: escape literal apostrophes in DigitBijectionConverter to avoid case-marker collision

pyrit/converter/bijection_converter.py

@@ -268,6 +268,12 @@ def _build_identifier(self) -> ComponentIdentifier:
     # the marker and restores the uppercase letter. Without this, capitalization is
     # silently destroyed at encode time (`"25".upper() == "25"`), not just mishandled
     # at decode.
+    #
+    # _CASE_MARKER is also a plain character that can appear in the plaintext itself
+    # (contractions, possessives: "it's", "don't"). Since every letter always encodes
+    # to a digit token, a passed-through marker immediately followed by an encoded
+    # letter is indistinguishable from a real case marker, so a literal marker is
+    # doubled on encode and collapsed back on decode.
     _CASE_MARKER = "'"
 
     def encode(self, *, prompt: str) -> str:
@@ -285,6 +291,8 @@ def encode(self, *, prompt: str) -> str:
             if char.lower() in self._mapping:
                 token = self._mapping[char.lower()]
                 encoded += (self._CASE_MARKER + token) if char.isupper() else token
+            elif char == self._CASE_MARKER:
+                encoded += self._CASE_MARKER * 2
             else:
                 encoded += char
         return encoded
@@ -321,6 +329,10 @@ def decode(self, encoded_text: str) -> str:
         decoded = ""
         i = 0
         while i < len(encoded_text):
+            if encoded_text[i] == self._CASE_MARKER and encoded_text[i + 1 : i + 2] == self._CASE_MARKER:
+                decoded += self._CASE_MARKER
+                i += 2
+                continue
             is_upper = encoded_text[i] == self._CASE_MARKER
             start = i + 1 if is_upper else i
             candidate = encoded_text[start : start + self._num_digits]

tests/unit/converter/test_bijection_converter.py

@@ -118,6 +118,26 @@ async def test_digit_converter_explicit_mapping_round_trip():
     assert converter.decode(encoded.output_text) == "abc xyz!"
 
 
+async def test_digit_converter_literal_apostrophe_round_trip():
+    custom_mapping = {letter: str(index + 10) for index, letter in enumerate(string.ascii_lowercase)}
+    converter = DigitBijectionConverter(mapping=custom_mapping)
+
+    encoded = await converter.convert_async(prompt="it's")
+
+    assert encoded.output_text == "1829''28"
+    assert converter.decode(encoded.output_text) == "it's"
+
+
+async def test_digit_converter_uppercase_letter_after_apostrophe_round_trip():
+    custom_mapping = {letter: str(index + 10) for index, letter in enumerate(string.ascii_lowercase)}
+    converter = DigitBijectionConverter(mapping=custom_mapping)
+
+    encoded = await converter.convert_async(prompt="I'm")
+
+    assert encoded.output_text == "'18''22"
+    assert converter.decode(encoded.output_text) == "I'm"
+
+
 def test_digit_converter_encodes_letters():
     converter = DigitBijectionConverter(num_digits=2)
     # encoding "hello" should produce digit strings