Raise ValueError for invalid PNG text keys

Tests/test_file_png.py

@@ -465,6 +465,24 @@ def test_roundtrip_itxt(self) -> None:
         assert im.text["eggs"].lang == "en"
         assert im.text["eggs"].tkey == "Eggs"
 
+    def test_text_key(self) -> None:
+        info = PngImagePlugin.PngInfo()
+
+        for length in (0, 80):
+            with pytest.raises(
+                ValueError, match="Text key must be 1 to 79 characters long"
+            ):
+                info.add_itxt("k" * length, "value")
+        with pytest.raises(ValueError, match="Text key cannot contain null characters"):
+            info.add_text("a\0b", "value")
+        assert info.chunks == []
+
+        info.add_itxt("i" * 79, "value")
+        info.add_text("t" * 79, "value")
+
+        im = roundtrip(Image.new("RGB", (1, 1)), pnginfo=info)
+        assert im.text == {"i" * 79: "value", "t" * 79: "value"}
+
     def test_nonunicode_text(self) -> None:
         # Check so that non-Unicode text is saved as a tEXt rather than iTXt
 

src/PIL/PngImagePlugin.py

@@ -310,6 +310,16 @@ def add(self, cid: bytes, data: bytes, after_idat: bool = False) -> None:
 
         self.chunks.append((cid, data, after_idat))
 
+    def _check_text_key(self, key: bytes) -> None:
+        if not 1 <= len(key) <= 79:
+            msg = "Text key must be 1 to 79 characters long"
+            raise ValueError(msg)
+
+        if b"\0" in key:
+            # The NULL character is the separator after the key
+            msg = "Text key cannot contain null characters"
+            raise ValueError(msg)
+
     def add_itxt(
         self,
         key: str | bytes,
@@ -320,7 +330,7 @@ def add_itxt(
     ) -> None:
         """Appends an iTXt chunk.
 
-        :param key: latin-1 encodable text key name
+        :param key: latin-1 encodable text key name. 1 to 79 characters long
         :param value: value for this key
         :param lang: language code
         :param tkey: UTF-8 version of the key name
@@ -330,6 +340,7 @@ def add_itxt(
 
         if not isinstance(key, bytes):
             key = key.encode("latin-1", "strict")
+        self._check_text_key(key)
         if not isinstance(value, bytes):
             value = value.encode("utf-8", "strict")
         if not isinstance(lang, bytes):
@@ -350,7 +361,7 @@ def add_text(
     ) -> None:
         """Appends a text chunk.
 
-        :param key: latin-1 encodable text key name
+        :param key: latin-1 encodable text key name. 1 to 79 characters long
         :param value: value for this key, text or an
            :py:class:`PIL.PngImagePlugin.iTXt` instance
         :param zip: compression flag
@@ -374,6 +385,7 @@ def add_text(
 
         if not isinstance(key, bytes):
             key = key.encode("latin-1", "strict")
+        self._check_text_key(key)
 
         if zip:
             self.add(b"zTXt", key + b"\0\0" + zlib.compress(value))