colornote: improve error handling and documentation

docs/formats/colornote.md

@@ -9,5 +9,6 @@ This page describes how to convert notes from ColorNote to Markdown.
 
 1. Export as described [at the website](https://www.colornote.com/faq-question/what-is-device-backup/)
 2. [Install jimmy](../index.md#installation)
-3. Convert to Markdown. Example: `jimmy-cli-linux colornote-20241013.backup --format colornote --password 1234`
+3. Convert to Markdown. Example: `jimmy-cli-linux colornote-20241013.backup --format colornote --password 0000`
+    1. The default password for automatic backups is `0000`.
 4. [Import to your app](../import_instructions.md)

src/formats/colornote.py

@@ -16,28 +16,44 @@
 import markdown_lib.common
 
 
-def decrypt(salt: bytes, password: bytes, ciphertext: bytes) -> bytes:
-    # decrypting is based on:
-    # https://github.com/olejorgenb/ColorNote-backup-decryptor/blob/61e105d6f13b2cd22b5141b6334bb098617665e1/src/ColorNoteBackupDecrypt.java
-    key = hashlib.md5(password + salt).digest()
-    iv = hashlib.md5(key + password + salt).digest()
-
-    cipher = Cipher(algorithms.AES128(key), modes.CBC(iv))
-    decryptor = cipher.decryptor()
-    plaintext_padded = decryptor.update(ciphertext) + decryptor.finalize()
-
-    unpadder = padding.PKCS7(cipher.algorithm.block_size).unpadder()
-    plaintext = unpadder.update(plaintext_padded) + unpadder.finalize()
-    return plaintext
-
-
 class Converter(converter.BaseConverter):
     accepted_extensions = [".backup"]
 
     def __init__(self, config):
         super().__init__(config)
         self.password = config.password
 
+    def parse_metadata(self, ciphertext: bytes):
+        # TODO: Is the reverse-engineered data correct?
+        # print(ciphertext[:8].decode("utf-8") == "NOTE")
+        major, minor, timestamp, note_count = struct.unpack(">LLQL", ciphertext[8:])
+        date = common.timestamp_to_datetime(timestamp / 1000).strftime(
+            "%Y-%m-%d %H:%M:%S"
+        )
+        self.logger.info(
+            f"Metadata: {note_count} notes exported at {date} "
+            f"with version {major}.{minor}"
+        )
+
+    def decrypt(self, salt: bytes, password: bytes, ciphertext: bytes) -> bytes | None:
+        # decrypting is based on:
+        # https://github.com/olejorgenb/ColorNote-backup-decryptor/blob/61e105d6f13b2cd22b5141b6334bb098617665e1/src/ColorNoteBackupDecrypt.java
+        key = hashlib.md5(password + salt).digest()
+        iv = hashlib.md5(key + password + salt).digest()
+
+        cipher = Cipher(algorithms.AES128(key), modes.CBC(iv))
+        decryptor = cipher.decryptor()
+        plaintext_padded = decryptor.update(ciphertext) + decryptor.finalize()
+
+        unpadder = padding.PKCS7(cipher.algorithm.block_size).unpadder()
+        try:
+            plaintext = unpadder.update(plaintext_padded) + unpadder.finalize()
+            return plaintext
+        except ValueError as exc:
+            self.logger.error("Decrypting failed. Wrong password?")
+            self.logger.debug(exc, exc_info=True)
+            return None
+
     def handle_wikilink_links(self, body: str) -> imf.NoteLinks:
         # only internal links
         # https://www.colornote.com/faq-question/how-can-i-link-a-note-with-another-note/
@@ -48,10 +64,13 @@ def handle_wikilink_links(self, body: str) -> imf.NoteLinks:
 
     def convert(self, file_or_folder: Path):
         ciphertext = file_or_folder.read_bytes()
-        # TODO: Meaning of ciphertext[:28]?
-        plaintext = decrypt(
+
+        self.parse_metadata(ciphertext[:28])
+        plaintext = self.decrypt(
             b"ColorNote Fixed Salt", self.password.encode("utf-8"), ciphertext[28:]
         )
+        if plaintext is None:
+            return
 
         # TODO: Meaning of plaintext[:16]? Looks similar to the iv.
         plaintext_stream = io.BytesIO(plaintext)
@@ -60,7 +79,7 @@ def convert(self, file_or_folder: Path):
             # parse binary colornote format
             # 4 bytes: chunk length
             # chunk length bytes: json data
-            (chunk_length,) = struct.unpack(">L", chunk_length_bytes)
+            chunk_length = struct.unpack(">L", chunk_length_bytes)[0]
             chunk_bytes = plaintext_stream.read(chunk_length)
             note_json = json.loads(chunk_bytes.decode("utf-8"))