ci: add linter task "ban unicode" to protect against malicious unicode

.cirrus.yml

@@ -174,6 +174,14 @@ task:
         ELECTRUM_LINTERS_IGNORE: ""
       allow_failures: true
 
+task:
+  name: "linter: ban unicode"
+  container:
+    image: python:3.10
+    cpu: 1
+    memory: 1G
+  main_script:
+    - contrib/ban_unicode.py
 
 # Cron jobs configured in https://cirrus-ci.com/settings/...
 # - job "nightly" on branch "master" at "0 30 2 * * ?"  (every day at 02:30Z)

contrib/ban_unicode.py

@@ -0,0 +1,58 @@
+#!/usr/bin/env python3
+#
+# Copyright (C) 2025 The Electrum developers
+# Distributed under the MIT software license, see the accompanying
+# file LICENCE or http://www.opensource.org/licenses/mit-license.php
+#
+# This script scans the whole codebase for unicode characters and
+# errors if it finds any, unless the character is specifically whitelisted below.
+# The motivation is to protect against homoglyph attacks, invisible unicode characters,
+# bidirectional and other control characters, and other malicious unicode usage.
+# Given that we mostly expect to use ASCII characters in the source code,
+# the most robust and generic fix seems to be to just ban all unicode usage.
+
+import os.path
+import subprocess
+import sys
+
+project_root = os.path.abspath(os.path.dirname(os.path.dirname(__file__)))
+os.chdir(project_root)
+
+EXCLUDE_PATH_PREFIX = {
+    "electrum/wordlist/",
+    "fastlane/",
+    "tests/",
+}
+EXCLUDE_EXTENSIONS = {
+    ".jpg", ".jpeg", ".png", ".ttf", ".otf", ".pdn", ".icns", ".ico", ".gif",
+}
+UNICODE_WHITELIST = {
+    "💬", "🗯", "⚠", chr(0xfe0f), "✓", "▷", "▽", "…", "•", "█", "™", "≈",
+    "á", "é", "’",
+    "│", "─", "└", "├",
+}
+
+exit_code = 0
+
+bfiles = subprocess.check_output(["git", "ls-files"])
+bfiles = bfiles.decode("utf-8")
+for file_path in bfiles.splitlines():
+    if os.path.isdir(file_path):
+        continue
+    if any(file_path.startswith(pattern) for pattern in EXCLUDE_PATH_PREFIX):
+        continue
+    _fname, ext = os.path.splitext(file_path)
+    if ext in EXCLUDE_EXTENSIONS:
+        continue
+    # open file
+    try:
+        with open(file_path, "r", encoding="utf-8") as f:
+            for line_no, line in enumerate(f.read().splitlines()):
+                for char in line:
+                    if ord(char)>0x7f and char not in UNICODE_WHITELIST:
+                        print(f"{file_path}:{line_no}. {line=}. hex={hex(ord(char))}. {char=}")
+                        exit_code = 1
+    except UnicodeDecodeError as e:
+        raise Exception(f"cannot parse file {file_path=}") from e
+
+sys.exit(exit_code)