diff --git a/weboob/tools/captcha/virtkeyboard.py b/weboob/tools/captcha/virtkeyboard.py
index d5b699ffb06705b91c88f8c7efccb089f1bff882..9091c21cefa729b50353a35913118752271f7771 100644
--- a/weboob/tools/captcha/virtkeyboard.py
+++ b/weboob/tools/captcha/virtkeyboard.py
@@ -284,7 +284,7 @@ def __init__(self, code_to_filedata):
                 raise VirtKeyboardError("Symbol '%s' not found; all symbol hashes are available in %s" % (char, path))
 
     def checksum(self, buffer):
-        return hashlib.md5(buffer).hexdigest()
+        return hashlib.md5(self.convert(buffer)).hexdigest()
 
     def dump(self, files, path):
         for dat in files:
@@ -298,6 +298,9 @@ def get_string_code(self, password):
             symbols.append(self.char_to_code[c])
         return self.codesep.join(symbols)
 
+    def convert(self, buffer):
+        return buffer
+
     @classmethod
     def create_from_url(cls, browser, code_to_url):
         code_to_file = {