def checkKey(self, credentials):
        """
        Retrieve files containing authorized keys and check against user
        credentials.
        """
        ouid, ogid = self._userdb.getpwnam(credentials.username)[2:4]
        for filepath in self.getAuthorizedKeysFiles(credentials):
            if not filepath.exists():
                continue
            try:
                lines = filepath.open()
            except IOError as e:
                if e.errno == errno.EACCES:
                    lines = runAsEffectiveUser(ouid, ogid, filepath.open)
                else:
                    raise
            with lines:
                for l in lines:
                    l2 = l.split()
                    if len(l2) < 2:
                        continue
                    try:
                        if _b64decodebytes(l2[1]) == credentials.blob:
                            return True
                    except binascii.Error:
                        continue
        return False

        Retrieve files containing authorized keys and check against user
        credentials.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/checkers.py