Retrieve the keys of the user specified by the credentials, and check
        if one matches the blob in the credentials.

def checkKey(self, credentials):
        """
        Retrieve the keys of the user specified by the credentials, and check
        if one matches the blob in the credentials.
        """
        filename = self._keyfile
        if not os.path.exists(filename):
            return 0
        lines = open(filename).xreadlines()
        for l in lines:
            l2 = l.split()
            if len(l2) < 2:
                continue
            try:
                if base64.decodestring(l2[1]) == credentials.blob:
                    return 1
            except binascii.Error:
                continue
        return 0