Suppose that someone suggests the following way to securely confirm that the two of you are both in possession of the same secret key. you create a random bit string the length of the key, xor it with the key, and send the result over the channel. your partner xors the incoming block with the key and sends it back. you check, and if what you receive is your original random string, you have verified that your partner has the same secret key. is this scheme secure?