lets say someone offers the following method for securely verifying that both of you have the same secret key. The length of the key is used to generate a random but string, which is then XORed with the key before being sent over the channel. The arriving block is XORed with the key by your partner before being sent back. you check and if the result match the random string you first generated, you may be certain that your partner also has access to the secret key. is this plan safe