For a sender and recipient to communicate securely using symmetric-key encryption, they must agree upon a key and keep it secret between themselves. If they are in different physical locations, they must trust a courier or some other secure communications medium to prevent the disclosure of the secret key during transmission. Anyone who overhears or intercepts the key in transit can later read, modify, and forge all information encrypted or authenticated with that key. Therefore, symmetric-key encryption alone as a means for transmitting secure data can be quite expensive simply due to the difficulty of secure key distribution.