Read more about it in this previous post: http://ha.xxor.se/2011/07/encryptse-beta-open-for-public.html
The Key Exchange feature enables users of Encrypt.se to communicate their secret crypto key to their friends over the phone, even if someone might be listening.
How it worksTo securely transmit a secret key over an insecure channel we utilize a well known method. This is how it works.
--------Sender-------- ------Recipient------ (Step 1) Input a secret key and apply a first level of encryption to the key. Send the encrypted key (Step 2) to the recipient. --------> The recipient applies a second level of encryption and (Step 3) transmits the double- The sender now removes <-------- encrypted key back. (decrypts) the first level of encryption and transfers the encrypted key back to (Step 4) the recipient. --------> The recipient can now remove (decrypt) the second level of encryption and read the secret key. --------Sender-------- ------Recipient------