So here is a private key crypto system in action. Two entities, A and B, have exchanged keys. A will use the key KA to send a message to B, and B can decrypt the message using the same key KA. And similarly, when B wants to send a message to A it encrypts it let’s say using another private key key B and when the message comes over here A will decrypt the message using the same key that was used for encryption namely key B. So one of the things that should be obvious is that, in order for this private key encryption system to work, both the entities need to know when they get an encrypted message who is the author of the message, because that is the only way they know what key to use in order to decrypt this message. So if a sends a message to b,it’s sending this encrypted message. When this cyphertext arrives here, for B to know that it has to use this key KA, it needs to know the identity of the sender. So the identity of the sender has to be sent in cleartext. So this is the format of a message that is going from A to B. The identity of the sender in cleartext Meaning that, looking at this, we will immediately know, this message is from sender A, and therefore, I should use key k A to decrypt the cyphertext, and vice versa when A gets a message from B. Of course, k A and k B can be the same, or in other words, it is the same key that is used for, A to send a message to B as well as for B to send a message to A. But the important thing to take away is that the key that is used for encryption and decryption of a given message is exactly the same, that’s the idea behind the private key encryption system.