Secure Channel - Encryption and Decryption Keys For A Message
how do we obtain the original message if we XOR the ciphertext with some other key stream, different from the one that we used to encrypt the message in Secure Channel: Sending a Message?


Answer: we don't use different keystreams.  When Alice sends Bob a message, she uses the 'Enc Alice to Bob' keystream,  When Bob receives that message, he uses that same 'Enc Alice to Bob' keystream.  The 'Enc Bob to Alice' keystream is used for messages that Bob send to Alice (both by Bob to encrypt those messages, and by Alice to decrypt those messages).

The MACs work the same way; the 'auth Alice to Bob' MAC keys are used by both sides to do integrity checking on the messages that Alice sends to Bob, and the 'auth Bob to Alice' MAC keys are used by both sides for messages that Bob sends to Alice.