# Signal Sender Keys: Why it works and which is the common key at the end?

As I has read upon "WhatsUp Encryption Overview" when a client receives a sender message then it performs the following actions:

1. Decomposes Sender Message to Chain Key and Signature Key.
2. Generates a Message Key from a Chain Key and update the Chain Key.
3. The sender encrypts the message using AES256 in CBC mode and signs it with Signature key.
4. The sender transmits the single ciphertext message to the server, which does server-side fan-out to all group participants.

So I wonder at the end which is the final group key each participant, as far as I understand has a single key so how others on a group chat are able to decrypt the message?

Also I cannot understand at step 2 which key is used in order to encrypt the message in AES CBC Mode?

Also how Chain Key is updated? As some sort of a DKF function is used to concat all single keys into one master key?

• I've got a lot of trouble reading your messages. Could you possibly read it again (imagine you're the one receiving the post). Things like "So I wonder at the end which is the final group key each participant as far as I understand has a single key so how others on a group chat are able to decrypt the message?" should definitely not be a single sentence. – Maarten Bodewes Sep 25 '18 at 15:53
• Thanks for taking my comment seriously, but the insertion of the comma alone doesn't do it for me. (Note that I may remove these comments later on; they are not important for readers question, just the formatting. Others may understand the question well even if I don't). – Maarten Bodewes Sep 26 '18 at 11:43