Does the authenticity of messages by Alice still holds if the state of bob is compromised in Double Ratchet In the double ratchet algorithm, let's say if bob's device is compromised including his long term key.

Does the ephemeral keys generated by Alice during the half Diffie–Hellman key exchange produces a new sending chain for Alice which is not yet known to Bob.

Also does the messages from Alice still have the non-repudiation property even if bob's state is completely compromised.
if bob's device is compromised including his long term key.
  
  Does the ephemeral keys generated by Alice during the half Diffie–Hellman key exchange produces a new sending chain for Alice which is not yet known to bob


Double Rachet is secure against an attacker who records everything than compromise the sender or the receiver. As always said, if the receiver or sender stored previously generated keys then there is no forward secrecy, the attacker gets the keys and reads the recorded messages. Securely delete the keys.

A compromised secret key or device has more attack vectors that can be catastrophic. 


The attacker can impersonate the attacked party and create new sessions.
The attackers can execute man-in-the-middle attacks constantly and maintain eavesdropping on the compromised session.


So the attacker will produce new chains that Alice and Bob are not aware of. The key and device must be replaced immediately if they are suspecting.


  if bob's device is compromised including his long term key.
  
  does the messages from Alice still has the non-repudiation property even if bob's state is completely compromised.


Alice will talk normally, so she cannot repudiate. The attacker has no power to forge a message that does not come from Alice. If they do, Bob can do, too and this is against the non-repudiation of the protocol.