Given this protocol, can you find some weakness? 1) A-&gt;B  A||B||{Na}Kab
2) B-&gt;A  {Na+1,Nb}Kab
3) A-&gt;B  {Nb+1}Kab
4) B-&gt;A  {K'ab,N'b}Kab


This protocol is used to refresh a symmetric session key.

EDIT:

A and B are the IDs of users A and B.
Na, Nb, N'b are nonces (pseudorandom values).
Kab is the old session key.
K'ab is the new session key.
|| means concatenation.

An attacker is only able to do man-in-the-middle.
Every message is encrypted with Kab
One weakness with this protocol is that the compromise of a past symmetric key, which can often be assumed over some sufficiently large period of time, compromises all keys and messages sent after the time that key was first used. 

edit:

If any $K_{ab}$ is compromised, a man in the middle attacker can resend message 4 corresponding to the compromised $K_{ab}$ and it will be accepted by $A$. The attacker can then impersonate $B$ communicating using the compromised key.