Can we have same security algo's for AS and NAS layers ,if we use different security algorithms will attach successfull?

Answer 1

In Attach Request, UE sends "UE Network Capability" to MME . Based on the UE network capability, MME chooses integrity and ciphering algorithms for NAS level.
In "Initial Context Setup Request" message, MME sends the "UE Security Capabilities " to eNodeB and it is up to eNodeB what algorithms it selects for AS level security.

Conclusion : MME choose NAS level security algorithms and eNodeB does for AS level. MME and eNodeB both can select the same combination or different combination of integrity and ciphering algorithm. Different pairs of security algorithms will not affect the attach procedure.

That is my understanding.

Answer 2

=> In Attach Request, UE sends "UE Network Capability" to MME and MME get UE security parameters from HSS. So, if there is a mismatch between UE sends to MME and MME received from HSS, Then the received from HSS is final for NAS level security. MME chooses integrity and ciphering algorithms for NAS level.

=>In "Initial Context Setup Request" message, MME sends the "UE Security Capabilities " to eNodeB and it is up to eNodeB what algorithms it selects for AS level security.

Conclusion : MME choose NAS level security algorithms and eNodeB does for AS level. MME and eNodeB both can select the same combination or different combination of integrity and ciphering algorithm. Different pairs of security algorithms will not affect the attach procedure.