Why does authentication procedure need to complete before security activation ?

Question

I saw a scenario where an Identity is requested by MME after successful Authentication and Security mode complete.. As a Response to this message UE is Sending IMEI? what is the need of checking IMEI number after successful authentication? If IMEI is in block list then ongoing Attach procedure is going to be Terminate...? Identity Request intention is mainly to check the IMEI belongs to any one of Black, while or Green list then why cannt MME check for this before Security Mode?

Answer 1

On behalf of Bart -

I can see two reasons.

1st there is no sense in running the security operation for someone we don't know we should serve. The security operations always cost the CPU where the memory is cheap by the $.

2nd auth operation is just passing the keys and XOR operation (cheap from CPU point of view). Long story short it's better to do one operation to check if subscriber is allowed on this MME than two.

Answer 2

As Salil has mentioned there is no point in serving for someone who the operator need not support.
And other point which I see is for security activation the operator and UE should have a common key K from which the other integrity and cipher keys are generated. Operator will have this key information in HSS. So If the operator is not aware of the UE itself. How comeit will be aware of the Key and further activate the security.