LTE security , ciphering activation time

Question

you may wonder why this Q now.
Because coming from 3G RAN side and moving to 4G Core side, I am not upto date with LTE RRC.
Now trying to understand the 5G RAN security.
Before that wanted to fill the LTE RAN security procedure GAP.

Correct me if I am wrong, I know ciphering and integrity is moved to PDCP from RRC in LTE.
Then my Q is, why is there no "Ciphering Activation time " in the RRC Connection Reconfig (for security mode command) in LTE ?

Basically in 3G, RRC suspends the RLC to get the sequence numbers . And puts these SNs (may add some offset, so the SNs used to send SMC are not included) in the Security mode
command in the IE, "Ciphering Activation time"
How does this happen in LTE ?

thanks
Pdk

Answer 1

Hi,

There is no specific activation time for ciphering in LTE, it simple mutual understanding that once UE receives NAS_security mode command then UE responds with security_mode_complete with ciphered message (This is mutual understanding that MME should decrypt).

Same way once AS_security_mode_command received at UE side UE will respond with security_mode_complete with ciphered.

So there is no need of activation timer here, because once the security mode command comes UE will respond with ciphered message. if UE doesn't support the algorithm the UE respond with security_mode_reject.

Comments

Thanks a lot.
But not sure why is there such a concept in 3G ? Is it because in 3G, RLC does the ciphering and doesnot know exactly which SN ends the SMC (RLC does segmentation and concatenation also)?
But in LTE, PDCP does the ciphering and entire SMC contained in one PDCP SDU /PDU ?

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Yes, even i think the same.

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Thanks a lot for the clarification . Helped me a lot :)