Authentication in LTE
Authentication is a process by which
both the UE and Network check whether the other party is authorized to
communicate with them.
In LTE and WCDMA, a mutual
authentication process is used i.e. the Network checks whether the UE is
authorized to camp on it and the UE also checks whether it is trying to camp on
is the correct Network.
The UE becomes authorized to camp on a
network by subscribing to the network.
In LTE, EPS AKA i.e. Authentication
Key Agreement procedure is used for Mutual Authentication.
Why
is Authentication required from both sides in LTE?
The UE needs to be authenticated by
the NW to check whether it has subscribed to the NW or not. Only UEs which have
a valid subscription should be allowed to camp on the NW.
The NW should also be authenticated by
the UE because if the UE does not check its authenticity, there is a risk that
the UE might camp on a fake NW (which has been designed to accept any UE) instead
of the one it is supposed to camp on.
What
is a difference between LTE authentication, NAS security and AS security?
LTE
Authentication: performs mutual authentication between UE
and NW.
NAS
Security: performs integrity protection/verification and
ciphering (encryption/decryption) of NAS signaling between UE and MME.
AS
Security: performs integrity protection/verification and ciphering
of RRC signaling and ciphering of user traffic between a UE and an eNB.
Give
a brief description of Authentication procedure in LTE?
The UE sends Attach Request to the MME
asking permission to camp on the Network.
In the Attach Request, the UE sends
its IMSI (or GUTI if the UE has camped on the NW before) as its identity.
The MME, upon receiving the request,
in turn requests Authentication Vectors from the HSS.
The HSS generates the following
Authentication Vectors using the EPS AKA Algorithm:
RAND
XRES
AUTNHSS
KASME
The HSS forwards these vectors to the
MME. The MME stores these vectors and
selects one of them to perform Mutual Authentication with the UE.
The MME forwards only RAND and AUTNHSS
to the UE.
The UE computes the RES, AUTNUE and
KASME using EPS AKA Algorithm.
The UE compares AUTNUE with
AUTNHSS to authenticate the Network.
If successfully authenticated, the UE
forwards the RES to the MME which compares it with the XRES received from the
HSS to authenticate the UE.
If the UE and the Network have successfully
authenticated each other, they share the same KASME.
Note: The KASME is
not transferred between the UE and the NW due to security reasons.
What
are the important Information Elements present in the Attach Request for
Authentication?
- IMSI: International
Mobile Subscriber Identity
- UE Network Capability:
security algorithms supported by
the UE
- KSIASME=7:
indicates that UE has no authentication key
What are the types of
Encryption and Integrity protection algorithms supported by UE?
The UE can support any of the
following EPS Encryption Algorithms and EPS Integrity Algorithms:
EEA
|
EIA
|
||
EEA0
|
Null ciphering algorithm.
|
EIA0
|
Null integrity protection algorithm.
|
128-EEA1
|
SNOW 3G.
|
128-EIA1
|
SNOW 3G.
|
128-EEA2
|
AES.
|
128-EIA2
|
AES.
|
128-EEA3
|
ZUC.
|
128-EIA3
|
ZUC.
|
What
is Authentication Information Request and when is it triggered?
When the MME receives an Attach
Request with KSIASME=7(111- the UE has no KASME available), it initiates Authentication
Information Request message to the HSS. The message contains the following
information elements:
- IMSI
- SN ID (Serving Network ID):
used to refer to the network accessed by the user. Consists of PLMN ID
(MCC+MNC).
- n (number of Authentication Vectors):
No. of authentication vectors requested by MME
- Network Type: type of the network accessed by UE (e.g. E-UTRAN)
What is the step taken by the HSS upon
receiving the Authentication Information Request?
On receiving the Authentication
Information Request message from the MME, the HSS takes the following
steps:
Generation of RAND and SQN, and
creation of XRES, AUTN, CK and IK using EPS AKA algorithm with LTE key (K), SQN
and RAND.
Derivation of a top-level key (KASME)
of the access network, using Key Derivation Function (KDF), and input elements
CK, IK, SQN and SN ID, to be delivered to the MME. KDF is a one-way
function.
Since SN ID is required when deriving
KASME, if the serving network is changed, KASME is derived again.
Formation of authentication vectors AVi=(RANDi,
AUTNi, XRESi, KASMEi), i=0..n.
All these vectors are delivered to the
MME in the Authentication Information Answer message.
What is the step taken by the MME on
getting the Authentication Information Answer message?
On receiving the Authentication
Vectors from the HSS, the MME:
stores them, and chooses one of them
for Mutual Authentication with the UE.
KASME - which is a base key of
MME and serves as a top-level key in the access network - stays within EPC only
and is not delivered to the UE through E-UTRAN, which is not secure.
Instead of KASME, the MME allocates
KSIASME, an index for KASME, and delivers it to the UE so that the UE and
the MME can use it as a substitute for KASME.
What Information Elements are
exchanged between the MME and the UE during the Mutual Authentication procedure?
The MME keeps with itself the KASME and
XRES in AV and delivers KSIASME, in substitution for KASME,
RAND and AUTN in the Authentication Request (KSIASME, RAND,
AUTN) message to the UE. XRES is used later in when authenticating
the user.
On receiving the Authentication
Request message from the MME, the UE delivers RAND and AUTN to
USIM. USIM, using the same EPS AKA algorithm used by the HSS, derives RES, AUTNUE,
CK and IK with the stored LTE key (K) and RAND and SQN generated from the
HSS. The UE then compares AUTNUE generated using EPS AKA algorithm and
AUTN received from MME to authenticate the network.
Once the UE has authenticated the
network, it delivers an Authentication Response message to MME, which
includes the RES generated using EPS AKA algorithm. If the network authentication
using AUTN fails, UE sends an Authentication Failure (CAUSE) message
that contains a CAUSE field stating reasons for such failure.
The MME, on receiving the Authentication
Response message from the UE, compares RES generated by the UE and
XRES of the AV received from the HSS to authenticate the user.
What process takes place in the UE
once it is authenticated by the Network?
The USIM delivers CK and IK to the UE
after its network authentication is completed. The UE derives KASME using
Key Derivation Function (KDF) with CK, IK, SQN and SN ID and stores it using
KSIASME which is received from the MME as its index. After this, KSIASME is
used instead of KASME during the NAS security setup between the UE and the
MME.
What happened for authentication
process when emergency bearer created by UE?
The UE has a PDN connection for
emergency bearer services established or is establishing a PDN connection for
emergency bearer services, the MME need not follow the procedures specified for
the authentication failure specified in the present sub-clause.
The MME may
respond to the AUTHENTICATION FAILURE message by initiating the security mode
control procedure selecting the "null integrity protection algorithm"
EIA0, null ciphering algorithm or may abort the authentication procedure and
continue using the current security context, if any.
The MME shall deactivate
all non-emergency EPS bearer contexts, if any, by initiating an EPS bearer
context deactivation procedure. If there is an ongoing PDN connectivity
procedure, the MME shall deactivate all non-emergency EPS bearer contexts upon
completion of the PDN connectivity procedure.
The network has considered the UE
to be attached for emergency bearer services only. If a UE has a PDN connection
for emergency bearer services established or is establishing a PDN connection
for emergency bearer services and sends an AUTHENTICATION FAILURE message to
the MME with the EMM cause appropriate for these cases (#20, #21, or #26,
respectively) and receives the SECURITY MODE COMMAND message before the timeout
of timer T3418 or T3420, the UE shall deem that the network has passed the
authentication check successfully, stop timer T3418 or T3420, respectively, and
execute the security mode control procedure.
If a UE has a PDN connection for
emergency bearer services established or is establishing a PDN connection for
emergency bearer services when timer T3418 or T3420 expires, the UE shall not
deem that the network has failed the authentication check and not behave as
described in item f.
Instead the UE shall continue using the current security
context, if any, deactivate all non-emergency EPS bearer contexts, if any, by
initiating UE requested PDN disconnect procedure. If there is an ongoing PDN
connectivity procedure, the UE shall deactivate all non-emergency EPS bearer
contexts upon completion of the PDN connectivity procedure.
The UE shall start
any retransmission timers (e.g. T3410, T3417, T3421 or T3430) if: - they were
running and stopped when the UE received the AUTHENTICATION REQUEST message and
detected an authentication failure; - the procedures associated with these
timers have not yet been completed. The UE shall consider itself to be attached
for emergency bearer services only.
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