Security Issues in Mobile WiMAX(IEEE

802.16e)Frank, A Ibikunle

Covenant University, Electrical and Information Engineering Department, Ota.

2009 IEEE Mobile WiMAX Symposium

Wimax☺802.16x☺802.16d (fixed)☺802.16e (mobile)

☺802.16e ----3G : 韓國 (WiBro), 美國 , 中國大陸 ( 專利 )☺802.16m ---- 4G : 起步

台灣與全球頻譜與執照北區 南區 全區全球

Map: http://www.wimaxmaps.org/

和 WiFi 的不同http://www.youtube.com/watch?v=chlGqhRKVjQ

基本名詞ỚMain roles involved in 802.16:ỚBase Station (BS)ỚMobile Station (MS) / Subscriber

Station (SS)ỚTwo security protocols of interest:

ỚAuthentication/Authorization protocol, establishes a shared Authorization Key (AK)

Ớ3-way Traffic Encryption Key (TEK) Handshake

ỚAK used to derive various other keys

ỚTraffic Encryption Keys are used to encrypt message data between the BS and MS

ỚKEK: key encryption key

ỚMBS: Multicast and Broadcast Service

3-way TEK HandshakeAfter authentication, BS initiates a 3-way handshake to transfer TEKs to MS

TEKs generated by BSHave a specified lifetime, after which new TEK is requested by MS

Structure of the 3-way handshake:Challenge: BS → MSRequest: MS → BSResponse: BS → MS

DefineSecurity: protection of data being transmitted over a wireless networks.

CIAConfidentiality: Allowing only that the intended legitimate recipients to read encrypted messages(information).Integrity: is referred to as ensuring that another party has not altered messages after it has been sent.Authentication: This is making sure that parties sending messages or receiving messages are who they say they are, and have the right to undertake such actions.

On wired networks:VPNsIPSecIDSFirewalls…

Major ProblemSecuring wireless signal is in its mode of transmission communicated through the air.

802.16e provides 2 authentication

RSA加密演算法 : 因數分解難度和 bits 有關EAP(Extensible Authentication Protocol)

EAP常用在無線網路Data link layerPacketAn authentication framework: only define message format and has many methods.Using EAP can define a way to encapsulate EAP messages.Provides some common functions and negotiation of authentication methods called EAP methods.

MACMessage Authentication CodeLike Digital Signature, but using symmetry key.

Initial network1. EAP-based authentication.2. The BS and BS set up a common AK.3. KEK is derived from the AK. (and AK is

used to securely transfer further keys)4. MAC digest and the transferred TEK is

encrypted by the KEK.5. After above, the 3-way TEK-exchange for

each data connection is executed.AK – KEK - TEK

Security Flaws1. Unauthenticated messages2. Unencrypted management

communication3. Shared keys in Multicast and

Broadcast Service

Unauthenticated messages

Most of the management messages defined in IEEE 802.16e are integrity protected. Done by:

HMAC: Hash based message authentication codeCMAC: Cipher based message authentication code

But some messages are not covered by any authentication mechanism.

Unauthenticated messages1. MOB_TRF-IND: Traffic Indication message2. MOB_NBR-ADV: Neighbor Advertisement message3. FPC: Fast Power Control message4. MSC-REQ: Multicast Assignment Request message5. DBPC-REQ: Downlink Burst Profile Change

Request message6. PMC-REQ: 7. MOB_ASC-REP: Association Result Report8. RNG-REQ: Ranging Request message

Unencrypted management communicationIn Mobile WiMAX management messages are still sent in the clear.Nearly all management information exchanged between MS and BS can be accessed by a listening adversary.Collecting management info can create detailed profiles about MSs.

Including capabilities of devices, security settings, associations with base stations…

Also can determine the movement and approximate position of the MS.

Shared keys in Multicast and Broadcast Service

Encrypted symmetrically with a shared key.Also message authentication is based on the same shared key.This alg. Contains the vulnerability that every group member can also encrypt and authenticate broadcast messages as if they originate from the real BS(be forged).

Shared keys in Multicast and Broadcast Service

GTEK: Group TEKMBRA: Multicast and Broadcast Rekeying Algorithm

The Solutions1. For Unauthenticated messages:

1. Easily using a HMAC or CMAC digit as well.2. But most messages are very short, a

tradeoff between the security and the effectiveness of the protocol has to be found.

3. Suggestion: use CMAC and Short HMAC to 64bits, and with all other needed parameters (i.e., packet number, key sequence number and reserved fields), both will be 104 bits in total.

HMAC詳細請看 : http://en.wikipedia.org/wiki/HMACUse SHA-1 (128bits ?)SHA: Secure Hash Algorithm

CMAC詳細請看 :

http://en.wikipedia.org/wiki/CMAC , too.( 有點複雜 ......)Use AES-128 ( 感覺也很偉大…… )AES: Advanced Encryption Standard

A symmetric-key encryption standard adopted by the U.S. government.

2. For Unencrypted management communication:

1. Encrypt directly after both sides have established a common key.

2. Doesn’t introduce any overhead to the connection.

3. Possible to use a symmetric key, decryption can be processed very fast.

3. For broadcasted/multicasted messages(shared keys):

1. From outside the group.2. Use asymmtric cryptography(but may

not effective).3. And for the MBS, see Figures as

followings.

Figure 1. Possible solutions to transmit GTEK in a secure way

Figure 2. Avoiding key forgery by a GTEK hash chain

Contributions1. Describe different security

vulnerabilities found in IEEE 802.16e.

2. Proffer possible solutions to eliminate them.

ThanksQ & A