1. Mobile Communications Mobile Security Fundamentals-3
– v4
W. Adi
Lecture-8
Mobile Security Fundamentals-3

2. Advanced Encryption Standard
AES
Advanced Encryption Standard
Proposed for 3G Mobile Authentication Functions
International Standard competition managed by NIST:
US National Institute of Science and Technology
AES Winner Algorithm:
The Rijndael Block Cipher, Decision Oct. 2000

3. AES Round-3 Finalist Algorithms
(finalized in 2001)
MARS : IBM (USA)
RC : R. Rivest (MIT), creator of the widely used RC4 (USA)
Twofish : Counterpane Internet Security, Inc. (USA)
Serpent : Ross Anderson, Eli Biham and Lars Knudsen (USA)
Rijndael: Designed by J. Daemen and V. Rijmen (Belgium)
Joan Daemen (of Proton World International)
Vincent Rijmen (of Katholieke Universiteit Leuven).

4. Rijndael: Basic concept
Key
Key Expansion
Round Keys K1 K2
... K9
K10 X
... R1 R2 R9
R10 Y
10 Encryption Rounds R1 … R10

5. Rijndael: Basic Encryption Round Functions
Byte sub a3 ..
a16 a1 a2
b16 b3 b2 b1
/ 8 bits
b = [M] a C
The only non-linear mapping !
A is 4x32 bits
Transposition B
Mix column
Linear mapping
B = [C] A
Round-Key Ki= 128 bits +

6. Security of AES/ Rijndael
Published to the scientific community 1998
Is still not broken !!
- No proof that Rijndael can not be broken !!

7. Important Lessons in Security Business
2nd Generation security lessons
Experts learned over the years that
the only way to assure security is:
follow an open design process
encourage public scientific review
Nobody is better than the rest of the research community.

8. New 3G Security Features 1/2
Network Authentication
The user can provably identify the network
Network Security
Mechanisms to support security within and between networks
Switch Based Security
More secrecy switch based rather than only to base station
IMEI Integrity
Integrity mechanisms for IMEI provided from login
Secure Services
Protect against misuse of services provided by Service Network
and Home Environment

9. New 3G Security Features 2/2
Secure Applications
Provide security for applications resident on USIM
Fraud Detection
Mechanisms to combating fraud in roaming situations
Flexibility
Security features can be extended and enhanced as required by new
threats and services
Visibility and Configurability
Users are notified whether security is on and what level of security is available. Users can configure security features for individual services
Lawful Interception
Mechanisms to provide authorized agencies with certain information
about subscribers

10. 3G User Confidentiality
Permanent user identity IMSI, user location, and user services cannot be determined by eavesdropping
Achieved by use of temporary identity (TMSI) which is assigned by VLR
(IMSI is sent in clear text when establishing TMSI)
Network
Mobile
Visiting Location Register

11. Mutual Authentication Mechanism 1/2
During Authentication and Key Agreement (AKA) the user and network
authenticate each other, and also they agree on cipher and integrity key
(CK, IK). CK and IK are used until their time expires.
Assumption: trusted HE and SN, and trusted links between them.
After AKA, security mode must be negotiated to agree on encryption and integrity algorithm.

12. 3G Mutual Authentication Mechanism 2/2
Generation of authentication
data at “Mobile” site
Generation of authentication
data at “Home Network” site
: Authentication Token
AES
K: subscriber seret key
SQN: Seuence Number
AK:Authentication Key
CK:Cipher Key
IK:Integrity Key
MAC: Message Authentication Code

13. 3G Data Integrity Mechanism
Integrity of data and source authentication of signaling data must be provided. The user and network agree on integrity key IK and algorithm during AKA and security mode set-up
KASUMI
Message authentic if equal

14. 3G Data Encryption Mechanism
Data Confidentiality
Signaling and user data should be protected from eavesdropping.
The user and network agree on cipher key CK and algorithm during AKA and security mode set-up
KASUMI

15. Problems with 3G Security
IMSI is sent in clear text when allocating TMSI to the user
The transmission of IMEI is not protected; Equipment identity is still not secured
A user can be brought to camp on a false BS. Once the user camps on the radio channels of a false BS, the user is out of reach of the paging signals of the network
Hijacking outgoing/incoming calls in networks with disabled encryption is possible. The intruder poses as a man-in-the-middle and drops the user once the call is set-up

16. Public Key Cryptography
Fundamentals of
Public Key Cryptography
Published 1976 by (Diffie &Hellman) at Stanford University
Breakthrough: Proved for the first time that it is
possible to share secrets without secret agreement
Many 3G mobile security applications in user layer
are expected to employ public key cryptography
(Mobile Commerce, mobile IP applications ...)

17. Secret Key Cryptography
(Symmetric System)
K-open = K-close
- Open and close with the same key !!
- Secret Key Agreement required

18. Public-Key Secrecy Systems
K-open
K-secret
- Open and close with different keys!!
- No Secret Key Agreement required
Two Major Schemes in Public Key Cryptography:
Diffie-Hellman Public Key exchange scheme
RSA public Key secrecy system

19. A B Public-Key Cryptography Breakthrough 1976
(Diffie-Hellman)
Shared Secret without exchange of secrets “Mechanical Scenario”
Open Register A B
Secret key-A
Secret key-B
injection
injection
SHIELD
! Same thing !
Shared Secret

20. How to “publicly” hide (shield) a secret ?
shielded secret
SHIELD = One Way Function
( commutative ! ) 6 9
How:
2 6 mod 11 = 9
log2 9 (mod 11) = 6
Discrete logarithm : no formula is known to compute log2 modulo 11 !

21. ( ) ( ) A B Example for Diffie-Hellman key exchange scheme 1976
Widely use in internet and banking ...
Open Agreement and Register
Shielding function is: y = (5 x) mod 7 A B
Secret key-A= 3
K-open-A= 6
5 3 = 6
K-open-B= 3
5 5 = 3
Secret key-B= 5
( ) 5
( ) 3
5 3
5 5
5 5 3
Shield
5 3 6
5 5.3
5 3.5
! same thing !
Z = 6

22. Basic Public Key Secrecy System (RSA system)
(Mechanical simulation: user B wants secured message from A)
User A
User B
Public register
Ko= Kc-1
close
open
( )Kc (mod m) Kc M
MKc.Ko = M
(MKc)Ko Ko
MKc

23. Mathematical Model of a Public-Key Crypto-system
(using asymmetric keys)
Sender
Receiver
Y = E (Zp,X) X X
Message
Message
E ( Zp,X )
D ( Zs,Y )
Channel
Secret-Key Zs
Public-Key Zp
Public Directory
Z.. Zp
Z... Zs

24. Cryptographic Protocols No Key Cryptography : Shamir 3-Pass Protocol
User A
User B A
Pass 1 A B A A B A A A B
Pass 2 A B A B B B A B
Pass 3 B B

25. ( ) ( ) ( ) Omura-Massey Lock* for: Shamir’s 3-Pass Protocol User A
Secrecy without Authenticity
User A
User B
p = large prime
All computations modulo p
Eb = secret key
Db = Eb-1
Ea = secret key
Da = Ea-1 Ea M
( ) Eb 1
= M Ea M
Ea Eb M 2 Da
( )
Ea Eb Db
( ) 3 M Eb Eb M
= M M
* J.L. Massey & J. K. Omura, US Patent, 1986

26. Non-Perfect Secret Sharing
Secret
10100
10010
Part A
Part B
10100
10010
Secret

27. Perfect Secret Sharing
Example: share the secret between users A and B
11101
Random
BSS
10100
Secret
01001
User B +
11101
User A
10100 +
10100 +
Common Secret