1

Anonymous and Authenti-cated Key Exchange for Roaming Networks

Guomin Yang et al.IEEE Transactions on Wireless Communication Vol. 6 No. 9 September 2007

2

Agenda

Preliminaries Introduction Security requirements Proposed Scheme : AAKE-R Analysis Conclusion

3

Preliminaries(1/9)Basic requirements of network security

Data Confidentiality – keeping infor-mation secret from all but who are authorized to it

Eavesdropping

A B

C

4

Preliminaries(2/9) Basic requirements of network security

Authentication – corroboration of identity of entity

Impersonation

A B

C

5

Preliminaries (3/9) Symmetric key cryptosystem

a.k.a Secret key cryptosystem Symmetric encryption for data confidentiality

Message Authentication Code (MAC) for Authentication

6

Preliminaries (4/9) Asymmetric key cryptosystem

a.k.a Public key cryptosystem Asymmetric encryption for data confidentiality

Digital signature scheme for Authentication

7

Preliminaries (5/9) Symmetric vs Asymmetric

Symmetric key cryptosystem

Asymmetric key cryptosystem

Computation speed Fast Slow

Key distribution Difficult Easy

The number of entire keys

n(n-1)/2 2n

Comparison table

8

Preliminaries (6/9) Authenticated Key Exchange (AKE)

Key distribution in symmetric setting is a problem

Two different types of key

Long-term key ▪ Set up initial key for each entity▪ Key Pre-distribution System

Session (short-term) key ▪ After long-term key set up, share secret information among 2 or

multi entities▪ Key Establishment System

Authenticated key exchange is a solution to estab-lish session key

9

Preliminaries (7/9) Authenticated key exchange (AKE)

In asymmetric setting, two entities au-thenticate each other and establish ses-sion key using digital signature scheme.

Key transport: one party creates and transfers it to the other(s)

Key exchange: a shared secret is derived by two or more parties as a function of information contrib-uted by. No party can determine the resulting value.

10

Preliminaries (8/9) Diffie-Hellman key exchange

To authenticate each other, these values should be signed using digital signature scheme

11

Preliminaries (9/9) Cryptographic hash function

A cryptographic hash function is a trans-formation that takes an input and returns a fixed-size string, which is called the hash value

One-wayness – calculating H(x) = y is easy, but given y, to find x is difficult

Collision free – Two different x1, x2 cannot have the same hash value y

12

Introduction (1/2) Roaming network

A technology lets a user originally sub-scribed to a network can travel to another network administrated by a different opera-tor and access services provided by this network as a visiting user or a guest

User can enjoy a much broader coverage in terms of services or geographical areas without being limited by that of their own networks

13

Introduction (2/2) Roaming network

Home server Foreign server

Roaming user

Home service area Foreign service

area

14

Security requirements

Server Authentication – The user is sure about the identity of the foreign server

Subscription validation – The foreign server is sure about the iden-tity of the home server of the user

Key Establishment – The user and the foreign server establish a random session key which is known only to them and is derived from contributions of both of them. In particular, the home server should not obtain the session key

User Anonymity – Besides the user and the home server, no one including the foreign serve can tell the identity of the user

User Untraceability – Besides the user and the home server, no one including the foreign server is able to identify any previous protocol runs which have the same user involved

15

Proposed AAKE-R(1/5)Notation

Notation table

16

Proposed AAKE-R (2/5) Building blocks

AKE (Authenticated Key Exchange)

AAKE (Anonymous Authenticated Key Exchange)

AKT (Authenticated Key Transport)

17

Proposed AAKE-R (3/5) Assumption

There is a direct link between roaming user and foreign server and another direct link between home server and foreign server

Roaming user know the public key of foreign server

Each user knows its home server’s public key and each server knows the public keys of all its sub-scribers

All servers know the public keys of all other servers in roaming network

18

Proposed AAKE-R (4/5)Proposed scheme

19

Proposed AAKE-R (5/5) optimized version

20

Security analysis

Server AuthenticationSubscription validation Key EstablishmentUser Anonymity & User traceabil-ity

21

Comparison with other pro-tocols

22

Conclusion

A secure and generic AAKE-R construction using AAKE and AKT as building blocks

It satisfies the security requirements of AAKE-R suggested by the authors

23

Weak points

User privacy violation – The home server can track roaming user

They do not suggest detailed performance evaluation. I think the overhead is big due to several asymmetric computation

24

Future work

Addressing user tracking problem by home server

Study of additional requirements such as supporting differentiated access

Try to find a way to reduce the number of asymmetric computation modifying AAKE-R or design novel AAKE-R that has lower computation overhead even though it satis-fies same requirements