Myagmar, Gupta UIUC G Security Principles Build on GSM security Correct problems with GSM security Add new security features Source: 3GPP

Myagmar, Gupta UIUC GSM Network Architecture BSC MS PSTN/ISDN BTS EIR AUC HLR VLR MSC OMC Um A-bis Circuit-switched technology Voice Traffic Mobility mgt A

Myagmar, Gupta UIUC GSM Security Elements, 1 Key functions: privacy, integrity and confidentiality Authentication Protect from unauthorized service access Based on the authentication algorithm A3(Ki, RAND)=> SRES Problems with inadequate algorithms Encryption Scramble bit streams to protect signaling and user data Ciphering algorithm A8(Ki, RAND) => Kc A5(Kc, Data) => Encrypted Data Need stronger encryption Confidentiality Prevent intruder from identifying users by IMSI Temporary MSI Need more secure mechanism

Myagmar, Gupta UIUC SIM A removable hardware security module Manageable by network operators Terminal independent Secure Application Layer Secure application layer channel between subscriber module and home network Transparency Security features operate without user assistance Needs greater user visibility Minimized Trust Requires minimum trust between HE and SN GSM Security Elements, 2

Myagmar, Gupta UIUC Problems with GSM Security, 1 Active Attacks Impersonating network elements such as false BTS is possible Key Transmission Cipher keys and authentication values are transmitted in clear within and between networks (IMSI, RAND, SRES, Kc) Limited Encryption Scope Encryption terminated too soon at edge of network to BTS Communications and signaling in the fixed network portion aren’t protected Designed to be only as secure as the fixed networks Channel Hijack Protection against radio channel hijack relies on encryption. However, encryption is not used in some networks.

Myagmar, Gupta UIUC Problems with GSM Security, 2 Implicit Data Integrity No integrity algorithm provided Unilateral Authentication Only user authentication to the network is provided. No means to identify the network to the user. Weak Encryption Algorithms Key lengths are too short, while computation speed is increasing Encryption algorithm COMP 128 has been broken Replacement of encryption algorithms is quite difficult Unsecured Terminal IMEI is an unsecured identity Integrity mechanisms for IMEI are introduced late

Myagmar, Gupta UIUC Problems with GSM Security, 3 Lawful Interception & Fraud Considered as afterthoughts Lack of Visibility No indication to the user that encryption is on No explicit confirmation to the HE that authentication parameters are properly used in SN when subscribers roam Inflexibility Inadequate flexibility to upgrade and improve security functionality over time

Myagmar, Gupta UIUC G Network Architecture Circuit/ Signaling Gateway 2G/2.5G 2G IN Services Call Agent Feature Server(s) RNC 3G Data + Packet Voice Circuit Switch Circuit Network Packet Network (Internet) Packet Gateway Radio Access Control Voice Mobility Manager IP Core Network IP RAN

Myagmar, Gupta UIUC New Security Features, 1 Network Authentication The user can identify the network Explicit Integrity Data integrity is assured explicitly by use of integrity algorithms Also stronger confidentiality algorithms with longer keys Network Security Mechanisms to support security within and between networks Switch Based Security Security is based within the switch rather than the base station IMEI Integrity Integrity mechanisms for IMEI provided from the start

Myagmar, Gupta UIUC New Security Features, 2 Secure Services Protect against misuse of services provided by SN and HE 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 ofsecurity is available Users can configure security features for individual services

Myagmar, Gupta UIUC New Security Features, 3 Compatibility Standardized security features to ensure world-wide interoperability and roaming At least one encryption algorithm exported on world-wide basis Lawful Interception Mechanisms to provide authorized agencies with certain information about subscribers

Myagmar, Gupta UIUC Summary of 3G Security Features, 1 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 cleartext when establishing TMSI

Myagmar, Gupta UIUC Summary of 3G Security Features, 2 Mutual Authentication 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. AKA process:

Myagmar, Gupta UIUC Summary of 3G Security Features, 3 Generation of authentication data at HLR:

Myagmar, Gupta UIUC Summary of 3G Security Features, 4 Generation of authentication data in USIM:

Myagmar, Gupta UIUC Summary of 3G Security Features, 5 Data Integrity Integrity of data and authentication of origin of signalling data must be provided The user and network agree on integrity key and algorithm during AKA and security mode set-up

Myagmar, Gupta UIUC Summary of 3G Security Features, 6 Data Confidentiality Signalling and user data should be protected from eavesdropping The user and network agree on cipher key and algorithm during AKA and security mode set-up

Myagmar, Gupta UIUC Summary of 3G Security Features, 7 IMEI IMEI is sent to the network only after the authentication of SN The transmission of IMEI is not protected User-USIM Authentication Access to USIM is restricted to authorized users User and USIM share a secret key, PIN USIM-Terminal Authentication User equipment must authenticate USIM Secure Applications Applications resident on USIM should receive secure messages over the network Visibility Indication that encryption is on Indication what level of security (2G, 3G) is available

Myagmar, Gupta UIUC Summary of 3G Security Features, 8 Configurability User configures which security features activated with particular services Enabling/disabling user-USIM authentication Accepting/rejecting incoming non-ciphered calls Setting up/not setting up non-ciphered calls Accepting/rejecting use of certain ciphering algorithms GSM Compatibility GSM user parameters are derived from UMTS parameters using the following conversion functions: cipher key Kc = c3(CK, IK) random challenge RAND = c1(RAND) signed response SRES = c2(RES) GSM subscribers roaming in 3GPP network are supported by GSM security context (example, vulnerable to false BTS)

Myagmar, Gupta UIUC Problems with 3G Security IMSI is sent in cleartext when allocating TMSI to the user The transmission of IMEI is not protected; IMEI is not a security feature A user can be enticed 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 SN 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

Myagmar, Gupta UIUC References 3G TS Security Principles and Objectives 3G TS Security Threats and Requirements Michael Walker “On the Security of 3GPP Networks” Redl, Weber, Oliphant “An Introduction to GSM” Artech House, 1995 Joachim Tisal “GSM Cellular Radio Telephony” John Wiley & Sons, 1997 Lauri Pesonen “GSM Interception” 3G TR A Guide to 3 rd Generation Security ftp://ftp.3gpp.org/TSG_SA/WG3_Security/_Specs/ pdf 3G TS Security Architecture ftp://ftp.3gpp.org/Specs/ /R1999/33_series/ zip 3G TR Vocabulary for 3GPP Specifications