3 G Security Principles Build on GSM

3 G Security Principles • Build on GSM security • Correct problems with GSM security • Add new security features Source: 3 GPP 1 Myagmar, Gupta UIUC 2001

GSM Network Architecture PSTN/ISDN MS Um MSC BTS BSC A-bis A OMC Voice Traffic Circuit-switched technology Mobility mgt VLR HLR AUC EIR 2 Myagmar, Gupta UIUC 2001

GSM Security Elements, 1 Key functions: privacy, integrity and confidentiality • Authentication Protect from unauthorized service access Based on the authentication algorithm A 3(Ki, RAND)=> SRES Problems with inadequate algorithms • Encryption Scramble bit streams to protect signaling and user data Ciphering algorithm A 8(Ki, RAND) => Kc A 5(Kc, Data) => Encrypted Data Need stronger encryption • Confidentiality Prevent intruder from identifying users by IMSI Temporary MSI Need more secure mechanism 3 Myagmar, Gupta UIUC 2001

GSM Security Elements, 2 • 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 4 Myagmar, Gupta UIUC 2001

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. 5 Myagmar, Gupta UIUC 2001

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 6 Myagmar, Gupta UIUC 2001

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 7 Myagmar, Gupta UIUC 2001

3 G Network Architecture Circuit Network Circuit Switch Circuit/ Signaling Gateway IN Services Feature Server(s) RNC Voice Radio Access Control Data + Packet Voice Mobility Manager Call Agent IP Core Network Packet Gateway Packet Network (Internet) IP RAN 2 G 2 G/2. 5 G 3 G 8 Myagmar, Gupta UIUC 2001

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 is based within the switch rather than the base station • IMEI Integrity mechanisms for IMEI provided from the start 9 Myagmar, Gupta UIUC 2001

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 of security is available Users can configure security features for individual services 10 Myagmar, Gupta UIUC 2001

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 11 Myagmar, Gupta UIUC 2001

Summary of 3 G 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 12 Myagmar, Gupta UIUC 2001

Summary of 3 G 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: 13 Myagmar, Gupta UIUC 2001

Summary of 3 G Security Features, 3 Generation of authentication data at HLR: 14 Myagmar, Gupta UIUC 2001

Summary of 3 G Security Features, 4 Generation of authentication data in USIM: 15 Myagmar, Gupta UIUC 2001

Summary of 3 G Security Features, 5 • Data 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 16 Myagmar, Gupta UIUC 2001

Summary of 3 G 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 17 Myagmar, Gupta UIUC 2001

Summary of 3 G Security Features, 7 • 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 resident on USIM should receive secure messages over the network • Visibility Indication that encryption is on Indication what level of security (2 G, 3 G) is available 18 Myagmar, Gupta UIUC 2001

Summary of 3 G 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 = c 3(CK, IK) random challenge RAND = c 1(RAND) signed response SRES = c 2(RES) GSM subscribers roaming in 3 GPP network are supported by GSM security context (example, vulnerable to false BTS) 19 Myagmar, Gupta UIUC 2001

Problems with 3 G 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 20 Myagmar, Gupta UIUC 2001

References • 3 G TS 33. 120 Security Principles and Objectives http: //www. 3 gpp. org/ftp/tsg_sa/WG 3_Security/_Specs/33120 -300. pdf • 3 G TS 33. 120 Security Threats and Requirements http: //www. arib. or. jp/IMT-2000/ARIB-spec/ARIB/21133 -310. PDF • Michael Walker “On the Security of 3 GPP Networks” http: //www. esat. kuleuven. ac. be/cosic/eurocrypt 2000/mike_walker. pdf • Redl, Weber, Oliphant “An Introduction to GSM” Artech House, 1995 • Joachim Tisal “GSM Cellular Radio Telephony” John Wiley & Sons, 1997 • Lauri Pesonen “GSM Interception” • http: //www. dia. unisa. it/ads. dir/corso-security/www/CORSO-9900/a 5/Netsec/netsec. html 3 G TR 33. 900 A Guide to 3 rd Generation Security ftp: //ftp. 3 gpp. org/TSG_SA/WG 3_Security/_Specs/33900 -120. pdf • 3 G TS 33. 102 Security Architecture ftp: //ftp. 3 gpp. org/Specs/2000 -12/R 1999/33_series/33102 -370. zip • 3 G TR 21. 905 Vocabulary for 3 GPP Specifications http: //www. quintillion. co. jp/3 GPP/Specs/21905 -010. pdf 21 Myagmar, Gupta UIUC 2001