1、CELLULAR TELEPHONE NETWORK SECURITY,Ari Vesanen, ari.vesanenoulu.fi Department of Information Processing Sciences, University of Oulu,Contents,Introduction to GSM GSM network structure and properties GSM network security model GSM network security threats GPRS vs. GSM Security UMTS vs. GSM Security,
2、Introduction to GSM,GSM worlds most widely used cellular phone system About 1000 million users First digital cellular phone standard 1982 GSM (Groupe Special Mobile) committee to create standard 1989 ETSI (European Telecommunications Standards Institute) responsible for development 1990 first specif
3、ications frozen,GSM specifications developed secretly No public evaluation according to scientific procedure Kerckhoffs principle violated: Algorithm strength should depend on secrecy of key and not on the secrecy of the algorithm itself GSM specifications and encryption algorithms have leaked and b
4、een subject to criticism,GSM Network Structure,Mobile station MS,SIM,PHONE,BTS,BTS,BSC,BSC,HLR,AuC,EIR,VLR,Base Station subsystem BSS,Network Switching Subsystem NSS,MSC,PLMN, PSTN, .,Abis,Um,A,Mobile Station = phone + SIM SIM = Subscriber Identity Module User identity IMSI (International Mobile Sub
5、scriber Identity) on SIM MSISDN (Mobile Subscriber International Integrated Services Digital Network) number = Phone number on SIM Phone identity IMEI (International Mobile Equipment Identity) in phone Got from phone: type *#06#,BSS components: Base Transceiver Station (BTS) and Base Station Control
6、ler (BSC) BTS controls radio communication with phone, encrypts calls and does decryption BSC can control several BTSs, tasks Initialization of radio channel Frequency hopping Handover (transferring user between cells) Traffic between BSS and MSC,NSS = MSC + SMSC + Registers (+ OSS) Mobile Services
7、Switching Centre (MSC) Main component of NSS Works as link to wired network Services for registering and authenticating mobile user Services related to mobility Short Message Service Centre (SMSC) Transmission of short messages Needs routing information - works in co-operation with HLR,HLR (Home Loc
8、ation Register) Information on subscribers registered in this GSM network Current location of users (location networks VLR address) One network can contain only one HLR VLR (Visitor Location Register) Relevant information on all active users in GSM network AuC (Authentication Center) User secret key
9、 information by IMSI EIR (Equipment Identity Register) Valid equipments by their IMEI code,GSM Network Radio Interface,Band control: combined TDMA/FDMA FDMA divides band into 200 kHz wide channels GSM 900 124 channels GSM 1800 374 channels Channels grouped and distributed to operators Carrier freque
10、ncy into time frames according to TDMA model TDMA frame = eight time intervals (slots) Message in one slot = burst Logical channel = one slot in one frame,Frequency hopping 216,7 hops/second After each burst frequency changed according to predefined pattern Spreads disturbances Makes eavesdropping m
11、ore difficultTDMA/FDMA model technically challenging,Establishing Call,Updating location Uses MSC, HLR and VLR When MS moves to new location area or to new operator area - must register for update Location update message to new MSC/VLR pair that registers new information and sends it to subscribers
12、HLR. HLR sends the previous VLR information that subscriber left its area,Phones home MSC,Phones location MSC,Incoming call,HLR,VLR,BTS,BSC,MS,Call Routing,1,6,2,3,4,5,GSM Network Security Model,Identification of subscriber IMSI IMSI consists of three components: Mobile Country Code (MCC) Mobile Net
13、work Code (MNC) Mobile Subscriber Identity Number (MSIN) TMSI temporary identifier, used instead of IMSI in communication Changed when location changed Makes IMSI capturing and subscriber communication monitoring more difficult,Authentication Actors: SIM card and (home networks) Authentication Cente
14、r (AuC) Authenticates user to network (not vice versa) Based on secret 128 bit key Ki (resides only on SIM and in AuC) Authentication always in home network! Authentication algorithm may be changed, yet works in visited networks Authentication method challenge-response Algorithm A3,MSC,HLR,AuC,MS,Re
15、gister to network,6. Check SRES,4. RAND,5. SRES,2. Request authentication triplet,3. Authentication triplet (RAND,SRES,Kc),Authentication in GSM Network,SRES = A3(RAND,Ki) Kc = Air interface encryption key,Air interface encryption Encryption algorithm A5 must reside in phone, for all network operato
16、rs common algorithm Key generated using algorithm A8 on SIM, hence may be operator specific Uses (64 bit) session key Kc = A8(RAND, Ki) and (22 bit) TDMA frame number A5 stream cipher, re-synchronized for each frame Kc rarely updated (in connection with authentication) Only air interface encrypted i
17、n GSM network, no encryption in operator network Relied on physical security,MS (A),BTS (B),Air Interface Encryption in GSM Network,A5,A5,Kc (64 bit),Frame no (22 bit),Kc (64 bit),CIPHER A-B,XOR,XOR,PLAIN A-B,CIPHER B-A,PLAIN B-A,XOR,XOR,PLAIN B-A,PLAIN A-B,Frame no (22 bit),114 bit,114 bit,114 bit,
18、114 bit,Algorithms,SAGE group under ETSI designed algorithms Composition secret A3, Device authentication algorithm Takes as parameters 128 bit key Ki and random number RAND, computes 32 bit fingerprint, SRES. Almost without exception: COMP128 algorithm used both as A3 and A8 COMP128 proposed in GSM
19、 specification,A8 air interface encryption key generation algorithm Mostly COMP128 Takes as parameters 128 bit key Ki and random number RAND, computes 64 bit session key Kc Kc used until MSC decides to re-authenticate device Both A3 and A8 on SIM card Operator can decide algorithms Authentication do
20、ne in subscribers home network - local network does not have to know algorithms, yet authentication works also when user roams,COMP128 not public, found out using SIM cards and leaked specifications http:/www.iol.ie/kooltek/a3a8.txt (Marc Briceno, Ian Goldberg and David Wagner) implementation Publis
21、hed in April 1998 Produces both SRES and Kc in one run Upper 32 bits SRES Lowest 54 bits + 10 zeros Kc - effectively Kc is 54 bit!,A5 Air Interface Encryption Algorithm,Stream cipher algorithm ”Original” European algorithm A5 leaked in general already in 1994, details in May 1999 (Briceno from GSM p
22、hone) Initialized each sent frame Key Kc used during call, but 22-bit frame number changed,European A5 Three feedback shift registers (LFSR = Linear Feedback Shift Register) of different lengths Register lengths 19, 22 and 23 bits Register values XORed and obtained bit XORed with plaintext bit Regis
23、ters initialized using session key Kc and frame number After initialization 228 bits pseudo random bit stream formed: 114 first bits to encrypt frame from device to base station, rest 114 bits from base station to device Cf. http:/cryptome.org/a51-bsw.htm,| | | | | | | | | | | | | | | | | |,| | | |
24、| | | | | | | | | | | | | | | | |,| | | | | | | | | | | | | | | | | | | | | |,XOR,XOR,XOR,XOR,R1 (19),R2 (22),R3 (23),A5 - cipher,18,13,C1,C2,21,22,C3,7,Rotation: Majority of C1,C2 and C3,0,0,0,Algorithm in many forms, original A5/1 Stronger than other A5/x s A5/0 = No encryption A5/2 decidedly weak
25、ened form (used e.g. in USA) Published and analyzed in August 1999 (very weak)Other A5/x s not become public (if any),GSM Network Security Defects,Network not authenticated Faking base station principally possible Algorithm weaknesses Both A5 and COMP128 defective Data integrity not checked Makes al
26、teration of data possible,Authentication data transmitted in clear both inside and between networks Contains also air interface encryption key Lack of visibility User can not know whether encryption used or not No confirmation to home network, whether serving network uses correctly authentication pa
27、rameters when user roams,Threats,Attacks against A5 A5 implementation (Mike Roe): http:/ Breaking air interface encryption - call eavesdropping Many methods proposed for breaking A5: Almost practical attack by Golic: ” Cryptanalysis of Alleged A5 Stream Cipher” cf. http:/ Birthday attack type time/m
28、emory -optimization,Attack applicable in real time: Biryukov, Shamir and Wagner (cf. http:/cryptome.org/a51-bsw.htm): Real time break algorithm on PC against the strong algorithm A5/1 Basic assumption: Attacker knows or guesses part of bit stream produced by cipher Basic idea: Great number of pre-co
29、mputed states stored (possible, since feedback registers can only be in 264 different states) Idea by Golic,Key can be deduced from initial state of each frame A5/1 can be effectively implemented on PC (each register small enough to store their states in computers memory as three cyclic arrays) A5/1
30、 can be run backwards effectively However, backward computation not entirely deterministic: one state can be arrived at from several states,Suitable 16-bit number alpha in advance chosen and only frames that include alpha considered The number of register states producing alpha is about 248 States c
31、omputed in advance and stored on disk - attack demands large amount of space Three different attacks (all require at least two 73GB hard drives),Estimate: First type attack (”biased birthday attack” two versions), needs about 2 minutes of call data Alpha appears sufficiently many times (ca. 71) in d
32、ata Direct collision with disk data and cipher data Encryption broken in one second Third type attack (”random subgraph attack”): call data 2 seconds Performing attack takes minutes No crypto attack carried out in practice (presumably),SIM card cloning (by physical contact) Subscribers secret key on
33、 SIM and security depends on this key - if attacker obtains SIM security can be broken An identical copy of SIM can be made If card noticed missing, it can quickly be shut out of services If copy and original simultaneously used, network notices and invalidates both In principal cloned card can be u
34、sed such that subscriber is billed,Revealing key Ki from SIM Based on weakness of COMP128 Inventors: SDA (Smartcard Developer Association) and ISAAC (Internet Security, Applications, Authentication and Cryptography) Cf. http:/www.isaac.cs.berkeley.edu/isaac/gsm-faq.html Flaw in algorithm - informati
35、on on Ki obtained by giving suitable random number inputs RAND as an argument to A8 Input RAND slightly changed and observed when identical answer obtained 217.5 inputs enough to deduce Ki,Test attack: SIM in card reader attached to PC; PC generated 150 000 challenges, using which SIM computed SRES
36、response and session key Kc - based on information Ki computed. Took ca. 8 hours April 1998 Used attack technique standard -like Cf. e.g. Serge Vaudenay ”FFT-Hash-II is not yet Collision-Free” http:/lasecwww.epfl.ch/pub/lasec/doc/liens-92-17.A4.ps,SIM cloning over-the-air ISAAC: According to experts
37、 possible in practice (faking base station) Cf. http:/www.isaac.cs.berkeley.edu/isaac/ gsm.html Type 1: Attacker builds fake base station, covering subscribers valid BTS - Subscribers SIM may be bombed with self-generated authentication requests,Estimate: Attack duration 8 13 hours, victim device ha
38、s to be in operating area of fake base station (not necessarily continuously) Subscriber can not detect attack Enhanced version of COMP128 exists (COMP128-2) Some operators use Not (known to be) brokenType 2: Attack from legal network Client outside home network (e.g. abroad) Attacker inside locatio
39、n network,Building fake (rogue) base station Cost estimate 10 000 euros Can capture IMSI Gathered information might be used in networks with more loose authenticationCounter: Temporary identifier TMSI, changed when subscriber location updated TMSI not entirely prevents IMSI capture since IMSI has to
40、 be sent once Also other attacks (e.g. mentioned SIM cloning),Cell change in GSM network Phone sends audibility reports to BTS BTS adds own information and sends to BSC BSC cell change request to MSC (if necessary) MSC resource allocation request to new BSC, that waits for MS to arrive New BSC send
41、acknowledgement to MSC that sends cell change command to old BSC, this forwards it to MS MS breaks connection to old base station and continues with new one,How to hook up a phone to my fake base station? Item 5: Cell change command from the network - Attacker may simulate command and force the phon
42、e to change No authentication for base stations - Device can not know communicating with a rogue base station,GPRS vs. GSM Security,GPRS transition phase to 3G, supports packet switched traffic Voice (circuit switched traffic) as in GSM GPRS data uses multiple slots Air interface encryption (differe
43、nces with GSM) New A5 algorithm GEA Yet secret GPRS traffic encryption extends further (base stations cannot cope with traffic using several slots),Authentication (differences with GSM) Separate authentication for circuit switched and packet switched traffic Packet switched backbone has own security
44、 features Not considered here,UMTS design applies open standardization Specs: 3GPP ( 3rd Generation Partnership Project) WWW site http:/www.3gpp.org, contains specifications etc. Cf. TTAE.3G-33.102 ”3G Security; Security Architecture” UMTS network constructed on (and parallel to) existing GSM networ
45、ks - Security model constructed on GSM security model,UMTS vs. GSM Security,Authentication method as in GSM Based on a secret key K, residing only on USIM and in home network AuC Comparison: in GSM network authentication vectors triplets (RAND, SRES ,Kc) in UMTS network quintets (RAND, XRES, CK, IK,
46、 AUTN) IK integrity key for data integrity AUTN authentication token for network authentication,Improvements to GSM security Encryption algorithms use longer keys Network also authenticated Signaling data authenticated and integrity checkedUMTS GSM compatible GSM users have GSM context GSM users have practically GSM security in UMTS network,
