2G/3G Authentication with SIM cards:

usage & roaming basics for the Internet challenged

Michael Haberler

Internet Foundation Austria

outline• a SIM card mini-tutorial

• features, protocol flow, usage, production, addressing

• UMTS authentication and key agreement• principles and protocol flow

• the universal integrated circuit card (UICC)• USIM app

• how 2G, 3G roaming works• „over the air“ (OTA) loading of UICC apps

• example: X.509 certificate download

• (U)SIM‘s and Internet access authentication – how SIMs and RADIUS roaming works

• (U)SIM‘s and SIP authentication – what the SIP server does

• How the parameter logistics works• a bonus business model thrown in• summary

what‘s a 2G SIM card• crypto smart card as per ISO 7816• access protected by a PIN code(s) („card holder verification“)• fixed storage of subscriber identity – IMSI (international mobile subscriber

identity) – „GSM MAC address“– E.164 number to IMSI mapping at the operator only

• safe storage for shared secret - accessible only through CHAP operation– not broken as of today except for most stupid CHAP algorithm known

• CHAP algorithm in hardware– operator chooses algorithm

• tree structured filesystem– stream, record, cyclic record files– can be readonly, read/write or none at all (for the key)

– some permission hierarchy

how are SIM cards produced• unprogrammed chips are „personalized“ and „closed“

(parameters written & sealed) • mass product - $5-$7 apiece at 1000+

– GEMplus, Giesecke & Devrient ....

• everybody can have SIM‘s made – even Mom&Pop ISP• not everybody may

– roam with other cellular operators– use the GSM algorithm „A3/A8“ – you wouldnt want it anyway– must be member of GSM association for that

• having your own algorithm in a chip mask is a circa $50K+ affair

• for testing & development unprogrammed castrated chips used (XOR algorithm for CHAP...)

how are (U)SIM cards accessed

• 2G, 3G use– builtin reader in the mobile handset

• for Internet use:– maybe builtin in PDA, PC (e.g.DELL)– external USB token – 20$ apiece– re-use a mobile SIM card via Bluetooth SIG SIM Access

Profile (only if roaming against 2G/3G operator)

• read 3G „(U)SIM Security Reuse by Peripheral Decices on local interfaces“ – contains some threat analysis

SIM usage in 2G authentication

2G GSMhandset

keys

access request – present IMSI

present challenge („RAND“)

send RESP (challenge response)

Authentication

Center

shared secret

IMSI structure

T0207420-98

MCC MNC MSIN

IMSI

MCC Mobile Country CodeMNC Mobile Network CodeMSIN Mobile Subscriber Identification NumberIMSI International Mobile Subscriber Identity

Threedigits

Two to threedigits

Maximum of tendigits

Maximum of fifteen digits

• MCC/MNC uniquely designates an operator and his authentication center

• when roaming, MCC/MNC tells the visiting network where to route the authentication request

• this is done via SS7 MAP (mobile application part)

what is „OTA“ (over the air) loading?• SIM cards are writable by mobile equipment

– if authenticated to network– if instructed by operator „over the air“– if file/directory is writable

• example: ISIM X.509 certificate „bootstrap“– AKA authenticated:

• let user visit PKI portal• download certificates through HTTP/Digest mechanism• certificates are stored in record structured files, as ar CA certifcates

• „The Air“ can also be an IP connection• download of executable applets possible

– SIM Toolkit, USAT (USIM Application toolkit)– bytecode instructions sent encrypted by 3DES, stored on card

• regularly used in 2G networks today – for functionality upgrades & parameter download

UMTS authentication and key agreement (AKA)

• substantially improved over 2G SIM

• protection against replay, MITM attacks

• sports also network-to-user authentication

• more complex algorithm

• compatibility functions 2G network/3G card, 3G network/2G card

3G AKA authentication flow

3G UMTShandset

keys

access request – present IMSI

challenge RAND || AUTN token

send RESP (challenge response)

Authentication

Center

shared secret,Sequence numbersresult:

Cipher keyIntegrity key

what‘s the universal integrated circuit card (UICC) about

• generic support mechanism for multiple applications on one card

• 2G,3G authentication become „applications“ selected as needed– USIM application implements AKA– 2G SIM app implements 2G CHAP– additional apps possible (ISIM, PKI certificate

storage etc)– ISIM is pretty close to SIP client needs!!

• mobile equipment chooses application

using (U)SIMs for Internet access authentication

• embed flow in EAP and tunnel in RADIUS• between 802.1x „supplicant“ in client and RADIUS

EAP backend using EAP-SIM or EAP-AKA• RADIUS server MAY gateway to SS7 MAP and

„roam“ – WiFi network looks like a GSM roaming partner– example: WiFi roaming through www.togewanet.com

• OR RADIUS server access an ISP-style database for keys – ISP is the SIM card issuer!

using (U)SIM for SIP authentication

• speak HTTP/AKA (RFC3310) between SIP UA and proxy• proxy translates into EAP-AKA-in-RADIUS• RFC specified only for AKA (3G auth)• no mapping of EAP-SIM onto HTTP/SIM for 2G auth• bad – almost all networks today use 2G auth – which

breaks SIP authentication through GSM/UMTS operators• we need to address this and spec HTTP/SIM

how 2G roaming works

• mobile equipment presents IMSI• visited network looks at MCC,MNC part of IMSI

– if no roaming agreement, drop him– otherwise send access request thru SS7 MAP to home

network– the home network verifies IMSI and sends a „triplet“:

(challenge, expected response, cipher key) authentication vector

– visited network presents challenge, reads response– if (response == expected response), service user

• the triplet is essentially an access ticket– note no replay detection – these fellows seem to trust each

other

how 3G roaming works

• not much different from 3G, just more parameters needed for AKA

• „triplets“ become „quintets“

how the 2G/3G user ids (IMSI‘s) are mapped to RADIUS authentication:

• take mobile country code, mobile network code• use them to create a realm• Example

– IMSI = 232011234567890• means mcc=232 (Austria) mnc=01 (Mobilkom)

– resulting realm• mnc01.mcc232.owlan.org

– resulting RADIUS user• 232011234567890@mnc01.mcc232.owlan.org• routing to Radius servers decided by „subdomain“

• convention established by Nokia• Nokia owns owlan.org domain pro-bono

from thereon this is vanilla RADIUS roaming• but its just fine if we call it mnc01.mcc232.visionNG.org if that sounds

better, realms just gotta be unique

how does 2G/3G address logistics work

• if you are a service provider and have E.164 ranges, get a MNC from your MCC administrator (FCC, regulator...)

• the E.164 range might also be, for example, from visionNG (+87810 ff) MCC = 901

• this doesnt mean you‘re part of 2G/3G roaming yet – contracts & regulatory prerequisites needed

• but the addressing is all set to go!!

a bonus business model thrown in:

• combine a SIP-based iTSP with a Mobile Virtual Network Operator (MVNO)– an MVNO has authentication, billing, customers, numbers,

but the radio network is outsourced from somewhere else

• issue (U)SIM cards which work both in a 2/3G handset AND as WiFi/SIP auth tokens – note the same card authenticates both uses!

• leave choice to user how to connect – Internet or cellular – using the same E.164 number

Summary

• 2G/3G has a strong/very strong authentication architecture• it is almost copy & paste for iTSP use at WiFi access, WiFi

roaming acces, SIP and other levels (TBD!)• it can serve to solve the X.509 certificate distribution problem• operator model (2G/3G home network, ISP home network) has

no impact on Internet-side terminals• numbering & addressing resources are compatible and

available (maybe not obviously so)• the Internet could become the biggest (U)SIM authenticated

mobile network ever to roam with 2G/3G land