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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

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2g 3g authentication with sim cards usage roaming basics for the internet challenged

2G/3G Authentication with SIM cards:usage & roaming basics for the Internet challenged

Michael Haberler

Internet Foundation Austria

outline
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
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
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 $50K+ affair
  • for testing & development unprogrammed castrated chips used (XOR algorithm for CHAP...)
how are u sim cards accessed
how are (U)SIM cards accessed
  • 2G, 3G use
    • builtin reader in the mobile handset
  • for Internet use:
    • maybe builtin in PDA, PC (e.g.DELL)
    • external USB token – 20$ 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

access request – present IMSI

present challenge („RAND“)

send RESP (challenge response)

SIM usage in 2G authentication

Authentication

Center

2G GSM

handset

keys

shared secret

imsi structure
IMSI structure
  • 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
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
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

access request – present IMSI

send RESP (challenge response)

3G AKA authentication flow

Authentication

Center

3G UMTS

handset

challenge RAND || AUTN token

keys

shared secret,

Sequence numbers

result:

Cipher key

Integrity key

what s the universal integrated circuit card uicc about
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
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
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
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
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
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-bonofrom 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
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
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
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