2G/3G Authentication with SIM cards: usage

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2G/3G Authentication with SIM cardsusage
roaming basics for the Internet challenged

• Michael Haberler
• Internet Foundation Austria

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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)SIMs and Internet access authentication
• how SIMs and RADIUS roaming works
• (U)SIMs and SIP authentication
• what the SIP server does
• How the parameter logistics works
• a bonus business model thrown in
• summary

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whats 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

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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 SIMs made even MomPop 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...)

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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

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SIM usage in 2G authentication
Authentication Center
2G GSM handset
keys
shared secret
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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)

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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

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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

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3G AKA authentication flow
Authentication Center
3G UMTS handset
challenge RAND AUTN token
keys
shared secret, Sequence numbers
result Cipher key Integrity key
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whats 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

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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!

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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

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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

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how 3G roaming works

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

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how the 2G/3G user ids (IMSIs) are mapped to
RADIUS authentication

• take mobile country code, mobile network code
• use them to create a realm
• Example
• IMSI 232011234567890
• means mcc232 (Austria) mnc01 (Mobilkom)
• resulting realm
• mnc01.mcc232.owlan.org
• resulting RADIUS user
• 232011234567890_at_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

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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 youre part of 2G/3G roaming yet
  contracts regulatory prerequisites needed
• but the addressing is all set to go!!

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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

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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