1. Michael Haberler Internet Foundation Austria
2G/3G Authentication with SIM cards: usage & roaming basics for the Internet challenged
Michael Haberler
Internet Foundation Austria

2. outline a SIM card mini-tutorial UMTS authentication and key agreement
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

3. 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

4. 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...)

5. 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

6. SIM usage in 2G authentication
access request – present IMSI
Authentication
Center
2G GSM
handset
present challenge („RAND“)
send RESP (challenge response)
keys
shared secret

7. 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)

8. 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

9. 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

10. 3G AKA authentication flow
access request – present IMSI
Authentication
Center
3G UMTS
handset
challenge RAND || AUTN token
send RESP (challenge response)
keys
shared secret,
Sequence numbers
result:
Cipher key
Integrity key

11. 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

12. 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
OR RADIUS server access an ISP-style database for keys
ISP is the SIM card issuer!

13. 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

14. 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

15. how 3G roaming works
not much different from 3G, just more parameters needed for AKA
„triplets“ become „quintets“

16. 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 =
means mcc=232 (Austria) mnc=01 (Mobilkom)
resulting realm
mnc01.mcc232.owlan.org
resulting RADIUS user
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

17. 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 ( 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!!

18. 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

19. 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