1. One-Pass GPRS and IMS Authentication Procedure for UMTS
By Yi-Bing, Ming-Feng Chang, Meng-Ta Hsu, and Lin-Yi Wu
An Overview, by Frank McCown
Mobile Computing CS Old Dominion University
October 22, 2004

2. Evolution of Mobile Networks
Analog  Digital  Multimedia
Generation
Type
Time
Standards 1G
Analog
1980s
AMPS, NMT, TACS 2G
Digital
1990s
GSM, CDMA, TDMA
2.5G
Higher data rate
Late 1990s
GPRS, EDGE 3G
Digital multimedia
2010s
UMTS
Analog: You could only easily use analogue cellular to make voice calls, and typically only in any one country.
Digital mobile phone systems added fax, data and messaging capabilities as well as voice telephone service in many countries.
Multimedia services add high speed data transfer to mobile devices, allowing new video, audio and other applications through mobile phones- allowing music and television and the Internet to be accessed through a mobile terminal. International support.
Moving from multiple standards to single standard.
Moving from unsecured communications to end-to-end secured communications.

3. UMTS Background
UMTS (Universal Mobile Telecommunications System) specification created by the 3GPP consortium
Progression from GSM (2G) to GPRS (2.5G)
Data rates: 144 kbps – 2 Mbps
10 million users as of Sept 20041
3GPP - 3rd Generation Partnership Project
GSM – (Global System for Mobile communication) Was primarily a European standard (2G). Has now spread throughout North America.
GPRS – (General Packet Radio Service) 2.5G. Involves overlaying a packet based air interface on the existing circuit switched GSM network 1

4. IMS Overview
IMS (IP Multimedia Subsystem) used by UMTS for providing IP telecommunications
Supports voice telephony, live video streaming, instant messaging, etc.
Performs signaling operations using the Session Initiation Protocol (SIP)
Uses CSCF to provide multimedia services
IMS is a framework for providing IP telecommunication services.
SIP, the Session Initiation Protocol (developed by IETF), is a signaling protocol for Internet conferencing, telephony, presence, events notification and instant messaging. SIP runs on top of different IP transport protocols like UDP and TCP.
CSCF – Call Session Control Function. The CSCF provides session control for subscribers accessing services within the IM (IP Multimedia) CN . In essence the CSCF is a SIP Server. It has responsibility for interacting with network databases such as the HSS for mobility and AAA (Access, Authorization and Accounting) Servers for security.

5. CSCF Overview Three types of Call Session Control Functions in IMS:
P-CSCF (Proxy)
Directs SIP messages from MS to home network.
S-CSCF (Serving)
Located in home network to provide session control of multimedia services.
I-CSCF (Interrogating)
Firewall for SIP messages directed toward home network. Selects S-CSCF for the MS.
CSCF – Call Session Control Function. The CSCF provides session control for subscribers accessing services within the IM (IP Multimedia) CN . In essence the CSCF is a SIP Server. It has responsibility for interacting with network databases such as the HSS for mobility and AAA (Access, Authorization and Accounting) Servers for security.

6. UMTS Architecture
SGSN – Serving GPRS Support Node. Responsible for the delivery of data packets from and to the mobile stations within its service area. Its tasks include packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The location register of the SGSN stores location information and user profiles of all GPRS users registered with this SGSN.
GGSN – Gateway GPRS Support Node. Interface between the GPRS wireless data network and other networks such as the Internet or private networks. To external packet data networks the GGSN performs the task of an IP router. Firewall and filtering functionality, to protect the integrity of the GPRS core network, are also associated with the GGSN along with a billing function.
CSCF – Call Session Control Function
P-CSCF (Proxy) - Directs SIP messages from MS to home network.
S-CSCF (Serving) - Located in home network to provide session control of multimedia services.
OSA – Open Service Access. Provides a standard interface through which developers can design services that may interact with functions within the network.
I-CSCF (Interrogating) - Firewall for SIP messages directed toward home network. Selects S-CSCF for the MS.
HSS – Home Subscriber Server. The master database for the wireless network, and while logically it is viewed as one entity, in practice it will be made up of several physical databases. The HSS holds variables and identities for the support, establishment and maintenance of calls and sessions made by subscribers. This includes the subscriber’s IMSI , security variables and location information.
AuC – Authentication Center. Provides authentication vectors (AVs) for the authentication process.

7. Goal: Security GPRS Authentication IMS Authentication
User authentication – between MS and SGSN
Network authentication – between MS and HSS/AuC
IMS Authentication
between MS and CSCF
USIM – Universal Subscriber Identity Module. Only emergency calls are permissible if not present in terminal.
ISMI – It’s a unique identifier allocated to each mobile subscriber in a GSM and UMTS network. It consists of a MCC (Mobile Country Code), a MNC (Mobile Network Code) and a MSIN (Mobile Station Identification Number).

8. USIM Smart Card Provides mobile station identification
Must be present in terminal
Contains data about subscriber
IMSI (International Mobile Station Identifier) for GPRS authentication
IMPI (IP Multimedia Private Identity) for IMS authentication
Encryption and integrity keys
Other: Identities, preferred languages, etc.
USIM – Universal Subscriber Identity Module. Only emergency calls are permissible if not present in terminal.
ISMI – It’s a unique identifier allocated to each mobile subscriber in a GSM and UMTS network. It consists of a MCC (Mobile Country Code), a MNC (Mobile Network Code) and a MSIN (Mobile Station Identification Number).

9. GPRS Authentication MS SGSN HSS/AuC Attach Request (IMSI)
Send Auth Info Request (IMSI)
Send Auth Info Response (AVs)
Select authentication vector
AV = (RAND, AUTN, XRES, CK, IK)
Auth & Ciphering Request (RAND, AUTN)
Two things happening here:
1) User Authentication – between MS and SGSN
2) Network Authentication – between MS and HSS/AuC
SGSN – Serving GPRS Support Node. Responsible for the delivery of data packets from and to the mobile stations within its service area. Its tasks include packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The location register of the SGSN stores location information and user profiles of all GPRS users registered with this SGSN.
HSS – Home Subscriber Server. The master database for the wireless network, and while logically it is viewed as one entity, in practice it will be made up of several physical databases. This depending on the number of subscribers and the extent of the services which need to be supported. The HSS holds variables and identities for the support, establishment and maintenance of calls and sessions made by subscribers. This includes the subscriber’s IMSI , security variables and location information.
AuC – Authentication Center. Provides authentication vectors (AVs) for the authentication process.
Step G.1. Consider an MS with the IMSI value imsi and the IMPI value impi. To access the GPRS services, the MS sends a GMM Attach Request (with the parameter IMSI = imsi) to the SGSN.
Step G.2. If the SGSN has the AVs of the MS, then Steps G.2 and G.3 are skipped. Otherwise, the SGSN must obtain the AVs from the HSS/AuC. That is, the SGSN invokes the authentication vector distribution procedure by sending a MAP SEND AUTHENTICATION INFO Request message to the HSS/AuC (with the parameter IMSI = imsi).
Step G.3. The HSS/AuC uses imsi to retrieve the record of the MS, and generates an ordered array of AVs (based on the pre-shared secret key K in the MS record). The generated AV array is sent to the SGSN through a MAP SEND AUTHENTICATION INFO Response message.
Each AV is good for one authentication and key agreement between the SGSN and the MS.
Step G.4. The SGSN selects the next unused authentication vector in the ordered AV array and sends the parameters RAND and AUTN (from the selected authentication vector) to the MS through a GMM Authentication and Ciphering Request message.
AV – Authentication Vector. Can only use one per authentication request.
RAND – Random number
AUTN – Authentication Token created by concatenating 3 fields and SQN added bit-by-bit to AK (Anonymity key), AMF (Authentication Management Field), and MAC (Message Authentication Code).
XRES – Signed Response
CK – Cipher Key
IK – Integrity Key
Step G.5. The MS checks whether the received AUTN can be accepted (uses pre-shared key K to decrypt) to authenticate the network. If so, it produces a response RES (using key K and RAND) that is sent back to the SGSN through a GMM Authentication and Ciphering Response message. The SGSN compares the received RES with the XRES. If they match, then the authentication and key agreement exchange is successfully completed.
Step G.6. The SGSN sends a GMM Attach Accept message to the MS, and the attach procedure is completed.
Verify AUTN, Compute RES
Auth & Ciphering Response (RES)
Compare RES and XRES
Attach Accept
Compute CK and IK
Select CK and IK

10. IMS Authentication MS SGSN HSS/AuC CSCF Register (IMPI)
Multimedia Auth Request (IMPI)
Multimedia Auth Answer (AVs)
Select authentication vector AV
Verify AUTN, Compute RES
401 Unauthorized (RAND, AUTN)
SGSN – Serving GPRS Support Node.
HSS – Home Subscriber Server.
AuC – Authentication Center.
CSCF - Call Session Control Function. Represents P-CSCF (Proxy-CSCF), S-CSCF (Service-CSCF), I-CSCF (Interrogating-CSCF)
IMPI – IP Multimedia Private Identity
Step I.1. The MS sends a SIP Register message to the CSCF (with the parameter IMPI = impi) through the SGSN.
Step I.2. Assume that the CSCF does not have the AVs for the MS (or the array has been used up and a new one is needed). The CSCF invokes the 7 authentication vector distribution procedure by sending a Cx Multimedia Authentication Request message to the HSS/AuC (with the parameter IMPI = impi).
Step I.3. The HSS/AuC uses impi to retrieve the record of the MS, and generate an ordered array of AVs. The HSS/AuC sends the AV array to the CSCF through a Cx Multimedia Authentication Answer message.
Step I.4. The CSCF selects the next unused authentication vector from the ordered AV array and sends the parameters RAND and AUTN (from the selected authentication vector) to the MS through a SIP 401 Unauthorized message. (Similar to HTTP 401 Unauthorized message)
Step I.5. The MS checks whether the received AUTN can be accepted. If so, it produces a response RES. The MS sends this response back to the CSCF through a SIP Register message. The CSCF compares the received RES with the XRES. If they match, then the authentication and key agreement exchange is successfully completed.
Step I.6. The CSCF sends a Cx Server Assignment Request message to the HSS/AuC.
Step I.7. Upon receipt of the Server Assignment Request, the HSS/AuC stores the CSCF name and replies a Cx Server Assignment Answer message to the CSCF.
Step I.8. The CSCF sends a 200 Ok message to the MS through the SGSN, and the IMS registration procedure is completed.
In the above procedure, Steps I.1-I.5 exercise authentication, and Steps I.6-I.8 perform registration.
Compare RES and XRES
Register (RES)
Server Assignment Request
Server Assignment Answer
200 OK

11. Similarities GPRS Authentication IMS Authentication SGSN  HSS/AuC
Send Auth Info Request (IMSI)
CSCF  HSS/AuC
Multimedia Auth Request (IMPI)
HSS/Auc  SGSN
Send Auth Info Response (AVs)
HSS/AuC  CSCF
Multimedia Auth Answer (AVs)
SGSN  MS
Auth & Ciphering Request (RAND, AUTN)
CSCF  MS
401 Unauthorized (RAND, AUTN)
MS  SGSN
Auth & Ciphering Request (RES)
MS  CSCF
Register (RES)
Attach Accept
200 OK

12. IMS One-Pass Authentication
Make use of GPRS authentication
SGSN implements a SIP Application Level Gateway (ALG)
Slight modification of SIP message format

13. IMS Registration: One-Pass Authentication
MS SGSN HSS/AuC CSCF
Register (IMPI)
Register (IMPI, IMSI)
Retrieve IMSI value
Store (IMSI, IMPI) pair
Server Assignment Request (IMPI)
SGSN – Serving GPRS Support Node.
HSS – Home Subscriber Server.
AuC – Authentication Center.
CSCF - Call Session Control Function. Represents P-CSCF (Proxy-CSCF), S-CSCF (Service-CSCF), I-CSCF (Interrogating-CSCF)
IMPI – IP Multimedia Private Identity
Step I*.1. The MS sends a SIP Register message to the SGSN with the parameter IMPI = impi. Note that after GPRS auth, the SGSN can identify the IMSI of the MS that transmits the GPRS packets. The SIP ALG in the SGSN adds the IMSI value of the MS in the Register message and forwards it to the CSCF.
Step I*.2. The CSCF stores the (imsi, impi) pair in the MS record, and sends a Cx Server Assignment Request message to the HSS/AuC with the parameter IMPI = impi. We note that if the CSCF has stored the (imsi, impi) pair before, then Steps I*.2 and I*.3 are skipped.
Step I*.3. The HSS/AuC uses the received IMPI value impi as an index to retrieve the IMSI and the user profile of the MS. We denote IMSI-HSS(IMPI) as the IMSI value retrieved from the HSS/AuC. The HSS/AuC stores the CSCF name and sends a Cx Server Assignment Answer to the CSCF (with the parameters IMSI-HSS(IMPI) and user profile).
Step I*.4. The CSCF checks whether the value IMSI-HSS(IMPI) and IMSI are the same. If so, the CSCF sends a SIP 200 Ok message to the SGSN and the authentication is considered successful. If they aren’t equal, the registration is illegal; possibly a MS is trying to illegally access the IMS service of some other MS.
Server Assignment Answer (IMSIHSS(IMPI), User Profile)
Check if IMSIHSS(IMPI) = IMSI
200 OK

14. IMS Authentication MS SGSN HSS/AuC CSCF No longer needed!
Register (IMPI)
Multimedia Auth Request (IMPI)
Multimedia Auth Answer (AVs)
Select authentication vector AV
Verify AUTN, Compute RES
Compare RES and XRES
401 Unauthorized (RAND, AUTH)
Register (RES)
No longer needed!
Server Assignment Request
Server Assignment Answer
200 OK

15. Cost Analysis How much signaling has one-pass procedure saved?
MS SGSN HSS/AuC CSCF
Cost = 1
β < 1 because 1) CSCF and HSS/AuC exchange messages through IP network. MS must transmit message via radio to SGSN and then through GPRS core network to HSS/AuC, then to CSCF.
Cost = β where β < 1
One-pass procedure: C1 = β
MS  CSCF, CSCF  HSS/AuC, HSS/AuC  CSCF, CSCF  MS

16. Cost Analysis Cost C2 for two-pass IMS procedure:
If AVs needed to perform authentication, C2,1 = 4 + 4β
If AVs not needed, C2,2 = 4 + 2β
AV array is of size n. 1 out of n IMS registrations incurs C2,2.
C2 = ( )C2,1 + ( )C2,2 = 4 + ( )2β
Improvement S = C2 - C1 = n + β
USIM – Universal Subscriber Identity Module. Only emergency calls are permissible if not present in terminal. 1 n
n -1 n
n +1 n C2
2n + (n + 1)β

17. Improvement S of One-Pass Over Two-Pass Procedureβ
n = AV array size
β = Cost HSS/AuC  CSCF
S = (n + β) / (2n + (n + 1)β)

18. Conclusion
After GPRS authentication, IMS two-pass authentication can be simplified using one-pass authentication.
New approach can save up to 50% of network traffic generated by IMS registration.
50% of the storage for buffering AVs is alleviated.
One-pass procedure is pending ROC and US patents.

19. Glossary 3GPP – Third Generation Partnership Project
AuC – Authentication Center
AV – Authentication Vector
RAND – Random number
AUTN – Authentication Token
XRES – Signed Response
CK – Cipher Key
IK – Integrity Key
CSCF – Call Session Control Function
P-CSCF – Proxy-CSCF
S-CSCF – Service-CSCF
I-CSCF – Interrogating-CSCF
GGSN – Gateway GPRS Support Node
GPRS – General Packet Radio Service
HSS – Home Subscriber Server
IMPI – IP Multimedia Private Identity
IMSI – International Mobile Station Identifier
IMS – IP Multimedia Core Network Subsystem
PS CN – Packet Switched Core Network
SGSN – Serving GPRS Support Node
SIP – Session Initiation Protocol
UMTS – Universal Mobile Telecommunications System
USIM - Universal Subscriber Identity Modules
UTRAN – UMTS Terrestrial Radio Access Network

20. References
“aSIP-Access Security for IP-Based Services”, presentation by Krister Boman, Apr
“Security in GSM, GPRS and 3GPP”, presentation by Tahar Ktari and David Mayor, Apr
“Security in the Mobile Internet”, tutorial by Ram Gopal and Lakshmi Narayanan,
UMTS Security, Boman, K., et. al., Electronics & Communication Engineering Journal, Oct 2002.