1. Security in GSM/GPRS and UMTS

2. Security in GSM/GPRS
The cellular network must warranty a secure transmission of voice and data without interception, and avoid fraud
Security in GSM/GPRS is implemented in the following elements:
SIM – This holds the IMSI, the ultrasecret MS key Ki, ciphering key generation algorythm (A8), authentication algorythm (A3) and PIN code
Handset – Implements the ciphering algorythms A5 (GSM), GEA1, GEA2, GEA3 (GPRS) in the hardware
GSM Network: The AUC (AUthentication Center) is a data base that holds the master keys Ki of users and generates the triplets (RAND, SRES & Kc) vectors.
The SGSN stores the triplets to use them during the authentication (RAND, SRES) and ciphering (Kc) and holds the temporary information about attached users (TLLI)
Core Network: Network layer (IP) IPSEC; Session layer: (AAA)* RADIUS, DIAMETER, SSL, WTLS (WAP)
*AAA: Authentication, Authorization & Accounting

3. Authentication and Ciphering in GPRS
RAI & TLLI or IMSI
Request Authentication
IMSI
Request Authentication Triplets Ki
Generate
RAND (1..n) A3 A8
SRES (1..n)
Kc (1..n)
RAND A8 A3
SRES
Pass
Fail
= ?
Authentication
Ciphering?
Ciphering ?
Store (1..n) RAND, SRES, Kc vectors
GPRS uses the following elements:
A random number called RAND
A secret Key Ki used for: (The Ki only exist in the HLR and the USIM)
Authenticating the subscriber
Generating another key for ciphering called Kc
An algorithm A3 generating a number SRES (Signed Result) from Ki and RAND input.
An algorithm A8 generating the key Kc from the Ki and RAND input.
An algorithm A5 (GEA1, GEA2) to apply the ciphering on the transmitted data using the key Kc.
ENCRYPTED
DATA

4. GPRS Authentication no encryption
Trace: Gb_noencrypted
Trace: Gr_noencrypted
GMM: IMSI Attach Request [IMSI], [RAI] 3
MAP: SendAuthenticationInfoArg [IMSI]
Request Authentication vectors [n] 1
MAP: SendAuthenticationInfoResArg
N times [RAND],[SRES] & [KC] 4
Authentication & Ciphering Request[RAND]
[Ciphering Algorithm not used] 6 Y
MAP: UpdateGPRSLocationArg [IMSI]
[SGSNnumber], [SGSN IP] 5
Authentication & Ciphering Response[SRES]
SRES =? 9
MAP: InsertSubscriberData Arg
[MISDN],[GPRS services and QoS contract] 6
Ack 7
MAP: UpdateGPRSLocatioRes
[HLR number] 8
GMM: Attach Accept [P-TMSI] 14
GMM: Attach Complete
New TLLI = P-TMSI 17
NOTE: See traces Gb_noencryption & Gr_noencryption

5. Why Encryption? Security of user data over the air interface
The encryption algorythm is installed in the MS and the SGSN.
This algorythm is restricted to MS to SGSN encrypted communications. Encryption is implemented at the LLC level.
Kc is never transmited over the radio interface.
Input: This is the LLC frame dependent input parameter (32 bits) for the ciphering algorithm. Depending on the frame type, this field is derived as follows:
- For I-frames carrying user data:
- The input value is set to a random initial value at LLC connection set-up and incremented by 1 for each new frame.
- For UI-frames carrying user data and signalling messages:
- the input parameter is a non-repeating 32-bit value derived from the LLC header.
Direction: (1 bit) uplink or downlink
Output: This is the output of the ciphering algorithm. The maximum length (1600 octets) of the output string is the maximum length of the payload of the LLC frame, including the FCS (Frame Check Sequence, 3 octets).
The minimum length of the output string is 5 octets.

6. Encrypted Protocols in GPRS
After GGM: Authentication & ciphering response. All protocols above LLC are encrypted,between MS and SGSN
ENCRYPTED

7. Non Ciphered Messages The following messages are never ciphered:
Attach Request
Attach Reject
Authentication and Ciphering Request
Authentication and Ciphering Response
Authentication and Ciphering Reject
Identity Request
Identity Response
Routing Area Update Request
Routing Area Update Reject
These messages are not ciphered so that the receiver (either SGSN or MS) can interpret the message

8. GPRS Authentication with encryption
Trace: Gb_ciphering
Trace: Gr_ciphering
GMM: IMSI Attach Request [IMSI], [RAI] 5
MAP: SendAuthenticationInfoArg [IMSI]
Request Authentication vectors [n] 1
MAP: SendAuthenticationInfoResArg
N times [RAND],[SRES] & [KC]
Authentication & Ciphering Request [RAND], [SQN]
[Ciphering Algorithm GEA/1] 4 6 Y
Authentication & Ciphering Response[SRES] 9
SRES =?
MAP: UpdateGPRSLocationArg [IMSI]
[SGSNnumber],[SGSN IP] 5
MAP: InsertSubscriberData Arg
[MISDN],[GPRS services and QoS contract] 6
ENCRYPTED 7
Ack
MAP: UpdateGPRSLocatioRes
[HLR number]
GMM: Attach Accept[P-TMSI] 8 16
GMM: Attach Complete 19
New TLLI = P-TMSI
NOTE: See traces Gb_encryption & Gr_encryption

9. Tools to analyze and troubleshoot a GPRS deciphered link

10. Deciphering a Capture file
PrismLite: offline only application
Posibility to merge up to 3 Gb links offline
Generates a raw .txt file <Gb01ciphered_dec.txt>
Encryption is activated above the LLC level for signaling (GMM/SM) SAPI=1 and data (SAPI= 3, 5, 9 or 11) Gb Gr

11. Online deciphering Performer: both offline and online application
Over 400,000 sessions online
You can also use:
An existing Gr File
Write the Kc keys into a Gr file.

12. Security in UMTS

13. Security in UMTS
Three entities are involved in the UMTS authentication
Home Network (HLR/Auc): holds the master keys K of all UEs. Generates the Quintuplets vectors (RAND, XRES, CK, IK and AUTN) using 5 one way functions.
Serving Network (VLR or SGSN): requests and stores the authentication vectors from the HLR, and sends the Authentication Request message to the UE with RAND and AUTN vectors.
The USIM: In the Smart Card of the terminal, holds the master key K (unique for this terminal).
When receives the Authentication Request message from VLR/SGSN with AUTN, and RAND vectors, uses these vectors together with the master key K to generate the vectors RES (used in the Authentication Response), CK (Ciphering Key) & IK (Integrity Key). After Authentication has been resolved, the corresponding CK & IK stored in the SGSN/VLR are transferred RNC using the RANAP: Security Mode procedure to start the integrity and encryption process between the UE and the RNC
*AAA: Authentication, Authorization & Accounting

14. Authentication Vectors
Initial Parameters:
K: Master Key (ultrasecret permanent 128 bits)
SQN: Incremental Sequence Number (48 bits)
RAND: Random bit Stream (128 bits)
AMF: Administrative Authentication Management Field (16 bits)
Calculated Parameters:
MAC: Message Authentication Code (64 bits)
XRES: Expected Authentication Response (4-64 bits)
CK: Ciphering Key (128 bits)
IK: Integrity Key (128 bits)
AK: Anonymous Key (48 bits)
Quintuplet Vectors: (1..n) RAND, AUTN, XRES, CK, IK
Generated in AuC, temporarily Stored in SGSN/VLR & verified with USIM.

15. Authentication, Integrity & ciphering in UMTS
Home
Network
Serving
Network
VLR
PS: GMM_Attach Request [RAI & IMSI or P-TMSI]
CS: MM_Location Update [LAI & IMSI or TMSI]
MAP_Send Auth Info Arg:[ IMSI & num of vectors]
Generate Auth Vectors
MAP_Send Auth Info Resp:
[(1..n) RAND,AUTN, XRES, CK, IK]
Store Auth Vectors
PS: GMM_Authentication & Ciphering Request [RAND & AUTN]
CS: MM_Authentication Request [RAND]
Verify AUTN
Generate RES
PS: GMM_Authentication & Ciphering Response [RES]
CS: MM_Authentication Response [RES]
RES=XRES
RANAP_Security Mode Command [CK & IK]
Encryption: Y/N
RRC_Security Mode Command
Encryption: Y/N
Store CK & IK
RRC_Security Mode Complete
Chosen Integrity Algorythm
RANAP_Security Mode Complete
Chosen Integrity Algorythm
MAP: UpdateGPRSLocationArg [IMSI]
[SGSNnumber],[SGSN IP]
MAP: InsertSubscriberData Arg
[MISDN],[GPRS services and QoS contract]
Ack
MAP: UpdateGPRSLocatioRes
[HLR number]
GMM: Attach Accept [P-TMSI]
GMM: Attach Complete
Example: Open PTMSI_Att_Iu_Gr

16. Authentication Keys generation: AUC & USIMf2
AMF
Generate
SQN 
RAND AK
MAC
XRES IK CK f5 f4 f3 f1
VLR K IK
XMAC
RES CK AK
SQN  f2 f3 f4 f1 f5
= ? *
AUTN: = SQN  AK || AMF || MAC
RAND
IMSI
Quintuplets: = RAND || XRES || CK || IK || AUTN
RES
 : XOR
|| : Concatenation

17. Ciphered Protocols in UMTS
After the RNC receives the Kc, the Security Mode Command is sent to the terminal to start the encryption
WCDMA Physical Channels
SDH or PDH
ATM
AAL2
MAC
RELAY
FP (Iub UP)
RLC
RRC Uu
Iub
ENCRYPTED
MAC SDU Ciphered
RLC PDU Ciphered

18. For tools to analyze and troubleshoot a UMTS deciphered link see: www
For tools to analyze and troubleshoot a UMTS deciphered link see: