ing two primary techniques, which are shown in Figure 5-18: Authentication Ciphering (encryption) authenticating the user Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Main GPRS Procedures Chapter 5 Figure 5-18 Security functions. Send Authentication Info (IMSI) Send Authentication Info Ack. Authentication Request (RAND) (SRES, RAND, Kc) Authentication Response Forbidden (SRES) Subscriber Authenticated Subscriber Authentication The Authentication Center (AuC) is responsible for generating a set of para- meters known as triplets. A triplet consists of a Cipher Key (K) Random Number (RAND) Signed Response (SRES) The RAND is a randomly generated number from a number pool con- taining 2128 numbers. The RAND, coupled with the Identification Key (Ki), is used to calculate K and SRES. K is a secret number allocated on a per- subscriber basis and is only held at the AuC and is based on the Subscriber Identity Module (SIM) card. Measures are taken to ensure that the Kj can- not be read from the SIM card. K is never transmitted over the network. Authentication procedure, based on the GSM, performs the selection of a ciphering algorithm. The SGSN may store the authentication triplets of the mobile station after detaching from the GPRS. If it does not have the pre- viously stored authentication triplets, they can be obtained from the HLR. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Main GPRS Procedures Main GPRS Procedures Ciphering The mobile station starts ciphering after sending the Authentication Response to the SGSN. On receipt of a valid response message, the SGSN starts ciphering. Ciphering is used over the air interface following the authentication procedure to provid