Authentication Principle

AUTHENTICATION PRINCIPLE

Authentication is a procedure used in checking the validity and integrity of subscriber data. With the help of the authentication procedure the operator prevents the use of false SIM modules in the network. The authentication procedure is based on an identity key, K_i, that is issued to each subscriber when his data are established in the HLR. The authentication procedure verifies that the K_i is exactly the same on the subscriber side as on the network side.

Authentication

Authentication is performed by the VLR at the beginning of every call establishment, location update and call termination (at the called subscriber side). In order to perform the authentication, the VLR needs the basic authentication information. If the mobile station was asked to broadcast its K_i, this would undermine the principle of authentication, because identification data would be sent across the air. The trick is to compare the K_i stored in the mobile with the one stored in the network without actually having to transmit it over the radio air interface. The K_i  is processed by a random number with a “one way” algorithm called A3 and the result of this processing is sent to the network. Due to the type of the algorithm A3, it is easy to get the result on the basis of K_i  and a random number, but it is virtually impossible to get the K_i on the basis of the result and random number (hence the name “one way” algorithm).

Since the security issue concerns confidentiality as well, the network uses more than one algorithm. These are introduced in the following sections.

Security Algorithms

The GSM system uses three algorithms for the purposes of authentication and ciphering. These algorithms are A3, A5 and A8. A3 is used in authentication, A8 is used in generating a ciphering key and A5 is used in ciphering.

Algorithms A3 and A8 are located in the SIM module and in the Authentication Center (AC). A5 is located in the mobile station and in the BTS.

Before an operator starts to use the security functions, the mobile subscriber is created in the Authentication Center. The following information is required in creating the subscriber:

o  IMSI of the Subscriber

o  Ki of the subscriber

o  Algorithm Version Used

The same information is also stored in the Mobile Subscriber's SIM. The basic principle of GSM security functions is to compare the data stored by the network to the data stored in the subscriber’s SIM. The IMSI number is the unique identification of the mobile subscriber. Ki is an authentication key with a length of 32 hexadecimal digits. The algorithms A3 and A8 use these digits as a basic value in authentication.

The Authentication Center generates information that can be used for all the security purposes during one transaction. This information is called an Authentication Triplet.

The authentication triplet consists of three numbers:

o  RAND

o  SRES

o  Kc.

RAND is a Random number, SRES (Signed Response) is a result that the algorithm A3 produces on the basis of certain source information and Kc is a ciphering key that A8 generates on the basis of certain source information.

Authentication Triplet

All the values included in the authentication triplet depend on each other i.e. a certain RAND inserted to the algorithms with a certain Ki always produces a certain SRES and a certain Kc.

When the VLR has this kind of three-value combination and the Mobile Subscriber authentication procedure is initiated, the VLR sends the random number RAND through the BSS to the SIM in the mobile station. As the SIM has (or it should have) exactly the same algorithms as used in triplet generation on the network side, the RAND number that the SIM receives and inserts to the algorithm should produce exactly the same SRES value as the one generated on the network side.

Authentication Procedure

If the SRES value in the authentication triplet is the same as the SRES calculated and sent by the mobile station, the authentication procedure is successful.

Services

Ciphering/Speech Encryption

Ciphering is used across the Air interface to provide speech and signalling encryption. When the authentication procedure has been completed successfully, the BTS and the mobile station are ready to start the ciphering procedure for further signalling and speech/data transmission.

The speech of the user and the ciphering key, K_c, are processed by the ciphering algorithm (A5) which produces the coded speech signal.