A3 ALGORITHM.pptx

7/27/2019 A3 ALGORITHM.pptx

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BY

G.KARTHIK REDDY(10BEC0308)

M.CHETANA SRI KRISHNA(10BEC0321)

R.VAMSHIDHAR REDDY(10BEC0332)


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A3 algorithm is a validation process used in a

GSM network for authentication of the mobile

user that is requesting service.

The A3 algorithm is a mathematical process

that combines a secret number (called Ki in

documents) stored in the SIM chip and a

second number that is transferred between thebase station and the mobile station.


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The numerical result of the A3 algorithmcalculation is transmitted back to the base

station. It is then compared to a previously internally

calculated answer also using the A3 algorithm

and the same two input quantities, in the basestation controller or in the MSC.

When the same matching result occurs for boththe mobile supplied information and previously

stored information, the customer has grantedaccess.


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Authentication- Whenever a MS requests accessto a network, the network must authenticate the MS.

Authentication verifies the identity and validity of the

SIM card to the network and ensures that the

subscriber is authorized access to the network.

Ki - The Ki is the individual subscriber authenticationkey. It is a 128-bit number that is paired with an IMSI

when the SIM card is created. The Ki is only stored on

the SIM card and at the Authentication Center (AuC).

The Ki should never be transmitted across thenetwork on any link.


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RAND - The RAND is a random 128-bit number that is generatedby the Auc when the network requests to authenticate asubscriber. The RAND is used to generate the Signed Response

(SRES) and Kc cryptovariables.

Signed Response - The SRES is a 32-bit cryptovariable used inthe authentication process. The MS is challenged by being giventhe RAND by the network, the SRES is the expected correct

response. The SRES is never passed on the Um (Air) interface. Itis kept at the MSC/VLR, which performs the authenticationcheck.

A3 Algorithm - The A3 algorithm computes a 32-bit SignedResponse (SRES). The Ki and RAND are inputted into the A3algorithm and the result is the 32-bit SRES. The A3 algorithmresides on the SIM card and at the AuC.


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1) The Mobile Station (MS) signs into the network.

2) The Mobile Services Switching Center (MSC) requests

5 triples from the Home Location Register (HLR).

3)The Home Location Register creates five triples utilizingthe A8 algorithm. These five triples each contain:

a) A 128-bit random challenge (RAND)

b) A 32-bit matching Signed Response (SRES)

c) A 64-bit ciphering key used as a Session Key (Kc).


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4)The Home Location Register sends the Mobile Services

Switching Center the five triples.

5)The Mobile Services Switching Center sends the randomchallenge from the first triple to the Base Transceiver Station

(BTS).

6) The Base Transceiver Station sends the random challenge from

the first triple to the Mobile Station.7) The Mobile Station receives the random challenge from the

Base Transceiver Station and encrypts it with the Individual

Subscriber Authentication Key (Ki) assigned to the Mobile

Station utilizing the A3 algorithm.

8) The Mobile Station sends the Signed Response to the Base

Transceiver Station.


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9) The Base Transceiver Station sends the Signed Response to the

Mobile Services Switching Center.

10)The Mobile Services Switching Center verifies the SignedResponse.

11)The Mobile Station generates a Session Key (Kc) utilizing the

A8 algorithm, the Individual Subscriber Authentication Key

(Ki) assigned to the Mobile Station, and the random challengereceived from the Base Transceiver Station

12) The Mobile Station sends the Session Key (Kc) to the Base

Transceiver Station.

13) The Mobile Services Switching Center sends the Session Key(Kc) to the Base Transceiver Station.


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14) The Base Transceiver Station receives the Session Key

(Kc) from the Mobile Services Switching Center.

15) The Base Transceiver Station receives the Session Key(Kc) from the Mobile Station.

16) The Base Transceiver Station verifies the Session Keys

from the Mobile Station and the Mobile Services switching

Center. 17) The A5 algorithm is initialized with the Session Key (Kc)

and the number of the frame to be encrypted.

18) Over-the-air communication channel between the Mobile

Station and Base Transceiver Station can now be encryptedutilizing the A5 algorithm.


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THANK

YOU

A3 ALGORITHM.pptx

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