name: A5 Ciphering Algorithm
full_form: Not explicitly defined in this text snippet, but refers to a ciphering algorithm used in GSM networks
context: The mobile station shall be ready to respond upon an AUTHENTICATION REQUEST message at any time whilst a RR connection exists. With exception of the cases described in subclause 4.3.2.5.1, it shall process the challenge information and send back an AUTHENTICATION RESPONSE message to the network. If a SIM is inserted in the MS, the MS shall ignore the Authentication Parameter AUTN IE if included in the AUTHENTICATION REQUEST message and shall proceed as in case of a GSM authentication challenge. It shall not perform the authentication of the network described in subclause 4.3.2.5.1. In a GSM authentication challenge, the new GSM ciphering key calculated from the challenge information shall overwrite the previous GSM ciphering key and any previously stored UMTS ciphering key and UMTS integrity key shall be deleted. The new GSM ciphering key shall be stored on the SIM/USIM together with the ciphering key sequence number. In a UMTS authentication challenge, the new UMTS ciphering key, the new GSM ciphering key and the new UMTS integrity key calculated from the challenge information shall overwrite the previous UMTS ciphering key, GSM ciphering key and UMTS integrity key. The new UMTS ciphering key, GSM ciphering key and UMTS integrity key are stored on the USIM together with the ciphering key sequence number. Furthermore, in A/Gb mode when after the authentication procedure an A5 ciphering algorithm that requires a 128-bit ciphering key is taken into use, then a new GSM Kc128 shall also be calculated as described in the subclause 4.3.2.3a. The SIM/USIM will provide the mobile station with the authentication response, based upon the authentication challenge given from the ME. A UMTS authentication challenge will result in the USIM passing a RES to the ME. A GSM authentication challenge will result in the SIM/USIM passing a SRES to the ME. A ME supporting UMTS authentication challenge may support the following procedure: In order to avoid a synchronisation failure, when the mobile station receives an AUTHENTICATION REQUEST message, the mobile station shall store the received RAND together with the RES returned from the USIM in the volatile memory and associate it with CS domain. When the MS receives a subsequent AUTHENTICATION REQUEST message, if the stored RAND value for the CS domain is equal to the new received value in the AUTHENTICATION REQUEST message, then the mobile station shall not pass the RAND to the USIM, but shall immediately send the AUTHENTICATION RESPONSE message with the stored RES for the CS domain. If, for the CS domain, there is no valid stored RAND in the mobile station or the stored RAND is different from the new received value in the AUTHENTICATION REQUEST message, the mobile station shall pass the RAND to the USIM, shall override any previously stored RAND and RES with the new ones and start, or reset and restart timer T3218. The RAND and RES values stored in the mobile station shall be deleted and timer T3218, if running, shall be stopped: - upon receipt of a SECURITY MODE COMMAND (Iu mode only), CIPHERING MODE COMMAND (A/Gb mode only), CM_SERVICE_ACCEPT, CM_SERVICE_REJECT, LOCATION_UPDATING_ACCEPT or AUTHENTICATION REJECT message; - upon expiry of timer T3218; or - if the mobile station enters the MM state MM IDLE or NULL.
description: The A5 ciphering algorithm is used in GSM networks for encryption and decryption.
explanation: Classified as an Algorithm since it represents a specific method of encryption.
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