Ciphering:

• Ciphering is one of the security procedures designed to protect the subscriber identity and data. It is an optional procedure in GSM.
• When ciphering is active, all information exchanged between the mobile and the network on the dedicated radio channels is encrypted. The key previously set between the network and the MS is used to encipher and to decipher the encrypted information.
• During the ciphering procedure the ciphering key Kc is set between the network and the MS. Ciphering is initiated after the ciphering key is set on the dedicated signalling channel (SDCCH or FACCH).
• The GSM uses stream ciphering, which means that the data flow transmitted on the SDCCH/TCH is the binary bit by bit addition of the user data flow to a ciphering bit stream.
• If ciphering algorithm is changed during Assignment or Handover then BSC passes the cipher mode setting to the MS in Assignment Command or Handover Command message.
• In an MSC-controlled handover, the MSC tells the new BSS whether encryption is required in the HANDOVER REQUEST message.
• In a BSC-controlled handover, the BSC passes the encryption information to the target BTS on the activation of the channels if the encryption information was received from the MSC.
• The CIPHER MODE COMMAND message (from the MSC to the BSC) contains the following data:

1. Message type
2. Layer 3 header information
3. Encryption information: permitted algorithms and ciphering key.
4. Cipher response mode.

• The message contains the encryption instructions and the L3 header information for the MS.
• When ciphering is required, the encryption information element contains information for the BTS to load the encryption device with the appropriate key.