Sign in Subscribe

What is the process of deciphering in GSM communication?

Last updated on 


The process of deciphering in GSM (Global System for Mobile Communications) communication is the reverse of the ciphering process. It involves the decryption of encrypted user data and signaling messages to recover the original information. The A5 algorithm family is used for ciphering and deciphering in GSM, and the specific algorithm employed depends on the network configuration. Here's a technical explanation of the deciphering process in GSM:

  1. Key Retrieval:
    • The process begins with the retrieval of the necessary keys. The Cipher Key (CK) used for ciphering is known by both the sender and the receiver. The Integrity Key (IK) is also retrieved during the authentication process.
    • The Sequence Number (SQN) is used to ensure the freshness of the keys and protect against replay attacks.
  2. Ciphering Key Generation:
    • The Ciphering Key (CK) is combined with the current value of the Sequence Number (SQN) to generate the actual key used for deciphering. This step ensures that the correct key is used for each communication session, preventing replay attacks.
  3. A5 Decryption:
    • The A5 decryption algorithm is applied to the received encrypted data using the generated Ciphering Key (CK). The specific A5 algorithm (A5/1, A5/2, or A5/3) depends on the GSM network configuration. A5/1 is the most widely used for voice and data decryption.
  4. Deciphering of User Data:
    • The decrypted data corresponds to the original user data, such as voice or text messages, that was initially encrypted before transmission.
    • The deciphered data is then processed and delivered to the intended recipient.
  5. Deciphering of Signaling Messages:
    • Signaling messages exchanged between the mobile station and the network are also deciphered using the A5 algorithm and the Ciphering Key. This ensures that the network can interpret and respond to the received signaling messages.
  6. Integrity Check:
    • The Integrity Key (IK) is used to perform an integrity check on the received data. The integrity check ensures that the data has not been tampered with during transmission.
  7. Verification of Sequence Number:
    • The received Sequence Number (SQN) is checked against the expected value. This step ensures that the received message is fresh and has not been replayed.
  8. Final Processing:
    • Once the data has been decrypted, integrity-checked, and the sequence number verified, it can be further processed or delivered to the intended destination within the GSM network.

In summary, the deciphering process in GSM involves retrieving the necessary keys, generating the Ciphering Key using the current Sequence Number, applying the A5 decryption algorithm, verifying the integrity of the received data, and checking the freshness of the data using the Sequence Number. This process ensures the confidentiality and integrity of user data and signaling messages in GSM communication.