A5 ENCRYPTION

GSM Glossary

GSM — A5 Encryption

 

 

 

 

 

 

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Information on A5 Encryption

The A5 is the algorithm used for encryption in GSM mobile phones. It can optionally be used on all voice and data connections. A5 is a stream cipher, which uses a 64-bit secret key. It’s implementation, however, is block based with a block length of 228 bits. A5 is designed to be efficiently implemented in hardware, involving only XOR and one-bit addition operations. A5 is notionally a secret algorithm, although it has now been widely publicised on the Internet.
     There are two versions of the A5 algorithm:

  • A5/1, which is used in Europe, and
  • A5/2, which is used in export systems.

    •      Known security vulnerabilities of the GSM A5 algorithm include:
  • In most deployed versions of GSM (employing COMP128), the 10 least significant bits of the key are set to zero, reducing the effective length of the key to 54 bits.
  • The keystream is frequently re-initialised, permitting attacks based on a known initial state.
  • While the state transition function of A5 is not uniquely invertible, it can be efficiently inverted because the number of possible parent states is small.
  • Cryptanalysis of A5/1 requires approximately 224 operations (with 248 pre-computed stored values.)
  • Cryptanalysis of A5/2 requires O(216) steps.


    • Other topics in our resources on GSM related to A5 Encryption include: 
     
  • GSM security
    		•
  • COMP128

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