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I read this Wikipedia article and I can't understand, why GSM do challenge-response authencation before encryption? Why it doesn't start encryption at the beggining and authencate with it?

Why this:

  • MS sends its IMSI to operator
  • Operator check IMSI and sends correspoinding RAND
  • MS make response and Kc with RAND and Ki, store the Kc and sends the response to operator
  • Operator check the response and send info about used encryption protocol etc to MS
  • And now all communication will be encrypted with Kc

Instead of this:

  • MS sends its IMSI to operator
  • Operator check IMSI and sends correspoinding RAND
  • MS make Kc with RAND and Ki, store the Kc and starts encrypt all communication with the Kc
  • Operator decrypt all communication with Kc and if decrypted content is ok, authencation is done
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  • There might be a real answer to this but most likely it's simply there's no need for encryption before the MS is authenticated. Commented Dec 4, 2023 at 11:26
  • Yes, but my point is, why is separate authencation needed? Encryption with Kc do authencate as well, right? Commented Dec 4, 2023 at 12:58

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