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I'm currently studying the A5/1 algorithm used for GSM encryption. I understood how thanks to the symmetric private Key and a public counter frame one can encrypt the communication between A and B.

However, what I don't really understand is how the symmetric private key K is managed ? If A wants to communicate with B. A and B have to share the same secret key K. Now, if A wants to communicate with C, A and C will have to share the same secret key. How does this key distribution is handled ? Does the phone company distribute them before the call ?

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GSM uses three different security algorithms - A3, A5, and A8. The A3 algorithm is used to authenticate the mobile device, the A5 algorithm to encrypt the data transmitted, and the A8 algorithm is used to generate the session key.

Hence, I believe you are interested in the A8 algorithm which deviates the session keys through a challenge and response handshake. However, I the session keys are shared between the user and his base station only and not between two communicating parties. The base station will decrypt the data and transfer it (most probably in plain text) to the other party's base station which then encrypts the data with the key shared with the other party of the conversation.

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The encrypion is between Mobile station and BTS, not between two Mobile Station, the encryption will be only between A and B, where A is your phone and B your BTS.

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