Fix key exchange

Ferdoranrequested to merge
Ferdoran/phoenix:fix-key-exchange into develop

Fixed the KeyExchange. There were multiple issues:

  • The KeyExchangeBase contained multiple errors calculating the challenges and keys, as well as sending them.
  • The KeyExchangeBase.GetSecureKey() did not transform the key correctly
  • The Message.EncryptionOffset was incorrectly initialized

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