[asterisk-security] register response calculation

[sipResearcher]

Hi,
   
  I have a simple question about SIP messaging. When a SIP client wants to register to SIP registrar (for example asterisk), it sends a REGISTER message and receives a Unauthorized message with a nonce value and it calculates a challenge response using username password and this nonce value. A looked up to the rfc about regsitration process but I couldn't understand how it computes this response value. 
   
  What is the formula for this calculation. Which parameters does it use exactly?

       
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