code: public SRTCPCryptoContext deriveContext(long ssrc)
      {
          SRTCPCryptoContext pcc = null;
          pcc = new SRTCPCryptoContext(ssrc, masterKey,
                  masterSalt, policy);
          return pcc;
      }
docs: Derive a new SRTPCryptoContext for use with a new SSRC
  
  This method returns a new SRTPCryptoContext initialized with the data of
  this SRTPCryptoContext. Replacing the SSRC, Roll-over-Counter, and the
  key derivation rate the application cab use this SRTPCryptoContext to
  encrypt / decrypt a new stream (Synchronization source) inside one RTP
  session.
  
  Before the application can use this SRTPCryptoContext it must call the
  deriveSrtpKeys method.
  
  @param ssrc
  The SSRC for this context
  @return a new SRTPCryptoContext with all relevant data set.