I believe it is the initial key, and then the checksum from the last packet is the new key. I never finished looking into it. There also might be a different key for both client and server. So far i've only looked at the crc function and not much on how it interacted with everything else too much.
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++[>++++++<-]>[<++++++>-]<.>++++[>+++++<-]>[<
+++++>-]<+.+++++++..+++.>>+++++[<++++++>-]<+
+.<<+++++++++++++++.>.+++.------.--------.>+.
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