Encryption process for any outgoing packet to the specified "remote IP":

-original payload P is encrypted (the bytes after the IP header) using RC4 with the preestablished key `KEY` to obtain encrypted payload `C=RC4(P, KEY)`

-The encrypted payload C is concatenated with the 16 byte output of `MD5(C|KEY)`, and the IP header is adjusted accordingly (increasing the packet total length, re-calculate the checksum) so that the outgoing packet will have payload `C | MD5(C|KEY)` and destination "remote IP"

Decryption process for any incoming packet from specified "remote IP":

-Check the packet payload for the MD5 authentication with the same given "key"
Specifically, divide the received packet payload (those bytes after the IP header) as two parts C, A where A is the last 16 bytes of the packet payload and C is the rest.

-If `MD5(C|KEY) == A`, decrypt C using RC4 to obtain
`P=RC4(C, KEY)`, change the payload of the incoming packet from C|A to P, and adjust the IP header accordingly (decrease the packet total length, re-calculate the checksum) so that the incoming packet will be restored to its original form without keyed md5 authentication before sent to the receiving process