help : about tcp data encrypt!

Now , I'm working under Linux7.2(kernel 2.4.9)
and I want to encrypt all transfer layer data with my own encrypt functions.
And let the Lan computers communcate under a VPN like envrionment.
I use the nf_register_hook(..) function to register a module function under netfilter
to get the transfer layer data and encrypt/decrypt them. but when I do this, the kernel allways panic. I don't what's going on?
do I give the wrong checksum, or the skb must be clone to operate?

somebody can help me???? thanks a lot! :)

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What type of Crypt scheme is it?

now may be just a user defined encrypt/decrypt function. to encrypt the transfer layer data to confuse sniffer tools.