[VPN] decrypting an ESP packet

[tictac toe]

hi 

I am trying to implement a vpn client. Now i am
implementing IPSEC and am facing a problem with the
ESP packet format. I understand that the payload and
ESP trailer are encrypted. I am able to identify the
SPI and Sequence number fields in the ESP header.

The problems are:

1) While stripping the payload which is of variable
length, i am unable to find out where the payload ends
since, there is no option of extracting the payload's
length in any field. I am using the AES algorithm for
encryption & SHA for hashing.

2) In the AES algorithm, i would like to know the size
of the output blocks irrespective of the key size
(128/192/256 bits) and the block size (128/192/256
bits).

Any inputs?

thanks and regards


	
	
		
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