Packaging and unpacking process of ESP message under IPSec transmission mode

First map

Packaging process

1. In the original IP In the message payload（ It may be carrying TCP message ） Add later ESP trailer Information .

• ESP trailer Including three parts ：Padding, Pad length, Next header. Since the selected encryption algorithm may be block encryption , So it may need to be filled （Padding）; Fill length attached （Pad length） It is convenient to remove the filled data when unpacking ;Next Head Used to indicate the original IP The message payload The type of agreement , Such as 6 Express TCP.

1. The original IP In the message payload And ESP trailer Proceed as a whole encryption , The encryption algorithm and key are provided by SA（Security Association) give .
2. For the first time 2 Add the encrypted data obtained in step ESP header, Encrypted data and ESP header Collectively known as “Enchilada”, Yes “Enchilada” Make one Abstract , Get an integrity measure （ESP Message Authentication Data）, And add to the end , Integrity measurement algorithms include the following The authentication key to be used is provided by SA give .

• ESP header It consists of two parts ：SPI (Security Parameter Index),Sequence number. SPI Used with SA Association , from SAD Get information about the IPSec Packet protocol algorithm, key and other information ;Sequence number It is initially established as 0, Each packet sent will increase automatically 1, Used to resist retransmission attacks .

1. Put the original IP The head of the protocol Change the field to 50（ Indicates that the upper layer agreement is ESP）, And add to ESP header front , Complete packaging .

Unpacking process

1. The receiver receives IP After the message , The protocol type found is 50, That means this is one ESP package . First see ESP header, adopt SPI Determine the corresponding... Of the data message SA , Get the corresponding pattern (tunnel/transport mode) And safety regulations .
2. Calculation “enchilada” A summary of the section , And attached at the end Authentication Data comparing , Verify data integrity ; Check the sequence number , Make sure the data is “ fresh ” Of , Prevent replay attacks .
3. according to SA Encryption algorithm and key provided , Decrypt Encrypted data , Get the original IP The message payload And ESP trailer.
4. according to ESP trailer Of Next Header Judge ESP The type of message protocol encapsulated （ Such as TCP）, And according to Padding Length Fill in length information , Find the length of the filled field , Delete and get the original IP The message payload.
5. take IP Tou and Yuan IP The message payload Combine , Then we get the complete original IP message .

Classic illustration

notes ：payload（ Load data ） Is relative , about IP Protocol message ,payload yes TCP header + TCP payload, So pay attention to distinguish between the above words and pictures payload describe .