IPSec ESP encryption

[adel81]

I have one question,

I am learning IPSec and I have the following question: the ESP Trailer is encrypted separately or together with the payload ?

Regards.

[MHM Cisco World]

The ESP trailer add after the ipsec encrypt the data frame' so it not include in encryption.

MHM

[adel81]

But when you read this Cisco article https://www.ciscopress.com/articles/article.asp?p=25477 and see the picture below, ESP Trailer is encrypted !

[MHM Cisco World]

Correct' I was confuse between esp auth (which add to end) and esp tail (which is add to end of original data packet).

Yes esp tail not auth is included in encrypt hash.

MHM

[Flavio Miranda · Accepted solution]

@adel81 

  I would say together because the ESP itself become part of the payload. 

https://www.ibm.com/docs/en/i/7.4?topic=protocols-encapsulating-security-payload

"The ESP trailer and the optional authentication data are appended to the payload. When you use both encryption and authentication, ESP completely protects the original datagram because it is now the payload data for the new ESP packet. ESP, however, does not protect the new IP header. Gateways must use ESP in tunnel mode."

http://www.tcpipguide.com/free/t_IPSecEncapsulatingSecurityPayloadESP-3.htm#:~:text=Trailer%20Calculation%20and%20Placement,Figure%20124%20and%20Figure%20125.

"The IPSec Encapsulating Security Payload protocol allows the contents of a datagram to be encrypted, to ensure that only the intended recipient is able to see the data. It is implemented using three components: an ESP Header added to the front of a protected datagram, an ESP Trailer that follows the protected data, and an optional ESP Authentication Data field that provides authentication services similar to those provided by the Authentication Header (AH)."

[M02@rt37 · Accepted solution]

Hello @adel81 

The ESP trailer, which includes padding, padding length, and the Next Header field, is appended to the original data packet and is encrypted together with the payload to ensure confidentiality. In contrast, the ESP Authentication Data, added at the very end of the ESP packet, is not encrypted but is used to verify the integrity and authenticity of the packet. The authentication hash is calculated over the ESP Header, encrypted payload, and ESP Trailer, excluding the Authentication Data itself. This separation ensures both confidentiality and integrity while maintaining proper packet structure.