Technical Tip: Decrypt ESP packets

[welnaggar]

Description

This article describes how to decrypt captured Encapsulated Security Payload (ESP) packets initiated or terminated on FortiGate using Wireshark.

Scope

FortiGate.

Solution

In some case, network administrators need to track specific packets that are encrypted and transferred through IPsec VPN tunnels.

ESP packets can be captured from the GUI under Network -> Packet capture or from the CLI with the following command:

diag sniffer packet any "esp and host 10.149.11.30" 6 0 a

To decrypt ESP packets, Security Association (SA) information needs to be available. List it from the CLI using this command:

diagnose vpn tunnel list

The output must be similar to this:

On Wireshark, open the PCAP file:

Edit the protocol preferences as follows:

1. Go to Edit -> Preferences -> Protocol -> ESP.
2. Enable the last 3 check-boxes and select 'Edit' next to ESP SAs.
3. Create two entries for the incoming and outgoing SAs.
4. For each line, add the information obtained from the VPN tunnel list.

It is necessary to add 0x to the very beginning of every key; eg. 9f7bff78bb0b9b38585dc7cfcc923491c974 -> 0x9f7bff78bb0b9b38585dc7cfcc923491c974.
Outgoing ESP.

• Protocol: Choose the IP protocol version; eg. IPv4.
• Src IP: The IP address of local gateway; eg. 172.16.1.2.
• Dst IP: The Ip address of the remote gateway; eg. 10.149.11.30.
• SPI: local SPI; eg. 0x7bfa58a1.
• Encryption: Choose the encryption type from the drop-down that matches the used algorithm; eg. AES-CBC [RFC3602].
• Encryption Key: The key after the encryption type; eg. 0x9f7bff78bb0b9b38585dc7cfcc923491c974481265468572b488cac5791450a5.
• Authentication: Choose the authentication type from the drop-down that matches the used algorithm; eg. HMAC-SHA-384-192 [RFC4868].
• Authentication Key: The key after the authentication type; eg. 0x8f94767df3524c694ccac5734db59ab55388dca78a55afe96b96fd4a04d62e642fd37f4b2ea03ff39a9a9125ce54f91f.

Incoming ESP.

• Protocol: Choose the IP protocol version; e.g. IPv4.
• Src IP: The IP address of local gateway; e.g. 10.149.11.30.
• Dst IP: The Ip address of the remote gateway; e.g. 172.16.1.2.
• SPI: remote SPI; e.g. 0x8bdd5fe9.
• Encryption: Choose the encryption type from the drop-down that matches the used algorithm; e.g. AES-CBC [RFC3602].
• Encryption Key: The key after the encryption type; e.g. 0xb8c64630a75f3a89721d448e789bf78cc84aa67a2b8d71836f5a050fa1310df5.
• Authentication: Choose the authentication type from the drop-down that matches the used algorithm; e.g. HMAC-SHA-384-192 [RFC4868].
• Authentication Key: The key after the authentication type; e.g. 0x04a99d4fc92dbfee1660c62c62defde2878bb163e50241ab96445924281060b2e1e3c3295c9c77e572bcd9818701dd3c.

5. Finally, select 'OK'. All encrypted packets can now be viewed.

Note:

Suppose the IPSec encryption algorithm (Phase 2 Proposal) is set to AES-GCM. In that case, it may be necessary to select 'AES-GCM with 16 octet ICV [RFC4106]' under the Wireshark ESP SAs dropdown for Encryption (if the option specifying the ICV length is not present on Wireshark, try updating the Wireshark version).

To check the exact IV length used, refer to the IKE debugs for the tunnel; otherwise, it will be necessary to use trial and error with different ICV lengths.

ESP preferences are local to the machine to which the details are added. To share the decrypted ESP packets with someone else, Go to Wireshark, select File, export PDUs to File, and select OS I Layer 3 from dropdown:

 

Related articles:

ESP preferences - Wireshark wiki

Technical Tip: Troubleshooting IPsec VPN tunnel errors with large size packets

Technical Tip: How to decrypt IPSec Phase-2 (ISAKMP) packets IKEv2

Technical Tip: How to decrypt IPSec Phase-1(ISAKMP) packets.

Decrypted ESP packets