IPsec Main and Quick Mode packet flow (Decrypted)

-

Main Mode 1st Message:

Generation of the initiator cookie — An 8-byte pseudo-random number used for anticlogging CKY-I = md5{(src_ip, dest_ip), random number, time, and date}

Generation of the responder cookie — An 8-byte pseudo-random number used for anticlogging CKY-R = md5{(src_ip, dest_ip), random number, time, and date}





Main Mode 2nd Message:





Before 3rd and 4th message:

  • Public key: Xa ,Xb
  • Nonce = Ni, Nr

How public key is created?
  • DH public value = Xa 
  • Xa = g^a mod p
  • g is the generator 
  • p is a large prime number 
  • a is a private secret known only to the initiator

Main Mode 3rd message:

  • Key exchange
  • Public key
  • NAT-D





Main Mode 4th Message:
  • Key exchange
  • Public key
  • NAT-D





NAT-D



Before 5th and 6th message:

Initiator secret = (Xb)a mod p = (Xa)b mod p = responder secret 

This value is the shared secret between the two parties and is also equal to g^ab

PRF => Pseudo random function based on negotiated hash
  • SKEYID's = PRF [Pre-shared key, Ni ,Nr ]   
  • SKEYID_d = PRF [SKEYID, g^ab | CKY-I | CKI-R| 0 ]  ==> To calculate key material
  • SKEYID_a = PRF [SKEYID, SKEYID_d, g^ab | CKEY-I | CKI-R | 1 ]  ==> Data Integrity and Authentication to subsequent message
  • SKEYID_e = PRF [SKEYID, SKEYID_a, g^ab | CKEY-I | CKI-R | 2 ] ==> Encrypt subsequent message


Main Mode 5th Message:






Main Mode 6th Message:






Quick Mode 1st Message:

If PFS (Perfect forward secrecy) is enabled, it will created new DH secret
  • New Nonce Generated: Nr'
  • New DH public value = Xb'
  • Xb' = g^b mod p 
  • g is the generator 
  • p is a large prime number 
  • b is a private secret known only to the responder







Quick Mode 2nd Message:







Initiator generates IPsec keying material ==>

Step 1. Generate new DH shared secret = (Xb')a mod p

Step 2. IPsec session key for incoming IPsec SA
= PRF (SKEYID_d, protocol (ISAKMP), new DH shared secret, SPIr, Ni', Nr')

Step 3. IPsec session key for outgoing IPsec SA 
= PRF (SKEYID_d, protocol (ISAKMP), new DH shared secret, SPIi, Ni', Nr')

Responder generates IPsec keying material ==>

Step 1. Generate new DH shared secret = (Xa')b mod p 

Step 2. IPsec session key for incoming IPsec SA 
= PRF (SKEYID_d, protocol (ISAKMP), new DH shared secret, SPIi, Ni', Nr')

Step 3. IPsec session key for outgoing IPsec SA 
= PRF (SKEYID_d, protocol (ISAKMP), new DH shared secret, SPIr, Ni', Nr')



Quick Mode 3rd Message:








Comments

Post a Comment

Popular posts from this blog

Backup your fortigate

-
         It is critical to take backup when it comes to upgrading or before making any configuration changes.  You should take the following backup: Configuration file Local certificates Why I should take Certificate backup? Because Unique SSL inspection CA and server certificates that are generated by your FortiGate by default are not saved in a system backup. Where I can save the backup? local PC, USB key, FTP, and TFTP server What if I have VDOM? If you have VDOMs, you can back up the configuration of the entire FortiGate or only a specific VDOM How to Backup? Click on the user name in the upper right-hand corner of the screen and select Configuration > Backup. Direct the backup to your Local PC or to a USB Disk.     The USB Disk option will not be available if no USB drive is       inserted in the USB port. If VDOMs are enabled, you can collect specific VDOM backup or the entire configuration Encryption. Encryption must ...
Read more

How NAT-Traversal works in IPsec

-
Image
In first two packet, it will negotiate NAT-T compatibility  1st Message in Main mode: 2nd Message in main mode: In 3rd and 4th Message, it will exchange hash of IP address and Port. 3rd Message main mode: 4th Message in main mode Debug on FGT: ike 0: comes 10.1.1.20:500->10.1.1.10:500,ifindex=4.... ike 0: IKEv1 exchange=Identity Protection id=987e2067d9c61bc0/3954de8f43e2c2c0 len=348 ike 0: in 987E2067D9C61BC03954DE8F43E2C2C004100200000000000000015C0A000104CD2F264257D538BA3EC01D4446EB541E8923 ike 0:VPN:14: responder:main mode get 2nd message... ike 0:VPN:14: received NAT-D payload type 20    ike 0:VPN:14: received NAT-D payload type 20 ike 0:VPN:14: NAT not detected   <<-----------------------       If hash matches, there is no NAT device in between and It continue to use same port 500 for remaining communication If there is NAT device in between both peer:==> 3rd message in main mode: 4th message in main mode: ike 0:VPN:3: ...
Read more