IPsec Packet by Packet

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Phase-1 of IPSEC

In 1st two message it negotiate proposal and IKE version and NAT-T capability 

Proposal/transform >>

HAGLE - HASH, AUTH, DH Group, Lifetime ,Encryption  

1st Message IPSec main mode:

SA Payload 

Proposal payload

Trasform payload

SPI/cookie




2nd IPsec Main mode message




3rd Message IPSec main mode:

In 3rd and 4th message, it will do key exchange and Nonce and NAT-D (IP & Port hash)

DH key exchange ::

DH algo used to agree on same shared secret and generate key material

DH public value is calculated  (public value is mathematical calculation of private keys)

-->>It generate public and private key's

--->>It share public key's (x^b)

(x^b)==>g^a mod p (MOD(generator of private key=a))

g^ab=>(x^b)^a mod p (large prime no)






==>Each side has their public key and nonce (Ni_b is the Initiator's Nonce, and Nr_B is the Responder's Nonce)

==>It generate  SKEYID = prf(pre-shared-key, Ni_b | Nr_b)

The Seed value is then combined with the DH Shared Secret (and a few other values) to create three Session Keys: 

1>>a Derevative key

 SKEYID_d = prf(SKEYID, g^ab | CKY-I | CKY-R | 0)

SKEYID_d (d for derivative): not used by Phase 1. 

-->Phase2 --->[It is used as seed key for Phase2 keys, i.e. seed key for production traffic keys in Plain English]

g^ab -->Shared secret in DH process


2>>Authentication key

SKEYID_a = prf(SKEYID, SKEYID_d | g^ab | CKY-I | CKY-R | 1)

(a for authentication) 

This key is used to protect message integrity in every subsequent packets as soon as both peers are authenticated (peers will authenticate each other in next 2 packets). Yes, I know, we verify the integrity by using a hash but throwing a key into a hash adds stronger security to hash and it's called HMAC.

HMAC=>keyed hash message authentication code (HMAC) is a specific type of message authentication code (MAC) involving a cryptographic hash function



[pre-shared +SKEYID_a +hash}

3>>an Encryption key. 

SKEYID_e = prf(SKEYID, SKEYID_a | g^ab | CKY-I | CKY-R | 2)

SKEYID_e (e for encryption): you'll see that the next 2 packets are also encrypted. 

As selected encryption algorithm for this phase was AES-CBC (128-bits) then we use AES with this key to symmetrically encrypt further data.


Next packet:


1. Hash of pre-shared key  

2. Identification 

ike 0:VPN:3: initiator: main mode get 3rd response...
ike 0:VPN:3: dec D3701B9BC03E3E03FA02FBC561FE57
ike 0:VPN:3: peer identifier IPV4_ADDR 20.1.1.20
ike 0:VPN:3: PSK authentication succeeded
ike 0:VPN:3: authentication OK
ike 0:VPN:3: established IKE SA d3701b9bc03e3e03/fa02fbc561fe5763

5th Message of Main mode:


6th Message of main mode:

5th and 6th Message in short



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