Internet Key Exchange

Architecture

Most IPsec implementations consist of an IKE daemon that runs in user space and an IPsec stack in the kernel that processes the actual IP packets.
User-space daemons have easy access to mass storage containing configuration information, such as the IPsec endpoint addresses, keys and certificates, as required. Kernel modules, on the other hand, can process packets efficiently and with minimum overhead—which is important for performance reasons.
The IKE protocol uses UDP packets, usually on port 500, and generally requires 4-6 packets with 2-3 turn-around times to create an SA on both sides. The negotiated key material is then given to the IPsec stack. For instance, this could be an AES key, information identifying the IP endpoints and ports that are to be protected, as well as what type of IPsec tunnel has been created. The IPsec stack, in turn, intercepts the relevant IP packets if and where appropriate and performs encryption/decryption as required. Implementations vary on how the interception of the packets is done—for example, some use virtual devices, others take a slice out of the firewall, etc.

IKE Phases

IKE consists of two phases: phase 1 and phase 2.^[10]
IKE phase 1's purpose is to establish a secure authenticated communication channel by using the Diffie–Hellman key exchange algorithm to generate a shared secret key to encrypt further IKE communications. This negotiation results in one single bi-directional ISAKMP Security Association (SA).^[11] The authentication can be performed using either pre-shared key (shared secret), signatures, or public key encryption.^[12] Phase 1 operates in either Main Mode or Aggressive Mode. Main Mode protects the identity of the peers; Aggressive Mode does not.^[10]
During IKE phase 2, the IKE peers use the secure channel established in Phase 1 to negotiate Security Associations on behalf of other services like IPsec. The negotiation results in a minimum of two unidirectional security associations (one inbound and one outbound).^[13] Phase 2 operates only in Quick Mode.<sup>[10]

 The ISAKMP security association negotiated during Phase 1 includes the negotiation of the following attributes used for subsequent negotiations:

    An encryption algorithm to be used, such as the Data Encryption Standard (DES).

    A hash algorithm (MD5 or SHA, as used by AH or ESP).

    An authentication method, such as authentication using previously shared keys.

    A Diffie-Hellman group. Diffie and Hellman were two pioneers in the industry who invented public-key cryptography. In this method, instead of encrypting and decrypting with the same key, data is encrypted using a public key knowable to anyone, and decrypted using a private key that is kept secret. A Diffie-Hellman group defines the attributes of how to perform this type of cryptography. Four predefined groups derived from OAKLEY are specified in IKE and provision is allowed for defining new groups as well.

Internet Key Exchange
IPSec Key Exchange (IKE)</sup>