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IPSec, like many secure networking protocol sets, is based on the concept of a “shared secret”. Two devices that want to send information securely encode and decode it using a piece of information that only they know. Anyone who isn't “in” on the secret is able to intercept the information but is prevented either from reading it (if ESP is used to encrypt the payload) or from tampering with it undetected (if AH is used). Before either AH or ESP can be used, however, it is necessary for the two devices to exchange the “secret” that the security protocols themselves will use. The primary support protocol used for this purpose in IPSec is called Internet Key Exchange (IKE).
IKE is defined in RFC 2409, and is one of the more complicated of the IPSec protocols to comprehend. In fact, it is simply impossible to truly understand more than a real simplification of its operation without significant background in cryptography. I don't have a background in cryptography and I must assume that you, my reader, do not either. So rather than fill this topic with baffling acronyms and unexplained concepts, I will just provide a brief outline of IKE and how it is used.
The purpose of IKE is to allow devices to exchange information required for secure communication. As the title suggests, this includes cryptographic keys used for encoding authentication information and performing payload encryption. IKE works by allowing IPSec-capable devices to exchange security associations (SAs), to populate their security association databases (SADs). These are then used for the actual exchange of secured datagrams with the AH and ESP protocols.
IKE is considered a “hybrid” protocol because it combines (and supplements) the functions of three other protocols. The first of these is the Internet Security Association and Key Management Protocol (ISAKMP). This protocol provides a framework for exchanging encryption keys and security association information. It operates by allowing security associations to be negotiated through a series of phases.
ISAKMP is a generic protocol that supports many different key exchange methods. In IKE, the ISAKMP framework is used as the basis for a specific key exchange method that combines features from two key exchange protocols:
- OAKLEY: Describes a specific mechanism for exchanging keys through the definition of various key exchange “modes”. Most of the IKE key exchange process is based on OAKLEY.
- SKEME: Describes a different key exchange mechanism than OAKLEY. IKE uses some features from SKEME, including its method of public key encryption and its fast re-keying feature.
IKE Operation
So, IKE doesn't strictly implement either OAKLEY or SKEME but takes bits of each to form its own method of using ISAKMP. Clear as mud, I know. Since IKE functions within the framework of ISAKMP, its operation is based on the ISAKMP phased negotiation process. There are two phases:
- ISAKMP Phase 1: The first phase is a “setup” stage where two devices agree on how to exchange further information securely. This negotiation between the two units creates a security association for ISAKMP itself; an ISAKMP SA. This security association is then used for securely exchanging more detailed information in Phase 2.
- ISAKMP Phase 2: In this phase the ISAKMP SA established in Phase 1 is used to create SAs for other security protocols. Normally, this is where the parameters for the “real” SAs for the AH and ESP protocols would be negotiated.
An obvious question is why IKE bothers with this two-phased approach; why not just negotiate the security association for AH or ESP in the first place? Well, even though the extra phase adds overhead, multiple Phase 2 negotiations can be conducted after one Phase 1, which amortizes the extra “cost” of the two-phase approach. It is also possible to use a simpler exchange method for Phase 2 once the ISAKMP security association has been established in Phase 1.
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.
Note that even though security associations in general are unidirectional, the ISAKMP SA is established bidirectionally. Once Phase 1 is complete, then, either device can set up a subsequent SA for AH or ESP using it.