Explanation: The Key management for IPSec is called the Internet Key Exchange (IKE)
Note: IKE underwent a series of improvements establishing IKEv2 with RFC 4306. The basis of this answer is IKEv2.
The IKE protocol is a hybrid of three other protocols: ISAKMP (Internet Security Association and Key Management Protocol), Oakley and SKEME. ISAKMP provides a framework for authentication and key exchange, but does not define them (neither authentication nor key exchange). The Oakley protocol describes a series of modes for key exchange and the SKEME protocol defines key exchange techniques.
IKE—Internet Key Exchange. A hybrid protocol that implements Oakley and Skeme key exchanges inside the ISAKMP framework. IKE can be used with other protocols, but its initial implementation is with the IPSec protocol. IKE provides authentication of the IPSec peers, negotiates IPSec keys, and negotiates IPSec security associations.
IKE is implemented in accordance with RFC 2409, The Internet Key Exchange.
The Internet Key Exchange (IKE) security protocol is a key management protocol standard that is used in conjunction with the IPSec standard. IPSec can be configured without IKE, but IKE enhances IPSec by providing additional features, flexibility, and ease of configuration for the IPSec standard.
IKE is a hybrid protocol that implements the Oakley key exchange and the SKEME key exchange inside the Internet Security Association and Key Management Protocol (ISAKMP) framework. (ISAKMP, Oakley, and SKEME are security protocols implemented by IKE.)
IKE automatically negotiates IPSec security associations (SAs) and enables IPSec secure communications without costly manual preconfiguration. Specifically, IKE provides these benefits:
•Eliminates the need to manually specify all the IPSec security parameters in the crypto maps at both peers.
•Allows you to specify a lifetime for the IPSec security association.
•Allows encryption keys to change during IPSec sessions.
•Allows IPSec to provide anti-replay services.
•Permits certification authority (CA) support for a manageable, scalable IPSec implementation.
•Allows dynamic authentication of peers.
About ISAKMP
The Internet Security Association and Key Management Protocol (ISAKMP) is a framework that defines the phases for establishing a secure relationship and support for negotiation of security attributes, it does not establish sessions keys by itself, it is used along with the Oakley session key establishment protocol. The Secure Key Exchange Mechanism (SKEME) describes a secure exchange mechanism and Oakley defines the modes of operation needed to establish a secure connection.
ISAKMP provides a framework for Internet key management and provides the specific protocol support for negotiation of security attributes. Alone, it does not establish session keys. However it can be used with various session key establishment protocols, such as Oakley, to provide a complete solution to Internet key management.
About Oakley
The Oakley protocol uses a hybrid Diffie-Hellman technique to establish session keys on Internet hosts and routers. Oakley provides the important security property of Perfect Forward Secrecy (PFS) and is based on cryptographic techniques that have survived substantial public scrutiny. Oakley can be used by itself, if no attribute negotiation is needed, or Oakley can be used in conjunction with ISAKMP. When ISAKMP is used with Oakley, key escrow is not feasible.
The ISAKMP and Oakley protocols have been combined into a hybrid protocol. The resolution of ISAKMP with Oakley uses the framework of ISAKMP to support a subset of Oakley key exchange modes. This new key exchange protocol provides optional PFS, full security association attribute negotiation, and authentication methods that provide both repudiation and non-repudiation. Implementations of this protocol can be used to establish VPNs and also allow for users from remote sites (who may have a dynamically allocated IP address) access to a secure network.
About IPSec
The IETF's IPSec Working Group develops standards for IP-layer security mechanisms for both IPv4 and IPv6. The group also is developing generic key management protocols for use on the Internet. For more information, refer to the IP Security and Encryption Overview.
IPSec is a framework of open standards developed by the Internet Engineering Task Force (IETF) that provides security for transmission of sensitive information over unprotected networks such as the Internet. It acts at the network level and implements the following standards:
IPSec
Internet Key Exchange (IKE)
Data Encryption Standard (DES)
MD5 (HMAC variant)
SHA (HMAC variant)
Authentication Header (AH)
Encapsulating Security Payload (ESP)
IPSec services provide a robust security solution that is standards-based. IPSec also provides data authentication and anti-replay services in addition to data confidentiality services.
For more information regarding IPSec, refer to the chapter "Configuring IPSec Network Security."
About SKEME
SKEME constitutes a compact protocol that supports a variety of realistic scenarios and security models over Internet. It provides clear tradeoffs between security and performance as required by the different scenarios without incurring in unnecessary system complexity. The protocol supports key exchange based on public key, key distribution centers, or manual installation, and provides for fast and secure key refreshment. In addition, SKEME selectively provides perfect forward secrecy, allows for replaceability and negotiation of the underlying cryptographic primitives, and addresses privacy issues as anonymity and repudiatability
SKEME's basic mode is based on the use of public keys and a Diffie-Hellman shared secret generation.
However, SKEME is not restricted to the use of public keys, but also allows the use of a pre-shared key. This key can be obtained by manual distribution or by the intermediary of a key distribution center (KDC) such as Kerberos.
In short, SKEME contains four distinct modes:
Basic mode, which provides a key exchange based on public keys and ensures PFS thanks to Diffie-Hellman.
A key exchange based on the use of public keys, but without Diffie-Hellman.
A key exchange based on the use of a pre-shared key and on Diffie-Hellman.
A mechanism of fast rekeying based only on symmetrical algorithms.
In addition, SKEME is composed of three phases: SHARE, EXCH and AUTH.
During the SHARE phase, the peers exchange half-keys, encrypted with their respective public keys. These two half-keys are used to compute a secret key K. If anonymity is wanted, the identities of the two peers are also encrypted. If a shared secret already exists, this phase is skipped.
The exchange phase (EXCH) is used, depending on the selected mode, to exchange either Diffie-Hellman public values or nonces. The Diffie-Hellman shared secret will only be computed after the end of the exchanges.
The public values or nonces are authenticated during the authentication phase (AUTH), using the secret key established during the SHARE phase.
The messages from these three phases do not necessarily follow the order described above; in actual practice they are combined to minimize the number of exchanged messages.
References used for this question:
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 4: Cryptography (page 172).
http://tools.ietf.org/html/rfc4306
http://tools.ietf.org/html/rfc4301
http://en.wikipedia.org/wiki/Internet_Key_Exchange
CISCO ISAKMP and OAKLEY information
CISCO Configuring Internet Key Exchange Protocol
http://www.hsc.fr/ressources/articles/ipsec-tech/index.html.en
