4.5. SA and Key Management

All IPsec implementations MUST support both manual and automated SA and cryptographic key management. The IPsec protocols, AH and ESP, are largely independent of the associated SA management techniques, although the techniques involved do affect some of the security services offered by the protocols.

Impact on Security Services

For example, the optional anti-replay service available for AH and ESP requires automated SA management. Moreover, the granularity of key distribution employed with IPsec determines the granularity of authentication provided. In general, data origin authentication in AH and ESP is limited by the extent to which secrets used with the integrity algorithm (or with a key management protocol that creates such secrets) are shared among multiple possible sources.

The following text describes the minimum requirements for both types of SA management.

4.5.1. Manual Techniques

The simplest form of management is manual management, in which a person manually configures each system with keying material and SA management data relevant to secure communication with other systems. Manual techniques are practical in small, static environments but they do not scale well.

Example Use Case: A company could create a virtual private network (VPN) using IPsec in security gateways at several sites. If the number of sites is small, and since all the sites come under the purview of a single administrative domain, this might be a feasible context for manual management techniques. In this case, the security gateway might selectively protect traffic to and from other sites within the organization using a manually configured key, while not protecting traffic for other destinations.

It also might be appropriate when only selected communications need to be secured. A similar argument might apply to use of IPsec entirely within an organization for a small number of hosts and/or gateways. Manual management techniques often employ statically configured, symmetric keys, though other options also exist.

4.5.2. Automated SA and Key Management

Widespread deployment and use of IPsec requires an Internet-standard, scalable, automated, SA management protocol. Such support is required to facilitate use of the anti-replay features of AH and ESP, and to accommodate on-demand creation of SAs, e.g., for user- and session-oriented keying. (Note that the notion of "rekeying" an SA actually implies creation of a new SA with a new SPI, a process that generally implies use of an automated SA/key management protocol.)

Default Protocol: The default automated key management protocol selected for use with IPsec is IKEv2 [Kau05]. This document assumes the availability of certain functions from the key management protocol that are not supported by IKEv1. Other automated SA management protocols MAY be employed.

Multiple Keys per SA

When an automated SA/key management protocol is employed, the output from this protocol is used to generate multiple keys for a single SA. This also occurs because distinct keys are used for each of the two SAs created by IKE. If both integrity and confidentiality are employed, then a minimum of four keys are required. Additionally, some cryptographic algorithms may require multiple keys, e.g., 3DES.

The Key Management System may provide a separate string of bits for each key needed for the SA, or it may generate a single string of bits from which all keys are extracted.