Cisco Systems OL-24201-01 Key Distribution Algorithm, EAP-FAST PAC-Opaque Packing and Unpacking

Appendix B Authentication in ACS 5.3

EAP-FAST

This scheme improves the security by reducing the amount of cryptographic sensitive material that is transmitted.

This section contains the following topics:

•Key Distribution Algorithm, page B-28

•EAP-FASTPAC-Opaque Packing and Unpacking, page B-28

•Revocation Method, page B-28

•PAC Migration from ACS 4.x, page B-29

Key Distribution Algorithm

The common seed-key is a relatively large and a completely random buffer that is generated by the primary ACS server. The seed-key is generated only once during installation, or it can be manually regenerated by an administrator. The seed-key should rarely be replaced, because if you change seed-key, of all the previous master-keys and PACs would automatically be deactivated.

The seed-key is generated by using a FIPS approved RNG generator that exists in the runtime cryptographic module (CryptoLib). The ACS primary server management determines when to generate the seed-key, and communicates with the ACS runtime to request a new seed-key to be generated.

The size of the seed-key may vary and should consist of at least 64 bytes (512 bit). A larger seed might have some performance implication as each master-key derivation is dependant on it subsequently.

At any given time, a single seed-key should be used by each ACS server and the primary ACS server should ensure to distribute the latest seed-key to all the servers. Old seed-keys must discarded.

The seed-key contains critical cryptographic sensitive information. Disclosing the seed-key information would expose the entire EAP-FAST PAC mechanism to a large set of possible identity vulnerabilities.

Because of that, the mechanism which transports the seed-key between the primary and the secondary ACS servers must be fully secured. Further security measures must be taken with respect to storing the seed-key in the data-base. The seed-key should be protected with the strongest means of security.

EAP-FAST PAC-Opaque Packing and Unpacking

When the server generates a new PAC, it must derive the master-key to be used. When the server accepts a new PAC the same algorithm should be used for deriving the master-key with some additional verification used to prevent possible attacks on the master-key scheme. The derivation calculation may be skipped if the master-key was already placed in the cache in the past.

Revocation Method

You can revoke all PACs and all Master-Keys. For this type of extensive revocation, all you need to do is to revoke the seed-key and replace it by a new one.

Having only a single seed-key to be used in the system facilitates implementation.

	User Guide for Cisco Secure Access Control System 5.3
B-28	OL-24201-01