IPsec, a Tutorial--Part IX

IKE Authentication Services
In an IPsec VPN using ISAKMP, IKE will be the channel over which security parameters are exchanged for IPsec SA negotiation. As such, it is absolutely critical that IKE SAs are established securely. To do this, IKE offers a number of robust authentication mechanisms to ensure that crypto endpoints are not exchanging information with would-be attackers instead of valid endpoints. Cisco IPsec VPN crypto endpoints support all of IKE's supported authentication protocols, which include:

• Pre-Shared Keys
• RSA Encryption (Encrypted Nonces)
• RSA Signatures (X.509 certificate based--requires X.509 CA)

For smaller networks on which keys can be manually defined, IKE preshared keys (PSKs) can be used. PSKs are manually defined in the IKE policy of each crypto endpoint. Once crypto and ISAKMP policies are attached to active crypto interfaces, IKE attempts to exchange PSKs with the appropriate crypto peer, or IPsec tunnel endpoint.

NOTE:
A defined, static PSK may specify a single peer to share a key with. Alternately, a range of peers can be specified for a single key using wildcard subnet masks in the ISAKMP key definition. A single IKE PSK defined for use with multiple peers using wildcard subnet masks is typically referred to as a wildcard preshared key.

To guarantee authenticity of this message exchange, IKE appends a message digest to the key through a user-defined hash algorithm (MD5 or SHA-1). Example 13 shows the required IKE configurations on James and Charlie.

While pre-shared keys are effective in smaller, trusted environments, the obvious pitfalls associated with manual keying exist. Somebody could verbally compromise the key, or the key could be a simple key that is easily guessed (administrator's home town, favorite baseball team, birthday, and so on). Because all peers share the same wildcard PSK, that key must be changed on all peers using that key, which can lead to increased administrative overhead. As such, wildcard PSKs have inherent security flaws primarily related to inability to effectively manage the eviction of a once-trusted IPsec peer in the group and are generally not recommended without the use of added authentication capabilities such as IKE extended authentication (x-Auth), IKE x-Auth and wildcard PSKs are discussed in greater detail in Chapter 12. In addition to IKE PSKs with x-Auth, IKE incorporates options for stronger keying in environments that require stronger security. Two such options are RSA Encryption (nonces), and RSA Signatures (CA-signed certificates), which enable administrators to have the crypto endpoint dynamically generate a pair of cryptographic keys for authentication.

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