The purpose of IKE Phase 2 is to negotiate IPSec SAs.
IKE phase 2 performs the following functions. For details, see Cisco,
“IPSec Overview Part Four: Internet Key Exchange (IKE)”.
- Negotiates
IPSec SA parameters protected by an existing IKE SA
- Establishes IPSec security associations
- Periodically renegotiates IPSec SAs to ensure security
- Optionally performs an additional Diffie-Hellman exchange
A negotiated shared IPSec Phase 2 policy includes:
- IPSec
Security protocols
When IKE is not used to establish SAs, a single
transform set must be used. Before a transform set can be included
in a crypto map entry, it must be defined. A transform set specifies
one or two IPSec security protocols (either Encapsulation Security
Protocol (ESP) or Authentication Header (AH)
To select a transform
set, consider the following:
- For data confidentiality,
include an ESP.
- For data authentication for the outer IP header as well as the
data, include an AH.
- For data authentication (either using ESP or AH), choose from
the MD5 or SHA (HMAC keyed hash variants) authentication algorithms.
The SHA algorithm is generally considered stronger than MD5, but
is slower.
- Encryption—AES Counter mode (AES-CTR) are used are also used.
AES-CTR use ESP confidentiality mechanism and require the encryptor
to generate a unique per-packet value and to communicate this value
to the decryptor. AES-CTR must be used in conjunction with an authentication
function, such as HMAC-SHA.
- Authentication
- IPSec Mode—the options are tunnel and transport modes.
- Perfect Forward Secrecy— Perfect forward secrecy means that
a piece of an encryption system automatically and frequently changes
the keys it uses to encrypt and decrypt information, such that if
the latest key is compromised, it exposes only a small portion of
the user’s sensitive data.
If perfect forward secrecy (PFS) is
specified in the IPSec policy, a new Diffie-Hellman exchange is
performed with each quick mode, providing keying material that has
greater entropy (key material life) and thereby greater resistance
to cryptographic attacks. Each Diffie-Hellman exchange requires large
exponentiations, thereby increasing CPU use and exacting a performance
cost.
For IKE Phase 2, see crypto map ipsec command.
For IPSec Mode, see crypto ipsec mode command.