IKE Overview
Internet Key Exchange (IKE) negotiates the IPSec security associations (SAs). This process requires that the IPSec systems first authenticate themselves to each other and establish ISAKMP (IKE) shared keys.
NOTE
A security association (SA) is a relationship between two or more entities that describes how the entities will use security services to communicate securely.
In phase 1 of this process, IKE creates an authenticated, secure channel between the two IKE peers, called the IKE security association. The Diffie-Hellman key agreement is always performed in this phase.
In phase 2, IKE negotiates the IPSec security associations and generates the required key material for IPSec. The sender offers one or more transform sets that are used to specify an allowed combination of transforms with their respective settings. The sender also indicates the data flow to which the transform set is to be applied. The sender must offer at least one transform set. The receiver then sends back a single transform set, which indicates the mutually agreed-upon transforms and algorithms for this particular IPSec session. A new Diffie-Hellman agreement may be done in phase 2, or the keys may be derived from the phase 1 shared secret.
Figure 1 shows the role that IKE takes in the IPSec VPN creation process.
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Figure 1 The function of IKE.
IKE authenticates the peer and the IKE messages between the peers during IKE phase 1. Phase 1 consists of main mode or aggressive mode. (These modes are described later in this article.) Potential peers in an IPSec session must authenticate themselves to each other before IKE can proceed. Peer authentication occurs during the main mode exchange during IKE phase 1. The IKE protocol is very flexible and supports multiple authentication methods as part of the phase 1 exchange. The two entities must agree on a common authentication protocol through a negotiation process.
IKE phase 1 has three methods to authenticate IPSec peers in Cisco products:
Pre-shared keys. A key value entered into each peer manually (out of band) and used to authenticate the peer.
RSA signatures. Uses a digital certificate authenticated by an RSA signature.
RSA encrypted nonces. Uses RSA encryption to encrypt a nonce value (a random number generated by the peer) and other values.
A common value used by all authentication methods is the peer identity (ID), which helps identify the peer. Some ID values used are as follows:
IP address of the peer (four octets), such as 172.30.2.2.
Fully qualified domain name (FQDN), such as student@cisco.com.
Pre-Shared Keys
With pre-shared keys, the same pre-shared key is configured on each IPSec peer. IKE peers authenticate each other by computing and sending a keyed hash of data that includes the pre-shared key. If the receiving peer is able to independently create the same hash using its pre-shared key, then it knows that both peers must share the same secret, thus authenticating the other peer.
Pre-shared keys are easier to configure than manually configuring IPSec policy values on each IPSec peer. But pre-shared keys don't scale well because each IPSec peer must be configured with the pre-shared key of every other peer with which it will establish a session.
RSA Signatures
RSA is a public-key cryptosystem used by IPSec for authentication in IKE phase 1. RSA was developed in 1977 by Ron Rivest, Adi Shamir, and Leonard Adelman.
The RSA signatures method uses a digital signature setup in which each device digitally signs a set of data and sends it to the other party. RSA signatures use a certificate authority (CA) to generate a unique-identity digital certificate that's assigned to each peer for authentication. The identity digital certificate is similar in function to the pre-shared key, but provides much stronger security.
Each initiator and responder to an IKE session using RSA signatures sends its own ID value (IDi or IDr), its identity digital certificate, and an RSA signature value consisting of a variety of IKE values, all encrypted by the negotiated IKE encryption method (DES or 3DES).
RSA Encryption
The RSA-encrypted nonces method uses the RSA encryption public key cryptography standard. It requires that each party generate a pseudo-random number (a nonce) and encrypt it in the other party's RSA public key. Authentication occurs when each party decrypts the other party's nonce with a local private key (and other publicly and privately available information) and then uses the decrypted nonce to compute a keyed hash. This system provides for deniable transactions. That is, either side of the exchange can plausibly deny that it took part in the exchange.
Cisco IOS software is the only Cisco product that uses RSA-encrypted nonces for IKE authentication. RSA-encrypted nonces use the RSA public key algorithm.
Certificate Authorities and Digital Certificates
The distribution of keys in a public key scheme requires some trust. If the infrastructure is untrusted and control is questionable (such as on the Internet), distribution of keys is troublesome. RSA signatures are used by certificate authorities (CAs), which are trusted third-party organizations. VeriSign, Entrust, and Netscape are examples of companies that are providing digital certificates. A client registers with a certificate authority; after the CA verifies the client's credentials, a certificate is issued.
The digital certificate is a package containing information such as a certificate bearer's identity: his or her name or IP address, the certificate's serial number, the certificate's expiration date, and a copy of the certificate bearer's public key. The standard digital certificate format is defined in the X.509 specification. X.509 version 3 defines the data structure for certificates, and is the standard that Cisco is supporting. Figure 2 identifies some key points of CA operation.
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Figure 2 CAs and digital certificates.
How IPSec Works
IPSec involves many component technologies and encryption methods. Yet IPSec's operation can be broken down into five main steps:
"Interesting traffic" initiates the IPSec process. Traffic is deemed interesting when the IPSec security policy configured in the IPSec peers starts the IKE process.
IKE phase 1. IKE authenticates IPSec peers and negotiates IKE SAs during this phase, setting up a secure channel for negotiating IPSec SAs in phase 2.
IKE phase 2. IKE negotiates IPSec SA parameters and sets up matching IPSec SAs in the peers.
Data transfer. Data is transferred between IPSec peers based on the IPSec parameters and keys stored in the SA database.
IPSec tunnel termination. IPSec SAs terminate through deletion or by timing out.
This five-step process is shown in Figure 3.
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Figure 3 The five steps of IPSec.
Step 1—Defining Interesting Traffic
What type of traffic is deemed interesting is determined as part of formulating a security policy for use of a VPN. The policy is then implemented in the configuration interface for each particular IPSec peer. For example, in Cisco routers and PIX Firewalls, access lists are used to determine the traffic to encrypt. The access lists are assigned to a cryptography policy; the policy's permit statements indicate that the selected traffic must be encrypted, and deny statements indicate that the selected traffic must be sent unencrypted. With the Cisco Secure VPN Client, you use menu windows to select connections to be secured by IPSec. When interesting traffic is generated or transits the IPSec client, the client initiates the next step in the process, negotiating an IKE phase 1 exchange.
Step 1 is shown in Figure 4.
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Figure 4 Defining "interesting traffic."
Step 2—IKE Phase 1
The basic purpose of IKE phase 1 is to authenticate the IPSec peers and to set up a secure channel between the peers to enable IKE exchanges. IKE phase 1 performs the following functions:
Authenticates and protects the identities of the IPSec peers
Negotiates a matching IKE SA policy between peers to protect the IKE exchange
Performs an authenticated Diffie-Hellman exchange with the end result of having matching shared secret keys
Sets up a secure tunnel to negotiate IKE phase 2 parameters
IKE phase 1 occurs in two modes: main mode and aggressive mode. These modes are described in the following sections.
Main Mode
Main mode has three two-way exchanges between the initiator and the receiver.
First exchange: The algorithms and hashes used to secure the IKE communications are agreed upon in matching IKE SAs in each peer.
Second exchange: Uses a Diffie-Hellman exchange to generate shared secret keying material used to generate shared secret keys and to pass nonces—random numbers sent to the other party and then signed and returned to prove their identity.
Third exchange: Verifies the other side's identity. The identity value is the IPSec peer's IP address in encrypted form. The main outcome of main mode is matching IKE SAs between peers to provide a protected pipe for subsequent protected ISAKMP exchanges between the IKE peers. The IKE SA specifies values for the IKE exchange: the authentication method used, the encryption and hash algorithms, the Diffie-Hellman group used, the lifetime of the IKE SA in seconds or kilobytes, and the shared secret key values for the encryption algorithms. The IKE SA in each peer is bi-directional.
Aggressive Mode
In aggressive mode, fewer exchanges are made, and with fewer packets. On the first exchange, almost everything is squeezed into the proposed IKE SA values: the Diffie-Hellman public key; a nonce that the other party signs; and an identity packet, which can be used to verify identity via a third party. The receiver sends everything back that is needed to complete the exchange. The only thing left is for the initiator to confirm the exchange. The weakness of using the aggressive mode is that both sides have exchanged information before there's a secure channel. Therefore, it's possible to "sniff" the wire and discover who formed the new SA. However, it is faster than main mode.
Step 2 is shown in Figure 5.
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Figure 5 IKE phase 1.
Step 3—IKE Phase 2
The purpose of IKE phase 2 is to negotiate IPSec SAs to set up the IPSec tunnel. IKE phase 2 performs the following functions:
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
IKE phase 2 has one mode, called quick mode. Quick mode occurs after IKE has established the secure tunnel in phase 1. It negotiates a shared IPSec policy, derives shared secret keying material used for the IPSec security algorithms, and establishes IPSec SAs. Quick mode exchanges nonces that provide replay protection. The nonces are used to generate new shared secret key material and prevent replay attacks from generating bogus SAs.
Quick mode is also used to renegotiate a new IPSec SA when the IPSec SA lifetime expires. Base quick mode is used to refresh the keying material used to create the shared secret key based on the keying material derived from the Diffie-Hellman exchange in phase 1.
Perfect Forward Secrecy
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.
Step 4—IPSec Encrypted Tunnel
After IKE phase 2 is complete and quick mode has established IPSec SAs, information is exchanged via an IPSec tunnel. Packets are encrypted and decrypted using the encryption specified in the IPSec SA. This IPSec encrypted tunnel can be seen in Figure 6.
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Figure 6 IPSec encrypted tunnel.
Step 5—Tunnel Termination
IPSec SAs terminate through deletion or by timing out (see Figure 7). An SA can time out when a specified number of seconds have elapsed or when a specified number of bytes have passed through the tunnel. When the SAs terminate, the keys are also discarded. When subsequent IPSec SAs are needed for a flow, IKE performs a new phase 2 and, if necessary, a new phase 1 negotiation. A successful negotiation results in new SAs and new keys. New SAs can be established before the existing SAs expire, so that a given flow can continue uninterrupted.
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Figure 7 Tunnel termination.
This brings us to the end of the fourth part of this five-part series of articles covering IPSec. Be sure to catch the next installment.