Knowing IPsec

There was a dire need of communicating data packets securely over large public WAN (mainly Internet). The solution was development of many networking protocols among which IPsec is one of the most deployed. It gains its benefits from the fact that it can be easily adapted without making any changes in the attached peers. In this blog we will look into the overview of IPsec, its deployment techniques and its working.

Introduction to IPsec

IPsec is a framework of related protocols that secure communications at the network or packet processing layer. It can be used to protect one or more data flows between peers. IPsec enables data confidentiality, integrity, origin authentication and anti-replay. It consists of two main protocols.

Authentication Header (AH)

In this protocol, IP header and data payload is hashed. From this hash, a new AH header is build which is appended to the packet. This new packet is transmitted via router where the router hashes the header and the payload. Both the hashes need to be exactly matched. Even a single bit is changed, the AH header will not match.

Encapsulating Security Payload (ESP)

This is a security protocol to provide encryption and integrity to the data packets. The ESP is added after the standard IP header. As it contains standard IP header, it can be routed easily with standard IP devices. This makes it backwards-compatible with IP routers and even those devices that were not designed to operate with IPsec. ESP is performed at the IP packet layer. It contains six parts of which two parts are only authenticated (Security Parameter Index, Sequence Number) whereas the rest of the four parts are encrypted during transmission (Payload Data, Padding, Pad Length and Next Header). It supports multiple encryption protocols and is up to the user to decide which one to opt.

Encryption Technologies

There are two encryption modes available for IPsec. Both modes have their own uses and should be used with caution depending upon the solution.

Tunnel Mode

This encrypts both the payload and the header. IPsec in tunnel mode is used when the destination of the packet is different than the security termination point. The most common use of this mode is between gateways or from end station to gateway. The gateway serves as a proxy for the hosts. So when the origin of the packets differs from the device that is providing security, tunnel mode is used.

Transport Mode

In this encryption mode, only the data portion of each packet is encrypted. This mode is applicable between end stations or between end station and gateway.

How does it work

IPsec makes use of tunneling. The data packets that we define sensitive or interesting are sent through the tunnel securely. By defining the characteristics of the tunnel, the security protection measures of sensitive packets are defined. IPsec offers numerous technologies and encryption modes. But its working can be broken into five major steps. A brief overview is given below:

Interesting Traffic Initiation

The sensitive traffic that needs to be monitored is deemed interesting. After deciding about the traffic, the security policy is implemented on the configuration interface for the peers. For example, in Cisco routers, access lists can be used to make decisions about encryption of packets by way of crypto map sets. The lists can be assigned to the policy stating that if the packets are permitted then they must be encrypted else send unencrypted data packet. When this traffic transits the IPsec client, the IKE phase one is triggered.

IKE Phase One

In this step, first the IPsec peers are authenticated thus protecting the identities of the peers. Then the Internet Key Exchange (IKE) Security Associations (SA) policy is negotiated among the peers. This results in both the parties to have a shared secret matching key that helps in the IKE phase two. Also, in this phase, there is setting up of a secure tunnel through which the exchange of information for phase two will occur. This phase has two operating modes

  • Main Mode: There are three exchanges among the initiator and the receiver. In first exchange, algorithms and hashes are exchanged. The second exchange is responsible for generations of shared secret keying using Diffie-Hellman exchange. The last exchange is for the verification of the other side’s identity. All three of these exchanges are bi-directional.

  • Aggressive Mode: There are fewer exchanges in this mode. All the required information is squeezed making it faster to use. The only trouble is that information is shared before there is a secure channel making this mode vulnerable.

IKE Phase Two

This phase negotiates information for IPsec SA parameters through the IKE SA. Here as well IPsec policies are shared and then establish IPsec SAs. There is only a single mode (quick mode) in this phase. It exchanges nonce providing replay protection. These nonces generate new shared secret key material. If the lifetime for IPsec expires, it can renegotiate a new SA.

Data Transfer

Here the data is safely and securely transmitted through the IPsec tunnel. The sent packets are encrypted and decrypted using the specified encryption in the IPsec SAs.

Tunnel Termination

The tunnel may terminate by either deletion or by time out. Time out occurs when the specified time (sec) has passed or when specified number of bytes will have passed through the tunnel.