CCNA-SEC Lec#7 | All about IPSec

What’s the IPsec?
The IPsec (Internet Protocol Security) Protocol Suite is a set of network security protocols, developed to secure the network traffic by establishing encrypted tunnels between two or more end points  across the public network.
IPsec provides the core benefits of confidentiality through encryption, data integrity through hashing and HMAC, and authentication using digital signatures or using a pre-shared key (PSK).
IPsec Goals
The goals can be described as follows:
Confidentiality: provided through encryption changing clear text into cipher text.
Data integrity: provided through hashing and/or through Hashed Message Authentication Code (HMAC) to verify that data has not been manipulated during its transit across the network.
Authentication: provided through authenticating the VPN peers near the beginning of a VPN session using pre-shared keys (PSK) or digital signatures (leveraging digital certificates). Authentication can also be done continuously through the use of an HMAC, which includes a secret known only to two ends of the VPN.
Antireplay protection: when VPNs are established, the peers can sequentially number the packets, and if a packet is attempted to be replayed again (perhaps by an attacker), the packet will not be accepted because the VPN device believes it has already processed that packet.
Internet Key Exchange (IKE) Protocol

IPsec uses the Internet Key Exchange (IKE) protocol to negotiate and establish secured site-to-site or remote access VPN tunnels.
Internet Key Exchange (IKE) is a protocol used to set up a IPSec Security Associations (SAs) by define security attributes like encryption key, encryption algorithm, and mode, between IPSec peers.
Security Association (SA) is an one-way virtual tunnel between the two endpoints peers. Thus, for full communication to occur, two SA’s must be established, one for each direction.

The establishment of an IPsec connection takes place in two phases, called IKE phases:
■ IKE Phase 1: The two endpoints authenticate one another and negotiate keying material. This results in an encrypted tunnel used by Phase 2 for negotiating the ESP security associations.
■ IKE Phase 2: The two endpoints use the secure tunnel created in Phase 1 to establishes the IPsec tunnel (IPsec SA), which used to secure the actual user data that is passed between the two endpoints.
IKE relies on ISAKMP to establish an initial secure channel over which the IPsec tunnel can be negotiated. An IKE policy determines the attributes of the ISAKMP session (typically called an IKE SA), including the encryption type and hashing methods.
At IKE Phase 1, There are  five basic items need to be agreed upon between the two VPN endpoints peers as below:
■ Encryption algorithm:  This could be AES, DES or 3DES.
■ Hash algorithm: This could be  MD5 or SHA.
■ Diffie-Hellman (DH) group to use: for creating and sharing keys.
■ Authentication method: This could be pre-shared key (PSK) or  RSA signatures.
■ The SA Lifetime: How long until this IKE Phase 1 tunnel should be torn down.
IKE Phase 1 negotiation can happen in two modes,either using Main Mode which be slower, but more secure or using Aggressive Mode which  faster, but less secure.
There are a  two primary methods for implementing the encapsulation of IPsec header as below:
■ Authentication Header (AH)
■ Encapsulating Security Payload (ESP)
Authentication Header (AH)
– While IPsec uses Authentication Header (AH) to provide Data Integrity, Authentication, and Anti-Replay functions for IPsec VPN. Authentication Header (AH) DOES NOT provide any Data Encryption at all.
– AH uses a hash algorithm to compute a hash value on both the payload and header of a packet which cause AH is incompatible with NAT which 
changes the IP header of a packet during translation which reflect on the receiving device will believe the packet has been altered in transit, and reject the packet.
Encapsulation Security Payload (ESP)
– While IPsec uses ESP (Encapsulating Security Payload) to provide Data Integrity, Encryption, Authentication, and Anti-Replay functions for IPsec VPN.
– ESP uses a hash algorithm to compute a hash value on the payload only and not include the header of a packet which enable it to be compatible with NAT.

ESP is more widely deployed than AH, because ESP provides all the benefits of IPsec, that is, Confidentiality, Integrity, Authentication and Re-Play attack protection….
IPsec Modes
IPsec uses two methods for encryption tunnel and transport mode.
– If IPsec tunnel mode is used, the both of IP header and payload are encrypted in addition a new header be injected.
– But When transport mode is used, only the packet payload is encrypted and the original IP header is left intact.
First let’s have a look at AH and ESP and how they tread original IP packetScreen Shot 2013-11-12 at 11.52.30 AM.png
And now about how those IP protocols fit in the two modes.

Screen Shot 2013-11-12 at 11.52.17 AM.png

In the next lecture we will go deeply into the IPsec configuration and troubleshooting, See you !
By: Hatem Farag | CCIE#54446
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