IPSec

IPSec Explained Visually | AH, ESP, Tunnel Mode, IKE & OSI Layers SimplifiedCyber connect · IPSec overview: AH, ESP, tunnel mode and IKE, 6 minIP Sec Fundamentals ESP Tunnel and Transport ModeFirewall Experts · ESP in transport mode and tunnel mode, 15 minIPSec Fundamentals -Authentication Header Introduction Deep DriveFirewall Experts · the Authentication Header in both modes, 14 min

IP security basicsSearch on YouTubeBookmarkSaved

Why IPSec existsSearch on YouTubeBookmarkSaved

There are many application-specific security mechanisms, for example S/MIME, PGP, Kerberos and SSL/HTTPS. But some security concerns cut across protocol layers. Thus, we want the network to supply security for all applications.

IPSec is the general IP security mechanism. It provides:

• Authentication.
• Confidentiality.
• Key management.

You can use IPSec over LANs, across public and private WANs, and for the Internet. A 1994 report identified the need. It asked for authentication and encryption in IPv4 and IPv6.

IP security usesSearch on YouTubeBookmarkSaved

The slide figure shows two places where IPSec can run:

On the public (Internet) or private network, each packet has an IP header, an IPSec header and a secure IP payload. On the local Ethernet behind the device, each packet has only an IP header and an IP payload.

Benefits of IPSecSearch on YouTubeBookmarkSaved

The slides give six benefits:

1. In a firewall or router, IPSec gives strong security to all traffic that crosses the perimeter.
2. In a firewall or router, IPSec is resistant to bypass.
3. IPSec is below the transport layer. Thus, it is transparent to applications.
4. IPSec can be transparent to end users.
5. IPSec can give security to individual users.
6. IPSec secures the routing architecture.

Architecture and servicesSearch on YouTubeBookmarkSaved

IP security architectureSearch on YouTubeBookmarkSaved

The specification is quite complex. It has these groups:

AH gives authentication and integrity, but no encryption. ESP gives encryption, and it can also give authentication. IKE makes and exchanges the keys for AH and ESP.

IPSec servicesSearch on YouTubeBookmarkSaved

The slides list six services:

1. Access control.
2. Connectionless integrity.
3. Data origin authentication.
4. Rejection of replayed packets. This is a form of partial sequence integrity.
5. Confidentiality (encryption).
6. Limited traffic flow confidentiality.

Transport and tunnel modesSearch on YouTubeBookmarkSaved

Mode comparisonSearch on YouTubeBookmarkSaved

The slide figures show the two uses:

• Transport-level security: an encrypted TCP session between two hosts, across an internal and an external network.
• A virtual private network through tunnel mode: encrypted tunnels carry IP traffic between corporate networks across the Internet.

Mode packet layoutsSearch on YouTubeBookmarkSaved

The protocol figure builds each packet layer by layer. Application data gets a TCP header, then an IP header, then IPSec.

ESP in transport mode:

orig IP hdr | ESP hdr | TCP hdr | Data | ESP trlr | ESP auth

ESP in tunnel mode:

new IP hdr | ESP hdr | orig IP hdr | TCP hdr | Data | ESP trlr | ESP auth

Tunnel mode puts the full original packet, with its header, inside a new IP packet. The outer header gives the tunnel endpoints. The inner header gives the true source and destination.

Security associationsSearch on YouTubeBookmarkSaved

What an SA definesSearch on YouTubeBookmarkSaved

A security association (SA) defines how the communication is secured. An SA is a logical connection. It defines:

• The encryption algorithm, for example AES or DES.
• The authentication algorithm, for example SHA or MD5.
• The keys.
• The lifetime.

An SA is one-way. Two-way traffic needs one SA in each direction.

SA parametersSearch on YouTubeBookmarkSaved

Three parameters define an SA:

An SA also has other parameters, for example the sequence number, AH and ESP information, and the lifetime. Each IPSec system keeps a database of security associations.

Encapsulating Security PayloadSearch on YouTubeBookmarkSaved

ESP servicesSearch on YouTubeBookmarkSaved

ESP provides these services:

• Message content confidentiality.
• Data origin authentication.
• Connectionless integrity.
• An anti-replay service.
• Limited traffic flow confidentiality.

The services depend on the options that you select when you establish the SA. They also depend on the network location. ESP can use a variety of encryption and authentication algorithms.

ESP packet formatSearch on YouTubeBookmarkSaved

The figure shows two spans. The encrypted span covers the payload data, the padding, the pad length and the next header. The ICV coverage starts at the SPI and stops before the ICV.

Encryption and paddingSearch on YouTubeBookmarkSaved

• ESP can encrypt the payload data, padding, pad length and next header fields.
• If the cipher needs an IV, the IV is at the start of the payload data.
• ESP can have an optional ICV for integrity.
• ESP computes the ICV after encryption.

ESP uses padding for two reasons:

1. To expand the plaintext to the necessary length.
2. To align the pad length and next header fields.

Anti-replay serviceSearch on YouTubeBookmarkSaved

In a replay, an attacker sends a copy of an authenticated packet again. ESP uses the sequence number to stop this attack:

1. The sender sets the sequence number to 0 when a new SA is established.
2. The sender increments the number for each packet.
3. The number must not exceed $2^{32} - 1$.
4. The receiver accepts packets with a sequence number in the window $(N - W + 1)$ to $N$.

$N$ is the highest sequence number received so far. $W$ is the window size.

ESP in IPv4 and IPv6Search on YouTubeBookmarkSaved

Before ESP, the packets are:

IPv4: orig IP hdr | TCP | Data
IPv6: orig IP hdr | extension headers (if present) | TCP | Data

Transport mode with ESP:

IPv4: orig IP hdr | ESP hdr | TCP | Data | ESP trlr | ESP auth
IPv6: orig IP hdr | hop-by-hop, dest, routing, fragment | ESP hdr | dest | TCP | Data | ESP trlr | ESP auth

• The authenticated span starts at the ESP header and stops before the ESP auth field.
• The encrypted span starts after the ESP header and ends with the ESP trailer.
• In IPv6, the hop-by-hop, destination, routing and fragment headers come before the ESP header. They are not authenticated or encrypted.
• A second destination header can come after the ESP header. ESP encrypts it.

Tunnel mode with ESP:

IPv4: new IP hdr | ESP hdr | orig IP hdr | TCP | Data | ESP trlr | ESP auth
IPv6: new IP hdr | ext headers | ESP hdr | orig IP hdr | ext headers | TCP | Data | ESP trlr | ESP auth

• The authenticated span starts at the ESP header and stops before the ESP auth field.
• The encrypted span covers the full inner packet and the ESP trailer.

Combining security associationsSearch on YouTubeBookmarkSaved

SA bundlesSearch on YouTubeBookmarkSaved

One SA can implement either AH or ESP. To implement both, you must combine SAs:

• The combined SAs form a security association bundle.
• The SAs in a bundle can end at different or the same endpoints.
• Two methods combine SAs: transport adjacency and iterated tunneling.

Transport adjacency applies more than one security protocol to the same IP packet, with no tunnel. Iterated tunneling puts one tunnel inside another tunnel.

There are three ways to combine authentication and encryption:

1. ESP with authentication.
2. A bundle of inner ESP and outer AH.
3. A bundle of inner transport and outer ESP.

Four combination casesSearch on YouTubeBookmarkSaved

Quick self-testSearch on YouTubeBookmarkSaved