IPSec (ESP, AH, DES, MD5, SHA, DH)

IPSEC:

Internet Protocol Security (IPsec) is a protocol suite for securing Internet Protocol (IP) communications by authenticating and encrypting each IP packet of a communication session. IPsec also includes protocols for establishing mutual authentication between agents at the beginning of the session and negotiation of cryptographic keys to be used during the session.

IPsec is an end-to-end security scheme operating in the Internet Layer of the Internet Protocol Suite. It can be used in protecting data flows between a pair of hosts (host-to-host), between a pair of security gateways (network-to-network), or between a security gateway and a host (network-to-host).

ESP:

Encapsulating Security Payload (ESP) is a security protocol used to provide confidentiality (encryption), data origin authentication, integrity, optional anti-replay service, and limited traffic-flow confidentiality by defeating traffic-flow analysis. Figure 3 shows that the data payload is encrypted with ESP.

ESP provides confidentiality by performing encryption at the IP packet layer. It supports a variety of symmetric encryption algorithms. The default algorithm for IPSec is 56-bit DES. This cipher must be implemented to guarantee interoperability among IPSec products. Cisco products also support use of 3DES for strong encryption. Confidentiality may be selected independent of all other services.

NOTE:
Deciding whether to use AH or ESP in a given situation may seem complex, yet can be simplified to a few rules. When you want to make sure that data from an authenticated source gets transferred with integrity and doesn't need confidentiality, use the AH protocol. If you need to keep data private (confidentiality), then you must use ESP. ESP will encrypt the upper-layer protocols in transport mode and the entire original IP datagram in tunnel mode so that neither is readable from the wire. However, ESP can also provide authentication for the packets.

AH: 

Authentication Header (AH) provides authentication and integrity to the datagrams passed between two systems. This is achieved by applying a keyed one-way hash function to the datagram to create a message digest. If any part of the datagram is changed during transit, this will be detected by the receiver when it performs the same one-way hash function on the datagram and compares the value of the message digest that the sender has supplied. The fact that the one-way hash also involves the use of a secret shared between the two systems means that authenticity can be guaranteed.

AH may also enforce anti-replay protection by requiring that a receiving host set the replay bit in the header to indicate that the packet has been seen. Without it, an attacker may be able to resend the same packet many times; for example, sending a packet that withdraws $100 from account X. Figure 1 shows two routers and confirms that the data between them is sent in cleartext.


The AH function is applied to the entire datagram except for any mutable IP header fields that change in transit, such as Time To Live (TTL) fields that are modified by the routers along the transmission path. AH works as follows:

1)The IP header and data payload is hashed.

2)The hash is used to build a new AH header, which is appended to the original packet.

3)The new packet is transmitted to the IPSec peer router.

4)The peer router hashes the IP header and data payload, extracts the transmitted hash from the AH header, and compares the two hashes. The hashes must match exactly. If even one bit is changed in the transmitted packet, the hash output on the received packet will change and the AH header will not match.

DES:

Data Encryption Standard (DES) is used to encrypt and decrypt packet data; it turns cleartext into ciphertext via an encryption algorithm. The decryption algorithm on the remote end restores clear-text from ciphertext. Shared secret keys enable the encryption and decryption. DES uses a 56-bit key, ensuring high-performance encryption.

MD5:

Message Digest 5 (MD5) is a hash algorithm used to authenticate packet data. Cisco routers and the PIX Firewall use the MD5 hashed message authentication code (HMAC) variant that provides an additional level of hashing. A hash is a one-way encryption algorithm that takes an input message of arbitrary length and produces a fixed-length output message. IKE, AH, and ESP can use MD5 for authentication.

SHA:

Secure Hash Algorithm 1 (SHA-1) is a hash algorithm used to authenticate packet data. Cisco routers and the PIX Firewall use the SHA-1 HMAC variant, which provides an additional level of hashing. IKE, AH, and ESP can use SHA-1 for authentication.

DH:

Diffie-Hellman (D-H) is a public-key cryptography protocol. It allows two parties to establish a shared secret key used by encryption algorithms (DES or MD5, for example) over an insecure communications channel. D-H is used within IKE (described later in this article) to establish session keys. 768-bit and 1024-bit D-H groups are supported in the Cisco routers and PIX Firewall. The 1024-bit group is more secure.

References:

http://en.wikipedia.org/wiki/IPsec

http://www.ciscopress.com/articles/article.asp?p=25470