IPsec

From Exampleproblems

IPsec (IP security) is a standard for securing Internet Protocol (IP) communications by encrypting and/or authenticating all IP packets. IPsec provides security at the network layer.

IPsec is a set of cryptographic protocols for (1) securing packet flows and (2) key exchange. Of the former, there are two: Encapsulating Security Payload (ESP) provides authentication, data confidentiality and message integrity; Authentication Header (AH) provides authentication and message integrity, but does not offer confidentiality. Originally AH was only used for integrity and ESP was used only for encryption; authentication functionality was added subsequently to ESP. Currently only one key exchange protocol is defined, the IKE (Internet Key Exchange) protocol.

1 Current status as a standard
2 Design intent
3 IPsec compared to other Internet security protocols
4 Technical details
- 4.1 Authentication Header
- 4.2 Encapsulated Security Payload (ESP)
5 Implementations
6 See also
7 Overview of IPsec-related RFCs
8 External links

Current status as a standard

IPsec is an obligatory part of IPv6, and is optional for use with IPv4. While the standard is designed to be agnostic to IP versions, current widespread deployment and experience concerns IPv4 implementations. IPsec protocols are defined by RFCs 2401–2412. As of 2005, work is progressing to release updated replacement documents, most notably for the key exchange protocol IKE. The year 2005 also saw the standardization of a UDP encapsulation mechanism for NAT traversal: NAT-T.

Design intent

IPsec was intended to provide either (1) tunnel mode: portal-to-portal communications security in which security of packet traffic is provided to several machines (even to whole LANs) by a single node, or (2) transport mode: end-to-end security of packet traffic in which the end-point computers do the security processing. It can be used to construct Virtual Private Networks (VPN) in either mode, and this is the dominant use. Note, however, that the security implications are quite different between the two operational modes.

End-to-end communication security on an Internet-wide scale has been slower to develop than many had expected. Part of the reason is that no universal, or universally trusted, Public Key Infrastructure (PKI) has emerged (DNSSEC was originally envisioned for this); part is that many users understand neither their needs nor the available options well enough to promote inclusion in vendors' products.

IPsec compared to other Internet security protocols

IPsec protocols operate at layer 3 of the OSI model, which makes them suitable for protecting both TCP and UDP-based protocols when used alone. This means that, compared with transport layer and above protocols such as SSL (OSI Layer 6), which cannot protect UDP level traffic, the IPsec protocols must cope with reliability and fragmentation issues, adding their complexity and processing overhead. SSL/TLS, in contrast, rely on a higher level layer TCP (OSI Layer 4) to manage reliability and fragmentation.

Technical details

Authentication Header

Authentication Header (AH) is intended to guarantee connectionless integrity and data origin authentication of IP datagrams. Further, it can optionally protect against replay attacks by using the sliding window technique and discarding old packets. AH tries to protect all fields of an IP datagram. Only fields changeable during transfer of an IP packet are excluded.

An AH packet diagram:

0	1	2	3
0 1 2 3 4 5 6 7	0 1 2 3 4 5 6 7	0 1 2 3 4 5 6 7	0 1 2 3 4 5 6 7
Next Header	Payload Length	RESERVED
Security Parameters Index (SPI)
Sequence Number
Authentication Data (variable)

Field meanings:

Next Header: Identifies the protocol of the transferred data.
Payload Length: Size of AH packet.
RESERVED: Reserved for future use (all zero until then).
Security Parameters Index (SPI): Identifies the security parameters in combination with IP address.
Sequence Number: A monotonically increasing number, used to prevent replay attacks.
Authentication Data: Contains the data necessary to authenticate the packet.

Encapsulated Security Payload (ESP)

The Encapsulating Security Payload (ESP) extension header provides origin authenticity, integrity, and confidentiality of a packet. Unlike the AH header, the IP packet header is not accounted for.

An ESP packet diagram:

0								1								2								3
0	1	2	3	4	5	6	7	0	1	2	3	4	5	6	7	0	1	2	3	4	5	6	7	0	1	2	3	4	5	6	7
Security Parameters Index (SPI)
Sequence Number
Payload * (variable)
								Padding (0-255 bytes)
																Pad Length								Next Header
Authentication Data (variable)

Field meanings:

Security Parameters Index (SPI): Identifies the security parameters in combination with IP address
Sequence Number: A monotonically increasing number, used to prevent replay attacks.
Payload Data: The data to be transferred.
Padding: Used with some block ciphers to pad the data to the full length of a block.
Pad Length: Size of padding in bits.
Next Header: Identifies the protocol of the transferred data.
Authentication Data: Contains the data used to authenticate the packet.

Implementations

IPsec support is usually implemented in the kernel with key management and ISAKMP/IKE negotiation carried out from user-space. Existing IPsec implementations tend to include both of these functionalities. However, as there is a standard interface for key management, it is possible to control one kernel IPsec stack using key management tools from a different implementation.

Because of this, there is confusion as to the origins of the IPsec implementation that is in the Linux kernel. The FreeS/WAN project made the first complete and open source implementation of IPsec for Linux. It consists of a kernel IPsec stack (KLIPS), as well as a key management daemon (pluto) and many shell scripts. The FreeS/WAN project was disbanded in March 2004. Openswan and strongSwan are continuations of FreeS/WAN. The KAME project also implemented complete IPsec support for NetBSD, FreeBSD, as well as Linux. Its key management daemon is called racoon. OpenBSD made its own ISAKMP/IKE daemon, simply named isakmpd (that was also ported to other systems, including Linux).

However, none of these kernel IPsec stacks were integrated into the Linux kernel. Alexey Kuznetsov and David S. Miller wrote a kernel IPsec implementation from scratch for the Linux kernel around the end of 2002. This stack was subsequently released as part of Linux 2.6.

Therefore, contrary to popular belief, the Linux IPsec stack did not originate from the KAME project. As it supports the standard PFKEY protocol and the native XFRM interface for key management, the Linux IPsec stack can be used in conjunction with either pluto from Openswan/strongSwan, isakmpd from OpenBSD project, racoon from the KAME project or without any ISAKMP/IKE daemon (using manual keying).

There are a number of implementations of IPsec and ISAKMP/IKE protocols. These include:

NRL [1] IPsec, one of the original sources of IPsec code [2]
OpenBSD, with its own code derived from NRL IPsec
the KAME stack, that is included in Mac OS X, NetBSD and FreeBSD
"IPSec" in Cisco IOS Software [3]
"IPSec" in Microsoft Windows, including Win2K and WinXP [4] [5]
SSH Sentinel [6] (now part of SafeNet [7]) provides toolkits
IPsec in Solaris [8]
IBM AIX operating system
IBM z/OS

Overview of IPsec-related RFCs

RFC 2367: PFKEY Interface
RFC 2401: Security Architecture for the Internet Protocol
RFC 2402: Authentication Header
RFC 2406: Encapsulating Security Payload
RFC 2407: IPsec Domain of Interpretation for ISAKMP (IPsec DoI)
RFC 2408: Internet Security Association and Key Management Protocol (ISAKMP)
RFC 2409: Internet Key Exchange (IKE)
RFC 2410: The NULL Encryption Algorithm and Its Use With IPsec
RFC 2411: IP Security Document Roadmap
RFC 2412: The OAKLEY Key Determination Protocol
RFC 3715: IPsec-Network Address Translation (NAT) Compatibility Requirements
RFC 3947: Negotiation of NAT-Traversal in the IKE
RFC 3948: UDP Encapsulation of IPsec ESP Packets