IPsec
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IPsec, an abbreviation of IP security, is a standard for securing Internet Protocol (IP) communications by encrypting and authenticating all IP packets. IPsec provides security at the network layer.
IPsec is a protocol suite (i.e., a set of interdependent protocols) consisting of (1) protocols for securing packet flows and (2) key exchange protocols used for setting up those secure flows. 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 (which is why it is not used as pervasively as ESP). Currently only one key exchange protocol is defined, the IKE protocol.
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Current status as a standard
IPsec is an obligatory part of IPv6, the new IETF Internet standard for Internet Protocol (IP) packet traffic, and is optional for use with IPv4. As a result, IPsec is expected to become more widely deployed as IPv6 becomes more popular. IPsec protocols are defined by RFCs 2401-2412. As of 2004, work is progressing to release updated replacement documents.
Design intent
IPsec was intended to provide either (1) 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) 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 has emerged (DNSSEC was originally envisioned for this), part is that many users understand neither their needs nor the available options well enough to force inclusion in vendors' products (which would lead to widespread adoption), and part is probably due to degradation (or anticipated degradation) of Net responsivity due to bandwidth loss from such things as spam.
IPsec vs. 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 the integrity and authenticity of the transferred packets. Further, it protects against replay attacks. 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 | |||
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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) protocol 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 | |||||||||||||||||||||||||||||||
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Payload * (variable) | |||||||||||||||||||||||||||||||
| Padding (0-255 bytes) | |||||||||||||||||||||||||||||||
| Pad Length | Next Header | ||||||||||||||||||||||||||||||
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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 GNU/Linux. It consists of a kernel IPsec stack (KLIPS), as well as a key management daemon (pluto). 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 GNU/Linux. Its key management daemon is called racoon.
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 or racoon from the KAME project.
There are a number of implementations of IPsec and ISAKMP/IKE protocols. These include:
- NRL IPsec, one of the original
- OpenBSD, with its own code derived from NRL IPsec
- Mac OS X, which includes the KAME IPsec code
- Cisco IOS
- Microsoft Windows Win2K and WinXP
- SSH Sentinel (now part of SafeNet) provides toolkits
- Solaris
See also
Overview of IPsec Related RFCs
- 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
External links
- IETF IPsec WG has "concluded", archive of the page is here (http://www.ietf.org/html.charters/OLD/ipsec-charter.html)
- IPsec WG still has important active drafts (https://datatracker.ietf.org/public/idindex.cgi?command=show_wg_id&id=1091)
- All IETF active security WGs (http://www.ietf.org/html.charters/wg-dir.html#Security%20Area)
- Securing Data in Transit with IPSec (http://www.windowsecurity.com/articles/Securing_Data_in_Transit_with_IPSec.html)
- Free S/WAN project homepage (http://www.freeswan.org/).
- Openswan project homepage (http://www.openswan.org/).
- The VPN Consortium (http://www.vpnc.org/).
- A long thread on the ipsec@lists.tislabs.com (http://www.netsys.com/ipsec/2000/msg00777.html) about whether uppercasing the S or not. The RFCs indicate that it is spelled "IPsec".de:IPsec