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In "computing, Internet Protocol Security (IPsec) is a network "protocol suite that "authenticates and "encrypts the "packets of data sent over a network. IPsec includes protocols for establishing "mutual authentication between agents at the beginning of the session and negotiation of "cryptographic keys for use during the session. IPsec can protect 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).[1] Internet Protocol security (IPsec) uses cryptographic security services to protect communications over Internet Protocol (IP) networks. IPsec supports network-level peer authentication, data-origin authentication, data integrity, data confidentiality (encryption), and replay protection.

IPsec is an end-to-end security scheme operating in the "Internet Layer of the "Internet Protocol Suite, while some other Internet security systems in widespread use, such as "Transport Layer Security (TLS) and "Secure Shell (SSH), operate in the "upper layers at the Transport Layer (TLS) and the Application layer (SSH). Hence, only IPsec protects all application traffic over an IP network. IPsec can automatically secure applications at the IP layer.



In the late 1980s, US NIST developed a set of security protocols for the Internet. One of these, Security Protocol at layer-3 (SP3) was implemented in IP encryption devices sold by Motorola. The IPsec Encapsulating Security Payload (ESP) is a direct derivative of the SP3 protocol. In 1992, both research and implementation began at the US "Naval Research Laboratory on IP encryption. This NRL work ultimately led to the IP Security protocols standardized by the IETF. Later, in December 1993, the Software IP Encryption protocol "swIPe (protocol) was researched at "Columbia University and "AT&T Bell Labs by John Ioannidis and others.

The IP Encapsulating Security Payload (ESP)[2] was researched at the "Naval Research Laboratory starting in 1992 as part of a "DARPA-sponsored research project, and was openly published by "IETF SIPP[3] Working Group drafted in December 1993 as a security extension for SIPP. This ESP was originally derived from the US Department of Defense SP3D protocol, rather than being derived from the ISO Network-Layer Security Protocol (NLSP). The SP3D protocol specification was published by "NIST in the late 1980s, but designed by the Secure Data Network System project of the US Department of Defense. The Security Authentication Header (AH) is derived partially from previous IETF standards work for authentication of the "Simple Network Management Protocol (SNMP) version 2.

In July 1992, the IETF was started to create an open freely available set of security extensions to the Internet protocol. This quickly became the IETF IP Security (IPsec) Working Group. The SDNS project had defined a Security Protocol Layer 3 (SP3) that had been published by NIST and was also the basis of the ISO Network Layer Security Protocol (NLSP).[4] Key management for SP3 was provided by the Key Management Protocol (KMP) that provided a baseline of ideas for subsequent work in the IPsec committee.

IPsec is officially standardised by the "Internet Engineering Task Force (IETF) in a series of "Request for Comments documents addressing various components and extensions. The original IETF specifications are in RFC-1825 through RFC-1827, which published in 1995. THe official spelling of the protocol name is IPsec.[5]

Security architecture[edit]

The IPsec suite is an "open standard. IPsec uses the following "protocols to perform various functions:[6][7]

Authentication Header[edit]

Authentication Header (AH) is a member of the IPsec protocol suite. AH guarantees connectionless "integrity and data origin "authentication of IP "packets. Further, it can optionally protect against "replay attacks by using the "sliding window technique and discarding old packets (see below).

AH operates directly on top of IP, using IP protocol number 51.[15]

The following AH packet diagram shows how an AH packet is constructed and interpreted:[8][9]

Authentication Header format
Offsets Octet16 0 1 2 3
Octet16 Bit10 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
0 0 Next Header Payload Len Reserved
4 32 Security Parameters Index (SPI)
8 64 Sequence Number
C 96 Integrity Check Value (ICV)
Next Header (8 bits) 
Type of the next header, indicating what upper-layer protocol was protected. The value is taken from the "list of IP protocol numbers.
Payload Len (8 bits) 
The length of this Authentication Header in 4-octet units, minus 2. For example, an AH value of 4 equals 3×(32-bit fixed-length AH fields) + 3×(32-bit ICV fields) − 2 and thus an AH value of 4 means 24 octets. Although the size is measured in 4-octet units, the length of this header needs to be a multiple of 8 octets if carried in an IPv6 packet. This restriction does not apply to an Authentication Header carried in an IPv4 packet.
Reserved (16 bits) 
Reserved for future use (all zeroes until then).
Security Parameters Index (32 bits) 
Arbitrary value which is used (together with the destination IP address) to identify the "security association of the receiving party.
Sequence Number (32 bits) 
A "monotonic strictly increasing sequence number (incremented by 1 for every packet sent) to prevent "replay attacks. When replay detection is enabled, sequence numbers are never reused, because a new security association must be renegotiated before an attempt to increment the sequence number beyond its maximum value.[9]
Integrity Check Value (multiple of 32 bits) 
Variable length check value. It may contain padding to align the field to an 8-octet boundary for "IPv6, or a 4-octet boundary for "IPv4.

Encapsulating Security Payload[edit]

Encapsulating Security Payload (ESP) is a member of the IPsec protocol suite. In IPsec it provides origin "authenticity, "integrity and "confidentiality protection of "packets. ESP also supports "encryption-only and "authentication-only configurations, but using encryption without authentication is strongly discouraged because it is insecure.[16][17][18] Unlike "Authentication Header (AH), ESP in transport mode does not provide integrity and authentication for the entire "IP packet. However, in "Tunnel Mode, where the entire original IP packet is "encapsulated with a new packet header added, ESP protection is afforded to the whole inner IP packet (including the inner header) while the outer header (including any outer IPv4 options or IPv6 extension headers) remains unprotected. ESP operates directly on top of IP, using IP protocol number 50.[15]

The following ESP packet diagram shows how an ESP packet is constructed and interpreted:[1][19]

Encapsulating Security Payload format
Offsets Octet16 0 1 2 3
Octet16 Bit10 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
0 0 Security Parameters Index (SPI)
4 32 Sequence Number
8 64 Payload data
  Padding (0-255 octets)  
  Pad Length Next Header
Integrity Check Value (ICV)
Security Parameters Index (32 bits) 
Arbitrary value used (together with the destination IP address) to identify the "security association of the receiving party.
Sequence Number (32 bits) 
A "monotonically increasing sequence number (incremented by 1 for every packet sent) to protect against "replay attacks. There is a separate counter kept for every security association.
Payload data (variable) 
The protected contents of the original IP packet, including any data used to protect the contents (e.g. an Initialisation Vector for the cryptographic algorithm). The type of content that was protected is indicated by the Next Header field.
Padding (0-255 octets) 
Padding for encryption, to extend the payload data to a size that fits the encryption's "cipher block size, and to align the next field.
Pad Length (8 bits) 
Size of the padding (in octets).
Next Header (8 bits) 
Type of the next header. The value is taken from the "list of IP protocol numbers.
Integrity Check Value (multiple of 32 bits) 
Variable length check value. It may contain padding to align the field to an 8-octet boundary for "IPv6, or a 4-octet boundary for "IPv4.

Security association[edit]

The IP security architecture uses the concept of a "security association as the basis for building security functions into IP. A security association is simply the bundle of algorithms and parameters (such as keys) that is being used to encrypt and authenticate a particular flow in one direction. Therefore, in normal bi-directional traffic, the flows are secured by a pair of security associations.

Security associations are established using the "Internet Security Association and Key Management Protocol (ISAKMP). ISAKMP is implemented by manual configuration with pre-shared secrets, "Internet Key Exchange (IKE and IKEv2), "Kerberized Internet Negotiation of Keys (KINK), and the use of IPSECKEY "DNS records.[14][20][21] RFC 5386 defines Better-Than-Nothing Security (BTNS) as an unauthenticated mode of IPsec using an extended IKE protocol.

In order to decide what protection is to be provided for an outgoing packet, IPsec uses the "Security Parameter Index (SPI), an index to the security association database (SADB), along with the destination address in a packet header, which together uniquely identify a security association for that packet. A similar procedure is performed for an incoming packet, where IPsec gathers decryption and verification keys from the security association database.

For multicast, a security association is provided for the group, and is duplicated across all authorized receivers of the group. There may be more than one security association for a group, using different SPIs, thereby allowing multiple levels and sets of security within a group. Indeed, each sender can have multiple security associations, allowing authentication, since a receiver can only know that someone knowing the keys sent the data. Note that the relevant standard does not describe how the association is chosen and duplicated across the group; it is assumed that a responsible party will have made the choice.

Modes of operation[edit]

IPsec can be implemented in a host-to-host transport mode, as well as in a network tunneling mode.

Transport mode[edit]

In transport mode, only the payload of the IP packet is usually "encrypted or authenticated. The routing is intact, since the IP header is neither modified nor encrypted; however, when the "authentication header is used, the IP addresses cannot be modified by "network address translation, as this always invalidates the "hash value. The "transport and "application layers are always secured by a hash, so they cannot be modified in any way, for example by "translating the "port numbers.

A means to encapsulate IPsec messages for "NAT traversal has been defined by "RFC documents describing the "NAT-T mechanism.

Tunnel mode[edit]

In tunnel mode, the entire IP packet is encrypted and authenticated. It is then encapsulated into a new IP packet with a new IP header. Tunnel mode is used to create "virtual private networks for network-to-network communications (e.g. between routers to link sites), host-to-network communications (e.g. remote user access) and host-to-host communications (e.g. private chat).[22]

Tunnel mode supports NAT traversal.

Cryptographic algorithms[edit]

Cryptographic algorithms defined for use with IPsec include:

Refer to RFC 7321 for details.

Software implementations[edit]

IPsec support is usually implemented in the "kernel with key management and "ISAKMP/"IKE negotiation carried out from user space. The openly specified "PF_KEY Key Management API, Version 2" is often used to enable the application-space key management application to update the IPsec Security Associations stored within the kernel-space IPsec implementation.[23]

Existing IPsec implementations usually include ESP, AH, and IKE version 2. Existing IPsec implementations on UNIX-like operating systems, for example, Solaris or Linux, usually include PF_KEY version 2.

Standards status[edit]

IPsec was developed in conjunction with "IPv6 and was originally required to be supported by all standards-compliant implementations of "IPv6 before RFC 6434 made it only a recommendation.[24] IPsec is also optional for "IPv4 implementations. IPsec is most commonly used to secure IPv4 traffic.

IPsec protocols were originally defined in RFC 1825 through RFC 1829, which were published in 1995. In 1998, these documents were superseded by RFC 2401 and RFC 2412 with a few incompatible engineering details, although they were conceptually identical. In addition, a mutual authentication and key exchange protocol "Internet Key Exchange (IKE) was defined to create and manage security associations. In December 2005, new standards were defined in RFC 4301 and RFC 4309 which are largely a superset of the previous editions with a second version of the Internet Key Exchange standard "IKEv2. These third-generation documents standardized the abbreviation of IPsec to uppercase “IP” and lowercase “sec”. “ESP” generally refers to RFC 4303, which is the most recent version of the specification.

Since mid-2008, an IPsec Maintenance and Extensions (ipsecme) working group is active at the IETF.[25][26]

Alleged NSA interference[edit]

In 2013, as part of "Snowden leaks, it was revealed that the US "National Security Agency had been actively working to "Insert vulnerabilities into commercial encryption systems, IT systems, networks, and endpoint communications devices used by targets" as part of the "Bullrun program.[27] There are allegations that IPsec was a targeted encryption system.[28]

The OpenBSD IPsec stack was the first implementation that was available under a permissive open-source license, and was therefore copied widely. In a letter which OpenBSD lead developer "Theo de Raadt received on 11 Dec 2010 from Gregory Perry, it is alleged that Jason Wright and others, working for the FBI, inserted "a number of "backdoors and "side channel key leaking mechanisms" into the OpenBSD crypto code. In the forwarded email from 2010, Theo de Raadt did not at first express an official position on the validity of the claims, apart from the implicit endorsement from forwarding the email.[29] Jason Wright's response to the allegations: "Every urban legend is made more real by the inclusion of real names, dates, and times. Gregory Perry's email falls into this category. … I will state clearly that I did not add backdoors to the OpenBSD operating system or the OpenBSD crypto framework (OCF)."[30] Some days later, de Raadt commented that "I believe that NETSEC was probably contracted to write backdoors as alleged. … If those were written, I don't believe they made it into our tree."[31] This was published before the Snowden leaks.

An alternative explanation put forward by the authors of the "Logjam attack suggests that the NSA compromised IPsec VPNs by undermining the "Diffie-Hellman algorithm used in the key exchange. In their paper[32] they allege the NSA specially built a computing cluster to precompute multiplicative subgroups for specific primes and generators, such as for the second Oakley group defined in RFC 2409. As of May 2015, 90% of addressable IPsec VPNs supported the second Oakley group as part of IKE. If an organization were to precompute this group, they could derive the keys being exchanged and decrypt traffic without inserting any software backdoors.

A second alternative explanation that was put forward was that the "Equation Group used "zero-day exploits against several manufacturers' VPN equipment which were validated by "Kaspersky Lab as being tied to the Equation Group[33] and validated by those manufacturers as being real exploits, some of which were zero-day exploits at the time of their exposure.[34][35][36] The "Cisco PIX and ASA firewalls had vulnerabilities that were used for wiretapping by the NSA.

Furthermore, IPsec VPNs using "Aggressive Mode" settings send a hash of the PSK in the clear. This can be and apparently is targeted by the NSA using offline dictionary attacks.[37][38][39]

IETF documentation[edit]

Standards Track[edit]

Experimental RFCs[edit]

Informational RFCs[edit]

Best Current Practice RFCs[edit]

Obsolete/Historic RFCs[edit]

See also[edit]


  1. ^ a b c Kent, S.; Atkinson, R. (November 1998). IP Encapsulating Security Payload (ESP). "IETF. RFC 2406. 
  2. ^ "SIPP Encapsulating Security Payload". IETF SIPP Working Group. 1993. 
  3. ^ "Draft SIPP Specification". IETF. 1993. p. 21. 
  4. ^
  5. ^ "RFC4301: Security Architecture for the Internet Protocol". Network Working Group of the IETF. December 2005. p. 4. The spelling "IPsec" is preferred and used throughout this and all related IPsec standards. All other capitalizations of IPsec [...] are deprecated. 
  6. ^ Thayer, R.; Doraswamy, N.; Glenn, R. (November 1998). IP Security Document Roadmap. "IETF. RFC 2411. 
  7. ^ Hoffman, P. (December 2005). Cryptographic Suites for IPsec. "IETF. RFC 4308. 
  8. ^ a b Kent, S.; Atkinson, R. (November 1998). IP Authentication Header. "IETF. RFC 2402. 
  9. ^ a b c d e Kent, S. (December 2005). IP Authentication Header. "IETF. RFC 4302. 
  10. ^ The "Internet Key Exchange (IKE), RFC 2409, §1 Abstract
  11. ^ Harkins, D.; Carrel, D. (November 1998). The Internet Key Exchange (IKE). "IETF. RFC 2409. 
  12. ^ Kaufman, C., ed. IKE Version 2. "IETF. RFC 4306. 
  13. ^ Sakane, S.; Kamada, K.; Thomas, M.; Vilhuber, J. (November 1998). Kerberized Internet Negotiation of Keys (KINK). "IETF. RFC 4430. 
  14. ^ a b Richardson, M. (February 2005). A Method for Storing IPsec Keying Material in DNS. "IETF. RFC 4025. 
  15. ^ a b "Protocol Numbers". IANA. "IANA. 2010-05-27. Archived from the original on 2010-07-27. 
  16. ^ "Bellovin, Steven M. (1996). "Problem Areas for the IP Security Protocols" ("PostScript). Proceedings of the Sixth Usenix Unix Security Symposium. San Jose, CA. pp. 1–16. Retrieved 2007-07-09. 
  17. ^ Paterson, Kenneth G.; Yau, Arnold K.L. (2006-04-24). "Cryptography in theory and practice: The case of encryption in IPsec" (PDF). Eurocrypt 2006, Lecture Notes in Computer Science Vol. 4004. Berlin. pp. 12–29. Retrieved 2007-08-13. 
  18. ^ Degabriele, Jean Paul; Paterson, Kenneth G. (2007-08-09). "Attacking the IPsec Standards in Encryption-only Configurations" (PDF). IEEE Symposium on Security and Privacy, IEEE Computer Society. Oakland, CA. pp. 335–349. Retrieved 2007-08-13. 
  19. ^ Kent, S. (December 2005). IP Encapsulating Security Payload (ESP). "IETF. RFC 4303. 
  20. ^ RFC 2406, §1, page 2
  21. ^ RFC 3129
  22. ^ William, S., & Stallings, W. (2006). Cryptography and Network Security, 4/E. Pearson Education India. p. 492-493
  23. ^ RFC 2367, PF_KEYv2 Key Management API, Dan McDonald, Bao Phan, & Craig Metz (July 1998)
  24. ^ RFC 6434, "IPv6 Node Requirements", E. Jankiewicz, J. Loughney, T. Narten (December 2011)
  25. ^ "ipsecme charter". Retrieved 2015-10-26. 
  26. ^ "ipsecme status". Retrieved 2015-10-26. 
  27. ^ "Secret Documents Reveal N.S.A. Campaign Against Encryption". New York Times. 
  28. ^ John Gilmore. "Re: [Cryptography] Opening Discussion: Speculation on "BULLRUN"". 
  29. ^ Theo de Raadt. "Allegations regarding OpenBSD IPSEC". 
  30. ^ Jason Wright. "Allegations regarding OpenBSD IPSEC". 
  31. ^ Theo de Raadt. "Update on the OpenBSD IPSEC backdoor allegation". 
  32. ^ David Adrian; Karthikeyan Bhargavan; Zakir Durumeric; Pierrick Gaudry; Matthew Green; "J. Alex Halderman; Nadia Heninger; Drew Springall; Emmanuel Thomé; Luke Valenta; Benjamin VanderSloot; Eric Wustrow; Santiago Zanella-Béguelink; Paul Zimmermann. "Imperfect Forward Secrecy: How Diffie-Hellman Fails in Practice" (PDF). 
  33. ^ Goodin, Dan (August 16, 2016). "Confirmed: hacking tool leak came from "omnipotent" NSA-tied group". Ars Technica. Retrieved August 19, 2016. 
  34. ^ Thomson, Iain (August 17, 2016). "Cisco confirms two of the Shadow Brokers' 'NSA' vulns are real". "The Register. Retrieved September 16, 2016. 
  35. ^ Pauli, Darren (August 24, 2016). "Equation Group exploit hits newer Cisco ASA, Juniper Netscreen". "The Register. Retrieved September 16, 2016. 
  36. ^ Chirgwin, Richard (August 18, 2016). "Fortinet follows Cisco in confirming Shadow Broker vuln". "The Register. Retrieved September 16, 2016. 
  37. ^
  38. ^
  39. ^

External links[edit]

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