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).^[1]

Some other Internet security systems in widespread use, such as Secure Sockets Layer (SSL), Transport Layer Security (TLS) and Secure Shell (SSH), operate in the upper layers of the TCP/IP model. In the past, the use of TLS/SSL had to be designed into an application to protect the application protocols. In contrast, since day one, applications did not need to be specifically designed to use IPsec. Hence, IPsec protects any application traffic across an IP network.

History

In December 1993, the experimental of IP Security swIPe (protocol) was researched at Columbia University and AT&T Bell Labs. In July 1994, Wei Xu at Trusted Information Systems continued this research. After several months, the research was completed successfully on BSDI system. By hacking the binary kernels, Wei had quickly extended his development on to Sun OS, HP UX, and other UNIX system. One of the challenges was slow performance of DES and 3DES. The software encryption can’t even support a T1 speed under the Intel 80386 architecture. By exploring the Crypto cards from Germany, Wei Xu further developed an automated device driver, known as plug-and-play today. By achieving the throughput for more than a T1s, this work made the commercial product practically feasible, that was released as a part of the well-known Gauntlet firewall. In December 1994, it was the first time in production for securing some remote sites between east and west coastal states of the United States.^{[citation needed]}

Another IP Security Protocol was developed in 1995 at the Naval Research Laboratory as part of a DARPA-sponsored research project. ESP was originally derived from the SP3D protocol, rather than being derived from the ISO Network-Layer Security Protocol (NLSP). The SP3D protocol specification was published by NIST, but designed by the Secure Data Network System project of the National Security Agency (NSA), AH is derived in part from previous IETF standards work for authentication of the Simple Network Management Protocol (SNMP).

Since 1996, the IP Security workshops were organized for standardizing the protocols. IPsec is officially specified by the Internet Engineering Task Force (IETF) in a series of Request for Comments documents addressing various components and extensions. It specifies the spelling of the protocol name to be IPsec.^[2]

Security architecture

The IPsec suite is an open standard. IPsec uses the following protocols to perform various functions:^[3][4]

• Authentication Headers (AH) provide connectionless integrity and data origin authentication for IP datagrams and provides protection against replay attacks.^[5][6]

• Encapsulating Security Payloads (ESP) provide confidentiality, data-origin authentication, connectionless integrity, an anti-replay service (a form of partial sequence integrity), and limited traffic-flow confidentiality.^[1]

• Security Associations (SA) provide the bundle of algorithms and data that provide the parameters necessary to operate the AH and/or ESP operations. The Internet Security Association and Key Management Protocol (ISAKMP) provides a framework for authentication and key exchange,^[7] with actual authenticated keying material provided either by manual configuration with pre-shared keys, Internet Key Exchange (IKE and IKEv2), Kerberized Internet Negotiation of Keys (KINK), or IPSECKEY DNS records.^{[8][9][10][11]}

Authentication Header

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).

• In IPv4, the AH protects the IP payload and all header fields of an IP datagram except for mutable fields (i.e. those that might be altered in transit), and also IP options such as the IP Security Option (RFC-1108). Mutable (and therefore unauthenticated) IPv4 header fields are DSCP/TOS, ECN, Flags, Fragment Offset, TTL and Header Checksum.^[6]
• In IPv6, the AH protects the most of the IPv6 base header, AH itself, non-mutable extension headers after the AH, and the IP payload. Protection for the IPv6 header excludes the mutable fields: DSCP, ECN, Flow Label, and Hop Limit.^[6]

AH operates directly on top of IP, using IP protocol number 51.^[12]

The following AH packet diagram shows how an AH packet is constructed and interpreted:^[5][6]

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 (a value of 0 means 8 octets, 1 means 12 octets, etcetera). 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.^[6]

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

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.^[13][14][15] 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.^[12]

The following ESP packet diagram shows how an ESP packet is constructed and interpreted:^[1][16]

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

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.^[11][17][18] 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

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

Transport mode

In transport mode, only the payload of the IP packet is usually encrypted and/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 translated, as this will invalidate the hash value. The transport and application layers are always secured by 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

In tunnel mode, the entire IP packet is encrypted and/or 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).

Tunnel mode supports NAT traversal.

Cryptographic algorithms

Cryptographic algorithms defined for use with IPsec include:

• HMAC-SHA1 for integrity protection and authenticity.
• TripleDES-CBC for confidentiality
• AES-CBC for confidentiality.

Refer to RFC 4835 for details.

Software implementations

IPsec support is usually implemented in the kernel with key management and ISAKMP/IKE negotiation carried out from user-space. Existing IPsec implementations often include both.

Standards status

IPsec was developed in conjunction with IPv6 and must be available in all standards-compliant implementations of IPv6 before the releasing of RFC 6434 which made it optional.^[19] Even though before the releasing of RFC 6434 not all IPv6 implementations include IPsec support;^[20] it is optional for IPv4 implementations. However, because of the slow deployment of IPv6, IPsec is most commonly used to secure IPv4 traffic. IPsec protocols were originally defined in RFC 1825 and RFC 1829, published in 1995. In 1998, these documents were superseded by RFC 2401 and RFC 2412 with incompatible aspects, 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”. It is unusual to see any product that offers support for RFCs 1825 and 1829. “ESP” generally refers to RFC 2406, while ESPbis refers to RFC 4303.

Since mid-2008, an IPsec Maintenance and Extensions working group is active at the IETF.^[21][22]

See also

Cryptography portal

• IPsec Passthrough
• Information security
• Opportunistic encryption
• Dynamic Multipoint Virtual Private Network
• tcpcrypt

References

External links

• All IETF active security WGs
  • IETF ipsecme WG ("IP Security Maintenance and Extensions" Working Group)
  • IETF btns WG ("Better-Than-Nothing Security" Working Group) (chartered to work on unauthenticated IPsec, IPsec APIs, connection latching)]
• Securing Data in Transit with IPsec WindowsSecurity.com article by Deb Shinder
• IPsec at the Open Directory Project
• IPsec on Microsoft TechNet
  • Microsoft IPsec Diagnostic Tool on Microsoft Download Center
• An Illustrated Guide to IPsec by Steve Friedl
• Security Architecture for IP (IPsec) Data Communication Lectures by Manfred Lindner Part IPsec
• Creating VPNs with IPsec and SSL/TLS Linux Journal article by Rami Rosen

Standards Track RFCs

• RFC 2403: The Use of HMAC-MD5-96 within ESP and AH
• RFC 2404: The Use of HMAC-SHA-1-96 within ESP and AH
• RFC 2405: The ESP DES-CBC Cipher Algorithm With Explicit IV
• RFC 2410: The NULL Encryption Algorithm and Its Use With IPsec
• RFC 2451: The ESP CBC-Mode Cipher Algorithms
• RFC 2857: The Use of HMAC-RIPEMD-160-96 within ESP and AH
• RFC 3526: More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)
• RFC 3947: Negotiation of NAT-Traversal in the IKE
• RFC 3948: UDP Encapsulation of IPsec ESP Packets
• RFC 4106: The Use of Galois/Counter Mode (GCM) in IPsec Encapsulating Security Payload (ESP)
• RFC 4301: Security Architecture for the Internet Protocol
• RFC 4302: IP Authentication Header
• RFC 4303: IP Encapsulating Security Payload
• RFC 4304: Extended Sequence Number (ESN) Addendum to IPsec Domain of Interpretation (DOI) for Internet Security Association and Key Management Protocol (ISAKMP)
• RFC 4307: Cryptographic Algorithms for Use in the Internet Key Exchange Version 2 (IKEv2)
• RFC 4308: Cryptographic Suites for IPsec
• RFC 4309: Using Advanced Encryption Standard (AES) CCM Mode with IPsec Encapsulating Security Payload (ESP)
• RFC 4543: The Use of Galois Message Authentication Code (GMAC) in IPsec ESP and AH
• RFC 4555: IKEv2 Mobility and Multihoming Protocol (MOBIKE)
• RFC 4806: Online Certificate Status Protocol (OCSP) Extensions to IKEv2
• RFC 4835: Cryptographic Algorithm Implementation Requirements for Encapsulating Security Payload (ESP) and Authentication Header (AH)
• RFC 4945: The Internet IP Security PKI Profile of IKEv1/ISAKMP, IKEv2, and PKIX
• RFC 5996: Internet Key Exchange Protocol Version 2 (IKEv2)

Experimental RFCs

• RFC 4478: Repeated Authentication in Internet Key Exchange (IKEv2) Protocol

Informational RFCs

• RFC 2367: PF_KEY Interface
• RFC 2412: The OAKLEY Key Determination Protocol
• RFC 3706: A Traffic-Based Method of Detecting Dead Internet Key Exchange (IKE) Peers
• RFC 3715: IPsec-Network Address Translation (NAT) Compatibility Requirements
• RFC 4621: Design of the IKEv2 Mobility and Multihoming (MOBIKE) Protocol
• RFC 4809: Requirements for an IPsec Certificate Management Profile
• RFC 6071: IPsec and IKE Document Roadmap

Obsolete RFCs

• RFC 2401: Security Architecture for the Internet Protocol (IPsec overview) Obsolete by RFC 4301
• RFC 2409: The Internet Key Exchange
• RFC 4306: Internet Key Exchange (IKEv2) Protocol (obsoleted by RFC 5996)
• RFC 4718: IKEv2 Clarifications and Implementation Guidelines (obsoleted by RFC 5996)