IP Encapsulating Security Payload (ESP)
draft-ietf-ipsec-esp-00

Network Working Group                                    Randall Atkinson
Internet Draft                                  Naval Research Laboratory
draft-ietf-ipsec-esp-00.txt                                 23 March 1995

                IP Encapsulating Security Payload (ESP)

STATUS OF THIS MEMO
     This document is an Internet Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas, and
   its working groups.  Note that other groups may also distribute working
   documents as Internet Drafts.

     Internet Drafts are draft documents valid for a maximum of 6 months.
   Internet Drafts may be updated, replaced, or obsoleted by other
   documents at any time.  It is not appropriate to use Internet Drafts as
   reference material or to cite them other than as "work in progress".

     This particular Internet Draft is a product of the IETF's IPng and
   IPsec working groups.  It is intended that a future version of this
   draft be submitted to the IPng Area Directors and the IESG for
   possible publication as a standards-track protocol.

0. ABSTRACT

     This document describes the IP Encapsulating Security Payload (ESP).
   ESP is a mechanism for providing integrity and confidentiality to IP
   datagrams.  In some circumstances it can also provide authentication
   to IP datagrams.  The mechanism works with both IPv4 and IPv6.  This
   document also describes the mandatory DES CBC encryption transform for
   use with ESP.

1. INTRODUCTION

     ESP is a mechanism for providing integrity and confidentiality to IP
   datagrams.  It may also provide authentication, depending on which
   algorithm and algorithm mode are used.  Non-repudiation and protection
   from traffic analysis are not provided by ESP.  The IP Authentication
   Header (AH) might provide non-repudiation if used with certain
   authentication algorithms. [Atk95b] The IP Authentication Header may
   be used in conjunction with ESP to provide authentication.  Users
   desiring integrity and authentication without confidentiality should
   use the IP Authentication Header (AH) instead of ESP.  This document

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   assumes that the reader is familiar with the related document "IP
   Security Architecture", which defines the overall Internet-layer
   security architecture for IPv4 and IPv6 and provides important
   background for this specification. [Atk95a]

1.1 Overview

     The IP Encapsulating Security Payload (ESP) seeks to provide
   confidentiality and integrity by encrypting data to be protected and
   placing the encrypted data in the data portion of the IP Encapsulating
   Security Payload.  Depending on the user's security requirements, this
   mechanism may be used to encrypt either a transport-layer segment
   (e.g. TCP, UDP, ICMP, IGMP) or an entire IP datagram.  Encapsulating
   the protected data is necessary to provide confidentiality for the
   entire original datagram.

     Use of this specification will increase the IP protocol processing
   costs in participating systems and will also increase the
   communications latency.  The increased latency is primarily due to the
   encryption and decryption required for each IP datagram containing an
   Encapsulating Security Payload.

     In order for ESP to work properly without changing the entire
   Internet infrastructure (e.g. non-participating systems), the original
   IP datagram is placed in the encrypted portion of the Encapsulating
   Security Payload and that entire ESP frame is placed within an
   datagram having unencrypted IP headers.  The information in the
   unencrypted IP headers is used to route the secure datagram from
   origin to destination. An unencrypted IP Routing Header might be
   included between the IP Header and the Encapsulating Security Payload.

     In the case of IP, an IP Authentication Header may be present both
   as an header of the unencrypted IP packet and also as a header within
   the encrypted IP packet.  In such a case, the unencrypted IPv6
   Authentication Header is primarily used to provide protection for the
   contents of the unencrypted IP headers and the encrypted
   Authentication Header is used to provide authentication for the
   encrypted IP packet.  This is discussed in more detail later in this
   document.

     The encapsulating security payload is structured a bit differently
   than other IP payloads. The first component of the ESP payload
   consist of the unencrypted field(s) of the payload.  The second
   component consists of encrypted data.  The field(s) of the unencrypted
   ESP header inform the intended receiver how to properly decrypt and
   process the encrypted data.  The encrypted data component includes
   protected fields for the security protocol and also the encrypted
   encapsulated IP datagram.

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     The concept of a "Security Association" is fundamental to ESP.  It
   is described in detail in the compaion document "Security Architecture
   for the Internet Protocol" which is incorporated here by
   reference. [Atk95a] Implementers should read that document before
   reading this one.

1.2 Requirements Terminology

     In this document, the words that are used to define the significance
   of each particular requirement are usually capitalised.  These words
   are:

   - MUST

     This word or the adjective "REQUIRED" means that the item is an
   absolute requirement of the specification.

   - SHOULD

     This word or the adjective "RECOMMENDED" means that there might
   exist valid reasons in particular circumstances to ignore this item,
   but the full implications should be understood and the case carefully
   weighed before taking a different course.

   - MAY

     This word or the adjective "OPTIONAL" means that this item is truly
   optional.  One vendor might choose to include the item because a
   particular marketplace requires it or because it enhances the product,
   for example; another vendor may omit the same item.

2. KEY MANAGEMENT

     Key management is an important part of the IP security
   architecture.  However, a specific key management protocol is not
   included in this specification because of a long history in the public
   literature of subtle flaws in key management algorithms and protocols.
   IP tries to decouple the key management mechanisms from the security
   protocol mechanisms.  The only coupling between the key management
   protocol and the security protocol is with the Security Association
   Identifier (SPI), which is described in more detail below.  This
   decoupling permits several different key management mechanisms to be
   used.  More importantly, it permits the key management protocol to be
   changed or corrected without unduly impacting the security protocol
   implementations. Thus, a key management protocol for IP is not
   specifed within this draft.  The IP Security Architecture describes
   key management in more detail and specifies the key management
   requirements for IP.  Those key management requirements are

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   incorporated here by reference. [Atk95a]

     The key management mechanism is used to negotiate a number of
   parameters for each security association, including not only the keys
   but other information (e.g. the cryptographic algorithms and modes,
   security classification level if any) used by the communicating
   parties.  The key management protocol implementation usually creates
   and maintains a logical table containing the several parameters for
   each current security association. An ESP implementation normally
   needs to read that security parameter table to determine how to
   process each datagram containing an ESP (e.g. which algorithm/mode and
   key to use).

3. ENCAPSULATING SECURITY PAYLOAD SYNTAX

     The Encapsulating Security Payload (ESP) may appear anywhere after
   the IP header.  The Internet Assigned Numbers Authority has assigned
   Protocol Number 50 to IP ESP. [STD-2] The header immediately
   preceding an ESP header will always contain the value 50 in its Next
   Header field.  ESP consists of an unencrypted header followed by
   encrypted data.  The encrypted data includes both the protected ESP
   header fields and the protected user data, which is either an entire
   IP datagram or an upper-layer protocol frame (e.g. TCP or UDP).  A
   high-level diagram of a secure IP datagram follows.

     |<--        Unencrypted              -->|<----    Encrypted   ------>|
     +-------------+--------------------+------------+---------------------+
     | IP Header   | Other IP Headers   | ESP Header | encrypted data      |
     +-------------+--------------------+------------+---------------------+

   A more detailed diagram of the ESP Header follows below.

     +-------------+--------------------+------------+---------------------+
     |             Security Association Identifier (SPI), 32 bits          |
     +=============+====================+============+=====================+
     |             Opaque Transform Data, variable length                  |
     +-------------+--------------------+------------+---------------------+

   Encryption and authentication algorithms, and the precise format of
   the Opaque Transform Data associated with them are known as
   "transforms".  The ESP format is designed to support new transforms in
   the future to support new or additional cryptographic algorithms.  The
   transforms are specified by themselves rather than in the main body of
   this specification.  The mandatory transform for use with IP is
   defined in Appendix A of this specification.  Other optional
   transforms exist in separate specifications and additional transforms
   might be defined in the future.

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3.1 Fields of the Encapsulating Security Payload

     This is a 32-bit pseudo-random value identifying the security
   association for this datagram.  If no security association has been
   established, the value of this field shall be 0x00000000.   An
   SPI is similar to the SAID used in other security protocols.  The
   name has been changed because the semantics used here are not
   exactly the same as those used in other security protocols.

     The set of SPI values in the range 0x00000001 though 0x000000FF
   are reserved to the Internet Assigned Numbers Authority (IANA) for
   future use.  A reserved SPI value will not normally be assigned by
   IANA unless the use of that particular assigned SPI value is openly
   specified in an RFC.

     The SPI is the only mandatory transform-independent field.
   Particular transforms may have other fields unique to the transform.
   Transforms are not specified in this document.

3.2 Security Labeling with ESP

     The encrypted IP datagram need not and does not normally contain any
   explicit Security Label because the SPI indicates the sensitivity
   label.  This is an improvement over the current practices with IPv4
   where an explicit Security Label is normally used with Compartmented
   Mode Workstations and other systems requiring Security Labels. [Ken91]
   [DIA] In some situations, users MAY choose to carry explicit labels
   (for example, IPSO labels as defined by RFC-1108 might be used with
   IPv4) in addition to using the implicit labels provided by ESP.
   Explicit label options could be defined for use with IPv6 (e.g. using
   the IPv6 End-to-End Options Header or the IPv6 Hop-by-Hop Options
   Header).  Implementations MAY support explicit labels in addition to
   implicit labels, but implementations are not required to support
   explicit labels.  Implementations of ESP in systems claiming to
   provide multi-level security MUST support implicit labels.

4. ENCAPSULATING SECURITY PROTOCOL PROCESSING
     This section describes the steps taken when ESP is in use between
   two communicating parties.  Multicast is different from unicast only
   in the area of key management (See the definition of the SPI, above,
   for more detail on this).  There are two modes of use for ESP.  The
   first mode, which is called "Tunnel-mode", encapsulates an entire IP
   datagram inside ESP.  The second mode, which is called
   "Transport-Mode", encapsulates a transport-layer (e.g. UDP or TCP)
   frame inside ESP.  The term "Transport-mode" must not be misconstrued
   as restricting its use to TCP and UDP. For example, an ICMP
   message MAY be sent either using the "Transport-mode" or the
   "IP-mode" depending upon circumstance.  This section describes

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   protocol processing for each of these two modes.

4.1 ESP in Tunnel-mode

     The sender takes the original IP datagram, encapsulates it into the
   ESP, locates the correct Security Association using the sending userid
   and the Destination Address, and then applies the appropriate
   encryption transform.  If host-oriented keying is in use, then all
   sending userids on a given system will have the same Security
   Association for a given Destination Address.  If no key has been
   established, then the key management mechanism is used to establish a
   encryption key for this communications session prior to the use of
   ESP.  The (now encrypted) ESP is then encapsulated in a cleartext IP
   datagram as the last payload.  If strict red/black separation is being
   enforced, then the addressing and other information in the cleartext
   IP headers and optional payloads MAY be different from the values
   contained in the (now encrypted and encapsulated) original datagram.

     The receiver strips off the cleartext IP header and cleartext
   optional IP payloads (if any) and discards them.  It then uses the
   combination of Destination Address and SPI value to locate the
   correct decryption key to use for this packet.  Then, it decrypts
   the ESP using the session key that has been established for this
   traffic.

     If no valid Security Association exists for this session (for
   example, the receiver has no key), the receiver MUST discard the
   encrypted ESP and the failure MUST be recorded in the system log or
   audit log.  This system log or audit log entry SHOULD include the SPI
   value, date/time, clear-text Sending Address, clear-text Destination
   Address, and the clear-text Flow ID.  The log entry MAY also include
   other identifying data.  The receiver might not wish to react by
   immediately informing the sender of this failure because of the strong
   potential for easy-to-exploit denial of service attacks.

     If decryption succeeds, the original IP datagram is then removed
   from the (now decrypted) ESP.  This original IP datagram is then
   processed as per the normal IP protocol specification.  In the case of
   system claiming to provide multilevel security (for example, a B1 or
   Compartmented Mode Workstation) additional appropriate mandatory
   access controls MUST be applied based on the security level of the
   receiving process and the security level associated with this Security
   Association.  If those mandatory access controls fail, then the packet
   SHOULD be discarded and the failure SHOULD be logged using
   implementation-specific procedures.

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4.2 ESP in Transport-mode

     The sender takes the original UDP or TCP or ICMP frame, encapsulates
   it into the ESP, locates the correct Security Association using the
   sending userid and Destination Address, and then applies the
   appropriate encryption transform.  If host-oriented keying is in use,
   then all sending userids on a given system will have the same Security
   Association for a given Destination Address. If no key has been
   established, then the key management mechanism is used to establish a
   encryption key for this communications session prior to the
   encryption.  The (now encrypted) ESP is then encapsulated as the last
   payload of a cleartext IP datagram.

     The receiver processes the cleartext IP header and cleartext
   optional IP headers (if any) and temporarily stores pertinent
   information (e.g. source and destination addresses, Flow ID, Routing
   Header).  It then decrypts the ESP using the session key that has been
   established for this traffic, using the combination of the destination
   address and the packet's Security Association Identifier (SPI) to
   locate the correct key.

     If no key exists for this session or the attempt to decrypt fails,
   the encrypted ESP MUST be discarded and the failure MUST be recorded
   in the system log or audit log.  If such a failure occurs, the
   recorded log data SHOULD include the SPI value, date/time received,
   clear-text Sending Address, clear-text Destination Address, and the
   Flow ID.  The log data MAY also include other information about the
   failed packet.

     If decryption succeeds, the original UDP or TCP frame is removed
   from the (now decrypted) ESP.  The information from the cleartext IP
   header and the now decrypted UDP or TCP header is jointly used to
   determine which application the data should be sent to.  The data is
   then sent along to the appropriate application as normally per IP
   protocol specification.  In the case of a system claiming to provide
   multilevel security (for example, a B1 or Compartmented Mode
   Workstation), additional Mandatory Access Controls MUST be applied
   based on the security level of the receiving process and the security
   level of the received packet's Security Association.

4.3. Authentication

     Some transforms provide authentication as well as confidentiality
   and integrity.  When such a transform is not used, then the
   Authentication Header might be used in conjunction with the
   Encapsulating Security Payload.  There are two different approaches to
   using the Authentication Header with ESP, depending on which data is
   to be authenticated.  The location of the Authentication Header makes

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   it clear which set of data is being authenticated.

     In the first usage, the entire received datagram is authenticated,
   including both the encrypted and unencrypted portions, while only the
   data sent after the ESP Header is confidential.  In this usage, the
   sender first applies ESP to the data being protected.  Then the other
   plaintext IP headers are prepended to the ESP header and its now
   encrypted data. Finally, the IP Authentication Header is calculated
   over the resulting datagram according to the normal method.  Upon
   receipt, the receiver first verifies the authenticity of the entire
   datagram using the normal IP Authentication Header process.  Then if
   authentication succeeds, decryption using the normal IP ESP process
   occurs.  If decryption is successful, then the resulting data is
   passed up to the upper layer.

     If the authentication process were to be applied only to the data
   protected by IP ESP and the protected data were an entire IP
   datagram, then the IP Authentication Header would be placed normally
   within that protected datagram.  However, if the protected data were
   less than an entire IP datagram, then the IP Authentication Header
   would be placed within the encrypted payload immediately after the ESP
   protected header and before any other header.

     If the Authentication Header is encapsulated within the ESP header,
   and both headers have specific security classification levels
   associated with them, and the two security classification levels are
   not identical, then an error has occurred.  That error SHOULD be
   recorded in the system log or audit log using the procedures described
   previously.  It is not necessarily an error for an Authentication
   Header located outside of the ESP header to have a different security
   classification level than the ESP header's classification level.  This
   might be valid because the cleartext IP headers might have a different
   classification level when the data has been encrypted using ESP.

5. CONFORMANCE REQUIREMENTS
     Implementations that claim conformance or compliance with this
   specification MUST fully implement the header described here, MUST
   support manual key distribution with this header, MUST comply with all
   requirements of the "Security Architecture for the Internet Protocol"
   [Atk95a], and MUST support the use of DES CBC as specified in the
   companion document entitled "The ESP DES-CBC Transform" [MS95].
   Implementers MAY also implement other ESP transforms.  Implementers
   should consult the most recent version of the "IAB Official Standards"
   RFC for further guidance on the status of this document.

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6. SECURITY CONSIDERATIONS

     This entire draft discusses a security mechanism for use with IP.
   This mechanism is not a panacea, but it does provide an important
   component useful in creating a secure internetwork.

     Users need to understand that the quality of the security provided
   by this specification depends completely on the strength of whichever
   encryption algorithm that has been implemented, the correctness of
   that algorithm's implementation, upon the security of the key
   management mechanism and its implementation, the strength of the key
   [CN94][Sch94, p233] and upon the correctness of the ESP and IP
   implementations in all of the participating systems.

     If any of these assumptions do not hold, then little or no real
   security will be provided to the user.  Use of high assurance
   development techniques is recommended for the IP Encapsulating
   Security Payload.

     Users seeking protection from traffic analysis might consider the
   use of appropriate link encryption.  Description and specification of
   link encryption is outside the scope of this note.

     If user-oriented keying is not in use, then the algorithm in use
   should not be an algorithm vulnerable to any kind of Chosen Plaintext
   attack.  Chosen Plaintext attacks on DES are described in [BS93] and
   [Mit94]. Use of user-oriented keying is recommended in order to
   preclude any sort of Chosen Plaintext attack and to generally make
   cryptanalysis more difficult.  Implementations MUST support
   user-oriented keying as is described in the IP Security
   Architecture. [Atk95a]

ACKNOWLEDGEMENTS
     This document benefited greatly from work done by Bill Simpson, Perry
   Metzger, and Phil Karn to make general the approach originally defined
   by the author for SIP, SIPP, and finally IPv6.

     Many of the concepts here are derived from or were influenced by the
   US Government's SP3 security protocol specification, the ISO/IEC's
   NLSP specification, or from the proposed swIPe security
   protocol. [SDNS89, ISO92a, IB93, IBK93, ISO92b] The use of DES for
   confidentiality is closely modeled on the work done for the
   SNMPv2. [GM93] Steve Bellovin, Steve Deering, Dave Mihelcic, and
   Hilarie Orman provided solid critiques of early versions of this
   draft.

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REFERENCES
   [Atk95a] Randall J. Atkinson, IP Security Architecture, Internet Draft,
            draft-ietf-ipng-sec-01.txt, 16 March 1995.

   [Atk95b] Randall J. Atkinson, IP Authentication Header, Internet Draft,
            draft-ietf-ipng-auth-01.txt, 16 March 1995.

   [Bel89]  Steven M. Bellovin, "Security Problems in the TCP/IP Protocol
            Suite", ACM Computer Communications Review, Vol. 19, No. 2,
            March 1989.

   [BS93]   Eli Biham and Adi Shamir, "Differential Cryptanalysis of the
            Data Encryption Standard", Springer-Verlag, New York, NY, 1993.

   [CN94]   John M. Carroll & Sri Nudiati, "On Weak Keys and Weak Data:
            Foiling the Two Nemeses", Cryptologia, Vol. 18, No. 23,
            July 1994. pp. 253-280

   [CERT95] Computer Emergency Response Team (CERT), "IP Spoofing Attacks
            and Hijacked Terminal Connections", CA-95:01, January 1995.
            Available via anonymous ftp from info.cert.org.

   [DIA]    US Defense Intelligence Agency (DIA), "Compartmented Mode
            Workstation Specification", Technical Report DDS-2600-6243-87.

   [GM93]   James Galvin & Keith McCloghrie, Security Protocols for Version 2
            of the Simple Network Management Protocol (SNMPv2), RFC-1446,
            DDN Network Information Center, April 1993.

   [Hin94]  Robert Hinden (Editor), IP Specification, Internet Draft,
            draft-hinden-ipng-IP-spec-00.txt, October 1994.

   [IB93]   John Ioannidis & Matt Blaze, "Architecture and Implementation
            of Network-layer Security Under Unix", Proceedings of the USENIX
            Security Symposium, Santa Clara, CA, October 1993.

   [IBK93]  John Ioannidis, Matt Blaze, & Phil Karn, "swIPe: Network-Layer
            Security for IP", presentation at the Spring 1993 IETF Meeting,
            Columbus, Ohio.

   [ISO92a] ISO/IEC JTC1/SC6, Network Layer Security Protocol, ISO-IEC
            DIS 11577, International Standards Organisation, Geneva,
            Switzerland, 29 November 1992.

   [ISO92b] ISO/IEC JTC1/SC6, Network Layer Security Protocol, ISO-IEC
            DIS 11577, Section 13.4.1, page 33, International Standards
            Organisation, Geneva, Switzerland, 29 November 1992.

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   [Ken91]  Steve Kent, "US DoD Security Options for the Internet Protocol
           (IPSO)", RFC-1108, DDN Network Information Center, November 1991.

   [Mit94] Matsui, M., "Linear Cryptanalysis method for DES Cipher",
            Proceedings of Eurocrypt '93, Berlin, Springer-Verlag, 1994.

   [MS95]   Perry Metzger & W.A. Simpson, "The ESP DES-CBC Transform",
            Work in Progress, March 1995.

   [NIST77] US National Bureau of Standards, "Data Encryption Standard",
            Federal Information Processing Standard (FIPS) Publication 46,
            January 1977.

   [NIST80] US National Bureau of Standards, "DES Modes of Operation"
            Federal Information Processing Standard (FIPS) Publication 81,
            December 1980.

   [NIST81] US National Bureau of Standards, "Guidelines for Implementing and
            Using the Data Encryption Standard", Federal Information
            Processing Standard (FIPS) Publication 74, April 1981.

   [NIST88] US National Bureau of Standards, "Data Encryption Standard",
            Federal Information Processing Standard (FIPS) Publication 46-1,
            January 1988.

   [STD-2]  J. Reynolds and J. Postel, "Assigned Numbers", STD-2,
            DDN Network Information Center, 20 October 1994.

   [Sch94]  Bruce Schneier, Applied Cryptography, John Wiley & Sons,
            New York, NY, 1994.  ISBN 0-471-59756-2

   [SDNS89] SDNS Secure Data Network System, Security Protocol 3, SP3,
            Document SDN.301, Revision 1.5, 15 May 1989, as published
            in NIST Publication NIST-IR-90-4250, February 1990.

DISCLAIMER

     The views and specification here are those of the author and are not
   necessarily those of his employer.  The Naval Research Laboratory has
   not passed judgement on the merits, if any, of this work.  The author
   and his employer specifically disclaim responsibility for any problems
   arising from correct or incorrect implementation or use of this
   specification.

AUTHOR INFORMATION

   Randall Atkinson <atkinson@itd.nrl.navy.mil>
   Information Technology Division

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   Naval Research Laboratory
   Washington, DC 20375-5320
   USA

   Telephone:      (DSN) 354-8590
   Fax:            (DSN) 354-7942

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