Network Working Group                                        R. Atkinson
Request for Comments: 1827                     Naval Research Laboratory
Category: Standards Track                                    August 1995

                IP Encapsulating Security Payload (ESP)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

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.

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 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 Tunnel-mode ESP, the original IP datagram is placed in the
   encrypted portion of the Encapsulating Security Payload and that
   entire ESP frame is placed within a 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 Transport-mode ESP, the ESP header is inserted into the IP
   datagram immediately prior to the transport-layer protocol header
   (e.g., TCP, UDP, or ICMP). In this mode bandwidth is conserved
   because there are no encrypted IP headers or IP options.

   In the case of IP, an IP Authentication Header may be present as a
   header of an unencrypted IP packet, as a header after the IP header
   and before the ESP header in a Transport-mode ESP packet, and also as
   a header within the encrypted portion of a Tunnel-mode ESP packet.
   When AH is present both in the cleartext IP header and also inside a
   Tunnel-mode ESP header of a single packet, 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 only 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.

   The concept of a "Security Association" is fundamental to ESP.  It is
   described in detail in the companion document "Security Architecture
   for the Internet Protocol" which is incorporated here by reference
   [Atk95a].  Implementors 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 Parameter Index (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
   specified within this memo.  The IP Security Architecture describes
   key management in more detail and specifies the key management
   requirements for IP.  Those key management requirements are
   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 and before the final transport-layer protocol.  The
   Internet Assigned Numbers Authority has assigned Protocol Number 50
   to ESP [STD-2].  The header immediately preceding an ESP header will
   always contain the value 50 in its Next Header (IPv6) or Protocol
   (IPv4) 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 a separate document [KMS95].  Other optional transforms
   exist in other separate specifications and additional transforms
   might be defined in the future.

3.1 Fields of the Encapsulating Security Payload

   The SPI is a 32-bit pseudo-random value identifying the security
   association for this datagram.  If no security association has been
   established, the value of the SPI 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
   level.  This is an improvement over the current practices with IPv4
   where an explicit Sensitivity 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, 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 "Tunnel-mode"
   depending upon circumstance.  ESP processing occurs prior to IP
   fragmentation on output and after IP reassembly on input.  This
   section describes protocol processing for each of these two modes.

4.1 ESP in Tunnel-mode

   In Tunnel-mode ESP, the ESP header follows all of the end-to-end
   headers (e.g., Authentication Header, if present in cleartext) and
   immediately precedes an tunnelled IP datagram.

   The sender takes the original IP datagram, encapsulates it into the
   ESP, uses at least the sending userid and Destination Address as data
   to locate the correct Security Association, 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 an 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 session key to use for this packet.  It then decrypts the ESP
   using the session key that was just located for this packet.

   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, cleartext Sending Address, cleartext Destination
   Address, and the cleartext 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.

4.2 ESP in Transport-mode

   In Transport-mode ESP, the ESP header follows the end-to-end headers
   (e.g., Authentication Header) and immediately precedes a transport-
   layer (e.g., UDP, TCP, ICMP) header.

   The sender takes the original transport-layer (e.g., UDP, TCP, ICMP)
   frame, encapsulates it into the ESP, uses at least the sending userid
   and Destination Address to locate the appropriate Security
   Association, 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 does not work properly for some reason,
   then the resulting data will not be parsable by the implementation's
   protocol engine.  Hence, failed decryption is generally detectable.

   If decryption succeeds, the original transport-layer (e.g., UDP, TCP,
   ICMP) frame is removed from the (now decrypted) ESP.  The information
   from the cleartext IP header and the now decrypted transport-layer
   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 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 Tunnel-mode ESP, then the IP Authentication Header would
   be placed normally within that protected datagram.  However, if one
   were using Transport-mode ESP, then the IP Authentication Header
   would be placed before the ESP header and would be calculated across
   the entire IP datagram.

   If the Authentication Header is encapsulated within a Tunnel-mode 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 after 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"
   [KMS95].  Implementors 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.

6. SECURITY CONSIDERATIONS

   This entire document 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.

   Cryptographic transforms for ESP which use a block-chaining algorithm
   and lack a strong integrity mechanism are vulnerable to a cut-and-
   paste attack described by Bellovin and should not be used unless the
   Authentication Header is always present with packets using that ESP
   transform [Bel95].

   Users need to understand that the quality of the security provided by
   this specification depends completely on the strength of whichever
   encryption algorithm 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
   [Mat94]. 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 SHOULD 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 memo.

REFERENCES

   [Atk95a] Atkinson, R., "Security Architecture for the Internet
            Protocol", RFC 1825, NRL, August 1995.

   [Atk95b] Atkinson, R., "IP Authentication Header", RFC 1826, NRL,
            August 1995.

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

   [Bel95]  Steven M. Bellovin, Presentation at IP Security Working
            Group Meeting, Proceedings of the 32nd Internet Engineering
            Task Force, March 1995, Internet Engineering Task Force,
            Danvers, MA.

   [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]   Galvin J., and K. McCloghrie, "Security Protocols for
            version 2 of the Simple Network Management Protocol
            (SNMPv2)", RFC 1446, Trusted Information Systems, Hughes LAN
            Systems, April 1993.

   [Hin94]  Bob Hinden (Editor), Internet Protocol version 6 (IPv6)
            Specification, Work in Progress, 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.

   [Ken91]  Kent, S., "US DoD Security Options for the Internet
            Protocol", RFC 1108, BBN Communications, November 1991.

   [KMS95]  Karn, P., Metzger, P., and W. Simpson, "The ESP DES-CBC
            Transform", RFC 1829, Qualcomm, Inc., Piermont, Daydreamer,
            August 1995.

   [Mat94]  Matsui, M., "Linear Cryptanalysis method for DES Cipher",
            Proceedings of Eurocrypt '93, Berlin, Springer-Verlag, 1994.

   [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]  Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
            RFC 1700, USC/Information Sciences Institute, 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'S ADDRESS

   Randall Atkinson
   Information Technology Division
   Naval Research Laboratory
   Washington, DC 20375-5320
   USA

   Phone:  (202) 404-7090
   Fax:    (202) 404-7942
   EMail:  atkinson@itd.nrl.navy.mil