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Network Working Group                                        R. AtkinsonRequest for Comments: 1827                     Naval Research LaboratoryCategory: Standards Track                                    August 1995IP 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.Atkinson                    Standards Track                     [Page 1]

RFC 1827             Encapsulating Security Payload          August 1995   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.Atkinson                    Standards Track                     [Page 2]

RFC 1827             Encapsulating Security Payload          August 19951.2 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,Atkinson                    Standards Track                     [Page 3]

RFC 1827             Encapsulating Security Payload          August 1995   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.Atkinson                    Standards Track                     [Page 4]

RFC 1827             Encapsulating Security Payload          August 19953.1 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 byRFC-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 MAYAtkinson                    Standards Track                     [Page 5]

RFC 1827             Encapsulating Security Payload          August 1995   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 theAtkinson                    Standards Track                     [Page 6]

RFC 1827             Encapsulating Security Payload          August 1995   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, aAtkinson                    Standards Track                     [Page 7]

RFC 1827             Encapsulating Security Payload          August 1995   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.Atkinson                    Standards Track                     [Page 8]

RFC 1827             Encapsulating Security Payload          August 19955. 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-Atkinson                    Standards Track                     [Page 9]

RFC 1827             Encapsulating Security Payload          August 1995   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.Atkinson                    Standards Track                    [Page 10]

RFC 1827             Encapsulating Security Payload          August 1995   [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.Atkinson                    Standards Track                    [Page 11]

RFC 1827             Encapsulating Security Payload          August 1995   [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.milAtkinson                    Standards Track                    [Page 12]