The NULL Encryption Algorithm and Its Use With IPsec

Network Working Group                                           R. Glenn
Request for Comments: 2410                                          NIST
Category: Standards Track                                        S. Kent
                                                                BBN Corp
                                                           November 1998


          The NULL Encryption Algorithm and Its Use With IPsec

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.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

   This memo defines the NULL encryption algorithm and its use with the
   IPsec Encapsulating Security Payload (ESP).  NULL does nothing to
   alter plaintext data.  In fact, NULL, by itself, does nothing.  NULL
   provides the means for ESP to provide authentication and integrity
   without confidentiality.

   Further information on the other components necessary for ESP
   implementations is provided by [ESP] and [ROAD].

1.  Introduction

   This memo defines the NULL encryption algorithm and its use with the
   IPsec Encapsulating Security Payload [ESP] to provide authentication
   and integrity without confidentiality.

   NULL is a block cipher the origins of which appear to be lost in
   antiquity.  Despite rumors that the National Security Agency
   suppressed publication of this algorithm, there is no evidence of
   such action on their part. Rather, recent archaeological evidence
   suggests that the NULL algorithm was developed in Roman times, as an
   exportable alternative to Ceaser ciphers. However, because Roman
   numerals lack a symbol for zero, written records of the algorithm's
   development were lost to historians for over two millennia.

   [ESP] specifies the use of an optional encryption algorithm to
   provide confidentiality and the use of an optional authentication
   algorithm to provide authentication and integrity.  The NULL
   encryption algorithm is a convenient way to represent the option of
   not applying encryption.  This is referred to as ESP_NULL in [DOI].

   The IPsec Authentication Header [AH] specification provides a similar
   service, by computing authentication data which covers the data
   portion of a packet as well as the immutable in transit portions of
   the IP header.  ESP_NULL does not include the IP header in
   calculating the authentication data.  This can be useful in providing
   IPsec services through non-IP network devices.  The discussion on how
   ESP_NULL might be used with non-IP network devices is outside the
   scope of this document.

   In this memo, NULL is used within the context of ESP.  For further
   information on how the various pieces of ESP fit together to provide
   security services, refer to [ESP] and [ROAD].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC 2119].

2. Algorithm Definition

   NULL is defined mathematically by the use of the Identity function I
   applied to a block of data b such that:

     NULL(b) = I(b) = b

2.1 Keying Material

   Like other modern ciphers, e.g., RC5 [RFC-2040], the NULL encryption
   algorithm can make use of keys of varying lengths.  However, no
   measurable increase in security is afforded by the use of longer key
   lengths.

2.2 Cryptographic Synchronization

   Because of the stateless nature of the NULL encryption algorithm, it
   is not necessary to transmit an IV or similar cryptographic
   synchronization data on a per packet (or even a per SA) basis.  The
   NULL encryption algorithm combines many of the best features of both
   block and stream ciphers, while still not requiring the transmission
   of an IV or analogous cryptographic synchronization data.

2.3 Padding

   NULL has a block size of 1 byte, thus padding is not necessary.

2.4. Performance

   The NULL encryption algorithm is significantly faster than other
   commonly used symmetric encryption algorithms and implementations of
   the base algorithm are available for all commonly used hardware and
   OS platforms.

2.5 Test Vectors

   The following is a set of test vectors to facilitate in the
   development of interoperable NULL implementations.

test_case =      1
data =           0x123456789abcdef
data_len =       8
NULL_data =      0x123456789abcdef

test_case =      2
data =           "Network Security People Have A Strange Sense Of Humor"
data_len =       53
NULL_data =      "Network Security People Have A Strange Sense Of Humor"

3. ESP_NULL Operational Requirements

   ESP_NULL is defined by using NULL within the context of ESP.  This
   section further defines ESP_NULL by pointing out particular
   operational parameter requirements.

   For purposes of IKE [IKE] key extraction, the key size for this
   algorithm MUST be zero (0) bits, to facilitate interoperability and
   to avoid any potential export control problems.

   To facilitate interoperability, the IV size for this algorithm MUST
   be zero (0) bits.

   Padding MAY be included on outgoing packets as specified in [ESP].

4. Security Considerations

   The NULL encryption algorithm offers no confidentiality nor does it
   offer any other security service.  It is simply a convenient way to
   represent the optional use of applying encryption within ESP.  ESP
   can then be used to provide authentication and integrity without
   confidentiality.  Unlike AH these services are not applied to any
   part of the IP header.  At the time of this writing there is no
   evidence to support that ESP_NULL is any less secure than AH when
   using the same authentication algorithm (i.e. a packet secured using
   ESP_NULL with some authentication algorithm is as cryptographically
   secure as a packet secured using AH with the same authentication
   algorithm).

   As stated in [ESP], while the use of encryption algorithms and
   authentication algorithms are optional in ESP, it is imperative that
   an ESP SA specifies the use of at least one cryptographically strong
   encryption algorithm or one cryptographically strong authentication
   algorithm or one of each.

   At the time of this writing there are no known laws preventing the
   exportation of NULL with a zero (0) bit key length.

5.  Intellectual Property Rights

   Pursuant to the provisions of [RFC-2026], the authors represent that
   they have disclosed the existence of any proprietary or intellectual
   property rights in the contribution that are reasonably and
   personally known to the authors.  The authors do not represent that
   they personally know of all potentially pertinent proprietary and
   intellectual property rights owned or claimed by the organizations
   they represent or third parties.

6.  Acknowledgments

   Steve Bellovin suggested and provided the text for the Intellectual
   Property Rights section.

   Credit also needs to be given to the participants of the Cisco/ICSA
   IPsec & IKE March 1998 Interoperability Workshop since it was there
   that the need for this document became apparent.

7.  References

   [ESP]        Kent, S., and R. Atkinson, "IP Encapsulating Security
                Payload", RFC 2406, November 1998.

   [AH]         Kent, S., and R. Atkinson, "IP Authentication Header",
                RFC 2402, November 1998.

   [ROAD]       Thayer, R., Doraswamy, N., and R. Glenn, "IP Security
                Document Roadmap", RFC 2411, November 1998.

   [DOI]        Piper, D., "The Internet IP Security Domain of
                Interpretation for ISAKMP", RFC 2408, November 1998.

   [IKE]        Harkins, D., and D. Carrel, "The Internet Key Exchange
                (IKE)", RFC 2409, November 1998.

   [RFC-2026]   Bradner, S., "The Internet Standards Process -- Revision
                3", BCP 9, RFC 2026, October 1996.

   [RFC-2040]   Baldwin, R., and R. Rivest, "The RC5, RC5-CBC, RC5-CBC-
                Pad, and RC5-CTS Algorithms", RFC 2040, October 1996

   [RFC-2119]   Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

6.  Editors' Addresses

   Rob Glenn
   NIST

   EMail: rob.glenn@nist.gov


   Stephen Kent
   BBN Corporation

   EMail: kent@bbn.com

   The IPsec working group can be contacted through the chairs:

   Robert Moskowitz
   ICSA

   EMail: rgm@icsa.net


   Ted T'so
   Massachusetts Institute of Technology

   EMail: tytso@mit.edu

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