Internet Engineering Task Force (IETF)                           S. Shen
Request for Comments: 5930                                        Huawei
Category: Informational                                           Y. Mao
ISSN: 2070-1721                             Hangzhou H3C Tech. Co., Ltd.
                                                             NSS. Murthy
                                                 Freescale Semiconductor
                                                               July 2010


       Using Advanced Encryption Standard Counter Mode (AES-CTR)
       with the Internet Key Exchange version 02 (IKEv2) Protocol

Abstract

   This document describes the usage of Advanced Encryption Standard
   Counter Mode (AES-CTR), with an explicit Initialization Vector, by
   the Internet Key Exchange version 2 (IKEv2) protocol, for encrypting
   the IKEv2 exchanges that follow the IKE_SA_INIT exchange.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5930.

Copyright Notice

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................3
   2. IKEv2 Encrypted Payload .........................................3
   3. IKEv2 Conventions ...............................................4
   4. Security Considerations .........................................4
   5. IANA Considerations .............................................4
   6. Acknowledgments .................................................4
   7. References ......................................................5
      7.1. Normative References .......................................5
      7.2. Informative References .....................................5

1.  Introduction

   The Internet Key Exchange version 2 (IKEv2) protocol [RFC4306] is a
   component of IPsec used for performing mutual authentication and
   establishing and maintaining security associations (SAs).  [RFC4307]
   defines the set of algorithms that are mandatory to implement as part
   of IKEv2, as well as algorithms that should be implemented because
   they may be promoted to mandatory at some future time.  [RFC4307]
   requires that an implementation "SHOULD" support Advanced Encryption
   Standard [AES] Counter Mode [MODES] (AES-CTR) as a Transform Type 1
   algorithm (encryption).

   Although [RFC4307] specifies that the AES-CTR encryption algorithm
   feature SHOULD be supported by IKEv2, no existing document specifies
   how IKEv2 can support the feature.  This document provides the
   specification and usage of AES-CTR Counter Mode by IKEv2.

   Implementers need to carefully consider the use of AES-CTR over the
   mandatory-to-implement algorithms in [RFC4307], because the
   performance improvements of AES-CTR are minimal in the context of

   IKEv2.  Furthermore, these performance improvements may be offset by
   the Counter Mode specific risk of a minor, hard-to-detect
   implementation issue resulting in total security failure.

1.1.  Conventions Used in This Document

   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 [RFC2119].

2.  IKEv2 Encrypted Payload

   Section 3.14 of IKEv2 [RFC4306] explains the IKEv2 Encrypted Payload.
   The Encrypted Payload, denoted SK{...}, contains other IKEv2 payloads
   in encrypted form.

   The payload includes an Initialization Vector (IV) whose length is
   defined by the encryption algorithm negotiated.  It also includes
   Integrity Checksum data.  These two fields are not encrypted.

   The IV field MUST be 8 octets when the AES-CTR algorithm is used for
   IKEv2 encryption.  The requirements for this IV are the same as what
   is specified for the Encapsulating Security Payload (ESP) in
   Section 3.1 of [RFC3686].

   IKEv2 requires Integrity Check Data for the Encrypted Payload as
   described in Section 3.14 of [RFC4306].  The choice of integrity
   algorithms in IKEv2 is defined in [RFC4307] or documents that update
   it in the future.

   When AES-CTR is used in IKEv2, no padding is required.  The Padding
   field of the Encrypted Payload SHOULD be empty, and the Pad Length
   field SHOULD be zero.  However, according to [RFC4306], the recipient
   MUST accept any length that results in proper alignment.  It should
   be noted that the ESP [RFC4303] Encrypted Payload requires alignment
   on a 4-byte boundary while the IKEv2 [RFC4306] Encrypted Payload does
   not have such a requirement.

   The Encrypted Payload is the XOR of the plaintext and key stream.
   The key stream is generated by inputting counter blocks into the AES
   algorithm.  The AES counter block is 128 bits, including a 4-octet
   Nonce, 8-octet Initialization Vector, and 4-octet Block Counter, in
   that order.  The Block Counter begins with the value of one and
   increments by one to generate the next portion of the key stream.
   The detailed requirements for the counter block are the same as those
   specified in Section 4 of [RFC3686].

3.  IKEv2 Conventions

   The use of AES-CTR for the IKE SA is negotiated in the same way as
   AES-CTR for ESP.  The Transform ID (ENCR_AES_CTR) is the same; the
   key length transform attribute is used in the same way; and the
   keying material (consisting of the actual key and the nonce) is
   derived in the same way.  See Section 5 of [RFC3686] for detailed
   descriptions.

4.  Security Considerations

   Security considerations explained in Section 7 of [RFC3686] are
   entirely relevant to this document as well.  The security
   considerations on fresh keys and integrity protection in Section 7 of
   [RFC3686] are totally applicable to using AES-CTR in IKEv2; see
   [RFC3686] for details.  As static keys are never used in IKEv2 for
   IKE_SA and integrity protection is mandatory for IKE_SA, these issues
   are not applicable for AES-CTR in IKEv2 when protecting IKE_SA.

   Additionally, since AES has a 128-bit block size, regardless of the
   mode employed, the ciphertext generated by AES encryption becomes
   distinguishable from random values after 2^64 blocks are encrypted
   with a single key.  Since IKEv2 SA cannot carry that much data
   (because of the size limit of the message ID of the IKEv2 message and
   the requirements for the message ID in Section 4 of [RFC4306]), this
   issue is not a concern here.

   For generic attacks on AES, such as brute force or precalculations,
   the key-size requirements provide reasonable security
   [Recommendations].

5.  IANA Considerations

   IANA [IANA-Para] has assigned an Encryption Algorithm Transform ID
   for AES-CTR encryption with an explicit IV for IKEv2: 13 as the
   number, and ENCR_AES_CTR as the name.  IANA has added a reference to
   this RFC in that entry.

6.  Acknowledgments

   The authors thank Yaron Sheffer, Paul Hoffman, Tero Kivinen, and
   Alfred Hoenes for their direction and comments on this document.

   This document specifies usage of AES-CTR with IKEv2, similar to usage
   of AES-CTR with ESP as specified in [RFC3686].  The reader is
   referred to [RFC3686] for the same descriptions and definitions.  The
   authors thank Russ Housley for providing the document.

   During the production and modification of this document, both Huawei
   and CNNIC supported one of the authors, Sean Shen.  Both are
   appreciated as affiliations of the author.

7.  References

7.1.  Normative References

   [RFC2119]         Bradner, S., "Key words for use in RFCs to Indicate
                     Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3686]         Housley, R., "Using Advanced Encryption Standard
                     (AES) Counter Mode With IPsec Encapsulating
                     Security Payload (ESP)", RFC 3686, January 2004.

   [RFC4306]         Kaufman, C., "Internet Key Exchange (IKEv2)
                     Protocol", RFC 4306, December 2005.

   [RFC4307]         Schiller, J., "Cryptographic Algorithms for Use in
                     the Internet Key Exchange Version 2 (IKEv2)",
                     RFC 4307, December 2005.

   [AES]             National Institute of Standards and Technology,
                     "Advanced Encryption Standard (AES)", FIPS PUB 197,
                     November 2001, <http://csrc.nist.gov/
                     publications/fips/fips197/fips-197.pdf>.

   [IANA-Para]       Internet Assigned Numbers Authority, "Internet Key
                     Exchange Version 2 (IKEv2) Parameters",
                     <http://www.iana.org>.

   [MODES]           Dworkin, M., "Recommendation for Block Cipher Modes
                     of Operation -- Methods and Techniques", NIST
                     Special Publication 800-38A, December 2001,
                     <http://csrc.nist.gov/publications/nistpubs/
                     800-38a/sp800-38a.pdf>.

7.2.  Informative References

   [RFC4303]         Kent, S., "IP Encapsulating Security Payload
                     (ESP)", RFC 4303, December 2005.

   [Recommendations] Barker, E., Barker, W., Burr, W., Polk, W., and M.
                     Smid, "Recommendation for Key Management - Part 1:
                     General (Revised)", NIST Special
                     Publication 800-57, March 2007, <http://
                     csrc.nist.gov/publications/nistpubs/800-57/
                     sp800-57-Part1-revised2_Mar08-2007.pdf>.

Authors' Addresses

   Sean Shen
   Huawei
   4, South 4th Street, Zhongguancun
   Beijing  100190
   China

   EMail: shenshuo@cnnic.cn


   Yu Mao
   Hangzhou H3C Tech. Co., Ltd.
   Oriental Electronic Bld., No. 2
   Chuangye Road
   Shang-Di Information Industry
   Hai-Dian District
   Beijing  100085
   China

   EMail: yumao9@gmail.com


   N S Srinivasa Murthy
   Freescale Semiconductor
   UMA PLAZA, NAGARJUNA CIRCLE, PUNJAGUTTA
   HYDERABAD  500082
   INDIA

   EMail: ssmurthy.nittala@freescale.com