RFC8750: Implicit Initialization Vector (IV) for Counter-Based Ciphers in Encapsulating Security Payload (ESP)

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Related keywords:  (CCM) (ChaCha20) (gcm) (IKE) (ipsec)

Internet Engineering Task Force (IETF)                        D. Migault
Request for Comments: 8750                                      Ericsson
Category: Standards Track                                    T. Guggemos
ISSN: 2070-1721                                               LMU Munich
                                                                  Y. Nir
                                                       Dell Technologies
                                                              March 2020

    Implicit Initialization Vector (IV) for Counter-Based Ciphers in
                  Encapsulating Security Payload (ESP)


   Encapsulating Security Payload (ESP) sends an initialization vector
   (IV) in each packet.  The size of the IV depends on the applied
   transform and is usually 8 or 16 octets for the transforms defined at
   the time this document was written.  When used with IPsec, some
   algorithms, such as AES-GCM, AES-CCM, and ChaCha20-Poly1305, take the
   IV to generate a nonce that is used as an input parameter for
   encrypting and decrypting.  This IV must be unique but can be
   predictable.  As a result, the value provided in the ESP Sequence
   Number (SN) can be used instead to generate the nonce.  This avoids
   sending the IV itself and saves 8 octets per packet in the case of
   AES-GCM, AES-CCM, and ChaCha20-Poly1305.  This document describes how
   to do this.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
   2.  Requirements Notation
   3.  Terminology
   4.  Implicit IV
   5.  IKEv2 Initiator Behavior
   6.  IKEv2 Responder Behavior
   7.  Security Considerations
   8.  IANA Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Authors' Addresses

1.  Introduction

   Counter-based AES modes of operation such as AES-CCM [RFC4309] and
   AES-GCM [RFC4106] require the specification of a nonce for each ESP
   packet.  The same applies for ChaCha20-Poly1305 [RFC7634].
   Currently, this nonce is generated thanks to the initialization
   vector (IV) provided in each ESP packet [RFC4303].  This practice is
   designated in this document as "explicit IV".

   In some contexts, such as the Internet of Things (IoT), it may be
   preferable to avoid carrying the extra bytes associated to the IV and
   instead generate it locally on each peer.  The local generation of
   the IV is designated in this document as "implicit IV".

   The size of this IV depends on the specific algorithm, but all of the
   algorithms mentioned above take an 8-octet IV.

   This document defines how to compute the IV locally when it is
   implicit.  It also specifies how peers agree with the Internet Key
   Exchange version 2 (IKEv2) [RFC7296] on using an implicit IV versus
   an explicit IV.

   This document limits its scope to the algorithms mentioned above.
   Other algorithms with similar properties may later be defined to use
   similar mechanisms.

   This document does not consider AES-CBC [RFC3602], as AES-CBC
   requires the IV to be unpredictable.  Deriving it directly from the
   packet counter as described below is insecure, as mentioned in
   Section 6 of [RFC3602], and has led to real-world chosen plaintext
   attacks such as BEAST [BEAST].

   This document does not consider AES-CTR [RFC3686], as it focuses on
   the recommended Authenticated Encryption with Associated Data (AEAD)
   suites provided in [RFC8221].

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   IoT:     Internet of Things

   IV:      Initialization Vector

   IIV:     Implicit Initialization Vector

   Nonce:   A fixed-size octet string used only once.  In this document,
            the IV is used to generate the nonce input for the

4.  Implicit IV

   With the algorithms listed in Section 1, the 8-byte IV MUST NOT
   repeat for a given key.  The binding between an ESP packet and its IV
   is provided using the Sequence Number or the Extended Sequence
   Number.  Figures 1 and 2 represent the IV with a regular 4-byte
   Sequence Number and an 8-byte Extended Sequence Number, respectively.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |                              Zero                             |
   |                      Sequence Number                          |

            Figure 1: Implicit IV with a 4-Byte Sequence Number

   Sequence Number:
      The 4-byte Sequence Number carried in the ESP packet.

      A 4-byte array with all bits set to zero.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |                         Extended                              |
   |                      Sequence Number                          |

       Figure 2: Implicit IV with an 8-Byte Extended Sequence Number

   Extended Sequence Number:
      The 8-byte Extended Sequence Number of the Security Association.
      The four low-order bytes are carried in the ESP packet.

   This document solely defines the IV generation of the algorithms
   defined in [RFC4106] for AES-GCM, [RFC4309] for AES-CCM, and
   [RFC7634] for ChaCha20-Poly1305.  All other aspects and parameters of
   those algorithms are unchanged and are used as defined in their
   respective specifications.

5.  IKEv2 Initiator Behavior

   An initiator supporting this feature SHOULD propose implicit IV (IIV)
   algorithms in the Transform Type 1 (Encryption Algorithm)
   Substructure of the Proposal Substructure inside the Security
   Association (SA) payload in the IKEv2 Exchange.  To facilitate
   backward compatibility with non-supporting peers, the initiator
   SHOULD also include those same algorithms with explicit IV as
   separate transforms.

6.  IKEv2 Responder Behavior

   The rules of SA payload processing require that the responder pick
   its algorithms from the proposal sent by the initiator, thus ensuring
   that the responder will never send an SA payload containing the IIV
   transform to an initiator that did not propose it.

7.  Security Considerations

   Nonce generation for these algorithms has not been explicitly
   defined.  It has been left to the implementation as long as certain
   security requirements are met.  Typically, for AES-GCM, AES-CCM, and
   ChaCha20-Poly1305, the IV is not allowed to be repeated for one
   particular key.  This document provides an explicit and normative way
   to generate IVs.  The mechanism described in this document meets the
   IV security requirements of all relevant algorithms.

   As the IV must not repeat for one SA when Counter-Mode ciphers are
   used, implicit IV as described in this document MUST NOT be used in
   setups with the chance that the Sequence Number overlaps for one SA.
   The sender's counter and the receiver's counter MUST be reset (by
   establishing a new SA and thus a new key) prior to the transmission
   of the 2^32nd packet for an SA that does not use an Extended Sequence
   Number and prior to the transmission of the 2^64th packet for an SA
   that does use an Extended Sequence Number.  This prevents Sequence
   Number overlaps for the mundane point-to-point case.  Multicast as
   described in [RFC5374], [RFC6407], and [G-IKEv2] is a prominent
   example in which many senders share one secret and thus one SA.  As
   such, implicit IV may only be used with Multicast if some mechanisms
   are employed that prevent the Sequence Number from overlapping for
   one SA; otherwise, implicit IV MUST NOT be used with Multicast.

   This document defines three new encryption transforms that use
   implicit IV.  Unlike most encryption transforms defined to date,
   which can be used for both ESP and IKEv2, these transforms are
   defined for ESP only and cannot be used in IKEv2.  The reason for
   this is that IKEv2 messages don't contain a unique per-message value
   that can be used for IV generation.  The Message-ID field in the
   IKEv2 header is similar to the SN field in the ESP header, but recent
   IKEv2 extensions [RFC6311] [RFC7383] do allow it to repeat, so there
   is not an easy way to derive unique IV from IKEv2 header fields.

8.  IANA Considerations

   IANA has updated the "Internet Key Exchange Version 2 (IKEv2)
   Parameters" registry [RFC7296] by adding the following new code
   points to the "Transform Type 1 - Encryption Algorithm Transform IDs"
   subregistry under the "Transform Type Values" registry [IANA]:

   | Number | Name                       | ESP Reference | IKEv2     |
   |        |                            |               | Reference |
   | 29     | ENCR_AES_CCM_8_IIV         | RFC 8750      | Not       |
   |        |                            |               | allowed   |
   | 30     | ENCR_AES_GCM_16_IIV        | RFC 8750      | Not       |
   |        |                            |               | allowed   |
   | 31     | ENCR_CHACHA20_POLY1305_IIV | RFC 8750      | Not       |
   |        |                            |               | allowed   |

      Table 1: Additions to "Transform Type 1 - Encryption Algorithm
                         Transform IDs" Registry

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3602]  Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
              Algorithm and Its Use with IPsec", RFC 3602,
              DOI 10.17487/RFC3602, September 2003,

   [RFC3686]  Housley, R., "Using Advanced Encryption Standard (AES)
              Counter Mode With IPsec Encapsulating Security Payload
              (ESP)", RFC 3686, DOI 10.17487/RFC3686, January 2004,

   [RFC4106]  Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
              (GCM) in IPsec Encapsulating Security Payload (ESP)",
              RFC 4106, DOI 10.17487/RFC4106, June 2005,

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,

   [RFC4309]  Housley, R., "Using Advanced Encryption Standard (AES) CCM
              Mode with IPsec Encapsulating Security Payload (ESP)",
              RFC 4309, DOI 10.17487/RFC4309, December 2005,

   [RFC5374]  Weis, B., Gross, G., and D. Ignjatic, "Multicast
              Extensions to the Security Architecture for the Internet
              Protocol", RFC 5374, DOI 10.17487/RFC5374, November 2008,

   [RFC6311]  Singh, R., Ed., Kalyani, G., Nir, Y., Sheffer, Y., and D.
              Zhang, "Protocol Support for High Availability of IKEv2/
              IPsec", RFC 6311, DOI 10.17487/RFC6311, July 2011,

   [RFC6407]  Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
              of Interpretation", RFC 6407, DOI 10.17487/RFC6407,
              October 2011, <https://www.rfc-editor.org/info/rfc6407>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC7383]  Smyslov, V., "Internet Key Exchange Protocol Version 2
              (IKEv2) Message Fragmentation", RFC 7383,
              DOI 10.17487/RFC7383, November 2014,

   [RFC7634]  Nir, Y., "ChaCha20, Poly1305, and Their Use in the
              Internet Key Exchange Protocol (IKE) and IPsec", RFC 7634,
              DOI 10.17487/RFC7634, August 2015,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8221]  Wouters, P., Migault, D., Mattsson, J., Nir, Y., and T.
              Kivinen, "Cryptographic Algorithm Implementation
              Requirements and Usage Guidance for Encapsulating Security
              Payload (ESP) and Authentication Header (AH)", RFC 8221,
              DOI 10.17487/RFC8221, October 2017,

9.2.  Informative References

   [BEAST]    Duong, T. and J. Rizzo, "Here Come The xor Ninjas", May
              2011, <https://www.researchgate.net/

   [G-IKEv2]  Weis, B. and V. Smyslov, "Group Key Management using
              IKEv2", Work in Progress, Internet-Draft, draft-ietf-
              ipsecme-g-ikev2-00, 8 January 2020,

   [IANA]     IANA, "Internet Key Exchange Version 2 (IKEv2)


   We would like to thank Valery Smyslov, Éric Vyncke, Alexey Melnikov,
   Adam Roach, and Magnus Nyström (security directorate) as well as our
   three Security ADs -- Eric Rescorla, Benjamin Kaduk, and Roman
   Danyliw -- for their valuable comments.  We also would like to thank
   David Schinazi for his implementation as well as Tero Kivinen and
   David Waltermire (the IPSECME Chairs) for moving this work forward.

Authors' Addresses

   Daniel Migault
   8275 Trans Canada Route
   Saint Laurent QC H4S 0B6

   Email: daniel.migault@ericsson.com

   Tobias Guggemos
   LMU Munich
   Oettingenstr. 67
   80538 Munich

   Email: guggemos@nm.ifi.lmu.de
   URI:   http://mnm-team.org/~guggemos

   Yoav Nir
   Dell Technologies
   9 Andrei Sakharov St
   Haifa 3190500

   Email: ynir.ietf@gmail.com