Implicit Initialization Vector (IV) for Counter-Based Ciphers in Encapsulating Security Payload (ESP)
RFC 8750

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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)

Abstract

   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
   https://www.rfc-editor.org/info/rfc8750.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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
   Acknowledgements
   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all