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