Implicit IV for Counter-based Ciphers in Encapsulating Security Payload (ESP)
draft-ietf-ipsecme-implicit-iv-06

Abstract

Encapsulating Security Payload (ESP) sends an initialization vector (IV) or nonce in each packet. The size of IV depends on the applied transform, being usually 8 or 16 octets for the transforms defined by the time this document is written. Some algorithms such as AES-GCM, AES-CCM, AES-CTR and ChaCha20-Poly1305 require a unique nonce but do not require an unpredictable nonce. When using such algorithms the packet counter value can be used to generate a nonce. This avoids sending the nonce itself, and saves in the case of AES-GCM, AES-CCM, AES-CTR and ChaCha20-Poly1305 8 octets per packet. This document describes how to do this.

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Copyright Notice

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Table of Contents

• 1. Requirements notation
• 2. Introduction
• 3. Terminology
• 4. Implicit IV
• 5. Initiator Behavior
• 6. Responder Behavior
• 7. Security Consideration
• 8. IANA Considerations
• 9. Acknowledgements
• 10. References
• 10.1. Normative References
• 10.2. Informational References
• Authors' Addresses

1. Requirements notation

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. Introduction

Counter-based AES modes of operation such as AES-CTR ([RFC3686]), AES-CCM ([RFC4309]), and AES-GCM ([RFC4106]) require the specification of an nonce for each ESP packet. The same applies for ChaCha20-Poly1305 ([RFC7634]). Currently this nonce is sent in each ESP packet ([RFC4303]). This practice is designated in this document as "explicit nonce".

In some context, such as 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 nonce is designated in this document as "implicit IV".

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

This document defines how to compute the nonce 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 this extension.

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 Security Consideration of [RFC3602] and has led to real world chosen plain-text attack such as BEAST [BEAST].

3. Terminology

• IoT: Internet of Things.
• IV: Initialization Vector.
• IIV: Implicit Initialization Vector.
• Nonce: a fixed-size octet string used only once. This is similar to IV, except that in common usage there is no implication of non-predictability.

4. Implicit IV

With the algorithms listed in Section 2, the 8 byte nonce MUST NOT repeat. The binding between a ESP packet and its nonce is provided using the Sequence Number or the Extended Sequence Number. Figure 1 and Figure 2 represent the IV with a regular 4-byte Sequence Number and with 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.
• Zero: 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 4 byte low order bytes are carried in the ESP packet.

5. Initiator Behavior

An initiator supporting this feature SHOULD propose implicit IV algorithms in the Transform Type 1 (Encryption Algorithm) Substructure of the Proposal Substructure inside the SA Payload. To facilitate backward compatibility with non-supporting peers the initiator SHOULD also include those same algorithms without Implicit IV (IIV) as separate transforms.

6. Responder Behavior

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

7. Security Consideration

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, AES-CTR and ChaCha20-Poly1305, the IV is not allowed being 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. Multicast as described in [RFC5374], [RFC6407] and [I-D.yeung-g-ikev2] is a prominent example, where 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 Sequence Number to overlap 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 is that IKEv2 messages don't contain unique per-message value, that can be used for IV generation. The Message-ID field in IKEv2 header is somewhat counterpart of SN field in ESP header, but recent IKEv2 extensions ([RFC6311], [RFC7383]) do allow it to repeat, so there is no an easy way to derive unique IV from IKEv2 header fields.

8. IANA Considerations

This section assigns new code points to the recommended AEAD suites provided in [RFC8221], thus the new Transform Type 1 - Encryption Algorithm Transform IDs [IANA] are as defined below:

-

ENCR_AES_CCM_8_IIV: 29

-

ENCR_AES_GCM_16_IIV: 30

-

ENCR_CHACHA20_POLY1305_IIV: 31

These algorithms should be added with this document as ESP Reference and "Not Allowed" for IKEv2 Reference.

9. Acknowledgements

We would like to thanks people Valery Smyslov for their valuable comments, David Schinazi for its implementation, as well as the ipseceme chairs Tero Kivinen and David Waltermire for moving this work forward.

10. References

10.1. Normative References

10.2. Informational References

Authors' Addresses